Analysis of the Dose Commitments Resulting from Atmospheric Transport and Deposition from Nuclear Risk Sites in the Russian Far East

The purpose of this study was to estimate the worst-case case dose commitments and potential consequences of accidental releases at nuclear risk sites in the Russian Far East. The nuclear risk sites of concern are near Petropavlovsk (52055'N & 158030'E) and Vladivostok (42055'N & 132025'E). The region of interest includes the territories of the Russian Far East, China, Japan, North and South Korea, State of Alaska, the Aleutian Islands, Mongolia, Burma, Hong Kong, Laos, Taiwan, Thailand, and Vietnam. The transboundary region (i.e., that outside of Russia) is of primary interest because the largest doses resulting from hypothetical releases from these sites would reside in Russia and would be examined using site specific information and detailed models that were unavailable for this study. However, the transboundary region can be examined, in general, using existing information and models. The The methodology from the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 1993 Report was used in this study to estimate effective dose commitments. It is recognized that this methodology is not the only acceptable manner to estimate such doses; the methodology was selected because it is independent, defensible, and, because it is based upon a multiplicative model, lends itself to a facile examination of parameter variation. The research tool used to generate the deposition data used as the basis of this study was a long-range transport model - the Danish Emergency Response Model of the Atmosphere (DERMA) which was used to simulate the 5-d atmospheric transport, dispersion and deposition of Cs-137 for a one-day release at a rate of 10Bq s-1 for a total "unit hypothetical release" of 8.64x10 14 Bq. The meteorological data from the European Center for Medium- Range Weather Forecasts (ECMWF, Reading, UK) based on the ECMWF global model forecast and analysis were used as input data for the model simulation. Using the DERMA model, the total Cs-137 depositions (i.e., sums of pertinent dry and wet deposition values) were computed for over 90% of the days in calendar year 2000. The necessary meteorological data was missing for the remaining days. In this report, Sr-90 and I-131 were radionuclides that might also have been of concern depending upon the conditions of the study. However, because of a lack of time and resources, the deposition values were not computed for these radionuclides for all calendar year 2000 days. There were Sr-90 and I-131 concentration and deposition data provided for selected days that were considered representative of the variation of the climactic condtions for the region for the year 2000. These data were used to generate simple, linear linear relationships between the unavailable Sr-90 and I-131 concentration and deposition data and the corresponding Cs-137 data. These relationships were found to be sufficiently accurate for the general examination undertaken in this report and were used to generate the necessary, unavailable data. From an examination of the appropriate source term information and deposition to dose transfer factors from both the UNSCEAR 1993 Report and the worst-case scenario, Cs-137 was determined to be the radionuclide of primary concern for this study. The Cs-137 deposition- to-dose transfer factor was dominated by the external exposure (to ground deposition) pathway. For the Petropavlovsk nuclear risk site, the maximum Cs-137 total deposition (locates in Russia) translated into a worst-case maximum effective dose commitment of 108 mSv per person for the maximum exposed individual (i.e., teen). For the transboundary region (i.e., that area outside of Russia), the maximum effective dose commitment was 5.0 mSv per teen. This maximum value was located in the State of Alaska; the maximum effective dose commitment for the Aleutian Islands was 3.3 mSv per teen. The maximum effective dose commitments in the effected U.S. territories were generally three to four times higher than those in Japan, the transboundary country with the next highest maximum dose commitments resulting from accidental releases from the Petropavlovsk risk site. For the Vladivostok nuclear risk site, the maximum Cs-137 137 total deposition (located again in Russia) translated into a worst-case maximum effective dose commitment of 102 mSv per teen. For the transboundary region, the maximum effective dose commitment for Cs-137 was 27 mSv per teen. These maximum values were located in China, which is proximate to the Vladivostok site. The maximum effective dose commitments for Japan and N. Korea are approximately the same (i.e., within a factor of two) as that for China. Note that the maximum effective dose commitments in the U.S. territories are generally more than a factor of 60 lower than those in China, the transboundary country with the highest maximum dose commitments resulting from accidental releases from the Vladivostok risk site. The maximum worst-case dose commitments corresponding to the potential Petropavlovsk and Vladivostok releases for both the regional and transboundary conditions were also compared to various annual reference levels (i.e., 0.15, 1.0, 10, and 100 mSv per person) discussed in the International Commission on Radiological Protection (ICRP) 82 Report pertaining to practices and interventions and the annual background radiation dose (i.e., 2.4 mSv per person) provided in the UNSCEAR 1993 Report. These comparisons were conservative because the effective dose commitments computed in this report are being compared to annual reference values and background doses. The worst-case maximum dose commitments from the Petropavlovsk site for the transboundary region on over 99% of all year 2000 days studied are less than the average annual background radiation dose. For the Vladivostok releases, the worst-case maximum dose commitments are less than the average annual background radiation dose for more than 44% of all year 2000 days studied. Furthermore, the maximum dose commitments corresponding to the Vladivostok releases for more than 90% of the year 2000 days studied are less than the annual 10 mSv per person level in which interventions are rarely justified and are all less than the annual 100 mSv per person level in which interventions are almost always justifiable according to ICRP 82. Therefore, the impacts from the adjusted Vladivostok releases would be, in general, more significant than those from Petropavlovsk (even though the Petropavlovsk releases translate into the maximum, worst-case dose commitment). The more significant impacts of the potential Vladivostok releases were compounded by the fact that many more people were impacted than from the corresponding hypothetical Petropavlovsk releases. However, the dose commitments from the potential Vladivostok releases could be considered negligible when compared to the 10 mSv per person level in which interventions are rarely justified. The maximum collective dose commitments corresponding to the worst-case dose commitments were also computed. The results indicate that even though the maximum effective dose commitments from the Petropavlovsk and Vladivostok releases were similar, the larger populations impacted by the Vladivostok releases generally resulted in significantly larger collective dose commitments and thus potential mortalities than those for the Petropavlovsk releases. For example, the maximum number of additional mortalities on a regional basis resulting from the worst-case Petropavlovsk scenario would be 355 with as many as 329 in Japan, 83 in China, 18 in the State of Alaska, and 10 in S. Korea. However, for the Vladivostok releases, there could be as many as 9771 additional mortalities on a regional basis, and the additional mortalities for Japan, China, N. Korea, S. Korea, Russia, and Taiwan would be 9501, 8575, 2485, 2436, 1614, and 318, respectively. The U.S. territories and Hong Kong might have an additional two mortalities each. However, even though these mortality numbers may appear large, it should be noted that none of the transboundary values exceed 9 mortalities per 100 000 persons, which is found in N. Korea resulting from the worst-case Vladivostok scenario. Because the aggregation of doses over large areas is contrary to the recommendation of the ICRP, a series of threshold values were imposed on the worst-case results to determine whether the conclusions would change dramatically. The impact on the maximum worst-case collective dose commitments for the Petropavlovsk releases would be significan. For example, if a threshold of 1 mSv per person is imposed on the collective dose computation, then the collective dose commitment for all transboundary areas except for the U.S. territories falls to zero (and this includes Japan, which had the largest collective dose commitment). However, the impact of imposing such thresholds on the collective dose commitments from the Vladivostok releases was much less profound than that for the corresponding Petropavlovsk dose commitments; in fact, the imposition of thresholds up to 1 mSv per person had little impact on the collective dose commitments for most countries in the region of interest. Even though the impact on the collective doses related to the Vladivostok releases was small, it remains true that the worst-case impacts of the effective dose commitments for the releases from both the Vladivostok and Petropavlovsk sites were negligible when compared to metrics such as the average annual background dose and other causes of death in the affected countries