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This is a followup to a prior question I asked about gaseous nuclear fission. What is the lowest nuclear fuel density required to initiate fission? This is critical for a gas core reactor with a solenoid magnetic bottle containment system in terms of how strong the magnetic field should be keep the fuel at a high enough density to trigger fission. Of course, the reactor vessel would be a cylinder with an inner jacket made of beryllium to reflect neutrons back into the core.

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Are you are asking about getting a self-sustaining chain reaction? Otherwise, those isotopes that spontaneous fission will do it at any density. –  dmckee Jul 13 '13 at 19:13
    
I am asking about a self-sustaining chain reaction. One idea might be to use deuterium-deuterium fusion as primer to initiate such a reaction by gamma ray and neutron emission. –  Mr X Jul 14 '13 at 6:37
    
See my answer to your previous question. Note, that density of fissile material for criticality depends on size of the reactor and the degree of enrichment and also on gas phase moderator/absorber as well as wall reflector/moderator material. –  user23660 Jul 19 '13 at 14:05
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There is always something new to learn from the questions here.

These gaseous reactors are being studied

The document is in PDF and is hard to copy figures, but, the gases used are :

• Uranium Tetrafluoride UF4 or Uranium Hexafluoride (UF6) are generally used as nuclear fuels in GCR.

• Berillyum Oxide (BeO) is used as a Moderator / Reflector in a GCR.

Density does not have much relevance for gases, it is the temperature pressure curve that is important at page 9 and it seems to work from quite low pressures up to tens of atmospheres .

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I would propose that perhaps instead of Uranium Fluoride gas(Which is corrosive and chemically unstable), what they could use instead is vaporize solid uranium pellets into plasma with intense microwaves and confine the plasma in a magnetic bottle. –  Mr X Jul 14 '13 at 6:39
    
I see that there has been plasma creation for separating isotopes en.wikipedia.org/wiki/Enriched_uranium#Plasma_separation . NASA seems to be studyng a reactor, i suppose it is classified naca.larc.nasa.gov/… –  anna v Jul 14 '13 at 7:00
    
Density of fissile and moderator materials (at least number density) definitely has relevance on neutronics and criticality calculations. –  user23660 Jul 19 '13 at 13:55
    
@user23660 I am just stating that gases are studied on temperature pressure curves. Density of course can be derived from there with some assumptions. –  anna v Jul 19 '13 at 16:18
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