Re: CANDU coolant reactivity feedback

[BERNARD L COHEN <blc+@PITT.EDU>]

On Tue, 12 Feb 2002, Steve Frantz wrote:



> Sure, I'll try. In a reactor, the coolant (water) acts as BOTH a

> moderator (to slow down neutrons which aids fission) and as a

> poison (to absorb neutrons which inhibits fission). Depending on

> the design of the reactor, one of the two characteristics is

> dominant. In all US designed reactors the moderating

> characteristics dominate (ie, they are overmoderated). As the

> reactor heats up and the coolant expands, it does less

> moderation and also less absorption of neutrons. Since

> moderation is dominant, the reduction in moderation exceeds the

> reduction in poison, and the reactor tends to shut down. In some

> other designs and under special circumstances (CANDU and RBMK)

> the poison can be dominant and heating up the coolant tends to

> cause fewer neutrons to be absorbed, and makes the power go up

> (positive feedback).



	--To make things more clear, if a better moderator is available --

graphite in RBMK and heavy water in CANDU -- ordinary water serves as a

poison.



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