CANDU Chain Reaction

["Ron L. Kathren" <rkathren@tricity.wsu.edu>]

Re the query from Jaroslav Franta:

CANDU reactors use natural uranium, which, of course is 0.72% by weight of
the fissile isotope U-235.  It is the U-235, in conjunction with the
deutrerium moderator, that results in the self sustaining chain reaction.
With ordinary light water, criticality would not be achieved.  While there
is some fast fission of U-238 in CANDU reactors, just as there is in
ordinary light water reactors whose fuel is more than 95% by weight U-238,
the amount of fast fission is insufficient to achieve a self-sustaining
chain reaction and actually competes with the fissile U-235 for neutrons.
Note that it is the U-235 that is fissile and not the U-238.  U-238 is
fissionable but not fissile (ie capable of sustaining a chain reaction).

Ron Kathren, CHP
rkathren@tricity.WSU.edu

----- Original Message -----
From: Franta, Jaroslav <frantaj@aecl.ca>
To: Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
Sent: Wednesday, August 16, 2000 12:01 PM
Subject: RE: What is Weapons Grade - CANDU


> You asked,
>
> I'm not an expert, but doesn't CANDU reactors use U-238?  I believe
> if they do use U-238 to generate electricity, then I would assume this to
be
> a chain reaction leading to a sustained criticality.
>


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