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The chance of fissioning on absorption of a thermal neutron is about 92%; the capture-to-fission ratio of 233 U, therefore, is about 1:12 – which is better than the corresponding capture vs. fission ratios of 235 U (about 1:6), or 239 Pu or 241 Pu (both about 1:3).
Fission occurs when a heavy nuclide such as uranium-235 absorbs a neutron and breaks into nuclides of lighter elements such as barium or krypton, usually with the release of additional neutrons. Like all nuclides with a high atomic number, these uranium nuclei require many neutrons to bolster their stability, so they have a large neutron-proton ...
This is not explainable by neutron reflection alone. An obvious explanation is resonance gamma rays increasing the fission and breeding ratio versus causing greater capture of uranium, and others over metastable conditions such as for isotope 235m U, which has a half-life of approximately 26 minutes.
In a nuclear reactor, the neutron population at any instant is a function of the rate of neutron production (due to fission processes) and the rate of neutron losses (due to non-fission absorption mechanisms and leakage from the system). When a reactor's neutron population remains steady from one generation to the next (creating as many new ...
These types of decay involve the nuclear capture of electrons or emission of electrons or positrons, and thus acts to move a nucleus toward the ratio of neutrons to protons that has the least energy for a given total number of nucleons. This consequently produces a more stable (lower energy) nucleus.
A less moderated neutron energy spectrum does worsen the capture/fission ratio for 235 U and especially 239 Pu, meaning that more fissile nuclei fail to fission on neutron absorption and instead capture the neutron to become a heavier nonfissile isotope, wasting one or more neutrons and increasing accumulation of heavy transuranic actinides ...
Fission cross section, measured in barns (a unit equal to 10 −28 m 2), is a function of the energy (so-called excitation function) of the neutron colliding with a 235 U nucleus. Fission probability decreases as neutron energy (and speed) increases.
Concentration/Density: Neutron reactions leading to scattering, capture or fission reactions are more likely to occur in dense materials; conversely, neutrons are more likely to escape (leak) from low density materials. Moderation: Neutrons resulting from fission are typically fast (high energy). These fast neutrons do not cause fission as ...