Abstract
Feynman-α analyses for a critical state and several subcritical states of the UTR-KINKI reactor have been carried out using two Bi14Ge3O12 (BGO) gamma-ray detectors free from radioactivation of the scintillator. As a statistical index of the analyses, the covariance-to-mean ratio of gamma counts between these detectors instead of the variance-to-mean ratio of each of the detectors is employed to get rid of a large negative correlation originating from the counting loss of a signal processing circuit. In the gate width dependence of the covariance-to-mean ratio measured at each reactor state, not only a familiar neutron-correlation component but also another small positive correlation between prompt gammas can clearly be observed. The prompt-neutron decay constant α determined considering the positive gamma correlation agrees very well with that obtained from a conventional Feynman-α analysis based on neutron detection. Neglecting the gamma correlation term, the decay constant is much overestimated with an increase in subcriticality, and the maximum overestimation reaches about 24% at a shutdown state with a subcriticality of 1.49%Δk/k.
Keywords:
Nomenclature
= | = residues of zero-power reactor transfer function | |
= | = zero-power reactor transfer function | |
= | = counting rate of a neutron counter (1/s) | |
= | = poles of zero-power reactor transfer function (1/s) | |
= | = gate width (s) | |
= | = velocity of gamma (cm/s) | |
= | = correlation amplitude defined by EquationEq. (8)(8) (8) |
Greek
= | = prompt-neutron decay constant (1/s) | |
= | = effective fraction of k’th delayed-neutron group | |
= | = detection efficiency of a neutron counter (counts per fission) | |
= | = detection efficiency of a gamma counter (counts per fission) | |
= | = decay constant of precursor of k’th delayed-neutron group (1/s) | |
= | = first factorial moment of number of prompt gammas released from a fission | |
= | = second factorial moment of number of prompt gammas released from a fission | |
= | = generation time (s) | |
= | = first factorial moment of number of neutrons released from a fission | |
= | = second factorial moment of number of neutrons released from a fission | |
= | = reactivity | |
= | = macroscopic gamma-absorption cross section (1/cm) | |
= | = dead time of a neutron counter |
Disclosure Statement
No potential conflict of interest was reported by the authors.