LANL's peer-reviewed Deimos design paper (Trellue et al., Nuclear Technology, published online 18 June 2025; accepted manuscript on DOE's OSTI) gives this history. The HALEU TRISO fuel was used in the 1980s Compact Nuclear Power Source (CNPS) critical experiment at Los Alamos and stayed in storage at Los Alamos until the 2000s, when it moved to NCERC in Nevada. LANL unpacked and reused it for the Deimos experiment in September 2024. The paper describes the kernels as 'CNPS fuel fabricated by General Atomics', uranium oxycarbide (UC0.3O1.7) at an average enrichment of 19.9% U-235. The compacts average 0.6214 cm in radius and 4.9426 cm long (called '2-in.' compacts) and weigh 14.73 g. Graphite cups about 95 cm long each hold nineteen compacts, and each cup held about 130 g of HALEU. Because of the fuel's unusually high particle packing, 'the compacts were crumbling and losing the small TRISO spheres', so they were placed in the graphite cups to protect them. The paper says extensive characterization was under way to verify the documented values. It does not mention Valar, and it does not say who owns the fuel. (Updates plan-014.)

• Design of a High-Assay Low-Enriched Uranium Tri-Structural Isotropic Critical Experiment for Advanced Reactor Validation (accepted manuscript), Sec. II (Los Alamos National Laboratory, in Nuclear Technology (via DOE OSTI), 2025-06-18) primary government “The kernels comprised UC0.3O1.7 and an average enrichment of 19.9% 235U”
• Design of a HALEU TRISO Critical Experiment for Advanced Reactor Validation, accepted manuscript, Sec. II (Los Alamos National Laboratory (via DOE OSTI), 2025-06-18) primary government “the compacts were crumbling and losing the small TRISO spheres”
• Design of a HALEU TRISO Critical Experiment for Advanced Reactor Validation, accepted manuscript, Sec. I (Los Alamos National Laboratory (via DOE OSTI), 2025-06-18) primary government “LANL recently unpacked and reused the CNPS fuel at NCERC”