Investigation of organic scintillators for neutron-gamma noise measurements in a zero power reactor

Abstract

Noise measurements in light water reactor systems aid in generating validation data for integral point kinetic parameter predictions and monitoring parameters for reactor safety and safeguards. The CROCUS zero-power reactor has been used to produce both data types to date, using thermal neutron detectors to observe neutron noise and inorganic scintillators to observe gamma noise. Also, the cross-correlation of gamma and neutron noise has been investigated at CROCUS with separate gamma and neutron detectors. Organic scintillators can be used to cross-correlate gamma and neutron noise with only one detector type, within a single detector volume, and provide nanosecond timing resolution for time-correlated measurements. Dual-particle measurements require particle-type discrimination and are hence possible with organic scintillators since such detectors have the property of presenting statistically different pulse shapes for gamma rays and fast neutrons. The fine timing precision increases the signal-to-noise ratio relative to moderated thermal neutron detectors for correlated measurements and the dual-particle sensitivity allows for multiple modalities of estimating the prompt neutron decay constant. In this work, we present data obtained with 5.08 cm-length by 5.08 cm-diameter trans-stilbene cylindrical detectors set in the water reflector of CROCUS. Preliminary results estimate the prompt neutron decay constant to be (155 ±5) s−1 at delayed critical.