Planning report for the safety assessment SR-Can

This document is a planning report for SKB's next assessment of long-term safety for a KBS 3 repository. The assessment, SR-Can, is to be finished by the end of 2005 and will be used for SKB's application to build an Encapsulation plant for spent nuclear fuel. Apart from outlining the methodology, the report discusses the handling in SR-Can of a number of important issues regarding the near field, the geosphere, the biosphere, the climatic evolution etc. The Swedish nuclear safety and radiation protection authorities have recently issued regulations concerning the final disposal of nuclear waste. The principal compliance criterion states that the annual risk of harmful effects must not exceed 10-6 for a representative individual in the group exposed to the greatest risk. There are also a number of requirements on methodological aspects of the safety assessment as well as on the contents of a safety report. The regulations are reproduced in an Appendix to this report. The primary safety function of the KBS 3 system is to completely isolate the spent nuclear fuel within copper canisters over the entire assessment period, which will be one million years in SR-Can. Should a canister be damaged, the secondary safety function is to retard any releases from the canisters. The main steps of the assessment are the following: 1. Qualitative system description, FEP processing: This step consists of defining a system boundary and of describing the system on a format suitable for the safety assessment. Databases of relevant features, events and processes influencing long-term safety are structured and used as one starting point for the assessment. 2. Initial state descriptions. 3. Process descriptions: In this step all identified processes within the system boundary involved in the long-term evolution of the system are described in detail. 4. Description of boundary conditions: This step is a broad description of the evolution of the boundaries of the system, focussing mainly on the climatic evolution and its influence on the repository system and future human actions. 5. Preliminary analyses: In this step, a number of analyses are carried out to understand the broad features of the system evolution for several combinations of initial and boundary conditions. 6. Scenario selection: In this step, a number of scenarios for detailed analysis are selected, drawing on information from the preceding steps of the analysis and other relevant sources. 7. Input data selection: In this sub-task, data to be used in the quantification of the repository evolution and dose calculations are selected and reported in a dedicated Data Report. 8. Analysis of scenarios: The temporal evolution of the system is analysed for each scenario in this step. 9. Integration of results and conclusions. The future climate evolution is an essential part of the description of the external conditions to the repository system. Past climate changes including permafrost and glacial conditions are highly likely to occur also in the future. The time sequence of these changes is governed by cyclic astronomical phenomena affecting insolation. A number of possible future climate evolutional pathways will be considered in the selection of scenarios in SR-Can. The biosphere chapter discusses the treatment of the general development of the biosphere, from the initial state as described by data from site investigations and onwards in different time periods. Thereafter the handling of groundwater discharge points is treated, followed by a description of modelling of radionuclide turn-over based on process understanding. Numerical modelling methods and ecosystem models are presented, as are the management of generic data and the handling of exposure to humans and the environment. The impact of geomechanical processes on the repository system might range from slight alterations of the hydraulic properties of the geosphere to jeopardising the integrity of the canister itself. Groundwater flow and radionuclide transport modelling in the geosphere is discussed in detail for current climate conditions and for altered climatic situations. Radionuclide transport in deposition tunnels and colloid facilitated transport are given particular attention. In the long term, the chemical properties of the groundwater, together with the properties of the buffer and the copper canister, determine for how long the buffer and canister will function properly. General near field issues addressed concern the near field temperature and the impact of earth quakes on the near field. Regarding the fuel, particular attention is given to fuel dissolution which will be thoroughly addressed in a state-of-the-art report as a supporting document to SR-Can. The derivation of input data regarding frequencies and sizes of initial canister sealing defects from test series of the canister production and quality control systems is discussed thoroughly. Other canister matters include the further evolution of defective seals, the internal evolution of a damaged canister, the isostatic collapse load of the canister insert and canister corrosion. Regarding the buffer and backfill, a number of different materials and backfilling concepts to be analysed in SR-Can are presented. Plans for modelling the initial THM unsaturated phase and the long-term chemical evolution of the saturated buffer and backfill are briefly outlined. Other matters addressed are radionuclide transport properties, colloid formation/erosion, gas issues and canister sinking. The handling of future human actions, most notably intrusion into the repository, will in SR-Can be restricted to actions that are carried out after the sealing of the repository, take place at or close to the repository site, are unintentional and that impair the performance of the repository's barriers