III. HEALTH PHYSICS

The American Nuclear Society Special Committee on Fukushima (the Committee) collected information that has been published in the open literature for the radiation exposure of workers, the release and deposition of radioactive materials over a wide area surrounding the Fukushima Daiichi NPS, and the contamination of water and food sources. It is important to note that data collection and analyses continue as this report is being written. It is too early to make any firm conclusions regarding these data and the definitive health impacts to workers or to members of the public. While these data do suggest that off-site health consequences may be minimal, it will take much longer to confirm the health impacts. These data do indicate that exposures to workers were significant during the first days of the emergency. Worker exposure controls were put into place, and with carefully planned worker protection practices during the recovery phase, exposures are being controlled. Measureable radioactive materials, mainly iodine-131 (131I), cesium-134 (134Cs), and cesium-137 (137Cs), were identified in public water supplies as well as in certain land areas. After peaking in middle to late March, the concentration of these radionuclides trended significantly downward, with cesium as the main concern.

III.A. On-Site Worker Dose

The Tokyo Electric Power Company (TEPCO) has been monitoring emergency workers for external dose throughout the accident and its aftermath. TEPCO has also performed whole-body counting on each worker to derive his or her internal dose. Over the period of time from March 2011 through July 2011, approximately 14,841 TEPCO employees and contractors were monitored. Slight discrepancies in the reported number of workers monitored are due to a handful of individuals where external and internal dose results are not both available.

The standard worker dose limit for Japanese workers is 50 mSv/year and 100 mSv over 5 years. Before the accident, the emergency dose limit was set at 100 mSv/year but was raised to 250 mSv/year to allow workers to respond to this serious accident.

As of the most recent monitoring period, no observable health effects have been reported in any of the workers. It should be noted that acute health effects are not expected at these doses to workers, although all are being closely monitored. For chronic health effects above 0.1 Sv (100 mSv), the cancer risk can be approximated as increasing by 10%/Sv (using the regulatory accepted linear no-threshold dose model used in radiation protection). For example, the occupational worker who received a dose of 0.1 Sv (100 mSv) has a 1% increased risk of developing a cancer later in his or her life. Estimating cancer risks to the general public is complicated by the low dose rates outside of the NPS and significant overall cancer rates from various environmental factors [2].

The maximum external dose recorded is 199 mSv (0.19 Sv), and the maximum internal dose that has been calculated is 590 mSv (0.59 Sv). The maximum total dose recorded to one worker was 670 mSv, and six workers have received doses in excess of the emergency dose limits established. Although 408 workers have received doses above the normal annual limit of 50 mSv, the average dose for emergency workers is still relatively low and has decreased steadily during the months following the accident. For workers performing emergency work since March, the average total accumulated dose is 22.4 mSv. For the months April through July, the average dose is <4 mSv. The total collective dose for all emergency workers is estimated to be 115 person-Sv. In addition to whole-body doses, two male employees received significant skin dose while laying electric cables from standing in contaminated water that flooded their boots. Their estimated skin dose was ~2 to 3 Sv.

The actions in the immediate aftermath of the accident on March 11 resulted in doses to a handful of workers in excess of established limits and elevated doses to a larger group, as noted above. Since that time, TEPCO has been improving the working conditions and safety measures for its workers. All TEPCO workers are required to wear protective clothing, gloves, and protection masks. TEPCO has established contamination-free rest areas throughout the NPS; installed water coolers; and introduced a “cool vest,” which aims to protect workers from heat exhaustion. Currently, seven designated rest areas have been created, and four additional rest areas are in preparation. Also, improvements in living conditions have been made at the gymnasium that houses these workers.

III.B. Off-Site Doses

At this time there are not enough data collected and publically reported by the Japanese government or the IAEA to reach any definitive conclusions on off-site health effects. The doses received by members of the public have come from four different pathways:

submersion dose from airborne radioactivity

inhalation dose from airborne radioactivity

consumption of contaminated water and foodstuffs

direct exposure from contaminated surface deposition.

The first two of these items cannot be measured retrospectively but can only be predicted from dispersion modeling. A few crude dispersion models have been made public, but no validated models have been made available for review to date in the United States. Airborne radioactivity is transitory, and the dose from inhalation is many times greater than the submersion dose for all but the inert gases.

Food and water contamination has been documented through extensive measurements. Most contaminated foodstuffs have been restricted, but there is no public information regarding their actual level of consumption at this time.

Conversely, the external exposure from ground contamination can be predicted with relative accuracy from the distribution of ground contamination. Using the relative mixture of 134Cs and 137Cs, the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) [3] has calculated the external dose for the first year after the accident at 16.6 mSv per MBq/m2 of total cesium. This is based upon an assumption of 12 hours/day inside structures, where the average dose rate is reduced 70%. A similar estimate was provided by the U.S. Department of Energy (DOE) National Nuclear Security Agency (NNSA). This estimate, although not definitive, suggests that health effects to the public will be minimal.

Japan’s Nuclear Safety Commission provides the latest evaluations of environmental radiation monitoring results [4]. The information provided below comes from the Japanese Health Monitoring Program.

By July more than 210,000 residents had been screened by experts from related organizations, universities, and local governments [5]. Two internal dose assessment surveys were started by Japan’s National Institute of Radiological Sciences (NIRS) and the Japan Atomic Energy Agency (JAEA). NIRS has completed an internal exposure survey on Fukushima Prefectural residents [6]. Initial measurements were taken between June 27 and July 16. The survey focused on residents who lived in areas associated with high doses. A total of 122 participants—90 residents from Namie Town, 20 residents from Iitate Village, and 12 residents from Kawamata Town—were initially enrolled in the survey, and 109 subjects were surveyed in follow-up examinations. Whole-body counters were used to detect activity from 134Cs, 137Cs, and 131I. Urine bioassays were used to determine a cutoff value for the whole-body counter measurements. Cesium-134 was detected in 52 out of 109 people (47.7%), with the highest value being 3,100 Bq. Cesium-137 was detected in 32 out of 109 people (29.4%), with the highest value being 3,800 Bq. Both 134Cs and 137Cs were detected in 26 out of 109 people (23.9%). Iodine-131 was not detected in any subject. Based on this survey, the combined internal dose from 134Cs and 137Cs was <1 mSv (100 mrem) for these individuals. JAEA began internal exposure surveying of 2,800 evacuees on July 11.

Appropriations were made for the “Health Fund for Children and Adults Affected by the Nuclear Accident,” created by Fukushima Prefecture to ensure the health of residents through mid-term and long-term projects. Currently, a two-step plan is being considered. First, a preliminary study began in early July on a sample of about 100 residents who were located in regions of high radiation levels. Those selected will undergo thorough testing for internal radiation contamination. All Fukushima residents will be considered in the primary study. Questionnaires will be distributed to all residents in order to help experts determine the radiation dose received by the residents. The data will be stored for 30 years to conduct follow-up health checks. An estimated 2 million residents need to be monitored. The United Nations Scientific Committee on the Effects of Atomic Radiation has also announced that it will conduct a study on the health impact to Fukushima residents [7].

III.C. Off-Site Contamination

The long-term land contamination off-site is due to the deposition of 134Cs and 137Cs because of their comparatively long half-lives (the half-life of 134Cs is 2.1 years, and the half-life of 137Cs is 30.1 years). The other radionuclides identified as being released have half-lives on the order of less than days or tens of days. The other isotopes of concern from a reactor accident include strontium (strontium-90) and yttrium (yttrium-90) and the actinides, but none of these have been measured in any detectable quantities within or beyond the established evacuation zone.

The initial measurement of ground contamination was performed by the Ministry of Education, Culture, Sports, Science and Technology–Japan (MEXT), with substantial assistance from the DOE NNSA and DOE Office of Nuclear Energy, by measuring exposure levels aboveground using fixed-wing airplanes and helicopter flyovers, extrapolating to the exposure rate at ground level, and converting that value to an area concentration of cesium, given the relative proportions of 134Cs and 137Cs expected. One example is shown in Fig. 8. From several of these maps, isodose/isoconcentration curves are generated, and a map over the entire survey area is produced, as in Fig. 9 from the NNSA. This method has the potential to miss small hot and cold spots in the survey area but provides a reasonable distribution of the deposition of these radionuclides.

A significant number of soil samples throughout the region have been collected and measured with gamma spectroscopy to obtain cesium concentration. A summary map is shown in Fig. 10 (measurements analyzed by Yasunari et al. [9]). This careful work provides a detailed quantification of 137Cs environmental contamination. Such data will be needed to better inform off-site cleanup or remediation activities.

A direct correlation between these various maps has not been completed at this time. But, the patterns observed are quite similar. Using the NNSA maps (Figs. 8 and 9), there is a total land area of ~874 km2 contaminated with 134Cs and 137Cs in a concentration >600 kBq/m2, which is the concentration that is predicted to correspond to a 10-mSv (1-rem) dose in the first year (this includes areas outside the 20-km evacuation zone). We were not able to precisely compare this to the Japanese government relocation land area, but the relocation area is larger.

III.D. Contamination of Foodstuffs

The Committee collected and compiled data for contamination of foodstuffs by 134Cs, 137Cs, and 131I. These detailed data, as well as detailed spreadsheets, are provided at the Japanese Ministry of Health, Labor and Welfare Web site [10]. The data for the period March 19, 2011, through September 21, 2011, indicate that typically <10% of samples were found to contain contamination levels exceeding the provisional regulation limits (regulatory action levels). Actions taken by the Japanese government to restrict consumption of contaminated meats are given in [11].

III.E. Water Monitoring

No data were available regarding the partition between public water supplies and bottled water that was used after the accident. Data do exist for some public water supplies. These data are provided in summary tables at the ANS Web site (http://fukushima.ans.org/). Summary graphs of the tap water measurements taken by MEXT [12] are shown in Fig. 11.

These measured data for public water supplies indicate that radiation levels were falling after March and were trending toward levels below allowable limits.

Early in the accident, radioactive materials were released with water coolant into the sea. The measurements taken near the Fukushima Daiichi NPS indicate these releases were dispersed quite quickly. The details of the movement of these radioactive materials in the fauna and flora of the ocean’s ecosystem are being monitored, and their effect will only become clear after longer-term monitoring and modeling.