[en] Full text of publication follows. The handling of uncertainties is a key element in the evaluation of long-term safety. There fire many categories of uncertainties and it is unavoidable that some of them will increase as a function time in the analysis of the extremely long timescales normally addressed in post-closure safety assessment. However, uncertainties must be shown to be constrained or otherwise appropriately handled prior to decisions to proceed with disposal programme. The Swedish Nuclear Power Inspectorate's regulation SKIFS 2002:1 states that the important requirement is that uncertainties are described and handled in a consistent and structured manner. The impact of uncertainties should be evaluated by sensitivity analysis, covering for instance the description of barrier performance and the analyses of consequences to human health and the environment.. The guidelines state that there should be a classification of uncertainties into different categories (e.g. scenario uncertainty, system uncertainty, model uncertainty, parameter uncertainty, spatial variation in the parameters used to describe the barrier performance of the rock.). Uncertainties may be handled in many different ways depending on their character: - eliminate if possible; - account for in the design; - reduce or constrain as much as is reasonable; - circumvent; - accept but discuss openly (regulator may prescribe stylized approach). Elimination of some uncertainties may be done by site selection (e.g. avoid permafrost in future climate states) and repository design (e.g. avoid canister failure from localised corrosion by selection of corrosion barrier material). For some aspects of harrier performance (e.g. mechanical integrity of canister) recommended safety factors may be used to account for conceptual uncertainties in models and geometric uncertainties. However, safety factors do not necessarily have to be applied for highly improbable loading conditions. Uncertainties can in some case be reduced by non-destructive testing (e.g. canister defects), more experimental data (e.g. site measurement, long-term experiments with engineered harrier components) and additional research. In spite of these efforts, a range of uncertainties related to the extreme complexity of the system in consideration must be handled through conservative simplifying assumptions. A thorough justification of such assumptions is needed, since there may be other implications of such assumption than those originally envisaged. Finally, some uncertainties are not readily reducible or possible to circumvent but are a consequence of the selection of geological disposal (future human actions scenarios, intrusion, etc.). They still need to be analysed and discussed. (authors)

[en] The Swedish Nuclear Fuel and Waste Management Company (SKB) is moving forward with plans for the disposal of spent nuclear fuel. SKB is planning to submit license applications for construction of an encapsulation plant in late 2006 and for construction of an underground waste repository for spent nuclear fuel in 2009. The latter will be based on results from currently ongoing site investigations at two sites in Sweden (Forsmark and Laxemar). SKB's concept for the disposal of spent nuclear fuel is known as KBS-3. According to the KBS-3 concept, SKB plans that after 30 to 40 years of interim storage, spent fuel will be placed in copper canisters and that these will be disposed of at a depth of about 500 m in crystalline bedrock. In the KBS-3 concept, the principal engineered barriers comprise an iron insert that will hold and support the spent fuel rods, a copper canister that will encapsulate the fuel and the insert, a layer of bentonite clay known as the buffer that will surround the canister, and a mixture of bentonite and crushed rock that will be used to backfill the waste deposition tunnels. As part of its programme, SKB has conducted a wide range of tests on engineered barriers within its underground laboratory at Aspo. (authors)

[en] Processes that prevent or delay the migration of radionuclides in the geosphere relative to ambient water velocity are called retention processes. These processes can be chemical (e.g., ion exchange, sorption, and mineral precipitation) or physical (e.g., matrix diffusion from fractures into rock matrix or filtration of colloidal particles). Such retention processes can significantly delay and reduce the rate at which radionuclides are released to the biosphere and, hence, reduce the dose to biota, including humans. In most countries, retention processes are considered important components of the safety case (e.g., radionuclide delay in the unsaturated and saturated zones). For example, retention processes are listed as principal factors in the U.S. Department of Energy Repository Safety Strategy for the proposed Yucca Mountain site, and significant performance was ascribed to the retention processes in recently completed performance assessments in Sweden and Finland. In this paper, we discuss the perspective from the United States and Swedish regulators on retention processes, with specific examples related to ion exchange (or sorption) and matrix diffusion. (authors)