Feasibility of ²⁵²Cf source driven neutron noise measurements in water moderated reactors

An experiment was performed at the Pool Critical Assembly to evaluate the possibility of ²⁵²Cf source driven neutron noise spectral density measurements in light water moderated reactors. This experiment showed that using commercially available detectors, such measurements can be performed in a reasonable time, that is, the measurement of the quantity of interest (the ratio of spectral densities directly related to reactivity) at ~12 dollars subscritical can be made to within 10% uncertainty in ~15 min using detectors with efficiencies of 10^-3 counts per reactor fission and ~3-4 h using detectors with efficiencies of 10^-4 counts per reactor fission.

Because of constraints on the location of the source and the detectors, slowly decaying higher modes affected the measured results and these effects increased with increasing subcriticality. Reactivities obtained from higher mode contaminated measurements do not compare favorably with reactivities obtained by other techniques.

It was found that a first order propagation of the standard errors of the measured coherence functions adequately described the standard error of the measured quantity of interest. An empirical equation was developed which describes the relationship of the standard errors of the measured coherence functions to the values of the 'true coherence functions and the measurement time. Therefore, the measurement time in a given system at a certain level of subcriticality using detectors with certain efficiencies can be estimated since the values of the true coherence functions can be estimated.

These first ²⁵²Cf source driven neutron noise spectral density measurements in a water moderated system showed that such measurements are possible and may be applicable to such water moderated systems as: light water reactors, fuel reprocessing, fuel fabrication, and fuel storage facilities. However, care is required in locating the source and the detectors to assure proper interpretability of the measured quantities.