Analysis of SKB MiniCan-Experiment 3

To ensure the safe encapsulation of spent nuclear fuel rods for geological disposal, SKB of Sweden is planning to use a system that consists of an outer copper canister and a cast iron insert (the KBS-3 concept). In 2007 Serco1 completed the set up of five model canister experiments at SKB's Aespoe laboratory and monitoring has continued since. The original aim of the model canister experiments was to examine how corrosion of the cast iron insert inside a copper canister would evolve with time, if water ingress through a small defect in the copper canister were to occur. Serco arranged manufacture and installation of five miniature copper canisters containing cast iron inserts, with 1 mm defects deliberately machined into the copper shell. The experiments use five small-scale model canisters (300 mm long x 150 mm diameter) that simulate the main features of the SKB canister design (hence the project name, 'MiniCan'). The main aim of the work is to examine how corrosion of the cast iron insert will evolve if a leak is present in the outer copper canister The experiments also included electrochemical equipment to monitor the corrosion behaviour of the model canisters in situ. In 2011 one of the experiments, Experiment 3, was removed f analysis. This report presents details of the procedures that were applied and the findings that were obtained from the analysis that was carried out on Experiment 3. To minimise exposure to air and to keep the contents of the experiment wet until the analysis was carried out, Experiment 3 was extracted from its borehole in August 2011 directly into a transfer tank that was filled with deaerated groundwater and placed in a purpose-built, water-filled and deoxygenated transfer flask. The transfer flask was then transported to the UK for dismantling and examination in a purpose-built anoxic glovebox that contained the appropriate lifting and cutting equipment for handling and sectioning the copper canister and the cast iron insert. The report presents details of the analysis of the water samples taken from the experiment before it was removed, followed by observations of the dismantled experiment and the results from the chemical and physical analytical techniques that were subsequently applied to samples taken from the experiment. Estimates of the corrosion rates of iron and copper were made by analysis of the weight loss specimens and these have been compared and contrasted with the in situ corrosion rates measured during the last five years. The water analysis showed that there had been an increase in the concentration of dissolved iron in the water inside the support cage of the experiment, accompanied by a fall in the pH of the water to approximately 6.6 and a decrease in the hydrogen concentration. Microbial analysis indicated that the surface of the canister was colonised by high concentrations of sulphate reducing bacteria (SRB). When the experiment was opened it was immediately apparent that the majority of the surfaces were covered with a layer of black deposit. The surface of the copper canister had turned black, although the coverage was not uniform. Corrosion product was visible extruding from the machined defect at the top of the canister and to a lesser extent from the machined defect at the bottom of the canister. This report presents the detailed results of the analyses