Power Company Fukushima Daiichi Research Articles

2023 Fukushima Medical University International Symposium on the Fukushima Health Management Survey, "Thinking Together about Health, Life, and our Future in Fukushima".

In response to the release of radioactive materials and evacuation of residents after the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant accident, caused by the Great East Japan Earthquake, the Fukushima Health Management Survey (FHMS) began in June of 2011. This survey aims to provide long-term follow-up of the physical and mental health of Fukushima residents and to maintain and improve their health for the future. Every year since 2019, Fukushima Medical University (FMU) has organized the FHMS International Symposium to share survey results with people in Fukushima Prefecture and beyond. The fifth annual symposium convened at FMU's Ekimae Campus on Saturday, 4 March 2023, with the theme, "Thinking Together about Health, Life and our Future in Fukushima."

Assessing the variability of tissue-free water tritium and non-exchangeable organically bound tritium in pine needles in Fukushima using atmospheric titrated water vapor

When conducting environmental tritium monitoring at nuclear and fusion facilities, it is important to understand how tritium concentrations vary within the soil-plant-atmosphere continuum. Past measurements of organically bound tritium (OBT) concentrations have been conducted from the standpoint of ascertaining the persistence of tritium in terrestrial vegetation, and it has been reported that OBT concentrations fluctuate depending on the influence of atmospheric sources and meteorological conditions. The present study provides information on the variability of tritium concentrations in vegetation growing close to atmospheric sources of tritiated water (HTO) in Japan. We conducted measurements of tissue-free water tritium (TFWT) and non-exchangeable OBT (NE-OBT) in pine needles collected from locations near the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant from 2018 to 2021. The results we obtained indicate that TFWT concentrations fluctuate in response to short-term changes in atmospheric HTO vapor concentrations and that NE-OBT concentrations vary primarily in response to the time-averaged TFWT concentration during the pine needle growing season, which is in turn mainly determined by atmospheric HTO vapor concentrations. Although it is difficult to ascertain the time-averaged TFWT concentration during the growing season by measuring plant materials, our results show that the measurement of atmospheric HTO vapor concentrations provides an effective alternative method of doing this. We believe our findings can lead to better understanding of the potential impact on the terrestrial environment associated with the atmospheric discharge of HTO and can contribute to improving the strategy of environmental tritium monitoring.

Posttraumatic stress response following the loss of significant close others in the Great East Japan Earthquake: Fukushima Health Management Survey.

During the Great East Japan Earthquake, many people experienced the loss of family and friends, among other traumatic events. This study sought to clarify the impact of the loss of significant close others on posttraumatic stress symptoms (PTSS), as well as the factors associated with PTSS, among individuals who experienced the loss of significant close others (i.e., bereaved group). Self-administered questionnaires were mailed to men and women (N = 180,604) aged 16 years and older living in municipalities that included evacuated areas surrounding the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station, which was significantly damaged following an earthquake and tsunami in March 2011, causing subsequent radiation leakage; this mailing yielded a 40.7% response rate, with 57,388 valid responses, in 2012. We used the Posttraumatic Stress Disorder Checklist-Specific (PCL-S) to measure PTSS as the outcome variable. Loss of significant close others in the disaster and respondents' relationship with the deceased constituted the explanatory variables. Basic characteristics and disaster-related factors were the confounding variables. Data were analyzed using chi-square tests and logistic regression analyses. The bereaved group showed more severe PTSS than the nonbereaved group, aOR = 1.58, 95% CI [1.50, 1.67]. The risk of developing PTSS increased if the deceased was a respondent's spouse, aOR = 1.67, 95% CI [1.22, 2.29]; child, 1.51 [1.01, 2.25]; or friend, 1.33 [1.16, 1.53]. Individuals who lose significant close others, including both family and friends, in disasters require close mental health care and, if necessary, should be referred for psychiatric treatment.

PEST Analysis Based on Japan's Nuclear Wastewater Discharge Event

On April 13, 2021, the Japanese government officially issued a statement saying that it will discharge the nuclear waste water stored in the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant into the sea around the spring of 2023 for a period of 30 years. Despite almost overwhelming opposition domestic and abroad, the Japanese government has not withdrawn or delayed the plan. Based on the six stakeholders and the PEST analysis method, this paper deeply discusses the reasons for the Japanese government's unilateral decision to discharge nuclear waste water into the sea. Research shows that Japan's discharge of nuclear waste water is not just an environmental protection issue, it is a consequence of Japan's political, economic, technological, cultural and even historical factors. The purpose of this paper is to provide a reference for effective measures in the future, in the hope of better protecting the legitimate rights and interests of stakeholders by introducing international environmental law to prevent and postpone the discharge of nuclear wastewater from Japan.

An iterative application of the Green's function approach to estimate the time variation in 137Cs release to the atmosphere from the Fukushima Daiichi Nuclear Power Station

The Green's function approach, which is an approach to optimize model parameters, was iteratively applied to a large number of pairs of observed and calculated values of surface air concentrations (677 pairs) and ambient dose rates (3279 pairs) to estimate the time variation in 137Cs release rate to the atmosphere from the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station, which was heavily damaged on March 11, 2011. Regarding surface air concentrations, observations suitable for the optimization were selected not to affect the derived release rates due to defects in the modeling framework rather than the release rates. It was found that the release rates of 97 out of a total of 497 segments with a width of 1 h in the target period were effectively optimized and that the modeled levels using the optimized release rates generally agreed well with the observed ones. The derived time variation was reasonable and consistent with that of preexisting works which were derived using another approach and broadly employed to assess the consequences of the Fukushima accident.

Large-scale sampling and radioactivity analysis of agricultural soil and food during nuclear emergencies in Japan: Variations over time in foodstuffs inspection and sampling

The measurement of radioactivity in food and agricultural ecosystems is an essential task for keeping the population safe after a nuclear emergency. Prior to the Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011, the probability of such an accident, combining complex effects of natural and technical factors, was not reflected in detail in national emergency preparedness guidelines. The lack of such guidelines resulted in a limited efficiency response to the Fukushima accident in agriculture. This outcome showed a need for the preparation of such guidelines as a part of emergency preparedness for nuclear and radiological emergences. This paper provides information and generic, non-country-specific guidance on approaches to sampling food. The paper is intended for scientists, policy makers and decision makers involved in nuclear emergency preparedness and responses, particularly on large scales and at different stages of nuclear emergency based on lessons learned from the FDNPP accident.

Factors relating to anxiety among medical teams dispatched to the Fukushima nuclear power plant disaster

At the time of the accident at Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant, medical teams and radiation experts were solicited from all over Japan by the Japanese government to go to Fukushima Prefecture. While several reports on the incident and its management have been published, there have been no reports on the causes of anxiety felt by the volunteers or problems foreseen by organizations that declined to send volunteers. To identify the factors contributing to the anxiety levels of the dispatched personnel and to understand the difficulties faced while dispatching volunteers to the affected area, two different questionnaires were sent out to the recruited volunteers and recruiting organizations. Among the professionals sent to Fukushima Prefecture, we found that previous knowledge of radiation emergency or disaster medicine was inversely proportional to their anxiety levels (p < 0.001). Factors giving rise to apprehension depended on the professional's occupation (p = 0.042). Possessing and sharing radiological information among team members helped alleviate anxiety once the professionals entered the hazardous area. It is thus desirable to establish systematic plans with guidelines for proactive education and training for each kind of professional and policies regarding the dispatch of volunteered professionals to alleviate anxiety levels prior to dispatch.

Monitoring 137Cs concentrations in bird species occupying different ecological niches; game birds and raptors in Fukushima Prefecture

This study was conducted to assess radiocesium accumulation in birds after the accident at Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station in 2011, with a particular focus on 137Cs, which has a long physical half-life. Results of 137Cs monitoring in four game bird species including two pheasant species and two duck species (copper pheasant, green pheasant, spot-billed duck and mallard) were assessed in Fukushima Prefecture. We also obtained samples from rescued raptors that died during treatment or rehabilitation at the Wildlife Symbiosis Centre in Fukushima Prefecture because of severe injury. We measured the muscle concentrations of 137Cs in four of these raptor species (black kite, northern goshawk, peregrine falcon and ural owl). Comparison of the two pheasant species showed that the copper pheasants inhabiting forested areas had higher 137Cs concentrations in muscle (Bq/kg, fresh mass) than the green pheasants inhabiting mountainous areas near human habitation (i.e., Satoyama). No clear tendencies were observed in 137Cs concentration in muscle of copper pheasants over time, but a tendency to decrease was observed in green pheasants over time. The difference in tendencies between species may be attributable to differences in their food habits and its 137Cs concentration, and also differences in the situation of 137Cs accumulation in their habitat. No significant differences were observed in 137Cs concentration in muscle between the resident spot-billed duck and migratory mallard because of the comparatively short biological of effective half-life of radiocesium. Analysis of 137Cs concentration in muscle of raptor revealed that the concentration was similar to, or lower than, those of pheasants and ducks.

Early Intake of Radiocesium by Residents Living Near the Tepco Fukushima Dai-ichi Nuclear Power Plant After the Accident. Part 2: Relationship Between Internal Dose and Evacuation Behavior in Individuals.

The Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident exposed members of the public to radiation. This study analyses the relation between personal behavior data obtained from 112 out of 174 subjects who underwent whole-body measurements by the National Institute of Radiological Sciences (NIRS) during the period from 27 June to 28 July 2011 and their committed effective doses (CEDs) from Cs and Cs. The whereabouts of the 112 persons living in municipalities near the FDNPP (mainly, Namie town) on several days in March 2011 are graphed on maps. It was confirmed that most subjects started evacuation promptly and had left the 20-km-radius of the FDNPP by the end of 12 March. The individual CEDs were poorly correlated with the person's distances from the FDNPP at any day in March. Meanwhile, the percentage of persons remaining within the 20-km radius of the FDNPP was 100% at 16:00 on 12 March and 42.9% at 0:00 on 15 March for those with CEDs > 0.1 mSv, whereas the corresponding values were much lower for those with CEDs ≤ 0.1 mSv. This suggests that the time of evacuation would be one of the crucial factors for the early intake; however, more personal behavior data are needed to be analyzed to clarify the relevance to the individual internal dose.

Using two detectors concurrently to monitor ambient dose equivalent rates in vehicle surveys of radiocesium contaminated land

In response to the accident at Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP), vehicle-borne monitoring was used to map radiation levels for radiological protection of the public. By convention measurements from vehicle-borne surveys are converted to the ambient dose equivalent rate at 1 m height in the absence of the vehicle. This allows for comparison with results from other types of survey, including surveys with hand-held or airborne instruments. To improve the accuracy of the converted results from vehicle-borne surveys, we investigated combining measurements from two detectors mounted on the vehicle at different heights above the ground. A dual-detector setup was added to a JAEA monitoring car and compared against hand-held survey meter measurements in Fukushima Prefecture. The results obtained by combining measurements from two detectors were within ±20% of the hand-held reference measurements. The mean absolute percentage deviation from the reference measurements was 7.2%. The combined results from the two detectors were more accurate than those from either the roof-mounted detector, or the detector inside the vehicle, taken alone. One issue with vehicle-borne surveys is that ambient dose equivalent rates above roads are not necessarily representative of adjacent areas. This is because radiocesium is often deficient on asphalt surfaces, as it is easily scrubbed off by rain, wind and vehicle tires. To tackle this issue, we investigated mounting heights for vehicle-borne detectors using Monte Carlo gamma-ray simulations. When radiocesium is deficient on a road compared to the adjacent land, mounting detectors high on vehicles yields results closer to the values adjacent to the road. The ratio of ambient dose equivalent rates reported by detectors mounted at different heights in a dual-detector setup indicates whether radiocesium is deficient on the road compared to the adjacent land.

The Japan Environment and Children’s Study (JECS) in Fukushima Prefecture-A progress report on the enrollment stage

The Japan Environment and Children's Study is an ongoing nationwide birth cohort study that is being conducted at 15 regional centers throughout Japan. The recruitment of subjects in the study area within Fukushima Prefecture, which includes Fukushima City, Minami Soma City and Futaba County, was begun on January 31, 2011 with the cooperation of the obstetrics and gynecology departments of local medical institutions. On March 11, soon after the start of recruitment, the Tohoku region was hit by an unprecedented disaster in the shape of the Great East Japan Earthquake, which was closely followed by the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant accident. As a result of the disaster, the study area in Fukushima Prefecture was expanded on October 1, 2012 to include 59 municipalities across Fukushima Prefecture. Three points are seen as particularly important: 1) Radiation risk communication, 2) The motto is "Be attentive. Be supportive.", and 3) Establishing cooperating partnerships. With the cooperation of all concerned, the recruitment period ended on March 31, 2014. The tentative total number of the participants enrolled at the Fukushima Regional Center was 34,666 (13,134 pregnant mothers, 8,695 fathers and 12,837 children born before November 30, 2014 as of June 2016).

Climate change and safety at work with ionizing radiations.

The accident at Tokyo Electric Power Company's (TEPCO's) Fukushima Daiichi nuclear power plant (NPP) has been one of the dominant topic in nuclear safety and it has brought new attention on the matter of accidents in NPPs due to external events related to natural causes. Climate change has risen new risks and the growing probability of extreme external events has increased exposure and vulnerability of workers in the nuclear sector. However extreme natural events are a threat not only to NPPs but to all facilities dealing with radioactive material and in an emergency scenario they can affect the effectiveness and implementation of safety devices and procedures and also prevent communications, causing delays in the readiness of response. It is clear that adaptation strategies are necessary to cope with emerging changes in climate and a new nuclear safety culture is growing, that addresses accidents initiated not only by internal but also by external events.

Early Intake of Radiocesium by Residents Living Near the TEPCO Fukushima Dai-Ichi Nuclear Power Plant after the Accident. Part 1: Internal Doses Based on Whole-body Measurements by NIRS.

The Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011 resulted in a release of radionuclides into the environment (I: 142.9 PBq, Cs:12.4 PBq). This study presents the results of internal doses to 174 residents living near the FDNPP at the time of the accident based on whole-body (WB) measurements performed by the National Institute of Radiological Sciences (NIRS) during the period between 27 June and 28 July 2011. The 174 subjects consisted of 125 adults (≥18-y) and 49 children (<18-y) and included 90 persons of Namie town, one of the municipalities heavily contaminated with the radionuclides. The number of subjects with significant detection of both Cs and Cs was relatively small: 28.8% for the adults and 4.1% for the children. A significant gender difference in the Cs detection rate (males > females) was observed in the adults but not the children. In this study, the committed effective dose (CED) from Cs and Cs was calculated based on individual WB contents (Cs) corrected against body size, the observed body content ratio of Cs to Cs, and the assumed intake scenario (namely, acute inhalation of Type F compounds on 12 March 2011 when the first explosive event occurred at the site of the FDNPP). The 90th-percentile CED value for the adults was around 0.1 mSv and the maximum CED (0.63 mSv) was found in an elderly male. Comparable CED results were obtained in other WB measurements subsequently performed by the Japan Atomic Energy Agency (JAEA) in a similar manner to that of the NIRS, suggesting that the contribution of ingestion to the WB content observed would be trivial for most of the JAEA subjects. The intake ratio of I to Cs was evaluated to be 3~5 based on the I thyroid measurement data of Tokonami et al. Using the average intake ratio of 3.8, the resulting median and maximum thyroid-equivalent doses to the adult subjects of this study were estimated at 3.5 mSv and 84 mSv, respectively.

Benchmark Study of the Accident at the Fukushima Daiichi NPS: Best-Estimate Case Comparison

The Great East Japan earthquake occurred on March 11, 2011, at 14:46, and the subsequent tsunami led Tokyo Electric Power Company's (TEPCO's) Fukushima Daiichi Nuclear Power Station (NPS) beyond a design-basis accident. After the accident, the Japanese government and TEPCO compiled a roadmap toward an early resolution to the accident including, among the main activities, the employment and improvement of existing severe accident (SA) computer codes. In the member countries of the Organisation for Economic Co-operation and Development/Nuclear Energy Agency (OECD/NEA), SA codes were developed after the accident at Three Mile Island Unit 2 and widely employed to assess NPS status in the postulated SA conditions. Therefore, working plans have been set up with the country members of the OECD/NEA to apply existing SA codes to analyze the accidents at the Fukushima Daiichi NPS Units 1, 2, and 3 and support the decommissioning, constituting an international program named Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Station (BSAF).The objectives of the BSAF project are to analyze the accident progression of Fukushima Daiichi NPS, to raise the understanding of SA phenomena, to contribute to the improvement of the methods and models of the SA codes, and to define the status of the distribution of debris in the reactor pressure vessels and primary containment vessels for decommissioning.The present technical paper summarizes the achievements obtained through a comparison of the results, emphasizing the portions of the accident where all the participants reached a common consensus and identifying still open questions where future work should be directed. Consensus exists on the current condition of Unit 1, where a large fraction of the fuel is assumed to have relocated ex-vessel. On the other hand, larger uncertainties exist for Units 2 and 3, where in-vessel and ex-vessel scenarios produce a reasonable prediction of the accident progression.

Four‐year decline in radioactive cesium transfer to perennial Gramineae candidate bioenergy crops in a field polluted by radioactive fallout from the Fukushima Daiichi Nuclear Power Plant in 2011

AbstractThe accident caused at the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant by the Great East Japan Earthquake of 11 March 2011 resulted in radioactive fallout over a wide area of eastern Japan. Production of bioenergy crops and their conversion to energy may be one way to resume agricultural production at an early date and to put renewable energy to practical use. We examined yearly changes in radioactive cesium concentrations in the harvested parts of six perennial Gramineae candidate bioenergy crops and the distribution of the cesium in plant organs and rhizosphere soil layers under no‐till conditions in an experimental field 112 km from the nuclear power plant. Average radioactivity of the sum of cesium‐134 and cesium‐137 in the harvested parts (foliage) of the six bioenergy crops on the day of harvest declined considerably from 193.3 Bq kg−1 in 2011 to 31.6% of the 2011 level in 2012, 28.8% in 2013 and 28.7% in 2014. Radioactivity was higher in the underground plant parts (subterranean stem and root) than in the aboveground parts (foliage and stubble). In 2014, radioactivity in the untilled rhizosphere soil was higher in the shallow layer (0–5 cm from the soil surface) than in the deeper layers, indicating that rainfall and perennial bioenergy crop cultivation for more than 3 years had a limited effect on the soil distribution of the fallout. These results provide unique and specific information on radioactive cesium transfer to bioenergy crops and could help in the planning of feedstock supplies in radiation‐polluted areas.

Influence of water management and fertilizer application on 137Cs and 133Cs uptake in paddy rice fields

Cesium-137 derived from the Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident contaminated large areas of agricultural land in Eastern Japan. Previous studies before the accident have indicated that flooding enhances radiocesium uptake in rice fields. We investigated the influence of water management in combination with fertilizers on 137Cs concentrations in rice plants at two fields in southern Ibaraki Prefecture. Stable Cs (133Cs) in the plants was also determined as an analogue for predicting 137Cs behavior after long-term aging of soil 137Cs. The experimental periods comprised 3 y starting from 2012 in one field, and 2 y from 2013 in another field. These fields were divided into three water management sections: a long-flooding section without midsummer drainage, and medial-flooding, and short-flooding sections with one- or two-week midsummer drainage and earlier end of flooding than the long-flooding section. Six or four types of fertilizer subsections (most differing only in potassium application) were nested in each water management section. Generally, the long-flooding treatment led to higher 137Cs and 133Cs concentrations in both straw and brown rice than medial- and short-flooding treatments, although there were some notable exceptions in the first experimental year at each site. Effects of differing potassium fertilizer treatments were cumulative; the effects on 137Cs and 133Cs concentrations in rice plants were not obvious in 2012 and 2013, but in 2014, these concentrations were highest where potassium fertilizer had been absent and lowest where basal dressings of K had been tripled. The relationship between 137Cs and 133Cs in rice plants was not correlative in the first experimental year at each site, but correlation became evident in the subsequent year(s). This study demonstrates a novel finding that omitting midsummer drainage and/or delaying drainage during the grain-filling period enhances uptake of both 137Cs and 133Cs.

Classification method of severe accident condition for the development of severe accident instrumentation and monitoring system in nuclear power plant

Instrumentation and monitoring systems in a nuclear power plant are very important to monitor plant conditions for safe operations and a plant shutdown. The severe accident at TOKYO ELECTRIC POWER COMPANY's Fukushima Daiichi Nuclear Power Station (hereinafter called as TF1) in March 2011 caused several severe situations such as core damage, hydrogen explosion, etc. Lessons learned from the severe accident at TF1 show that an appropriate operable instrumentation and monitoring system for a severe accident should be developed so that the system will deliver an appropriate performance for mitigation of severe accident condition in a nuclear power plant.This paper proposes the classification method of severe accident condition for the development of an appropriate operable instrumentation and monitoring system for a severe accident based on the problem analysis of monitoring variables during the severe accident at TF1. The classification is formed on the basis of the integrity of boundary for plant safety and the successful (or unsuccessful) condition of the cooling water injection, and is used for an establishment of defining severe accident environmental conditions for the instrumentation and monitoring system. Examples of the establishment method are also shown in this paper.

THE BASIC DATA FOR RESIDENTS AGED 15 YEARS OR YOUNGER WHO RECEIVED A COMPREHENSIVE HEALTH CHECK IN 2011-2012 AS A PART OF THE FUKUSHIMA HEALTH MANAGEMENT SURVEY AFTER THE GREAT EAST JAPAN EARTHQUAKE.

To assist in the long-term health management of residents and evaluate the health impacts after the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant accident in Fukushima Prefecture, the Fukushima prefectural government decided to implement the Fukushima Health Management Survey. This report describes the results for residents aged 15 years or younger who received health checks and evaluates the data obtained from 2011 and 2012. The target group consisted of residents aged 15 years or younger who had lived in the evacuation zone. The health checks were performed on receipt of an application from any of the residents. The checks, which included the measurements of height, weight, blood pressure, biochemical laboratory findings, and peripheral blood findings, were performed as required. 1) A total of 17,934 (64.5%) and 11,780 (43.5%) residents aged 15 years or younger received health checks in 2011 and 2012, respectively. 2) In both years, a number of male and female residents in the 7-15 year age group were found to suffer from obesity, hyperlipidemia, hyperuricemia, or liver dysfunction. Furthermore, pediatric aged 15 years or younger were commonly observed to suffer from hypertension or glucose metabolic abnormalities. 3) A comparison of data from 2012 and 2011 demonstrated that both males and females frequently showed increased body height and decreased body weight in 2012. The prevalence of hypertension, glucose metabolic abnormalities, or high γ-GTP values in males and females in the 7-15 year age group in 2012 was lower than that in 2011. However, the prevalence of hyperuricemia among residents in the 7-15 year age group was higher in 2012 than in 2011. These results suggested that some residents aged 15 years or under who had lived in the evacuation zone had developed obesity, hyperlipidemia, hyperuricemia, liver dysfunction, hypertension, or glucose metabolic abnormalities. Therefore, we think that it is necessary to continue the health checks for these residents in order to ameliorate these lifestyle-related diseases.

Effective half-lives of 137Cs from persimmon tree tissue parts in Japan after Fukushima Dai-ichi Nuclear Power Plant accident

To estimate the radiocesium decreasing rates from persimmon trees during a period of about 3 y following the accident at Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we conducted measurements of tree tissue parts collected in 2011–2013. The sampling was carried out in Chiba, 220 km south of FDNPP; radioactive fallouts discharged from FDNPP had mainly been observed in March–April 2011 on the sampling site. We measured 137Cs concentrations in the tree tissue parts, i.e., fruits (flesh, skin and seeds), leaves and newly emerged branches, and then the effective half-lives (Teff) of 137Cs were calculated. Leaf samples were classified into two types by sampling months according to the growing stages, that is, immature (April–May) and mature (June–November) leaves. All these parts showed exponential declines in 137Cs concentration with good adjusted contribution ratios of higher than ca. 0.7. The calculated Teff values from all tissue parts were similar with the average of 229 d (range: 216–243 d). From these results, we concluded that each tree tissue was representative for the calculation of Teff. For comparison to these observation results, open source food monitoring data from 2011 to 2013 including 137Cs data for persimmon fruits collected in Fukushima Prefecture were used to calculate Teff for persimmon trees. Values of Teff were obtained for persimmon fruits grown in each local government area in Fukushima Prefecture and they ranged from 303 to 475 d.

Analyses and countermeasures for decreasing radioactive cesium in buckwheat in areas affected by the nuclear accident in 2011

Tokyo Electric Power Company's Fukushima Dai-ichi nuclear power plant released radioactive cesium (Cs) into the environment after the Great East Japan Earthquake of March 11, 2011. After radioactive Cs fell onto agricultural fields, radioactive nuclide levels in some buckwheat (Fagopyrum esculentum Moench) grain subsequently exceeded new standard limits for radioactive materials, including that for radioactive Cs (100Bqkg−1) established by the Japanese government in 2012. A survey of soils and buckwheat grain from 68 farmers’ fields in 2012 revealed that soil exchangeable potassium (K) concentration was significantly and negatively correlated with radioactive Cs concentration in the grain. The effect of K application on the reduction of radioactive Cs concentration in the grain was confirmed by pot and field experiments conducted respectively in 2012 and 2013. This effect might result from the similarity of ion forms of K and Cs and/or the lower exchangeable Cs concentrations at higher exchangeable K concentrations in soil. Based on these results, farmers were recommended to establish a soil exchangeable K concentration of 250mgkg−1 (300mgkg−1 in K2O) before applying basal fertilizer. After this recommendation, the soil K concentrations of farmers’ fields increased, causing an overall decrease in radioactive Cs concentrations of buckwheat grain produced in 2013. Consequently, no grain sample from this year exceeded the standard limits for radioactive Cs level across the affected area in Japan. These efforts contributed greatly to the rehabilitation and reconstruction of agriculture in the area contaminated with radioactive Cs.