the residual plutonium in such a system is proportional to the product of the neutron flux and the amount of plutonium remaining in the core, so the destruction of the residual plutonium is like exponential decay. A finite time can be reported only if the extent of depletion is specified (see, e.g., Hebel et al. 1978, Choi and Pigford 1993).

A campaign to eliminate, for example, 99 percent of the WPu using LWRs would take many decades and would involve substantial costs and risks. No facilities designed for reprocessing LWR spent fuel are currently operational in the United States. Reopening of closed facilities or construction of new ones in the United States would be costly and time-consuming, and it would be expected to be the subject of considerable political controversy and intense regulatory scrutiny.¹⁴ Alternatively, reprocessing in other countries could be considered. Facilities capable of processing and fabricating multiple recycle plutonium (which is far more radioactive than WPu) would be needed, and technologies for that purpose would have to be demonstrated and licensed. The costs of repeated reprocessing and refabrication, and the costs of fuel development and licensing for multiple recycle fuel, would add a substantial increment (in the range of billions of dollars) to the subsidy required for the once-through plutonium disposition approach.

If the traditional PUREX process for separating plutonium from LWR fuel were used, such an elimination campaign could have the effect of increasing rather than decreasing net proliferation risk, as repeated and extensive handling of fully separated weapon-usable plutonium would be required. (The steady-state material that would eventually exist, however, with its high admixture of higher plutonium isotopes and other actinides, would be even less attractive for use in weapons than ordinary reactor-grade plutonium.) It is possible that some of the reprocessing approaches that have been suggested for other reactor types, which do not fully separate the plutonium, could be adapted to LWR fueling, but such approaches would have to be developed.

The net rate of plutonium destruction would be increased if additional plutonium was not being produced by absorption of neutrons in the uranium in the MOX fuel. Uranium-free "nonfertile" fuels for LWRs have been proposed for this purpose, containing plutonium, an inert material such as zirconium, and burble absorbers such as erbium (see ABB-CE 1993, pp. 111-51 ff.; and INEL 1993c). Use of such nonfertile fuels in LWRs has not been demonstrated on any substantial scale, and a substantial fuel development program would be required, involving significant cost and time. A variety of safety issues would require careful examination, and licensing of the new fuel could be expected to be difficult and time-consuming.