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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