![]() ![]() Depending on how subcritical the reactor is this could take a while. Subcritical neutron flux does not mean that fission has stopped it only means that fission will eventually drop to the level expected with only spontaneous fission. With the overabundance of fuel it is only highly improbable that a local slurry reactor zone could form in the corium blob. If it were not for the fact that much more fuel is present for the minimal critical configuration it would be so statistically unlikely to even have local areas of critical flux that it would really be impossible. If you change the arrangement the fuel should go subcritical also. If you remove the moderator the fuel is not critical by itself. By arraigning the fuel rods and moderators in an optimal configuration the closest to perfect flux is achieved. This would allow the fuel to “burn” most efficiently. The perfect nuclear reactor would have surface flux where all the fast neutrons escaping the fuel rod were reflected back as slow neutrons. The fuel in the rods has both an internal and a surface neutron flux. Modern reactors use fuel rods and both solid and liquid moderators. I suppose if a slurry reactor melted down the corium would be critical. Some of the first reactor designs were slurry reactors where finely ground moderator and fuel were mixed together. Some nuclei will absorb fast neutrons and give off slow neutrons, and a gamma ray. Fission produces mostly fast neutrons which are not absorbed at high probability by fuel nuclei, but if you have enough fuel nuclei this problem is obviated. ![]() Perhaps a better term would be critical concentration, but I think “critical mass” sounds better don’t you? In the supercritical mass the fuel can act as moderator and neutron generator. “Why don’t they create a nuclear explosion?” A good understanding of why they don’t provides a good framework to understand why corium should not be critical, and why subcritical can be fairly bad.Ĭritical mass is more a description of neutron flux than actual mass of material. One of the questions often being answered is “why don’t meltdowns go supercritical?” ie. ![]()
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