Power spectral analysis for a thermal subcritical reactor system driven by a pulsed 14 MeV neutron source

Abstract

A series of power spectral analyses for a thermal subcritical reactor system driven by a pulsed 14 MeV neutron source was carried out at Kyoto University Critical Assembly (KUCA), to determine the promptneutron decay constant of the accelerator-driven system (ADS). The cross-power spectral density between time-sequence signal data of two neutron detectors was composed of a familiar continuous reactor noise component and many delta-function-like peaks at the integral multiple of pulse repetition frequency. The prompt-neutron decay constant inferred from the reactor noise component of the cross-power spectral density was consistent with that obtained by a pulsed neutron experiment. However, the reactor noise component of the auto-power spectral density of each detector was hidden by a white chamber noise in the higher-frequency range and this feature resulted in a considerable underestimation of the decay constant. For several runs with a low pulse-repetition frequency, furthermore, we attempted to infer the decay constant from point data of the delta-function-like peaks. The analysis for a run under a slightly subcritical state resulted in the consistent decay constant; however, those for other runs under significantly subcritical states underestimated the decay constant. Considering the contribution of a spatially higher mode to the point data, the above underestimation was solved to obtain the consistent decay constant. While the Feynman-α formula for a pulsed neutron source is too complicated to be fitted directly to variance-to-mean ratio data, the present analysis on frequency domain is much simpler and the conventional formula based on the first-order reactor transfer function is available for fitting to power spectral density data.

Recent activities in the field of reactor physics

“…Advanced concepts of FR cores have been proposed by several authors [29,30]. On ADSs, some fundamental research works have been carried out using an experimental facility [31,32].…”

Determination of prompt-neutron decay constant from phase shift between beam current and neutron detection signals for an accelerator-driven system in the Kyoto University Critical Assembly

“…A wide variety of reactor physics experiments on an ADS using the pulsed spallation neutrons is now in progress [10,11]. In the preparation for connecting the proton-beam accelerator at KUCA, a series of static and kinetic experiments on an ADS with pulsed 14 MeV neutrons has been conducted [12][13][14][15][16][17][18][19][20]. As a basic study for online monitoring of the subcriticality in the operation of an ADS, various Feynman-α and power spectral analyses for a subcritical reactor system driven by a pulsed 14 MeV neutron source have been performed.…”

Measurement of large negative reactivity of an accelerator-driven system in the Kyoto University Critical Assembly

“…Sakon et al derived the decay constant not only from the correlated component but also from the uncorrelated peak point [19]. Here, their analysis technique is briefly described.…”

“…A wide variety of reactor physics experiments on ADS using the pulsed spallation neutrons is now in progress [10,11]. In the preparation for connecting the proton-beam accelerator at KUCA, a series of static and kinetic experiments on ADS with pulsed 14 MeV neutrons has been conducted [12][13][14][15][16][17][18][19]. As a basic study for on-line monitoring of the subcriticality in operation of ADS, various power spectral analyses for a subcritical reactor system driven by a pulsed 14 MeV neutron source were carried out [19].…”

“…In the preparation for connecting the proton-beam accelerator at KUCA, a series of static and kinetic experiments on ADS with pulsed 14 MeV neutrons has been conducted [12][13][14][15][16][17][18][19]. As a basic study for on-line monitoring of the subcriticality in operation of ADS, various power spectral analyses for a subcritical reactor system driven by a pulsed 14 MeV neutron source were carried out [19]. The measured cross-power spectral density between time-sequence signal data of two neutron detectors consisted of a familiar reactor-noise component and many delta-functionlike peaks at the integral multiple of pulse repetition frequency.…”