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Abstract
Protactinium-231 is a key radionuclide, formed in the thorium- 233 U 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 235 U. The movement of 231 Pa 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 231 Pa to that of 235 U was measured in low-grade uranium ore samples using high-resolution gamma-spectrometry. The activity ratio of 231 Pa to 235 U can be used to study the disequilibrium relationship between the 231 Pa and 235 U, 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 ( 231 Th 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 231 Pa/ 235 U is used to find out the age of marine geological samples. 1 Inequilibrium of radionuclides 231 Pa and 235 U 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 235 U, 231 Pa and 227 Th in natural uranium ore samples. The equilibrium status of the radionuclides in the 238 U series was also studied by analyzing the activity ratios between 214 Pb and 234m Pa, 214 Bi and 234m Pa, as well as between 231 Pa and 223m Pa.
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