CANDU coolant reactivity feedback

[Steve Frantz <sfrantz@YAHOO.COM>]

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).



At 02:28 PM 02/12/2002 -0500, Franta, Jaroslav wrote:

>Losing the cooling water in the current Candu actually speeds

up the 

>nuclear fission



Could someone please briefly explain why?



mike



Mike McNaughton

Los Alamos National Lab.

email: mcnaught@LANL.gov or mcnaughton@LANL.gov

phone: 505-667-6130





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