Simulation and optimisation of gas storage tanks filled with capacitance

Abstract

The Pebble Bed Modular Reactor (PBMR) is a revolutionary small, compact and safe nuclear power plant. It operates on a direct closed Brayton cycle. One of the unique features of this concept is its load following capability enabled by extracting or injecting helium from or to the system during operation. This characteristic of the PBMR requires that extracted helium must be stored during load following periods. When more power is required from the system, this stored helium can be injected into the system again. The attempt to make the PBMR as small and compact as possible ended up in problems with large storage tanks. A proposal was made to fill the tanks with heat capacitance. This would reduce the necessary gas storage area dramatically. Helium is injected in to the tanks at 120°C. The capacitance would absorb the energy that the gas contains and consequently the gas would experience a decrease in temperature. This implies that the density of the gas will increase and result that more helium can be stored in the same tank before the tank's maximum operating pressure is reached. A Computational Fluid Dynamics (CFD) simulation was done to determine how feasible the proposal was. The simulation showed that the capacitance reduced the total pressure in the tank significantly. This implied that more helium can be stored in the same tank or that a smaller tank can be used to store the same mass of helium. The large heat transfer area that the capacitance provides result that this kind of system has a quick thermal response. Since, the system experiences short injection periods (60 seconds), it is a very useful characteristic. In order to make optimal use of this advantage, the gas must be distributed evenly throughout the tank and no local high temperature regions must occur in the tank. A few injection concepts were investigated in order to optimise for this requirement.