Hirokuni Yamanishi

An electrostatic integrating 222Rn monitor with cellulose nitrate film for environmental monitoring.

This paper describes a new type of electrostatic integrating 222Rn monitor designed for the environmental 222Rn monitoring. The window area of the monitor was selected to make the exchange rate optimal. The collecting electrode was positioned on the basis of calculating the internal electric field. A drying agent, P2O5, was placed in the bottom of the monitor, since the collection efficiency of 218Po+ atoms depends on the humidity of the air. The monitors have been calibrated against known 222Rn exposures. The detection limit is 1.2 Bq m-3 for an exposure time of 2 mo. In a small survey, annual mean 222Rn concentrations between 3.7 and 9.5 Bq m-3 in outdoor air and between 6.4 and 11.9 Bq m-3 in indoor air were measured.

Radiation shielding analysis of a large helical device

Abstract Radiation shielding analysis of a large helical device (LHD) has been performed to provide information on a radiation environment needed for equipment design and operation planning. First, the applicability of the JENDL-3 library which was applied to the shielding calculations of the LHD was tested by means of a benchmark experiment on the shielding efficiency of ordinary concrete carried out at the fusion neutronics source facility at JAERI. Benchmark analysis was carried out with the MCNP continuous energy Monte Carlo code to test the JENDL-3 library itself. The same benchmark calculation was carried out with the DOT3.5 code to examine the groupwise cross-section library of fusion-40 processed from JENDL-3. Secondly, the evaluation of the following items with the DOT3.5 code was revised, using fusion-40 instead of the formerly used GICX-40 library: (1) dose distribution inside and outside the LHD building; (2) dose distribution inside the cellar of the LHD building resulting from steamed radiations; (3) Environmental dose distribution resulting from sky-shine effect; (4) activity and dose rate levels of vacuum vessel and superconducting magnet.

Separation of Natural Background by using Correlation of Time-series Data on Radiation Monitoring

Increase of dose rate due to rainfall often becomes an interfering factor for radiation monitoring outdoors. However, the nature of this interference is different at each place. We obtained a good correlation of dose rate between monitoring points by analysis of 10 minutes-series data of rainy days. The regression line formula for each point was derived from the relation of its dose rate with that of a basis point selected from among the monitoring points. Dose rate drifting due to rain at one point could be reconstructed from this formula and the measured value of the basis point. The estimated dose differed from the measured data by less than 1.2 nSv/10 min. In addition, a simulation was conducted by which virtual doses from the facility were added to actual measured values at each point. By applying the method in this study, virtual doses were calculated within 10% error when they were larger than 33 nSv. This procedure of dose evaluation can be applied to outdoor radiation monitoring if there are several radiation monitors in a site.

Accuracy of neutron dose evaluation in the area monitoring for LHD experiments

Abstract The error in the evaluation of neutron dose during calculation of the neutron field around the large helical device (LHD) in D–D operation is discussed. The expected neutron dose at each monitoring point was derived from the dose conversion factor and neutron fluence data, which was calculated with the radiation transport code DOT-3.5. In contrast, the detected dose at the neutron counter was obtained from the fluence data and the detector response given by calculation with MCNP-4b. The neutron counter used in these calculations consisted of a helium-3 proportional counter with a cylindrical polyethylene moderator. According to the results of the calculations, the ratio of the detected dose to the expected dose was found to lie in the range 1.0–3.0 on the outdoor monitoring points. Since the response of a single neutron counter may lead to inconsistencies in the dose conversion factor, we attempted to minimize these inconsistencies by using a pair of counters with moderators of different thickness. The ratio of the detected dose to the expected dose ranged from 1.5 to 2.1 at the site boundary, indicating that the use of a paired counter allows a more accurate evaluation of dose than the use of a single counter.

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

Development of a Simplified Treatment for Measuring Tritium Concentration in the Environmental Water-Removal of Dissolved Ions by Reverse Osmosis Membrane for Electrolysis Enrichment-

近年開発された固体高分子電解質膜式電解濃縮装置は電解の際に電解質を加える必要がなく, その利点を活かすため, 電解前の試料水の蒸留を濾過にて代替し, 電解後の蒸留を省略する方法を提案し, 検討した。純水を用いた実験により, 電解装置からの溶出イオンはトリチウム濃度測定に影響がないことを確認した。電解前の蒸留の代替法として逆浸透法と精密濾過法を用いて河川水などの環境水中トリチウム濃度測定を行った。精密濾過では試料水中の溶存イオンが除去できず, 電解装置の電極や電解質膜に付着及び吸着し電解装置の故障を招いたが, 逆浸透法では故障しなかった。どちらの濾過法もトリチウム計測への影響がないことを確認した。電解の前処理に逆浸透法を用い, 後処理を割愛することで, 電解前後の処理時間を2日間程度から1.5時間程度に短縮することができた。液体シンチレーション計測法による電解濃縮法を必要とする環境水中トリチウム濃度測定法を簡素化できた。

Three-dimensional radiation shielding analysis of the large helical device

Three-dimensional radiation shielding analysis around a large helical device (LHD) has been performed in order to confirm the margin of the present shielding design and to provide more detail information on a radiation environment for machine design and operation planning. Calculations were carried out using three-dimensional Sn code TORT and 25 group cross-section data based on the JENDL-3 library, and shielding characteristic were examined. Consequently, the radiation leakage effect due to the helical-shaped plasma and coils is necessary to be considered in the evaluation of the radiation flux distribution and its related evaluation: nuclear heat generation inside the cryostat. The present design method based on two-dimensional calculations was compared with three-dimensional calculations, and turned out to be an appropriate method with a suitable conservativeness with regard to the leakage source term calculation.