Decay heat is the residual thermal power generated by radioactive decay of fission products and transuranic elements accumulated in nuclear fuel during reactor operation. Even after a reactor is shut down and the fission chain reaction ceases, the fuel continues to produce significant heat: approximately 6-7% of full thermal power immediately after shutdown, declining to about 1% within one hour and continuing to decrease over days, weeks, and months following well-characterized decay curves. The inability to adequately remove decay heat is the root cause of the most consequential nuclear accidents in history, including the Three Mile Island partial meltdown in 1979 and the Fukushima Daiichi triple meltdown in 2011, where loss of electrical power disabled the active cooling systems needed to remove post-shutdown decay heat.
The management of decay heat is the central design challenge that passive safety systems in SMRs and advanced reactors are engineered to address. NuScale's VOYGR modules are submerged in a large below-grade pool that provides an essentially indefinite passive heat sink through natural convection and conduction, requiring no electrical power or operator action. TerraPower's Natrium uses a Reactor Vessel Air Cooling System (RVACS) that passively removes decay heat through natural air circulation around the reactor vessel. The BWRX-300 employs an isolation condenser system that passively condenses reactor steam and returns it to the vessel through gravity. These systems are designed to maintain fuel integrity during extended station blackout scenarios exceeding 72 hours, addressing the specific failure mode that overwhelmed Fukushima's defenses.
For TRISO-fueled designs like X-energy's Xe-100 and Kairos Power's KP-FHR, the decay heat challenge is mitigated by the fuel's ability to retain fission products at temperatures far above those achievable during any credible loss-of-cooling scenario. The low power density of gas-cooled and salt-cooled reactors, combined with large thermal masses and passive heat rejection pathways, ensures that fuel temperatures remain well below the 1,600 degree Celsius limit of TRISO coatings even in beyond-design-basis events. Decay heat also determines the cooling requirements for spent fuel storage, influencing how long fuel assemblies must remain in spent fuel pools before transfer to dry cask storage, a factor in back-end fuel cycle economics and facility decommissioning planning.