Quantifying measurement uncertainties of MIT Research Reactor's new digital nuclear safety system

Abstract

Accurate measurement of reactor power is one of the most important requirements in order to ensure the reliable and safe operation of a nuclear reactor. The MIT research reactor, (MITR-II), currently has outdated analog reactor period and power monitoring instruments that are difficult to read and in need of frequent adjustment. Replacing these old monitors with digital ones is an important step in continuing to ensure MITR-II's reliable and safe operation. Four Mirion DWK 250's, wide-range neutron ux monitors that provides reactor period and power level monitoring, have been assembled and connected to the MITR for parallel testing. Before they can fully replace the existing analog monitors, the accuracy of measurement of the DWK's must be quantified to meet the performance requirements described in the Safety Analysis Report (SAR) and obtain Nuclear Regulatory Commission (NRC) approval. As part of the calibration tests, reactor power measured by the four DWK channels was recorded and compared to the reactor thermal power obtained through calorimetry calculations. The measurement uncertainties of the four DWK channels were quantified by taking into account the statistical uncertainties of data recorded over two separate operating periods in 1 minute intervals after thermal and xenon equilibrium were reached. The reactor neutron power measured by each channel was found to t a normal distribution during steady-state operation, and the 3 values, 99.7% condence level, were found to be less than 1.6% of the average power detected for each DWK indicating each device's high level of precision. The average percent error between DWK and thermal power was determined to be 4% for each DWK, thus a thorough review of calibration procedures should be performed to ensure an accurate indication of reactor power. Furthermore, as the setpoints of the DWK's may be adjusted over the core cycle of the reactor, the relationship between shim blade height and DWK adjustment was also determined by parallel testing and modelling. The adjustment of each of the DWK's over a fuel cycle was found to be minimal.