Re: Use of heavy water in nuclear weapons

I can't think of a realistic way that heavy water would be used in a
nuclear weapon.  The fusion pit in most thermonuclear weapons is
(supposedly) lithium deuteride.  With all of the excess neutrons flying
around, the lithium would get split into various combinations of the
isotopes of helium and hydrogen, added to the deuterium there, to be fused
for the extra energy kick by the fission event.  It would be possible to
use deuterated or tritiated water as the pit, but I wouldn't expect that to
work very well.  The water would have a negative impact on the energy of
the reaction in the opening few nanoseconds and I expect the oxygen would
get in the way.  Even if the oxygen got broken up like the lithium, parts
of it would still be too heavy to undergo fusion, even at the temperatures
and pressures at the core of a fission reaction.  This would naturally have
an undesireable effect on yield and would also very likely result in
incomplete fission of some of the material around the core.  Not a problem
that you want your strategic weapon having.  

You are quite right, heavy water is used in commerical nuclear power plants
in Canada as the moderator/coolant.  With a commerical nuclear power plant,
the goal is to split U-235.  In US reactors, we enrich the fuel to ~4-7%
U-235 depending on the reactor.  Regular (though admittedly very clean)
water is sufficient to slow down the neutrons to sustain the chain
reaction.  In Canada, the desire was to not have to enrich the uranium, but
to use it in it's natural state (~0.72% U-235).  Deuterium water is a much
better moderator.  This gives the neutrons the ability to slow down much
faster than "regular" water, thus allowing the chain reaction to continue.
 The irony is that it costs about the same ammout to "enrich" the water as
it does to enrich the uranium.  Tap water is only about 0.015 atom%
deuterium.  

One final thought.  As far as I know, a heavy water moderated reactor
hasn't been used to make plutonium for nuclear weapons, but I suppose one
could be with a lot of modifications.  

Scott Kniffin
NASA GSFC