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-233U 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 235U. The movement of 231Pa 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 231Pa to that of 235U was measured in low-grade uranium ore samples using high-resolution gamma-spectrometry. The activity ratio of 231Pa to 235U can be used to study the disequilibrium relationship between the 231Pa and 235U, 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 1.5.104 years (231Th 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 ratioPa/-35U is used to find out the age of marine geological samples.1 Inequilibrium of radionuclides 231Pa and 235U 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 235U, 231Pa and 227Th in natural uranium ore samples. The equilibrium status of the radionuclides in the 235U series was also studied by analyzing the activity ratios between 214Pb and 234mP, 2148i and 234mPa, as well as between 231Pa and 234mPa.