NEUTRON POISONS

Nuclear Physics and Reactor Theory Volume 2 of 2
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NEUTRON POISONS DOE-HDBK-1019/2-93 Reactor Theory (Nuclear Parameters) NEUTRON POISONS In some reactors, neutron-absorbing materials called poisons are intentionally designed into the reactor for specific purposes. Some of these poisons deplete as they absorb neutrons during reactor operation, and others remain relatively constant. EO 3.1 DEFINE the following terms: a. Burnable poison b. Non-burnable poison c. Chemical shim EO 3.2 EXPLAIN the use of burnable neutron poisons in a reactor core. EO 3.3 LIST the advantages and disadvantages of chemical shim over fixed burnable poisons. EO 3.4 STATE two reasons why fixed non-burnable neutron poisons are used in reactor cores. EO 3.5 STATE an example of a material used as a fixed non-burnable neutron poison. Fixed Burnable Poisons During operation of a reactor the amount of fuel contained in the core constantly decreases. If the reactor is to operate for a long period of time, fuel in excess of that needed for exact criticality must be added when the reactor is built. The positive reactivity due to the excess fuel must be balanced with negative reactivity from neutron-absorbing material. Moveable control rods containing neutron-absorbing material are one method used to offset the excess fuel. Control rods will be discussed in detail in a later chapter. Using control rods alone to balance the excess reactivity may be undesirable or impractical for several reasons. One reason for a particular core design may be that there is physically insufficient room for the control rods and their large mechanisms. NP-03 Rev. 0 Page 30

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