During a routine inspection of Reactor 2 at Chapel cross nuclear power station in 1997, a crack was found in the pressure retaining shell of heat exchanger 6. Further examination revealed that the crack was large, extending about 230mm along the surface and up to 90% of the wall thickness. There was no threat to nuclear safety with the reactor shutdown, but it was clear that full investigation and repair weld were necessary before the reactor could safely re-start.
BNFL consulted TWI at an early stage. The repair was to be made by completely cutting out the crack and core holes leaving an elongated opening through the shell. The opening was then filled by controlled deposition of weld metalfrom outside the boiler in a sequence that would optimise residual stresses and minimise the risk of cracking.
With so many difficulties and potential problems, the strategy was to take all possible steps to maximise the integrity of the repair. The key lay in achieving a defect free weld with maximum grain refinement to limit embrittlement in the HAZ regions. Welding trials conducted at TWI confirmed that the weld repair procedure had been optimised in terms of HAZ microstructural refinement and tolerance to welding variables.
Once completed, the weld had to demonstrate to be defect free by a programme of non-destructive examinations in which there could be the utmost confidence. The diverse inspection techniques applied included magnetic particle, radiography and ultrasonic manual pulse echo and time of flight diffraction (TOFD). An Inspection Qualification Body (IQB) led by TWI generated the necessary confidence in the inspection by proving the capability of the inspection in accordance with an established European methodology.
In order to develop a safety case, BNFL had to show by an engineering critical assessment (ECA) that the limiting crack size was well in excess of the threshold size for the detection capability of the NDE. The ECA required data about the fracture toughness of the weld metal and HAZ regions and the operational and residual stresses in the repaired condition. TWI obtained this data from programmes of fracture toughness testing and residual stress analysis.
The project demonstrated that a significant opening in a safety critical boiler could be repaired by filling with weld metal in a way that can meet the pressure systems regulations and additional requirements appropriate to nuclear plant. Vintage silicon killed boiler quality steel, equivalent to BS 1501:161C, can be successfully welded using a controlled deposition weld buttering procedure. The normal requirement for a pressure test after such a repair could be avoided on a basis including a rigorous reliable inspection proved by formal qualification. The repair was judged by BNFL to have been an outstanding success that had returned an old vessel to full working order.