Speaker
Description
Measurements of reactors inform nuclear data through validation benchmarks and serve to monitor reactor behavior during critical, startup, and shutdown phases. Reactor measurements contribute to the accuracy of reactor modeling, support safe operational practices, and provide insights for design and operating teams, as well as regulatory bodies. The CROCUS zero-power research reactor provides an excellent platform for these experiments. CROCUS is a light water, low enriched uranium (1% Uranium-235) facility that provides a flexible environment for detector measurements in a thermal reactor setting, with a maximum power of 100 W and neutron flux of 2.5 x 10$^9$ n/cm$^2$-s. With options for detector deployment at various reactor positions—such as control rod locations, water-tight tubing, and outside the vessel, along with cable ports to the control room—this reactor supports extensive analysis of fission kinetics and gamma-ray fields. We conducted a measurement campaign named the Simultaneous Analysis of Noise Detectors and Spectroscopy of a Low-power Experiment (SANDcaStLE) to (1) use various detection systems to derive kinetic parameter estimates from fission chain “noise”, and (2) to evaluate the gamma-ray energy spectrum both at power and post-shutdown. We deployed neutron detectors across multiple positions, including four CFUL-01 fission chambers around the core perimeter and the SAFFRON array, consisting of 160 compact ZnS:$^6$Li(Ag) scintillators distributed across three main axial planes (15, 50, and 85 cm). The distribution of the SAFFRON array provides high spatial resolution, while the detectors and fiber optic readout enable precise temporal tracking, making SAFFRON well-suited for detailed mapping of core kinetics and reactivity changes, especially near control rod regions. Gamma-ray detectors were similarly positioned to assess both kinetic parameters and the gamma-ray energy spectrum. A CeBr$_3$ detector in a control rod position measured in-core, while two BGO detectors and one HPGe detector outside the reactor offered additional data ex-core. Initial analysis of both gamma and neutron systems has provided estimates of the prompt neutron decay constant at critical (155-160 s$^{-1}$), aligning with benchmark values. Ongoing analysis will extend to estimating the effective delayed neutron fraction using both fission chambers and, for the first time, the SAFFRON. Our kinetic analysis will include comparisons across neutron and gamma-ray detectors to evaluate convergence times, investigating the efficiency and spatial advantages of distributed systems for kinetic estimates. Additionally, the HPGe detector resolved the post-shutdown gamma-ray energy spectrum, capturing isotope behavior for over 56 gamma-ray emitting fission products. Quantification of isotopes post-shutdown will offer insights for modeling isotope decay and simulating resultant reactivity changes, validating our understanding of the fission product populations in low-power thermal systems.
