Question

Explain why burnable poisons are used in the reactor at the beginning of life. Include the boron letdown curve and a discussion of under versus over moderated in your discussion.

Answer 1

to explain Burnable poison .There use,significance , boron letdown curve & under vs over moderation.

Burnable Poisons it is also known as burnable absorbers .Burnable poisons are the materials that posses a high neutron absorption cross section. They can be converted into materials of relatively low absorption cross section which is the result of radiative capture..

Because of the burnup of the Burnable poison,the negative reactivity of the Burnable poison decreases over core life .These poisons should decrese their negative reactivity at same rate the fuel excess positive reactivity is depleted.In pressurised water reactors to decrese initial concentration of boric acid burnable poisons are used.It also decrease relative power of fresh fuel assemblies.

Burnable poisons are used generally in the form of boron or gadolinium compounds that are shaped in separate lattice plate .it can be introduced as additive to the fuel .Since they usually disributes more uniformly than control rods , these poisons are less disruptive to core power distribution.

boron letdown curve

boron ten depletion during period of 12-month fuel cycle.At the begging of specific fuel cycle conc. of boric acid is highest & at the end is almost zero .Reacter must be re fueled, as there is no positiv reactivity that can be compensate with negative reactivity of fuel burnup.

UNDER MODERATION VS OVER moderation

when there is a less than optimum amount of moderator between fuel plates or fuel rods its termed as under moderated.As moderator temperature increases void decreases.It inserts negative reactivity .Void ffedback is require for stable system

while when there is more than optimum amount of moderator between fuel plate or fuel rods its known to be over moderated .An increase in moderator temperature void increases ,system insert positive reactivity .It result in unstable system unless other negative feedback overrides positive effect