The WP-CAVE concept for long-term storage of spent nuclear fuel in an isolated rock cavern has been further developed. Installations and operations have been suggested for horizontal and vertical transportation and for remote handling and final emplacement of 10 years old spent fuel. The fuel assemblies are stored in the same condition as en their removal from the reactor (no encapsulation). The displacements and stress redistributions in the rock mass during the excavation and construction of a WP-CAVE have been calculated by use of the finite element method. The conclusion is that no extensive cracking due to tensile stresses or spalling due to excessive compressive stresses should be expected.
Das Konzept WP-CAVE fur die Einlagerung von gebrauchtem Kernbrennstoff in einer isolierten Felskaverne ist weiterentwickelt worden. Einrichtungen und Bedienungsverfahren werden fuer horizontale und vertikale Transporte und fuer Fernsteuerung und endgueltige Unterbringung von 10-jahrigem gebrauchtem Kernbrennstoff vorgeschlagen. Die Brennstoffelemente werden in demselben Zustand eingelagert wie sie aus dem Reaktor ausgenommen worden sind (kein Einkapseln). Die Verschiebungen und Spannungsumlagerungen in der Felsenmasse wahrend des Ausbaues einer WP-CAVE sind mit der Methode der finiten Elemente berechnet worden. Die Schlussfolgerung ist dass keine umfassende Rissbildung wegen Zugspannungen oder Abplatzung infolge grosser Druckspannungen zu erwarten sind.
Le concept de WP-CAVE pour le stockage de longue duree de combustible nucleaire usage dans une caverne rocheuse isolee a ete developpe. Des installations et des operations ont ete suggerees pour des transports horizontaux et verticaux et pour le pilotage a distance et l''emplacement final du combustible nucleaire de dix ans d''age. Les batteries de combustible usage sont stockees dans les mêmes conditions qu''à leur enlèvement du reacteur (pas d''enrobement). Les deplacements et les redistributions de tensions dans la masse rocheuse pendant l''excavation et construction d''une WP-CAVE ont ete calcules par la methode des elements finis. La conclusion est qu''aucune formation etendue de fissures due à des contraintes de tension et qu''aucun ecaillement par des tensions de compression excessive ne sont attendues.
Fig. 1. Group of three (or more) WP-CAVEs and joint receiving station for spent nuclear fuel. Depth to centre of repository is 300 m. Diameter of central ball-filled cavern is 40 m. Surrounding clay barrier (watertight hentonite-quartz compound) has thickness 5 m. Its distance from cavern wall is 40 m. Cylindrical heat stack in porous cavern enhances natural convection of enclosed air (which evens out temperature peaks). Helical tunnel with horizontal adits is used during excavation only.
In receiving station, each concrete ball arrives on a rail-bound wagon and each waste canister in a radiation-shielded transport cask on a truck. Ball is slowly preheated before being remotely loaded with three waste canisters and being remotely lowered through fuel shaft to horizontal transfer tunnel.
One WP-CAVE can accommodate nuclear waste with a total initial thermal power of 300 kW. This means a storing capacity of one year''s unreprocessed, spent nuclear fuel (about 350 tons) from 12 nuclear 1000 MWe reactors when assuming 10 years of interim storage from time of removal of spent fuel from reactor.
Maximum bulk temperature of circulating air and maximum surface temperature of metal canisters in sealed and abandoned cavern have been calculated to be 200 °c and 330 °c, respectively. These maxima will be reached 10–15 years from time of deposition of waste and closure of cavera. Maximum temperature of clay barrier will be about 40 °c (after 50 years). Maximum increase of temperature at ground surface above WP-CAVE will be less than 1 °C.
Central cavern of WP-CAVE has been estimated to stay completely dry during about 1000 years. No groundwater flow through repository will occur within 100,000 years or more. WP-CAVE has been designed to permit easy retrieval of stored waste for a period of 200 years from time of deposition of waste-carrying balls in heat stack (and sealing of cavern and barrier). No monitoring from outside is needed at any time.
The WP-CAVE storage facility (fig. 1) has been developed since 1976, see Akesson, Bergman & Sagefors (1979, 1980, 1981).
Fig. 2. Assemblies of spent nuclear fuel are taken directly from interim storage (for instance, from Swedish central storage facility CLAB) and are placed (after drying) in a specially designed cylindrical canister having a closed lower end and an open upper end. Fuel channels mayor may not accompany assemblies in canister. Filled canister (with a provisional upper end cap) is transported to WP-CAVE receiving station Where upper and lower collars are remotely mounted and fastened. Canister is then remotely inserted into hole of concrete ball intends to solve two of the most important technical problems met with in any scheme for underground disposal of nuclear wastes: how to keep th0 store dry for a long enough time and how to dispose of the residual heat during the first decades and centuries.
