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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 ...
In a normal thermal reactor, tin-121m has a very low fission product yield; thus, this isotope is not a significant contributor to nuclear waste. Fast fission or fission of some heavier actinides will produce 121m Sn at higher yields. For example, its yield from U-235 is 0.0007% per thermal fission and 0.002% per fast fission. [10]
Uranium-235 (235 U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nature as a primordial nuclide. Uranium-235 has a half-life of 703.8 million years.
Since the nuclei that can readily undergo fission are particularly neutron-rich (e.g. 61% of the nucleons in uranium-235 are neutrons), the initial fission products are often more neutron-rich than stable nuclei of the same mass as the fission product (e.g. stable zirconium-90 is 56% neutrons compared to unstable strontium-90 at 58%).
Fission products of 235 U by yield [6] 4n 4n + 1 4n + 2 4n + 3 4.5–7% 0.04–1.25% <0.001% 228 Ra № 4–6 a: 155 Eu þ: 248 Bk [7] > 9 a: 244 Cm ƒ: 241 Pu ƒ: 250 Cf 227 Ac № 10–29 a: 90 Sr 85 Kr 113m Cd þ: 232 U ƒ: 238 Pu ƒ: 243 Cm ƒ: 29–97 a: 137 Cs 151 Sm þ: 121m Sn 249 Cf ƒ: 242m Am ƒ: 141–351 a No fission products have ...
Initially only about 0.7% of it is fissile U-235, with the rest being almost entirely uranium-238 (U-238). They are separated by their differing masses. Highly enriched uranium is considered weapons-grade when it has been enriched to about 90% U-235. [citation needed] U-233 is produced from thorium-232 by neutron capture. [19]
129 I is one of the seven long-lived fission products that are produced in significant amounts. Its yield is 0.706% per fission of 235 U. [7] Larger proportions of other iodine isotopes such as 131 I are produced, but because these all have short half-lives, iodine in cooled spent nuclear fuel consists of about 5/6 129 I and 1/6 the only stable iodine isotope, 127 I.
A uranium-235 tamper will fission even with slow neutrons. A highly enriched uranium tamper is therefore more efficient than a depleted uranium one, and a smaller tamper can be used to achieve the same yield. The use of enriched uranium tampers therefore became more common once enriched uranium became more plentiful.