Spent nuclear fuel (SNF) is fuel that has been irradiated in a reactor core until its fissile content and reactivity have decreased to the point where continued operation is no longer economically or physically viable. Despite being called "spent," the fuel retains approximately 95% of its original uranium content, along with newly created plutonium and other transuranic elements, plus highly radioactive fission products. Immediately after removal from the reactor, spent fuel generates intense radiation and significant decay heat, requiring storage in water-filled spent fuel pools adjacent to the reactor building. After several years of pool cooling (typically 5-10 years), the fuel's radioactivity and heat generation decrease sufficiently for transfer to passive dry cask storage systems.
The management of spent nuclear fuel is a defining policy and engineering challenge for the nuclear industry. The United States has accumulated approximately 90,000 metric tons of spent fuel across more than 70 operating and decommissioned reactor sites, stored in a combination of spent fuel pools and dry cask storage installations. The Nuclear Waste Policy Act of 1982 mandated a federal repository (the proposed Yucca Mountain site in Nevada), but political opposition has prevented its development, leaving spent fuel in interim storage at reactor sites indefinitely. Each SMR deployment adds incrementally to the spent fuel inventory, and the total quantity is directly related to fuel burnup: higher-burnup advanced fuels generate more decay heat per assembly but produce fewer assemblies per unit of energy generated.
For the advanced reactor sector, spent fuel considerations vary significantly by design. Fast-spectrum reactors like TerraPower's Natrium and Oklo's Aurora produce spent fuel with a different isotopic composition than thermal reactors, potentially amenable to reprocessing and recycling in closed fuel cycle configurations. Newcleo's business model explicitly depends on reprocessing spent fuel to produce MOX fuel for its lead-cooled fast reactor. TRISO fuel from HTGRs and FHRs presents unique back-end challenges: the ceramic-coated particles are extremely durable (which is their safety advantage) but also difficult to reprocess, and their dispersed form factor differs from conventional spent fuel assemblies. The SMR industry's long-term sustainability depends on credible spent fuel management strategies, whether through interim dry cask storage, consolidated interim storage facilities, geological repositories, or advanced reprocessing and recycling technologies.