Abstract−A considerable fraction of radioactivity entering the environment from different nuclear events is associated with particles. The impact of these events can only be fully assessed where there is some knowledge about the mobility of particle bound radionuclides entering the environment. The behavior of particulate radionuclides is dependent on several factors, including the physical, chemical and redox state of the environment, the characteristics of the particles (e.g., the chemical composition, crystallinity and particle size) and on the oxidative state of radionuclides contained in the particles. Six plutonium-containing particles stemming from Runit Island soil (Marshall Islands) were characterized using non-destructive analytical and microanalytical methods. By determining the activity of 239,240 Pu and 241 Am isotopes from their gamma peaks structural information related to Pu matrix was obtained, and the source term was revealed. Composition and elemental distribution in the particles were studied with synchrotron radiation based micro X-ray fluorescence (SR-µ-XRF) spectrometry. Scanning electron microscope equipped with energy dispersive X-ray detector (SEM-EDX) and secondary ion mass spectrometer (SIMS) were used to examine particle surfaces. Based on the elemental composition the particles were divided into two groups; particles with plain Pu matrix, and particles where the plutonium is included in Si/O-rich matrix being more heterogenously distributed. All of the particles were identified as fragments of initial weapons material. As containing plutonium with low 240 Pu/ 239 Pu atomic ratio, ~2-6 %, which corresponds to weapons grade plutonium, the source term was identified to be among the safety tests conducted in the history of Runit Island.
IntroductionFrom 1946 through 1958 the United States conducted a total of 65 atmospheric nuclear weapons tests on Bikini and Enewetak Atolls within the Republic of the Marshall Islands [UNSCEAR 2000;Hamilton, 2004]. The nuclear test program included air drops (4), barge detonations (35), tower detonations (13), surface detonations (10), and underwater detonations (3). The total yield for all tests conducted on Bikini and Enewetak Atolls was 109 Mt with an estimated fission yield of 53 % [UNSCEAR, 2000;Hamilton 2004]. At Bikini, the total yield was approximately 76.8 Mt with a fission yield of approximately 55 %. At Enewetak, the total and fission yields were 30.2 Mt and 49%, respectively [UNSCEAR, 2000]. A surface safety test and one additional unsuccessful detonation at Enewetak Atoll resulted in little of no fission yield [DOE, 2000].The nuclear test program in the Marshall Islands produced both global and localized forms of fallout contamination. The terrestrial and marine environments of each atoll are contaminated with a range of fission and activation products, and unfissioned nuclear materials [Hamilton, 2004] In general, a considerable fraction of radioactivity entering the environment from nuclear events such as nuclear weapons accidents, auth...