Signal-to-Noise Ratio of CABRI Hodoscope: Monte Carlo Calculation Versus Experiments.

The CABRI experimental pulse reactor is devoted to the study of reactivity-initiated accidents (RIAs), for the purpose of the CABRI International Program (CIP), managed by the French Safety and Radiation Protection Institute (IRSN). CABRI’s hodoscope equipment detects the fast neutrons emitted during a power pulse by a tested rod, positioned inside a dedicated test loop reproducing either sodium reactor or pressurized water reactor (PWR) conditions. One of the most important parameters measured by the hodoscope detectors is the signal-to-noise ratio (SNR), characterizing the fraction of neutrons directly coming from the test rod (“signal”) over neutrons coming from the core (“noise”). In this article, the method used to calculate the SNR using a 2-D model of CABRI, with the MCNP6.2 Monte Carlo code, will be detailed. Comparisons between the calculated and measured SNRs for different configurations are in quite good agreement. Another parameter of interest is the so-called “scattering coefficient,” which corresponds to the fraction of neutrons coming from the test rod and being scattered between their birth and their detection. This parameter is used to enhance the analysis of the fuel displacement that may happen during the power transient. To estimate this coefficient, an innovative method using a combination of different options available in MCNP6.2 has been used. Computed coefficients show a slight discrepancy with the measurements. Finally, the sensitivity of the SNR and scattering coefficient to technological parameters and nuclear data libraries is discussed. [ABSTRACT FROM AUTHOR]