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Uranium-238 (238 U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor.
Uranium-235 makes up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a fission chain reaction. It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature. Uranium-235 has a half-life of 703.8 million years.
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.463 × 10 9 years, [7] roughly the age of the Earth. Uranium-238 is predominantly an alpha emitter, decaying to thorium-234. It ultimately decays through the uranium series, which has 18 members, into lead-206. [17]
Pu-239 is produced artificially in nuclear reactors when a neutron is absorbed by U-238, forming U-239, which then decays in a rapid two-step process into Pu-239. [22] It can then be separated from the uranium in a nuclear reprocessing plant. [23] Weapons-grade plutonium is defined as being predominantly Pu-239, typically about 93% Pu-239. [24]
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235 U) has been increased through the process of isotope separation.Naturally occurring uranium is composed of three major isotopes: uranium-238 (238 U with 99.2732–99.2752% natural abundance), uranium-235 (235 U, 0.7198–0.7210%), and uranium-234 (234 U, 0.0049–0.0059%).
96% of the mass is the remaining uranium: most of the original 238 U and a little 235 U. Usually 235 U would be less than 0.8% of the mass along with 0.4% 236 U. Reprocessed uranium will contain 236 U, which is not found in nature; this is one isotope that can be used as a fingerprint for spent reactor fuel.
uranium-238: 4.468 141.0 thorium-232: 14.05 443 10 18 seconds (exaseconds) isotope half-life 10 9 years 10 18 seconds lutetium-176: 37.64 1.188 rhenium-187: 41.22 1.301
The critical mass for lower-grade uranium depends strongly on the grade: with 45% 235 U, the bare-sphere critical mass is around 185 kilograms (408 lb); with 19.75% 235 U it is over 780 kilograms (1,720 lb); and with 15% 235 U, it is well over 1,350 kilograms (2,980 lb). In all of these cases, the use of a neutron reflector like beryllium can ...