Nuclear Physics (Sukarna Banik, Department of Physics, Comilla University, Bangladesh) Control rods and chemical shims Control rods are rods, plates, or tubes containing a neutron absorbing material (material with high absorption cross-section for thermal neutron) such as boron, hafnium, cadmium, etc., used to control the power of a nuclear reactor. A control rod is removed from or inserted into the reactor core to increase or decrease the reactor’s reactivity (increase or decrease the neutron flux). This, in turn, affects the reactor’s thermal power, the amount of steam produced, and hence the electricity generated. Control rods are used for maintaining the desired state of fission reactions within a nuclear reactor (i.e., subcritical state, critical state, power changes). They constitute a key component of an emergency shutdown system (SCRAM). Control rods usually constitute cluster control rod assemblies (PWR) inserted into guide thimbles within a nuclear fuel assembly. The cladding protects the absorbing material (e.g.,, pellets of Boron Carbide), usually made of stainless steel. They are grouped into groups (banks), and the movement usually occurs by the groups (banks). The typical total number of clusters is 70. This number is limited, especially by the number of penetrations of the reactor pressure vessel head. In PWRs, they are inserted from above, with the control rod drive mechanisms being mounted on the reactor pressure vessel head. Due to the necessity of a steam dryer above the core of a boiling water reactor, this design requires the insertion of the control rods from underneath the core. Control Rods usage • Reactor startup. • Control of the reactor and power maneuvering. • Axial offset control. • Reactor shutdown. • Emergency shutdown – SCRAM. Moreover, isotope 10B has a high (n, alpha) reaction cross-section along the entire neutron energy spectrum. The cross-sections of most other elements become very small at high energies, as in the case of cadmium. The cross-section of 10B decreases monotonically with energy. For fast neutrons, its cross-section is on the order of barns. Boron, as the neutron absorber, has another positive property. The reaction products (after a neutron absorption), helium and lithium, are stable isotopes. Therefore there are minimal problems with decay heating of control rods or burnable absorbers used in the reactor core. Criticality of a Power Reactor For power reactors, the reactor can behave differently at power conditions due to the presence of reactivity feedbacks. Power reactors are initially started from hot standby mode (a subcritical state at 0% of rated power) to power operation mode (100% of rated power) by withdrawing control rods and boron dilution from the primary source coolant. During the reactor startup and up to about 1% of rated power, the reactor kinetics is exponential as in a zero-power reactor. This is due to the fact all temperature reactivity effects are minimal. On the other hand, temperature reactivity plays a very important role during further power increase from about 1% up to 100% of rated power. As the neutron population increases, the fuel and the moderator increase their temperature, which results in a decrease in reactivity of the reactor (almost all reactors are designed to have the temperature coefficients negative). The negative reactivity coefficient acts against the initial positive reactivity insertion, and this positive reactivity is offset by negative reactivity from temperature feedbacks. Positive reactivity must be continuously inserted (via control rods or chemical shim) to keep the power increasing. After each reactivity insertion, the reactor power stabilizes itself on the power level proportionately to the reactivity inserted. The following inherent characteristics are required in accident tolerant control rods: • The reactivity worth of ATCR should be comparable to or exceed that of conventional CR. • The neutron-absorbing materials used in ATCR should have a sufficiently high melting point and high eutectic temperature with cladding to prevent CR breakage from extensive fuel rod failure in a severe accident, thus avoiding uncontrollable recriticality even if coolant without boron is injected for emergency cooling of the core. Boric Acid – Chemical Shim By chemical shim, we mean that boric acid is dissolved in the coolant/moderator. Boric acid (molecular formula: H3BO3) is a white powder that is soluble in water. In pressurized water reactors, chemical shim (boric acid) is used to compensate for an excess of reactivity of reactor core along the fuel burnup (long-term reactivity control). At the beginning of the specific fuel cycle concentration of boric acid is highest (see picture). At the end of this cycle concentration of boric acid is almost zero, and a reactor must be refueled. In certain cases also fine power changes can be controlled by the chemical shim. If it is desired to increase power, the boric acid concentration must be diluted, removing 10B from the reactor core and decreasing its poisoning effect. Compared with burnable absorbers (long-term reactivity control) or with control rods (rapid reactivity control), the boric acid avoids the unevenness of neutron-flux density in the reactor core because it is dissolved homogeneously in the coolant in the entire reactor core. On the other hand, high concentrations of boric acid may lead to a positive moderator temperature coefficient, which is undesirable. In this case, more burnable absorbers must be used. Moreover this method is slow in controlling reactivity. Normally, it takes several minutes to change the concentration (dilute or borate) of the boric acid in the primary loop. For rapid changes of reactivity control rods must be used.
