Current mode neutron noise measurements in the zero power reactor CROCUS - 18185

The present article is an overview of developments and results regarding neutron noise measurements in current mode at the CROCUS zero power facility. Neutron noise measurements offer a non-invasive method to determine kinetic reactor parameters such as the prompt decay constant at criticality α = β_eff/Λ, the effective delayed neutron fraction β_eff, and the mean generation time Λ for code validation efforts. At higher detection rates, i.e. above 2*10⁴ events in the used configuration at 0.1 W, the previously employed pulse charge amplification electronics with BF₃ detectors yielded erroneous results due to dead time effects. Future experimental needs call for higher sensitivity in detectors, higher detection rates or higher reactor powers, and thus a generally more versatile measurement system. We, therefore, explored detectors operated with current mode acquisition electronics to accommodate the need. We approached the matter in two ways: First way: by using the two compensated ¹⁰B-coated ionization chambers available in CROCUS as operational monitors. The compensated current signal of these chambers was extracted from core monitoring output channels. Second way: by developing a new current mode amplification station to be used with other available detectors in core. Characteristics and first noise measurements of the new current system are presented. We implemented post-processing of the current signals from first and second ways with the APSD/CPSD method to determine α. At two critical states (0.5 and 1.5 W), using the ¹⁰B ionization chambers and their CPSD estimate, the prompt decay constant was measured after 1.5 hours to be α = (156.9±4.3) s^-1 (1σ). This result is within 1σ of statistical uncertainties of previous experiments and MCNPv5-1.6 predictions using the ENDF/B-7.1 library. The new system connected to a CFUL01 fission chamber using the APSD estimate at 100 mW after 33 min yielded α = (160.8 ± 6.3) s^-1, also within 1σ agreement. The improvements to previous neutron noise measurements include shorter measurement durations that can achieve comparable statistical uncertainties and measurements at higher detection rates. (authors)