Impact of water chemistry operations on the deposit of corrosion products in the fuel assemblies at Paks Nuclear Power Plant

摘要

Numbers of Steam Generator decontamination operations were carried out prior to feed-water header replacement work for three of the four VVER-440 type Units of Paks Nuclear Power Plant. The internal surface of the Steam Generator heat exchanger tubes had not been sufficiently passivated after the decontamination, which caused the release of excess corrosion products into the main coolant loop. The deposited corrosion products had reduced the flow cross section within the fuel rod bundles and, as a consequence, increased the hydraulic resistance of the fuel assemblies. Decision was made to remove the deposit from the fuel assemblies with the use of a chemical cleaning method. During these cleaning operations carried out in a separate tank at the reactor, 30 fuel assemblies have seriously damaged as a consequence of insufficient cooling due to a design error of the cleaning tank. To minimise the rate of deposit, a new, originally not used water regime was developed for the start-up/shut-down operation conditions, for which we have a two year long operational experience now. The water regime parameters used for normal operation conditions have also been revised and international experts were requested for expert opinion on the established modifications. The introduction of the modifications is in progress now.