An almost spherical rock cavern is surrounded at some distance by a fairly thick clay barrier. Nuclear waste canisters are mounted in cylindrical holes traversing large concrete balls. These are stored in the cavern together with similar concrete balls filling up the entire cavern space. The clay barrier prevents ground water circulation for a very long time, enhances ion-exchange, and increases the repository resistance against future tectonic movements and earthquakes. Calculations of heat balance, thermal effects, clay barrier function and costs have been performed. One repository (Fig. 1) accommodates one year's unreprocessed waste from 12 nuclear 1000 MWe reactors assuming 10 years of interim storage. Construction operations are outlined. Various advantages of the concept are emphasized (see list at end of paper).
The WP-CAVE concept was first published in the ROCKSTORE 77 Proceedings (Akesson and Hök, 1977). From that on the concept has been developed through theoretical calculations and feasibility studies. The present state of the project is given in a fairly comprehensive report which is available (Akesson, Bergman and Sagefors, 1979).
An overall-view of a WP-CAVE repository is given in Fig. 1. As shown in Fig. 2., the storage cavern proper is almost spherical and has a cylindrical heat stack placed in its centre. The space around the stack is filled with porous concrete balls (Fig. 2). The waste canisters to be stored are porously mounted in large concrete balls (Fig. 2). The balls loaded with the waste are lowered into the cavern through the shaft (Fig. 1) and placed in the bottom half of the stack (Fig. 2). The upper half of the stack is filled with empty balls. The shaft can then be plugged and sealed (no ventilation is needed). Without any monitoring from the outside the sealed and abandoned cavern will remain dry and porous for more than 1000 years. Natural convection through the stack and the ball filling (Fig. 2) will transport the heat released from the loaded balls (dotted in Fig. 2) to the cavern wall. The stack and the balls are designed to stay substantially intact for the first 100-200 years. If desired the loaded balls can during this period of time be hoisted through the shaft (the seal first having been broken) and the stored waste retrieved. After these first 100-200 years the heating power of the stored waste will have decreased to only a few percent of its starting value and there will be no further need for the (convective mechanism. A gradual collapse of the balls, the stack and the cavern wall will then do no harm and can be accepted. The excavation of the slot for the barrier will start from below (Fig. 1) and the slot will successively be filled with a compacted compound of sodium bentonite and quartz sand. This compound tends to expand when absorbing water and will therefore gradually develop a strong pressure against the slot walls. The clay barrier will thus remain continuous and tight for a very long time.