Re: Design of Nuclear Power Plants

[Bernard L Cohen <blc+@pitt.edu>]

	Bob Flood is right, but the problem goes further: the hydrogen in 
water absorbs neutrons and thus serves as a slight "poison", so losing 
the water in a Chernobyl type reactor not only does not remove tne 
moderator, but it does remove the poison, which speeds up the chain 
reaction. That is the source  of the positive reactivity coefficient---if 
the chain reaction speeds up, the temperature rises, more water boils 
into steam, so poison is removed and the chain reaction speeds up further. 
Thus, a rise in temperature causes a further rise in temperature, which is 
an unstable situation. 
	In normal operation, an additional effect, Doppler broadening, 
keeps the over-all reactivity coefficient negative, but the Chernobyl 
accident occurred at a very low power, abnormal operating conditions.

Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu


On Tue, 30 Apr 1996, Bob Flood wrote:

> >2)  The RBMK reactors are carbon moderated
> >while the remaining operating US commercial
> >power reactors are water moderated.  A basic
> >explanation of the end result is that carbon
> >moderation leads to situations in which the
> >RBMK reactors can/will have an overall positive
> >reactivity coefficient.
> >
> 
> More important, from my point of view, is the fact that a moderator is
> necessary to sustain the chain reaction, and a coolant is necessary to
> remove the heat of the reaction. Use of water as both coolant and moderator
> (in all US power reactors) means that loss of coolant carries with it loss
> of the moderator, which reduces or stops the chain reaction. The RMBK design
> allows for loss of coolant without losing the moderator, which can (and did)
> result in an uncooled chain reaction.
> 
> Bob Flood
> Unless otherwise noted, all opinions are mine alone.
> (415) 926-3793
> bflood@slac.stanford.edu
>