Fallout Technetium - 99 Distribution and Behavior in Japanese Soils

Environmental amounts of Tc have been accumulating because of its long half-life of 2.1 × 10 y. This pure β-emitter (Emax= 0.29 MeV) is produced in the fission of U or Pu with a relatively high fission yield of ca. 6%, and it has been released to the environment through nuclear weapons tests and nuclear industries. The quantity of Tc produced by atmospheric nuclear weapon explosions was estimated to be 140 TBq using an equal fission yield to that of Cs and actual Cs deposition data. The release of Tc by nuclear industries through 1986, as estimated by Luykx, was of the order of 1000 TBq. This was mainly from nuclear fuel reprocessing and most of the Tc was discharged into the sea. The most significant discharge of Tc nowadays is from Sellafield to the Irish Sea; the reported total amount of Tc was ca. 550 TBq in 1995-1999. Compared to this, the amount introduced via nuclear medical use of Tc (half-life: 6.01 h) is negligible. For example, the amounts from Mo/Tc generators and Tc used during 2000 in Japan were 180 TBq and 285 TBq, respectively, which corresponds to ca. 7 MBq of Tc generation in total. Technetium is known to exist in all valence states from +7 to –1 and the dominant species in natural aqueous solution in equilibrium with atmosphere is the highly soluble chemical form pertechnetate, TcO4. Thus, Tc reaching the sea would remain in seawater for a long time. However, in the terrestrial environment, TcO4 has a high geochemical mobility and availability for plants. For this reason, it is thought to be one of the most important radionuclides for long-term dose assessment. From a radioecological viewpoint, analysis data on global fallout Tc in environmental samples should give useful information for predicting the nuclide behavior. At present, however, due to very low concentration and analytical difficulties for determination of the nuclide in environmental samples, there is a general lack of data on Tc levels in the literature. Therefore, the behavior of the nuclide in the terrestrial environment is not well understood. In this study, we determined concentrations of Tc in surface soil samples. Also Cs was used as a comparative indicator for discussion of the sources of Tc, because the fission yields from U and Pu are about the same (ca. 6%) for the two isotopes, and the behavior and distribution of Cs in the environment is reasonably well understood. The activity ratio of Tc/Cs was calculated to understand Tc mobility in the soil environment.