Power spectral analysis for a subcritical reactor system driven by a pulsed spallation neutron source in Kyoto University Critical Assembly

ABSTRACT

For a subcritical reactor system driven by a periodically pulsed spallation neutron source in Kyoto University Critical Assembly, a series of power spectral analysis on frequency domain is carried out to determine the prompt-neutron decay constant and quantitatively to confirm a non-Poisson characteristics of the neutron source. Not only the cross-power but also the auto-power spectral density has a considerable correlated noise component even at a deeply subcritical state, where no correlated component could be observed under a pulsed DT(14MeV) neutron source. The non-Poisson character of the spallation source must enhance the correlation amplitude of these power spectral densities. Moreover, the Degweker’s factor (m₂-m₁²)/m₁² of 0.082 ± 0.021, which is a quantitative indication of the non-Poisson character, is determined from the present analysis and is consistent with that obtained by a previous Rossi-α analysis for the same subcritical system driven by the spallation source. The non-zero value of the factor convinces us again that the present source has a different statistical distribution from the Poisson. The prompt-neutron decay constant obtained from the present analysis well agreed with that done from the above previous Rossi-α analysis.

KEYWORDS:

• KUCA
• ADS
• spallation source
• power spectral analysis
• non-Poisson
• MVP

Acknowledgments

The present work was performed as a joint research program of the KUCA at the Institute for Integrated Radiation and Nuclear Science, Kyoto University.

Disclosure statement

No potential conflict of interest was reported by the authors.

Nomenclature

fR	=	Pulse repetition frequency [Hz]
H(f)	=	Power spectral density of an impulse response of detector and processing-circuit system
$m_1$	=	First factorial moment of source multiplicity distribution (average number of neutrons in a pulsed bunch)
$m_2$	=	Second factorial moment of source multiplicity distribution
N	=	Number of protons in a pulse bunch
q	=	Charge produced in a detector due to absorption of a neutron
Greek	=
α	=	Prompt-neutron decay constant of fundamental mode [1/s]
${\alpha }_H$	=	Prompt-neutron decay constant of higher mode [1/s]
$\epsilon$	=	Detection efficiency of a neutron counter
$\bar{\nu }$	=	First factorial moment of number of neutrons generated in fission event
$\overline{\nu \left(\nu -1\right)}$	=	Second factorial moment of number of neutrons generated in fission event
$\overline{{\nu }_{sp}}$	=	First factorial moment of number of neutrons produced by spallation event
$\overline{{\nu }_{sp}\left({\nu }_{sp}-1\right)}$	=	Second factorial moment of number of neutrons produced by spallation event
${\lambda }_d$	=	Probability per unit time that a neutron is detected by a neutron counter $\left(=\epsilon {\lambda }_f\right)$ [1/s]
${\lambda }_f$	=	Probability per unit time that a neutron induces a fission event [1/s]
$\rho$	=	Reactivity