Re: Nuclear Weapons Material

[Phil Karn <karn@qualcomm.com>]

At the risk of pushing this even further from cryptography, I should
say that tritium is used in the "boosting" of *fission* weapons. A
mixture of tritium and deuterium is injected into the exploding
fission core to increase the "alpha" (neutron multiplication "gain")
of the system.  The D-T thermonuclear reactions themselves contribute
relatively little energy, but the increase in fission efficiency can
be dramatic.

Thermonuclear boosting was the second major improvement made to US
fission weapons after WWII. The first was the "levitated pit", a gap
between the conventional explosive/tamper assembly and the fissile pit
to allow the former to gain significant momentum before slamming into
the latter.

Both techniques result in considerably more efficient use of fissile
material, but are not absolutely necessary to make a usable weapon (as
shown at Hiroshima and Nagasaki). I believe the simple uranium gun
used at Hiroshima only fissioned a few percent of its U-235. Fat Man
did better, but not that much.

Phil