Year
1995
Abstract
The principles on which the Modular Vault Dry Store (MVDS) technology is based now date back a quarter of a century. The original dry store was designed and built by GEC ALSTHOM at Wylfa Nuclear Power Plant in North Wales in the UK and became operational in 1970. The Wylfa station has twin magnox reactors generating a total of 1160 MW electrical output. Fig 1 shows the original Dry Store Cells 1, 2 and 3, which are located midway between the two reactors. The spent fuel is discharged directly from the \"on-load\" reactor into the store. The atmosphere in the storage tubes is carbon dioxide which is also the reactor coolant. At the time of loading into the store, each fuel element is radiating approx IkW of heat. The fuel is stored 12 high in each of 588 storage tubes. The decay heat from the fuel is conducted through the wall of the fuel storage tube and transferred by convection to the air within the cell, which rises by natural buoyancy and discharges to atmosphere through an exhaust stack. Outside air is drawn into the cell through the bottom air intake, producing a continuous self-regulating thermo-syphon. Since the original three dry store cells were built, approximately one third of a million fuel elements have passed through the cells. The store has been very successful and has operated for the past 25 years (the original design life of the station) without any significant problem. Only after completing the design life did some localised wear become apparent. In 1994 when routinely discharging a fuel element from storage Cell No.l, a fuel handling grab generated an error signal. On investigation it was found that the external guide-fins on the grab had worn through the carburised layer and wall of the load/unload tube to produce slots 8 feet long which prevented passage of the grab. This particular tube had seen approximately 200,000 fuel movements. The remedial action taken was to cut away a 13 feet section of the tube and replace it (Fig 2). Before this could be undertaken, all fuel within the cell had to be discharged (approx 6000 elements). To ensure that the repair operation went smoothly and with minimum downtime, a mock-up was built where operators wearing breathing suits could rehearse the procedure. The actual radiation levels in the work area of the empty dry storage cell were quite low; the main hazards were airborne particulates and residual CO2 which could not be vented from the piping system. The repair went smoothly and to programme using three four-men teams working 8 hour shifts. The operation took 7 days from entering the cell to completion of the closing weld on the vessel access hatch. This is the only repair work within the dry store that has been necessary in the 25 years since start-up.