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239 Pu has a higher probability for fission than 235 U and a larger number of neutrons produced per fission event, so it has a smaller critical mass. Pure 239 Pu also has a reasonably low rate of neutron emission due to spontaneous fission (10 fission/s·kg), making it feasible to assemble a mass that is highly supercritical before a detonation ...
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]
Fission product yields by mass for thermal neutron fission of U-235 and Pu-239 (the two typical of current nuclear power reactors) and U-233 (used in the thorium cycle). This page discusses each of the main elements in the mixture of fission products produced by nuclear fission of the common nuclear fuels uranium and plutonium.
239 Pu is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are uranium-235 and uranium-233. 239 Pu is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons.
In contrast, for example, a generic civilian Pressurized water reactor's spent nuclear fuel isotopic composition, following a typical Generation II reactor 45 GWd/tU of burnup, is 1.11% plutonium, of which 0.56% is Pu-239, and 0.28% is Pu-240, which corresponds to a Pu-239 content of 50.5% and a Pu-240 content of 25.2%. [46]
The first production reactor that made 239 Pu was the X-10 Graphite Reactor. It went online in 1943 and was built at a facility in Oak Ridge that later became the Oak Ridge National Laboratory. [42] [note 5] In January 1944, workers laid the foundations for the first chemical separation building, T Plant located in 200-West.
The plutonium-239 (or the fissile uranium-235) fissile cross-section is much smaller in a fast spectrum than in a thermal spectrum, as is the ratio between the 239 Pu/ 235 U fission cross-section and the 238 U absorption cross-section.
239 U rapidly decays into 239 Np which in turn rapidly decays into 239 Pu. The small percentage of 239 Pu has a higher neutron cross section than 235 U . As the 239 Pu accumulates the chain reaction shifts from pure 235 U at initiation of the fuel use to a ratio of about 70% 235 U and 30% 239 Pu at the end of the 18 to 24 month fuel exposure ...