Nuclear energy optimism is rising again, but its durability is uncertain.

Construction is under way on small modular reactor projects in Oak Ridge and other sites as federal funding and private backers increase; experts say high costs, new fuels like TRISO and a history of delays make long-term success uncertain.

Concrete foundations are rising at Oak Ridge, Tenn., for what could be among the first small modular reactors in the United States. Companies such as Kairos Energy are building new designs that use molten salt coolants and TRISO fuel and aim to be smaller and more modular than traditional plants. The federal government has increased funding and loan guarantees for advanced reactors while private backers, including technology firms, have also joined some projects. The industry’s recent history of high costs and long delays remains central to debates about whether these efforts will scale successfully. Key facts: - Kairos Energy is constructing a test reactor at Oak Ridge, with a target completion for the test unit in 2028 and a demonstration unit capable of producing electricity by about 2030. - Kairos says its commercial design would occupy about 60 acres, the reactor structure would rise roughly 32 feet, and the company has a contract to supply about 500 megawatts of capacity to Google by 2035. - New small reactors use TRi-structural ISOtropic (TRISO) particle fuel and, in some designs, molten salt coolant; proponents say these features change containment needs, while some safety experts express reservations about heat behavior and containment. - The United States has added few new large reactors in recent decades; in contrast, China completed more than three dozen reactors in the last decade, and U.S. projects such as Vogtle faced large cost overruns (about $35 billion) and delays. - The Energy Department has awarded funds for advanced reactor technologies and provided loan guarantees, and some companies such as NuScale, TerraPower and Radiant Energy Group are pursuing small or micro-reactor projects in different locations. - Past projects have been canceled or delayed when expected costs rose, and estimates of costs for first-of-a-kind small reactor projects vary and are subject to change. Summary: Builders and policymakers are moving forward with a new generation of smaller nuclear reactors and increased public and private funding, while developers set near-term milestones for test and demonstration units. The broader outcome depends on whether projects can be completed on time and at sustainable cost, and on how technical questions about new fuels and designs are resolved; undetermined at this time.