Regardless of the mix of isotopes in plutonium, there exist the potential for using it to make nuclear weapons. Analyses have been made of the relative risk of proliferation for a wide variety of scenarios of which two are most important: With reprocessing, the separated plutonium must be successfully safeguarded and not diverted, even by government action.

The alternative scenario involves keeping the plutonium in the spent fuel and disposing of it directly as nuclear waste - a once-through fuel cycle. Most of the spent fuel consists of inert uranium oxide. Less than 1% is fissionable U-235 and the plutonium oxide content is also about 1%. All of the intensely radioactive fission products also remain in the spent fuel. For this reason, commercial reprocessing must be done remotely, using massive radiation shielding and sophisticated automated handling equipment. The radioactive waste itself serves as an effective barrier.

Since there is so much spent fuel in existence, analysts planning the disposition of excess weapons plutonium say that plutonium as inaccessibleas in spent fuel meets a "spent fuel standard" and requires no more immediate changes to reduce proliferation risk.

The long-term risk with direct disposal of spent fuel develops as radioactivity of fission products decays. The dangers to persons from retrieving spent fuel and chemically separating the plutonium can be assessed as a function of time. Decades from now the radioactivity would be low enough to permit mining and reprocessing to separate the plutonium. Also, the plutonium itself becomes less rich in the shorter half-life heavier isotopes, making it a somewhat more attractive weapons material.

Sir Walter Marshall, when he headed Britain's nuclear power program, called a spent fuel repository a "plutonium mine." However, in either scenario--burning plutonium as MOX fuel or direct disposal--the proliferation risks are low. Neither scenario has an obvious nonproliferation advantage over the other.

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