Answer to Question #9039 Submitted to "Ask the Experts"

Category: Nuclear Power, Devices, and Accidents — Nuclear Devices

The following question was answered by an expert in the appropriate field:

Q

It appears that the dosimetry in studies of the survivors of the Hiroshima and Nagasaki atomic bombings in the recent BEIR report includes the immediate effects of the blast and does not include any of the effects of induced radioactivity or fallout. Based on a question and answer I found on your Web site, the dose rate even a few days after the blast could be substantial. At the dose rates mentioned in the response, a dose of between 10-14 mSv could result if someone spent some time in the area after the bombing. Do the BEIR reports and the DS02 system ignore this contribution to overall dose? These doses are in the same range of the "low-dose" portion of the study population delivered by the blast. It would seem to confound the variables.

A

The estimates of radiation exposure risks from studies of the survivors of the Hiroshima and Nagasaki atomic bombings in the recent BEIR report are based on the so-called Life Span Study (LSS) cohort (NRC 2006). This cohort includes both a large proportion of survivors who were within 2.5 km of the hypocenters of the bombs at the time of bombings and a similar-size sample of survivors between 3 and 10 km from the hypocenters, whose initial radiation doses coming directly from the bombs were negligible. For the survivors within 2.5 km of the hypocenters of the bombs, their doses were estimated based on the DS02 system (Cullings et al. 2006; Young and Kerr 2005). The DS02 system provides only an estimate of a survivor's dose from the initial radiations coming directly from the bombs and does not provide an estimate of a survivor's dose from residual radiation due to neutron activation of the soil near the hypocenters of the bombs or radioactive fallout. It was not possible to provide a reassessment of the residual radiation doses because of time limitations placed on the DS02 study, which was made possible through a congressional mandate and a special congressional budget appropriation (Young and Kerr 2005). 
 

External exposures from neutron-induced radionuclides in soil have recently been reassessed based on DS02 calculations as functions of both distance from the hypocenters and elapsed time after the explosions (Imanaka et al. 2008, Imanaka 2010). An exposure rate of 4 and 6 Sv h-1 was estimated at the hypocenter starting at one minute after the explosion at Nagasaki and Hiroshima, respectively. These exposure rates were found to decrease by factors of one thousand after one day and one million after one week. Maximum cumulative exposures at the hypocenter from the time of explosion to infinity were 0.6 Sv at Nagasaki and 1.2 Sv at Hiroshima. The estimated exposures over time also decrease with distance from the hypocenters by a factor of about 10 at 500 m and a factor of 300 to 400 at 1,000 m. Consequently, a significant exposure was received from neutron activation of soil only by those individuals, or so-called early entrants, who entered an area at a distance less than 1,000 m from the hypocenter within a few days after the bombings (Imanaka et al. 2008). It was impossible to track the action of each of the survivors over the days or weeks following the bombings in order to make reliable dose estimates for their exposures to soil activation or fallout.

Results have been reported previously of a study of chromosome aberrations in blood samples collected from a volunteer army who entered Hiroshima one day after the bombing and worked within 1,000 m of the hypocenter for seven days in search and rescue operations (Kadama 1991). Ten control subjects who were the same age and resided in the same area 30 km from the hypocenter were also selected for the study. All individuals in the study were asked for a history of their medical exposures, and the members of the volunteer army or "early entrants" were asked for additional information regarding their activities during their stay at Hiroshima in August 1945. Ten of the volunteer army and two of the 10 controls had not received severe medical exposures and were deemed acceptable for use in the study (Kamada 1991). Five members of the voluntary army were estimated to have received doses of 60 to 130 mSv, and the remaining five volunteer army members and controls were estimated to have received doses of less than 20 mSv (Kamada 1991). Most of the volunteer army members had undergone stomach examinations from one to five times during the previous five years, and the actual radiation doses in 1945 should be less than the estimated doses from this study (Kamada 1991). The radiation doses for four survivors have also been estimated recently by Imanaka and his colleagues using available information on the movements of these survivors in the hypocenter area at Hiroshima within a few days of the bombing on 6 August 1945 (Imanaka 2010). These estimated doses were as follows: 24 mSv (survivor entered on August 6 and 7), 9.4 mSv (survivor entered on August 7 and 8), 3.1 mSv (survivor entered on August 7 and 9), and 2.6 mSv (survivor entered on August 7). Thus, the recently estimated doses for early entry by several Hiroshima survivors into the hypocenter based on DS02 soil-activation calculations agree closely with previous results from the study by Kamada (1991).
 
At Hiroshima, numerous fires broke out within 30 minutes after the explosion, and a massive fire storm formed about two to three hours later (Ishikawa and Swain 1981). A firestorm also occurred in Nagasaki about two hours after the explosion, but it was not as violent as the one in Hiroshima. The totally burned and demolished area in Hiroshima extended out to approximately 2,000 m in all directions (Ishikawa and Swain 1981, Figure 4.2). At Nagasaki, the totally burned and destroyed area extended out approximately 2,000 m toward the north end of the Urakami Valley and 2,500 m toward the south end of the valley (Ishikawa and Swain 1981, Figure 4.3). Because the main residential area of Nagasaki was in an adjoining valley and was protected by a row of hills between the two valleys, the structural damage was confined to an area about three times smaller than that at Hiroshima (Ishikawa and Swain 1981). The fires smoldered for a couple of days and helped to limit the number of people who entered the hypocenter areas immediately after the bombings, particularly at Hiroshima. It would appear that few survivors were likely to have received substantial doses as a result of spending time near the hypocenters in the weeks after the bombings and that the doses from this source are likely to have a low correlation with the doses received from the initial radiations coming directly from the bombs and thus not likely to have much of an impact on risk estimates.  Similar arguments can be made for fallout because the areas of high fallout are not large and do not include many survivors, particularly at Nagasaki.

George D. Kerr, PhD
 

References
Cullings HM, Fujita S, Funamoto S, Grant EJ, Kerr GD, Preston DL. Dose estimation for atomic bomb survivor studies: Its evolution and present status. Radiat. Res. 166(1), Part 2:219-254; 2006.

Imanaka T, Endo S, Tanaka K, Shizuma K. Gamma-ray exposure from neutron-induced radionuclides in soil in Hiroshima and Nagasaki based on DS02 calculations. Radiat. Environ. Biophys. 47(3):331-336; 2008.

Imanaka T. Personal communication. 6 May 2010.

Ishikawa E, Swain DL. Hiroshima and Nagasaki - The physical, medical, and social effects of the atomic bombings. Tokyo: Iwanami Shoten, Publishers; 1981.

Kamada N. Chromosome abnormality in early entrants. J. Radiat. Res. 32 (Toyko), Suppl.:275-277; 1991. Available at: http://www.jstage.jst.go.jp/browse/jrr/. Accessed 19 August 2010.

National Research Council. Health risks from exposure to low levels of ionizing radiation - BEIR VII Phase 2. Washington, DC: The National Academies Press; 2006.

Young RW, Kerr GD. Reassessment of the atomic bomb radiation dosimetry for Hiroshima and Nagasaki - Dosimetry System 2002 (DS02). Hiroshima: Radiation Effects Research Foundation; 2005. Available at: http://www.rerf.jp/shared/ds02/index.html. Accessed 19 August 2010.

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