Improved quality of heavy steels and their welds as related to the integrity of RPCs

The improvements of heavy structural steels and their welds for reactor pressure components (RPCs) of LWRs achieved in the past decade are reviewed. These include the steady improvement of the quality of the following specific properties of heavy structural steels for RPCs: uniformity and isotropy of mechanical properties, including less mass effect in the mid-section; fracture toughness; internal defects; weldability; degradation by neutron irradiation. Further, requirements for the easier execution of inspections such as: fewer weld seams in RPCs; larger and more integral design of component materials; change of cast steels to forged steels. As regards main welds, the experience of underclad cracking (UCC) and high levels of Cu content in the weld of some RPCs were two of the incentives for reviewing the welding techniques for RPCs. This has led to a change of SA508, Cl.2 steel to Cl.3 (equivalent designations of steel, too) and a change of welding wire coating from Cu to Al or Cr. A further progress was the application of narrow gap welding processes and control of welding residual stresses. UCCs of two different origins were basically solved by the improvement of base materials and proper welding sequences. The inspectability of RPCs was further secured by the improved internal quality of steels and welds. (author)