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Answer to Question #1574 Submitted to "Ask the Experts"Category: Doses and Dose Calculations — External dose calculations The following question was answered by an expert in the appropriate field: Q
How much radiation would a person be exposed to if they were in Hiroshima three days after the atomic bomb was dropped and were within 300-400 meters from the hypocenter? Would this be significant enough to induce cancer?
A
Due to neutron-induced activation of soil and building materials, one could receive some dose from gamma rays near the hypocenters of the bombs in Hiroshima and Nagasaki in weeks following the bombings. These exposures from radioactivity induced directly by bomb neutrons represent a mechanism that is distinct from radioactive fallout, which had little impact on areas near the hypocenters, but could have resulted in small exposures in some more distal areas. Both induced radioactivity and fallout in Hiroshima and Nagasaki are discussed at length in Chapter 6 of the DS86 Final Report (DS86 1987). Appendix 2 of that chapter gives tables and graphs that allow calculation of the estimated dose received at one meter above ground as a function of time and distance from the hypocenter for either city. This reference is available online at RERF's Web site. The index for Appendix 2 is currently located at this URL.
(The resolution of the plots is somewhat limited in the present online version. As an aid to reading them, please note that in Fig. 7 the X-axis ranges from 0 km ground distance on the left to 3 km on the right, linear scale. The Y-axis ranges from 10-7 cGy on the bottom to 103 on the top in logarithmic decades, that is, from 10-9 Sv to 10 Sv. In Fig. 8 the X-axis ranges from 10-2 hours on the left to 105 hours on the right in logarithmic decades. The Y-axis ranges from 10-2 at the bottom to 100 = 1 at the top in two logarithmic decades.) According to Table 3 of Appendix 2 the estimated dose-equivalent rates three days after the bombing in Hiroshima are 480 µSv/hr at 300 m and 140 µSv/hr at 500 m from the hypocenter. The temporal decline in dose-equivalent rates is a step-like combination of decay curves that was determined by the decay rates for the combination of the activation product radionuclides produced in soil by the bomb neutrons. The dose-equivalent rate at 300 m is estimated as 7.1 µSv/hr one week after the bombing but has decreased to 1.2 µSv/hr after one month. The worldwide average dose-equivalent rate due to natural background from cosmic rays and terrestrial radionuclides at the surface of the earth is estimated to be about 0.1 µSv/hr according to a contemporary reference (UNSCEAR 2000). According to Table 3, this level of dose-equivalent rate due to bomb-induced soil radioactivity was reached at about ten months after the bombing in Hiroshima. Cumulative dose-equivalents after the bombing in Hiroshima can be estimated by using Figures 7 and 8 in the same Appendix 2. Fig. 7 can be used to determine the infinite-time dose-equivalent Dinf(d) (dose-equivalent received for continuous exposure, integrated from the time of the bomb detonation until infinity) as a function of distance, while Fig. 8 indicates the fraction ft+ of that total infinite-time dose-equivalent that is received after time t, that is, starting any specified time t after the bombing and integrating from t to infinity with respect to time. We estimate the dose-equivalent received at distance d in a specified time interval from t1 to t2, or D(d,t1,t2), t1 < t2, as D(d,t1,t2) = Dinf(d) * (ftl+ – ft2+). As an example, suppose we want to calculate the dose-equivalent received for a two-week stay beginning three days after the bombing. For a distance of 300 m the infinite time dose-equivalent estimate Dinf(300) is 600 mSv, while for 400 m this value is 300 mSv. Specifying t in days and d in m, we would calculate the dose-equivalent at 300 m as Dinf(300) * (f3+ – f17+), which is approximately equal to (600 mSv)*(0.110 – 0.098) = 7.2 mSv. The dose-equivalent estimate at 400 m is about half this, or 3.6 mSv. Using risk coefficients such as those published by NAS/BEIR (1990) or UNSCEAR (2000), one finds that the estimated lifetime risk for solid cancers associated with doses of this magnitude would be on the order of 1 case per 1,000 people exposed to a dose equivalent of 100 mSv, or 1 case per 10,000 people at 10 mSv. One might further observe that it is unlikely for a person to have spent 24 hours a day in a location so close to the hypocenter so soon after the bombing, so that the actual dose received would probably need to be corrected downward for an occupancy factor that might at most be on the order of 33% for eight hours a day or 50% for 12 hours per day. Radiation Effects Research Foundation Dr. Shoichiro Fujita Dr. Dale Preston Harry M. Cullings, PhD References
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