Chapter 2
Subcriticality
Kengo Hashimoto
Abstract For a subcritical reactor system driven by a periodically pulsed spallation
neutron source in KUCA, the Feynman-α and the Rossi-α neutron correlation analyses are conducted to determine the prompt neutron decay constant and quantitatively
to confirm a non-Poisson character of the neutron source. The decay constant determined from the present Feynman-α analysis well agrees with that from a previous
analysis for the same subcritical system driven by an inherent source. Considering
the effect of a higher mode excited, the disagreement can be successfully resolved.
The power spectral analysis on frequency domain is also carried out. Not only the
cross-power but also the auto-power spectral density have a considerable correlated
component even at a deeply subcritical state, where no correlated component could
be previously observed under a 14 MeV neutron source. The indicator of the nonPoisson character of the present spallation source can be obtained from the spectral
analysis and is consistent with that from the Rossi-α analysis. An experimental technique based on an accelerator-beam trip or restart operation is proposed to determine
the subcritical reactivity of ADS. Applying the least-squares inverse kinetics method
to the data analysis, the subcriticality can be inferred from time-sequence neutron
count data after these operations.
Keywords Subcriticality · Prompt neutron decay constant · Feynman-α method ·
Rossi-α method · Power spectral analyses
K. Hashimoto (B)
Atomic Energy Research Institute, Kindai University, Osaka, Japan
e-mail: kengoh@pp.iij4u.or.jp
© The Author(s) 2021
C. H. Pyeon (ed.), Accelerator-Driven System at Kyoto University Critical Assembly,
https://doi.org/10.1007/978-981-16-0344-0_2
13
Subcriticality
Kengo Hashimoto
Abstract For a subcritical reactor system driven by a periodically pulsed spallation
neutron source in KUCA, the Feynman-α and the Rossi-α neutron correlation analyses are conducted to determine the prompt neutron decay constant and quantitatively
to confirm a non-Poisson character of the neutron source. The decay constant determined from the present Feynman-α analysis well agrees with that from a previous
analysis for the same subcritical system driven by an inherent source. Considering
the effect of a higher mode excited, the disagreement can be successfully resolved.
The power spectral analysis on frequency domain is also carried out. Not only the
cross-power but also the auto-power spectral density have a considerable correlated
component even at a deeply subcritical state, where no correlated component could
be previously observed under a 14 MeV neutron source. The indicator of the nonPoisson character of the present spallation source can be obtained from the spectral
analysis and is consistent with that from the Rossi-α analysis. An experimental technique based on an accelerator-beam trip or restart operation is proposed to determine
the subcritical reactivity of ADS. Applying the least-squares inverse kinetics method
to the data analysis, the subcriticality can be inferred from time-sequence neutron
count data after these operations.
Keywords Subcriticality · Prompt neutron decay constant · Feynman-α method ·
Rossi-α method · Power spectral analyses
K. Hashimoto (B)
Atomic Energy Research Institute, Kindai University, Osaka, Japan
e-mail: kengoh@pp.iij4u.or.jp
© The Author(s) 2021
C. H. Pyeon (ed.), Accelerator-Driven System at Kyoto University Critical Assembly,
https://doi.org/10.1007/978-981-16-0344-0_2
13