Long-term evolution of the multi-layer cover: project near surface disposal of category a waste at Dessel

During the current project phase of the disposal of category A waste – short-lived low and intermediate level waste – ONDRAF/NIRAS must, following the governmental decision of 23 June 2006, develop a near surface disposal facility in Dessel, which is based on the near surface facility disposal design developed by the STOLA-Dessel partnership during the previous preliminary project phase. The present report is an integral part of the discussions between ONDRAF/NIRAS and the regulatory body FANC (Federal Agency for Nuclear Control) on the methodology and tools used for the long-term radiological safety assessments. These discussions aim in general at informing and asking the advice of the FANC about the long-term radiological safety assessment methodology, the general principles to be employed in the treatment of uncertainties, and the scenarios that will be considered in assessing the long-term safety of the near surface disposal system. Safety is the most important and driving requirement in planning and implementing such facilities. This report NIROND-TR 2010-03 describes the components of the multi-layer cover and their functioning and provides an estimate of their long-term evolution. Gradual degradation processes and major disruptive events are described. Examples of the latter are earthquakes and erosion. Long-term evolution of multi-layer cover performance is governed by two components: the earth cover - a series of natural materials including sand, gravel, clay - and (mainly) a fiber reinforced concrete slab which forms the structural basis of the earth cover. The multi-layer cover fulfills the safety function limitation of water infiltration through the prevention barriers (R2a). It is difficult to define an exact timing for failure of the multi-layer cover to fulfill the safety function limitation of water infiltration through the prevention barriers. Gradual degradation processes alone would not quickly deteriorate the effectiveness of the earth cover, certainly not the first few thousands of years. However, its behaviour – after release of nuclear regulatory control - will be mainly affected by arthquakes, as the earth cover is not designed to withstand design basis earthquakes (DBE). Available information on impact assessments for earthquakes indicates that disruption of top layers will happen within the next 1 000 years; performance may further deteriorate as a result of sequences of earthquakes in combination with extreme erosion events. These observations have led to the decision to conservatively put the performance of the earth cover to zero as concerns safety function R2a at the end of the nuclear regulatory control phase. Infiltration in the earth cover is assumed to increase to its maximum value, determined by the evapotranspiration of any remaining vegetation (i.e., net rainfall = total precipitation - evapotranspiration). Unlike the earth cover, the fiber reinforced concrete slab will resist design basis earthquakes, although it is not designed to resist beyond design basis earthquakes (BDBE). A BDBE with a return period of 20 000 years and a probability of non-exceedance of 4% at 816 y AP was considered. It is assumed that when the BDBE happens, i.e. at 816 years AP, the fiber reinforced concrete slab will no longer be effective in reducing infiltrating water. From then onwards, the overall multi-layer cover infiltration is maximal, determined by the net rainfall. For the multi-layer cover as a whole, a gradual increase in permeability (and hence infiltration) is assumed from the end of the nuclear regulatory control phase till the time of complete failure of R2a. This assumption will be imposed for safety and capacity calculations. For screening calculations, a much more conservative approach is taken: complete failure 40 years after start of cover placement.