High sensitivity of the neutron noise analysis method to the presence of cadmium in HFIR fuel storage racks

Verification of the presence of neutron-absorbing materials in fuel storage racks and nuclear fuel process tanks is in many cases essential for criticality safety. Neutron counting techniques have been used previously for this purpose. Previous measurements with high-flux isotope reactor (HFIR) fuel elements using neutron noise analysis methods have demonstrated the high sensitivity of coherence measurements to changes in fissile loading or variations in neutron-absorbing materials. To investigate the usefulness of neutron noise analysis techniques for assessing the presence of neutron-absorbing material in fuel storage racks, a study was performed using a Monte Carlo simulation of the {sup 252}Cf-source-driven noise analysis measurement for an empty submerged HFIR fuel storage rack using the methods of Ficaro and Wehe. This simulation uses an analog version of the KENO V.a code to calculate the time sequence of pulses in the detectors that detect neutrons from the scattering/absorption processes in nonfissile systems (such as the empty fuel storage rack) or from the fission chain multiplication process in fissile systems initiated by neutrons from the spontaneous fission of {sup 252}Cf. The Monte Carlo calculations provide the autopower spectral densities and the real and imaginary parts of the cross-power spectral densities from which the coherences andmore » ratios of spectral densities are obtained and can be compared to the measured values.« less