[ExI] Interesting book

[John Clark]

On Sun, Nov 18, 2018 at 12:22 PM <spike at rainier66.com> wrote:

*> he is missing some key notions which a formal physics education can
> supply: that bit about making uranium from thorium for example (that’s one
> of those yes, but… comments) and making a critical mass from Americium.   *
>

You can can turn Thorium into Uranium 233 in a reactor and theoretically
you can make a bomb from U-233 but even if you somehow avoid Gama Rays from
U-232 contamination it's much harder to make a bomb with U-233 than with
U-235 or Plutonium 239.  A U-233 bomb was attempted only twice, in 1955 the
USA set off a plutonium-U-233 composite bomb, it was expected to produce 33
kilotons but only managed 22; the only pure U-233 bomb I know of was set
off in 1998 by India, but it was a fizzle, a complete flop, it produced a
minuscule explosion of only 200 tons due to pre-detonation. You could make
a bomb with Americium-242 (half life about a century) and it would be small
enough to fit in your pocket because its critical mass is less than 1% that
of Plutonium. But you could do other things with Americium-242, like make a
super efficient rocket.

The efficiency of a rocket depends on its exhaust velocity, the faster the
better. The space shuttle's oxygen hydrogen engine had a exhaust velocity
of about 4500 meters per second and that's very good for a chemical rocket,
the nuclear heated rocket called NERVA tested in the 1960's had a exhaust
velocity of 8000 meters per second, and ion engines are about 80,000. With
the help of Americium you could do better, much better, say around
200,000,000 meters per second. And then you could get to Mars in 2 weeks
instead of 2 years.

The primary products of a fission reaction are about that fast, but if you
use Uranium 235 or Plutonium 239 the large bulk of the material will absorb
the primary fission products and just heat up the material, that slows
things way down. However the critical mass for Americium-242 is so small
that wouldn't be a problem.

In the January 2001 issue of Nuclear Instruments and Methods Physics
Research A Yigal Ronen and Eugene Shwagerous calculate that a metallic film
of Americium 242 less than a thousandth of a millimeter thick would undergo
fission. This is so thin that rather than heat the bulk material the energy
of the process would go almost entirely into the speed of the primary
fission products, they would go free.

Engineering the rocket would be tricky and I'm not sure I'd want to be on
the same planet as a large scale Americium 242 production facility, but
it's an interesting idea.

John K Clark
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