Nuclear weapons reductions have resulted in 34 metric tons of declared excess weapons-grade plutonium in both the United States and Russia. To deal with this material both countries recently signed an agreement to disposition 68 metric tons of weapons-grade plutonium. Stocks of separated plutonium have been growing in the European, Russian, and Japanese nuclear energy complexes due to the failure of the breeder reactor, the high cost of using the material as mixed oxide (MOX) fuel, and political opposition to use of the material. The total stock of separated civilian reactor-grade plutonium is 200 metric tons. Not only does separated plutonium pose a threat to international security, but also over the long run it poses a hazard to the environment. Therefore, it is necessary to select a disposition method that is both proliferation resistant and will perform well over the millennia in a repository.