Fuel Depletion and Neutron Poisons
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Operator Generic Fundamentals Reactor Theory – Neutron Poisons © Copyright 2014 Operator Generic Fundamentals 2 Neutron Poisons Introduction • This lesson looks at fuel depletion and methods used to lengthen time between refueling outages • Will review fission product poisons, including xenon-135 and Samarium-149 • Xenon-135 – Most significant fission product poison – Creates numerous operational issues for operators • Samarium-149 – Second most significant fission product poison – Production and removal processes – Operational issues associated with Sm © Copyright 2014 INTRO Operator Generic Fundamentals 3 Terminal Learning Objectives At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80% score on the following TLOs: 1. Describe how fuel depletion and neutron poison concentration affect reactivity in a reactor core. 2. Describe the behavior of xenon-135 in a nuclear reactor and its effects on reactor operation. 3. Describe the production, removal, and effects of samarium-149 on the operation of a nuclear reactor. © Copyright 2014 INTRO Operator Generic Fundamentals 4 Fuel Depletion and Neutron Poisons TLO 1 – Describe how fuel depletion and neutron poison concentration affect reactivity in a reactor core. • Reactivity effects from nuclear fuel depletion and methods used for increasing core life discussed • Fuel depletion and neutron poison terms explained • Types of neutron poisons and methods used for kexcess control discussed • Core life and neutron poison affects on moderator and doppler coefficients, thermal flux, and control rod worth explained © Copyright 2014 TLO 1 Operator Generic Fundamentals 5 Enabling Learning Objectives for TLO 1 1. Describe fuel depletion for a nuclear reactor and how it impacts reactivity over the life of the core. 2. Explain the following terms: fuel cycle, fuel exposure, conversion ratio, burnable poison, non-burnable poison, and chemical shim. 3. Explain the concept and use of burnable neutron poisons in a reactor core. 4. Describe the design of installed burnable poisons for a nuclear reactor. 5. Explain the advantages and disadvantages of chemical shim over fixed burnable poisons. © Copyright 2014 TLO 1 Operator Generic Fundamentals 6 Enabling Learning Objectives for TLO 1 6. Describe fixed non-burnable poisons used in reactor cores, include an example of material used as non-burnable poison. 7. Explain how the following nuclear reactor core parameters change over core life due to fuel depletion and neutron poison concentration: moderator temperature coefficient , Doppler coefficient, control rod worth, and core thermal flux. 8. Explain the change in the value of excess reactivity over core life. 9. Describe the effect of changes in boron concentration on reactivity during natural circulation conditions. © Copyright 2014 TLO 1 Operator Generic Fundamentals 7 Reactivity Effects of Fuel Depletion ELO 1.1 – Describe fuel depletion for a nuclear reactor and how it impacts reactivity over the life of the core. • Fission process in a nuclear reactor’s fuel results in burnup of fissionable nuclei and a gradual depletion of fuel • Depletion of fissionable nuclei results in negative reactivity addition and a very slow decrease in kexcess • To maintain keff=1.0, dilution of boron from RCS/ moderator is performed – Control rods normally maintained fully out at 100%, therefore boron is only variable positive reactivity source available to operators – Fixed burnable poisons also installed to aid control of kexcess, but not under operator control © Copyright 2014 ELO 1.1 Operator Generic Fundamentals 8 Reactivity Effects of Fuel Depletion • As fuel depletes, – Fuel mix of uranium, plutonium, and other fuel nuclei changes – Fission products production affected by different fuels – Negative reactivity from fuel burnout • To maintain keff=1, positive reactivity must be added to reactor – Soluble boron used throughout core life for positive reactivity – Fixed poison, used primarily during first third of core life © Copyright 2014 ELO 1.1 Operator Generic Fundamentals 9 Reactivity Effects of Fuel Depletion Except for non-leakage terms, fuel depletion will affect factors of sixfactor formula and keff as follows: • Fast fission factor (ε) – Uranium-235 depletion results in fewer fast fissions occurring from uranium-235 (fast fission factor decreases) – Result is small impact for ε – May drop from 1.04 in new core to 1.03 in depleted core © Copyright 2014 ELO 1.1 Operator Generic Fundamentals 10 Reactivity Effects of Fuel Depletion • Resonance escape probability – As core ages, some uranium-238 is converted to plutonium-240 by following reaction 238 1 239 π½−239 π½−239 π+ π→ π ππ ππ’ 92 0 92 93 94 239 1 240 ππ’ + π → ππ’ 94 0 94 – Urainium-238, with high resonance absorption cross-section peaks, depletes and plutonium-240 increases – Plutonium-240 has higher resonance absorption cross-section peaks – Results in decrease in resonance escape probability over core life © Copyright 2014 ELO 1.1 Operator Generic Fundamentals 11 Reactivity Effects of Fuel Depletion • Thermal utilization – ƒ – Decreasing number of uranium-235 atoms causes a decrease in thermal utilization factor – As fuel concentration decreases, moderator-to-fuel ratio increases o Increases probability of neutron absorption by moderator o Fewer neutrons available for absorption in fuel, further decrease for ƒ – Soluble boron, control rods, and burnable poisons used in reactor to control kexcess from fuel loading © Copyright 2014 ELO 1.1 Operator Generic Fundamentals 12 Reactivity Effects of Fuel Depletion • Thermal utilization – ƒ (cont’d) – Boron concentration and burnable poison concentrations reduced to compensate for fuel burnup and fission product poisons over core life – Lowering concentration of boron decreases probability that thermal neutrons will be absorbed by boron o Results in an increase to thermal utilization factor – Plutonium-239 builds up over core life from U-238 neutron capture – Plutonium-239 is a fissionable fuel, its increase causes an increase to thermal utilization factor – Overall effect over core life of these effects may be a slight increase in thermal utilization factor because of decreasing boron concentration © Copyright 2014 ELO 1.1 Operator Generic Fundamentals 13 Reactivity Effects of Fuel Depletion • Reproduction factor – Plutomiun-239 production from neutron capture by uranium-238 – Neutron yield per fission for Pu-239 slightly higher than U-235, but production of plutonium-239 lags depletion of uranium-235 o Slight decrease in reproduction factor occurs over core life • Non-leakage factors have no significant changes over core life • Overall effects on keff ππππ = π β πΏπ → π ↓ πΏπ‘β → π β π β – keff decreases without operator action © Copyright 2014 ELO 1.1 Operator Generic Fundamentals 14 Reactivity Effects of Fuel Depletion Knowledge Check Fuel depletion in a nuclear reactor causes thermal utilization to decrease unless soluble boron is diluted to add positive reactivity. A. True B. False Correct answer is A. © Copyright 2014 ELO 1.1 Operator Generic Fundamentals 15 Fuel Depletion Common Terms ELO 1.2 – Explain the following terms: fuel cycle, fuel exposure, conversion ratio, burnable poison, non-burnable poison, and chemical shim. • This section introduces terms related to fuel loads, fuel depletion, fuel production, and control of excess reactivity (kexcess) • Fuel metallurgical limits and kexcess determine fuel cycle length and maximum amount of energy a reactor can produce • Metallurgical limits ensure integrity of fuel and its cladding © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 16 Fuel Depletion Common Terms Fuel Cycle • PWRs loaded with enough fuel to operate at 100% power for an 18or 24-month fuel cycle • Core life is occasionally extended by using a power coastdown • Fuel cycle describes cycle of core operation between refuelings • Maximize fuel cycle to achieve maximum economic benefits from fuel – Minimizing down time and extending time between RFO’s © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 17 Fuel Depletion Common Terms Fuel Cycle Continued • Fuel cycles and maximum energy available from fuel determined by – Fuel metallurgical limits – Installed kexcess • Metallurgical limits needed to ensure integrity of fuel and cladding – Contain fission products, many of which are gases • Reactivity limits consider – Shutdown margin – Temperature coefficients – Fuel thermal limits (restrict temperatures) © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 18 Fuel Depletion Common Terms Fuel Exposure • Describes amount of energy released per unit weight of fuel • Measured in megawatt days per metric ton uranium (Mwd/MTU) • Megawatts relate to core thermal power output, with a metric ton equaling 1,000 kg of uranium Examples: – 2,940 Mw thermal for one day = 2,940 Mwd of thermal output – 1,470 Mw thermal for two days = 2,940 Mwd of thermal output – If loaded with 71.5 metric tons of uranium, then fuel exposure = 2,940 Mwd/71.5MTU = 41 Mwd/MTU © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 19 Fuel Depletion Common Terms Fuel Exposure Continued • For an 18-month cycle, EOL exposure (entire cycle) is approximately 20,000 Mwd/MTU – Used for three cycles, may reach 55,000 Mwd/MTU • Twice used fuel assemblies loaded into low power regions to reduce extended life thermal stresses • Fuel exposure may be applied to: – An individual fuel element – Average value of a section of the core – Entire reactor core • Fuel design efforts improve thermal and nuclear design characteristics for longer life and higher output – Desired for financial benefits – Non-proliferation fuel designs desired © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 20 Fuel Depletion Common Terms Conversion Ratio • Advantage of using low enrichment fuel is it breeds its own fuel • When uranium-238 absorbs a neutron, it normally doesn’t fission, but instead, from neutron capture, becomes uranium-239, which decays to neptunium-239 and decays to plutonium-239 : 238 1 239 π½−239 π½−239 π+ π→ π ππ ππ’ 92 0 92 93 94 • Microscopic thermal fission cross-section of plutonium-239 is even larger than uranium-235, making it a good fissionable fuel © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 21 Fuel Depletion Common Terms Conversion Ratio Continued • When plutonium-239 absorbs a neutron and doesn’t fission, it forms plutonium-240, which becomes plutonium-241 from neutron capture: 239 1 240 1 241 ππ’ + π → ππ’ + π → ππ’ 94 0 94 0 94 • Plutonium-241 has an even larger microscopic thermal fission crosssection than either uranium-235 or plutonium-239 • Additional fuel produced while uranium-235 is depleting because of these neutron captures and beta decays © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 22 Fuel Depletion Common Terms Conversion Ratio Continued • Ratio of fuel production to depletion called conversion ratio or breeding ratio • Must be one or greater to be self-sustaining – Fast breeder reactors capable of this • May be expressed as a percent or a decimal • PWRs in range of 50% to 70% or 0.5 to 0.7 Example: • A conversion ratio of 50 percent – 50 fissionable plutonium-239/241 nuclei created per 100 uranium235 nuclei consumed © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 23 Fuel Depletion Common Terms Conversion Ratio Continued • At EOL, enrichment of uranium-235 is reduced considerably, with an appreciable amount of plutonium accumulated Example: • 4.0% enriched uranium-235 fuel element with 40,000 MWd/MTU of burnup, will have: – EOL enrichment (U-235) 0.8% – About 0.6% plutonium-239 – Some small amount of plutonium-241 © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 24 Fuel Depletion Common Terms Burnable Poison • Burnable poisons utilized to control large amounts of reactivity from excess fuel • Good burnable poisons have – Large microscopic absorption cross-section for neutrons – Do not fission – Create products having low microscopic absorption crosssections • Boron-10 has a 3,838 barn thermal neutron microscopic absorption cross-section – After absorbing a neutron, normally produces Helium-4 and Lithium-7 – Both have very low microscopic neutron absorption cross-section © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 25 Fuel Depletion Common Terms Burnable Poison Continued • Negative reactivity added from burnable poisons decreases over core life (depletion) – Ideally, this decrease should match depletion of excess reactivity from fuel, cancel each other out • Generally used in form of boron or gadolinium manufactured as rods inserted into a fuel element assembly – Or contained within fuel pellets • Distributed more evenly than control rods – Less disruptive to core power distribution © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 26 Fuel Depletion Common Terms Non-Burnable Poison • A non-burnable poison: – Has a relatively large absorption cross-section for neutrons – Does not fission – Creates products with medium to large absorption cross-sections – Maintains constant negative reactivity worth over life of core • Hafnium has six stable isotopes, all having medium to large microscopic capture cross-sections – When a hafnium isotope absorbs a neutron, it forms another hafnium isotope – Consequently, macroscopic cross-section of hafnium remains large for entire fuel cycle © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 27 Fuel Depletion Common Terms Non-Burnable Poison Continued • Absorbers with low neutron absorption cross-sections can also be treated as non-burnable poisons • Self-shielding used for more even depletion of a burnable poison – Thick materials used – From self-shielding, absorptions take place in outer layers first then work inward as outer layers convert to non-poisonous – Allows for an even negative reactivity insertion over a longer period of time as poison depletes • Fixed non-burnable poisons used for: – Power shaping (flux) – Prevent power peaking near high moderator regions of reactor © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 28 Fuel Depletion Common Terms Chemical Shim (Soluble Boron) • Name given to adjusting concentration of boric acid dissolved in reactor coolant system (RCS) – Used in RCS to control large amount of kexcess • Primarily Boron-10, allows much finer control of reactivity than control rods • Boron-10, 20% of naturally occurring boron, is neutron-absorbing poison and is 17.5% of boric acid weight – Can adjust reactivity by varying concentration of boric acid (comprised of boron-10 isotope) in RCS © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 29 Fuel Depletion Common Terms Chemical Shim (Soluble Boron) Continued • Increasing boron concentration – Moderator absorbs more neutrons – Adds negative reactivity • Diluting boron concentration – Adds positive reactivity • Changing boron concentration in a PWR is a slow process – Used primarily to compensate for fuel burnout – Slower power transients, or fission product poison buildup • Soluble boron produces a spatially uniform neutron absorption method when dissolved in moderator © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 30 Fuel Depletion Common Terms Chemical Shim (Soluble Boron) Continued • Adjusting boron concentration – Minimizes control rod use – Allows for rod positioning at 100% withdrawn o Flatter flux profile for even fuel burnout and reduced power peaking • Flatter flux profile created by: – No regions of depressed flux like those produced in vicinity of inserted control rods © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 31 Fuel Depletion Common Terms Knowledge Check Adjusting the concentration of boric acid dissolved in the reactor coolant system is called . A. soluble hafnium B. chemical shim C. conversion ratio D. boron ratio Correct answer is B. © Copyright 2014 ELO 1.2 Operator Generic Fundamentals 32 Use of Burnable Neutron Poisons ELO 1.3 – Explain the concept and use of burnable neutron poisons in a reactor core. • Burnable poisons installed to work with soluble boron (chemical shim) for controlling large amount of kexcess (excess fuel) needed for long duration fuel cycles • This session discusses the concept and use of burnable poisons • Poison designed to burnup or deplete at a rate approximately equal to or faster than negative reactivity addition rate from fuel depletion • At fuel cycle EOL, burnable poisons should be fully depleted © Copyright 2014 ELO 1.3 Operator Generic Fundamentals 33 Use of Burnable Neutron Poisons • Burnable poisons used in nuclear reactors for following purposes: – Shape neutron flux for more uniform power density – Allow higher fuel enrichment – Preclude high concentrations of soluble boron at BOL (mitigates positive MTC) • Commercial PWRs load fuel for 18-month or longer fuel cycles – Higher enrichment fuel used – To offset kexcess, burnable and soluble poisons used – Burnable poison depletion rates need to be balanced with rate of fuel depletion and burn-up prior to EOL © Copyright 2014 ELO 1.3 Operator Generic Fundamentals 34 Use of Burnable Neutron Poisons • Designed to: – Minimize need for soluble boron adjustments and control rod repositioning – Leave little negative reactivity remaining at EOL • Any negative reactivity at EOL shortens core life • Fixed burnable poisons may be located in specific patterns for shaping and controlling flux profiles in core © Copyright 2014 ELO 1.3 Operator Generic Fundamentals 35 Use of Burnable Neutron Poisons IMPORTANT, by using installed burnable poisons: • Soluble boron concentrations at BOL can be decreased to prevent significant positive MTC Recall from previous lessons that a positive MTC is undesirable and limited by plant technical specifications. At BOL, a small positive MTC may be allowed, but will generally become negative by the time power is increased to 100%. © Copyright 2014 ELO 1.3 Operator Generic Fundamentals 36 Burnable Neutron Poisons Knowledge Check – NRC Bank Instead of using a higher concentration of soluble boric acid, burnable poisons are installed in a new nuclear reactor core to ___________. A. prevent boron precipitation during normal operation B. establish a more negative moderator temperature coefficient C. allow control rods to be inserted farther upon initial criticality D. maintain reactor coolant pH above a minimum acceptable value Correct answer is B. © Copyright 2014 ELO 1.3 Operator Generic Fundamentals 37 Burnable Neutron Poison Design ELO 1.4 – Describe the design of installed burnable poisons for a nuclear reactor. • Installed burnable poisons are effective, along with soluble boron, to counter kexcess needed for extended fuel cycles • This section discusses – Design – Materials used – Construction – Installation practices for installed burnable poisons © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 38 Burnable Neutron Poison Design • Two general types: – Burnable poison rod assemblies (BPRAs) – Burnable poison integral to fuel pellet itself • BPRAs resemble control rods, but are fixed within fuel elements and therefore cannot be moved during the cycle • Westinghouse has a design where BPRA has an annular hole in middle of rodlet – Called a Wet Annular Burnable Poison Assembly (WABPA) – Advantage is that poison exposed to neutron flux from inside out and outside in © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 39 Burnable Neutron Poison Design • Most popular installed burnable poison is compound that contains boron-10 • Boron-10 has large thermal neutron cross-section of 3,838 barns – Greater than absorption cross-sections of U-235, Pu-239, or Pu241 • Westinghouse has designed a fuel pellet with a thin coating of a Boron compound on surface of pellet – Location burns out neutron poison very fast © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 40 Burnable Neutron Poison Design • Gadolinium is also been used as a burnable poison • Gadolinium-155 has a cross-section of 61,000 barns • Gadolinium-157 has a cross-section of 255,000 barns for thermal neutrons – Gadolinium is usually applied throughout the fuel pellet – Provides self-shielding effect and slowing burnout rate – Still depletes quickly – Burns out quicker than xenon builds in, requires boration © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 41 Burnable Neutron Poison Design • Westinghouse Boron-10 coated fuel also burns up very fast – Similar effect to gadolinium, but a closer match to xenon, less boration needed – After about 2500 Mwd/MTU, dilution is again needed to balance fuel burnout © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 42 Burnable Poisons Materials • Materials most commonly used are boron and gadolinium • Both have much higher cross-section for absorption than U-235 © Copyright 2014 Isotope Cross-Section Uranium-235 683 barns Boron-10 3,838 barns Gadolinium-155 61,000 barns Gadolinium-157 255,000 barns ELO 1.4 Operator Generic Fundamentals 43 Burnable Neutron Poison Design • Boron-10 undergoes a neutron-alpha (n, ο‘) reaction • Gadolinium-155 and gadolinium-157 undergo neutron-gamma (n, ο§) reactions • Neutron poison capture reactions generally result in nuclei that have relatively small neutron absorption cross-sections and stable daughter products EXCEPTION • Hafnium, sometimes used in control rods • Has a decay chain of five successive isotopes with high thermal neutron cross-sections – Considered non-burnable poison © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 44 Burnable Poison Rods • Longer fuel cycles (18 months or more) use burnable poison rods • In PWRs, account for approximately 6% to 8% Δk/k reactivity Figure: Burnable Poison Depletion Over Core Life © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 45 Burnable Poison Rod Construction • Consists of a borosilicate glass tube (borated silicate glass 12.5 wt.% without B2O3) – Contained in 304 stainless steel cladding – Plugged and seal welded at both ends to encapsulate glass – Glass tube supported along its inside diameter by a thin walled tubular inner liner • Borosilicate glass is very similar to Pyrex® • Construction ensures that by-products of neutron absorption reactions are contained within the rods © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 46 Burnable Neutron Poison Design Figure: Borosilicate Glass Burnable Poison Road © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 47 Burnable Poison Rod Positioning • Radially spaced to provide a flatter radial power distribution Figure: Burnable Poison Rod Positioning in a PWR Core © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 48 Integrated Fuel Burnable Absorbers • Borosilicate glass rods effective at controlling kexcess • Limitation is that they display relatively high residual absorption rate at EOL – Boron in rods is not completely depleted • Consequence is that EOL will still have fuel that cannot be used to produce power © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 49 Integrated Fuel Burnable Absorbers • To minimize these consequences, Integrated Fuel Burnable Absorbers (IFBA) are manufactured into commercial PWR reactor fuel pellets • IFBA pellets are coated with a thin film of zirconium diboride (ZrB2) – Coating reduces thermal neutron flux available for fission as compared to an uncoated pellet • Boron coating is normally depleted prior to end of one fuel cycle © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 50 Integrated Fuel Burnable Absorbers • IFBA pellets located in middle of fuel rod where highest flux exists • Regular pellets located at ends with low flux • Provides a more even axial flux pattern • Allows increased fuel enrichment without associated higher power peaks • Improved uniform fuel burnup with lower peak centerline fuel temperatures Figure: IFBA Pellet © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 51 Burnable Neutron Poisons Knowledge Check The reactor engineering group has noticed that the critical boron versus Mwd/MTU curve is indicating a lower boron concentration than expected. Which one of the following could be a cause for this? Note: the core is half-way through its fuel cycle and power has been at 100% for 2 months. A. Fuel enrichment is higher than design B. Installed fixed burnable poisons are depleting faster than anticipated C. Dropped control rod is undetected D. Xenon has not reached equilibrium Correct answer is C. © Copyright 2014 ELO 1.4 Operator Generic Fundamentals 52 Chemical Shim vs. Fixed Burnable Poisons ELO 1.5 – Explain the advantages and disadvantages of chemical shim over fixed burnable poisons. • This section will summarize advantages and disadvantages of – Soluble poisons – Fixed burnable poisons © Copyright 2014 ELO 1.5 Operator Generic Fundamentals 53 Chemical Shim Advantages • Cost effective method for balancing kexcess to increase fuel loading and fuel cycle time • Reactivity contribution to kexcess can be varied • Produce a spatially uniform neutron absorption – Flatter flux profile than can be produced by rod insertion • Finer control of reactivity than control rods • Minimizes control rod use and allows positioning for – Control of flux profiles – Improved fuel performance, lower fuel temperatures, even fuel burnout, and reduced power peaking • Allows for fewer control rods for kexcess control © Copyright 2014 ELO 1.5 Operator Generic Fundamentals 54 Chemical Shim Disadvantages • In high concentrations, can result in undesirable positive moderator coefficient • Additional costs involved in boric acid handling, storage, processing, etc. • Metallurgical issues (corrosion) • Changing boron concentration in a PWR is a slow process and therefore slow reactivity changes © Copyright 2014 ELO 1.5 Operator Generic Fundamentals 55 Fixed Burnable Poisons Advantages • May be discretely loaded in specific locations to shape or control flux profiles in the core • By adding fixed burnable poisons, soluble boron concentrations can be reduced • Reduces potential positive moderator temperature coefficient • Allows increased kexcess for improved fuel cycle length © Copyright 2014 ELO 1.5 Operator Generic Fundamentals 56 Fixed Burnable Poisons Disadvantages • If not completely depleted at the end of the fuel cycle, remaining fuel will not be completely depleted (burned) – Undesirable for economic considerations – Fuel not completely exhausted and shorter fuel cycles • Fixed, therefore no ability to change poison loading during fuel cycle © Copyright 2014 ELO 1.5 Operator Generic Fundamentals 57 Chemical Shim vs. Fixed Burnable Poisons Knowledge Check Adding fixed burnable poisons to the core allows soluble boron concentrations to be reduced, reducing potential positive moderator temperature coefficient. A. True B. False Correct answer is A. © Copyright 2014 ELO 1.5 Operator Generic Fundamentals 58 Fixed Non-Burnable Neutron Poisons ELO 1.6 – Describe fixed non-burnable poisons used in reactor cores and include an example of material used as non-burnable poison. • Fixed non-burnable poisons maintain their reactivity worth throughout the fuel cycle • Not useful for negating kexcess, they are useful for shaping neutron flux levels within the core for an entire fuel cycle © Copyright 2014 ELO 1.6 Operator Generic Fundamentals 59 Fixed Non-Burnable Neutron Poisons • A non-burnable poison maintains a constant negative reactivity worth over life of the core – No neutron poison is completely non-burnable – Certain materials are considered as non-burnable because of a long life time as a poison. – A good example of this is hafnium • Fixed non-burnable poisons are used to suppress neutron flux levels in specific regions of the core for an entire cycle © Copyright 2014 ELO 1.6 Operator Generic Fundamentals 60 Fixed Non-Burnable Neutron Poisons • Hafnium has a long useful life as a neutron absorber – Absorption of neutrons from one isotope – Leads to production of another neutron absorber isotope – Continuing through a chain of five absorbers • Results in long-lived burnable poison with non-burnable characteristics • In some cases, absorbers with low neutron absorption cross-sections also used as non-burnable poisons • Self-shielding can make reactivity of a burnable poison material more uniform over core life © Copyright 2014 ELO 1.6 Operator Generic Fundamentals 61 Fixed Non-Burnable Neutron Poisons • Purpose – Power shaping – Prevent excessive flux and power peaking near heavy moderator regions of the reactor • Maintain their reactivity worth throughout fuel cycle and therefore cannot be used to balance kexcess of the fuel load © Copyright 2014 ELO 1.6 Operator Generic Fundamentals 62 Fixed Non-Burnable Neutron Poisons Knowledge Check Which of the following negative reactivities is not used for controlling kexcess? A. Soluble boron B. Hafnium C. Control rods D. IFBA pellets Correct answer is B. © Copyright 2014 ELO 1.6 Operator Generic Fundamentals 63 Fixed Non-Burnable Neutron Poisons Knowledge Check – NRC Bank Which one of the following is not a function performed by burnable poisons in an operating nuclear reactor? A. Provide neutron flux shaping B. Provide more uniform power density C. Offset the effects of control rod burnout D. Allow higher fuel enrichment of initial core load Correct answer is C. © Copyright 2014 ELO 1.6 Operator Generic Fundamentals 64 Changes Over Core Life From Fuel Depletion ELO 1.7 – Explain how the following nuclear reactor core parameters change over core life due to fuel depletion and neutron poison concentration: moderator temperature coefficient, Doppler coefficient, control rod worth, and core thermal flux. • Fuel burnup over core life causes a slow addition of negative reactivity and indirectly affects other reactivity parameters – Soluble boron reactivity coefficient – Moderator temperature reactivity coefficient – Fuel temperature reactivity coefficient – Control rod reactivity worth – Fission product poison concentration and reactivity worth – Neutron flux levels © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 65 Moderator Temperature Coefficient of Reactivity • Boron concentration in coolant/moderator is a function of fuel burnup – Boron concentration decreases due to fuel burnup • Boron results in reduction of thermal utilization factor (f) • In an undermoderated core with lower boron concentrations, change in thermal utilization factor with respect to moderator temperature change (Δf/ΔT) decreases • f is positive factor to MTC – As it reduces in magnitude, MTC becomes more negative © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 66 Moderator Temperature Coefficient of Reactivity For these reasons, MTC becomes more negative over core life. • Resonance escape probability factor is negative influence to MTC and becomes slightly more negative over core life due to buildup of plutonium-240 (higher resonance peaks) © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 67 Doppler Coefficient (FTC) • Magnitude of Doppler coefficient becomes larger with fuel burnup – Buildup of isotopes having substantial resonance absorption peaks, > uranium-238 – Most important plutonium-240 – Plutonium-240 has a large resonance peak at 1 eV − − 238 1 239 π½ 239 π½ 239 1 240 π+ π→ π ππ ππ’ + π → ππ’ 92 0 92 93 94 0 94 • Result is an increase in resonance capture or decrease in resonance escape probability over core life • Therefore, a more negative FTC over core life © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 68 Doppler Coefficient (FTC) Figure: Fuel Temperature Coefficient Changes Over Core Life Due to Buildup of Plutonium-240 and Other Fission Products © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 69 Doppler Coefficient (FTC) • However, over core life, there may be an improvement of heat transfer from fuel pellet to cladding from pellet expansion (swelling) (plant and fuel dependent) – Results in lower fuel temperatures at full power and smaller coefficient • So even though the coefficient (per °F) may become more negative from Pu-240 buildup, overall defect may not change appreciably because of lower fuel temperatures at high power © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 70 Doppler Coefficient (FTC) Figure: Average Fuel Temperature (°F) • From buildup of Pu-240 over core life, FTC becomes more negative, BUT decrease in fuel temperature over core life means smaller temperature change • Check your plant data on how FTC (Doppler) changes over core life © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 71 Control Rod Worth • Control rods considered a non-burnable poison • Neutron thermal diffusion length increases from: – Reduction of burnable and soluble poison concentrations – Fuel depletion • Neutrons spend more time in vicinity of control rods to thermalize and therefore a greater chance of absorption in control rods • As fuel depletion increases, thermal flux also increases to maintain power, higher neutron flux levels near control rods also increases control rod reactivity worth Therefore, control rod worth increases over core life. © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 72 Core Thermal Flux • Core power output is proportional to product of neutron flux and fuel macroscopic cross-section for fission • To maintain 100% power with a decreasing fuel macroscopic fission cross-section (fuel burnup) – Average thermal neutron flux must increase due to fuel depletion • However, creation of Pu-239 and Pu-241 fuel isotopes causes thermal flux level to increase less than without the fuel conversion To operate at the same power over core life, thermal flux must increase. © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 73 Changes Over Core Life From Fuel Depletion Knowledge Check Which of the following statements accurately describes the magnitude of nuclear reactor control rod worth over core life? A. Control rod worth is constant over core life because the effects of fuel depletion and boron dilution offset one another. B. Control rod worth is constant over core life because control rods are made of non-burnable neutron poisons. C. Control rod worth increases over core life as a result of fuel depletion and burnable poison concentration reduction. D. Control rod worth decreases over core life as a result of fuel depletion and burnable poison concentration reduction. Correct answer is C. © Copyright 2014 ELO 1.7 Operator Generic Fundamentals 74 Excess Reactivity Over Core Life ELO 1.8 – Explain the change in value of excess reactivity over core life. • Excess multiplication factor (kexcess) varies over core life • At BOL with fuel in the core, keff will be greater than one if – No control rods – No soluble boron – No burnable poisons installed in core • Excess multiplication factor (kexcess), is needed to make up for fuel depletion and fission product buildup over core life Excess fuel © Copyright 2014 ELO 1.8 Operator Generic Fundamentals 75 kexcess Versus Core Life Curve • 100% power, all rods out, and a xenon-free condition is chosen as starting point for kexcess and critical boron concentration curves Figure: kexcess Over Core Life © Copyright 2014 ELO 1.8 Operator Generic Fundamentals 76 kexcess Versus Core Life Curve • Excess fuel load provides large amount of positive reactivity, raising kexcess well above one • kexcess decreases at BOL due to buildup of fission product poisons to equilibrium levels • After buildup of fission product poisons, positive reactivity is added from depleting burnable poisons, overcomes negative reactivity from fuel depletion Figure: kexcess Over Core Life • kexcess increase continues to about one-third cycle • Then, fuel burnout catches up and surpasses burnout of burnable poisons © Copyright 2014 ELO 1.8 Operator Generic Fundamentals 77 kexcess Versus Core Life Curve • This kexcess tends to be more representative for a PWR using gadolinium for fixed burnable poisons • Higher capture cross-section for gadolinium causes it to burn up more rapidly during first third of fuel cycle, causing increase of kexcess • However, many commercial PWRs tend to use boron instead of gadolinium • In this case, the graph of kexcess over core life tends to be shaped more like the following figure of critical boron concentration over core life Figure: kexcess Over Core Life © Copyright 2014 ELO 1.8 Operator Generic Fundamentals 78 Critical Boron Curve • Like kexcess curve at BOL, value of critical boron concentration drops sharply due to buildup of fission product poisons xenon and samarium Figure: Critical Boron Concentration Over Core Life © Copyright 2014 ELO 1.8 Operator Generic Fundamentals 79 Critical Boron Curve • After fission products reach equilibrium, boron concentration (and kexcess) remain relatively constant due to the combined effects of fuel burnup and burnable poison burnup • After period of constant value, critical boron and kexcess drop linearly to EOL due to decrease in critical boron required to compensate for steady depletion of fuel © Copyright 2014 Figure: Critical Boron Concentration Over Core Life ELO 1.8 Operator Generic Fundamentals 80 Excess Reactivity Over Core Life Knowledge Check Excess reactivity (excess multiplication factor) decreases early in core life due to ______________ and then increases or levels out as the core approaches middle-of-life conditions due to _______________. A. fission product poison buildup; burnout of installed poisons B. burnout of installed poisons; fission product poison buildup C. boration; dilution D. fission product poison buildup; chemical shim Correct answer is A. © Copyright 2014 ELO 1.8 Operator Generic Fundamentals 81 Boron and Natural Circulation ELO 1.9 – Describe the effect of changes in boron concentration on reactivity during natural circulation conditions. • This section is a short discussion of boron concentration changes during natural circulation conditions © Copyright 2014 ELO 1.9 Operator Generic Fundamentals 82 Boron and Natural Circulation • During natural circulation conditions, boration or dilution not normally performed • Concern is that without the reactor coolant pumps (RCPs) running, the boron mixture will not be uniformly distributed throughout the core – Much lower RCS flow rate during natural circulation • This could create loss of shutdown margin condition • Any boron changes in natural circulation will have the same reactivity effect as with forced circulation once completely mixed © Copyright 2014 ELO 1.9 Operator Generic Fundamentals 83 Boron and Natural Circulation • Another concern is when residual heat removal (RHR) is placed in service for RCS cooldown, boron concentration in the RHR system may be less than boron in the RCS – For this reason, RHR loops are warmed up, circulated, and sampled for boron concentration prior to placing in service with the RCS • During a large break loss of coolant accident, boron can plate out on the fuel cladding – For this reason, safety injection is transferred from cold leg to hot leg injection and back again to flush any boron that may have plated out © Copyright 2014 ELO 1.9 Operator Generic Fundamentals 84 Boron and Natural Circulation Knowledge Check A nuclear reactor has been shut down for 8 hours following a loss of offsite power. The reactor coolant system is in hot standby on singlephase natural circulation. Compared to adding boric acid to the RCS during forced circulation, adding boric acid during natural circulation requires _________ time to achieve complete mixing in the RCS; once completely mixed, a 1 ppm increase in RCS boron concentration during natural circulation will cause a/an ________ change in core reactivity. A. more; smaller B. more; equal C. less; smaller Correct answer is B. D. less; equal © Copyright 2014 ELO 1.9 Operator Generic Fundamentals 85 Boron and Natural Circulation Knowledge Check – NRC Bank Which one of the following describes whether reactor power can be increased from 50 percent to 100 percent in a controlled manner faster near the beginning of core life (BOL) or near the end of core life (EOL)? (Assume all control rods are fully withdrawn just prior to beginning the power increase.) A. Faster near EOL, because faster changes in boron concentration are possible. B. Faster near EOL, because integral control rod worth is greater. C. Faster near BOL, because faster changes in boron concentration are possible. D. Faster near BOL, because integral control rod worth is greater. Correct answer is C. © Copyright 2014 ELO 1.9 Operator Generic Fundamentals 86 TLO 1 Summary • Describe fuel depletion for a nuclear reactor and how it impacts reactivity over life of core – Fission results in destruction of fissionable nuclei in reactor fuel, fuel depletion – Depletion of fuel over core life results in gradual insertion of negative reactivity • Define the following terms: – Fuel cycle - time between refueling – Fuel exposure - fuel depletion or burnup – Conversion ratio - number of plutonium-239 nuclei produced per 100 uranium-235 nuclei consumed – Burnable poison – used with soluble boron to control excess fuel needed for fuel cycles – Non-burnable poison - used in reactor cores to shape power and prevent excessive flux and power peaking near moderator regions – Chemical shim - name for adjusting concentration of boric acid dissolved in reactor coolant system © Copyright 2014 TLO 1 Operator Generic Fundamentals 87 TLO 1 Summary • Explain concept and use of burnable neutron poisons in a reactor core – Installed during refueling, compensate for excess positive reactivity of fuel – Uses: shape neutron flux for more uniform power density, allow higher fuel enrichment, and allow for lower concentrations of soluble boron to mitigate positive MTC • Describe design of installed burnable poisons for a nuclear reactor – BPRAs and burnable poison integral to fuel pellet o BRPA - one type of installed burnable poison is borosilicate glass tube contained within type 304 stainless steel cladding o Large number of burnable poison rods are positioned near center of core to flatten radial neutron flux distribution in core – IFBA - fuel pellet coated with thin film of zirconium diboride (ZrB2) o Normally burned out prior to end of one fuel cycle o Loaded in middle of fuel rod assemblies to control neutron flux patterns o Allows increased fuel loading without higher power peaks © Copyright 2014 TLO 1 Operator Generic Fundamentals 88 TLO 1 Summary • Chemical shim advantages over fixed burnable poisons – Cost effective method of balancing kexcess – Reactivity can be varied during reactor operation – Has a spatially uniform effect, flatter profile – Finer control and allows for fewer control rods • Chemical shim disadvantages over fixed burnable poisons – In high concentrations, can result in positive MTC – Additional costs in boric acid handling storage and processing – Creates metallurgical issue of corrosion – Changing concentration/reactivity is slow process • Fixed burnable poison advantages – Can be loaded to specific locations – Allows for soluble concentrations to be reduced – Reduces potential for positive MTC – Allows increased kexcess for longer fuel length • Fixed burnable poison disadvantages – If not completely depleted prior to EOL, reduces fuel cycle – Fixed poisons, can’t adjust during fuel cycle © Copyright 2014 TLO 1 Operator Generic Fundamentals 89 TLO 1 Summary • Describe fixed non-burnable poisons used in reactor cores, include an example of material used as non-burnable poison – Maintain reactivity worth constant throughout fuel cycle – Hafnium is an example of a non-burnable poison, absorption of a neutron by one isotope chains to multiple isotopes, also with high absorption cross-sections • Explain how the following nuclear reactor core parameters change over core life due to fuel depletion and neutron poison concentration: – Moderator temperature coefficient - MTC becomes more negative largely due to significant drop in boron concentration as fuel depletes – Doppler coefficient - Doppler coefficient become larger as core ages primarily due to increase in Pu 240, which has larger resonance capture – Control rod worth – increases over core from fuel burnup, less boron, and a higher neutron flux – Core thermal flux - due to fuel burnup © Copyright 2014 TLO 1 Operator Generic Fundamentals 90 TLO 1 Summary • Explain the change in the value of excess reactivity over core life – Initial drop in magnitude due to buildup of fission product poisons – kexcess increases from about day 25 to 1/3 fuel cycle due to burnout of installed burnable poisons – From 1/3 fuel cycle to EOL reactivity from fuel burnout dominates and kexcess decreases © Copyright 2014 TLO 1 Operator Generic Fundamentals 91 TLO 1 Summary • Describe the effect of changes in boron concentration on reactivity during natural circulation conditions – Boron concentration and dilution takes longer when RCS is operating in a natural circulation mode versus forced circulation – Once boron is completely mixed (or diluted), same change in reactivity in the core occurs during either forced or natural circulation © Copyright 2014 TLO 1 Operator Generic Fundamentals 92 Crossword Puzzle • It’s crossword puzzle time! © Copyright 2014 TLO 1 Operator Generic Fundamentals 93 Xenon-135 Fission Product Poison TLO 2 – Describe the behavior of xenon-135 in an operating nuclear reactor and its effects on reactor operation. • Xenon-135 has a very significant negative reactivity component – Large impact on operation of a nuclear reactor • Understanding how it is produced and removed and its response during various reactor operation transients is important to operators for predicting reactor response • Xenon transients can limit reactor operations, create unacceptably high power peaking in fuel, and even prevent reactor from starting up • These are discussed and explained in this chapter © Copyright 2014 TLO 2 Operator Generic Fundamentals 94 Enabling Learning Objectives for TLO 2 1. Describe fission product poisons and how fission product poisons affect the neutron life cycle. 2. List the most important fission product poisons to the operation of a nuclear reactor. 3. Explain how xenon-135 is produced and removed in the core of a nuclear reactor. 4. Explain the following terms: equilibrium iodine, equilibrium xenon , transient xenon, peak xenon, xenon free, xenon precluded startup, and xenon dead time. © Copyright 2014 TLO 2 Operator Generic Fundamentals 95 Enabling Learning Objectives for TLO 2 5. Explain how xenon-135 concentration reacts during the following nuclear reactor operations: xenon free initial reactor startup, reactor shutdown, decrease in reactor power, increase in reactor power, and reactor startup with xenon present in core. 6. Describe the causes and effects of a xenon oscillation. 7. State the approximate time following a reactor shutdown at which the reactor can be considered xenon free. 8. Explain the effects of xenon concentration on a nuclear reactor core’s thermal flux profile for control rod motion and core life. © Copyright 2014 TLO 2 Operator Generic Fundamentals 96 Fission Product Poison Effect on 0N1 Life Cycle ELO 2.1 – Describe fission product poisons and how fission product poisons affect the neutron life cycle. • Fission fragments resulting from fission events decay to produce a variety of fission products • Fission product poisons are of major concern because they absorb of neutrons, removing them from the neutron life cycle • There are dozens of long-lived and stable fission product poisons – Small to large neutron absorption cross-sections – Build up to equilibrium values over core life – Add negative reactivity decreasing thermalization factor (f) © Copyright 2014 ELO 2.1 Operator Generic Fundamentals 97 Most Abundant Fission Product Poisons • Although several fission products have significant neutron absorption crosssections, xenon-135 and samarium-149 have most substantial impact on reactor operation • Fission yield curve shows probability of nuclei yield for U-235 fission Figure: Fission Yield Curve for Uranium-235 © Copyright 2014 ELO 2.1 Operator Generic Fundamentals 98 Fission Product Poison Effect on Neutron Life Cycle • Xenon-135 and samarium-149 both have high absorption crosssections: – 2.6 x 106 barns for xenon-135 – 4.0 x 104 barns for samarium-149 • Because they absorb neutrons that could be absorbed in the fuel, they impact the thermal utilization factor (ƒ), keff and reactivity © Copyright 2014 ELO 2.1 Operator Generic Fundamentals 99 Fission Product Poison Effect on Neutron Life Cycle • Equation for ƒ shows that an increase in macroscopic cross-section for absorption by any neutron poison will result in a decrease in ƒ π= © Copyright 2014 πΉπ’ππ + π ππ’ππ π πππ + ππ‘βππ + π π ELO 2.1 ππππ ππ π Operator Generic Fundamentals 100 Fission Product Poison Effect on Neutron Life Cycle • Fission product poisons present in core at any given time depends: – Poison’s production and removal rate • Fission product poisons may be produced: – Directly from fission – From decay (or decay chain) of certain fission products • Removal of fission product poisons occurs by: – Radioactive decay – Neutron absorption • Removal generally results in an isotope with a much lower neutron absorption cross-section © Copyright 2014 ELO 2.1 Operator Generic Fundamentals 101 Fission Product Poison Effect on 0N1 Life Cycle • Term often associated with fission product poisons is equilibrium • At equilibrium, production rate of poison equals removal rate of poison, therefore concentration of poison is constant • Depending on poison, equilibrium levels are power dependent and time to reach equilibrium is related to both power and decay rates © Copyright 2014 ELO 2.1 Operator Generic Fundamentals 102 Other Fission Product Poisons • Many other fission products besides xenon and samarium have appreciable cross-sections for neutron absorption – Concentration of these poisons is not necessarily depleted by neutron absorption • Due to moderate cross-sections and continued production by fission, these poisons may be referred to as permanent poisons • In a thermal reactor, these poisons may accumulate at rate of about 50 barns per fission – Has effect of adding negative reactivity over core life © Copyright 2014 ELO 2.1 Operator Generic Fundamentals 103 Reactivity Effects of Fission Product Poisons • Reactivity effects of fission product poisons such as xenon and samarium occur relatively slowly – Compared to other reactivities such as control rods, fuel, and moderator temperatures • Except for a situation of rapid burnout from peak xenon, changes in reactivity from fission product poisons occur over time periods of hours to days to years, rather than seconds or minutes © Copyright 2014 ELO 2.1 Operator Generic Fundamentals 104 Fission Product Poison Effect on Neutron Life Cycle Knowledge Check Fission product poisons contribute ___________ reactivity to a nuclear reactor as they buildup in the core and ____________ reactivity to a nuclear reactor as they burnout in the core. A. negative; positive B. negative; negative C. positive; negative D. positive; positive Correct answer is A. © Copyright 2014 ELO 2.1 Operator Generic Fundamentals 105 Xenon and Samarium ELO 2.2 – List the most important fission product poisons to the operation of a nuclear reactor. • Most important fission product poisons to operation of nuclear reactor – Xenon-135 — 2.6 x 106 barns – Samarium-149 — 4.0 x 104 barns • At equilibrium levels they add considerable negative reactivity: – Xenon – approximately -3,000 pcm (varies with power level) – Samarium – approximately -700 - 1,000 pcm (100% BOL/EOL) © Copyright 2014 ELO 2.2 Operator Generic Fundamentals 106 Xenon and Samarium • On a reactor trip – Xenon will peak with a negative reactivity of almost 5,000 pcm and decay in 3 days back to 0 pcm • Samarium, once it reaches equilibrium, does not decrease in its negative reactivity worth – Will add another approximately 400 pcm on a reactor trip (dependent on power history) © Copyright 2014 ELO 2.2 Operator Generic Fundamentals 107 Xenon and Samarium Knowledge Check Which of the following fission product poisons are of the greatest concern to the operation of a nuclear reactor? A. Xenon and boron B. Zirconium and samarium C. Xenon and samarium D. Boron and gadolinium Correct answer is C. © Copyright 2014 ELO 2.2 Operator Generic Fundamentals 108 Xenon and Samarium Knowledge Check – NRC Check Fission product poisons can be differentiated from other fission products in that fission product poisons... A. have a longer half-life. B. are stronger absorbers of thermal neutrons. C. are produced in a larger percentage of fissions. D. have a higher fission cross-section for thermal neutrons. Correct answer is B. © Copyright 2014 ELO 2.2 Operator Generic Fundamentals 109 Production and Removal of Xenon ELO 2.3 – Explain how xenon-135 is produced and removed in the core of a nuclear reactor. • Xenon-135 is produced from two sources: – Directly as a fission fragment – Beta-minus tellurium-135 decay chain (I-135 usually considered decay source) • Xenon-135 is removed by: – Beta-minus decay to Cesium-135 (Cs-135) – Neutron capture reaction which creates Xe-136 (burnout) © Copyright 2014 ELO 2.3 Operator Generic Fundamentals 110 Xenon-135 Production • Xenon-135 production decay chain 135 π½ 135 π½ 135 π½ 135 π½ 135 ππ → πΌ→ ππ → πΆπ → π΅π (π π‘ππππ) 52 53 54 55 56 19 π ππ 6.57 βπ 9.10 βπ 2.36 × 106 π¦ππππ • Approximately 0.3% of all fissions yield Xe-135 as a fission fragment • Approximately 6.0 % of all fissions yield tellarium-135 directly, with beta-minus to I-135 – Because of short half-life of Te-135 and longer half-life of I-135, Te-135 is ignored and I-135 is assumed the source from fission © Copyright 2014 ELO 2.3 Operator Generic Fundamentals 111 Xenon-135 Production • Of the total xenon-135 production: – Approximately 5% is directly as a fission fragment – 95% is from the beta-minus decay of I-135 • With 95% of the xenon coming from I-135 and I-135 having a half-life of 6.6 hours, a significant delay in production of Xe-135 occurs – Important when considering changing power levels, since production of Xe-135 via I-135 is dependent on power level (number of fissions occurring) © Copyright 2014 ELO 2.3 Operator Generic Fundamentals 112 Xenon-135 Removal • Xenon-135 is removed by beta-minus decay to Cs-135 or neutron absorption to Xe-136 (burnout) – Cs-135 and Xe-136 have small neutron absorption cross-sections and therefore are not neutron poisons • Neutron capture cross-section of Xe-135 is between 1 and 1.3 x 106 barns • Ratio of Xe-135 removal due to burnout and decay vary with power level – Significant in that it governs equilibrium values and transient response – Half-life of Xe-135 is 9.1 hours © Copyright 2014 ELO 2.3 Operator Generic Fundamentals 113 Xenon-135 Production and Removal • Rate of change of xenon concentration equal to rate of production minus rate of removal • When in balance, xenon is said to be in equilibrium π ππ‘π ππ πβππππ π₯ππππ_ 135 π¦ππππ ππππππ _ 135 π₯ππππ_ 135 π₯ππππ_ 135 _ ππ π₯ππππ 135 = ππππ πππ π πππ + πππππ¦ − πππππ¦ − ππ’πππ’π πππππππ‘πππ‘πππ βπππ = πΎππ βπ‘ ππ’ππ π Φ + ππΌ ππΌ − πππ πππ − π ππ π Φ π ππ Where: NI = iodine-135 concentration NXe = xenon-135 concentration λI = decay constant for iodine-135 γxe = fission yield of xenon-135 λXe = decay constant for xenon-135 ππ π = microscopic absorption cross-section π for xenon-135 ππ’ππ π = macroscopic cross-section in fuel Φ = thermal neutron flux βπ‘ = change in time © Copyright 2014 ELO 2.3 Operator Generic Fundamentals 114 Xenon-135 Production and Removal • Xenon burnup term is neutron capture of xenon-135 to xenon-136 1 136 135 ππ + π → ππ + πΎ 0 54 54 • Xenon-136 not significant neutron absorber; neutron absorption by xenon-135 means removal of xenon poison from reactor – Burnup rate of xenon-135 dependent upon neutron flux and xenon-135 concentration • Xenon-135 decays (second to last term in equation) by beta emission to cesium-135 (9.10 hour half-life) – Cesium-135 has a very long half-life (>106 years) and a small absorption cross-section for neutrons – Cesium-135 not considered a poison © Copyright 2014 ELO 2.3 Operator Generic Fundamentals 115 Xenon-135 Production and Removal Knowledge Check Which of the following is the greatest source of xenon-135 production in an operating nuclear reactor core? A. The decay of tellarium-135 fission fragments B. Direct xenon-135 production from fission C. Neutron capture by samarium-134 fission fragments D. The decay of neodymium-149 fission fragments Correct answer is A. © Copyright 2014 ELO 2.3 Operator Generic Fundamentals 116 Xenon Transient Terms ELO 2.4 – Explain the following terms: equilibrium iodine, equilibrium xenon, transient xenon, peak xenon, xenon free, xenon precluded startup, and xenon dead time. • An understanding of various terms related to xenon transients will provide a deeper understanding of xenon characteristics and limitations during reactor operations © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 117 Equilibrium Iodine (NI eq) • Equilibrium iodine is constant I-135 concentration reactor eventually occurs after a power change – Directly proportional to reactor power – Takes 20 to 25 hours after power change to reach equilibrium • I-135 has to reach equilibrium before Xe-135 can reach equilibrium © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 118 Equilibrium Iodine (NI eq) • Rate of change of iodine concentration equal to rate of production minus rate of removal πππππ ππππ π ππ‘π ππ πβππππ ππ = − π·ππππ¦ πππ‘π − π΅π’πππ’π πππ‘π πππ π πππ ππππππ πππππππ‘πππ‘πππ or βππΌ βπ‘ = πΎπΌ ππ’ππ π πΌ Φ − ππΌ ππΌ − π ππΌ Φ π Where: NI = iodine-135 concentration ϒI = fission yield of iodine-135 ππ’ππ = macroscopic cross-section in fuel π Φ = thermal neutron flux λI = decay constant for iodine-135 πΌ π = microscopic absorption cross-section for iodine-135 π βπ‘ = change in time © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 119 Equilibrium Iodine (NI eq) • Since iodine-135 microscopic absorption cross-section (σIa) is very small, burnup rate term may be ignored, simplify expression to: βππΌ = πΎπΌ βπ‘ ππ’ππ π Φ − ππΌ ππΌ Where: NI = iodine-135 concentration ϒI = fission yield of iodine-135 ππ’ππ = macroscopic cross-section in fuel π Φ = thermal neutron flux λI = decay constant for iodine-135 σIa = microscopic absorption cross-section for iodine-135 βπ‘ = change in time © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 120 Equilibrium Iodine (NI eq) • When rate of iodine production equals rate of iodine removal, equilibrium exists • To obtain equilibrium concentration from the formula on last slide, βππΌ = πΎπΌ βπ‘ ππ’ππ Φ − ππΌ ππΌ π • Can be solved for N (concentration) ππΌ ππ = πΎπΌ ππ’ππ Φ π ππΌ • Equilibrium iodine concentration proportional to fission reaction rate (γI ππ’ππ Φ), π © Copyright 2014 therefore proportional to reactor power level ELO 2.4 Operator Generic Fundamentals 121 Equilibrium Xenon (NXe eq) • When production and removal rates of xenon-135 are equal, equilibrium is established – Concentration (atomic density) of xenon-135 present in reactor during this condition is referred to as equilibrium xenon – Equilibrium concentration of xenon-135 is designated NXe (eq) πππ ππ = © Copyright 2014 πΎππ π ππ’ππ Φ π + ΖπΌ ππΌ ππ Φ + πππ π ELO 2.4 Operator Generic Fundamentals 122 Equilibrium Xenon (NXe eq) • For xenon-135 to be in equilibrium, iodine-135 must also be in equilibrium • Substituting expression for equilibrium iodine-135 concentration into the equation for equilibrium xenon results in: πππ ππ = ππ’ππ (πΎππ π Φ π + πΎπΌ ) ππ Φ + πππ π • It can be seen that equilibrium for xenon-135 increases as power increases, because numerator is proportional to fission reaction rate © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 123 Equilibrium Xenon (NXe eq) • Equilibrium xenon at 25% power is approximately equal to 50% of 100% power equilibrium • Equilibrium xenon at 50% power is between 70 and 80% of 100% power equilibrium • These values are due to magnitude of each of the removal terms in denominator of equilibrium equation – At low powers, decay is major removal term – At high powers, burnout is major removal term • At 100% power and 100% equilibrium, 70% of Xe-135 removal is burnout and 30% decay • At 30% power, removal terms are approximately equal © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 124 Equilibrium Xenon (NXe eq) • Equilibrium iodine-135 and xenon-135 concentrations as a function of neutron flux are: Figure: Equilibrium Iodine-135 and Xenon-135 Concentrations Versus Neutron Flux © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 125 Equilibrium Xenon (NXe eq) • Equilibrium xenon-135 concentration is reached quicker at higher reactor power levels, faster production rate of xenon – Approximately 40 hours after startup to 100% – Approximately 44 hours after startup to 50% – Approximately 48 hours at lower reactor power level © Copyright 2014 Figure: Equilibrium Iodine-135 and Xenon-135 Concentrations Versus Neutron Flux ELO 2.4 Operator Generic Fundamentals 126 Transient Xenon • Whenever a power change in reactor occurs, Xe-135 undergoes a transient • It takes 40 to 50 hours after power change for Xe-135 to reach equilibrium – Negative reactivity due to Xe-135 is directly proportional to Xe135 concentration • On a power change, after operator has compensated for power defect change with control rods and/or boron, Xe-135 continues to affect reactivity for hours afterward © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 127 Peak Xenon • Anytime the reactor undergoes a rapid down power, Xe-135 concentration will peak and then decrease to a new lower equilibrium value or go to zero (on a shutdown) • Reactor trip is most rapid down power, results in largest xenon peaks • Xenon peaks because thermal neutron flux goes down with power reducing burnout rate • Xe-135 production from I-135 decay remains almost the same for a period of time (6.5 hour half-life) adding to xenon peak © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 128 Peak Xenon • Eventually, production slows, decay become a closer match for new power level, and xenon peaks • Following peak, removal rate exceeds production rate until a new equilibrium is reached or zero is reached if the reactor is shutdown • Peak xenon reached in approximately 6 to 10 hours after a shutdown – Time to reach peak xenon in shutdown reactor can be estimated: o Time to peak xenon (in hours) equal to square root of percent reactor power prior to shutdown (trip) © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 129 Peak Xenon • Using this rule, a shutdown (trip) from 100% power will result in peak xenon about 10 hours later • A shutdown from 50% power will result in peak xenon 7 hours later • The greater the flux level (power) prior to shutdown, the greater the concentration of iodine-135 at shutdown – Therefore, greater peak after shutdown © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 130 Peak Xenon Figure: Xenon-135 Reactivity After Reactor Shutdown © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 131 Peak Xenon • Increased concentration of xenon-135 immediately following reactor shutdown increases shutdown margin by adding additional negative reactivity to the core • After peak xenon-135 is reached, concentration slowly decreases until a xenon-free condition reached • Removal of xenon-135 by decay after peak results in a decrease in shutdown margin (positive reactivity added) • After approximately 20-24 hours, shutdown margin in reactor will have returned to its initial value (at time of shutdown) • As xenon-135 decays, shutdown margin will continue to decrease © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 132 Peak Xenon • Shutdown margin requirements should not be exceeded because of control rod negative reactivity insertion • However, any additional cooldown adding positive reactivity could reduce shutdown margin further to point of exceeding its limits • Adding boron would likely be performed © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 133 Xenon Free • Just after refueling, no critical operation Or • Following reactor shutdown (or trip), xenon peaks then decays off to essentially zero after 70-80 hours © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 134 Xenon Precluded Startup • Following a reactor trip, xenon peaks – May provide sufficient negative reactivity to prevent a reactor restart or maintaining criticality • Could occur with insufficient positive reactivity available from control rods and/or dilution rate • Most likely EOL on shutdown from a high power level with excess reactivity levels as low as 1% Δk/k due to fuel depletion – Inability to restart reactor because of xenon called a xenon precluded startup © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 135 Xenon Precluded Startup and Xenon Dead Time • Inability to restart reactor due to xenon would persist for several hours until xenon-135 peak is decayed sufficiently for rods and boron dilution to overcome the negative xenon reactivity • Period of time where the reactor is unable to override the effects of xenon is called xenon dead time – A xenon precluded startup is an example of this © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 136 Xenon Transient Terms Knowledge Check Two commercial pressurized water reactors are being shut down from power range operation. Reactor A has been operated for several days at 100% power. Reactor B has been operated for several days at 75% power. Which of the following correctly describes the behavior of xenon-135 that can be expected for each reactor after shutdown? Assume each reactor will be shut down for several days. A. The peak xenon concentration for Reactor A will be smaller in magnitude and will occur earlier than the peak xenon concentration for Reactor B. B. The peak xenon concentration for Reactor A will be smaller in magnitude and will occur later than the peak xenon concentration for Reactor B. C. The peak xenon concentration for Reactor A will be greater in magnitude and will occur later than the peak xenon concentration for Reactor B. D. The peak xenon concentration for Reactor A will be greater in magnitude and will occur earlier than the peak xenon concentration for Reactor B. Correct answer is C. © Copyright 2014 ELO 2.4 Operator Generic Fundamentals 137 Xenon Response During Reactor Operations ELO 2.5 – Explain how xenon-135 concentration reacts during the following nuclear reactor operations: initial reactor startup (xenon free), reactor shutdown, decrease in reactor power, increase in reactor power, and reactor startup with xenon present in core. • Changes in xenon-135 concentration affect amount of reactivity present in the core • Reactor operations can result in significant changes in concentration of xenon-135 and, therefore, reactivity • In order to achieve or maintain a safe and desired reactor power level, reactor operators must be able to recognize and account for these effects © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 138 Initial Reactor Startup (Xenon Free) • Xenon-free condition exists at beginning of core life (BOL) prior to reactor operation, when no xenon (or iodine-135) has been produced in the core Or • It can also occur anytime in core life when the reactor has been shutdown long enough to allow any xenon-135 to completely or almost completely decay away – Approximately 70-80 hours after reactor shutdown © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 139 Initial Reactor Startup (Xenon Free) Figure: Time to Reach Equilibrium Xenon for Various Power Levels © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 140 Initial Reactor Startup (Xenon Free) • Immediate production of xenon-135 directly from fission • Immediate production of iodine-135 (Te-135) directly from fission – Delayed (half-life 6.5 hours) decay of I-135 to Xe-135 • These two mechanisms lead to an increase in concentration of xenon-135 in the core Figure: Time to Reach Equilibrium Xenon for Various Power Levels © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 141 Initial Reactor Startup (Xenon Free) • As concentration is building, xenon removed by burnout and decay – Eventually, production and removal rates for xenon-135 balance – Equilibrium concentration of xenon-135 occurs • If no operator action is taken during this time, xenon is building in (constant steam demand) – RCS temperature will decrease to compensate for xenon negative reactivity Figure: Time to Reach Equilibrium Xenon for Various Power Levels © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 142 Reactor Shutdown • When reactor is shutdown from power: – Iodine-135 decaying to xenon-135 is adding to Xe-135 already in the core – Thermal neutron flux drops to essentially zero o Xe-135 removal now only by decay • Sudden drop in burnout and I-135 decay still temporarily producing xenon at a rate equal to pre-shutdown power level causes Xe-135 to peak – Equilibrium iodine, like xenon, is directly proportional to power level before shutdown, therefore I-135 will continue for sometime to be producing xenon-135 at pre-shutdown levels © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 143 Reactor Shutdown • Initially after shutdown – Burnup decreases significantly – Production from I-135 decay remains almost constant o Causes large xenon peaking • Largest peaks occur with reactor trips, not gradual shutdowns – Xe-135 is burning out during shutdown, reducing peak Remember that 95% of xenon production is from I-135 decay. © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 144 Reactor Shutdown • Largest magnitude Xe-135 peak occurs – Reactor trips from 100% power – 100% equilibrium xenon and iodine • The greater the flux level prior to shutdown, the greater the concentration of iodine-135 at shutdown; therefore, the greater the peak in xenon-135 concentration after shutdown • Xe-135 concentration peaks after trips do not change much through core life (for a given power level) • However, reactivity of peaks become more negative as core ages – Reduced competition for thermal neutrons as Boron-10 is reduced by dilution © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 145 Reactor Shutdown Figure: Xenon-135 Reactivity After Reactor Shutdown © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 146 Reactor Shutdown • Equilibrium Xe-135 reactivity for 100% power – Between -2,500 pcm and -3,000 pcm • Peak Xe-135 reactivity for a trip from 100% power – Between -4,500 to -5,000 pcm • For advanced cycle cores with lower thermal flux levels, time to peak: – 7.5 hrs to trip from 100% – 5 hrs to trip from 50% © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 147 Decrease in Reactor Power • Assume a reactor operating at 100% power with equilibrium xenon – Decrease power to 50% – Immediate decrease in xenon burnup – Increase in xenon-135 concentration • With a constant Xe-135 decay rate, reduced burnup rate, and iodine135 concentration still at equilibrium level for 100% power – 95% of xenon is produced at 100% power rate – Production > removal © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 148 Decrease in Reactor Power • Xenon-135 concentration continues to rise at a decreasing rate until production equals rate of removal – Roughly 8-10 hours after initial reduction in power level • Xenon-135 concentration then gradually decreases to new equilibrium level in about 50-60 hours – Greater power change will require longer times for xenon to reach equilibrium © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 149 Decrease in Reactor Power • With some neutron flux in reactor, magnitude of decrease in burnout is less than for a reactor trip – Magnitude of peak to be lower and earlier • The more gradual the down power, the: – Lower the Xe-135 peak – Earlier the peak • If power is lowered very gradually, no Xe-135 peak © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 150 Decrease in Reactor Power Figure: Xenon-135 Variations During Power Changes © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 151 Increase in Reactor Power • Reactor power raised from 50% to 100% power (up power transient) • Xenon burnout increases from higher power level • Decay of xenon remains constant • Immediate increase in direct production of Xe-135 occurs (5% of production) • Concentration of xenon in core initially decreases © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 152 Increase in Reactor Power • 95% of xenon production is from iodine-135 decay, 6.5 hour half-life, increased xenon production is delayed • After roughly 4-6 hours (power level dependent): – Rate of production of xenon from iodine and fission equals rate of removal of xenon by burnup and decay – Xenon reaches a minimum and begins to build to equilibrium – Equilibrium will be reached in 20-30 hours • Most rapid possible burnout of xenon occurs when a reactor is started up and operated at full power while a maximum peak xenon condition exists • Power increases can also be made so gradually that no dip in xenon occurs © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 153 Increase in Reactor Power Figure: Xenon-135 Variations During Power Changes © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 154 Reactor Startup With Xenon Present • Commercial reactor operations may require reactor to be started up after short-term shutdown and prior to time where all xenon-135 has decayed Figure: Xenon Behavior During Reactor Startup With Xenon Present in the Core © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 155 Reactor Startup with Xenon Present • This accelerated decrease in xenon concentration during startup is a result of two factors: – Burnout rate very high and even above normal for an up power transient due to high concentration of xenon-135 – Direct production of iodine-135 and xenon-135 from fission is again occurring, but takes several hours for equilibrium conditions to re-establish • Lag in recovery of xenon-135 concentration during this kind of startup directly attributed to half-life of iodine-135 • Although iodine-135 concentration starts to recover immediately, production of xenon-135 from iodine-135 lags due to 6.5 hour half-life © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 156 Xenon Response During Reactor Operations Knowledge Check Equilibrium iodine-135 concentration in an operating nuclear reactor is directly proportional to which of the following? A. The rate of decay of xenon-135 B. The decay constant for iodine-135 C. The fission reaction rate D. The rate of neutron capture by tellurium-135 Correct answer is C. © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 157 Xenon Response During Reactor Operations Knowledge Check – NRC Bank Reactor power is increased from 50% to 60% in 1 hour. The most significant contributor to the initial change in core xenon reactivity is the increase in xenon . A. production of xenon-135 from fission B. production of xenon-135 from iodine-135 decay C. loss of xenon-135 due to absorption of neutrons D. loss of xenon-135 due to decay to cesium-135 Correct answer is C. © Copyright 2014 ELO 2.5 Operator Generic Fundamentals 158 Xenon Oscillations ELO 2.6 – Describe the causes and effects of a xenon oscillation. • Axial xenon oscillations fairly common in PWR operations • Main significance is not so much xenon oscillations, but resulting local fuel power peaking • Power peaking can result in fuel exceeding thermal limits, uneven burnout, high fuel temperature, cladding failure, etc. © Copyright 2014 ELO 2.6 Operator Generic Fundamentals 159 Xenon Oscillations • Large thermal reactors with little flux coupling between regions may experience spatial power oscillations because of non-uniform presence of xenon-135 • Mechanism described in following four steps: 1) An initial lack of symmetry in the core power distribution causes an imbalance in fission rates within the reactor core o Affecting iodine-135 buildup and xenon-135 burnup o Example, individual control rod movement or misalignment 2) In the high-flux region, (higher burnout) xenon-135 burnout allows the flux to increase further o In low-flux region, increase in xenon-135 (lower burnout) causes a further reduction in flux o Iodine concentration increases where flux is high and decreases where flux is low © Copyright 2014 ELO 2.6 Operator Generic Fundamentals 160 Xenon Oscillations 3) As soon as iodine-135 levels build up sufficiently, I-135 beta minus decay to xenon reverses initial situation o Flux decreases in higher power area causing power to decrease and former low-flux region increases in power 4) Repetition of these patterns leads to xenon oscillations moving about the core with periods on the order of 15 hours • Xenon oscillations can either converge or diverge – Converging oscillations die out by themselves – Diverging oscillations continue to grow unless corrected by operator action © Copyright 2014 ELO 2.6 Operator Generic Fundamentals 161 Xenon Oscillations • Xenon oscillations can be radial (side to side) or axial (top to bottom) – Radial oscillations uncommon – Axial xenon oscillations do occur • Any rapid change in axial power distribution can cause an axial xenon oscillation • A change in axial power distribution is identified by Axial Flux Difference (AFD) indicators. – Also called Delta Flux or Delta I © Copyright 2014 ELO 2.6 Operator Generic Fundamentals 162 Xenon Oscillations • Core is not as strongly coupled axially as it is radially – Reason axial flux tilts and xenon oscillations are more likely • Xenon oscillations can cause AFD to approach and exceed technical specification limits – Can require reactor power to be reduced to less than 50% – In extreme cases, requires a reactor shutdown © Copyright 2014 ELO 2.6 Operator Generic Fundamentals 163 Xenon Oscillations Knowledge Check Which of the following could result in a xenon oscillation in the core of a nuclear reactor? A. A change in reactor power level due to a change in steam demand B. Individual control rod insertion C. A reactor trip from a high power level D. Reactor coolant system flow oscillations Correct answer is B. © Copyright 2014 ELO 2.6 Operator Generic Fundamentals 164 Xenon Free ELO 2.7 – State the approximate time following a reactor shutdown at which the reactor can be considered "xenon free." • Section reviews xenon response following a shutdown or trip • After an S/D, xenon-135 concentration will peak in 7 to 10 hours • Xenon-135 concentration decreases at a rate controlled by decay of iodine-135 to xenon-135 and the decay rate of xenon-135 – Iodine-135 has a half-life of 6.5 hours – Xenon-135 has a half-life of 9.1 hours © Copyright 2014 ELO 2.7 Operator Generic Fundamentals 165 Xenon Free • In most cases, xenon-135 concentration in 20 to 24 hours after shutdown equals full power equilibrium xenon-135 concentration (level at time of trip) • Three days (70 to 80 hours) after shutdown, xenon-135 concentration has decreased to a small percentage of its pre-shutdown level – Reactor can be assumed to be xenon free without a significant error introduced into reactivity calculations • The higher the reactor power level at the start of a shutdown, the longer the time required to reach a xenon-free condition © Copyright 2014 ELO 2.7 Operator Generic Fundamentals 166 Xenon Free Knowledge Check A commercial pressurized water reactor that has been operated at 100% for several weeks is being shut down. How much time must elapse prior to a subsequent startup attempt to ensure that reactivity due xenon-135 is no longer present in the core? A. 80 hours B. 40 hours C. 20 hours D. 10 hours Correct answer is A. © Copyright 2014 ELO 2.7 Operator Generic Fundamentals 167 Xenon Effects to Thermal Flux Profile ELO 2.8 – Explain the effects of xenon concentration on a nuclear reactor core’s thermal flux profile for control rod motion and core life. • When core neutron flux levels change from reactivity inputs, Xenon too is affected, in turn affecting flux distribution • This section reviews and adds to some previously covered topic on xenon oscillations © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 168 Xenon Effects to Thermal Flux Profile Control Rod Motion • When control rods are inserted a small amount (while maintaining a constant reactor power), thermal flux in top half of core decreases, while thermal flux in bottom half of core increases – Axial Flux Difference (AFD) will go strongly negative • Rate of xenon-135 burnup in lower portion of core increases immediately – Exaggerates power peak in bottom of core over next several hours – Xe-135 peak (less flux) in top of core will further suppress power in top at the same time – AFD goes more negative © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 169 Xenon Effects to Thermal Flux Profile Control Rod Motion • Initial rod motion pushes flux pattern lower in core • Power peak in bottom of core creates a larger I-135 concentration in the bottom Figure: Thermal Flux Versus Xenon Concentration After Control Rod Insertion © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 170 Xenon Effects to Thermal Flux Profile Control Rod Motion • About 6.5 hours after power peaks in bottom, Xe-135 from I-135 will produce so much Xe-135 that power will be suppressed in bottom of core and increase in top • After about 19.5 hours, axial thermal flux and xenon concentration profiles shown in figure (D) will exist in core Figure: Thermal Flux Versus Xenon Concentration After Control Rod Insertion © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 171 Xenon Effects to Thermal Flux Profile Control Rod Motion • After power increases in top of reactor, I-135 concentration increases and eventually produces a Xe-135 peak in top of reactor • Period of cycles (peak in top to peak in top, or peak in bottom to peak in bottom) is 24-28 hours Figure: Thermal Flux Versus Xenon Concentration After Control Rod Insertion © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 172 Xenon Effects to Thermal Flux Profile Control Rod Motion • Axial xenon oscillations can also be caused by rapid down power transients • If RCS Tavg is kept on program by boration only, rods 100% withdrawn, power distribution will peak in top of core relative to core average power – Due to RCS Thot decreasing far more than RCS Tcold • Since MTC is negative and delta temperature change larger in top, more positive reactivity added to top of core than bottom © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 173 Xenon Effects to Thermal Flux Profile Control Rod Motion • Rapid change in power distribution can trigger a xenon oscillation similar to control rod insertion • However, down power xenon oscillation is opposite in phase to rod insertion xenon oscillation – By inserting control rods judiciously while decreasing power, AFD can be kept in tight band to prevent xenon oscillation from starting © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 174 Xenon Effects to Thermal Flux Profile Control Rod Motion • If a xenon oscillation is occurring, do not insert control rods just as power is peaking at top of core – Causes power peak in bottom of core 12-14 hours later even greater in magnitude, and following power peak in top (24-28 hours later) even greater in magnitude • Correct time to insert control rods is just after power peaks in bottom of core – Can dampen oscillation if performed correctly © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 175 Core Age Effects • Xenon oscillations converge when neutrons from high power region are shifted to region of high xenon concentration – Neutrons reduce size of Xe-135 peaks by burnout – This in turn dampens oscillations • Actually, no neutron makes it from top to bottom of reactor or from bottom to top – Shifted neutrons cause fissions in center of reactor, and those fissions release neutrons that cause more fissions • Eventually, neutrons reach region of high xenon concentration and reduce size of peak by burnout © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 176 Core Age Effects • At BOL, reactivity effects due to MTC tend to dampen xenon oscillations • Xenon oscillations are more prevalent at EOL because: – Fuel mostly depleted in axial center of core – Less neutronic coupling (neutron sharing) between upper and lower halves of core • If left unchecked, magnitude of these flux shifts could continue to increase and could result in violations of reactor thermal limits © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 177 Core Age Effects • Westinghouse has created a fuel design that reduces the probability of creating diverging xenon oscillations • Fuel pellets near top and bottom of fuel element have annular holes – Reduces disparity in fuel concentration at EOL between edges and axial center, better neutron coupling • Also creates more space for helium and fission product gasses when clad creep and fuel pellet swell closes the gap between fuel pellets and cladding © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 178 Effects of Xenon on Core Profiles • Production of xenon-135 and its precursor, iodine-135, is dependent on thermal neutron flux – Production rate of these isotopes at any local point in a reactor depends on thermal flux level on fuel at that location • Since neutron flux is not uniform over entire volume of core, xenon135 and iodine-135 will not be uniform either – Result is xenon-135 can produce a local reactivity effect that tends to change thermal flux profiles across the core © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 179 Effects of Xenon on Core Profiles • In areas of core where local xenon concentration is relatively high, thermal neutron flux will be depressed • In areas of core where local xenon concentration is relatively low, thermal neutron flux will tend to be greater – Due to effects of xenon-135 on neutron life cycle – Areas of high xenon-135 absorb a greater number of thermal neutrons, removing them from neutron life cycle – Opposite effect in areas of lower xenon-135 concentration • An example of this localized effect is a xenon oscillation © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 180 Effects of Xenon on Core Profiles • During core operation, MTC causes equilibrium thermal neutron flux to shift towards bottom of core – Colder water in bottom better neutron thermalizer • Results in thermal flux at bottom of core greater than thermal flux at top • Equilibrium iodine and xenon concentrations directly proportional to local thermal neutron flux level • Since thermal flux is greater in bottom half of core from MTC, concentration of iodine and xenon also greater near bottom of core © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 181 Xenon Effects to Thermal Flux Profile Knowledge Check A nuclear reactor is experiencing a xenon oscillation. At what time in core life would this xenon oscillation be of greatest concern from a reactor operational standpoint? A. Just after initial core startup B. Early in core life C. Toward the middle of core life D. Late in core life Correct answer is D. © Copyright 2014 ELO 2.8 Operator Generic Fundamentals 182 Xenon Effects to Thermal Flux Profile Knowledge Check – NRC Bank Xenon-135 poisoning in a nuclear reactor core is most likely to prevent a reactor startup following a reactor shutdown from ____________ power at the ____________ of core life. A. high; beginning B. low; beginning C. high; end D. low; end Correct answer is C. © Copyright 2014 ELO 2.8 Operator Generic Fundamentals Solution 183 Xenon Graph Project • Using a page of blank graph paper, draw the response of xenon-135 during the following transient. Plot time on horizontal axis and reactivity in pcm on vertical axis. Provide approximate pcm and time values for all peak and equilibrium conditions. – – – – – – – Start with power at 0% and no xenon Perform startup and increase power to 50% At 50 hours, increase to 100% power At 90 hours, trip reactor At 98 hours, perform a reactor startup and increase power to 100% At 150 hours, decrease power to 50% Stop plot at 200 hours Note: Realizing that power changes take time, for demonstration of xenon response and to minimize assumptions, assume power changes occur rapidly. © Copyright 2014 Class Project Operator Generic Fundamentals 184 TLO 2 Summary • Describe fission product poisons and how fission product poisons affect neutron life cycle – Fission product poisons act as parasitic absorbers of neutrons, removing neutrons from neutron life cycle – An increase in macroscopic cross-section for absorption by any neutron poison will result in a decrease in the value of ƒ – At equilibrium, production rate of poison equals removal rate of poison, therefore concentration of poison is constant – Depending on poison, equilibrium levels power dependent and time to reach equilibrium related to both power and decay rates © Copyright 2014 TLO 2 Operator Generic Fundamentals 185 TLO 2 Summary • List the most important fission product poisons to the operation of a nuclear reactor – Most important fission product poisons to operation of a nuclear reactor o Xenon-135 — 2.6 x 106 barns o Samarium-149 — 4.0 x 104 barns – At equilibrium levels, add considerable negative reactivity o Xenon – approximately -3,000 pcm (varies with power) o Samarium – approximately -700 - 1,000 pcm (100% BOL/EOL) • Explain how xenon-135 is produced and removed in the core of a nuclear reactor – Xenon-135 is produced from two sources: o Directly as a fission fragment o Beta-minus tellurium-135 decay chain © Copyright 2014 TLO 2 Operator Generic Fundamentals 186 TLO 2 Summary • Continued – Xenon-135 removed by: o Beta-minus decay to Cesium-135 (Cs-135) o Neutron capture reaction which creates Xe-136 (burnout) – Approximately 5% directly as a fission fragment – 95% from beta-minus decay of I-135 – With 95% of xenon coming from I-135, and I-135 having a half-life of 6.6 hours, significant delay in production of Xe-135 occurs – Rate of change of xenon concentration equal to rate of production minus rate of removal (when in balance, xenon said to be in equilibrium) © Copyright 2014 TLO 2 Operator Generic Fundamentals 187 TLO 2 Summary • Explain the following terms: equilibrium iodine, equilibrium xenon, transient xenon, peak xenon, xenon free, xenon precluded startup, and xenon dead time. – Equilibrium iodine is constant I-135 concentration in reactor that occurs after a power change – Equilibrium xenon when xenon concentration is not liner with power level; at 25% power, xenon is approximately equal to its 50% of its 100% power level o At low powers, decay is major removal term o At high powers, burnout is major removal term – Transient xenon - when a power change in reactor occurs, Xe135 undergoes a transient – Peak xenon - anytime reactor undergoes a rapid down power, Xe135 concentration will peak and then decrease to a new lower equilibrium value or go to zero (on a shutdown) © Copyright 2014 TLO 2 Operator Generic Fundamentals 188 TLO 2 Summary • Continued – Xenon dead time is period of time where reactor is unable to override effects of xenon • Explain how xenon-135 concentration reacts during the following nuclear reactor operations: – Initial reactor startup o When reactor attains a 5-10% power, xenon-135 starts to decrease due to burnout with increasing flux levels o Decrease much faster than normal xenon decay or rate changes and must be monitored very carefully as very large reactivity addition rates can be created – Reactor shutdown o The greater the flux level prior to shutdown, the greater the concentration of iodine-135 at shutdown; therefore, the greater the peak in xenon-135 concentration after shutdown © Copyright 2014 TLO 2 Operator Generic Fundamentals 189 TLO 2 Summary • Describe the causes and effects of a xenon oscillation – A xenon oscillation may be caused by a rapid change in the core power distribution o Can change local power levels in core by a factor of three or more o More prevalent at end of core life because fuel is mostly depleted in axial center of core and there is less neutronic coupling between upper and lower halves of core • State approximate time following a reactor shutdown at which reactor can be considered "xenon free" – 3 days (70 to 80 hours) after shutdown, xenon-135 concentration has decreased to a small percentage of its pre-shutdown level – The higher the reactor power level at start of a shutdown, the longer the time required to reach a xenon-free condition © Copyright 2014 TLO 2 Operator Generic Fundamentals 190 TLO 2 Summary • Explain the effects of xenon concentration on a nuclear reactor core's thermal flux profile for CR motion and core life – When control rods inserted a small amount (while maintaining a constant reactor power), thermal flux in top half of core decreases, while thermal flux in bottom half of core increases – Control rod movement can result in concentration of xenon-135 oscillating between top and bottom portions of core, resulting in a change in axial thermal flux profile within core © Copyright 2014 TLO 2 Operator Generic Fundamentals 191 TLO 2 Summary • Xenon effects on thermal flux – At BOL, reactivity effects due to MTC tend to dampen xenon oscillations – Xenon oscillations are more prevalent at EOL • Xenon effects on core flux profiles – In areas of core where local xenon concentration is relatively high, thermal neutron flux will be depressed – In areas of core where local xenon concentration is relatively low, thermal neutron flux will tend to be greater © Copyright 2014 TLO 2 Operator Generic Fundamentals 192 Xenon 135 Fission Product Poison Knowledge Check – NRC Bank Slow changes in axial power distribution in a nuclear reactor that has operated at a steady-state power for a long time can be caused by xenon . A. peaking B. override C. burnup D. oscillation Correct answer is D. © Copyright 2014 TLO 2 Operator Generic Fundamentals 193 Samarium-149 TLO 3 – Describe the production, removal, and effects of samarium-149 on the operation of a nuclear reactor. • In this section, reactivity effects from samarium-149 are discussed and compared with xenon-135 • Samarium-149 is second most significant fission product poison – However, its effect to neutron life cycle is much less significant than xenon-135 • Samarium is not as visible to operator as xenon – Its response to reactor transients is important for operator to understand and predict © Copyright 2014 TLO 3 Operator Generic Fundamentals 194 Enabling Learning Objectives for TLO 3 1. Explain how samarium-149 is produced and removed from the reactor core during reactor operation. 2. Describe equilibrium samarium-149 concentration. 3. Explain how equilibrium samarium-149 concentration varies with the following reactor operations: initial reactor startup, reactor shutdown, and reactor startup after shutdown. 4. Describe the effects of samarium-149 concentration on reactor operation over core life. 5. Compare the effects of samarium-149 to the effects of xenon-135 on reactor operation. © Copyright 2014 TLO 3 Operator Generic Fundamentals 195 Samarium Production and Removal ELO 3.1 – Explain how samarium-149 is produced and removed from the reactor core during reactor operation. • Next to xenon-135, samarium-149 is most significant fission product poison • Samarium-149 behaves significantly different from xenon-135 due to its different nuclear properties • This session will explain where samarium comes from and how it is removed © Copyright 2014 ELO 3.1 Operator Generic Fundamentals 196 Samarium-149 Production • Negligible amounts of samariun-149 produced directly from fission – 1.1% of all fissions result in production of either neodymium-149 or promethium-149 – Samarium-149 is end product of a decay chain containing both of these isotopes π½−149 π½−149 149 60 ππ 61 1.73 βππ’ππ ππ 62 ππ 53 βππ’ππ • Neodymium-149 half-life – 1.7 hours • Promethium-149 half life – 53 hours © Copyright 2014 ELO 3.1 Operator Generic Fundamentals 197 Samarium-149 Removal • Samarium-149 has half-life of 1016 years and is considered stable – Only removal mechanism for samarium-149 is by neutron capture – Results in conversion of samarium-149 to samarium-150 149 1 150 ππ + π → ππ + πΎ 62 0 62 • Microscopic cross-section for absorption for samarium-149 is 4.1 x 104 barns • Samarium-150 also considered stable, but has a low absorption cross-section for neutrons of approximately 103 barns © Copyright 2014 ELO 3.1 Operator Generic Fundamentals 198 Samarium Production and Removal Knowledge Check Samarium-149 is removed from a reactor by which one of the following processes? A. It is stable, so it is only removed when the reactor is defueled B. Neutron capture reactions producing samarium-150 C. Beta minus decay to promethium-149 D. Beta minus decay to neodymium-149 Correct answer is B. © Copyright 2014 ELO 3.1 Operator Generic Fundamentals 199 Equilibrium Samarium ELO 3.2 – Describe equilibrium samarium-149 concentration and how it varies with reactor power changes. • Equilibrium concentrations of promethium-149 and samarium-149 are discussed in this session • Equilibrium Sm-149 concentration is independent of power level (time to equilibrium is not) – Promethium-149 with a half-life of 53 hours is considered fission product precursor with a 1.1% fission yield – Promethium-149 burnup is negligible – Samariun-149 fission yield is negligible © Copyright 2014 ELO 3.2 Operator Generic Fundamentals 200 Equilibrium Samarium • Rate of change of Pm-149 = yield from fission - decay of Pm-149 ππππ = πΎππ ππ‘ ππ’ππ ΙΈ − Ζππ πππ π Where: NPm = Pm-149 concentration γPm = Pm-149 fission yield λPm = decay constant for Pm-149 • Solving equilibrium value of Pm-149 gives: πππ ππ = © Copyright 2014 πΎππ ππ’ππ ΙΈ π πππ ELO 3.2 Operator Generic Fundamentals 201 Equilibrium Samarium • Rate of samarium-149 formation: Sm-149 rate of change = yield from fission + Pm-149 decay - Sm-149 burnup ππππ = πΎππ ππ‘ ππ’ππ π ΙΈ + πππ πππ − πππ π ππ ΙΈ π Where: NSm = Sm-149 concentration γSm = Sm-149 fission yield σaSm = microscopic absorption cross-section of Sm-149 • Since fission yield of samarium-149 nearly zero, equation shortened: ππππ ππ = πππ πππ − πππ π ΙΈ π ππ‘ © Copyright 2014 ELO 3.2 Operator Generic Fundamentals 202 Equilibrium Samarium • Equation further solved for equilibrium samarium-149 by substituting for formula for equilibrium concentration of promethium-149 πππ ππ = ππ’ππ ΙΈ π πΎππ πππ • Yielding: πππ = © Copyright 2014 πΎππ π ππ’ππ π ππ π ELO 3.2 Operator Generic Fundamentals 203 Equilibrium Samarium • Decay of promethium-149 to samarium-149 has half-life of 53 hours • Decay of neptunium-239 to plutonium-239 has a half-life of 56.4 hours (fuel conversion) • Conversion to plutonium-239 adds positive reactivity, while samarium-149 production adds negative reactivity at very close to same decay rate • These two reactivities are very close in magnitude and therefore cancel each other out • Because of this, reactivity effects from building samarium-149 to equilibrium can be ignored © Copyright 2014 ELO 3.2 Operator Generic Fundamentals 204 Equilibrium Samarium • Similar to xenon-135 equilibrium requiring iodine-135 in equilibrium, promethium-149 must be in equilibrium for samarium-149 to reach equilibrium Equilibrium samarium-149 concentration is independent of neutron flux and power level. © Copyright 2014 ELO 3.2 Operator Generic Fundamentals 205 Equilibrium Samarium Knowledge Check Which one of the following is correct concerning equilibrium samarium149 concentration? A. Equilibrium Sm-149 concentration at 10% power is much less than at 100% power. B. Equilibrium Sm-149 concentration at 50% power is approximately half of the equilibrium Sm-149 concentration at 100% power. C. Promethium-149 concentration has to reach equilibrium before Sm-149 can reach equilibrium. D. Since samarium-149 is stable it takes years of reactor operation before it reaches equilibrium. Correct answer is C. © Copyright 2014 ELO 3.2 Operator Generic Fundamentals 206 Samarium Concentration Transients ELO 3.3 – Explain how equilibrium samarium-149 concentration varies with the following reactor operations: initial reactor startup, reactor shutdown, and reactor startup after shutdown. • This session discusses samarium-149 and promethium-149 response for startup and shutdown scenarios • This is good information for operator to understand about second most important fission product poison © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 207 Samarium Concentration Transients • Although equilibrium samarium-149 is independent of power level, amount of time required to reach equilibrium samarium is related to power level • When a new core is taken critical and power increased first time, production of promethium-149 begins with no samarium-149 present – As promethium-149 builds, it decays to samarium-149 © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 208 Samarium Concentration Transients Figure: Samarium-149 Buildup to Equilibrium • Equilibrium samarium-149 concentration is reached in about 20-25 days if reactor is operated at significant power levels – Time to equilibrium is power dependent 25-35 days © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 209 Samarium Concentration Transients • As mentioned, during decay of promethium-149 to samarium-149, neptunium-239 decays to plutonium-239 • Since these two reactivities negate each other, during this period, operator is not likely to need to make reactivity adjustments (may need to for XE-135 buildup) • Reactivity from equilibrium samarium-149 is on order of -700 to 1,000 pcm (BOL – EOL) © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 210 Samarium Concentration Transients • Sm-149 will peak after shutdown due to decay of equilibrium promethium-149 – No decay and burnout stops • More promethium-149 in core, higher the peak – Samarium-149 peak is therefore dependent on power level prior to shutdown (equilibrium promethium-149) • Promethium-149 will completely decay off to samarium-149, but samarium-149 peak levels at a constant higher value – Sm-149 does not decay – Power is 0%, so no burnout © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 211 Samarium Concentration Transients Figure: Behavior of Samarium-149 in a Pressurized Water Reactor © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 212 Samarium Concentration Transients • Rate of samarium-149 production at shutdown is: ππππ = πππ πππ ππ‘ • Plutonium-239 peaks after shutdown from decay of neptunium-239 • These two reactivity effects, along with other minor effects, sometimes combined into one curve of reactivity added during shutdown – Could be used in shutdown margin calculations and estimated critical position calculations © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 213 Samarium Concentration Transients • With these two reactivities combined, net reactivity change can be positive or negative – Typically, a value of +60 pcm to -250 pcm may be expected – Usually, samarium-149 is largest effect • Both samarium-149 and plutonium-239 are considered stable and remain in used fuel after shutdown • Actual reactivity added from samarium-149 peaking is on the order of -860 pcm to -1,300 pcm (100% power trip) – Includes equilibrium samarium – Power dependent, the higher the power history at shutdown, the higher the peak © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 214 Samarium Concentration Transients • After startup and increasing thermal flux levels: – Samarium-149 peak from shutdown will immediately start burndown to equilibrium Sm-149 concentration – Due to low inventory and long half-life of promethium-149, significant formation of additional samarium-149 is delayed © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 215 Samarium Concentration Transients Figure: Behavior of Samarium-149 in a Pressurized Water Reactor © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 216 Samarium Concentration Transients Knowledge Check Every samarium-149 transient is partially offset in terms of reactivity by a transient of which of the following? A. Pu-239 from Np-239 decay B. Pu-241 from Np-241 decay C. Pu-242 from Np-242 decay D. U-233 from Pa-233 decay Correct answer is A. © Copyright 2014 ELO 3.3 Operator Generic Fundamentals 217 Samarium-149 Effects Over Core Life ELO 3.4 – Describe the effects of samarium-149 concentration on reactor operation over core life. • This session provides information about samarium-149 effects over core life • Greatest change in samarium-149 concentration occurs immediately following initial startup of a new reactor core that contains no previously used fuel • However, samarium-149 and other unspecified poisons continue to increase in concentration throughout core life and remain in the core – Presents a constant source of negative reactivity © Copyright 2014 ELO 3.4 Operator Generic Fundamentals 218 Samarium-149 Effects Over Core Life • Reactivity values due to equilibrium and peak samarium-149 may also become increasingly more negative as core ages due to: – Less thermal neutron competition from decreasing concentration of soluble boron (boron-10) – Depleting burnable poisons – Increasing thermal neutron flux levels over core life • While samarium-149 reactivity should never prevent a restart, probability would be greatest at EOL – Negative reactivity added by Samarium-149 combined with lack of positive reactivity available due to fuel burnout at EOL could prevent the reactor from immediately being taken critical © Copyright 2014 ELO 3.4 Operator Generic Fundamentals 219 Samarium-149 Effects Over Core Life Knowledge Check At what point in core life will samarium-149 concentration in the core of a nuclear reactor have the greatest impact on the operator’s ability to start up the reactor? A. Toward the end of core life B. Toward the middle of core life C. Toward the beginning of core life D. Samarium-149 is never a concern during a reactor startup Correct answer is A. © Copyright 2014 ELO 3.4 Operator Generic Fundamentals 220 Samarium-149 to Xenon-135 Comparison ELO 3.5 – Compare the effects of samarium-149 to the effects of xenon135 on reactor operation. • This section gives a brief review and overview of differences between xenon and samarium neutron poisoning © Copyright 2014 ELO 3.5 Operator Generic Fundamentals 221 Samarium-149 to Xenon-135 Comparison • Xe-135 has substantial effect on net reactivity of the core – At power – During startup o When shutting down, Xe-135 reactivity must be accounted for • If xenon concentration is changing during a reactor startup, estimated critical position must anticipate estimated xenon at the time of expected criticality • Shutdown margin calculations must also consider xenon • Power transients, especially up power, can add considerable amount of reactivity from xenon burnout • Reactor operator must always be aware of what xenon-135 is doing © Copyright 2014 ELO 3.5 Operator Generic Fundamentals 222 Samarium-149 to Xenon-135 Comparison • Samarium-149 neutron effects minor when compared to xenon • Its reactivity effect is opposed by Pu-239 from Np-239 decay • It is of lesser magnitude and much slower in changing reactivity • For these reasons, its effect on reactor operations, estimated critical position calculations, and shutdown margin are small compared to xenon © Copyright 2014 ELO 3.5 Operator Generic Fundamentals 223 Samarium-149 to Xenon-135 Comparison Effect Xenon-135 Samarium-149 Microscopic Cross-Section for Absorption (σa) Time to Peak Concentration 2.6 x 106 Barns 4.1.x 104 Barns Square root of power prior to S/D or trip ο» 20 days Time to Equilibrium Concentration 40 – 48 hours 25 – 35 days Reactivity Worth (these are approximate reactivity values — change over core life) -2.7% Δk/k at 100% -0.7% Δk/k at power equilibrium power equilibrium -4.7% Δk/k at peak -1.1% Δk/k at peak Removal by Decay Yes No Equilibrium Dependent on Power Yes No Distribution Problem (Oscillations) Yes No © Copyright 2014 ELO 3.5 Operator Generic Fundamentals 224 Other Fission Product Poisons • Numerous other fission products, as a result of their concentrations and thermal neutron absorption cross-sections, have a neutron poisoning effect on reactor operations • Individually they are fairly small, but lumped together they also have a significant impact • May accumulate at an average rate of 50 barns per fission event in the reactor © Copyright 2014 ELO 3.5 Operator Generic Fundamentals 225 Samarium-149 to Xenon-135 Comparison Knowledge Check A commercial pressurized water reactor experienced a reactor trip from 100% reactor power. Which of the following would contribute the greatest magnitude of negative reactivity to the reactor core, assuming the reactor remains shutdown for an extended period of time? A. Samarium-149 concentration approximately 12.5 days after the trip B. Samarium-149 concentration 20-25 days after the trip C. Xenon-135 concentration approximately 10 hours after the trip D. Xenon-135 concentration approximately 70 hours after the trip Correct answer is C. © Copyright 2014 ELO 3.5 Operator Generic Fundamentals 226 TLO 3 Summary • Explain how samarium-149 is produced and removed from the reactor core during reactor operation – Produced directly from fission and decay of promethium-149 during reactor operation π½−149 149 ππ ππ 60 61 1.73 βππ’ππ π½−149 62 ππ 53 βππ’ππ – Removed from the core by neutron absorption, does not decay 149 1 150 ππ + π → ππ + πΎ 62 0 62 © Copyright 2014 TLO 3 Operator Generic Fundamentals 227 TLO 3 Summary • Describe equilibrium samarium-149 concentration and how it varies with reactor power changes – Equation for equilibrium samarium-149 concentration: πππ = πΎππ ππ’ππ π ππ π – Independent of power level (time to equilibrium is not) π – During initial reactor startup, production of promethium-149 begins, promethium decays to samarium-149 – Samarium equilibrium concentration reached in about 25-35 days if reactor is operated at significant power levels © Copyright 2014 TLO 3 Operator Generic Fundamentals 228 TLO 3 Summary • Explain how equilibrium samarium-149 concentration varies with the following reactor operations: – Initial reactor startup o Equilibrium samarium-149 concentration reached in about 20 to 25 days if reactor is operated at significant power levels – Reactor shutdown o Following a reactor shutdown, samarium-149 concentration increases due to decay of promethium-149 inventory of the core and loss of the burnup factor © Copyright 2014 TLO 3 Operator Generic Fundamentals 229 TLO 3 Summary • Explain how equilibrium samarium-149 concentration varies with the following reactor operations: – Reactor re-start o If reactor restarted following a shutdown, Sm-149 concentration decreases as samarium is burned up and delay in promethium149 producing samarium-149 catches up to return samarium to equilibrium • Describe the effects of samarium-149 concentration on reactor operation over core life – Remains in core as constant source of negative reactivity – Reactivity values due to equilibrium and peak samarium-149 become increasingly more negative as core ages – While samarium-149 reactivity should not prevent a restart, its value is greatest at EOL © Copyright 2014 TLO 3 Operator Generic Fundamentals 230 TLO 3 Summary • Compare the effects of samarium-149 to the effects of xenon-135 on reactor operation Effect Xenon-135 Samarium-149 2.6 x 106 Barns 4.1.x 104 Barns ο» 20 days Time to Equilibrium Concentration Square root of power prior to S/D or trip 40 – 48 hours Reactivity Worth (these are approximate reactivity values — change over core life) -2.7% Δk/k at 100% -0.7% Δk/k at power equilibrium power equilibrium -4.7% Δk/k at peak -1.1% Δk/k at peak Microscopic Cross-Section for Absorption (σa) Time to Peak Concentration 25 – 35 days Removal by Decay Yes No Equilibrium Dependent on Power Yes No Distribution Problem (Oscillations) Yes No © Copyright 2014 TLO 3 Operator Generic Fundamentals 231 Crossword Puzzle • It’s crossword puzzle time! © Copyright 2014 TLO 2-3 Operator Generic Fundamentals 232 Neutron Poisons Summary At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80% score on the following TLOs: 1. Describe how fuel depletion and neutron poison concentration affect reactivity in a reactor core. 2. Describe the behavior of xenon-135 in a nuclear reactor and its effects on reactor operation. 3. Describe the production, removal, and effects of samarium-149 on the operation of a nuclear reactor. © Copyright 2014 Summary Operator Generic Fundamentals
