Annual Report

54. Measurement of Technetium-99 in Marshall Islands Soil Samples by ICP-MS

Keiko Tagami and Shigeo Uchida

Keywords: technetium-99, Marshall Islands, ICP-MS, soil samples


In order to improve our understanding of the behavior of 99Tc in the environment and to develop appropriate waste storage/disposal options, it is essential that we obtain more reliable information on the distributions and fate of 99Tc in the environment. Generally it is difficult to obtain information from a global fallout area due to the extremely low 99Tc concentrations. However, there are several contaminated sites where elevated levels of 99Tc are expected because of inputs from nuclear facilities and/or nuclear weapons test programs. One of the sites is the northern Marshall Islands where the United States carried out over 60 nuclear test detonations during the 1950's. There is little or no information available on the levels of 99Tc contamination in and around the atolls. In this study, soil samples collected from various sites in the Marshall Islands were used as the basis for developing a refined analytical method for determination of 99Tc by ICP-MS.

The following three Tc extraction techniques were examined: (M1) acid leaching of Tc from incinerated soil; (M2) acid leaching of Tc from raw dry soil; and (M3) Tc volatilization from incinerated soil using a combustion apparatus. The chemical recovery was measured by counting 95mTc in the sample with a NaI (Tl) scintillation counter (Aloka, ARC-380) and comparing the result with standard solutions. The 99Tc content of the sample solution was then determined by ICP-MS (Yokogawa, PMS-2000) with 180 s counting time at mass 99.

Total chemical recoveries of Tc for extraction techniques M1, M2 and M3 were 49.6 - 98.5%, 39.7 - 76.4% and 7.6 - 16.9%, respectively. The low recoveries obtained using the combustion apparatus (M3 extraction method) appeared to show that Tc is not very efficiently volatilized from Marshall Islands soils due to the high carbonate contents.

The concentrations of 99Tc in the seven soil samples collected from the Marshall Islands are listed in Table 8 as determined by M1 and M2. These measurements represent the first analyses of 99Tc in environmental samples collected from former U.S. test sites in the Marshall Islands. The concentrations of 99Tc ranged from 0.1 to 1.1 mBq g-1 dry weight. In order to provide a preliminary assessment of the levels and spatial variability of 99Tc in soils we also measured 137Cs activity concentrations by gamma-spectrometry. Cesium-137 was used as a comparative indicator of the source term of 99Tc, because the fission yields from 235U and 239Pu are about the same (ca. 6%) for the two isotopes, and the behavior and distribution of 137Cs in the environment is reasonably well understood. It is interesting to note that the 99Tc concentrations in the three soil samples taken from Bikini Island (94BB, 95BB_1 and 95BB_2) were all very similar, within the range of the uncertainties of the measurements (i.e., 0.73 - 1.11 mBq g-1). These three 99Tc values were consistent with similar levels of 137Cs observed in the same samples. Using a reference date of 1954 the 99Tc /137Cs activity ratios in Bikini surface soils ranged from 0.7 - 1.1 x 10-4 or around 50 - 70% of the theoretical fission yield production ratio of 1.4 x 10-4.

Publication
Tagami, K., Uchida, S., Hamilton, T. and Robison, W.: Appl. Radiat. Isot. 53, 75-79, 2000.

Table 8. 99Tc and 137Cs activity concentrations and activity ratios of 99Tc/137Cs in Marshall Islands soil samples.
Sample
code
Location n 99Tc
(mBq g-1)
137Cs
(mBq g-1)
Activity Ratio
(99Tc/137Cs)

92RR Rongelap Island
(Rongelap Atoll)
3 0.29 + 0.10 2159 + 11 (1.3 + 0.5) x 10-4
94EP Lujor Island
(Enewetak Atoll)
3 0.46 + 0.08 464 + 4 (9.9 + 1.7) x 10-4
95EY Runit Island
(Enewetak Atoll)
6 0.10 + 0.02 112 + 2 (8.7 + 2.2) x 10-4
96EO Aej Island
(Enewetak Atoll)
3 0.60 + 0.15 1059 + 7 (5.6 + 1.4) x 10-4
94BB Bikini Island
(Bikini Atoll)
7 0.93 + 0.14 10094 + 20 (9.3 + 1.4) x 10-5
95BB_1 Bikini Island
(Bikini Atoll)
7 0.73 + 0.14 10058 + 20 (7.3 + 1.4) x 10-5
95BB_2 Bikini Island
(Bikini Atoll)
3 1.11 + 0.23 10128 + 21 (1.1 + 0.2) x 10-4

(Note) +: 1 sigma statistical error for 99Tc and activity ratio and counting error for 137Cs.
Reference date for 137Cs activities is 1 March 1954.


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