Subcriticality determination of nuclear fuel assembly by Mihalczo method.

Abstract

To establish a technique of on-site subcriticality determination suitable for the criticality safety management of nuclear fuel assembly, the applicability of the method proposed by Mihalczo was examined with the Tank-type Critical Assembly (TCA) of the Japan Atomic Energy Research Institute. In the Mihalczo method, cross power spectral densities and auto power spectral densities are evaluated from the output currents of an ionization chamber containing 252Cf neutron source and two neutron detectors. The principle of this method is that the spectral ratio formed by the power spectral densities mentioned can be related to the subcriticality by the help of a stochastic theory.Throughout our data processing, an improved formula taking account of the neutron extinction at a detection process was used. Up to the subcriticality of 15 dollars, the Mihalczo method agreed with the water-level worth method, which has been a standard method of reactivity determination at the TCA facility. The systems treated in the present report hold symmetry concerning the nuclear fuel configuration and the 252Cf chamber position. It was clarified that, contrary to Mihalczo's assertion, the factor converting the spectral ratio to a subcriticality depends on subcriticality itself.

Higher harmonic analyses of the 252 Cf source driven noise analysis method

“…However, this effect is neglected throughout this paper. There have been controversies among researchers over the interpretation of the ratio of spectral densities and how the ratio is related to the reactivity of a subcritical system (Yamane and Nishina, 1986;Difilippo, 1990;Sutton and Doub, 1991;Akcasu and Stolle, 1993). The measured quantities in subcritical measurements include the higher harmonic modes as well as the fundamental mode.…”

“…The responses of the electronic components of the detection system, denoted by h 1 ðxÞ; h 2 ðxÞ, and h 3 ðxÞ in (Mihalczo et al, 1990), are not considered in this paper because they are all eventually eliminated in the ratio of spectral densities (Eq. (1)) (Yamane and Nishina, 1986). The auto-correlation function of detector 1 for a time lag between two detection times, (i.e., s ¼ t 2 À t 1 ) is given by…”

Application of stochastic diffusion theory to the interpretation of the 252Cf source-driven noise analysis method for subcriticality determination

“…The method has been implemented for the past 15 years by Mihalczo and his collaborators at ORNL for various compositions and geometries with seemingly satisfactory results (Mihalczo et al, 1978(Mihalczo et al, , 1986(Mihalczo et al, , 1988(Mihalczo et al, , 1990King and Mihalczo, 1983). In recent years, however, a controversy has developed between the ORNL group and others over the correct analytic expression needed to relate the experimental data, the power spectral densities (PSDs) of the outputs of two neutron detectors and a third external source (252Cf) detector, to the desired result, k (Yamane et al, 1986 ;Difilippo, 1988 ;Akcasu and Stolle, 1989;Sutton and Doub, 1991). The origin of this controversy has been explained through a careful interpretation of the experiment using the Langevin equation description of fluctuations at the transport level (Akcasu and Stolle, 1991).…”