Carbon-14 in the Palo Duro basin repository

Operation of nuclear reactors results in production of the radioactive nuclide carbon-14. In a typical light water pressurized reactor using oxide fuel, the rate of C-14 formation is 10 to 20 Ci per GWe-year. Thermal neutrons can form C-14. However, the lower isotopic concentration (1.1%) combined with the low reaction cross section (less than one millibarn) makes this a minor pathway for C-14 production. Fast neutrons can form C-14 from the oxygen present by the reaction 16O(n, 3He) 14C but the cross section for this reaction is smaller than for the 14N(n, p) 14C reaction. This latter is the major mode of C-14 formation and occurs because of the small amount, typically 10 to 60 ppM, of nitrogen dissolved in the nuclear fuel. Reprocessing of the spent fuel eliminates the C-14 as CO2 off-gas in the dissolving step. However, in the once-through mode, the carbon would be retained in the encapsulated fuel placed in the geologic repository. This report addresses the possible species and behavior of the C-14 which might be released in time from the waste package. 8 refs