机器人在福岛核事故中的应用和启示

福岛核事故发生至今,至少已经有30多种机器人应用于福岛第一核电厂1/2/3号机组,开展反应堆厂房外航测、作业区域和人员通道去污、反应堆厂房内部探查、安全壳外部泄露探查和安全壳内部探查;其中,8个机器人在作业时因辐射损伤、障碍物阻挡等原因而发生故障。福岛核电厂的经验教训和良好实践表明,在核工业机器人研制和实际操作使用中值得借鉴:在观念上应该重视核工业机器人的作用,并进行系列化发展;除了几十Gy/h以上的强辐射场,核工业机器人实际应用中的制约因素往往是空间可达性和控制性能而不是耐辐射性能;应用于10 Gy/h以下的辐射场时,通过累积剂量管理和关键部件更换,可弥补通用机器人在耐辐射设计方面的不足。     

Atmospheric 85Kr and 133Xe activity concentrations at locations across Japan following the Fukushima Dai-ichi Nuclear Power Plant accident

Atmospheric ⁸⁵Kr and ¹³³Xe activity concentrations were determined from weekly air samples collected at Sapporo, Akita and Chiba, Japan, throughout 2011. The results demonstrated that the Fukushima Dai-ichi Nuclear Power Plant accident in early March 2011 resulted in high ¹³³Xe activity concentrations as well as elevated levels of ⁸⁵Kr activity; there was a striking increase in the concentrations of both isotopes over the week running from 14 to 22 March as the radioactive plume released from the plant was captured. At Chiba, following the accident, the ⁸⁵Kr activity concentration increased from 1.38 to 17.7 Bq/m³, while the ¹³³Xe levels increased from below the minimum detectable concentration (MDC ≤ 1.9 × 10^?3 Bq/m³) to 1.3 × 10³ Bq/m³. Conversely, at Sapporo and Akita, high ⁸⁵Kr activity concentrations were not observed, due to differences in air transportation mechanisms based on wind directions. Duplicate samples were collected at Chiba to allow the simultaneous analyses of ⁸⁵Kr and ¹³³Xe at the Japan Chemical Analysis Center and the Bundesamt für Strahlenschutz in Germany and the results were in good agreement. The external effective radiation doses resulting from ⁸⁵Kr and ¹³³Xe releases following the accident were estimated to be approximately 7.0 × 10^?3 and 1.3 μSv, respectively, based on the activity concentrations of these nuclides from March to June in 2011 at Chiba.     

浅议福岛核电事故对我国核电发展的影响及借鉴

日本福岛核电事故虽然未对我国的环境造成严重影响,但是该事故的发生对我国后续核电的发展必然产生重大影响。我国庞大、密集的核电发展规划在世界能源发展领域是绝无仅有的,无论是在技术路线、标准制订还是在建造、运行的组织管理上,我国核电建设者和管理者都将面临巨大考验。通过分析和借鉴本次日本福岛核电事故的发生、处理过程,将对我国核电发展在多方面提供重要参考,从而促进我国核电规划及核电事故应急体系的持续完善。     

Field photon energy spectra in Fukushima after the nuclear accident

After the accident of the Fukushima Daiichi Nuclear Power Plant, radionuclide were spread out over large area. It has been past almost two years since the nuclear accident. Therefore, Cs-137 and Cs-134 nuclei are the main sources of gamma rays. The field gamma rays, however, are not mono-energy due to photons scattered from the ground, the air, etc. The effective dose for external exposure depends on the energy of radiations, thus photon energy spectra are important for the evaluation of effective dose for the people who live in Fukushima. In the present study, the photon energy spectra have been measured at several points in Fukushima to evaluate reference energy spectra after the nuclear accident. The energy spectra in Fukushima area were measured using a cadmium zinc telluride (CdZnTe) detector, and an unfolding method was applied to evaluate photon energy spectra. The fraction of low-energy photon is increased by decontamination around the measurement point. This can be used to estimate the efficiency of decontamination effect by removing the radionuclides on the target surface. The photon energy spectra measured by the present study would be useful for the evaluation of the effective dose for the people who live in Fukushima.     

One-year time variations of anthropogenic radionuclides in aerosols in Tokyo after the Fukushima Dai-ichi Nuclear Power Plant reactor failures

We report on the results of monitoring of environmental radiation for one year (13 March 2011 to 12 March 2012), including air dose rates and the concentrations of radionuclides in aerosols in Tokyo, after the reactor failures at the Fukushima Dai-ichi Nuclear Power Plant. The air dose rates began to increase at 4:00–5:00 JST on 15 March 2011, and the maximum rate was observed at 10:00–11:00 JST. Two peaks were observed before 23 March 2011, and then the air dose rates decreased until March 2012. The time variations of concentrations of radionuclides in aerosols showed tendencies similar to those of air dose rates. Short-lived radionuclides (⁹⁹Mo (^99mTc), ^129mTe (¹²⁹Te), ^131mTe, ¹³²Te (¹³²I), ¹³³I and ¹³⁶Cs) were under the detection limit during April 2011. Iodine-131 was detected until early June 2011, and long-lived radionuclides (¹³⁴Cs and ¹³⁷Cs) were detected intermittently for one year. Based on our results, gamma doses and committed effective doses resulting from inhalation were estimated.     

A simple mass and heat balance model for estimating plant conditions during the Fukushima Dai-ichi NPP accident

A simple evaluation method for the analysis of thermal-hydraulic transients in reactor pressure vessel (RPV) and primary containment vessel (PCV) is proposed to support understanding the accident behaviors of the Fukushima Dai-ichi nuclear power plant (NPP). Since most of the measurements of the plants were unavailable especially in the early stage of the accident, and the accessibility to the plants had been limited by radiation, analytical investigation for the plant was required to understand the plant conditions such as the magnitude of the damages. In order to provide easy-to-use technical tools to support the analytical investigation, we developed a simplified analysis code, named “HOTCB”, based on total mass and heat balances in a lamped parameter system. The HOTCB code has capabilities to treat two-phase fluid including water, steam, and non-condensable gas in a wide range of temperatures up to highly superheated conditions, and to consider heat structures, i.e. heat capacities and heat transfer to the fluid. The code was provided to Tokyo Electric Power Company (TEPCO) and was practically used for the analysis on the accident. This paper provides the details of the code and simulations of Unit 1 and Unit 2 reactors of Fukushima Dai-ichi nuclear power plant (NPP) as examples to show the usefulness of the code.     

Validation of MAAP model enhancement for Fukushima Dai-ichi accident analysis with Phenomena Identification and Ranking Table (PIRT)

The Modular Accident Analysis Program (MAAP) model enhancement items to improve the simulation capability for molten corium behavior in the accidents at the Fukushima Dai-ichi nuclear power plants were validated with the Phenomena Identification and Ranking Table (PIRT). The importance ranks of the identified phenomena were evaluated for each time phase through brainstorming and discussion with the experts in the Atomic Energy Society of Japan and the members of the MAAP model enhancement project. When the current MAAP evaluation models were reviewed with the PIRT, it is found that 95 of the 386 high-ranked phenomena were not considered in MAAP 5.0.1. While 62 of these phenomena will have been addressed in the MAAP enhancement project and 25 others are not suitable to be analyzed by MAAP, 8 important phenomena should be considered in post-MAAP enhancement project with additional experiments or fundamental studies.     

Study on long-term radiation exposure analysis after the Fukushima Dai-ichi nuclear power plant accident: validation of the EU long-term radiation exposure model (ERMIN)

The Fukushima Dai-ichi nuclear power plant accident led to the dispersion of radioactive material by wind that resulted in soil and air pollution over a wide area. Even now, more than 150,000 people in Fukushima prefecture are still relocated from their homes. In order to estimate how long such relocation might continue, we estimated radiation exposures for the 10 years following the accident that occurred on 12 March 2011, using European model for inhabited areas (ERMIN) developed by a partnership of several European organizations. We validated the estimated results of long-term radiation exposure with the observed data at 1, 2 and 3 years after the nuclear accident. We analyzed the deposition velocity and re-suspension coefficient using the observed data on air concentration, amount of falling dust and soil contamination, and compared them with the published data.     

Analysis for iodine release from unit 3 of Fukushima Dai-ichi nuclear power plant with consideration of water phase iodine chemistry

The accident at unit 3 of the Fukushima Dai-ichi nuclear power plant was analyzed with THALES2 for the progression of severe accident coupled with Kiche for the iodine chemistry in aqueous phase. The analysis indicated that, compared with the analysis without the aqueous phase iodine chemistry, a significantly larger amount of iodine was released from the water pool of the suppression chamber (S/C) with forms of molecular iodine and organic iodine due to the repeated operation of the containment venting system. It was also implied in the sensitivity analysis that the late phase release of the volatile iodine species was largely influenced by the pH of the S/C water and gas–liquid mass transfer coefficient under a gas–liquid two-phase flow condition.     

基于福岛核事故监测数据的辐射剂量场重构方法研究

辐射剂量场的重构是核事故后开展应急工作的一种重要手段。为了比较不同插值方法用于辐射剂量场重构的优劣,基于日本福岛第一核电站事故后发布的监测数据,分别利用反距离权重插值法、克里金插值法、线性三角网插值法和径向基函数插值法4种方法进行辐射剂量场重构。通过划分不同数量的测试集,以均方根误差、平均绝对误差以及标准偏差为判别依据...     

Source term estimation of atmospheric release due to the Fukushima Dai-ichi Nuclear Power Plant accident by atmospheric and oceanic dispersion simulations

The source term of the atmospheric release of ¹³¹I and ¹³⁷Cs due to the Fukushima Dai-ichi Nuclear Power Plant accident estimated by previous studies was validated and refined by coupling atmospheric and oceanic dispersion simulations with observed ¹³⁴Cs in seawater collected from the Pacific Ocean. By assuming the same release rate for ¹³⁴Cs and ¹³⁷Cs, the sea surface concentration of ¹³⁴Cs was calculated using the previously estimated source term and was compared with measurement data. The release rate of ¹³⁷Cs was refined to reduce underestimation of measurements, which resulted in a larger value than that previously estimated. In addition, the release rate of ¹³¹I was refined to follow the radioactivity ratio of ¹³⁷Cs. As a result, the total amounts of ¹³¹I and ¹³⁷Cs discharged into the atmosphere from 5 JST on March 12 to 0 JST on March 20 were estimated to be approximately 2.0 × 10¹⁷ and 1.3 × 10¹⁶ Bq, respectively.     

福岛核事故场外环境修复综述及启示

概述了日本福岛核事故后放射性物质的释放和沉积情况。归纳综述了日本福岛核事故后场外环境修复的方针、环境修复法规、环境修复计划以及环境修复的进展和现状。最后提出了我国今后在核事故发生后对于场外放射性环境修复过程中需要关注和探讨的问题。     

福岛第一核电站事故对近岸海域鱼类的辐射影响

利用福岛第一核电站事故后不同沿岸和离岸距离处的海水中放射性核素137Cs的监测数据,以鱼类为参考水生生物,通过核素137Cs的累积动力学模式对鱼体内核素的动态浓集进行了评估,给出了福岛第一核电站事故后液态放射性释放所致近岸海域水生动物体内核素水平的变化规律。     

Long-term predictions of ambient dose equivalent rates after the Fukushima Daiichi nuclear power plant accident

To analyze radiation protection strategies and rehabilitation programs in Fukushima, prediction models have been developed for ambient dose equivalent rate distributions within the 80 km-radius around the Fukushima Daiichi nuclear power plant. The prediction models characterized by ecological half-lives of radioactive caesium for land-use, enable Fukushima residents to obtain distribution maps of ambient dose equivalent rates after the Fukushima Daiichi nuclear power plant accident. Model parameters such as the ecological half-lives for the short-term component and the fractional distribution of short-term component were evaluated using ambient dose equivalent rates through car/vehicle-borne surveys. It was found that the ecological half-lives among land-use differ only slightly, whereas the fractional distributions of the short-term component are clearly dependent on land-use. In addition, uncertainties concerning predictions of ambient dose equivalent rates arising from variability in model parameters were assessed using Monte Carlo simulations. Long-term changes of ambient dose equivalent rates were predicted for different land-use areas. Distribution maps of ambient dose equivalent rates for the next 30 years after the accident, created by the prediction models are expected to be useful for follow-up of the radiological situation since they provide information on the space variation of the ambient dose equivalent rates in inhabited areas.     

Development of ocean dispersion concentration maps of the contaminated water released from the Fukushima Dai-ichi Nuclear Power Plant

Ocean dispersion concentration maps were developed to gain an understanding of the migration behavior of contaminated water leaking from the Fukushima Dai-ichi Nuclear Power Plant. We used ensemble mean values to define the semi-climatological seasonal field in our calculations of the trajectories of the contaminated water. The maps were validated by conducting actual release events in two different seasons. The results showed that despite their simplicity, the proposed methods yield relatively good results.     

Measurements of 131I in the thyroids of employees involved in the Fukushima Daiichi nuclear power station accident

The Great East Japan Earthquake Disaster on 11 March 2011 caused an unprecedented accident at the Fukushima Daiichi nuclear power station operated by Tokyo Electric Power Company (TEPCO). Nuclear Fuel Cycle Engineering Laboratories of Japan Atomic Energy Agency performed internal dose measurements of 560 employees involved in the accident during the period from 20 April to 5 August in 2011 at the request of TEPCO. The present paper describes our measurements of ¹³¹I in the thyroid that is the predominant contributor to the internal dose. These measurements were carried out using an HPGe detector installed in a low-background shielded chamber made of 20-cm-thick steel and the detector was placed adjacent to the subject's neck. The typical minimum detectable activity of this technique was 10 Bq for a counting time of 10 min; however, this sensitivity made it difficult to identify a residual thyroid content of ¹³¹I corresponding to a committed effective dose of 20 mSv for late subjects. This paper discussed technical issues experienced through the measurements such as the influence of ¹³¹I in the rest of the body, the calibration phantom of use, and so on.     

Radiation protection lessons learned from the TEPCO Fukushima No.1 NPS accident

Lessons learned from the TEPCO Fukushima No.1 NPS accident are discussed from the viewpoint of radiation protection in the situation of nuclear emergency. It became clear from the discussion that the protective measures should be practiced by taking into account the time profiles of the radiological disaster after the nuclear accident and that the land and coastal sea areas monitoring had to be practiced immediately after the nuclear accident and the communication methods to tell the public about the radiation information and the meaning of protective measures should be developed for mitigation of the sociological aspects of disaster impacts. And it was pointed out from the view point of practicing countermeasures that application of the reference levels, above which it was judged to be inappropriate to plan to allow exposure to occur, played an important role for practicing protective measures in an optimized way and that the quantities and units used for quantifying radiation exposure of individuals in terms of radiation doses have caused considerable communication problems. Finally, the occupational exposures and the public exposures that have been reported so far are shown, and it is concluded that there is no conclusive evidence on low dose exposures that would justify a modification of the radiation risk recommended by the International Commission on Radiological Protection.     

Production cross-sections for the residual radionuclides from the Cd(p, x) nuclear processes

We measured the production cross-sections of the ^{107,111m,115g}Cd, ^{108m,108g,109g,110m,110,111g,113m,114m,115m,116m}In and ^{104g,105g,106m,110m,111g,113g}Ag radionuclides for proton-induced reactions on cadmium by using a stacked-foil activation technique in the energy rangy between 3 and 40 MeV at the MC-50 cyclotron of the Korea Institute of Radiological and Medical Science. The measured cross-sections were compared with the available literature data and the theoretical calculations by the model codes TALYS and ALICE-IPPE. The integral yields for thick targets were also obtained from the measured cross-sections of the produced radionuclides. The measured cross-sections, especially the indium (In) radionuclides have a significance for various practical applications; thin layer activation analysis, nuclear medicine, nuclear technology, radioactive waste handling, etc.