On the quantitative interpretation of uranium series disequilibria

The quantitative interpretation of radioactive disequilibria developed between radionuclides in natural decay chains has been discussed. Due to the wide spectrum of radioactive half lives of the member nuclides, uranium series can retain information long times (350 000 years) about changes in the bedrock. Radioactive disequilibrium is the result of different physico-chemical behaviour of nuclides under natural conditions. The geochemical behaviour of important nuclides is known rather well, which provides a sound basis for using uranium series disequilibrium (USD) method. A mathematical model was developed to simulate inventories of a 4-member (or shorter) radioactive decay chain in simplified systems. The systems can be periodically opening or continuously open in thermodynamical sense, which means that outside mass flows are periodical or continuous in nature, respectively. Both types of mass flow can be combined into continuous scenarios. The computer code URSE (uranium series disequilibria) was developed from the mathematical model. URSE was tested extensively by reference calculations with another code and by calculating relevant cases presented in the literature