The Swedish and Finnish plan for deep geological disposal of high active spent nuclear fuel is based on the multi-barrier KBS-3 concept. According to the KBS-3 concept, the spent nuclear fuel will be deposited in copper canisters surrounded by compacted bentonite clay at approximately 400–500 m depth in crystalline rock at a selected geological domain. Currently, two design alternatives of the KBS-3 concept exist according to the alignment of the deposited canisters, the vertical KBS-3V variant and the horizontal KBS-3H variant, as illustrated in Fig. 1.
Posiva Oy is conducting detailed repository site characterization studies at the ONKALO underground research facility, located on the Olkiluoto Island, Western Finland. The site characterization studies are conducted for the purposes of an upcoming safety case, submitted to the reviewing authority as a part of the repository operational license application using the vertical KBS-3V deposition variant as a reference design. However, the horizontal KBS-3H design variant has been kept as an alternative to the vertical KBS-3V case, as the horizontal variant holds the advantage of reducing the need for excavation and subsequent backfilling of deposition tunnels, thus reducing overall costs of the repository closure (Posiva 2014).
The rock mechanics site characterization program of Olkiluoto aims to describe the initial state and properties of the host rock barrier associated with the repository volume. The initial state of the repository acts as a basis for any forward evolution model of the repository system, as it represents the state of the host rock volume in which the canisters containing the spent nuclear fuel are deposited and sealed into after repository closure. Understanding the initial state and subsequent forward evolution of the repository system is required for demonstrating the operational- and long term safety of the KBS-3 repository design when submitting the operational license application. This paper describes the main outcomes of a rock mechanics evolution study conducted for the horizontal KBS- 3H repository design alternative for the Olkiluoto site. For establishing quantitative estimates of the deposition drift stability evolution, the following phases of the repository lifespan were studied and boundary conditions for each phase were derived as the loads, imposed on the repository system and deposition drifts, vary over the whole lifespan.
