Neutron Poissons (cont'd)

Outline of the Lecture

•Fixed Burnable Poisons

•Soluble Poisons

•Non-burnable Poisons

•Fission Product Poisons

•Production and Removal of Xenon-135

•Xenon-135 Response to Reactor Power Changes and Shutdown

•Xenon-135 Oscillations

•Production and Removal of Samarium-149

•Samarium-149Response to Reactor Shutdown

•Other Neoutron Poisons

Fixed Burnable Poisons (part 1)

•During operation of a reactor the amount of fuel contained in the core constantly decreases

•If the reactor is to operate during long periods, fuel in excess of that needed for exact criticality must be added

•The positive reactivity due to the excess fuel must be balanced with negative reactivity from neutron-absorbing material

Fixed Burnable Poisons (part 2)

•Moveable control rods containing neutron-absorbing materials are one method used to offset the excess fuel

•However, using control rods alone may be impractical

–E.g. there is physically insufficient room for the control rods and their large mechanisms

•To control large amounts of excess fuel burnable poisons are used

•Burnable poisonsare materials that have a high neutron absorption cross section that are converted into materials of relatively low absorption cross section as a result of neutron absorption

Fixed Burnable Poisons (part 3)

•Due to the burnupof the poison material, the negative reactivity of the poison decreases over core life

•Ideally, these poisons should decrease their negative reactivity at the same rate the fuel’s excess positive reactivity is depleted

•Fixed burnable poisons are usually used in the form of compounds of boron or gadolinium that are shaped into separate lattice pins or plates, or introduced as additives to the fuel

Soluble Poisons (part 4)

•Soluble poisons,also called chemical shim,produce as patially uniform neutron absorption when dissolvedin the watercoolant

•The most common soluble poisonin PWR sisboric acid (”soluble boron”or ”solbor”)

•The boric acidin thecoolant decreasesthethermal utilization factor,causingthedecreaseinreactivity

Soluble Poisons (part 5)

•By varying the concentration of boric acidin the coolant (a process referred to asboration and dilution) there activity of the core can be easily varied

•If the boron concentration is increased (boration), the coolant/ moderator absorbs more neutrons,adding negative reactivity

•If the boron concentration is reduced (dilution), positive reactivity is added