Analysis of Channel Blockage of MNSR Reactor Using the System Thermal-Hydraulic Code RELAP5/MOD3.3

The increased extensive use of research reactors and improved regulatory and operational safety requirements have increased the use of more realistic simulations of the plant phenomena involved during steady-state and transient conditions. The earlier adopted conservative model assumptions in the reactor safety analysis which were based on conservatism are now been replaced with best-estimate methodologies. The best-estimate approach aims at providing a detailed realistic description of postulated accident scenarios based on best-available modelling methodologies and numerical solution strategies sufficiently verified against experimental data from differently scaled separate and integral effect test facilities. The core behaviour of Ghana Research Reactor one (GHARR-1) Miniature Neutron Source Reactor (MNSR) during the loss of flow has been investigated. Steady-state and transient analysis were done with best estimate code RELAP5/MOD3.3. The simulated transient characterizes a Loss-of- Flow-Accident (LOFA) type transient. The study forms part of the ongoing core conversion program that is currently ongoing at the facility to convert the reactor from highly-enriched uranium (HEU) to low-enriched uranium (LEU) fuel. Partial and total blockage of coolant to the reactor core transients were performed to study the behaviour of the reactor. It was observed in the case of partial blockage that although boiling occurred in the blocked channels, which lead to increase in both coolant and cladding temperature, the reactor presented a safer steady again due to in-flow of coolant from adjacent channels to the blocked channels. The calculations showed that cladding and coolant temperatures of blocked channels are below the melting point of the assembly. For total blockage the calculations ended abruptly at about 70 s after the start of transient. Therefore we could not observe the whole transient but the observed phenomena indicate unsafe behaviour of the reactor.