Figure 11.26 illustrates the three basic options for repository design:

1. Rooms, mined, accessible to store waste containers

2. Caverns, mined or leached, not accessible, to be charged through a shaft either from the surface or from a lower level

3. Galleries with storage boreholes in the floor, mined, accessible to charge the boreholes

The choice among these options depends on the type of waste and the filling technique appropriate for the type.

Accessible storage rooms are simple but useful only for waste with a low surface dose rate. Otherwise the waste would have to be stored with lost shielding, which is usually uneconomic.

Caverns may be used for wastes with higher surface dose rates because the waste container can be dropped from a shielded cask into the cavern. The heat generation, however, must be very moderate, because dropping the containers into the cavern leads to a random array not optimized in terms of heat dissipation. In West Germany an In situ solidification process for non-high-level waste is being investigated where granulated waste mixed with a binder is to be pumped into a cavern.

Single boreholes in the floors of galleries are provided to hold high-level glass cylinders. The cylinders are carried in heavily shielded casks and are then lowered into the boreholes. Single boreholes can be arranged in a way that the heat is sufficiently dissipated to maintain maximum permissible peak temperatures in the salt.

Rode mechanical stability. The main potential hazard to the integrity of an underground repository has its roots in rock mechanical failures. The stability of the repository depends on many factors, such as the volume of the rooms relative to the pillars. Convergence of rooms due to the plasticity of the salt and enhanced by the elevated temperature may cause stresses within the rock salt. It is therefore important that a repository at least for HLW should be built in a salt formation not mined before. Moreover, only the space required for a minimum number of years should be mined at the same time, and every room used up should be backfilled with crushed salt. On the other hand, convergence will help to eliminate open space in the rock salt quickly after rooms have been backfilled and will thereby be beneficial.

Another factor affecting rock mechanics is the temperature. The rock has attained a quasi-equilibrium state corresponding to the geothermal temperature gradient over millions of years. Only formations having this tectonic stability are eligible as waste repositories. Inserting high-level waste will inevitably disturb this equilibrium by raising the temperature in the salt and by creating new gradients. The natural temperature at depths of 1000 m is in the neighborhood of 40 to 45°C with gradient of a little more than 2°C per 100 m.

According to suggestions made in an Oak Ridge study [Cl], the following temperature criteria are to be met:

Waste temperature should not exceed the temperature of the solidification process.

No more than 1 percent of the salt shall be at a temperature above 250°C.

No more than 25 percent of the salt shall be at a temperature above 200°C.

If camallite interspersion is expected, the maximum temperature shall be limited to 100°C.

In West Germany 200°C is presently envisaged as an upper limit of the waste canister surface temperature.

In general, the temperature increase caused by the waste should be kept low to ensure that the quasi-equilibrium is disturbed as little as possible. It may turn out as a result of further thermomechanical analyses that it is desirable to age the solidified HLW for quite a while in engineered storage before it is put into a geologic repository.

Thermal analysis. The temperature distribution in space and time is given by the following differential equation:

cp = div (к grad T) + q’ at

where c = specific heat p = density к = heat conductivity

q’ = heat-production rate of the source per unit volume c, p, and к are functions of space and temperature. Equation (11.7) has been solved numerically [С1].

A parametric analysis has been conducted with room width, waste package array (pitch), waste characteristics, and diameter of HLW container as parameters.

Optimization leads to a set of parameters indicated in the schematic cross sections of the repository presented in Fig. 11.27. The diameter of the glass cylinders is 6 in (15.24 cm). These parameters will permit storage of the 20-year HLW production of a 1400-MT/year reprocessing plant for 20 years in an area of about 0.5 km2.

Figure 11.28 illustrates the temperature distribution throughout the repository. The maximum temperature rise at the hottest spot of the salt, according to this calculation, will be about 175°C and will be reached after about 50 years.