Abstract
ABSTRACTRadiocarbon (14C or carbon-14, half-life 5730 yr) is a key radionuclide in the assessment of the safety of a geological disposal facility (GDF) for radioactive waste. In particular, the radiological impact of gaseous carbon-14 bearing species has been recognized as a potential issue. Irradiated steels are one of the main sources of carbon-14 in the United Kingdom’s radioactive waste inventory. However, there is considerable uncertainty about the chemical form(s) in which the carbon-14 will be released. The objective of the work was to measure the rate and speciation of carbon-14 release from irradiated 316L(N) stainless steel on leaching under high-pH anoxic conditions, representative of a cement-based near field for low-heat generating wastes. Periodic measurements of carbon-14 releases to both the gas phase and to solution were made in duplicate experiments over a period of up to 417 days. An initial fast release of carbon-14 from the surface of the steel is observed during the first week of leaching, followed by a drop in the rate of release at longer times. Carbon-14 is released primarily to the solution phase with differing fractions released to the gas phase in the two experiments: about 1% of the total release in one and 6% in the other. The predominant dissolved carbon-14 releases are in inorganic form (as 14C-carbonate) but also include organic species. The predominant gas-phase species are hydrocarbons with a smaller fraction of 14CO (which may include some volatile oxygen-containing carbon-species). The experiments are continuing, with final sampling and termination planned after leaching for a total of two years.
Highlights
Radiocarbon (14C or carbon-14, half-life 5730 yr) is a key radionuclide in the assessment of the safety of a geological disposal facility (GDF) for radioactive waste in the United Kingdom and the need to better understand the possible importance of gaseous carbon-14 bearing species has been recognized (NDA 2012)
This paper describes the design of leaching experiments to measure the release of carbon-14 to both gas and solution phases from irradiated stainless steel under high-pH, anoxic conditions, and the results obtained during the first year of operation
In the case of TD14C the cumulative releases have been calculated using an assumption that the ratio of inorganic to organic carbon-14 species released remains constant throughout the experiments
Summary
Radiocarbon (14C or carbon-14, half-life 5730 yr) is a key radionuclide in the assessment of the safety of a geological disposal facility (GDF) for radioactive waste in the United Kingdom and the need to better understand the possible importance of gaseous carbon-14 bearing species has been recognized (NDA 2012). A number of radioactive gases will be generated from waste materials within a GDF, with carbon-14 bearing methane (14CH4) likely to be the dominant carbon-14 species transported in the gas phase, potentially reaching the biosphere at low activity concentrations. The main sources of carbon-14 in UK radioactive wastes are irradiated graphite, irradiated steels, irradiated reactive metals (primarily Magnox and uranium), and spent nuclear fuel. There are 17,700 TBq of carbon-14 in the 2013 UK Derived Inventory of which 7090 TBq is associated with irradiated steel wastes (RWM 2015)
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