Due to the currently insufficient storage capacity for fuel assemblies from the SE-EBO units´ production through their assumed life cycle, a project of an extension of the storage capacity was started and implemented using the compacting approach. An ISFS safety improvement program was started by seismic upgrading of the building and of the technological structures, based on the project developed and the safety-related documentation submitted. Slovak regulatory authorities and organizations approved the starting of the ISFS seismic upgrading and storage capacity extension in late 1997; the completion is planned for 1998 aimed at increasing the ISFS storage capacity and extending the operation life to 50 years.
A new compact cask container is the key element to extend ISFS' storage capacity and to store a total of 14,112 fuel assemblies. The new cask has been designed for 48 pcs (as compared to the current 30 psc) WWER 440 fuel assemblies or 18 hermetically sealed casks with assemblies containing damaged fuel elements. The fuel assemblies are placed in 48 hexagonal storage absorption casks of boron steel arranged in a shape which at the same time corresponds to the container shape and is most convenient for the storage in step ponds. The layout, dimensions of the storage casks, the storage container grid all comply with the criteria for subcriticality. The subcritical value of the fuel assemblies which is given by the compact container grid geometry, does not exceed the value of kef of 0.95 even in pure condensate. Maximum temperature of the cooling water is 500 C.
A handling equipment for the transfer of spent fuel assemblies from the old casks into the new compact casks will be added as a new lifting equipment at ISFS. The handling equipment will be computer-controlled, with the fuel assembly grasping guidance being electronically controlled with an accuracy of ± 2 mm.
In the view of the more stringent requirements concerning the removal of residual heat from spent fuel, the original cooling system of pond water will be replaced by a new system. The new system comprises two plate coolers (one serving as a 100% backup) and 4 pumps (one per pond, the stand-by pond pump serving as backup pump of the other pumps). Heat removal from cooling water will be provided for by an autonomous system of coolant water cooling comprising 3 cooling micro-towers and 2 circulation pumps (one serving as a 100% backup).
The original ISFS project was designed in accordance with Czechoslovak standards (CSN 730036 - Seismic Load of Constructions) applicable at the time of the project design. It does not comply with the present requirements put on seismic resistance of nuclear power plants.
The aim of the ISFS seismic upgrade project has been to increase its seismic resistance to the level required by the present international regulations, according to the geological and seismic surveys performed. Since, at the time of the original project preparation, the results of the SAV-performed surveys were not available, the assignment developed by EQE International was used; it is currently being used also for seismic assessment of other premises (NPP V-1, V-2) at Jaslovské Bohunice. The seismicity-related specifications are defined in detail in the EQE International Report of September 1996 (EQE Project Number 745 009.01).
Building structures, technological equipment, electric equipment and category 1 I&C (1a, 1b, 1c) were classified at the IRLE and/or STN 730036 category 2 level using the "Requirements" method. The calculations made have shown the need for certain adjustments of building structures and technologies the implementation of which is necessary in the framework of the "ISFS Bohunice Seismic Upgrading and Storage Capacity Extension" project.
The implementation of the above project will provide that even upon a seismic event (values of MVZ = 80 MSK 64 with peak acceleration of 0.25 g horizontal and 0.13 g vertical at the ground surface level apply to the EBO site), all safety functions of ISFS will be maintained, and the storage capacity will be sufficient to store spent fuel from NPP V-1 and V-2 units throughout the their planed life cycle.
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