Sin-ya Hohara

An improved Feynman-α analysis with a moving–bunching technique

ABSTRACT The bunching technique has been widely utilized in Feynman-α neutron correlation analysis to synthesize neutron counts within longer gate widths by bunching time-sequence neutron counts stored in multichannel scaler channels. An alternative technique referred to as “moving–bunching technique” was proposed to reduce a statistical scatter of variance-to-mean ratio of neutron counts. The conventional bunching technique has no overlap of adjacent bunches, while the present technique makes adjacent bunches overlap as long as possible similarly to the moving average technique. A Feynman-α experiment was performed in the UTR-KINKI, to confirm the advantage of the proposed bunching technique. When a neutron detector was placed far from the core, a Feynman-α analysis with the conventional bunching technique led to a scattered variance-to-mean ratio from which the prompt-neutron decay constant was never determinable. However, another analysis with the proposed technique remarkably reduced the above scatter and enabled the determination of the decay constant. For a neutron detector close to the core, the proposed technique also reduced statistical error of the decay constant.

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 unique power spectral analysis for a subcritical reactor system driven by a pulsed 14 MeV neutron source was carried out at the Kyoto University Critical Assembly (KUCA). In this analysis, a complex cross-power spectral density between time-sequence signal data from an accelerator beam ammeter and a neutron detector was measured to determine the prompt-neutron decay constant of an accelerator-driven system (ADS) from the phase data of the spectral density. Assuming the one-point kinetics model, in theory, the decay constant can be arithmetically derived from the phase at the integral multiples of the pulse repetition frequency. However, the actual derivation from the phase at a pulse repetition frequency of 20 Hz considerably underestimated the prompt-neutron decay constant, compared with that obtained by a previous pulsed neutron experiment, and the derived decay constant apparently decreased with an increase in the multiple of the pulsed repetition frequency. Considering a lag time in detector response, the above underestimation and the above apparent decrease were solved to obtain the consistent decay constant. While both previous power spectral analysis and Feynman-α analysis for pulsed neutron source require non-linear least-squares fits of the respective complicated formulae, the present analysis makes the fitting unnecessary except at regular calibration of the lag time. This feature is advantageous for a robust online monitoring of subcritical reactivity of an actual ADS.

Time Trend Change of Air Dose Rate on Paved Areas in Fukushima City After the Fukushima Daiichi NPP Accident

The Kinki University Atomic Energy Research Institute investigated radioactive contamination resulting from the Fukushima Daiichi Nuclear Power Plant accident in urban areas of Fukushima City, Fukushima Prefecture, Japan. Activity measurement of the surface soil and a survey of the dose rate distribution in urban areas were performed. From the results of this research, dose rate changes in paved areas became clear, and gradients of the dose rate decrease for different paving materials were measured and analyzed.

An applied limit of the bunching method for the Feynman-α analysis

ABSTRACT Feynman-α method is used as the representative method in reactor noise analysis for the criticality monitoring. Feynman-α analysis needs a large amount of measurement time in its original process, though many researchers use the bunching method and its derived methods for the experimental data processing to shorten the measurement time. However, the detailed characteristics and the application limit of the bunching method have not been researched and discussed enough. This paper shows a possibility that the Bunching method is a method to reduce the probability fluctuation with the Y value only in the appearance. Moreover, the criteria for determining that the Y value is not an accidental product are also provided in this paper.

Migration of Radioactive Cesium to Water from Grass and Fallen Leaves

The TEPCO Fukushima Daiichi Nuclear Power Plant accident in March 2011 led to high amounts of emitted radioactive Cs being deposited on land by both rainwater and snowfall. In addition, a significant amount of Cs was deposited on the surface of leaves, and after the accident, both trees and grasses absorbed radioactive Cs through their roots. In order to assess the effect on water sources, it is therefore important to evaluate the amount of radioactive Cs migrating to the water from both grass and fallen leaves.

Effects of irradiation of radioactive rays in nuclear reactor on residual space charge of polyethylene

This paper describes residual space charge formation after 10 kV of voltage was removed in neutron irradiated 0.1mm thick polyethylene film. The neutron irradiation to polyethylene was performed at Kinki University reactor (UTR-KINKI) in Kindai University Atomic Energy Research Institute. The time of neuron irradiation to polyethylene was changed in the range of 3 hours. After the irradiation treatment, 10kV was applied to polyethylene for 5 minutes. After the voltage was switched off, positive polarity of residual space charge was mainly formed in neutron irradiated polyethylene. On the other hand, a small positive space charge was observed in non-irradiated polyethylene. It was also found that the total amount of residual charge in neuron irradiated polyethylene was larger than that in non-irradiated polyethylene. These results indicate that neuron irradiation to polyethylene enhances space charge accumulation in polyethylene bulk.

Study on Radioactive Cesium in Wild Mushroom

2011年 3月 11日の東日本大震災に伴う東京電力福島第 1 原子力発電所の事故によって、大量の放射性セシウムが大 気中に放出された、その一部は現在も福島県の浪江町、飯 舘村、南相馬市、川俣町などの地面に沈着しており、近畿 大学は震災直後から原子力研究所を中心として、川俣町に おける環境放射線の調査を川俣町と共同で行い、実態の把 握と対策の提言に資するデータの収集を行っている 。一 方、2012年 12月の朝日新聞の記事によると、青森県で採 取したきのこからも放射性セシウムが検出されたとしてい るが、検出された放射性セシウムは Cs-137のみで Cs-134 はなかったとしており 、事故後においても東北地方北部 で採取される野生きのこには、福島第一原発事故ではなく、 チェルノブイリ事故によってもたらされた放射性セシウム が検出されている可能性が高い。しかし、これまでに川俣 町の環境調査で測定された放射性セシウムとしては、Cs134も含まれていたことより 、検出された放射性セシウム は福島第一原発の事故により放出され、環境中に残留して いるものであることが強く示唆されている。 福島第一原発の事故以来、きのこは放射性物質の移行係 数が高い食品であるとの風評が流れたこともあり、特に野 生きのこや野外で栽培されているきのこは放射性物質濃度 が高い食品であるとされ、採取や販売が規制されてきた。 しかし、実際にはきのこの種類によっては放射性物質の濃 度は異なってくるといった研究報告 や、栽培方法の検 討により、野外で栽培されるきのこでも基準値以下に放射 セシウム濃度を低減した栽培法が開発されてきている に もかかわらず、いまだにきのこは放射性物質濃度、移行係 数ともに高い食品であると認識されている。また、マツタ ケやホンシメジのように植物の根に共生し、土から発生す る菌根性のきのこ (ectomycorrhizal mushroom; EM)は、シ イタケやエノキタケなどのように枯木や落葉から発生する タイプの腐生性きのこ (saprobic mushroom; SA)に比べて移 行係数が高く、高濃度の放射性物質を含むとため、食用に 供することができないとされている。 本研究では、福島県川俣町の計画的避難区域に指定され ている山木屋地区に自生しているきのこの放射能レベルの 研究論文