Chapter II Basic concept of the engineered barrier system and disposal facility

2.1 Basic safety concept

2.1.1 Concept of geological disposal in Japan

The multibarrier concept forms the basis for geological disposal concepts in most countries(OECD/NEA, 1991). The manufactured components of the multibarrier system constitute anengineered barrier system (EBS), consisting of a solid waste form (e.g. vitrified high-levelradioactive waste (HLW) or spent fuel), an overpack (or container) and materials placedbetween the overpack and the surrounding rock during emplacement (backfill or buffermaterials). In this system, the individual barriers have redundant safety functions in order tocompensate for inevitable uncertainties in their long-term performance. However, they arenot totally independent, in that the disposal system consists of passive components that acttogether in a complementary and compatible way.

The present generic study considers a wide range of geological environments throughoutJapan. In this context, an engineered barrier system (EBS) is selected with a highperformance margin to cover the broad range of possible geological conditions. Thisapproach reduces the requirements on the barrier functions of the geosphere, allowing a widerange of geological environments to be considered as candidate disposal sites.

The Japanese multibarrier system is illustrated in Figure 2.1-1. When a potential disposal siteis selected in the future, the optimum design for the engineered barrier system will bedetermined in the light of safety and economic considerations, which take into account thegeological conditions specific to the site.

Buffer

Host Rock

Vitrified Waste(in stainless steel canister)

Overpack(Carbon steel)

• Physical containment of vitrified wasteduring the period of high heatgeneration and radiation field

• Ensures reducing conditions around thevitrified waste

• Sorption of radionuclides onto corrosionproducts

• Containment of radionuclides inhomogeneous glass matrix

• Low radionuclide release to groundwaterdue to high chemical durability

• Stable under thermal and radiation loads

Long-term stability avoiding• Volcanic activity• Significant fault movement• Significant effects due to uplift/denudation• Significant effects due to climate/sea-level change• Presence of natural resources

Favorable geological environment• Favorable geochemistry (e.g. chemically reducing)• Low groundwater flux• Rock mechanical stability• Isolation from the human environment

Function as a natural barrier system• Retardation and dilution/dispersion of released

radionuclides

Geological Environment• Low permeability• Low solute diffusivity• Retardation of radionuclide

migration (Sorption)• Swelling and plasticity• Chemical buffering• Low solubility of many

radionuclides in porewater• Filtration of colloids, organic

matter and microbes

Buffer (Mixture of bentonite and sand)

Figure 2.1-1 The basic concept of geological disposal in Japan