Project Sapphire (1994) covertly airlifted ~600 kg of Kazakh weapons-grade uranium to Oak Ridge, and its declassified after-action report is an unusually candid Cold War primary source

Project Sapphire was a covert November 1994 US–Kazakhstan operation, run under the Cooperative Threat Reduction (Nunn–Lugar) program, to remove weapons-grade highly enriched uranium from the Ulba Metallurgical Plant before it could be diverted. The declassified DTRA after-action report gives the total material mass as roughly 2,200 kg, containing roughly 600 kg of U-235 (~90% enriched) — so the commonly cited "~600 kg" figure is the contained fissile U-235, not the gross material mass. The material was flown to the Oak Ridge Y-12 complex for storage under IAEA safeguards and later downblended, feeding the same pipeline as the Megatons to Megawatts program. [quant-verified 2026-07-12 against the DTRA after-action report: '600 kg of uranium-235'; see question-verify-project-sapphire-heu-tonnage-dtra-report]

What distinguishes the source is candor rare in official Cold War history. The declassified Defense Threat Reduction Agency after-action report records that OSIA funding "was obtained through DOE the day before the OSIA team departed," and that "OPSEC was a grave concern... classified technical conversations occurred at the hotel, within earshot of foreign nationals" — alongside a hostile plant employee who reportedly walked team members into a room with an active metal fire.

Sapphire bridges the vault's nonproliferation cluster: it is a materials-security counterpart to the Soviet Scientists Immigration Act of 1992, both post-Soviet instruments aimed at keeping weapons capability — expertise in one case, fissile material in the other — from leaking to proliferant states.