It was intended at the time of acceptance of the Technical Design of Paks Nuclear Power Plant, that following a three-year decay in the decay pool, the spent fuel assemblies would be taken back to the Soviet Union on free of charge basis. The spent fuels were to be reprocessed in the Soviet Union with all reprocessing residues remaining in the Soviet Union. This was a worldwide unique service from the Soviet partner to the owner of the power plant, since the commercial reprocessing procedures all around the world included that the end-products (uranium, plutonium, low- and intermediate-level conditioned wastes) should be returned to the country of origin of the spent fuel. The use of this returning services involved at the same time that closed fuel cycle option, along with above detailed services, was adopted by Hungary with regard to the back-end of the fuel cycle.
Following the commissioning of the first power plant unit, the conditions of the return shipment were unilaterally amended by the Soviet Union. This included the increase of the required decay period to five years and the claim of the Russian party on continually increasing payment for the acceptance of the returned fuels.
In its efforts to meet the new requirements, Paks Nuclear Power Plant has doubled the capacities of the decay pools by decreasing the lattice pitch of the storage racks, and the return shipments were managed in the framework of treaties. Between 1989 and 1998, 2331 spent fuel assemblies were shipped back to the Soviet Union (later Russia).
Upon the effects of the problems that appeared in the field of the return shipments, studies were initiated by professionals of Paks Nuclear Power Plant to find alternative options for the storage, re-processing and disposal of the spent nuclear fuels. Following its decision to construct an interim spent fuel store, a contract was made by Paks Nuclear Power Plant in September 1992 with GEC Alsthom from the UK, for the construction of a Modular Vault Dry Store. One advantage of using this type of construction that the modular design of the store facility allows the extension at any time, as required.
The first three modules and the reception building of the facility, which is capable of storing the spent fuels, at least for a period of 50 years, have been constructed by 1997. This was followed by the hand-over of a four-vault module in 2000 and another one in 2003, and finally the construction of a new phase including five vaults was completed in 2007, thus providing a total storage capacity for 7200 spent fuel assemblies.