Dynamic Subcriticality Measurements Using the252Cf-Source-Driven Noise Analysis Method

Abstract

This paper reports on dynamic measurements of the subcritical neutron multiplication factor K{sub eff} using the {sup 252}Cf-source-driven neutron noise analysis method performed for an unreflected 25.1-cm-i.d. cylindrical tank containing aqueous uranyl nitrate as the solution height changed at rates of 1 to 23 cm/min, with corresponding changes in k{sub eff} from 4 {times} 10 {sub {minus} 4} to 0.01/s. These experiments, which were the first test of the method to measure k{sub eff} while it is changing, showed the following: this method has the capability to measure subcriticality for a multiplying system to a k{sub eff} as low as 0.30; experimental k{sub eff} values can be obtained from the ratio of spectral densities with as little as 6 s of data accumulation and a small fraction of a second analysis time while the solution tank is drained from a height of 29.5 to 6.5 cm in {approx} 60 s, with corresponding changes in k{sub eff} from 0.95 to 0.30; the measured k{sub eff} values obtained do not depend on the speed at which the solution height is changed or whether it is filling or draining; the results of the dynamic measurements agreed with the static measurements; where static measurementsmore » were practical (limited to k{sub eff} down to {approx} 0.5 by detection efficiency) with {sup 3} He proportional counters sensitive to leakage neutrons only, the results agreed with those from measurements with scintillation detectors sensitive to gamma rays and neutrons escaping from the system; as in previous experiments the ratios of spectral densities at low frequency were used successfully to obtain k{sub eff} values using a modified point kinetics interpretation of the data; the neutron multiplication factors from independent measurements using the break frequency noise analysis method agree with the value of k{sub eff} from the measured ratios of spectral densities of k{sub eff} values of 0.65.« less