Evaluation of Maximum Allowable Power Density and Thermal Limitations of Fuel Assembly of the Miniature Neutron Source Reactor as a Tool for Its Core Performance Measurements

Abstract

 The ratio of peak heat flux to the average heat flux around a reactor core is known to be the Power Peaking Factor of the reactor. This parameter was reported to be the key value that dictates the maximum allowable power density of a fuel assembly. Investigation shows that reactor core performance largely depends on the power peaking factor. It was in recognition of the role of this parameter that measurements were performed in this work to verify the Power Peaking Factor and core performance of the Nigeria Research Reactor–1 (NIRR-1) core, which is a Miniature Neutron Source Reactor (MNSR). Our results show that the Power Peaking Factor for a preset neutron flux of 5.0×10¹¹ cm^-2s^-1 ranges from 0.707 ± 0.293 to 1.014 ± 0.014 with an average coolant temperature difference of 12.1˚C. This agrees with the inherent feature of MNSRs that compensates for the high negative temperature coefficient of reactivity in order to keep the reactor stable at its preset power level. This is also in agreement with the safety requirements of the MNSR, which does not permit power excursion and occurrence of boiling. However, our results show that the reactor stability is not applicable during startup and shutdown conditions. It means that utilization of the Reactor must wait some few minutes after startup to achieve stability and must stop some few minutes before the shutdown. The results also indicate the need to obtain a correction factor for samples that will stay in the reactor for a cyclic or longer period of irradiation provided there will be shut down and start-up in between the irradiations. Our results also revealed that there is a strong dependence of the reactor power on coolant temperature and rod position, which is in perfect agreement with the design of the MNSR and findings of many workers in the area. The computer program developed in this work for the determination of power peaking factor using moderator parameters will not only serve as a new source code for microcomputer control of the reactor peak power, maximum power factor and flux distribution but will also make it possible for the microcomputer console to display real time peak power level, maximum power factor thermal limitation of the reactor core, a tool that is lacking in MNSR design.