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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