Security concerns
The PRISM reactors are intended to be used with a small fuel recycling facility on-site to service every cluster of PRISMs. Once uranium or plutonium enters the site, it would never come out, but would all be fissioned to create electricity. At no point in the entire fuel cycle is plutonium ever isolated. All materials with even the remote potential of being used in fission weaponry is always contaminated with sufficient highly radioactive material as to be not only useless for weapons but also too radioactive to handle except with remote handling equipment. Proliferation resistance is built in to the PRISM's design. You can read all about it in my book, Prescription for the Planet.
