A thermo-hydraulic model of the inventory control system for load following in the PBMR

Abstract

This dissertation is aimed at the development of a thermo-hydraulic model of the Inventory Control System (ICS) for the Pebble Bed Modular Reactor (PBMR), which is a helium cooled nuclear power plant. The model is used as a design and simulation tool. The ICS is a storage facility whose function is to perform load following through helium mass transfer to and from the PBMR p e r cycle. This mass transfer generally uses pressure differential as the driving force. With a view to minimize the storage volume of the ICS, and hence improve the economics of the design, several concepts were investigated, many of which attain storage effectiveness by employing a heat transfer mechanism for cooling or heating the helium appropriately during mass transfer. From this, a suitable concept for the ICS was found to be a multi-tank arrangement, with a heat capacitance within each tank. This heat capacitance is in the form of perforated steel, and provides the heat transfer mechanism that aids mass transfer under pressure differential. The thermodynamic model of the multi-tank concept, owes its foundation to the conservation principles of mass, energy and momentum, and includes a cost structure to address the economic aspect of the design. This model is developed in the Engineering Equation Solver (EES) software environment. For a specified power reduction down to 45% of the full power (166 MW), the ICS must store at least 2.4 tonnes (from a total of 4.7 tonnes) of helium. To do this, the optimal design entails an arrangement of six similarly sized tanks with a total storage volume of 609 m3, 2.5% of which is occupied by the heat capacitance. An alternative model developed in Flownex was used to validate the EES model. Good correlation was obtained overall, particularly with regards to the tank pressures during helium mass transfer. As a result, the thermo-hydraulic model was deemed suitable as a design and simulation tool.