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Weapons of Mass Destruction (WMD)




Plutonium Production

Plutonium, one of the two fissile elements used to fuel nuclear explosives, is not found in significant quantities in nature. Plutonium can only be made in sufficient quantities in a nuclear reactor. It must be "bred," or produced, one atomic nucleus at a time by bombarding 238 U with neutrons to produce the isotope 239 U, which beta decays (half-life 23 minutes), emitting an electron to become the (almost equally) radioactive 239 Np (neptunium). The neptunium isotope again beta decays (half-life 56 hours) to 239 Pu, the desired fissile material. The only proven and practical source for the large quantities of neutrons needed to make plutonium at a reasonable speed is a nuclear reactor in which a controlled but self-sustaining 235 U fission chain reaction takes place. The graphite-moderated, air- or gas-cooled reactor using natural uranium as its fuel was first built in 1942. Scale-up of these types of reactors from low power to quite high power is straightforward. ccelerator-based transmutation to produce plutonium is theoretically possible, and experiments to develop its potential have been started, but the feasibility of large-scale production by the process has not been demonstrated.

The "size" of a nuclear reactor is generally indicated by its power output. Reactors to generate electricity are rated in terms of the electrical generating capacity, MW(e), meaning megawatts of electricity. A more important rating with regard to production of nuclear explosive material is MW(t), the thermal power produced by the reactor. As a general rule, the thermal output of a power reactor is three times the electrical capacity. That is, a 1,000 MW(e) reactor produces about 3,000 MW(t), reflecting the inefficiencies in converting heat energy to electricity.

A useful rule of thumb for gauging the proliferation potential of any given reactor is that 1 megawatt-day (thermal energy release, not electricity output) of operation produces 0.9-1.0 gram of plutonium in any reactor using 20-percent or lower enriched uranium; consequently, a 100 MW(t) reactor produces about 100 grams of plutonium per day and could produce roughly enough plutonium for one weapon every 2 months. In practice, reactors have a "capacity factor" -- the percentage of time that they are actually operating, that would typically range from 60 percent to up to 85 percent. And light-water power reactors make fewer plutonium nuclei per uranium fission than graphite-moderated production reactors. Separated reactor-grade plutonium from a light-water reactor can be used in a nuclear weapon, with a about eight kilograms needed for a simple nuclear explosive. Less weapon-grade plutonium from a graphite production reactor is needed per nuclear weapon, with each needing perhaps 5 kilograms of weapon-grade plutonium.

In addition to production of plutonium, nuclear reactors can also be used to make tritium, 3 H, the heaviest isotope of hydrogen. Tritium is an essential component of boosted fission weapons and multi-stage thermonuclear weapons. The same reactor design features which promote plutonium production are also consistent with efficient tritium production, which adds to the proliferation risk associated with nuclear reactors.




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