Conceptual studies for underground siting of high-temperature gas-cooled power reactors

Contrary to foreign activities aiming at 'siting deep in rock', in the FRG research is done on 'cut and fill' technique in soil because of topographical and geological conditions. The HTR offers itself because of high safety and variety of applications. Underground material acts as additional confinement of radioactivity, becoming effective after hypothetical failure of containment. The transport of contaminants is extremely delayed, decisively influenced by actual underground conditions. The most important radioactive materials are retained almost totally by way of ion exchange (Cs, Sr) and physical adsorption (Iodine, noble gas to a certain extent). Because of mitigated spread of activity an inadmissible contamination of groundwater will not occur especially as counter-measures are possible. Ground-level release of radioactive noble gases can be prevented by the lay-out of back-filling and coverage. Underground-siting guarantees better protection against extreme external effects which must be taken into account to avoid 'common mode failure'. Shock waves can be eliminated as well as area conflagration and aircraft impact by adequate coverage. Solid layers of soil will have a positive effect on seismic behaviour of the embedded structure, not at last because of damping. Underground-siting causes easier solution of decommissioning problems, high adaptibility to future requirements, and better protection against sabotage and direct radiation. For the realisation of large pits different techniques are available. They will be discussed in connection with a concept for underground siting of a HTR plant. An approximate site-independent safety is ensured by a firm and unpermeable back-filling of the clearance between the pit wall and the containment structure. Costs and construction time must be estimated in detail taking into account all influencing factors. (orig.)