Valar's NSDA sets target performance for decay-heat removal: capacity for 100% of decay heat, natural circulation within 30 minutes, and 'No operator action required for 72 hours' (page 23); it credits a reactor cavity cooling system, a vessel auxiliary cooling path and conduction to the structures. DOE's categorical exclusion (April 2026), describing the project's attributes, states that the passive system keeps fuel below safety limits 'indefinitely' without operator action or outside power. Valar's Castle Country page says that on 26 June 2026 it shut Ward 250 down, switched off every active cooling system, simulated a total loss of electrical power and watched the reactor cool itself. That test came eight days after the first criticality DOE announced (18 June). ANS reported Valar's announcement that it reached 10 kWt on 22 June, and Valar's Castle Country page says full power came 'days later' than criticality; no DOE or independent record of the test, and no power log before it, was found; how much heat a reactor must shed after shutdown depends on how long and how hard it ran, which the NSDA's own accident method ties to operating history (safe-009). The claim of passive cooling is consistent with the design documents; the June 26 demonstration rests on the company's account.

• Ward250 Nuclear Safety Design Agreement, No. 100403, Rev 02 (status: Draft; issued for Public Release) (Valar Atomics, 2025-10) primary company “No operator action required for 72 hours”
• NEPA Determination DOE-ID-26-005: Valar Ward250 Research Reactor (categorical exclusion B5.26) (U.S. Department of Energy, Office of Nuclear Energy (Idaho), 2026-04-22) primary government “maintains fuel temperatures below safety limits indefinitely without operator action or external power”
• Castle Country | Valar Atomics (Valar Atomics) primary company “Then we watched the reactor cool itself safely, exactly as designed.”
• Valar's Ward 250 reaches criticality in Utah (American Nuclear Society, Nuclear Newswire, 2026-06-22) news “on June 22 announced it had reached a 10 kWt output”