The use of heat balance method in the thermal power calibration of Nigeria Research Reactor-1 (NIRR-1)

The knowledge of the reactor thermal power calibration is important for neutron flux and burn-up determination. Burn-up is linearly dependent on the thermal power of the reactor and its accuracy is crucial for the determination of the mass of burned 235U, fission products and decay heat power generation. Power monitoring of reactors is done by means of nuclear instrumentation, but its calibration is always done by thermal procedures. The aim of this paper is to present the result of the thermal power calibration of Nigeria Research Reactor-1 (NIRR-1), a low power Miniature Neutron Source Reactor (MNSR). In order to cover the whole range of the reactor, the calibration was performed at three different power levels: low power (3.6 kW), half power (15 kW) and full power (30 kW). The method used in the calibration involved the steady state energy balance of the cooling loop of the reactor. For this method, the measurement of the inlet temperature, outlet temperature and temperature difference were carried out and also flow-rate of the coolant passing through the core was determined. The heat losses by (conduction, convection and evaporation) were evaluated and the values added to the power calculated as a function of flow rate and temperature difference gives the total thermal power. In a quest to fully understand the behavior of the reactor and ascertain its maximum operable time, a special cadmium string of 1.16 mk inserted during commissioning into one of the inner irradiation channels, was removed and the reactor operated at full power in this condition. The thermal power dissipated in the core during this test period was also computed. The values of the thermal power obtained at low, half, full power and when the cadmium string was removed were: (3.7 ± 0.3) kW, (15.2 ± 1.2) kW, (30.7 ± 2.5) kW and (31.3 ± 8.2) kW respectively. Comparing the results obtained with NIRR-1 flux parameters and preset power on the console, it was observed that calibration at lower power yielded more accurate result with a deviation of only 6.8%.