control rod matereal questoin 1

[bksp]

i have been learning about nuclear engineering for a while now. i feel i have a fairly good understanding of the consepts ect. there is one thing i dont get, the control rods in both rbmk-1000 reactors and some of our own pwr's and bwr's are made of boron. i don't understand how there coud be positive void coeficient in the rbmk reactor and not the pwr/ bwr if the control rods in both of them are moderators!

 

[electricpete]

I'm not familiar with all the reactors you mention. However Boron is an absorber, not a moderator.

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[hydrae]

Boron is not a neutron moderator it is a neutron poison,

To maintain a chain reaction in natural enrichment you need fuel, large moderation, and absence of poisons. Large moderation can be reduced by enrichment. Presence of poisons can be offset by enrichment.

Natural water is slightly poisonous.
Pure graphite has a very lower poison value.
Pure heavy water has a very low poison value.
Heavy water, natural water, and graphite are moderators.

The RBMK uses graphite as the moderator and a low enrichment and some designs can use natural enrichment. PWR and BWR use regular water as the moderator and a higher enrichment.

Positive void coefficient refers to the stability of the power level.

In PWR and BWR since the water is the moderator, when you remove the water by heating the system up (in a BWR the steam fraction is larger and in a PWR the pressurized water is less dense) the reaction slows down because there is less moderator, this produces a self throttling effect.
In the RBMK, the graphite is the moderator and the water in the tubes has a greater poison effect since the enrichment is lower, when the water is removed the slight poison of the water is not absorbed so the reaction speeds up creating the positive void coefficient. To control this positive void coefficient more control rods are required and a greater complexity in the control logic.
Hydrae