No, this is not going to work.
You've missed the difficultly of reaching critical mass by using compressed UF6 gas. First, you have interspersed a bunch of fluorine in the middle of your reaction mass. This alone will increase the average atomic difference considerably, which is terrible for your purpose.
And you converted your fuel to a gas. Your average atomic distance just went up massively. You will not be able to overcome this by injecting neutrons - there is no way to control enough neutrons sufficient to support a massive injection rate at the time of the spark, and turn it off at other times in the power cycle. In the reaction chamber won't be able to play the neutron reflector card to make a real difference either.
If you want a nuclear bang to power your engine, you will have to use solid fuel nuclear core bombs and feed them into the chamber for each power stroke. I.e., you now have a nuclear pulse ICE instead of a nuclear pulse rocket.
<PERSON> is correct, and I should have made that point clearer. 479
It is also a very non-linear relationship. IIRC, the implosion in a PU-239 bomb only makes the fuel about 10% denser, but this is key to its function.
Never considered including a moderator as part of your fuel source in a bomb - and not qualified to do so, but since no bomb uses this approach (as far as unclassified info reveals), I have no reason to believe it is significantly beneficial to the process.
I did a little research to confirm that use of a moderator does not benefit nuclear explosion. This is taken from the notes in the Los Alamos Primer.
Slow neutrons cannot play an essential role in an explosion process since they require about a microsecond to be slowed down in hydrogenic materials and the explosion is all over before they are slowed down.
Slow neutrons, a.k.a. thermal spectrum neutrons, are those that have been slowed down by the moderator. This is useful in a nuclear reactor because it increases the cross-sectional area of the fuel - which makes it significantly more likely to trigger a fission reaction before the free neutron escapes the fuel source.
I mentioned in a comment that critical mass is not linear with fuel density. As a first order approximation, critical mass is proportional the 1 over density squared.
The reason Pu-239 requires an implosion to make a bomb is that all plutonium is poisoned by the presence of Pu-240 - an artifact of making plutonium in a reactor. The Pu-240 fissions more readily and a simple bullet plug bomb will make a fizzle explosion before the supercritical plutonium has a chance to involve the bulk of the Pu-239 in the explosion.
I did not mean to imply that you could not make a critical mass of plutonium without compression, just that the sub-critical mass is converted into a critical mass by the implosion even though the compression effect is relatively minor. 435