For the plutonium pathway, the key technology is the reprocessing of spent nuclear fuel. The goal is to extract from this spent fuel the fissile plutonium-239, the ingredient of atomics bombs. This plutonium-239 is formed in the fuel, when neutrons are captured by uranium-238 nuclei.
The weapon-grade plutonium used to manufacture bombs contains more than 93% of the fissile isotope plutonium 239. Concentration of plutonium 240 must not exceed 6%. Plutonium 240 has a significant rate of spontaneous fission and emits neutrons which trigger unwanted chain reactions. A concentration exceeding this threshold would cause pre-detonation of the bomb embarrassing for its reliability and bad for its effectiveness.
The plutonium found in the spent fuel of civilian reactors is made up of around 60% of the 239 isotope. The remaining 40% consists of heavier isotopes, around 25% of which is plutonium 240 - a high percentage that makes this plutonium inadequate for military uses.
Would it be possible to follow the uranium pathway by enriching this civilian plutonium so that it contains more than 93% of plutonium 239? Raising the concentration of plutonium from 60% to 93% seems easier than raising the concentration of uranium from 0.7% to 90%. So why not adapt and pass a gaseous plutonium compound through thousands of centrifuges? This is not possible. The safety problems would be insurmountable. Uranium hexafluoride can be used owing to its low radioactivity. A plutonium gas would be several million times more radioactive !
Even low power reactors can produce small quantities of plutonium: a 10MW reactor, for instance, can produce as much as 2kg of plutonium per year. This is the method currently employed by North Korea in its attempts to develop openly and produce a nuclear bomb.
The fourth generation reactors of the future will be designed to be resilient to proliferation. They should not be prone to the extraction of weapons-grade plutonium.
The threat of nuclear proliferation is currently a matter of great concern to the United States and elsewhere. For example they required through the AIEA that the Russians which provide the nuclear fuel of the Bushehr reactor in Iran, will take it back once spent. Such regulations may well be commonplace in tomorrow's world, where nations will be forbidden access to fuel unless they offer satisfactory guarantees.
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