NBSLM03E (2010)
Low Carbon Technologies and Solutions: Sections 6 - 8
N.K. Tovey (杜伟贤) M.A, PhD, CEng, MICE, CEnv
Nuclear Power
Norwich Business
Business
Norwich
School
School
1
NBSLM03E (2010)
Low Carbon Technologies and Solutions
N.K. Tovey (杜伟贤) M.A, PhD, CEng, MICE, CEnv
Section 6: Nuclear Power:- The Basics
6. Nature of Radioactivity
• Structure of the Atom
• Radioactive Emissions
• Half Life of Elements
• Fission
• Fusion
• Chain Reactions
• Fertile Materials
7. Fission Reactors
8. The Nuclear Fuel Cycle
9. Fusion Reactors
(in notes)
2
Norwich Business
Business
Norwich
School
School
2
Section 6: Nature of Radioactivity (1)
Structure of Atoms.
• Matter is composed of atoms which consist
primarily of a nucleus of:
– positively charged PROTONS
– and (electrically neutral) NEUTRONS.
• The nucleus is surrounded by a cloud of
negatively charged ELECTRONS which balance
the charge from the PROTONS.
• PROTONS and NEUTRONS have approximately
the same mass
+
+ +
3p
4n
• ELECTRONS are about 0.0005 times the mass of
the PROTON.
• A NUCLEON refers to either a PROTON or a
NEUTRON
Norwich Business
School
Lithium Atom
3 Protons 4 Neutrons
3
Section 6: Nature of Radioactivity (2)
Structure of Atoms.
• Elements are characterized by the number of PROTONS present
– HYDROGEN nucleus has 1 PROTON
– HELIUM has 2 PROTONS
– OXYGEN has 8 PROTONS
– URANIUM has 92 PROTONS.
• Number of PROTONS is the ATOMIC NUMBER (Z)
• N denotes the number of NEUTRONS.
• The number of neutrons present in any element varies.
• 3 isotopes of hydrogen all with 1 PROTON:– HYDROGEN itself with NO NEUTRONS
– DEUTERIUM (heavy hydrogen) with 1 NEUTRON
– TRITIUM with 2 NEUTRONS.
Symbol D
Symbol T
• only TRITIUM is radioactive.
• Elements up to Z = 82 (Lead) have at least one isotope which is stable
Norwich Business
School
4
Section 6: Nature of Radioactivity (3)
Structure of Atoms.
• URANIUM has two main ISOTOPES
•
235U
•
238U
which is present in concentrations of 0.7% in naturally
occurring URANIUM
which is 99.3% of naturally occurring URANIUM.
• Some Nuclear Reactors use Uranium at the naturally occurring
concentration of 0.7%
• Most require some enrichment to around 2.5% - 5%
• Enrichment is energy intensive if using gas diffusion
technology, but relatively efficient with centrifuge technology.
• Some demonstration reactors use enrichment at around 93%.
Norwich Business
School
5
Section 6: Nature of Radioactivity (4)
Structure of Atoms.
• Protons have strong nuclear forces to overcome the strong
repulsive forces from the charges on them. This is the energy
released in nuclear reactions
+ +
+
+
+
+
Stable elements plot close to blue line.
Those isotopes plotting away from
line are unstable.
For elements above Lead (Z = 82),
there are no stable isotopes.
Norwich Business
School
6
Section 6: Nature of Radioactivity (5)
Radioactive emissions.
• FOUR types of radiation:• 1) ALPHA particles ()
- large particles consisting of 2 PROTONS and 2 NEUTRONS
the nucleus of a HELIUM atom.
• 2) BETA particles (β) which are ELECTRONS
• 3) GAMMA - RAYS. ()
– Arise when the kinetic energy of Alpha and Beta particles is lost
passing through the electron clouds of atoms. Some energy is used to
break chemical bonds while some is converted into GAMMA -RAYS.
• 4) X - RAYS.
– Alpha and Beta particles, and gamma-rays may temporarily dislodge
ELECTRONS from their normal orbits. As the electrons jump back
they emit X-Rays which are characteristic of the element which has
been excited.
Norwich Business
School
7
Section 6: Nature of Radioactivity (6)

β

 - particles are stopped by a thin sheet of paper
β – particles are stopped by ~ 3mm aluminium
 - rays CANNOT be stopped – they can be attenuated to safe
limits using thick Lead and/or concrete
Norwich Business
School
8
Section 6: Nature of Radioactivity (7)
Radioactive emissions.
• UNSTABLE nuclei emit Alpha or Beta particles
• If an ALPHA particle is emitted, the new element will have an
ATOMIC NUMBER two less than the original.
e
231
235
90
92
93
Th
U
Np
4
2
He
• If an ELECTRON is emitted as a result of a NEUTRON
transmuting into a PROTON, an isotope of the element ONE
HIGHER in the PERIODIC TABLE will result.
Norwich Business
School
9
Section 6: Nature of Radioactivity (8)
Radioactive emissions.
•
235U
consisting of 92 PROTONS and 143 NEUTRONS is one of SIX
isotopes of URANIUM
• decays as follows:-
235U
alpha
URANIUM
231Th
THORIUM
beta
231Pa
PROTACTINIUM
alpha
227Ac
ACTINIUM
• Thereafter the ACTINIUM - 227 decays by further alpha and
beta particle emissions to LEAD - 207 (207Pb) which is stable.
• Two other naturally occurring radioactive decay series exist.
One beginning with 238U, and the other with 232Th.
• Both also decay to stable (but different) isotopes of LEAD.
Norwich Business
School
10
Section 6: Nature of Radioactivity (9)
HALF LIFE.
• Time taken for half the remaining atoms of an element to undergo their
first decay e.g:•
•
•
238U
4.5 billion years
235U
0.7 billion years
232Th
14 billion years
• All of the daughter products in the respective decay series have much
shorter half - lives some as short as 10-7 seconds.
• When 10 half-lives have expired,
– the remaining number of atoms is less than 0.1% of the original.
• 20 half lives
– the remaining number of atoms is less than one millionth of the
original
Norwich Business
School
11
Section 6: Nature of Radioactivity (10)
HALF LIFE.
From a radiological hazard point of view
• short half lives - up to say 6 months have intense radiation,
but
• decay quite rapidly. Krypton-87 (half life 1.8 hours)- emitted
from some gas cooled reactors - the radioactivity after 1 day is
insignificant.
• For long half lives - the radiation doses are small, and also of
little consequence
• For intermediate half lives - these are the problem - e.g.
Strontium -90
– has a half life of about 30 years which means it has a relatively high
radiation, and does not decay that quickly.
• Radiation decreases to 30% over 90 years
Norwich Business
School
12
Section 6: Nature of Radioactivity (11): Fission
Some very heavy UNSTABLE elements exhibit FISSION e.g. 235U
n
235U
93Rb
n
n
13
140Cs
This reaction is one of several which might
take place. In some cases, 3 daughter
products are produced.
Norwich Business
Business
Norwich
School
School
13
13
Section 6: Nature of Radioactivity (12): Fission
• Nucleus breaks down into two or three fragments accompanied by
a few free neutrons and the release of very large quantities of
energy.
• Free neutrons are available for further FISSION reactions
• Fragments from the fission process usually have an atomic mass
number (i.e. N+Z) close to that of iron.
• Elements which undergo FISSION following capture of a neutron
such as URANIUM - 235 are known as FISSILE.
• Diagrams of Atomic Mass Number against binding energy per
NUCLEON enable amount of energy produced in a fission
reaction to be estimated.
• All Nuclear Power Plants currently exploit FISSION reactions,
• FISSION of 1 kg of URANIUM produces as much energy as
burning 3000 tonnes of coal.
Norwich Business
School
14
Section 6: Nature of Radioactivity (13): Fusion
Fusion of light elements e.g. DEUTERIUM and TRITIUM produces
even greater quantities of energy per nucleon are released.
3H
Deuterium – Tritium fusion
Tritium
4He
(3.5 MeV)
2H
Deuterium
n
(14.1 MeV)
In each reaction 17.6 MeV is liberated or 2.8 picoJoules (2.8 * 10-15J)
15
Norwich
Business
Norwich
Business
08/04/2015
School
School
15
Section 6: Nature of Radioactivity (14): Binding Energy
Atomic Mass Number
Binding Energy per nucleon [MeV]
0
50
100
150
200
250
-2
-4
Fusion Energy
release per
nucleon
1 MeV per nucleon is
equivalent to 96.5 TJ per
kg
-6
-8
-10
Iron
56
Range of Fission
Products
Uranium
235
Fission Energy
release per
nucleon
Redrawn from 6th report on Environmental Pollution – Cmnd. 6618 - 1976
1) The energy released per nucleon in fusion reaction is much greater than the
corresponding fission reaction.
2) In fission there is no single fission product but a broad range as indicated.
Norwich Business
School
16
Section 6: Nature of Radioactivity (15): Fusion
• Developments at the JET facility in Oxfordshire have achieved the
break even point.
• Next facility (ITER) will be built in Cadarache in France.
• Commercial deployment of fusion from about 2040 onwards
• One or two demonstration commercial reactors in 2030s perhaps
• No radioactive waste from fuel
• Limited radioactivity in power plant itself
• 8 litres of tap water sufficient for all energy needs of one
individual for whole of life at a consumption rate comparable to
that in UK.
• Sufficient resources for 1 – 10 million years
Norwich Business
School
17
Section 6: Nature of Radioactivity (16): Chain Reactions
n
Fast Neutrons are
unsuitable for sustaining
further reactions
fast
neutron
235U
n
n
Slow neutron
235
U
n
fast neutron
n
Norwich Business
School
Slow neutron
18
Section 6: Nature of Radioactivity (17): Chain Reactions
CHAIN REACTIONS
• FISSION of URANIUM - 235 yields 2 - 3 free neutrons.
• If exactly ONE of these triggers a further FISSION, then a
chain reaction occurs, and continuous power can be
generated.
• UNLESS DESIGNED CAREFULLY, THE FREE
NEUTRONS WILL BE LOST AND THE CHAIN
REACTION WILL STOP.
• IF MORE THAN ONE NEUTRON CREATES A NEW
FISSION THE REACTION WOULD BE SUPER-CRITICAL
(or in layman's terms a bomb would have been created).
Norwich Business
School
19
Section 6: Nature of Radioactivity (18): Chain Reactions
• IT IS VERY DIFFICULT TO SUSTAIN A CHAIN REACTION,
• Most Neutrons are moving too fast
• TO CREATE A BOMB, THE URANIUM - 235 MUST BE
HIGHLY ENRICHED > 93%,
• Normal Uranium is only 0.7% U235
• Material must be LARGER THAN A CRITICAL SIZE and
SHAPE OTHERWISE NEUTRONS ARE LOST.
• Atomic Bombs are made by using conventional explosive to bring
two sub-critical masses of FISSILE material together for sufficient
time for a SUPER-CRITICAL reaction to take place.
• NUCLEAR POWER PLANTS CANNOT EXPLODE LIKE AN
ATOMIC BOMB.
Norwich Business
School
20
Section 6: Nature of Radioactivity (19): Fertile Materials
• FERTILE MATERIALS
• Some elements like URANIUM - 238 are not FISSILE, but
can transmute:n
fast
neutron
e
e
238U
Uranium - 238
+n
238
239Np
239
U
Pu
239U
Uranium - 239
beta239Np
beta 239Pu
Neptunium - 239
Plutonium - 239
PLUTONIUM - 239 is FISSILE and may be used in place of URANIUM - 235.
Materials which can be converted into FISSILE materials are FERTILE.
Norwich Business
School
21
Section 6: Nature of Radioactivity (20): Fertile Materials
• URANIUM - 238 is FERTILE as is THORIUM - 232 which can
be transmuted into URANIUM - 233.
• Naturally occurring URANIUM consists of 99.3% 238U which is
FERTILE and NOT FISSILE, and 0.7% of 235U which is
FISSILE. Normal reactors primarily use the FISSILE properties
of 235U.
• In natural form, URANIUM CANNOT sustain a chain reaction:
free neutrons are travelling fast to successfully cause another
FISSION, or are lost to the surrounds.
• MODERATORS are thus needed to slow down/and or reflect the
neutrons in a normal FISSION REACTOR.
• The Resource Base of 235U is only decades
• But using a Breeder Reactor Plutonium can be produced from
non-fissile 238U producing 239Pu and extending the resource base
by a factor of 50+
Norwich Business
School
22
n
Section 6: Nature of Radioactivity (21): Chain Reactions
Sustaining a reaction in a Nuclear Power Station
fast
neutron
Fast Neutrons are
unsuitable for sustaining
further reactions
235U
n
n
fast
neutron
Slow neutron
n
235
U
n
fast neutron
Insert a moderator to
slow down neutrons
n
Norwich Business
School
Slow neutron
23
NBSLM03E (2010)
Low Carbon Technologies and Solutions
N.K. Tovey (杜伟贤) M.A, PhD, CEng, MICE, CEnv
Section 7: Fission Reactors
6.
7.
Nuclear Power – The Basics
Nuclear Power: Fission reactors
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
8.
9.
24
General Introduction
MAGNOX Reactors
AGR Reactors
CANDU Reactors
PWRs
BWRs
RMBK/ LWGRs
FBRs
Generation 3 Reactors
Generation 3+ Reactors
Nuclear Fuel Cycle
Fusion Reactors
Norwich Business
Business
Norwich
School
School
24
Fission Reactors (1)
FISSION REACTORS CONSIST OF:i)
ii)
iii)
a FISSILE component in the fuel
a MODERATOR
a COOLANT to take the heat to its point of use.
The fuel elements vary between different Reactors
• Some reactors use unenriched URANIUM
– i.e. the 235U in fuel elements is at 0.7% of fuel
– e.g. MAGNOX and CANDU reactors,
• ADVANCED GAS COOLED REACTOR (AGR) uses 2.5 – 2.8% enrichment
• PRESSURISED WATER REACTOR (PWR) and BOILING WATER REACTOR
(BWR) use around 3.5 – 4% enrichment.
• RMBK (Russian Rector of Chernobyl fame) uses ~2% enrichment
• Some experimental reactors - e.g. High Temperature Reactors (HTR) use highly
enriched URANIUM (>90%) i.e. weapons grade.
Norwich Business
School
25
Fission Reactors (2): Fuel Elements
PWR fuel assembly:
UO2 pellets loaded into fuel
pins of zirconium each ~ 3 m
long in bundles of ~200
AGR fuel
assembly:
UO2 pellets loaded
into fuel pins of
stainless steel each
~ 1 m long in
bundles of 36.
Whole assembly in
a graphite
cylinder
Magnox fuel rod:
Natural Uranium metal bar
approx 35mm diameter and
1m long in a fuel cladding
made of MagNox.
Burnable
poison
Norwich Business
School
26
Fission Reactors (3)
• No need for the extensive coal handling plant.
• In the UK, all the nuclear power stations are sited on the coast so
there is no need for cooling towers.
• Land area required is smaller than for coal fired plant.
• In most reactors there are three fluid circuits:1) The reactor coolant circuit
2) The steam cycle
3) The cooling water cycle.
• ONLY the REACTOR COOLANT will become radioactive
• The cooling water is passed through the station at a rate of tens of
millions of litres of water and hour, and the outlet temperature is
raised by around 10oC.
Norwich Business
School
27
Fission Reactors (4)
REACTOR TYPES – summary 1
• MAGNOX - Original British Design named after the magnesium alloy
used as fuel cladding. Four reactors of this type were built in France,
One in each of Italy, Spain and Japan. 26 units were built in UK.
• They are only in use now in UK. On December 31st 2006, Sizewell
A, Dungeness A closed after 40 years of operation leaving Oldbury
with two reactors is now continuing beyond its original extended 40
year life. Wylfa (also with 2 reactors) will close this year or next. All
other units are being decommissioned
• AGR - ADVANCED GAS COOLED REACTOR - solely British
design. 14 units are in use. The original demonstration Windscale
AGR is now being decommissioned. The last two stations Heysham
II and Torness (both with two reactors), were constructed to time and
have operated to expectations.
Norwich Business
School
28
Fission Reactors (5)
REACTOR TYPES - summary
• SGHWR - STEAM GENERATING HEAVY WATER
REACTOR - originally a British Design which is a hybrid
between the CANDU and BWR reactors.
• PWR Originally an American design of PRESSURIZED
WATER REACTOR (also known as a Light Water Reactor
LWR). Now most common reactor.• BWR BOILING WATER REACTOR - a derivative of
the PWR in which the coolant is allowed to boil in the reactor
itself. Second most common reactor in use.
• RMBK LIGHT WATER GRAPHITE MODERATING
REACTOR (LWGR)- a design unique to the USSR which
figured in the CHERNOBYL incident. 16 units still in operation
in Russian and Lithuania with 9 shut down.
Norwich Business
School
29
Fission Reactors (6)
REACTOR TYPES - summary
• CANDU - A reactor named initially after CANadian
DeUterium moderated reactor (hence CANDU), alternatively
known as PHWR (pressurized heavy water reactor). 41
currently in use.
• HTGR HIGH TEMPERATURE GRAPHITE REACTOR
- an experimental reactor. The original HTR in the UK started
decommissioning in 1975. The new Pebble Bed Modulating
Reactor (PBMR) is a development of this and promoted as a 3+
Generation Reactor by South Africa.
• FBR FAST BREEDER REACTOR - 'breeds'
PLUTONIUM from FERTILE 238U
– extends resource base of URANIUM over 50 times. Mostly
experimental at moment with FRANCE, W. GERMANY and
UK, Russia and JAPAN having experimented with them.
Norwich Business
School
30
MAGNOX REACTORS (also known as Gas Cooled Reactors (GCR)
•
•
•
•
FUEL TYPE - unenriched URANIUM
METAL clad in Magnesium alloy
MODERATOR - GRAPHITE
COOLANT - CARBON DIOXIDE
DIRECT RANKINE CYCLE
- no superheat or reheat efficiency ~ 20%
to 28%.
DISADVANTAGES:• CANNOT LOAD FOLLOW – [Xe
poisoning]
• OPERATING TEMPERATURE
LIMITED TO ABOUT 250oC - 360oC
limiting CARNOT EFFICIENCY to ~40 50%, and practical efficiency to ~ 28-30%.
• LOW BURN-UP - (about 400 TJ per
tonne)
• EXTERNAL BOILERS ON EARLY
DESIGNS.
31
ADVANTAGES:• LOW POWER DENSITY - 1 MW/m3.
Thus very slow rise in temperature in fault
conditions.
• UNENRICHED FUEL
• GASEOUS COOLANT
• ON LOAD REFUELLING
• MINIMAL CONTAMINATION FROM
BURST FUEL CANS
• VERTICAL CONTROL RODS - fall by
gravity in case of emergency.
Norwich Business
School
31
ADVANCED GAS COOLED REACTORS (AGR)
•
•
•
•
FUEL TYPE - enriched URANIUM
OXIDE - 2.3% clad in stainless steel
MODERATOR - GRAPHITE
COOLANT
- CARBON DIOXIDE
SUPERHEATED RANKINE CYCLE (with
reheat) - efficiency 39 - 41%
ADVANTAGES:• MODEST POWER DENSITY - 5 MW/m3.
slow rise in temperature in fault conditions.
• GASEOUS COOLANT (40- 45 BAR cf 160
bar for PWR)
• ON LOAD REFUELLING under part load
• MINIMAL CONTAMINATION FROM
BURST FUEL CANS
• RELATIVELY HIGH
THERMODYNAMIC EFFICIENCY 40%
• VERTICAL CONTROL RODS - fall by
gravity in case of emergency.
DISADVANTAGES:• MODERATE LOAD FOLLOWING
CHARACTERISTICS
• SOME FUEL ENRICHMENT NEEDED. 2.3%
OTHER FACTORS:• MODERATE FUEL BURN-UP - ~
1800TJ/tonne (c.f. 400TJ/tonne for
MAGNOX, 2900TJ/tonne for PWR).
• SINGLE PRESSURE VESSEL with presstressed concrete walls 6m thick. Prestressing tendons can be replaced if
necessary.
Norwich Business
32
School
32
CANDU REACTOTS (PHWR)
•
ADVANTAGES:• MODEST POWER DENSITY - 11
MW/m3.
• MODERATOR - HEAVY WATER
• HEAVY WATER COOLANT - low neutron
COOLANT
- HEAVY WATER
absorber hence no need for enrichment.
DISADVANTAGES:• ON LOAD REFUELLING - and very
efficient indeed permits high load factors.
• POOR LOAD FOLLOWING
CHARACTERISTICS
• MINIMAL CONTAMINATION from burst
fuel can - defective units can be removed
• CONTROL RODS ARE HORIZONTAL, and
without shutting down reactor.
cannot operate by gravity in fault conditions.
• MODULAR: - can be made to almost any
• MAXIMUM EFFICIENCY about 28%
size
OTHER FACTORS:• MODERATE FUEL BURN-UP - ~
MODEST FUEL BURN-UP - about
1000TJ/tonne
• FACILITIES PROVIDED TO DUMP
HEAVY WATER MODERATOR from
reactor in fault conditions
• MULTIPLE PRESSURE TUBES instead of
one pressure vessel.
FUEL TYPE - unenriched URANIUM
OXIDE clad in Zircaloy
33
Norwich Business
School
33
PRESSURISED WATER REACTORS – PWR (VVER)
•
•
•
FUEL TYPE - 3 – 4% enriched URANIUM
OXIDE clad in Zircaloy
MODERATOR - WATER
COOLANT
- WATER
DISADVANTAGES:•
•
•
•
•
ADVANTAGES:• GOOD LOAD FOLLOWING CHARACTERISTICS
- claimed for SIZEWELL B. - most PWRs are
NOT operated as such.
• HIGH FUEL BURN-UP- about 2900TJ/tonne –
• VERTICAL CONTROL RODS - drop by gravity
in fault conditions.
ORDINARY WATER as COOLANT pressure to prevent boiling (160 bar). If
break occurs then water will flash to
steam and cooling will be less effective.
OTHER FACTORS:ON LOAD REFUELLING NOT
POSSIBLE - reactor must be shut down. • LOSS OF COOLANT also means LOSS
OF MODERATOR so reaction ceases - but
SIGNIFICANT CONTAMINATION OF
residual decay heat can be large.
COOLANT CAN ARISE FROM BURST
FUEL CANS - as defective units cannot be • HIGH POWER DENSITY - 100 MW/m3,
removed without shutting down reactor.
and compact. Temperature can rise
rapidly in fault conditions. NEEDS active
FUEL ENRICHMENT NEEDED. - 3-4%.
ECCS.
MAXIMUM EFFICIENCY ~ 31 - 32%
• SINGLE STEEL PRESSURE VESSEL 200
latest designs ~ 34%
mm thick.
34
Norwich Business
School
34
BOILING WATER REACTORS – BWR
•
ADVANTAGES:FUEL TYPE - 3% enriched URANIUM
• HIGH FUEL BURN-UP- about
OXIDE clad in Zircaloy
2600TJ/tonne
• MODERATOR - WATER
• STEAM PASSED DIRECTLY TO
• COOLANT
- WATER
TURBINE therefore no heat exchangers
DISADVANTAGES:needed. BUT SEE DISADVANTAGES..
• ORDINARY WATER as COOLANT – but
OTHER FACTORS:designed to boil: pressure ~ 75 bar.
• CONTROL RODS MUST BE DRIVEN • LOSS OF COOLANT also means LOSS
OF MODERATOR so reaction ceases - but
UPWARDS - POWER NEEDED IN
residual decay heat can be large.
FAULT CONDITIONS. Water can be
• HIGH POWER DENSITY - 100 MW/m3,
dumped in such circumstances.
and compact. Temperature can rise
• ON LOAD REFUELLING NOT
rapidly in fault conditions. NEEDS active
POSSIBLE - reactor must be shut down.
ECCS.
• SIGNIFICANT CONTAMINATION OF
COOLANT CAN ARISE FROM BURST • SINGLE STEEL PRESSURE VESSEL 200
mm thick.
FUEL CANS RADIOACTIVE STEAM
WILL PASS DIRECTLY TO TURBINES.
• FUEL ENRICHMENT NEEDED. - 3%.
• MAXIMUM EFFICIENCY ~ 34-35%
35
Norwich Business
School
35
RMBK (LWGR): - involved in Chernobyl Incident
• FUEL TYPE - 2% enriched
URANIUM OXIDE clad in Zircaloy
• MODERATOR - GRAPHITE
• COOLANT
- WATER
DISADVANTAGES:• ORDINARY WATER as COOLANT flashes to steam in fault conditions
hindering cooling.
• POSITIVE VOID COEFFICIENT !!! positive feed back possible in some fault
conditions -other reactors have negative
voids coefficient in all conditions.
• IF COOLANT IS LOST moderator will
keep reaction going.
• FUEL ENRICHMENT NEEDED. - 2%
• PRIMARY COOLANT passed directly
to turbines. This coolant can be slightly
radioactive.
• MAXIMUM EFFICIENCY ~30% ??
RMBK (LWGR):
ADVANTAGES:• ON LOAD REFUELLING
• VERTICAL CONTROL RODS which
can drop by GRAVITY in fault
conditions.
NO THEY CANNOT!!!!
OTHER FACTORS:• MODERATE FUEL BURN-UP - ~
MODEST FUEL BURN-UP - about
1800TJ/tonne
• LOAD FOLLOWING
CHARACTERISTICS UNKNOWN
• POWER DENSITY probably
MODERATE?
• MULTIPLE PRESSURE TUBES
Norwich Business
(involved
in Chernobyl
School
incident)
36
FAST BREEDER REACTORS (FBR OR LMFBR)
•
FUEL TYPE - depleted Uranium or UO2
surround PU in centre of core. All elements
clad in stainless steel.
• MODERATOR - NONE
• COOLANT
- LIQUID METAL
DISADVANTAGES:• DEPLETED URANIUM FUEL ELEMENTS
REPROCESSED to recover PLUTONIUM
and sustain the breeding for future use.
• CURRENT DESIGNS have SECONDARY
SODIUM CIRCUIT
• WATER/SODIUM HEAT EXCHANGER.
If water and sodium mix a significant
CHEMICAL explosion may occur which
might cause damage to reactor itself.
ADVANTAGES:•
•
•
•
LIQUID METAL COOLANT - at
atmospheric pressure. Will cool by natural
convection in event of pump failure.
BREEDS FISSILE FUEL from non-fissile
238U – increases resource base 50+ times.
HIGH EFFICIENCY (~ 40%)
VERTICAL CONTROL RODS drop by
GRAVITY in fault conditions.
OTHER FACTORS:• VERY HIGH POWER DENSITY - 600
MW/m3 but rise in temperature in fault
conditions limited by natural circulation of
sodium.
Norwich Business
School
37
GENERATION 3 REACTORS: EPR1300: PWR
• Schematic of Reactor is very similar to later PWRs (SIZEWELL) with 4 Steam
Generator Loops.
• Main differences? from earlier designs.
– Output power ~1600 MW from a single turbine
(cf 2 turbines for 1188 MW at Sizewell).
– Each of the safety chains is housed in a separate building.
•
Efficiency claimed at 37%
• But no actual experience
and likely to be less
Construction is under way at
Olkiluoto, Finland.
Second reactor under
construction in Flammanville,
France
Possible contender for new UK
generation
Norwich Business
School
38
GENERATION 3: AP1000: PWR
• A development from SIZEWELL
• Power Rating comparable with SIZEWELL
Possible Contender for
new UK reactors
• Will two turbines be used ??
• Passive Cooling – water tank on
top – water falls by gravity
• Two loops (cf 4 for EPR)
• Significant reduction in
components e.g. pumps etc.
Norwich Business
School
39
GENERATION 3: ACR1000: Advanced Candu Reactor
•A development from CANDU with added safety features less Deuterium
needed
•Passive emergency cooling as with AP1000
See Video Clip of on-line
refuelling
Norwich Business
School
40
Generation 3 ESBWR: Economically Simple BWR
• A derivative of Boiling Water Reactor for which it is claimed has several safety
features but which inherently has two disadvantages of basic design
•Vertical control rods which must be driven upwards
•Steam in turbines can become radioactive
Norwich Business
School
41
GENERATION 3+ REACTORS: the PBMR
• Pebble Bed Modulating Reactors are a development from Gas Cooled
Reactors.
• Sand sized pellets of Uranium each coated in layers of graphite/silicon
carbide and aggregated into pebbles 60 mm in diameter.
• Coolant: Helium
• Connected directly to closed circuit gas turbine
• Efficiency ~ 39 – 40%,
possibility of CCGT??
• Graphite/silicon
carbide effective
cladding
• very durable at high
temperatures
Norwich Business
School
42
GENERATION 3+ REACTORS: the PBMR
• Unlike other Reactors, the PBMR uses a closed circuit high temperature
gas turbine operating on the Brayston Cycle for Power. This cycle is
similar to that in a JET engine or the gas turbine section of a CCGT.
• Normal cycles exhaust spent gas to atmosphere.
• In this version the helium is in a closed circuit.
PBM
Reactor
Combustion
Chamber
Fuel In
Compressor
Turbine
Generator
Open
Brayston
Closed
Brayston
Cycle
Cycle
Air In
Heat
Exchanger
Norwich Business
School
Exhaust
43
GENERATION 3+ REACTORS: the PBMR
•
•
•
•
Efficiency of around 38 – 40%, but possibility of CCGT???
Helium passes directly from reactor to turbine
Pebbles are continuously fed into reactor and collected.
Tested for burn up and recycled as appropriate ~ typically 6 times
Norwich Business
School
44
NBSLM03E (2010)
Low Carbon Technologies and Solutions
N.K. Tovey (杜伟贤) M.A, PhD, CEng, MICE, CEnv
Section 8: Nuclear Fuel Cycle
6. Nuclear Power – The Basics
7. Nuclear Power: Fission reactors
8. Nuclear Fuel Cycle
9. Fusion Reactors
45
Norwich Business
School
45
Section 8: Nuclear Fuel Cycle
•
•
•
TWO OPTIONS AVAILABLE:1. ONCE-THROUGH CYCLE,
2. REPROCESSING CYCLE
CHOICE DEPENDS primarily on:1. REACTOR TYPE IN USE (more or less essential for
MAGNOX),
2. AVAILABILTY OF URANIUM TO COUNTRY IN
QUESTION,
3. DECISIONS ON THE POSSIBLE USE OF FBRs.
4. DECISIONS ON HOW RADIOACTIVE WASTE IS TO BE
HANDLED.
• Reprocessing leads to much less HIGH LEVEL
radioactive waste, but more low level radioactive waste
ECONOMIC CONSIDERATIONS done 10 years ago show
little difference between two types of cycle except that for
PWRs, ONCE-THROUGH CYCLE appeared MARGINALLY
more attractive.
Norwich Business
School
46
Section 8: Nuclear Fuel Cycle
NUCLEAR FUEL CYCLE divided into two parts:•
FRONT-END - includes MINING of Uranium Ore,
EXTRACTION, CONVERSION to "Hex", ENRICHMENT, and
FUEL FABRICATION.
•
BACK-END - includes TRANSPORTATION of SPENT FUEL,
STORAGE, REPROCESSING, and DISPOSAL.
NOTE:
1. Transportation of Fabricated Fuel elements has negligible cost
as little or no screening is necessary.
2. Special Provisions are needed for transport of spent fuel for
both cycles.
3. For both ONCE-THROUGH and REPROCESSING
CYCLES, the FRONT-END is identical. The differences are
only evident at the BACK- END.
Norwich Business
School
47
Section 8: Simplified Fuel Cycle for a PWR (1)
Cooling
Ponds
1T
0.9m3
Fuel
Rods
Enrichment
Reactor
~ 70000 homes
1PJ 60 x 2MW wind turbines
9 kg
Plutonium
REPROCESSING
UF6
Once Through
Liquid 5m3
0.96 t
Uranium
Storage
HL Waste
Concentrate
0.5m3
U3O8
0.4m3 IL
waste
Mining
Spoil
Ore
1500 m3
0.7m3
LL
waste
0.8m3
IL waste
Norwich Business
School
0.15m3
solid
0.9m3 HL
waste
48
Section 8: Simplified Fuel Cycle for a PWR (2)
• MINING - ore > 0.05% by weight of U3O8 to be economic.
– Typically at 0.5%, 500 tonnes (250 m3) must be excavated to
produce 1 tonne of U3O8 ("yellow-cake") which occupies about
0.1 m3.
• URANIUM leached out chemically
– resulting powder contains about 80% yellow-cake. The
'tailings' contain the naturally generated daughter products.
• PURIFICATION/CONVERSION
- dissolve 'yellow-cake' in nitric acid and conversion to Uranium
tetrafluoride (UF4)
• UF4 converted into URANIUM HEXAFLOURIDE (UF6) or
"HEX" if enrichment is needed.
Norwich Business
School
49
Section 8: Simplified Fuel Cycle for a PWR: Enrichment (1)
ENRICHMENT.
proportion of URANIUM - 235 is artificially increased.
GAS DIFFUSION - original method still used in FRANCE.
• "HEX" is allowed to diffuse through a membrane separating
the high and low pressure parts of a cell.
•
235U
diffuses faster than 238U through this membrane.
• Outlet gas from lower pressure is slightly enriched in 235U (by
a factor of 1.0043) and is further enriched in subsequent cells.
• HUNDREDS / THOUSANDS of such cells are required in
cascade depending on the required enrichment.
• Pumping demands are very large as are the cooling
requirements between stages.
Norwich Business
School
50
Section 8: Simplified Fuel Cycle for a PWR: Enrichment (2)
ENRICHMENT: GAS DIFFUSION.
• Outlet gas from HIGH PRESSURE side is slightly depleted
URANIUM and is fed back into previous cell of sequence.
• AT BACK END, depleted URANIUM contains only 0.2 0.3% 235U,
– NOT economic to use this for enrichment.
• This depleted URANIUM is currently stockpiled, but could be
an extremely value fuel resource should we decide to go for
the FBR.
Norwich Business
School
51
Section 8: Simplified Fuel Cycle for a PWR: Enrichment (3)
ENRICHMENT.
GAS CENTRIFUGE ENRICHEMENT
• similar to the Gas diffusion in that it requires many stages.
•
"HEX" is spun in a centrifuge, and the slightly enriched
URANIUM is sucked off near the axis and passed to the next
stage.
• ENERGY requirements for this process are only ~10% of the
GAS DIFFUSION method.
• All UK fuel is now enriched by this process at Capenhurst.
Norwich Business
School
52
Simplified Fuel Cycle for a PWR: Fuel Fabrication (1)
• FUEL FABRICATION • MAGNOX reactors: URANIUM metal is machined into bars using normal
techniques.
– CARE MUST BE TAKEN not to allow water into process as this acts as a
moderator and might cause the fuel element to 'go critical'.
– CARE MUST ALSO BE TAKEN over its CHEMICAL TOXICITY
although this is not a much a problem as PLUTONIUM
– URANIUM METAL bars are about 1m in length and about 30 mm in
diameter.
• OXIDE Fuels for Other Reactors
– Because of low thermal conductivity of oxides of uranium, fuels of this form
are made as small pellets which are loaded into stainless steel cladding in the
case of AGRs, and ZIRCALLOY in the case of most other reactors.
• TRANSPORT of FUEL Elements
– Little screening is needed as URANIUM is an alpha emitter and even a thin
layer of paper is sufficient to stop such particles.
– No special precaution are needed as even enriched fuel is unsuitable for
bomb making
Norwich Business
School
53
Simplified Fuel Cycle for a PWR: Fuel Fabrication (2)
PLUTONIUM
Fuel fabrication presents much greater problems.
• Workers require more shielding from radiation.
• Chemically toxic.
• Metallurgy is complex.
• Can reach criticality on its own WITHOUT a MODERATOR.
• Care must be taken in manufacture and ALL subsequent storage that the fuel
elements are not of size and shape which could cause a criticality.
NOTE:Transport of PLUTONIUM fuel elements
• a potential hazard, as a crude atomic bomb could be made without the need for a
large amount of energy cf enriched URANIUM.
• DELIBERATE 'spiking' of PLUTONIUM with some fission products is
considered to make the fuel elements very difficult to handle.
Norwich Business
School
54
Simplified Fuel Cycle for a PWR: Fuel Fabrication (3)
• 1 tonne of enriched fuel for a PWR produces ~1PJ of
energy.
• 1 tonne of unenriched fuel for a CANDU reactor
produces about ~0.2 PJ in a single pass.
• However, because of losses, about 20-25% MORE
ENERGY PER TONNE of MINED URANIUM can be
obtained with CANDU if the spent fuel is reprocessed.
Norwich Business
School
55
Simplified Fuel Cycle for a PWR: BACKEND (1)
BOTH ONCE-THROUGH and REPROCESSING CYCLES
• SPENT FUEL ELEMENTS from the REACTOR
• FISSION PRODUCTS mostly with SHORT HALF LIVES. heat is
evolved
– spent fuel elements are normally stored under water
– at least in the short term.
• 100 days, the radioactivity reduced to about 25% of its original
value, and after 5 years the level will be down to about 1%.
• Early reduction comes from the decay of radioisotopes such as
IODINE - 131 and XENON - 133 -half-lives (8 days and 1.8 hours
respectively).
• CAESIUM - 137 decays to only 90% of its initial level even after 5
years.
– accounts for less than 0.2% of initial radioactive decay, but 15%
of the activity after 5 years.
Norwich Business
School
56
Simplified Fuel Cycle for a PWR: BACKEND (2)
BOTH ONCE-THROUGH and REPROCESSING CYCLES
• SPENT FUEL ELEMENTS stored under 6m of water
– also acts as BIOLOGICAL SHIELD.
– Water may become radioactive from corrosion of fuel cladding causing
leakage - so water is conditioned –
– kept at pH of 11 - 12 (i.e. strongly alkaline in case of MAGNOX). Other
reactor fuel elements do not corrode so readily.
– Any radionucleides escaping into the water are removed by ION
EXCHANGE.
• Subsequent handling depends on whether ONCE-THROUGH or
REPROCESSING CYCLE is chosen.
– Spent fuel can be stored in dry caverns,
– drying the elements after the initial water cooling is a problem.
– Adequate air cooling must be provided, and this may make air - radioactive
if fuel element cladding is defective. WYLFA power station stores
MAGNOX fuel elements in this form.
Norwich Business
School
57
Simplified Fuel Cycle for a PWR: No Reprocessing
ADVANTAGES:• NO REPROCESSING needed - therefore much lower discharges of
low level/intermediate level liquid/gaseous waste.
• FUEL CLADDING NOT STRIPPED - therefore less solid
intermediate waste created. (although sometimes it is)
• NO PLUTONIUM in transport so no danger of diversion.
DISADVANTAGES:• CANNOT RECOVER UNUSED URANIUM - 235, PLUTONIUM
OR URANIUM - 238. Thus fuel cannot be used again.
• VOLUME OF HIGH LEVEL WASTE MUCH GREATER (5 - 10
times) than with reprocessing cycle.
• SUPERVISION OF HIGH LEVEL WASTE needed for much longer
time as encapsulation is more difficult than for reprocessing cycle.
Norwich Business
School
58
Simplified Fuel Cycle for a PWR: Reprocessing Cycle (1)
• ADVANTAGES:• MUCH LESS HIGH LEVEL WASTE - therefore less problems with
storage
• UNUSED URANIUM - 235, PLUTONIUM AND URANIUM - 238 can
be recovered and used again, or used in a FBR thereby increasing resource
base 50 fold.
• VITRIFICATION is easier than with spent fuel elements. Plant at
Sellafield now operational although technical problems are preventing
vitrification at full capcity.
DISADVANTAGES:• Greater volumes of both Low Level and Intermediate Level Waste are
created.
• Historically, routine emissions from reprocessing plants have been greater
than storage of ONCE-THROUGH cycle waste.
Norwich Business
School
59
Simplified Fuel Cycle for a PWR: Reprocessing Cycle (2)
Dealing with liquid effluents
• At SELLAFIELD the ION EXCHANGE plant
• SIXEP (Site Ion EXchange Plant)
– commissioned in early 1986,
– substantially reduced the radioactive emissions in the
effluent discharged to Irish Sea since that time by a factor
of 500+ times
• Further improvements with more advance waste treatment
have now been installed.
• PLUTONIUM is stockpiled or in transport if used in FBRs.
(although this can be 'spiked').
Norwich Business
School
60
Simplified Fuel Cycle for a PWR: Reprocessing Cycle (3)
Fuel stored in cooling ponds
to allow further decay
Fuel decanned
cladding to
intermediate level
waste storage
Dissolve Fuel in
Nitric Acid
The Chemistry
add tributyl phosphate (TBP)
in odourless ketone (OK)
further treatment with TBP/OK
High Level Waste
medium level waste
reduced with ferrous
sulphamate
** PLUTONIUM – converted
for storage or fuel fabrication
for MOX or FBR
**Pipes in this area are of small diameter to prevent CRITICALITIES.
URANIUM – converted into
UO3 and recycled
Norwich Business
School
61
Radioactive Waste Disposal – An Introduction
LOW LEVEL WASTE.
• CONTAINS MATERIALS CONTAMINATED WITH
RADIOISOTOPES
– either very long half lives indeed,
– or VERY SMALL quantities of short lived radioisotopes.
• FEW SHIELDING PRECAUTIONS ARE NECESSARY DURING
TRANSPORTATION.
• PHYSICAL BULK MAY BE LARGE as its volume includes items
which may have been contaminated during routine operations.
– Laboratory Coats, Paper Towels etc.
– Such waste may be generated in HOSPITALS, LABORATORIES,
NUCLEAR POWER STATIONS, and all parts of the FUEL CYCLE.
Norwich Business
School
62
Radioactive Waste Disposal – An Introduction
OPTIONS FOR DISPOSAL OF LOW LEVEL WASTE.
• BURYING LOW LEVEL WASTE SURROUNDED BY A THICK CLAY
BLANKET IS A SENSIBLE OPTION.
• If clay is of the SMECTITE type acts as a very effective ion exchange
barrier which is plastic and deforms to any ground movement sealing any
cracks.
• IN BRITAIN IT IS PROPOSED TO BURY WASTE IN STEEL
CONTAINERS AND PLACED IN CONCRETE STRUCTURES IN A
DEEP TRENCH UP TO 10m DEEP WHICH WILL BE SURROUNDED
BY THE CLAY.
• IN FRANCE, THE CONTAINERS ARE PILED ABOVE GROUND AND
THEN COVERED BY A THICK LAYER OF CLAY TO FORM A
TUMULUS.
• Energy Field Courses in 1999 and 2001 visited the site at ANDRA near
Cherbourg. (Agence National de Déchets Radioactive)
Norwich Business
School
63
Radioactive Waste Disposal – An Introduction
INTERMEDIATE LEVEL WASTE.
– contains HIGHER quantities of SHORT LIVED RADIOACTIVE
WASTE,
– or MODERATE QUANTITIES OF RADIONUCLEIDES OF
MODERATE HALF LIFE
– - e.g. 5 YEARS - 10000 YEARS HALF LIFE.
• IN FRANCE SUCH WASTE IS CAST INTO CONCRETE
MONOLITHIC BLOCKS AND BURIED AT SHALLOW DEPTH.
• IN BRITAIN, it was originally proposed to bury similar blocks at the
SAME SITES to those used for LOW LEVEL WASTE.
• UNSATISFACTORY AS CONFUSION BETWEEN THE TWO TYPES
OF WASTE WILL OCCUR.
• SEPARATE FACILITIES ARE NOW PROPOSED.
Norwich Business
School
64
Radioactive Waste Disposal – An Introduction
HIGH LEVEL WASTE.
• At Sellafield, high level waste is now being encapsulated and stored on site in
specially constructed vaults.
• A building about the size of the UEA swimming pool house in area and about twice
as high houses all the high level radioative waste from the UKs Civil Nuclear
Program with space for decommissioning of all final fuel from MAGNOX.
• MOST RADIONUCLEIDES IN THIS CATEGORY HAVE HALF LIVES OF UP
TO 30 YEARS, and thus ACTIVITY in about 700 years will have decayed to
around natural background radiation level.
• PROPOSALS FOR DISPOSAL INCLUDE
– burial in deep mines in SALT;
– burial 1000m BELOW SEA BED and BACKFILLED with SMECTITE;
– burial under ANTARCTIC ICE SHEET,
– shot INTO SPACE to the sun!
Norwich Business
School
65
Radioactive Waste Disposal – An Introduction – A Dilemma
UK processes waste from Overseas Countries.
• Should we send back exact quantities of each of:
– High Level Waste
– Intermediate Level Waste
– Low Level Waste
• Or should we:
– Send back same amount of radioactivity
• i.e. a larger amount of a small volume of High Level
Waste
• and no Intermediate and Low Level Waste?
Norwich Business
School
66
Fission Reactors: Fuel Elements (MAGNOX)
Magnox fuel rod:
Natural Uranium metal bar
approx 35mm diameter and
1m long in a fuel cladding
made of MAGNOX.
Norwich Business
School
67
Fission Reactors: Fuel Elements (AGR)
AGR fuel
assembly:
UO2 pellets loaded
into fuel pins of
stainless steel each
~ 1 m long in
bundles of 36.
Whole assembly in
a graphite
cylinder
Burnable
poison
Norwich Business
School
68
Fission Reactors: Fuel Elements (PWR)
PWR fuel assembly:
UO2 pellets loaded into fuel
pins of zirconium each ~ 3 m
long in bundles of ~200
Norwich Business
School
69