Analysis of the Reasons for Damage in the Welded Assembly Joining the Hot Collector of First-Loop Coolant to the Du 1200 Tube of the Steam Generators in No. 5 Unit of the Novovoronezh NPP

Resumo

The results of scientific research performed in search of the reasons for the high degree of damage in the welded joint connecting the hot coolant collector in the first loop to the Du 1200 connection piece of steam generators in the No. 5 unit of the Novovoronezh NPP with VVER-1000 (welded connection 111) are presented. The computational and experimental approach used to analyze the accumulation of actual operational damage in the metal in the zone of welded connection 111, adopted in the No. 5 unit of the Novovoronezh NPP, is described. The results of operating the continuous monitoring system for three fuel runs shed light on the high damage in the zone of the welded joint 111 owing to the influence of beyond design basis thermal power loads on the fillet of the annular pocket of the coolant manifold, where operational cracks start and grow. Measures to change the technical specifications for operating the blowdown system of the steam generators, which will make it possible to reduce the operational damage in the welded joint 111, have been developed and adopted in order to rule out beyond design basis actions.