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.     

Analysis of cesium isotope compositions in environmental samples by thermal ionization mass spectrometry – 1. A preliminary study for source analysis of radioactive contamination in Fukushima prefecture

Cesium was recovered from plant samples obtained from Fukushima prefecture. The isotopic ratios of ¹³⁴Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs were analyzed by thermal ionization mass spectrometry with a single filament method using a TaO activator. Samples containing 5 Bq of ¹³⁷Cs were analyzed with typical analytical errors of approximately 0.5% for ¹³⁴Cs/¹³⁷Cs and approximately 0.1% for ¹³⁵Cs/¹³⁷Cs. Measurements of both ratios showed profiles that were characteristic of the measurements of among other environmental samples reported in the literature. The results showed the isotopic ratios of ¹³⁴Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs were applicable for the source analysis of radioactive Cs in Fukushima prefecture.     

Physical speciation of Pu-239 + 240 and Cs-137 in oligotrophic and organic rich ground water of tropical environment

Laboratory experiments were carried out in oligotrophic and organic rich ground water to study the fractionation of Pu-239 + 240 and Cs-137 in suspended particulate matter and in various dissolved fraction. The experimental solutions were spiked with Pu-239 + 240 and Cs-137 solution. The total concentration of Cs-137 and Pu-239 + 240 in unfiltered water were partitioned into suspended particulates by using conventional filtration assembly. The colloidal components were concentrated with ultra-filtration stirred cell ranging from <0.45 μm to 1.1 nm in six different stages. Fractionation of Cs-137 and Pu-239 + 240 associated with the particulate matter in the size range of 2.7 μm − 0.22 μm in oligotrophic and organic rich ground water are <30% and <20%, respectively. For the dissolved state in the oligotrophic environment, the maximum concentrations of Cs-137 and Pu-239 + 240 were observed in fraction concentrated with ?1.1 nm. In organic rich ground water, Cs-137 and Pu-239 + 240 were distributed in all the dissolved fractions with a marginally higher concentration associated with 1.6 nm (10,000 NMWL). The possibility of formation of pseudo colloid of Cs-137 and Pu-239 + 240 by complexation with naturally occurring organic and inorganic colloids was investigated in detail.     

Quantities of I-131 and Cs-137 in accumulated water in the basements of reactor buildings in process of core cooling at Fukushima Daiichi nuclear power plants accident and its influence on late phase source terms

During the process of core cooling at Fukushima Daiichi nuclear power plants accident, large amount of contaminated water was accumulated in the basements of the reactor buildings at Units 1–4. The present study estimated the quantities of I-131 and Cs-137 in the water as of late March based on the press-opened data. The estimated ratios of I-131 and Cs-137 quantities to the core inventories are 0.51%, 0.85% at Unit 1, 74%, 38% at Unit 2 and 26%, 18% at Unit 3, respectively. According to the Henry's law, certain fraction of iodine in water could be released to atmosphere due to gas–liquid partition and contribute to increase in the release to environment. A lot of evaluations for I-131 release have been performed so far by the MELCOR calculation or the SPEEDI reverse estimation. The SPEEDI reverse predicted significant release until 26 March, while no prediction in MELCOR after 17 March. The present study showed that iodine release from accumulated water may explain the release between 17 and 26 March. This strongly suggests a need for improvement of current MELCOR approach which treats the release only from containment breaks for several days after the core melt.     

基于LaBr3(Ce)晶体耦合SiPM阵列的可变角分辨率伽玛相机的设计与研制

编码孔径成像技术由于探测效率高、信噪比高、角分辨率好、成像质量稳定可靠等优点而广泛应用于核安全、核设施的去污及退役的测量、核医学等领域。建立通过改变编码准直器和探测器之间距离进而实现可变角分辨的伽玛成像系统。整个成像系统主要由编码准直器、位置灵敏探测(position sensitive detector, PSD)、数据采集卡以及图像重建系统组成。该成像系统的编码准直器采用修正均匀冗余阵列(modified uniformly redundant array, MURA)编码方式,为了保障对较高能量射线的探测能力,编码准直器的材料采用含钨量90%的钨铜合金,PSD通过LaBr₃(Ce)晶体耦合SiPM阵列组成,重建算法采用的是直接互卷积算法,快速高效。测试结果显示,整个位置灵敏探测器的平均能量分辨率为4.96%(662 keV);该辐射成像系统可以准确地对Am-241、Cs-137、Co-60进行清晰成像,并通过改变编码准直器和探测器之间的距离成功分辨出两个Cs-137点源的位置。     

Predictability of a short-term emergency assessment system of the marine environmental radioactivity

ABSTRACT

A Short-Term Emergency Assessment system of the Marine Environmental Radioactivity (STEAMER) was developed at the Japan Atomic Energy Agency (JAEA) to forecast or reanalyze the oceanic dispersion of radionuclides that are released into the ocean around Japan from nuclear facilities during routine operation or in an emergency. STEAMER is currently in daily operation at JAEA to conduct single forecast simulation. The predictability of STEAMER is validated by utilizing oceanographic forecast and reanalysis data in this study. The oceanic dispersion simulations that use oceanographic reanalysis data as input data are assumed to have true solutions. Reanalysis data that has been optimized by data assimilation is the most reliable input for post-analysis. Rigorous oceanic dispersion simulations are conducted for the hypothetical release of Cs-137 from the Fukushima Daiichi Nuclear Power Plant. The predictability of the Cs-137 oceanic dispersion is quantitatively estimated over a forecast period. Moreover, ensemble forecast simulations are also performed applying the Lagged Average Forecast methodology and they successfully improve the predictability of the Cs-137 oceanic dispersion over that obtained using single forecast simulation. The ensemble forecast simulations need to be installed in STEAMER in the future.     

Distribution of the 134Cs/137Cs ratio around the Fukushima Daiichi nuclear power plant using an unmanned helicopter radiation monitoring system

Large amounts of radioactive substances were released into the environment by the Fukushima Daiichi nuclear power plant (FDNPP) accident. Several research institutes have mapped the distribution of nuclides with long half-lives, such as ¹³⁴Cs and ¹³⁷Cs. Although the ratio of ¹³⁴Cs and ¹³⁷Cs has been believed to be equal without depending on the location of the contaminated area, several researchers report that it is different depending on places quite a little. We measured the energy spectrum of gamma rays in high resolution within an approximately 3-km radius of the FDNPP by using an unmanned helicopter equipped with a LaBr₃(Ce) scintillation detector. Then, we analyzed the ¹³⁴Cs/¹³⁷Cs ratio in the area from these measured data in detail. The results show that the ¹³⁴Cs/¹³⁷Cs ratio is different between the plume trace extending north and the other plume traces. We have obtained valuable data for identification of which radioactive substances were released by individual reactor units.     

TRIGA fuel burn-up calculations and its confirmation

The Cesium (Cs-137) isotopic concentration due to irradiation of TRIGA Fuel Elements FE(s) is calculated and measured at the Atominstitute (ATI) of Vienna University of Technology (VUT). The Cs-137 isotope, as proved burn-up indicator, was applied to determine the burn-up of the TRIGA Mark II research reactor FE. This article presents the calculations and measurements of the Cs-137 isotope and its relevant burn-up of six selected Spent Fuel Elements SPE(s). High-resolution gamma-ray spectroscopy based non-destructive method is employed to measure spent fuel parameters. By the employment of this method, the axial distribution of Cesium-137 for six SPE(s) is measured, resulting in the axial burn-up profiles. Knowing the exact irradiation history and material isotopic inventory of an irradiated FE, six SPE(s) are selected for on-site gamma scanning using a special shielded scanning device developed at the ATI. This unique fuel inspection unit allows to scan each millimeter of the FE. For this purpose, each selected FE was transferred to the fuel inspection unit using the standard fuel transfer cask. Each FE was scanned at a scale of 1 cm of its active length and the Cs-137 activity was determined as proved burn-up indicator. The measuring system consists of a high-purity germanium detector (HPGe) together with suitable fast electronics and on-line PC data acquisition module. The absolute activity of each centimeter of the FE was measured and compared with reactor physics calculations. The ORIGEN2, a one-group depletion and radioactive decay computer code, was applied to calculate the activity of the Cs-137 and the burn-up of selected SPE. The deviation between calculations and measurements was in range from 0.82% to 12.64%.     

Plate-out of fission products and its effect on maintenance and repair

With the existing models and data sets, the transport and deposition behaviour of fission products in the primary circuit of a HTR can be predicted with sufficient accuracy for the layout of effective maintenance and repair procedures. As an example, the distribution of the plate out activity of the most important nuclides, Ag-110m, Cs-134 and Cs-137, and the corresponding dose rates at selected locations of the primary circuit have been investigated for the reference Nuclear Process Heat plant PNP-500. In the low temperature region of the primary circuit the expected gamma dose rates lie below 0.4 rem, predominantly caused by directly activated nuclides such as Co-60. In the high temperature region gamma dose rates up to 10 rem/h are expected — mainly due to plate out. Here remote handling devices are required, whereas in the low temperature region mobile shielding enables relatively easy access to the components for maintenance and repair work.     

基于极低本底γ谱分析的海水直接测量研究

为实现便捷、灵敏的海水放射性测量,建立了基于中国锦屏地下实验室低本底高纯锗γ谱仪GeTHU的海水直接测量方法。使用GeTHU直接测量了分别来自西太平洋及上海、青岛、厦门近岸海域的4个体积均接近8.8 L的海水样品,约18小时的直接测量能够探测到⁴⁰K及来自²³⁸U、²³²Th天然放射系的一些关键放射性核素。基于GeTHU的直接测量方法对人工放射性核素同样具有较好的灵敏度,⁵⁷Co、¹³¹I、¹³⁴Cs、¹³⁷Cs的最小可探测活度浓度经过12小时的测量可以降低到25 mBq/L以下,其中¹³⁷Cs的最小可探测活度浓度在48小时的测量后可以达到约8.6 mBq/L。GeTHU将会被持续用于海水放射性的直接测量,尤其是用于需要测量大量样品的紧急监测任务。     

海水中134Cs、137Cs和60Co的联合分析

建立了小体积海水中¹³⁴Cs、¹³⁷Cs和⁶⁰Co的联合分析方法,确定了最佳实验条件。采用磷钼酸铵富集法对海水中放射性铯进行浓集后,其上清液利用氢氧化钴沉淀载带海水中的⁶⁰Co,用γ能谱仪进行测量。结果表明：该法对海水中放射性¹³⁴Cs、¹³⁷Cs和⁶⁰Co的回收率分别为87%~95%、87%~95%和89%~93%,检测限分别为0.048、0.051、0.046 Bq/L。另外,对2017年IAEA国际比对（IAEA-RML-2017-01）海水样品中的¹³⁴Cs、¹³⁷Cs和⁶⁰Co进行分析测量,核素分析结果的最终评价均为“通过”,验证了本实验室采用的¹³⁴Cs、¹³⁷Cs和⁶⁰Co联合分析方法的可行性和可靠性,为今后该方法在常规海洋环境放射性监测中的应用推广奠定了基础。     

Development of a Cherenkov light imaging system for studying the dynamics of radiocesium in plants

High-resolution images of radiocesium (¹³⁷Cs) distribution are required to study cesium kinetics in plants. A Cherenkov light imaging system can visualize fine distributions of radionuclides emitting beta particles using an optical camera. To evaluate the linearity of the system, an imaging test was performed with point sources of ¹³⁷Cs, with a radioactivity of 10–2000 kBq. The results indicated that the system has a good linearity between the image intensity and the radioactivity of ¹³⁷Cs. We developed an imaging system for plants using this system to study radiocesium movement in intact plants. To demonstrate the ability to image radiocesium in a plant, an experiment was performed with an intact soybean plant for four days. The root of an 11-day-old soybean plant was dipped in 20 mL of a culture solution containing ¹³⁷Cs with a radioactivity of 10 MBq without potassium. After one day, the solution was replaced with one with potassium but no ¹³⁷Cs. The soybean plant was in healthy condition in the system, and the high-resolution serial images indicated that ¹³⁷Cs was transported to the shoot and accumulated in the node. Therefore, Cherenkov light imaging is promising for imaging radiocesium in intact plants.     

混合放射性核素参考溶液的研制

将^103Ru标准溶液中的^103RuCl3转化为Na2[^103RuNO(NO2)4OH]，并通过选择适宜的化学组成和酸度，建立了制备^103Ru,^131I,^137Cs,^134Cs和^40K等混合放射性核素参考溶液的方法。经实验检验和放射性比对测量，结果表明，采用该方法制得的混合放射性核素参考溶液是均匀、稳定的、符合放射性计量标准的要求。     

Radiological risk assessment for an urban area: Focusing on an air contamination event

This paper specifically discusses an atmospheric dispersion modeling and health risk assessment for Cs-137 to assess the potential and actual effects on human health from an inhalation event due to a radiological terrorist attack in the Seoul metropolitan area, Korea. The source term was assumed to be 5 TBq of Cs-137, introduced into the central part of Seoul’s metropolitan area by a terrorist attack. Atmospheric dispersion models can be used to support the decision making and risk assessments when terrorist attacks have happened in an urban area. The concepts of Gaussian plume modeling and computational fluid dynamics modeling were used to calculate the Cs-137 concentration in the air. Mortality risk and morbid risk coefficients for the inhalation of contaminated air were used to assess the human health risk. The mortality and morbidity are 1.12E−2 and 1.64E−2, respectively in case of the Gaussian plume, while 6.23E−3 and 9.13E−3 in case of the computational fluid dynamics model. The results of the modeling are dependent on the terror scenarios and dispersion models. Accordingly, the optimization process is needed for final decision making.     

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.     

Rapid measurement of radiocesium in water using a Prussian blue impregnated nonwoven fabric

We developed a rapid method for concentrating and measuring radiocesium (¹³⁴Cs and ¹³⁷Cs) dissolved in fresh water using nonwoven fabric impregnated with Prussian blue (PB) as a radiocesium absorber in combination with gamma-ray spectrometry using a germanium (Ge) detector. Utilizing this method, dissolved radiocesium in a 20–100 L freshwater sample could be concentrated within a period of 20–60 min by passing the sample through 10–12 columns, connected in series, that had been fitted with nonwoven fabric disks impregnated with PB. Laboratory tests using water samples containing known amounts of radiocesium confirmed that the overall recovery rate of the isotope was 100%–108%, and that the first six columns recovered 84%–97% of the isotope. The detection limit of this method was determined to be 0.002 Bq/L with a sample of 100 L and measurement time of 43,200 s. In comparison with traditional methods using ion-exchange resin, co-precipitation with ammonium phosphomolybdate, etc., our method has the advantages of reduced cost and a significantly shorter concentration time. Since water samples can be treated in short periods of time, it is now possible to conduct radiocesium pre-concentration in situ, thus eliminating the need to transport large-volume water samples to laboratories.     

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.     

行李放射性检测装置的研制

行李放射性检测装置可与现有X射线安全检测装置联合用于放射性安全检测。该装置采用NaI(Tl)探测器，用移动平均法进行放射性检测。在本底为0.1μGy/h、行李移动速度小于0.2m/s条件下，该装置对 ¹³⁷Cs、⁶⁰Co、¹³³Ba核素的可探测活度下限分别为6.7×10³、3.7×10³、6.7×10³Bq，对富集度为90%的U样品，探测质量下限为2g。     

基于剂量监测的船用反应堆破损燃料元件燃耗分析

分析了船用堆燃料元件破损后冷却剂中¹³⁴Cs、¹³⁷Cs的放射性活度与破损燃料元件中¹³⁴Cs、¹³⁷Cs的放射性活度之间的关系,同时也分析了燃料元件中¹³⁴Cs、¹³⁷Cs的放射性活度与燃料元件燃耗之间的关系。由分析得到破损燃料元件燃耗的计算公式,为进一步定位破损元件提供理论依据。     

Estimation of release rate of iodine-131 and cesium-137 from the Fukushima Daiichi nuclear power plant

The atmospheric release rates of I-131 and Cs-137 from the Fukushima Daiichi nuclear power plant in March 2011 were estimated by comparing environmental monitoring data of air concentration and deposition rate on a regional scale with calculated values from an atmospheric dispersion model. Although the release rates were not estimated for all days after 11 March, because of lack of monitoring data, temporal changes in the release rates were reasonably estimated with estimated uncertainties in a factor of 3.3 and 2.9 for I-131 and Cs-137, respectively. A large release was estimated from the night of 14 March to at least the afternoon of 15 March, with maximum values of 7.2 × 10¹⁵ Bq h^?1 for I-131 and 1.5 × 10¹⁴ Bq h^?1 for Cs-137. The release rates during other periods were estimated at one- to two-orders of magnitude smaller than the largest release rate on 15 March. Uncertainty in the estimated release rate for 15 and 20 March was larger than for other periods. The significant release during 14 and 15 March and the trend of the release rate by the end of March were consistent with previous reports. This agreement, despite using different datasets, shows robustness of the temporal changes estimated in the studies.