Feasibility of a Radially Cooled Thorium Breeder Pebble Bed type High Temperature Reactor

The world demand on energy, combined with environmental concerns, constitutes the need for a plentiful supply of safely produced energy without carbon emissions. The High Temperature Reactor (HTR) is the predecessor of the Gen-4 VHTR design, and has a large thermal efficiency and unsurpassed safety features. Thorium offers more natural resources, more proliferation resistance, and better waste characteristics than uranium. The safety features of the HTR and the resourcefulness of thorium can be combined in a thorium breeder pebble bed reactor. In this research, radial cooling is proposed as an improvement to this reactor type, specifically to solve or strongly mitigate the problems involved with the high pressure drop, temperature variations and reactivity control. The system was compared to the axially cooled case to investigate the effects of the change in coolant direction, and studies have been performed to research the physicalfeasibility of radial cooling within practical and safety constraints. MATLAB models were built to compare the thermal hydraulic behaviour of axially and radially cooled core designs. In order to verify the thermal hydraulics and include neutronics in the simulation, the feasibility of the radially cooled core was investigated in more detail using coupled THERMIX / DALTON steady state calculations. Transient behaviour was researched with THERMIX in stand alone mode. It was found that radial cooling is a feasible design option for this reactor type. The placement of a central reflector allows sufficient regulation of the mass flow distribution, leading to small helium outflow temperature variations. The control rods can be placed in the central reflector, especially for the system with outward cooling, where the rods are constantly cooled by fresh helium. The pressure drop was found to decrease from 1.2 bar to 38 millibar. Simulations on both pressurized as well as depressurized loss of forced cooling incidents with SCRAM have been performed, and the maximum temperature remains well under 1600 degrees Celsius in both these cases. Reversing the coolant flow direction to inward cooling enhances the radial spread in power density, which lowers the maximum transient temperatures. However, it has a negative effect on reactivity and makes the placement of the reactivity control system in the central reflector more troublesome due to the higher centre core temperature. This means that the system with outward radial cooling is considered more favourable.NERAReactor Institute DelftApplied Science