Re: Nuclear Weapons Material

[Phil Karn <karn@unix.ka9q.ampr.org>]

>We aren't discussing fission bombs. Please reread.

Sigh. At the risk of furthering a way-off-topic discussion, I should
elaborate on what I said earlier. My understanding is that the tritium
produced for nuclear weapons is used only to "boost" the *fission*
reactions in the "primary" that is in turn used to trigger the main
fusion reaction in the "secondary".

Although the main fusion reaction in a thermonuclear device *is*
between tritium and deuterium, the much larger quantities of tritium
needed for this stage are produced during the actual detonation by
neutron irradiation of lithium-6. That's why lithium-6 deuteride is
used as the fusion fuel.

Once again, these materials are distinct from the small amounts of
gaseous tritium and deuterium used in the fission boosting stage.

To summarize the steps (page 22, "US Nuclear Weapons" by Hansen):

1. High explosives detonate and compress the fission fuel in the primary.

2. At the right moment, neutrons are injected from an external
generator to start the chain reaction.

3. Small amounts of gaseous tritium and deuterium are injected into
the exploding fission core to boost the fission reaction, resulting in
much more rapid and complete fission.

4. X-rays from the exploding primary, traveling at the speed of light,
are focused onto a physically separated "secondary", the fusion fuel
assembly, rapidly compressing and heating it by radiation pressure.
Physical separation is essential to give the secondary time to react
before the exploding primary physically blows it apart. *This* is the
"breakthrough" that Ulam came up with that made the H-bomb practical;
before then, Teller had wanted to simply pile deuterium closely around
an A-bomb, which clearly wouldn't work.

5. At the center of the rapidly imploding *secondary* is a "sparkplug"
of fissionable material. Neutrons from the primary cause this material
to fission, producing even more neutrons that breed large amounts of
tritium from the lithium-6 in the fusion fuel.

6. The newly produced tritium fuses with the deuterium in the main
fusion reaction.

7. Fast neutrons from the fusion reaction may then fission a jacket
of U-238 (yes, U-238) surrounding the secondary, producing an even
greater yield using material that would otherwise be useless.

8. Additional fusion stages may then react (if present).

As you can see, the fission and fusion reactions in a modern
thermonuclear weapon are very closely interwined.

Just to bring this back somewhat to cryptography, an interesting topic
for speculation is the operation of the "permissive action links"
(PALs) that control these weapons. The complexity of the procedure
suggests that the precise timing of many events is crucial if a
high-yield nuclear explosion is to result. This is particularly true
for the timing of the many HE detonators, the neutron generator and
the fusion boost injector.  Perhaps these parameters are stored in
encrypted form in the weapon and can be decrypted for use only with
the proper externally-provided key? Considering that a brute force key
search would consume one weapon per trial key, perhaps this technique
isn't too bad against dictionary attacks? :-)

Phil