Tetsuo Horiguchi

Multimodal analysis of prompt neutron spectra for 237Np(n,f)

Abstract An attempt was made to analyzed the systematic variation of the prompt neutron spectra of 237 Np(n,f) on the basis of the multimodal analysis of mass and kinetic energy distributions. The spectra of neutrons emitted from fragments of each mode (standard I, II, III, superlong) were calculated independently and the total spectra were synthesized. It was found that the partial spectrum for standard I mode is the softest, getting harder in ascending order for standard II, standard III and superlong modes. The calculated total spectra well represented the experimental data and were found to be more sensitive to the variation of the incident energy than the conventional treatment of the Madland-Nix model.

Multimodal analysis of prompt neutron spectra for (sf), (sf), (sf) and (n,f)

Abstract The prompt neutron spectra for 238 Pu (sf), 240 Pu (sf), 242 Pu (sf) and 239 Pu (n th ,f) were calculated with the modified Madland–Nix model with consideration to the multimodal nature of the fission process. The spectra of neutrons for each mode (Standard-I, -II, -III) were calculated independently and the total spectra were synthesized. The partial spectra for the three modes were found to be considerably different from each other. Different mode branching ratios resulted in different total spectra for the three even-mass spontaneously fissioning isotopes. Comparison of spectra for 240 Pu (sf) and 239 Pu (n th ,f), for which the fissioning nucleus is the same, revealed that the total spectra were harder for 239 Pu (n th ,f) due to the higher excitation energy of the fissioning nucleus. The calculated total spectra well represented the experimental data.

Variance-to-mean method generalized by linear difference filter technique

Abstract The conventional variance-to-mean method (Feynman-α method) seriously suffers the divergency of the variance under such a transient condition as a reactor power drift. Strictly speaking, then, the use of the Feynman-α is restricted to a steady state. To apply the method to more practical uses, it is desirable to overcome this kind of difficulty. For this purpose, we propose an usage of higher-order difference filter technique to reduce the effect of the reactor power drift, and derive several new formulae taking account of the filtering. The capability of the formulae proposed was demonstrated through experiments in the Kyoto University Critical Assembly. The experimental results indicate that the divergency of the variance can be effectively suppressed by the filtering technique, and that the higher-order filter becomes necessary with increasing variation rate in power.

Measurement of subcritical reactivity in unsteady state with Digital Time-Series Data Acquisition System using difference filter technique

The Digital Time-Series Data Acquisition System using first-in-first-out (FIFO) memory module has been developed for the monitoring of subcritical reactivity. The system records arriving time of the signal from a neutron detector, and time-series data are saved in a personal computer (PC). Using this system, the measurement of time-series data is carried out more efficiently than that using a conventional multichannel scaler (MCS) system, and it has been applied for the monitoring of subcritical reactivity of the reactor in a steady state using the conventional Feynman-/spl alpha/ technique. In the present work, this system was applied for the monitoring of subcritical reactivity in an unsteady state using the difference filter technique. The test experiment was carried out with the research reactor of Atomic Energy Institute of Kinki University, Japan. From the experimental results, it was shown that the present system was useful for the continuous monitoring of varying subcritical reactivity by application of the difference filter technique.

Rapid estimation of core-power ratio in coupled-core system by rod drop method

To determine rapidly the core-power ratio in a coupled-core system, a method is proposed on the basis of the control rod drop experiment. A formula of an asymmetrical two-point version was derived to deduce the core-power ratio and subcriticalities of the individual cores. It requires only a familiar measurement technique and tools for the conventional rod drop experiment to apply this formula for the purpose of obtaining these quantities. The present method was applied to the rod drop data measured in coupled-core systems, where the core-power ratio sensitively depended on the rod patterns. The validity of the proposed method was experimentally demonstrated through the comparison between the measured core-power ratios obtained by the present method and that derived from the flux distribution measurement.

Feynman-α correlation analysis by prompt-photon detection

Abstract Two-detector Feynman-α measurements were carried out using the UTR-KINKI reactor, a light-water-moderated and graphite-reflected reactor, by detecting high-energy, prompt gamma rays. For comparison, the conventional measurements by detecting neutrons were also performed. These measurements were carried out in the subcriticality range from 0 to

Measurement of the gamma-ray energy spectrum of the educational Kinki University Reactor (UTR-KINKI)

1.8. The gate-time dependence of the variance-and covariance-to-mean ratios measured by gamma-ray detection were nearly identical with those obtained using standard neutron-detection techniques. Consequently, the prompt-neutron decay constants inferred from the gamma-ray correlation data agreed with those from the neutron data. Furthermore, the correlated-to-uncorrelated amplitude ratios obtained by gamma-ray detection significantly depended on the low-energy discriminator level of the single-channel analyzer. The disriminator level was determined as optimum for obtaining a maximum value of the amplitude ratio. The maximum amplitude ratio was much larger than that obtained by neutron detection. The subcriticality dependence of the decay constant obtained by gamma-ray detection was consistent with that obtained by neutron detection and followed the linear relation based on the one-point kinetic model in the vicinity of delayed critical. These experimental results suggest that the gamma-ray correlation technique can be applied to measure reactor kinetic parameters more efficiently.

Experimental investigation on secondary-count effect in Feynman-α measurement by fission counter

The gamma-ray energy spectrum of the Kinki University Reactor (UTR-KINKI) was estimated from Ge detector measurements combined with Monte Carlo N-particle transport criticality calculations. The gamma rays mainly originated from prompt fission components, although small amounts of gamma rays from (n,γ) reactions, fission product gamma rays, and activation gamma rays were detected. The averaged gamma-ray tissue kerma rate in the irradiation port during UTR-KINKI operation at 1W was calculated as 10.5cGy/h based on the estimated gamma-ray energy spectrum. This value is consistent with a previous measurement with paired ionization chambers and a tissue equivalent gas proportional counter. This result demonstrates the reliability of the estimated gamma-ray energy spectrum.

Dosimetry of fission neutrons in a 1-W reactor, UTR-Kinki

Abstract The Feynman-α (variance-to-mean ratio) measurements were carried out by using fission counters, which contain fissile material of around 2 g, to investigate experimentally the secondary-count effect caused by fission events in the counter. The secondary-count effect was observed when the fission counter was covered with graphite, whereas it was not observed when the bare counter was used and when the counter was surrounded by light-water. Therefore, it was concluded that the secondary-count effect depends not only on fission events in the counter but also on the surrounding material of the counter. A formula to correct the secondary-count effect was derived, and it was applied to obtain the inherent decay constant α. The validity of this formula demonstrated through the comparison with the results obtained by using 3 He and BF 3 counters which are free from the secondary-count effect.