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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).
The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low it may take a long time (in 235 U reactors, as long as many minutes) before a chance neutron encounter starts a chain reaction even if the reactor is supercritical.
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.
These nuclides tend to be unstable to β + decay or electron capture, since such decay converts a proton to a neutron. The decay serves to move the nuclides toward a more stable neutron-proton ratio. On the other side of the valley of stability, this ratio is large, corresponding to an excess of neutrons over protons in the nuclides.
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.
Fission product yields by mass for thermal neutron fission of U-235, Pu-239, a combination of the two typical of current nuclear power reactors, and U-233 used in the thorium fuel cycle If a graph of the mass or mole yield of fission products against the atomic number of the fragments is drawn then it has two peaks, one in the area zirconium ...
For both thermal neutrons and fast neutrons, the capture-to-fission ratio of uranium-233 is smaller than those of the other two major fissile fuels, uranium-235 and plutonium-239. [ 3 ] Fissile material