Demonstration of the 3SBD Concept for the Pebble Bed Fluoride Salt-Cooled HighTemperature Reactor (PB-FHR)

The 3S by Design (3SBD) concept is designed to capture, and potentially optimize, interrelationships between the safety, security, and safeguards of a nuclear facility. Although 3SBD has been discussed at many levels there has not been sufficient research done to prove that 3SBD can be used as a quantitative approach rather than just being a high-level concept. This paper intends to show quantitatively the inter-relationships between safety, security, and safeguards by focusing on a notional low-pressure advanced reactor concept called a Pebble Bed Fluoride salt-cooled High-temperature Reactor (PB-FHR). The PB-FHR is a reactor concept within a class of FHR reactor concepts currently under development for the U. S. Department of Energy (DOE). The FHR reactor class is defined as a Generation IV reactor that features low-pressure liquid fluoride salt cooling, coated-particle fuel, a high-temperature power cycle, and fully passive decay heat rejection. The PB-FHR utilizes a similar ‘pebble’ fuel form and packed bed core as the gas-cooled Pebble Bed Modular Reactor (PBMR) concept. The utilization of a semi-random packed bed core configuration presents several operational and safeguards challenges as compared with static core configurations. In addition, the PB-FHR also utilizes a Direct Reactor Auxiliary Cooling System (DRACS) for decay heat removal under a variety of accident scenarios. This heavy reliance on passive safety systems for ultimate heat rejection creates several synergistic and sometimes competing objectives from a safety and security standpoint. In this paper, design choices for the PB-FHR are quantitatively investigated and evaluated within this 3SBD framework.