Report on 14C release speciation from carbon steel under alkaline reducing conditions (D2.7)

In the safety assessment of the geological disposal of high level nuclear waste, 14C is a critical radionuclide because of its long half-life (5730 years) and its high mobility in the geosphere and biosphere. It is possible for 14C to be released as the result of corrosion or waste degradation processes. The speciation of 14C will depend on the speciation of its precursor (it is generally assumed that the main precursor of 14C in irradiated ferrous materials is 14N, but there are 14C production pathways with 13C and 17O as precursors) and the chemical conditions in the repository. The question also arises whether the chemical bond of the precursor is maintained during irradiation. The international project CAST (Carbon-14 Source Term), which is partially funded through the Euratom Seventh Framework Programme, aims at understanding the generation and release of 14C containing species in conditions relevant for waste packaging and geological disposal. The project is focusing on 14C releases from irradiated metals (steels and zirconium alloys), irradiated graphite, and spent ion-exchange resins, as dissolved and gaseous species. The results of these studies will be transferred to the national programmes where they will be used for further development of the national safety cases. Within the CAST project, Work Package 2 focuses on irradiated steels – both stainless steels representative for reactor internals and carbon steels representative for the reactor pressure vessel material. The aim of the work at SCK•CEN in the framework of Work Package 2 of the CAST project was to investigate the release of 14C from carbon steels representative for the reactor pressure vessel (RPV) steel of Belgian nuclear power plants and the 14C speciation in a cementitious environment, which is relevant for the Belgian geological dispoal concept, which involves placing the RPV steel in a concrete monolith. To achieve this, we designed static and accelerated corrosion tests, and we obtained irradiated reference material representative for the RPV steel at end-of-life in the Belgian power plants. The static tests consisted of exposing the sample to a representative environment (anaerobic, high pH) for several months without imposing a potential, while in the shorter accelerated tests an electrochemical setup was used to fix the potential of the sample at a more anodic value than the corrosion potential (in order to accelerate the corrosion). It is well known that in highly alkaline environments carbon steel develops a thin, coherent passive film leading to very low corrosion rates, in the order of 0.1 µm/year [DIOMIDIS, 2014; FUJISAWA, 1997; FUJIWARA, 2002; GRAUER, 1991a; GRAUER, 1991b; HONDA, 2009; KREIS, 1993; KANEKO, 2004; KURSTEN, 2014; MIHARA, 2002; NAISH, 1990; NAISH, 1993; NAISH, 2001; NEWMAN, 2010; NEWMAN, 2015; NISHIMURA, 2003; RWMC, 1998; SMART, 2002; SMART, 2009]. Therefore, we expect a very low release of carbon species through corrosion of the carbon steel. This report gives an overview of the entire experimental programme carried out at SCK•CEN in the framework of WP2 of the CAST project. It contains information on the samples (both unirradiated and irradiated), the properties of the samples, the results from corrosion tests, and the results from speciation studies.