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The density of plutonium-238 at room temperature is about 19.8 g/cc. [3] The material will generate about 0.57 watts per gram of 238 Pu. [4] The bare sphere critical mass of metallic plutonium-238 is not precisely known, but its calculated range is between 9.04 and 10.07 kilograms. [5]
This is an extended version of the energy density table from the main Energy density page: Energy densities table ... Pu-238 α-decay: 2,200,000: Hf-178m2 isomer:
Plutonium dioxide is a stable ceramic material with an extremely low solubility in water and with a high melting point (2,744 °C). The melting point was revised upwards in 2011 by several hundred degrees, based on evidence from rapid laser melting studies which avoid contamination by any container material.
The higher the density, the lower the critical mass. The density of a material at a constant temperature can be changed by varying the pressure or tension or by changing crystal structure (see allotropes of plutonium). An ideal mass will become subcritical if allowed to expand or conversely the same mass will become supercritical if compressed.
Trace amounts of plutonium-238, plutonium-239, plutonium-240, and plutonium-244 can be found in nature. Small traces of plutonium-239, a few parts per trillion , and its decay products are naturally found in some concentrated ores of uranium, [ 54 ] such as the natural nuclear fission reactor in Oklo , Gabon . [ 55 ]
238 Pu has become the most widely used fuel for RTGs, in the form of plutonium(IV) oxide (PuO 2). [37] However, plutonium(IV) oxide containing a natural abundance of oxygen emits neutrons at the rate of roughly 2.3 × 10 3 n/sec/g of plutonium-238. This emission rate is relatively high compared to the neutron emission rate of plutonium-238 metal.
In a fission nuclear reactor, uranium-238 can be used to generate plutonium-239, which itself can be used in a nuclear weapon or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power comes from the fission of 239 Pu, which is not supplied as a fuel to the reactor, but rather, produced from 238 U. [5] A certain amount of production of 239
Plutonium-239 and plutonium-241 are fissile, like uranium-235. Small quantities of uranium-236 , neptunium-237 and plutonium-238 are formed similarly from uranium-235. Normally, with low-enriched uranium fuel being changed every five years or so, most of the plutonium-239 is "burned" in the reactor.