Kairos Power LLC, committed to developing a safe and affordable technology in pursuit of its mission to enable the world’s transition to clean energy, is actively engaged in demonstrating a series of major hardware advancements. Notably, this includes the introduction of Hermes, a low-power test reactor, which represents a groundbreaking achievement as the first operational pebble bed fluoride-salt-cooled high-temperature reactor (FHR). At the time of this paper’s preparation, the construction of the Hermes reactor had begun in Oak Ridge, Tennessee. The initial startup process of Hermes, encompassing prefuel loading, fuel loading, and the transition to a low-power core, signifies a historic milestone in the realm of advanced nuclear energy on a national scale.

The procedures employed for initiating fuel loading and conducting low-power testing for the Hermes reactor, which were informed by the experience of historical reactors with a high degree of resemblance, are demonstrated in this paper. These procedures were methodically designed to validate the critical components for criticality prediction capabilities within Kairos Power’s nuclear design framework. By doing so, this process also ensures that the safety analysis of the Hermes reactor is conducted within the appropriate range of parameters derived from nuclear design analysis.

The first key technical finding of this paper emphasizes the essential role of in-vessel detectors for monitoring fuel loading during Hermes startup. These detectors allow for conservative predictions as criticality is approached, leveraging their proximity to the active region of the core. Additionally, it was confirmed that the proven inverse subcritical multiplication factor (1/M) approach for reaching criticality is applicable to Hermes fuel loading. Finally, the work in this paper demonstrates the unique capability of a pebble bed FHR to safely achieve criticality through both pebble fuel loading and the withdrawal of reactivity control banks, while adjusting the fuel-to-moderator composition to reach a targeted critical fuel height.