This research work presents the results of the thermal power calibration of Nigeria Research Reactor-1 (NIRR-1), a low power Miniature Neutron Source Reactor (MNSR), using calorimetric and heat balance methods. The calibration was performed at two different power levels: half power (15kW) and full power (30kW). The calorimetric method involved operating the reactor at a constant power and isolating coiling system to avoid cooling of the pool water. For this method, the reactor pool temperature rise with time was measured, the reactor heat capacity constant was calculated and the heat losses from the reactor pool to the environment were evaluated. The power was then evaluated as a function of temperature-rise rate. The total thermal power is the sum of heat losses from the reactor pool to the environment and the power calculated as a function of temperature-rise rate. The heat balance method consisted of 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 were evaluated and the values added to the power calculated as a function of flow rate and temperature difference gives the total thermal power. The Average water temperature rise during these experiments were: (29.5 to 32.0)oC and (27.0 to 29.6)oC for calibration at half and full power levels respectively .The thermal power obtained by the calorimetric method at half and full power were: (15.8±0.7) kW and (30.2±1.5) kW respectively. For heat balance method, the values of the thermal power obtained at half and full power were: (15.2±1.2) kW and (30.7±2.5) kW respectively. It was observed that the calorimetric method is more accurate with deviations of only 4% and 5% for calibrations at half and full power respectively. It is therefore recommended that the calorimetric method should be adopted for routine thermal power calibration of NIRR-1.

Fuel burn-up was shown to be linearly dependent on the reactor thermal power (Podvratnik, 2011). It is therefore obvious that the reactor thermal power calibration is very important for precise fuel burn-up calculation. The reactor power can then be determined from measuring the absolute thermal neutron flux distribution across the core in horizontal and vertical planes (Musa et al., 2012). It has also been established that flux distributions can be measured with activation of cadmium covered and bare foils irradiated at steady reactor power (Souza, 2002). But Shaw (1969) demonstrated that this method consumes a lot of time and is not accurate. It can therefore be said that the foil activation method is most suited for zero power reactors and seldom applied to bigger reactors. In the case of high power reactors in which a temperature rise across the core is produced and measured then a heat balance method is the most common and accurate method of determining the power output of the core (Mesquita et al., 2007).

Accurate reactor thermal power calibration is important for: safe monitoring and evaluation of reactor dynamic behavior, determination of fuel burn-up and normalization of neutron fluxes and dose rate. (Mesquita et al., 2007; 2009; 2011, Podvratnik, 2011). Power excursion of any reactor is of great concern to reactor physicist for safe operation reasons. As power is related to the neutrons population and to the mass of fissile material present, its measurement is essential to the safe control and operation of the reactor as well as the reliability of the research reactor (DOE, 1993, Podvratnik, 2011). It therefore became imperative to undertake power measurements and calibration from time to time to establish.

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