[PaulHoule]

Early on the fission products were stored in acid solutions in tanks. Circa 1980 the technology was developed to evaporate the solution and trap the fission products inside glass.

Plutonium from either a LWR or FBR fuel cycle is heavily contaminated with isotopes that will cause a bomb to predetonate or get really hot. Somebody with advanced technology (say Japan's government) could probably use electromagnetic separation to remove the unwanted isotopes, but you wouldn't expect ISIS to be able to do it.

The real thing terrorists would want to nick from a reprocessing plant is Neptunium 237; it has a large critical mass compared to plutonium, but it can be separated by chemical means and will not predetonate.

In the 1970s people wrote hang-wringing papers wondering if inventory control could be made good enough to detect diversion, a 2000s accident at Sellafield's THORP plant showed that it probably can't. They lost an Olymptic size swimming pool worth of fluid containing upwards of 50kg of Pu and around 1000kg of U and did not notice for months.

To be fair, it drained into a containment area and did not threaten anyone. They were able to clean it up. But obviously the inventory control was nonexistent.

THORP has been successful at producing plutonium oxide powder but the UK was unable to fabricate it into fuel elements and had to ship it to France.