DEVELOPMENT OF AN MCNP-BASED CALCULATIONAL MODEL FOR SEGMENTED TYPE SELF-POWERED NEUTRON DETECTORS

Recently a new idea has been proposed on constructing Self Powered Neutron Detectors (SPNDs) of more complex geometry, which could provide information not only on the local neutron flux, but also about its gradient. Such detectors could be used for neutron current measurements, which makes it possible to localize the vibrations in the reactor core. The development of these segmented type SPNDs requires the application of a modelling tool that is capable of calculating in complex geometries the coupled neutron, gamma and electron transport processes taking place in SPNDs. This paper outlines the first part of the investigation, the main objective of which is to elaborate an MCNP based computational model and code system satisfying the above requirements. Since no benchmarktype measurement data have been found in the literature, the authors performed a reference measurement that made it possible to separately study the electron transport processes from the neutron and background gamma photon reactions. The measurement was performed on a conventional cylindrical geometry SPND with rhodium emitter. During the evaluation of the measurement a current component of unknown origin was revealed. This component may have significant influence on the interpretation of the detector signal occurring in transient conditions. Modelling of the measurement using the code MCNP, supplemented with a self-developed program for the calculation of the electrostatic field, have proven to be suitable for the detailed tracking of the simultaneous neutronphoton-electron transport processes. However, due to the discrepancy between the calculated and measured detector current, furthermore the uncertainty of some nuclear data and parameters (cross sections, half-lives, beta spectrum etc.) the authors believe that it would be useful to initiate a calculational SPND intercomparison project on a well-established benchmark measurement.