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| Fig. 1: Map of Marshall Islands. Nuclear took place in Bikini and Enewetok, shown in red. (Source: Wikimedia Commons) |
In the period between 1946 and 1958 the United States conducted 67 nuclear tests over the Marshall Islands. [1] (See Fig. 1.) Nuclear tests mostly took place in northern latitude islands, notably the Bikini Atoll, which required evacuation of the local Marshallese population to prevent exposure of radioactive fallout. Despite efforts to minimize environmental damage of populated islands and protect the health of its inhabitants; nuclear testing resulted in long-term harm to the Marshallese people.
Castle Bravo was the largest thermonuclear device detonated by the United States. It was a significant contributor to radiation exposure of the Marshall Islands. The Castle Bravo test exceeded expectations of total yield, releasing 15 megatons of TNT equivalent energy, and unintentionally releasing fallout over a much longer distance than anticipated. [2] Fig. 2 shows the total extent of nuclear fallout from Castle Bravo. [18] The radioactive fallout extended downwind as far west as Utirik. It contaminated populated atolls of Rongelap which required emergency evacuation to Kwajalein.
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| Fig. 2: Calculated fallout pattern from the Castle Bravo test. [18] The numbers are expressed as "unshielded" exposures that would have occurred in the first 96 hours after detonation if the exposed person had not taken any shelter. The innermost contour, cutting the northerm part of the Rongelap Atoll (which was uninhabited), corresponds to 33 Gray (3300 rad), a lethal exposure many times over. The outermost contour, cutting the southern part of Rongelap Atoll (which was inhabited) corresponds to 2.2 Gray (220 rad). Individuals from this part of Rongelap were evacuated after the detonation. The yield of Castle Bravo was 2.5 times larger than expected. (Adapted from Glasstone and Dolan. [18]) (Source: Wikimedia Commons) |
Nuclear detonation disperses a large variety of radionuclides into the surrounding environment, which then insinuate themselves into both organic and inorganic matter. Dosage reconstruction for individuals affected by radioactive fallout depends on both external exposure, predominantly from γ-rays, and internal dosage from ingestion and inhalation of radionuclides. Chronic health complications result from radionuclide absorption into organs; particularly red bone marrow, thyroid gland, stomach wall, and colon. [3] Risks of internal radiation exposure include radionuclides that may persist in the environment for decades following testing. Long-lived radionuclides of interest include 90Sr (t1/2 = 28.9 yr), 137Cs (t1/2 = 30.2 yr), 55Fe (t1/2 = 2.7 yr), and 66Co (t1/2 = 5.3 yr). [4] In particular, 90Sr is a commonly measured isotope in human bones given the elements chemical similarity to calcium. [5] Adverse health consequences are also attributed to shorter-lived isotopes, including 131I (t1/2 = 8 days), whose immediate absorption into human tissue was documented via urine samples taken from individuals in the days following Castle Bravo. [6] Long-lived 137Cs is important because it is a γ-emitter (see Fig. 3) and thus easy to measure. [8] This measurement may then be used to estimate the amounts of other radioisotopes using known fission waste composition.
Environmental fallout contamination is evaluated using the deposition density, which is a measure of activity per unit area, where activity is the rate of nuclear decays. [7] Deposition density is measured in Becquerels per square meter (Bq m-2), where 1 Becquerel refers one nuclear decay per second. Estimated deposition densities of significant radionuclides from nuclear testing provides a basis for subsequent analyses of the risks of internal exposure.
Literature investigating deposition density in the Marshall Islands consists of both historical field measurements and theoretical estimates calculated from mathematical models. For instance, in 1989 the Republic of the Marshall Islands commissioned 137Cs soil profiles using in situ γ-spectrometry measurements with high-purity germanium (HPGe) detectors. [9] These measurements were subsequently used to verify mathematical and computational tools that calculate deposition densities in regions where data acquisition was limited. The most recent studies use time-of-arrival (TOA) data and meteorological models, notably HYSPLIT, which was developed by NOAA. [10]
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| Fig. 3: Decay pathways for 137Cs showing high probablility of 0.6617 MeV γ-emission. [11] (Source: Wikimedia Commons) |
Recent studies focused on mapping spatial variations in deposition density by categorizing islands into northern latitude, mid-latitude, and southern latitude islands. Northern latitude islands were closest in proximity to nuclear tests and include Bikini, Enewetak, and Rongelap. Central latitude islands include Kwajalien (where the inhabitants of Rongelap were evacuated), and southern latitude atolls which include the modern day capital of Majuro and was geographically the furthest from nuclear testing. Table 1 shows data of cumulative 137Cs deposition for islands in the northern, mid-latitude, and southern atolls of the Marshall Islands. Estimates for the northern regions may be considered conservative given that site-specific deposition densities reached as high as 1300 kBq m-2 in part of Rongelap. [12]
Thyroid disease assessment from exposure to nuclear testing can be estimated by estimating the intake of radio-iodines from the exposed population. The thyroid gland, responsible for regulating physiological processes via hormone production, cannot distinguish stable iodine from radio-iodines. It has been shown that young children are most susceptible to radio-iodines as they tend to accumulate in dairy products. [13] Due to the short half-life of 131I, long-term measurements of this isotope is not feasible.Fortunately, the Los Alamos Scientific Laboratory (LASL) collected urine samples from the exposed adult population on Rongelap island within a month of the Castle Bravo test and performed radioactivity assays to determine the activity concentration from 131I. [6] Urine samples were collected on 16 March 1954 (35 adults), 17 March 1954 (31 adults), and 19 March 1954 (9 adults). In addition to these initial sample collection efforts, urine samples were collected from 9 U.S military observers who took temporary residence on Rongerik, located slightly farther out than Rongelap. The samples were then tested for gamma activity using a photomultiplier-based scintillation detector on aggregated urine of volume 500 mL. The measurement, performed on 30 March 1954, showed an radio-iodine count rate of ~70 counts per second per 500 mL of urine among adults in Rongelap, and approximately 20 counts per second per 500 mL of urine among military observers in Rongerk. Therefore, the limited data available from the immediate aftermath of Castle Bravo show that Marshallese citizens indeed faced radio-iodine exposure. We now investigate whether this exposure resulted in a cluster of thyroid cancer cases among the Marshallese.
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| Table 1: Estimated 137Cs deposition densities for differing geographical regions in the Marshall Islands. [8] |
A study conducted between 1993-1997 performed a thyroid screening program of 5821 Marshallese citizens to determine the historical prevalence of thyroid cancer. [14] This cohort was categorized based on whether its participants were born prior to Castle Bravo (1936- 1954) or following Castle Bravo (1954-1965). Among those born prior to Castle Bravo 57/3709 (1.5%) participants had a thyroid cancer diagnosis, whereas participants born after Castle Bravo reported 11/2112 (0.5%) thyroid cancer diagnoses. The three-fold difference in thyroid cancer diagnosis rates between individuals born prior to and following Castle Bravo suggests that increased fallout exposure contributed to a cluster of thyroid disease among those affected by nuclear testing. This conclusion is corroborated by the presence of radioactive iodine count rates in the urine sample studies.
Other cancers of interest to those exposed to nuclear fallout include leukemia, stomach cancer, and colon cancer. Much of the literature focuses on projected numbers of cancer cases based on calculated dosages; rather than actual medical screenings of the affected community. [12,15] The absence of quantitative measurements on cancer rates forces us to rely on theoretical estimates on the number of cancers attributable to fallout exposure in the Marshall islands. Future work may verify the robustness of these cancer rate projections by confirming the actual number of cancer cases in the Marshall Islands.
© Martin Gonzalez. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. K. I. L. Abella et al., "Background Gamma Radiation and Soil Activity Measurements in the Northern Marshall Islands." Proc. Natl. Acad. Sci. (USA) 116, 15425 (2019)
[2] T. A. Ruff, "The Humanitarian Impact and Implications of Nuclear Test Explosions in the Pacific Region," Int. Rev. Red Cross 97, 775 (2015).
[3] S. L. Simon et al., "Acute and Chronic Intakes of Fallout Radionuclides By Marshallese From Nuclear Weapons Testing at Bikini and Enewetak and Related Internal Radiation Doses," Health Phys. 99, 157 (2010).
[4] S. L. Simon et al., "Dose Estimation For Exposure to Radioactive Fallout From Nuclear Detonations," Health Phys. 122, 1 (2022).
[5] J. Mangano and J. D. Sherman, "Elevated in vivo Strontium-90 From Nuclear Weapons Test Fallout Among Cancer Decedents: A Case-Control Study of Deciduous Teeth," Int. J. Health Serv. 41, 137 (2011).
[6] P. S. Harris, S. L. Simon, and S. A. Ibrahim, "Urinary Excretion of Radionuclides from Marshallese Exposed to Fallout From the 1954 Bravo Nuclear Test," Health Phys. 99, 217 (2010).
[7] H. L. Beck et al., "A Method For Estimating the Deposition Density of Fallout on the Ground and on Vegetation From a Low-Yield, Low-Altitude Nuclear Detonation," Health Phys. 122, 21 (2022).
[8] H. L. Beck et al., "Fallout Deposition in the Marshall Islands from Bikini and Enewetak Nuclear Weapons Tests," Health Phys. 99, 124 (2010).
[9] S. L. Simon and J. C. Graham, "A Comparison of Aerial and Ground Level Measurements of 137Cs in the Marshall Islands," Environ. Monit. Assess. 53, 363 (1998).
[10] B. E. Moroz et al., "Predictions of Dispersion and Deposition of Fallout From Nuclear Testing Using the NOAA-HYSPLIT Meteorological Model," Health Phys. 99, 252 (2010).
[11] E. B. Podgorsak, Radiation Physics for Medical Physicists, 3rd Ed. (Springer 2016).
[12] S. L. Simon et al., "Radiation Doses and Cancer Risks in the Marshall Islands Associated With Exposure to Radioactive Fallout From Bikini and Enewetak Nuclear Weapons Tests: Summary," Health Phys. 99, 105 (2010).
[13] S. L. Simon and A. Bouville, "Health Effects of Nuclear Weapons Testing," Lancet 386, 407 (2015).
[14] T. Takahashi et al., "The Relationship of Thyroid Cancer With Radiation Exposure From Nuclear Weapon Testing in the Marshall Islands," J. Epidemiol. 13, 99 (2003).
[15] C. E. Land et al., "Projected Lifetime Cancer Risks From Exposure to Regional Radioactive Fallout in the Marshall Islands," Health Phys 99, 201 (2010).
[16] S. Glasstone and P. U. Dolan, The Effects of Nuclear Weapons, 3rd Ed (U.S. Department of Defense, 1977), p. 437.
