Application of Statistical Correlation Techniques for Measurement of the Keff of Highly Subcritical Systems
The possibility of measurement of the Keff of highly subcritical (0.4 < Keff < 0.8) systems by statistical correlation techniques is demonstrated. Measurements were carried out using three different techniques on the storage tank of the U-233 uranyl nitrate solution reactor PURNIMA II at Trombay. The first of these is an adaptation of the neutron coincidence counting technique familiar in passive assay of plutonium for safeguards applications. An equation relating the system Keff and the coincidence counts obtained using a shift register has been derived along the lines of Bonnels 1985 paper. The second technique is the so called Dead Time a (first moment) method. Here the loss in count rate measured following introduction of an artificial dead time in the pulse path is related to the kinetic parameters of the subcritical assembly. The complimentarity of these two techniques is pointed out. The third technique adopted is the well known Asymptotic Prompt Variance method of Feynman. In all these techniques it is shown that the subcritical reactivity [(1-Keff)/Keff)] is inversely proportional to a directly measurable quantity characteristic of the technique such as the real coincidence probability or the V/m ratio. The results of the three experiments with the storage tank filled up to maximum solution height as well as those of the shift register coincidence experiment spanning a Keff range of 0.4 to 0.8 are found to be in good agreement with Monte Carlo calculations.
KeywordsStorage Tank Dead Time Uranyl Nitrate Spontaneous Fission Coincidence Count Rate
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