A Thermal-Hydraulics Analysis of Pipe Breaks in the Pebble Bed Modular Reactor Main Power System

This paper presents observations from analyses of pipe breaks presumed to occur in the Pebble Bed Modular Reactor’s (PBMR’s) Main Power System (MPS), and a brief insight into the implications of evaluated shear ratio values on dust lift-off and dust deposition in the MPS. The PBMR is a multibillion rand project tasked with the design, construction, commissioning and marketing of first-of-a-kind pebble-bed, high-temperature gas-cooled nuclear technology. The main loop of the envisaged nuclear power plant comprises a single Brayton thermodynamic cycle that integrates the pebble-packed reactor to a system of heat exchangers and to a single-shaft turbine-generator-compressor system. The reactor coolant is helium which, under 9 000 kPa, exits the reactor at approximately 900 °C. This drives the downstream turbine in a direct action set-up, supplying the grid with roughly 165 MWe of electricity. The PBMR’s MPS and support systems have progressed to advanced design phases. Thermal-hydraulics analysis has played an integral role in providing input data to design and systems engineering functions. Twenty-eight critical positions around the MPS have been identified for the capture of parameters used to calculate shear ratios, which are used to estimate dust lift-off fractions that could result from the pipe break scenarios, and to predict nuclear doses likely to affect the reactor’s adjacent compartments and the immediate environment.