Determination of the activity ratios of 231Pa to 235U and 227Th to 235U in ore samples using gamma-spectrometry

Protactinium-231 is a key radionuclide, formed in the thorium-U-233 nuclear fuel cycle. Being of the high level waste (HLW), formed in this fuel cycle, it poses long-term radiation hazard. It is also observed in nature as a member of the natural radioactive series (actinium series) starting with U-235. The movement of Pa-231 in soil or water is important from the point of view of long-term hazards of high-level waste stored in repository. In a preliminary study the activity ratio of Pa-231 to that of U-235 was measured in low-grade uranium ore samples using high-resolution gammas-spectrometry. The activity ratio of Pa-231 to U-235 can be used to study the disequilibrium relationship between the Pa-231 and U-235, which in turn will help in age determination of marine geological samples. The resultant isotopic disequilibrium. i.e.. their activity ratio being not equal to one can further provide a means for dating the two different processes mentioned above on a time scale up to 1.5-10(4) years (Th-231 has a half-life of 26 hours and can be neglected). Examples of application of this chronometer include studies of marine carbonates. phosphates. bones and young volcanic rocks. For example, the measurement of the activity ratio Pa-231/U-235 is used to find out the age of marine geological samples.(1) Inequilibrium of radionuclides Pa-231 and U-235 observed from activity measurement is highlighted. An absolute activity measurement method was used for the measurement of activity ratios among different nuclides. Computer program SAMPO was used for the analysis of closely spaced peaks. The paper describes the equilibrium status among important nuclides namely U-235. Pa-231 and Th-227 in natural uranium one samples. The equilibrium status of the radionuclides in the U-238 series was also studied by analyzing the activity ratios between Pb-214 and Pa-214m. Bi-214 and Pa-234m, as well as between Pa-231 and Pa-234m.