Interim initial state report for the safety assessment SR-Can

A thorough description of the initial state of the engineered parts of the repository system is one of the main bases for the SR-Can safety assessment. The initial state refers to the state at the time of deposition for the spent fuel and the engineered barriers and the natural, undisturbed state at the time of beginning of excavation for the repository for the geosphere and the biosphere. The repository system is based on the KBS-3 method, where copper canisters with a cast iron insert containing spent nuclear fuel are surrounded by bentonite clay and deposited at approximately 500 m depth in saturated, granitic rock. For the purpose of the safety assessment the engineered portion of the repository system has been divided into a number of consecutive barriers or sub-systems. The importance of a particular feature for safety has influenced the resolution into components. In principle, components close to the source term and those that play an important role for safety are treated in more detail than more peripheral components. For the option with 40 years of reactor operation, the quantity of BWR fuel is estimated at 7200 tonnes and the quantity of PWR fuel at 2300 tonnes. The fuel burn-up may vary from 15 MWd/kgU up to 60 MWd/kg. Geometric aspects of the fuel cladding tubes of importance in the safety assessment are, as a rule, handled sufficiently pessimistically in analyses of radionuclide transport that differences between different fuel types are irrelevant. The relative differences in radionuclide inventory with respect to burn-up are small. Deviations in inventory and deviating or damaged fuel are not considered in the SR-Can interim reporting but will be handled in the final reporting of SR-Can. The canister consists of an inner container, the insert of cast iron and an outer shell of copper. The cast iron insert provides mechanical stability and the copper shell protects against corrosion in the repository environment. The copper shell is 5 cm thick and the cylindrical canister has a length of approximately 4.8 metres and a diameter of 1.05 metres. The copper shell is made of pure oxygen-free copper. The insert is cast from spheroidal graphite cast iron and has channels where the fuel assemblies are placed. The insert is presently available in two versions: one for 12 BWR assemblies and one for 4 PWR assemblies. A canister holds about two tonnes of spent fuel. Canisters with BWR and PWR assemblies weigh 25 and 27 tonnes, respectively. The decay heat in the spent fuel deposited in one canister is limited to 1700 W, to fulfil temperature requirements at the canister surface in the deposition hole. A total of about 4500 canisters will be produced according to current estimates. In the deposition holes, the copper canister is surrounded by a buffer of clay. The buffer is deposited as bentonite blocks and rings. The final decision on excavation technique for the deposition tunnels has not been taken and two possible techniques, drill and blast or mechanical excavation (tunnel boring machine), are analysed in SR-Can. The excavation technique will have implications on the dimensions and shape of the deposition tunnels. The cross section in a drill and blast deposition tunnel is a square with an arched roof, whereas the cross section in a mechanically excavated tunnel is circular. Two different backfill concepts are analysed. One is an in situ compacted mixture of crushed rock and bentonite of the same type as in the buffer and the other comprises pre-compacted blocks of Friedland clay. A number of more or less vertical investigation or surface-based characterisation boreholes are to be drilled during site investigations in order to obtain, e.g. data on the properties of the rock. These boreholes will be sealed, no later than at the closure of the deep repository. Some holes will be bored from the repository tunnels during the construction phase, meaning that horizontal and upwards-directed holes also have to be sealed. The borehole seals must prevent short-circuiting of flow of contaminated groundwater from the repository. They should, therefore, not be more permeable than the undisturbed, surrounding rock. Time-dependent degradation must be accepted, but the goal is to use plug materials that maintain their constitution and tightness for a long time. Seals for boreholes are under development as part of SKB's RDandD programme. The concept adopted for surface-based boreholes in SR-Can comprises the following materials at different depths: compacted moraine (0-3 m), close-fitting rock cylinders from the site (3-50 m), compacted moraine (50-60 m), smectite pellets (60-100 m), and highly compacted smectite clay contained in perforated copper tubes (below 100 m). Tunnel-based boreholes are assumed to be filled with highly compacted smectite clay in perforated copper tubes. These boreholes are plugged with concrete at the tunnel