The spent fuel that was put into storage was reported to contain "about 3,000 kg of weapon-grade plutonium and 10,000 kg of HEU (with enrichment less than 33%)" The reactor used fuel with uranium enriched to 17%, 21%, and 26% of U-235. The material has been placed under IAEA safeguards.
UPDATE: The facility, Baikal-1, is located in Northeastern Kazakhstan, near the former Soviet nuclear test site in Semipalatisk. Here is the facility on Google Maps.
UPDATE: NNSA posted a set of photos from the event.