Description of the Mk1 PB-FHR

The Mk1 PB-FHR is a pre-conceptual design of a fluoride salt cooled high temperature reactor. Its objective is to study numerous potential benefits of the FHR technology, with regards to efficiency, safety and construction. Within the scope of the virtual test bed, we aim to assess its safety performance in design-basis and beyond design-basis accidents.

The Mk1 PB-FHR uses an air Brayton cycle to produce 100MWe from 236MWth, with the possibility of natural gas co-firing for additional power output. It uses a Direct Reactor Auxiliary Cooling System, or DRACS, to passively cool the core in the case of a loss of active cooling.

Flow diagram for the Mk1 reactor, main salt loop, and power conversion systems, from UCB FHR website

The Mk1 PB-FHR has an annular cylindrical core. The salt flows upwards and pebbles are buoyant and float at the top of the core. Pebbles enter through a fueling chute, while the fluoride salt coolant flows from both the fueling chute and the inner reflector. Pebbles leave the core through a defueling chute at the top of the core. Their burnup is examined before they are re-inserted or withdrawn from the core. There is significant coolant bypass flow in the outer reflector, which is further detailed in this model.

Cross section of the Mk1-FHR core, from UCB FHR website