THOR puts fast reactor fuel to the test in U.S.-Japan TREAT collaboration

A researcher working on the THOR-C-2 assembly at Idaho National Laboratory. (Photo: INL)

THOR: The newly developed THOR capsule (short for Temperature Heat-Sink Overpower Response) houses fuel experiments designed to mimic the conditions of fast reactors during temperature transients, complete with specialized instrumentation to monitor how fuel pins respond to the conditions in real time.

According to the DOE, the experiments mark the first time in 20 years that irradiated transient experiments on fast reactor fuel have been conducted anywhere in the world. Japan and the U.S. previously performed tests on high-burnup fast reactor fuels in the late 1980s, before EBR-II-a sodium-cooled fast reactor-shut down and TREAT was placed on standby in 1994. TREAT, which first reached criticality in 1959, was restarted in 2017.

Testing plans: Researchers at INL used fresh fuel pins developed for the decommissioned EBR-II sodium fast reactor to commission and test the THOR capsules and are now preparing transient experiments using high-burnup materials from historic irradiation testing in EBR-II. These tests include both mixed oxide fuel samples used in Japanese and French fast reactor designs and metallic alloy fuel samples similar to the fuel planned for TerraPower's Natrium design.

INL is currently loading the first of four irradiated fuel experiments into TREAT, with the first transient test expected to start in February, according to the DOE. The experiments are part of a four-year cost-shared facility sharing initiative between the DOE and the Japan Atomic Energy Agency (JAEA) under the Civil Nuclear Energy Research and Development Working Group.

"Execution of these unique experiments is an important step toward developing global confidence in the enhanced performance and safety of advanced nuclear reactor technologies," said Daniel Wachs, the national technical director for the U.S. Advanced Fuels Campaign. "It's also a remarkable example of how critical international collaborations will enable the next generation of energy technology development."

High-level talks: On January 9, energy secretary Jennifer Granholm and Yasutoshi Nishimura, head of Japan's Ministry of Economy, Trade, and Industry (METI), [SG1][DS2]met in Washington, D.C., to affirm their commitment to nuclear energy cooperation in the context of discussions on global energy security and clean energy transitions.

In addition to talks on natural gas exports from the United States, renewable energy technologies, and cooperation on the utilization of hydrogen and ammonia, the DOE noted that "METI and DOE intend to cultivate opportunities for cooperation on nuclear energy, such as development and construction of next-generation advanced reactors including small modular reactors (SMRs) both within each country and [in third-party] countries. Both countries also intend to work on maximizing use of existing reactors and building robust nuclear component and fuel supply chains, including uranium fuel, among like-minded countries."

TerraPower looks to Japan for data: About one year ago, in January 2022, TerraPower announced that it had signed its own memorandum of understanding with the JAEA, Mitsubishi Heavy Industries, and Mitsubishi FBR Systems to share data and resources on the testing of sodium fast reactor components in an agreement that the parties anticipate will support the deployment of advanced reactors in both countries. TerraPower is planning to build a 345-MWe Natrium sodium fast reactor with a molten salt energy storage system with 50-50 cost-shared funding of about $2 billion from the DOE's Advanced Reactor Demonstration Program.

JAEA's Oarai Research and Development Institute houses the Joyo test reactor and other facilities, including AtheNa, a large-scale sodium test facility for component development and demonstration.