5.
DOSIMETRY AND ITS APPLICATION

As stated earlier, radiation dose attributable to fallout contamination of Rongelap Atoll results from external exposure from gamma-emitting radionuclides distributed through the environment and from internal exposure due to the intake of radionuclides through inhalation, ingestion, and absorption through the skin. Owing to the natural processes of weathering and the decay of many elements with short half-lives only five of the radionuclides originally deposited on Rongelap as a consequence of the BRAVO test are still present in sufficient quantities to contribute significantly to the estimated dose to individuals who may return to live on Rongelap (Kercher and Robison, 1993). These are: cesium-137, strontium-90, plutonium-239, plutonium-240, and americium-241. Of these radionuclides, cesium-137 accounts for more than 90% of the projected radiation dose.

Potential radiation doses from exposure to external radiation can be predicted directly from measurements of the radiation field. Prediction of radiation doses from internal sources is more complicated. When radioactive material is taken in by inhalation or ingestion or is presented to the skin, a fraction is absorbed and reaches the blood stream. The absorbed material is then distributed to various organs and tissues. The radioactivity is eventually removed from the fluids, organs, and tissues by radioactive decay and by biological processes. The fraction absorbed, the distribution after absorption, and the rates and pathways of radionuclide metabolism are determined by the elements and chemical forms.

In doses calculated prospectively, intake of each radionuclide is estimated as the product of its concentration in air, food, water, or other medium and the quantity of such medium taken into the body through breathing, ingestion, or skin absorption. The absorption (uptake), distribution, and retention of a radionuclide is estimated from biokinetic models of the element in humans. Radiation dose rates to various organs and tissues are then calculated from the characteristics of the radiation and physical dosimetric models of the transport and absorption of the radiation in the human body. Cumulative (time-integrated) doses can be calculated from the expected residence time of the radionuclide in the body.

After a single intake, the quantities of radioactivity in individual organs (organ burdens) and the whole body (body burden), and hence the associated radiation dose rates, rapidly reach a maximum and then decrease because of radioactive decay and biological elimination. In the case of continuous, chronic exposure, organ and body burdens continue to build up until a maximum is reached at which the increase is balanced by the loss. The burdens (and associated doses) then follow the concentrations in the relevant environmental media. The time to reach this maximum varies with the radionuclide and its chemical form. The maximum occurs earlier for radionuclides with short effective half-lives (as determined by radioactive decay and biological retention time). It occurs later for radionuclides with long effective half-lives. For example, cesium-137 has an effective half-life of about 100 d in the adult human; under constant (or slowly decreasing) continuous chronic intake, the maximum dose rate can



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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands 5. DOSIMETRY AND ITS APPLICATION As stated earlier, radiation dose attributable to fallout contamination of Rongelap Atoll results from external exposure from gamma-emitting radionuclides distributed through the environment and from internal exposure due to the intake of radionuclides through inhalation, ingestion, and absorption through the skin. Owing to the natural processes of weathering and the decay of many elements with short half-lives only five of the radionuclides originally deposited on Rongelap as a consequence of the BRAVO test are still present in sufficient quantities to contribute significantly to the estimated dose to individuals who may return to live on Rongelap (Kercher and Robison, 1993). These are: cesium-137, strontium-90, plutonium-239, plutonium-240, and americium-241. Of these radionuclides, cesium-137 accounts for more than 90% of the projected radiation dose. Potential radiation doses from exposure to external radiation can be predicted directly from measurements of the radiation field. Prediction of radiation doses from internal sources is more complicated. When radioactive material is taken in by inhalation or ingestion or is presented to the skin, a fraction is absorbed and reaches the blood stream. The absorbed material is then distributed to various organs and tissues. The radioactivity is eventually removed from the fluids, organs, and tissues by radioactive decay and by biological processes. The fraction absorbed, the distribution after absorption, and the rates and pathways of radionuclide metabolism are determined by the elements and chemical forms. In doses calculated prospectively, intake of each radionuclide is estimated as the product of its concentration in air, food, water, or other medium and the quantity of such medium taken into the body through breathing, ingestion, or skin absorption. The absorption (uptake), distribution, and retention of a radionuclide is estimated from biokinetic models of the element in humans. Radiation dose rates to various organs and tissues are then calculated from the characteristics of the radiation and physical dosimetric models of the transport and absorption of the radiation in the human body. Cumulative (time-integrated) doses can be calculated from the expected residence time of the radionuclide in the body. After a single intake, the quantities of radioactivity in individual organs (organ burdens) and the whole body (body burden), and hence the associated radiation dose rates, rapidly reach a maximum and then decrease because of radioactive decay and biological elimination. In the case of continuous, chronic exposure, organ and body burdens continue to build up until a maximum is reached at which the increase is balanced by the loss. The burdens (and associated doses) then follow the concentrations in the relevant environmental media. The time to reach this maximum varies with the radionuclide and its chemical form. The maximum occurs earlier for radionuclides with short effective half-lives (as determined by radioactive decay and biological retention time). It occurs later for radionuclides with long effective half-lives. For example, cesium-137 has an effective half-life of about 100 d in the adult human; under constant (or slowly decreasing) continuous chronic intake, the maximum dose rate can

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands be expected to occur within about 2 years. In contrast, a portion of the plutonium in the human body has an effective half-life of around 50 y, and under continuous chronic intake, the body burden (and associated dose rates) would be expected to increase throughout the lifetime of a person. For the purposes of radiation protection, radiation dose is expressed in terms of equivalent dose, which, for a given type of radiation, is the product of the energy deposited (absorbed dose) and the effectiveness of the type of radiation (radiation weighting factor). Doses resulting from nonuniform spatial distribution in the body, as occurs with the selective deposition of radionuclides in various organs or tissues, can be expressed in terms of effective dose, a summation of organ doses in a manner calculated to be biologically equivalent to the dose from a uniform whole-body exposure. Effective dose is calculated by multiplying the individual tissue equivalent doses by specific tissue weighting factors and summing the weighted tissue doses. Once radionuclides enter a person's body, the person is "committed" to the dose resulting from the radioactive decay of the radionuclides for as long as they remain in the body. For assessing radionuclide intakes by members of the public, the International Commission on Radiological Protection (ICRP) Report 56 (ICRP, 1989) recommends calculating committed doses from the time of intake to age 70. In contemporary terminology, the dose commitments from an intake are expressed in terms of the committed equivalent dose to organs and tissues and committed effective dose to the whole body. For the purposes of dose limitation, the committed dose (for 50 y in adults and to age 70 for children) from the annual intake of radionuclides is considered part of that year's radiation dose (ICRP, 1991a). In the international system (SI) of units, equivalent dose, effective dose, committed equivalent dose, and committed effective dose are expressed in sieverts (Sv). Before adoption of SI units, the unit of dose in the traditional system was the rem (1 rem = 0.01 Sv). Reconstruction of past radiation doses and prediction of future doses also involves knowledge, or prediction, of the time-dependent nature of the exposure pattern. In circumstances where no continuous individual record of external exposure or radionuclide intake is available, the exposure pattern must be modeled. A frequently used approach to estimating exposure is to develop scenarios for different exposure patterns, which in turn might include different estimates of intake. The scenarios might be for a "reasonable" exposure, a "maximum" exposure, or any other desired situation. Although it is difficult to verify any individual scenario, reasonable boundary conditions for exposure can be established if several are developed. Dosimetry Approaches External Dose The dose rate from external radiation is readily measurable. Instruments can identify the type, energy, and intensity of radiation. From these measurements, the radiation doses to tissues and organs can be calculated. Alternatively, dose rates can be calculated from the measured or projected concentrations of radionuclides in air or soil. Dose-rate coefficients for these quantities are cited in Federal Guidance Report 12 (EPA, 1993), but age-specific

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands coefficients are not given. Internal Dose As noted earlier, environmental data, physiological-anatomical data, and element-specific biokinetic computer models can be used to estimate body burden and radiation dose at any time after an intake. These element-specific models are used to estimate tissue burdens and associated dose rates after ingestion and (when combined with a respiratory tract dosimetry model) inhalation of radionuclides. The biokinetic models can also be coupled to intake through the skin to estimate organ burdens and dose rates. Many models have been developed to estimate the metabolism of elements. The most generally applicable for dosimetry are those developed by the ICRP. The ICRP biokinetic models have evolved from the simple retention models of ICRP-2 (ICRP, 1959) through the more complex retention models in ICRP-30 (ICRP, 1982, 1988) and to models that include age-specific absorption and organ transfer coefficients, as exemplified in ICRP-56 (ICRP, 1989). Intake by Inhalation and Ingestion The intake from inhaled radionuclides can be estimated from measurements of body burdens or from projections of radionuclide concentrations in air. Measurements of intake or projections based on radionuclide concentrations in the air, water, and soil are used as the basis for calculating radiation dose from ingested radionuclides. For convenience, ICRP has published the results of biokinetic models, for "reference" persons, for radionuclide uptake and retention by the various tissues, factors relating organ burdens to dose rates in various target tissues, and dose coefficients for converting inhalation and ingestion intakes to committed doses in ICRP-30 (ICRP, 1982, 1988). Age-specific biokinetic and dosimetric models were developed later. Calculations for selected radionuclides were performed and inhalation and ingestion dose coefficients (committed dose per unit intake) for generic persons age 3 mo, 1 y, 5 y, 10 y, and 15 y old and adults were published in ICRP-56 (ICRP, 1989). The dose coefficients, although developed for generic models, provide a consistent set of calculations and are recommended for comparison purposes. Estimates of the radiation dose to the uterus and the embryo or fetus from intake of radionuclides for new exposures and for pre-existing conditions have been published as NUREG/CR-5631 (Sikov et al., 1992; Sikov and Hui, 1993) Intake through the skin Intake through the skin can be either through the intact skin or through wounds. Of particular interest in the Marshall Islands resettlement is the potential for uptake from contaminated soil. Uptake through intact skin can be estimated from skin contamination (expected to be a function of skin surface area and skin adherence of contaminants) and uptake fractions (EPA, 1989). For chronic exposure to contaminated soil, skin contamination for an individual of age p can be estimated from

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands where Sc is total-body skin contamination in Bq, Cs is concentration in soil in Bq/kg, Sa is skin adherence of soil in kg/m2 and Ap is age-specific skin area in m2. Daily uptake, Du, in the equilibrium, chronic-exposure situation can then be estimated from where Uf is the daily uptake fraction and Sc is defined in Eq. 8 above. In the case of resettlement of Rongelap, the soil contaminant of interest is plutonium. Studies of plutonium absorption through the skin have usually used plutonium as the nitrate in acid solution. Reported uptake fractions through the skin in humans range from 0.0002%/h (4.8 × 10-5/d) for plutonium nitrate in 0.4 M HNO3 (Langham, 1964; Khodyreve, 1966) to 0.01%/h (2.4 × 10-3/d) for plutonium in 9% HCl (ICRP, 1986). Oakley and Thompson (1956), e.g. as cited in Langham (1964, pp. 565-582), reported that uptake of plutonium nitrate from wounds in animals was 3 times that through intact skin. Overall systemic absorption was less than 1% of the amount applied. In the environmental-contamination scenario, the quantity of soil in wounds would be substantially less than the quantity adhering to the overall skin, so uptake from wounds should not add substantially to the uptake from skin contamination. Uptake of plutonium from abrasion-type wounds should be negligible. Plutonium, particularly plutonium oxide, is trapped in the tissue exudate and immobilized in the eschar. The plutonium will be eliminated when the eschar detaches and drops off (Langham, 1964; Langham et al. 1962). Appraisal of Current DOE Assessments In estimating doses and potential risks to the people of the Marshall Islands, metabolic and dosimetric models have been used to calculate contributions from external sources and from inhalation and ingestion of radionuclides. LLNL personnel have used some of the most current biokinetic models when estimating metabolism and dosimetry for inhaled or ingested radionuclides. The most recent re-evaluation of the potential dose (Robison et al., 1993) uses ICRP-30 (ICRP, 1982, 1988), ICRP-56 (ICRP, 1989), and, for some cases, ICRP-61 (ICRP, 1991b). Complete anatomical and biokinetic factors were not examined, but two indicators—body weight and the biological half-life of cesium-137—do not suggest any serious problems with application of the ICRP models for external and internal dosimetry for the Marshall Islands population. Body weight. Robison et al. (1993) provide data on the weights of males and females from four atolls, including Rongelap. Average values are comparable to those for the reference 70-kg male and 58-kg female given in ICRP-23 (ICRP, 1975).

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands Biological half-life. Robison et al., (1993) quote BNL data on cesium-137 biological half-life in Marshallese (23 males and 21 females). The half-life for males (median, 115 d; mean, 119 d) is not greatly different from the 110 d given for the long-lived component in ICRP-56 (ICRP, 1989). A shorter half-life is reported for females (83 d); this is consistent with the discussion of cesium-137 retention in females given in ICRP-56. The nuclides of primary importance have been cesium-137, strontium-90, plutonium-239, plutonium-240 and americium-241 (Robison et al., 1982, 1993; Kohn, 1989; Robison and Phillips, 1989). Those references and other documents by LLNL personnel have reported estimated doses received by Marshall Islands people for many years. Except where otherwise noted, the following discussion addresses the 1993 LLNL dose assessment for Rongelap Island (Robison et al., 1993). External Dose Estimates of external dose rate have been consistent when repeated measurements have been made (Kohn, 1989, Table 5.1). Gamma-ray exposure rates as a function of location were estimated from an aerial survey made in 1978 and from indoor and outdoor gamma spectroscopy conducted in 1988. The major current contributors to external gamma dose were identified as cesium-137 (more than 99%) and cobalt-60 and corresponding radioactive-decay corrections were applied. The annual dose equivalent to the typical resident of the island was estimated with a scenario involving occupancy-time weighting factors for four indoor and outdoor locations of different radiation levels. A conversion factor of 0.0075 sievert/roentgen (Sv/R) was used to convert measured exposure in air to dose equivalent to the testes. Doses were decay-corrected for the projected resettlement date of 1995, and annual doses and accumulated doses were calculated for each of the next 70 y. The maximum external gamma-ray dose rate was projected to be 0.11 mSv/y (11 mrem/y) in 1995; this will decrease as cesium-137 continues to decay and to migrate in the environment. The source of data is appropriate, and the aerial-survey and ground-based results are consistent with each other when decay-corrected to a common date. The occupancy scenario for Rongelap Island appears reasonable; however, because of the different dose rates on the island, the dose will be lower if less time is spent in the island's interior. The exposure rate from fallout is higher on most other islands of Rongelap Atoll than on Rongelap Island itself (Kohn, 1989), so estimates for external gamma-ray dose for the maximally exposed resident should include consideration of time spent on other islands. For the purposes of projecting the post-resettlement dose, it would have been more appropriate and more consistent to convert measured values to effective dose, using factors such as presented in ICRP-51 (ICRP, 1987), rather than dose to the testes. However, the conversion factors would not be greatly different from the value used, and the results would not be significantly different. On the basis of the data presented, the decay-corrected external gamma dose of 0.11 mSv/y for 1995 appears reasonable. The projected doses for future decades are probably overestimated, inasmuch as the corrections were based only on physical half-life of the radionuclides with no consideration of the effective environmental half-life. The LLNL report points out that the external beta dose would be a shallow dose only and

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands that the beta contribution would add only slightly to the gamma shallow dose. It concludes, considering the relatively low risk-based weighting factor for the skin, that the beta contribution to the total effective dose would be extremely small. This appears to be a reasonable conclusion. Intake and Internal Dose Inhalation Potential inhalation exposure due to resuspension of radionuclide-bearing soil was assessed. Airborne radionuclide concentrations were projected for Rongelap from measured concentrations in Rongelap soil and enhancement factors developed in the resuspension experiments conducted on other Marshall Islands atolls. Because the average concentrations of transuranic radionuclides in surface soils over the entire island were observed to be roughly twice those in the vicinity of the village and housing area, location was an important variable in projecting airborne concentration and inhalation intake. Inhalation intake for the typical Rongelap resident was projected with a scenario involving time weighting for four activity-location combinations. Average plutonium and americium concentrations reported for Rongelap surface soil and the projected average annual intake of these radionuclides are presented in Table 5-1 for the reference year 1995. Concentrations of cesium-137 and strontium-90 in the soil were also measured but these radionuclides were not included in the inhalation dose assessment; the product of the soil concentration and the inhalation dose factors for the various radionuclides indicates that the relative dose contributions of these two will be orders of magnitude less than the dose contributions from plutonium and americium. ICRP dose methods and models were used to convert inhalation intake to dose. Projected intakes of plutonium and americium were presented as fractions of the intake limit corresponding to 1 mSv/y (numerically equivalent to the committed effective dose in millisieverts/year—see Table 5-1). From the information provided, the resuspension experiments appear to be reasonable and add an important dimension to the dose assessment. The algorithm for computing intake accounts for the important factors. The occupancy scenario is difficult to evaluate, but there are no obvious problems. The breathing rates and the distribution between resting and active are similar to, but not identical with, the values for the ICRP reference adult male. The committed effective doses are on the order of a scale of microsieverts/year; annual doses will initially be much lower and will slowly approach this value. There were no age-dependent calculations. Although the use of reference-man breathing patterns should give a conservative estimate of nonadult intake—the ICRP inhalation model is the same (ICRP, 1989)—an age-dependent scaling of breathing rates would have given a better approximation. Comparison of the 70-y effective doses calculated for children for plutonium and americium with those for adults in ICRP-56 suggests that the dose rates for americium might go down by a factor of about 2, but those for plutonium might go up by a factor of 3. Uncertainties in inhalation deposition and dose by nuclide were not calculated.

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands Table 5-1 Surface-Soil Radioactivity, Projected Inhalation Intakes and Projected Inhalation Dose of Selected Radionuclidesa   Plutonium Americium Soil concentration Bq/g (pCi/g) Bq/g (pCi/g) Village mean 0.063 (1.7) 0.046 (1.2) Island mean 0.13 (3.5) 0.097 (2.6) Average individual inhalation intake   Bq/y (pCi/y) Bq/y (pCi/y)   0.044 (1.2) 0.023 (0.62) Average individual committed effective dose   mSv/y (mrem/y) mSv/y (mrem/y)   0.0027 (0.27) 0.0016 (0.16) a Adapted from Robison et al. (1993), projected to 1995. Ingestion Local sources of ingested radionuclides considered in the LLNL assessment include soil (incidental ingestion), drinking water (from rainfall and groundwater), and diet (marine and terrestrial foods). The radionuclides of potential importance, because of their presence in the environment are cesium-137, strontium-90, plutonium-239, plutonium-240, and americium-241. The relative contributions to projected radionuclide intake from local sources are summarized in Table 5-2 for two LLNL scenarios: A, imported-foods-available, and B, imported-foods-unavailable. The assessment assumed an average soil ingestion of 100 mg/d. For the models used, soil makes a small contribution to the intake of cesium-137 and strontium-90 but it is the major contributor of transuranics in Scenario A and an important contributor in Scenario B. Thus, a dose from ingestion of transuranics is sensitive to the assumptions about the quantity of soil intake. The assessment used the average island-soil radioactivity values, and the doses are

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands Table 5-2. Local Source Contributions to Intake by Ingestion of Various Radionuclides     Contributions     Component mass intake, g/d 137Cs 90Sr 239, 240Pu 241Am Scenario A-imported foods available: Soil 0.1 0.2% 3.6% 72.3% 81.1% Watera 977 0.05% 3.6% 1.0% 0.5% Local food 375 >99% 92.8% 26.7% 18.4% Intake, Bq/d — 31 0.47 0.018 0.012 Scenario B-imported foods unavailable: Soil 0.1 0.07% 1.1% 35.0% 54.0% Waterb 530 0.01% 1.0% 0.4% 0.3% Local food 1,011 99.9% 97.9% 64.6% 45.7% Intake, Bq/d — 78 1.5 0.037 0.018 a Including rainwater, well water, and drinking water in coffee or tea and malolo. b Including rainwater and well water. higher than would be estimated with the lower village-vicinity values. The LLNL assessment assumes that the drinking-water source is rainwater 60% of the time and groundwater 40% of the time. The diet model for Scenario A uses 947 g for the total daily intake of rainwater, well water, and water used to brew coffee or tea and B. Thus, the dose from ingestion of transuranics is sensitive to the assumptions about the reconstitute dried drinks (malolo). The quantity of water specified for Scenario B is only 530 g with the absence of coffee or tea and malolo. These quantities are less than the daily water reguirements given for the ICRP-23 reference persons (ICRP, 1975), fluid water values of 1,700 g/day for the male

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands and 1,200 for the female. The quantity of water specified in these scenarios therefore seems low. For the water-consumption quantities used, the radionuclide intakes are probably overestimated—groundwater (with higher radionuclide concentrations than rainwater) is used only in conditions of extreme drought, but the assessment assumed relatively high groundwater use of 40%. However, neither the daily quantity nor the percentage of groundwater is critical, in that it appears that drinking water would contribute only a few percent, at most, of the projected radionuclide intake. The remainder of the diet constitutes the major source of the projected fission-product intake and contributes a large fraction of the projected transuranic intake. Average radionuclide concentrations for the various food items vary over a range of 105; consequently, the composition of the diet model is critical to the dose assessment. Diet models were reviewed in previous sections of this report; this section addresses primarily the dose-estimation method and doses estimated with the LLNL diet model. In the LLNL assessment, the scenario-based radionuclide intakes were converted to doses with the methods and models of ICRP-30, -48, -56, and-61 (ICRP, 1979, 1986, 1989, 1991b). One special consideration was the use of gastrointestinal-to-blood transfer factors (f1 values) of 10-4 for plutonium ingested in soil and the ICRP value of 10-3 for plutonium in other media. Intakes of these radionuclides were presented as fractions of the intake limit corresponding to 1 mSv/y (in effect, the committed effective dose in millisieverts/year). Ingestion intakes and the associated committed effective doses projected for the initial year are summarized by radionuclide for the two scenarios in Table 5-3. The LLNL assessment did not make age-specific dose estimates. However, doses from ingestion of the major contributor, cesium-137, are fairly constant with age from 1 y through adult life (see, for example, ICRP-56, 1989). Also, Robison and Phillips (1989) calculated doses from continuous intake, beginning at various ages, of cesium-137 and strontium-90 from an intake source decaying with a 30-y half-life. These calculations indicated that, although maximum dose rates occurred when intake began at an earlier age,4 estimated integral effective doses for adults due to intake of cesium-137 and strontium-90 can be used as conservative estimates for intake beginning at earlier ages. In summary, estimates of radionuclide ingestion were based on a substantial database. All the credible routes of ingestion appear to have been considered. The estimate of the soil-ingestion route would be improved by stronger data on quantities of soil ingested, particularly as a function of age. The estimation of ingestion due to water is not clear, particularly in Scenario B; however, drinking water does not appear to be a major dose contributor. The diet model plays an important role and has been discussed in an earlier section. The conversion of intake to dose uses accepted ICRP models and methods. The assessment was limited to a reference adult; however, evidence was given to support the position that this provides a conservative assessment for the broader population. A possible exception, not thoroughly 4   The maximum dose rate for cesium-137 was calculated to occur when intake begins at age 4 months and to have a value about twice that for intake beginning as an adult. For strontium-90 the maximum rate occurred when intake began about age 13; this value was slightly greater than for intake beginning as an adult.

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands Table 5-3. Summary of Annual Whole-Body Doses Attributable to Local Fallout Projected for 1995† Route Scenario A. Imported Food Available Scenario B. Imported Food Unavailable Radionuclide Intake Bq/y Dose* mSv/y % of Total Dose Intake Bq/y Dose* mSv/y % of Total Dose Ingestion 137Cs 11,000. 0.16 56 28,000 0.40 73 90Sr 170. 0.0057 2 550 0.018 3 239, 240pu 6.6 0.0033 1 14 0.008 2 241Am 4.4 0.0029 1 6.6 0.0044 1 Subtotal   0.17 60   0.43 79 Inhalation 239, 240Pu 0.044 0.0027 1 0.044 0.0027 < 1 241Am 0.023 0.0016 < 1 0.023 0.0016 < 1 Subtotal   0.0043 2   0.0043 1 Intake Subtotal — 0.18 62 — 0.43 80 External — 0.11 38 — 0.11 20 Total   0.29 100   0.54 100 -   (29 mrem/y)     (54 mrem/y)   † Data derived from Robison et al., 1993. * Dose = Effective dose (external) and committed effective dose (intake). covered, is the question of intake of transuranics through the ingestion of soil by various age groups. The dietary scenarios developed in Table 3-1 can be used to estimate the effects of changing dietary assumptions on dose calculations. In Table 5-4, the dose contribution of all

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands Table 5-4. Calculated Average Annual Whole-Body Dose to Rongelap Residents Associated with Potential Dietary Scenarios Considering Variations in Energy Intake, Food Choices, and Remediation with KCl (Projected for 1995)   Ujelang local & imported diet Ujelang local only Local only1 Coconut collector's diet2 Local only from northern islets3 Local only KCl applied4   (A) (B) (C) (D) (E) (F) 137Cs intake, kBq\y 11 28 66 56 106-396 12 137Cs Dose, mSv/y 0.16 0.40 0.92 0.78 1.50-5.57 0.17 Other sources,5 mSv/y 0.13 0.14 0.14 0.14 0.14 0.14 Total local source dose: mSv/y 0.29 0.54 1.06 0.92 1.64-5.71 0.31 mrem/y 29 54 106 92 164-571 31 * Robison et al., 1992; 1993, Tables 5 and 6. 1 Ujelang local-food-only diet adjusted by the committee to provide energy equal to the Ujelang local-and-imported-food diet (Column B × 2.305) to bring caloric intake to a sustainable level. 2 Energy intake of Ujelang local diet increased by increasing intake of fish (two-fold), coconut juice (five-fold), drinking coconut meat (five-fold), sprouted coconut (five-fold), consumption of turtle eggs reduced (90g) and of water reduced (300g) to make diet quantities logical. 3 Local diet in Column C but with pandanus, coconut, and arrowroot collected in northern islets of Rongelap and Rongerik Atolls. Kohn (1989) estimated whole body doses 2-to 9-fold greater than on Rongelap Island. 4 Local diet in Column C with an assumed 90% loss of cesium-137 in coconut breadfruit and pandanus because of KCl fertilization (Robison et al., 1993). 5 All internal and external sources of radiation other than those due to cesium-137. components of the diet other than those from cesium-137 have been held constant. For a first approximation, the dose associated with cesium-137 (projected for 1995) has been calculated on the basis of the estimated daily cesium-137 intake for the various dietary scenarios. Comparison of the results of the dietary scenarios listed in Table 5-4 shows the impact of dietary assumptions. Although the average annual dose calculated in columns A and B

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Radiological Assessments for Resettlement of Rongelap in the Republic of the Marshall Islands is less than 1 mSv (100 mrem), the average annual doses calculated in columns C and D are around 1 mSv, indicating that a sizable fraction of the population will exceed the 100-mrem limit of the MOU if these diet scenarios are valid. In the scenario presented in column E, the average annual dose exceeds 1 mSv (100 mrem). The effect of mitigation can be seen in column F; all assumptions are identical with those of column C, but KCl is applied to reduce cesium-137 uptake in food plants. In this case, the calculated annual dose is similar to that calculated if the local-food diet is supplemented with imported foods. Obviously, there is benefit to mitigation with KCl, and various exposure scenarios can be used to investigate potential effects of other actions that might be taken. Intake through skin The effect of skin contamination with plutonium has not been addressed in the published LLNL reports. Application of the previously cited absorption factors from the literature would project substantial intake of plutonium from Rongelap soil. Such projections applied to Rongelap, however, would be expected to produce an overestimate, because the plutonium in that soil is of a fired-oxide form that is substantially less soluble than the nitrate forms used in most of the experiments in the literature. This is supported by the history of low excretion of plutonium measured in persons who resided on Rongelap 1957 to 1985. However, because of the uncertainties in the projections, assessments should be performed on the potential intake of plutonium through the skin.