Theoretical investigations of two-phase flow and heat transfer in parallel multichannel core of a low power reactor

Abstract

Ghana Research Reactor-1 (GHARR-1) is a low power reactor of 30 kW (th) tank-in-pool type with parallel multichannel core, operating presently with a Highly Enriched Uranium (HEU). The data on GHARR-1 was used for this research work. The GHARR-1 under normal operating conditions experiences single-phase flow. Under abnormal operating conditions GHARR-1 experiences two-phase flow regime which is caused by large reactivity insertions associated with the abnormal operating conditions of the low power reactor. This theoretical research purposely investigated two-phase flow properties (parameters) which are normally associated with nucleate boiling that develops at abnormal operating conditions of the low power reactor. A FORTRAN code, conservation laws and correlations were employed to compute the two-phase flow parameters, which include boiling boundary, vapor quality, pressure drop, single-phase and nucleate boiling heat transfer coefficients, onset of nucleate boiling (ONB) heat flux, critical heat flux and safety margins (ONB ratio (ONBR) and DNB ratio (DNBR)). The boiling boundary of 229.2 mm from the inlet at 37 kW reactor power, ONBR of 1.23 and DNBR of 134.44 were obtained for 30 kW GHARR-1. For abnormal reactor powers from 40-260 kW, the values of -0.005-0.106, 2.203-1.153 kPa, 1.161-1.795 kW/m²K, 4.013-14.878 kW/m²K, 11.949-158.730 kW/m² and 2951.917-2953.687 kW/m² were obtained for exit vapor quality, pressure drop, single-phase and nucleate boiling heat transfer coefficients, ONB heat flux and critical heat flux respectively. These results indicated that subcooled nucleate boiling will commence if the 30 kW GHARR-1 is operated above 37 kW reactor power, the coolant in the GHARR-1 core will not evaporate if it is operated below 40 kW, coolant heat removal from the core at low reactor powers would be more effective and low power reactors operating at low reactor powers can hardly experience nuclear accident. At abnormal operating conditions with high reactor powers, the low power GHARR-1 is safe and can have longer life span if it is continued to be operated at low reactor powers.

Keywords: Low power reactor, heat transfer, single-phase flow, two-phase flow, safety margins.