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.     

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.     

福岛核事故后上海气溶胶中~(131)I、~(134)Cs和~(137)Cs的来源途径分析

利用对气溶胶中典型放射性核素(~(131)I和134,~(137)Cs)的分析,可以评估福岛核事故产生的放射性物质对上海及全球的大气放射性本底水平造成的影响。本工作结合核事故释放过程、核素的天然衰变以及气象条件等因素,获得核事故期间上海的气溶胶中~(131)I和134,~(137)Cs活度浓度及其比值的分布特征:~(131)I被检出的时间(2011-03-27)早于~(134)Cs(2011-04-06)和~(137)Cs(2011-04-08),~(131)I的活度浓度(0.01~1.20 mBq/m3)比~(134)Cs(0.01~0.58mBq/m3)和~(137)Cs(0.01~0.65mBq/m3)大2~10倍,而且在不同的时间段出现相应的多峰值现象;~(131)I/~(137)Cs活度浓度比值(1.3~10.6)在2011年4月5日之后呈递减趋势,但是~(134)Cs/~(137)Cs活度浓度比值(0.8~2.9)则一直在1.1左右波动。利用HYSPLIT模型模拟放射性气团运移轨迹的分析方法,表明在核事故期间输入到上海的放射性气溶胶的途径有东北和西北两条主要迁移路径。同时通过结合国内相关城市核事故期间大气放射性监测数据,证实了东北路径在中国境内的控制地位。另外,通过总结和分析北半球大气监测数据中~(131)I/~(137)Cs和~(134)Cs/~(137)Cs活度浓度比值最大值的分布特征,验证了日本核事故产生的放射性气溶胶在北半球的传输过程。     

Transformation of the forms of90Sr and137Cs in soil and bottom deposits

A method is proposed for estimating the distribution factors of¹³⁷Cs and⁹⁰Sr in soil and deposits. The method is based on predicting the forms in which these radionuclides occur and estimating the exchange distribution factor. It is shown that fixation of the radionuclides is a reversible process. A model is proposed for the transformation of the chemical forms of¹³⁷Cs and⁹⁰Sr. including leaching from fuel particles, fixation, and remobilization. It is shown that a first-order kinetic model satisfactorily describes the transformation of the forms of⁹⁰Sr as well as¹³⁷Cs for time periods of about one month. For longer periods of time, a diffusion fixation model is best for¹³⁷Cs. Recommendations are made for the key parameters for various types of soils. 1 figure, 2 tables, 11 references. Scientific and Industrial Association “Taifun.” Translated from Atomnaya énergiya, Vol. 88, No. 1, pp. 55–60, January, 2000.     

Measurements of 134Cs and 137Cs in urine and estimation of the inter nal dose of an adult exposed to the Chernobyl Accident

To estimate the internal dose of a Chinese visiting scholar internally contaminated because of the Chernobyl Accident, the contents of 134Cs and 137Cs in urine were measured using a Ge（Li） γ-spectrometer. The internal doses were calculated based on data from the ICRP Publications. The effective doses from ^134Cs and ^137Cs were estimated to be 61 μSv and 98 μ Sv, respectively. The sum of 159 μSv was lower than the total effective dose （310 μSv）, from the inhalation and ingestion of natural radionuclides. The dose of ^131I was also reviewed referring to the UNSCEAR 2000 Report. The equivalent effective dose of ^131I was estimated to be 2.9 mSv, 18 times more than the amount of ^134Cs and ^137Cs. Therefore, it is considered that the earlier estimation of internal doses of ^131I is important in evaluating radiation injuries from a nuclear reactor accident.     

Analysis of cesium isotope compositions in environmental samples by thermal ionization mass spectrometry-3

Cesium was recovered from soil samples obtained in Fukushima prefecture. Isotopic analysis of Cs was performed by γ-spectrometry to determine the activity ratio of ¹³⁴Cs/¹³⁷Cs and thermal ionization mass spectrometry was used to determine the isotopic ratios of ¹³³Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs. The analytical results showed that both the activity ratio of ¹³⁴Cs/¹³⁷Cs and the isotopic ratio of ¹³⁵Cs/¹³⁷Cs were within the expected values for the Fukushima Daiichi Nuclear Power Plant estimated using the ORIGEN-II code, suggesting that most of the radioactive Cs in the soil sample originated from the Fukushima Daiichi Nuclear Power Plant. The concentration of ¹³⁷Cs and the contribution of radioactive Cs from global fallout were correlated to the distance from the Fukushima Daiichi Nuclear Power Plant, while the contribution of radioactive Cs from each reactor did not show any similar distance dependence.     

Development of a Non-conservative Radionuclides Dispersion Model in the Ocean and its Application to Surface Cesium-137 Dispersion in the Irish Sea

A numerical simulation model system that consists of an ocean current model, Princeton Ocean Model (POM), and a particle random-walk model, SEA-GEARN, has been developed to describe the migration behavior of non-conservative radionuclides in a shallow water region. Radionuclides in the ocean are modeled in three phases, i.e., the dissolved phase in seawater, the adsorbed with large particulate matter (LPM) and the adsorbed with active bottom sediment. The adsorption and desorption processes between the dissolved and solid phases are solved by the stochastic method with the kinetic transfer coefficients. Deposition of the LPM and re-suspension from bottom sediment are also considered. The system was applied to simulate the long-term (24-year) dispersion of 137Cs actually released from the BNFL spent nuclear fuel reprocessing plant at Sellafield in United Kingdom. The calculation well reproduced the main characteristics of migration of dissolved ¹³⁷Cs concentration in the Irish Sea.     

Air concentration estimation of radionuclides discharged from Fukushima Daiichi Nuclear Power Station using NaI(Tl) detector pulse height distribution measured in Ibaraki Prefecture

Air concentrations of ¹³³Xe, ¹³¹I, ¹³²I, ¹³³I, ¹³²Te, ¹³⁴Cs, ¹³⁶Cs, and ¹³⁷Cs in the early stage of the Fukushima Daiichi Nuclear Power Station (FNPS1) accident were estimated for six locations in Ibaraki Prefecture from pulse height distributions measured with NaI(Tl) scintillation detectors continuously operated as a part of monitoring station systems. Air concentration of ¹³³Xe in Ibaraki was estimated for the first time, which showed the maximum value of 150 kBq m^?3 in the plume arrived at the southern part of Ibaraki around noon of 15 March 2011. The plume was found to consist mainly of noble gases without other nuclides at significant level and to correspond to the hydrogen explosion at Unit 3. Spatiotemporal distributions of ¹³¹I and other nuclides were discussed for the plumes detected during the periods of 15–16 and 20–21 March. Variations in radionuclide composition among the plumes and within each plume were also discussed.     

Estimation of Total Amounts of Anthropogenic Radionuclides in the Japan Sea

We estimated the total amounts of anthropogenic radionuclides, consisting of ⁹⁰Sr, ¹³⁷Cs, and ^239+240 Pu, in the Japan Sea for the first time based on experimental data on their concentrations in seawater and seabed sediment. The radionuclide inventories in seawater and seabed sediment at each sampling site varied depending on the water depth, with total inventories for ⁹⁰Sr, ¹³⁷Cs, and ^239+240Pu in the range of 0.52–2.8 kBq m^?2, 0.64--4.1 kBqm^?2, and 27-122Bqm^?2, respectively. Based on the relationship between the inventories and the water depths, the total amounts in the Japan Sea were estimated to be about 1:2 ± 0:4PBq for ⁹⁰Sr, 1:8 ± 0:7PBq for ¹³⁷Cs, and 69 ± 14TBq for ^239+240Pu, respectively; the amount ratio, ⁹⁰Sr:¹³⁷Cs:^239+240Pu, was 1.0:1.6:0.059. The amounts of ⁹⁰Sr and ¹³⁷Cs in the Japan Sea were in balance with those supplied from global fallout, whereas the amount of ^239+240Pu exceeded that supplied by fallout by nearly 40%. These results suggest a preferential accumulation of the plutonium isotopes. The data used in this study were obtained through a wide-area research project, named the “Japan Sea expeditions (phase I),” covering the Japanese and Russian exclusive economic zones.     

Radiological assessment of the South Sakhalin Region during the Fukushima-1 accident (Japan)

The results of ¹³¹I, ¹³⁷Cs, and ¹³⁴Cs measurements in the South Sakhalin atmosphere performed with a gamma spectrometer placed in a mobile radiation surveying laboratory are presented. The measurements were performed in March–April 2011 as part of radiation monitoring performed by Rosgidromet in connection with the Fukushima-1 accident in Japan. The maximum recorded ¹³¹I volume activity of inhaled air in South Sakhalin is 8700 times lower than the admissible yearly-average value for the general public. The values for ^137,134Cs are 10⁵ times lower than the corresponding norms.     

Atmospheric radionuclides from Fukushima Dai-ichi nuclear accident detected in Lanzhou,China

After the Fukushima Dai-ichi nuclear power plant accident on March 11,2011,the radioactivity released from the accident was transported around the globe by atmospheric processes.The radioactivity monitoring program on atmospheric particulate in Lanzhou,China was activated by GSCDC to detect the input radionuclides through atmospheric transport.Several artificial radionuclides were detected and measured in aerosol samples from March 26 to May 2,2011.The peaked activity concentrations(in m Bq/m3)were:1.194(131I),0.231(137Cs),0.173(134Cs)and 0.008(136Cs),detected on April 6,2011.The average activity ratio of131I/137Cs and134Cs/137Cs in air were 13.5 and 0.78.The significant increase of137Cs activity concentration,one order of magnitude higher than pre-Fukushima accident levels,in ground level aerosol was observed in 2013,as its resuspension from soil.The back-trajectory analysis simulated by NOAA-ARL HYSPLIT shows a direct transfer of the air masses released from Fukushima to Lanzhou across the Pacific Ocean,North America and Europe at the height close to 9000 m AGL.The value of effective dose for inhalation is close to one millionth of the annual limit for the general public.     

Cold Trapping of Fission Products in a Stainless Steel Sodium Loop

The behavior of fission products in sodium system was studied using a cold trap installed in the Toshiba Fission Product Loop, which is a stainless steel inpile sodium loop. The fission products were generated within the liquid sodium by irradiating UO² fuel under sodium convection. The cold trapping behavior of ⁹⁵Zr, ⁹⁵Nb, ¹³²Te, ¹³¹I ¹³⁷Cs and ¹⁴⁰La (¹⁴⁰Ba) was examined by measuring the γ-rays in the cold trap and in the sodium dump tank by means of Ge(Li) detectors. Such fission product nuclides as ^{131 132}Te and ¹³⁷Cs were found to be reversively removed by the cold trap at lower temperatures, with negligible hysteresis. The distribution coefficient K(cm) for the cold trap surface was found tobe expressible by log K=(3.08±0.17)-(0.0112±0.0006)×T(°C) for ¹³¹I, and by log K:=-(4.55±0.29) + (1830±1/140) × 1/T(K) for ¹³⁷Cs. The behavior of ¹³¹I and ¹³⁷Cs during cold trapping can be explained clearly by adsorption model using these K values. While ⁹⁵Zr, ⁹⁵Nb, ¹⁰³Ru and ¹⁴⁰La(¹⁴⁰;Ba) also were detected in the cold trap, the efficacy of cold trapping is far lower for these nuclides than for ¹³¹I and ¹³⁷Cs, and changing the cold trap temperature did not appear to have any appreciable effect on the cold trapping. Presence of ¹⁴¹Ce and ¹⁴⁴Ce was not detectable in the cold trap, although small amounts of these nuclides were detected in the sodium dump tank.     

日本福岛核电站事故后的海洋放射化学

由于对环境影响的关注,日本福岛核电站事故后,人们进行了大气、陆地和海洋环境人工放射性核素变化监测与研究,研究的主要核素是131I、137 Cs、134 Cs和129I。除了关注浓度水平的变化外,还进行了通过大气和海流对事故释放的放射性核素运行路径的模拟研究。研究表明,受气候条件的控制,事故释放进入大气的放射性核素先经过太平洋到达北美,然后越过大西洋到达欧洲,最后绕北半球一周后到达中国。除事故核电站周边外,全球大气中131I活度浓度在mBq/m3量级,137 Cs活度浓度在0.1~1mBq/m3量级。事故释放进入海洋的放射性核素将随海流向东输运,然后在北太平洋随环流输运。研究也发现在离开源地不远的海区,由于混合进入200m水深以下的次表层水,在远离事故核电站海区水体的137 Cs活度浓度可达100Bq/m3,但大部分水体137 Cs活度浓度在Bq/m3量级,仅稍高于本底水平。     

Development of regional downscaling capability in STEAMER ocean prediction system based on multi-nested ROMS model

Oceanic regional downscaling capability was implemented into Short-Term Emergency Assessment system of Marine Environmental Radioactivity (STEAMER) developed by Japan Atomic Energy Agency to enable us to predict more realistically the oceanic dispersion of radionuclides at higher spatiotemporal resolutions for broader applications. The system consisted of a double-nested oceanic downscaling circulation model with tidal forcing and an oceanic radionuclide dispersion model. This system was used to comparatively examine downscaling and tidal effects on the dispersion of radionuclides hypothetically released from the Fukushima Daiichi Nuclear Power Plant in the colder season. The simulated dissolved ¹³⁷Cs distribution was different from that obtained using coarser-resolution models because downscaling enhanced both horizontal and vertical mixing. The suppression of horizontal mixing and the promotion of vertical mixing by tidal forcing synergistically reduced offshore ¹³⁷Cs transport. In addition, the submesoscale effects strengthened the three-dimensional ¹³⁷Cs fluctuations by <10 times, while the tidal effects promoted slightly increased the intensity of three-dimensional ¹³⁷Cs fluctuations by approximately 3%. This indicated that the submesoscale effects substantially surpassed tidal forcing in oceanic mixing in the coastal margin off Fukushima in the colder season.     

福岛核事故期间广西地区辐射环境应急监测

福岛核事故发生后,广西环保部门按照环境保护部核与辐射事故应急办公室的应急指令组织有关力量展开应急监测.监测内容包括γ辐射剂量率连续监测,以及空气、沉降灰、雨水、江河水、海水、蔬菜、土壤等环境介质中人工放射性核素水平监测.监测结果表明:气溶胶样品中监测到微量的131 I 、137 Cs 、134 Cs ,蔬菜和雨水样品中监测到微量的131 I .这些核素的活度浓度极低,公众无需采取防护措施.     

通过分析放射性核素活度鉴别燃料初始富集度的可行性研究

高温气冷堆核电站示范工程（HTR-PM）的反应堆在达到平衡状态前要经过一个较长时间的过渡过程。该过程中堆芯将装入两类燃料球,它们在设计上只有燃料初始富集度不同。反应堆运行要求在过渡过程中要鉴别出装有低富集度燃料的燃料球,并按其燃耗水平不同将其卸出。本文针对此问题,讨论了通过分析燃料球中放射性核素活度（或其比值）以鉴别两类燃料球的方法。堆物理分析软件和KORIGEN软件针对过渡过程的计算结果初步肯定了该方法的理论可行性,并可看出最有可能的鉴别指征量是¹³⁴Cs活度、¹²⁵Sb与¹³⁷Cs的活度比值和¹³⁴Cs活度与¹³⁷Cs活度平方的比值。     

Emission of Radionuclides from the Destroyed Unit of the Chernobyl Nuclear Power Plant

Investigations of radionuclide emission resulting from the Chernobyl accident are briefly reviewed. Three ways to estimate emission are examined: direct investigations of radionuclides emitted from the destroyed unit; study of the quantity and composition of radionuclides emitted on the territory after the active stage of the accident was over; study of the quantity and composition of radionuclides remaining in the No. 4 unit. All three methods suffer from serious difficulties which are examined in the paper. New data are presented. These data concern estimates of the emission of fuel particles, ¹³⁷Cs, and ¹³¹I. 6 tables, 17 references.     

Exact solutions to gaseous flow time and temperature dependent parent and first-daughter fission-product plateout problems

Exact solutions are obtained to the parent and first-daughter time and temperature dependent convective-diffusion equations with radioactive decay in an axisymmetric cylindrical conduit. Time invariant nominal gaseous flow conditions are assumed for which piecewise polynomial spatial representations of the axial temperature distribution are considered. The time dependence of the daughter-product inlet concentration due to parent decay is included. A mass transfer model incorporating these solutions is developed to predict the fission-product plateout on the conduit wall. The temperature dependence of both the diffusion and wall mass transfer coefficients is characterized using Arrhenius relationships. Comparison with experimental ¹³⁴Cs, ¹³⁷Cs, and ¹³¹I data demonstrates model viability in correctly predicting temperature dependent plateout under relevant flow conditions.     

Examination of 131I and 137Cs releases during late phase of Fukushima Daiichi NPP accident by using 131I/137Cs ratio of source terms evaluated reversely by WSPEEDI code with environmental monitoring data

To investigate what happened in reality during the Fukushima Daiichi Nuclear Power Plant accident, the phenomena within reactor pressure vessel and the discussion of ties with the environmental monitoring measurement are very important. However, the previous study that treats phenomena of the both has not necessarily advanced up to the present time. The source terms predicted by simulation codes such as MELCOR has not yet been consistent with the reverse estimation by WSPEEDI code using environmental measurement data. This study investigated ¹³¹I and ¹³⁷Cs release behaviors during the late phase of the accident to contribute to such examination using the ¹³¹I/¹³⁷Cs ratio of the new source terms predicted by Katata. The ¹³¹I release by the gas–liquid partition from the contaminated water in the 1F2 and 1F3 reactor buildings which was pointed out in the previous study was reevaluated using the new source terms. In addition, paying attention to the similarity of the core conditions between the Fukushima accident and the Phébus FPT3 experiment using the B₄C control rods, the release of organic iodine (CH₃I) during the 1F3 suppression pool venting, formation of CsBO₂ and its release behavior were examined which have not yet been sufficiently studied so far.     

A review of radionuclide behaviour in the primary system of a very-high-temperature reactor

During normal operation of (V)HTRs radiologically-significant contamination of the primary system will occur this being of prime importance in relation to depressurization accidents. This paper reviews information relevant to radiocontaminant transport in (V)HTR primary systems paying particular attention to chemical forms, interactions with dust and overall distribution. The primary-system environment comprises nuclear graphites, alloys, dust and high-purity helium into which low releases of the isotopes ¹³⁴Cs, ¹³⁷Cs, ⁹⁰Sr, ^110mAg, ¹³¹I, ¹³⁵Xe and ⁸⁵Kr can be anticipated. Additionally, proper treatment of radiological risk requires accounting for tritium.A likely gas-phase speciation of the chemically-active fission products is proposed:

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for caesium and strontium, hydroxides would be dominant with iodides as minor species if a relatively low iodine concentration can be assumed;

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for iodine, a split between CsI and HI are likely to dominate with atomic iodine as a minor species.

Strong sorption of radionuclides onto carbonaceous dust can be expected. Such dust is likely to cover all surfaces in a pebble-bed (V)HTR so radionuclides will principally associate with this dust rather than underlying alloys. This is unlikely in prismatic (V)HTRs with lower and uneven dust deposits. Where caesium interacts with alloys strong implanting of a large fraction can occur via adsorption and reaction with low-concentration silicon. Silver shows no special affinity for carbonaceous dust but may interact preferentially with nickel-rich alloys, i.e., in the IHX and/or the gas turbine. Quantitative evaluations of radionuclide distribution are hampered by a lack of data regarding sorption onto the graphites, alloys and carbonaceous dust of modern (V)HTR systems; a long time will elapse before sufficient data are forthcoming. In the meantime, some form of best-estimate distribution and upper-bound concentration for contamination is needed if deterministic safety evaluations are to begin. This distribution will be different for pebble-bed and prismatic designs.