BWR In-Core Instrumentation Sensitivity to Material and Geometrical Distortions

Abstract: Several BWR utilities are adopting practices for determining the state of control rods based on the neutron sensitive LPRM and gamma sensitive TIP measurement systems of the nuclear reactor core. This method is in this study evaluated by quantitatively analyzing the detector sensitivity to several material and geometrical distortions in the detector vicinity, using the Los Alamos National Laboratory stochastic Monte Carlo code MCNP5 and the Westinghouse 3D core simulator POLCA7. These results are used to determine whether or not there are potential pitfalls that need to be considered when applying the in-core detector based diagnostic methods to determine the state of control rods. This study also addresses the utility concern about shutdown margin deterioration due to potential loss of neutron absorbing material from control rods. It is found that indication of potential control rod leakage by means of the LPRM and the gamma TIP probe detector systems is feasible, provided that both detector systems show similar deviations that correspond to the potential loss of neutron absorbing material from the control rod. Furthermore, it is possible to correlate the observed detector signaldeviation to the magnitude of the distortion. The study lays out criteria that need to befulfilled for indication of control rod leakage to be reasonable, but it is to be noted that nodefinitive stand is or can be taken regarding the state of individual control rods at anyutility. The shutdown margin is found not to be significantly deteriorated in any of the utility cases that are studied under an assumption of control rod loss of neutron absorbing material as apotential cause for anomalous detector readings. In order to provoke a significant effect, asection of the top fifth of the control rod must have lost neutron absorbing material next to a control rod position with already low shutdown margin.