Dynamic subcriticality measurements using the 252Cf-source-driven noise analysis method

This paper reports on dynamic measurements of the subcritical neutron multiplication factor Keff using the 252Cf-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 keff from 4 x 10 -4 to 0.01/s. These experiments, which were the first test of the method to measure keff while it is changing, showed the following: this method has the capability to measure subcriticality for a multiplying system to a keff as low as 0.30; experimental keff 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 ∼ 60 s, with corresponding changes in keff from 0.95 to 0.30; the measured keff 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 measurements were practical (limited to keff down to ∼ 0.5 by detection efficiency) with 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 keff 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 keff from the measured ratios of spectral densities of keff values of 0.65