Carbon-14 (half-life 5,730 years) 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 recognised as a potential issue. Carbon 14 is expected to be released from a GDF over a timescale of several thousand years. A number of radioactive gases will be generated from waste materials within a GDF, with carbon 14 bearing methane (14CH4) likely to dominate any carbon 14 transported in a gas phase, potentially reaching the biosphere at low activity concentrations. Sources of carbon-14 include irradiated graphite, irradiated steels and Zircaloys, irradiated reactive metals, spent ion–exchange resins and spent fuel.
The objective of this work is to measure the rate and speciation of carbon-14 release from irradiated stainless steel on leaching under high-pH anaerobic conditions, representative of a cement based near field for intermediate- and some low-level wastes (ILW/LLW). In particular, this includes measurements of releases to the gas phase as well as to solution. The gas phase carbon-14 collection method allows for the discrimination of carbon-14 released as 14CO2, 14CO (and volatile oxidized species) or 14C-hydrocarbons. The carbon-14 solution analysis method used to date has measured the inorganic carbon-14 release only. Work is in progress to measure the total carbon-14 release to solution that includes any dissolved organic carbon-14 species. Three experiments have been in progress in NRG’s Hot Cell Laboratory at Petten for a period of 20 months: two contain irradiated stainless steel with similar total inventories of carbon-14 (and also cobalt-60); the third is a control experiment with unirradiated stainless steel from the same batch. The steel samples are being leached in 0.1 mol dm 3 NaOH solution. The three experiments have been sampled six times to date and analytical data are now available for the carbon-14 releases to the gas phase, inorganic carbon-14 to solution and cobalt-60 releases to solution up to 13 months leaching. The experiments are still running and further sampling is planned after 2 years.
The presentation will cover: initial characterisation of irradiated steel samples to assess their suitability for use in the experiments; the approach selected and the conceptual design of the experiments for measuring carbon-14 releases from irradiated steel samples in a hot cell (i.e. a shielded cell); the preparation, installation and commissioning of the equipment in a hot cell. The main part will focus on the presentation and discussion of the results up to 13 months leaching.
This project is funded by Radioactive Waste Management and the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no. 604779, the CAST project.