Current Mode Neutron Noise Measurements in the Zero Power Reactor CROCUS

LRS, EPFL
LRS, Paul Scherrer Institut (PSI)
LRS, EPFL and NES, PSI

email: oskari.pakari@epfl.ch

Published online: 10 January 2018

Abstract

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 cps 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:

1) 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 coremonitoring output channels.

2) 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 1)and 2) 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 newsystem connected to a CFUL01 fission chamber using the APSDestimate at 100 mW after 33 min yielded α = (160.8 ± 6.3) s^-1, also within 1σ agreement.

The improvements to previous neutron noise measurementsinclude shorter measurement durations that can achievecomparable statistical uncertainties and measurements at higherdetection rates.