Analysis Of The Effect Of Krsko NPP Ex-Core Detector Position On Their Response

During an earthquake on the 29th of December 2020, the Krsko NPP automatically shut down due to the trigger of the negative neutron flux rate signal on the power range nuclear instrumentation (PRNI). From the time course of the detector signal it can be concluded that there is a possibility that the fluctuation in the detector signal was caused by the mechanical movement of the ex-core neutron detectors or pressure vessel components, and not by the actual change in the reactor power. The aim of the analysis was to assess the difference in neutron flux at the ex-core detector position if the detector is moved for 5 cm in the radial or axial direction. In addition, the effect of core barrel movement for 5 mm in the radial direction was analysed. The analysis is supplemented with the thermal and total neutron flux gradient calculation in radial, axial and azimuthal directions. Monte Carlo particle transport code MCNP was used to study changes in the ex-core detector response for the above-mentioned scenarios. Power and intermediate-range detectors were analysed separately since they are constructed differently and exhibit different response characteristics. It was found that the power range ex-core detector movement has a negligible effect on the value of thermal neutron flux at the active part of the detector. However, the 5 cm radial movement of the intermediate-range detector leads to 7 % - 8 % change in thermal neutron flux within the active intermediate-range detector region. The analysis continued with the evaluation of the effect of core barrel movement on the ex-core detector response. It was determined that the 5 mm core barrel radial oscillation can lead to 9 % - 10 % change in thermal neutron flux within the active detector region. Analysis showed that the mechanical movement of ex-core neutron detectors could not explain the fluctuations in the ex-core detector signal. However, the core barrel oscillations could be a probable reason for the observed fluctuations in the ex-core detector signal during an earthquake.