Would this fissioning method be viable?

[Searinox]

If the fission process in a nuclear bomb can be sustained until nearly all of the material has fissioned, that means that the process continues even as the mass goes subcritical from the disintegration. As each reaction in the chain shrinks the mass of the supercritical fuel and turns it subcritical, there will be enough neutrons released up to that point to continue the reaction even under subcritical conditions, until nearly all the fuel is depleted.

If my understanding of the process is correct, then this is another way to keep a supercritical reaction going, and is very similar to a neutron reflector, except instead of "recycling" escaping neutrons from the main reaction, you could have a secondary source, and if that source produces enough neutrons, nuclear detonation is still possible even as the mass and density of the main fuel remain subcritical.

Where am I going with all this?

Well, suppose you had a sphere of Pu-239 that has almost critical mass. If the deductions above are correct, then you could set it off by bombarding it with a powerful enough shower of neutrons. What if you could surround it with a secondary, subcritical fuel, and then bring that secondary fuel to critical mass(not supercritical, just critical)?

Neutron initiators are used in fission bombs to speed up the reaction by emitting a quick few dozen neutrons, but in this case you'd be heavily bombarding a near-supercritical fuel with billions of neutrons from the secondary fuel. The shower of neutrons could bring the average of reactions inside the main fuel to >2 reactions from each fission and you'd have a runaway fission process.

There would be two huge advantages from this process. One of them would be that you don't have to build a complicated detonation system that ensures even, simultaneous compression of the plutonium sphere. The second would be that as the main fuel begins to heat up and expand, it doesn't stop the chain reaction because the excess neutrons are coming from outside. The secondary fuel is also by far the best neutron initiator, kickstarting the fission process with billions of neutrons instead of a few dozen and ensuring a rapid pace of fission. The downside to this design would be the need for more fissile material. But once the reaction is well under way, it may also fission the secondary fuel, boosting the explosion. The secondary fuel needs to break up much slower from the neutron shower, so that it remains active as the reaction reaches extremely high levels.

Here is a drawing of what I mean.

I realize that the hollow sphere in the picture is perhaps the worst possible shape to achieve critical mass(most material needed), but perhaps someone here could have a better idea? :)

 

[mfb]

You are talking about fission bombs?

If the fission process in a nuclear bomb can be sustained until nearly all of the material has fissioned, that means that the process continues even as the mass goes subcritical from the disintegration.

While some fission processes can happen after the material is subcritical, the supercritical phase is required to generate enough neutrons.
Producing neutrons without a chain reaction requires energy - and if the whole construction is subcritical, this energy is a significant fraction of the total released energy. You would need energy equivalent to some tons of TNT to get a bomb of some kilotons TNT equivalent. And you cannot use TNT, as this does not release its energy as neutrons ;). This would make the bomb quite heavy and large.

Your draft would still need a high-precision compression, otherwise the neutron emitter would fly away too quickly. Oh, and I think the neutron flux would be a bit low.

The idea to use subcritical fuel and an external neutron source is discussed - but not for bombs, but for burning radioactive waste via transmutation.

 

[QuantumPion]

If the fission process in a nuclear bomb can be sustained until nearly all of the material has fissioned, that means that the process continues even as the mass goes subcritical from the disintegration. As each reaction in the chain shrinks the mass of the supercritical fuel and turns it subcritical, there will be enough neutrons released up to that point to continue the reaction even under subcritical conditions, until nearly all the fuel is depleted.

If my understanding of the process is correct, then this is another way to keep a supercritical reaction going, and is very similar to a neutron reflector, except instead of "recycling" escaping neutrons from the main reaction, you could have a secondary source, and if that source produces enough neutrons, nuclear detonation is still possible even as the mass and density of the main fuel remain subcritical.

Where am I going with all this?

Well, suppose you had a sphere of Pu-239 that has almost critical mass. If the deductions above are correct, then you could set it off by bombarding it with a powerful enough shower of neutrons. What if you could surround it with a secondary, subcritical fuel, and then bring that secondary fuel to critical mass(not supercritical, just critical)?

Neutron initiators are used in fission bombs to speed up the reaction by emitting a quick few dozen neutrons, but in this case you'd be heavily bombarding a near-supercritical fuel with billions of neutrons from the secondary fuel. The shower of neutrons could bring the average of reactions inside the main fuel to >2 reactions from each fission and you'd have a runaway fission process.

There would be two huge advantages from this process. One of them would be that you don't have to build a complicated detonation system that ensures even, simultaneous compression of the plutonium sphere. The second would be that as the main fuel begins to heat up and expand, it doesn't stop the chain reaction because the excess neutrons are coming from outside. The secondary fuel is also by far the best neutron initiator, kickstarting the fission process with billions of neutrons instead of a few dozen and ensuring a rapid pace of fission. The downside to this design would be the need for more fissile material. But once the reaction is well under way, it may also fission the secondary fuel, boosting the explosion. The secondary fuel needs to break up much slower from the neutron shower, so that it remains active as the reaction reaches extremely high levels.

Here is a drawing of what I mean.

I realize that the hollow sphere in the picture is perhaps the worst possible shape to achieve critical mass(most material needed), but perhaps someone here could have a better idea? :)

Your understanding is not quite correct. A reactor or bomb cannot fission all of its mass using its own neutrons as once it becomes subcritical, the power exponentially decays to zero. A reactor with a multiplication factor less than 1 can still multiply source neutrons - this is called subcritical multiplication. But the most fissions you can generate is $\frac{S}{1-k}$ and even the strongest neutron sources are nothing compared to the neutron flux of an at-power reactor or detonating bomb (maybe 8-10 orders of magnitude less).

The initiators in a bomb emit more than just a "few dozen" neutrons. The source just due to spontaneous fission of Pu-240 is significant. Even so, billions of source neutrons is nothing when you are talking about critical nuclear reactors. Typical flux range for a neutron source is maybe 10⁶-10⁹ neutrons/s where as the flux of a reactor can be 10¹²-10¹⁴ n/cm²/s over a core volume of ~25 m³. A bomb detonation would have an even higher flux - maybe 10²⁰ n/cm²/s or more.

Your main confusion comes from a misunderstanding of the multiplication factor. Adding source neutrons does not increase the multiplication factor - it simply adds a zero baseline of neutrons.

You can calculate the power generated by source neutrons quite easily. The energy released by fission is about 200 MeV or 3.2*10^-11 J. If your multiplication factor is 0.995 (about $1 short of critical) than your source multiplication will be about 200. If your source strength was exceptionally strong at 10¹⁰ n/s, then your fission rate would be ~2¹² s^-1. This produces a total power of 65 watts, which is not very much even compared to the heat your very strong source would be producing due to radioactive decay.

 

[Searinox]

Your main confusion comes from a misunderstanding of the multiplication factor. Adding source neutrons does not increase the multiplication factor - it simply adds a zero baseline of neutrons.

That's what was on my mind for the longest time when I made this post. Thanks!