US-based Kairos Power has completed the final verification and validation (V&V) reports for its KP-BISON fuel performance code. As the first code of its kind tailored for a molten salt reactor (MSR), it evaluates behaviour and structural integrity of TRISO (Tristructural Isotropic) fuel particles under high temperatures.
Kairos Power is developing Fluoride Salt-Cooled High-Temperature Reactor (KP-FHR) technology, following an iterative approach, moving from non-nuclear engineering test units to demonstration plants and finally a commercial fleet. First concrete for Hermes 1, a 35 MWt non-power reactor designed to demonstrate nuclear heat production, was poured in May 2025. The US Nuclear Regulatory Commission (NRC) issued a construction permit for Hermes 2, a two-unit plant (35 MWt each) in November 2024 and Kairos broke ground for the facility in Oak Ridge, Tennessee in April.
Validating the KP-BISON code ensures the TRISO pebbles will perform safely within the Flibe molten salt coolant environment. Flibe is a specialised molten salt mixture made of lithium fluoride and beryllium fluoride. The fuel pebbles contain thousands of poppyseed-sized TRISO particles. Each particle consists of a uranium oxycarbide fuel kernel coated with multiple protective layers of ceramic and carbon materials to prevent the release of fission products.
The V&V reports will be submitted to NRC as part of the licensing basis the Hermes 2 demonstration plant. Kairos Power’s safety case is based on the combination of robust TRISO fuel Flibe. TRISO fuel particles cannot melt in a KP-FHR and act as individualised containment systems that trap fission products inside. Flibe provides an additional layer of protection by absorbing any fission products that may escape during operation. This combination allows for high-temperature output at near-atmospheric pressure, eliminating the need for costly containment systems.
KP-BISON was developed to measure how TRISO particles perform under different stressors within a KP-FHR reactor, including temperature and burnup (or energy extracted from the fuel), which increase pressure inside the fuel kernel.
Kairos Power’s Modelling and Simulation team began working with Idaho National Laboratory (INL) in 2018 to develop the models and tools within KP-BISON, a modification of the laboratory’s BISON code. BISON is a fuel performance code based on experimental data that can be applied to various fuel forms, including light-water reactor fuel rods, TRISO fuel, metallic rod, plate fuel and others.
Kairos Power received federal funding in 2019 to tailor the BISON code to assess the performance of its fuel kernel design within the unique operating environment of the KP-FHR and its Flibe coolant.
The team selected from existing TRISO models historically used in high-temperature gas reactors to be incorporated into KP-BISON. This enabled the code to analyse and predict fuel performance margins across a range of normal and off-normal KP-FHR operating conditions.
To verify KP-BISON, the team confirmed the code’s accuracy by performing studies across an extensive test matrix of models and properties to ensure the code computed the correct solutions. The team then validated the code by comparing KP-BISON fuel performance results to real-world irradiation experiment data from the Department of Energy’s Advanced Gas Reactor (AGR) programme, which used the same fuel specification as Kairos Power’s fuel type.
“This is the first set of TRISO codes related to the safety of the system that have met this milestone in the U.S., and the first time ever for an application to a salt-cooled reactor,” said Ed Blandford, Kairos Power’s co-founder and chief technology officer. “All of this work, which involved thousands of pages of calculations and analysis, boils down to the final tool we use to model the fuel and defend against underpredictions of failure.”
Because KP-FHRs operate outside of the current AGR qualification envelope, Kairos Power will use KP-BISON to support fuel irradiation testing at NRG PALLAS in the Netherlands to demonstrate the performance envelope for KP-FHR operation in line with regulatory requirements.
