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For such use, the concentration of fissile isotopes uranium-235 and plutonium-239 in the element used must be sufficiently high. Uranium from natural sources is enriched by isotope separation, and plutonium is produced in a suitable nuclear reactor.
Less than a formula quantity of strategic special nuclear material but more than 1,000 grams of uranium-235 (contained in uranium enriched to 20 percent or more in the U-235 isotope) or more than 500 grams of uranium-233 or plutonium-239, or in a combined quantity of more than 1,000 grams (2.2 pounds) when computed by the equation grams ...
Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA.This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", consisting of enriched uranium (), uranium-233, and plutonium-239.
Plutonium is a chemical element; it has symbol Pu and atomic number 94. ... The mixture is not sufficiently enriched for efficient nuclear weapons, ...
The pits of the first nuclear weapons were solid, with an urchin neutron initiator in their center. The Gadget and Fat Man used pits made of 6.2 kg of solid hot pressed plutonium-gallium alloy (at 400 °C and 200 MPa in steel dies – 750 °F and 29,000 psi) half-spheres of 9.2 cm (3.6 in) diameter, with a 2.5 cm (1 in) internal cavity for the initiator.
In all of these cases, the use of a neutron reflector like beryllium can substantially drop this amount, however: with a 5 centimetres (2.0 in) reflector, the critical mass of 19.75%-enriched uranium drops to 403 kilograms (888 lb), and with a 15 centimetres (5.9 in) reflector it drops to 144 kilograms (317 lb), for example.
The odd numbered fissile plutonium isotopes present in spent nuclear fuel, such as Pu-239, decrease significantly as a percentage of the total composition of all plutonium isotopes (which was 1.11% in the first example above) as higher and higher burnups take place, while the even numbered non-fissile plutonium isotopes (e.g. Pu-238, Pu-240 and ...
Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233. Plutonium-239 has a half-life of 24,110 years. [1]