Irradiated graphite (i-graphite) is a typical nuclear waste arising from the decommissioning of some nuclear power plants and research reactors. According to the current nuclear waste management strategy in Germany, i-graphite discharged from commercial and research reactors is to be disposed of in a deep geological repository for low and intermediate level radioactive wastes, which requires reliable information on radionuclide inventory and their release behavior under repository relevant conditions. Typically, i-graphite contains a number on radionuclides (activation products), like 3H, 14C or 36Cl, which may potentially be released during storage and disposal. Among them 14C, a weak beta-emitter with a half-life of 5,700 years, is of major concern due to radiation hazards by potential incorporation into the human body. Despite years of research activities on i-graphite, the binding forms of 14C as well as the mechanisms of 14C release from i-graphite and its speciation remain unclear; therefore the focus of the present work is on the understanding of 14C release behavior from i-graphite, during the operational phase of a repository, as well as under the long-term conditions relevant to final disposal.
Outcomes of earlier studies demonstrated that 14C can be present in i-graphite in different chemical forms, like elemental C, CO2 absorbed or trapped in the closed pores or as organic C species. These 14C-species have different mobility in i graphite, which can be affected by a number of factors, e.g. graphite microstructure, temperature, composition and pH of infiltrating groundwater, promoting the 14C release into the gaseous phase. The behavior of 14C in i-graphite was investigated by means of model leaching tests using i-graphite acquired from the Rossendorf Research Reactor (RFR). The effects of various storage conditions, such as temperature and composition of leaching solution, were investigated. For the quantitative determination of volatile 14C species at relatively low concentrations a development of a customized analytical method was required, which will be discussed in details.
The results of the study demonstrated that the main volatile 14C fraction (> 83%) is in form of CO2; the remaining - 17 %, are comprised of 14CO or/and organic 14C species. Model experiments with i-graphite also revealed almost negligible 14C release into the gaseous phase, when i-graphite is stored under cementitious conditions (i.e. in a basic media with pH>12.9). In this case a dominant fraction of 14C remains in the liquid phase in form of carbonate species, which can be immobilized due to reaction with the hydrated cement phases. Consistent with earlier studies on AVR-graphite, a small fraction of formic acid was detected in the leaching solution. The annual release rate of volatile 14C for RFR-graphite into the gas phase was determined to be <1%/year, which allows RFR-graphite disposal with a maximum permitted container loading in the German repository Konrad. In addition, pros and cons of a thermal pre-treatment of RFR-graphite for a selective separation 14C were evaluated.
The work leading to these results has received funding from the European Union’s European Atomic Energy Community’s (Euratom) Seventh Framework Programme FP7/2007-2013 under grant agreement no. 604779, the CAST project.