Further Developments in Nuclear Pressure Vessel Manufacture Using the Hot Isostatic Pressing Process and Thick-Section Electron Beam Welding

Abstract

Hot Isostatic Pressing (HIPing) – Powder Metallurgy has been used by Rolls-Royce to successfully manufacture nuclear pressure boundary components such as valves, piping, and pump casings; the majority of these components being manufactured in stainless steels, typically 316L. Rolls-Royce has pioneered the use of this technology in the nuclear field in order to provide cost and lead-time reductions. Rolls-Royce considers there to be significant potential benefits in applying the HIP process, together with Thick Section Electron Beam Welding (TSEBW), to the manufacture of large Low Alloy Steel (LAS) pressure vessels. These benefits would include cost savings, lead-time reductions, an increase in the material quality, and improved inspectability of the as-manufactured material. TSEBW offers the opportunity to dramatically reduce vessel section welding time, and, as no filler material is used, the potential to have a weld microstructure very similar to the parent material, thus providing the opportunity to eliminate through-life vessel weld inspections to reduce plant in-service costs. Production cost and timescale reductions are of particular interest with large vessel manufacture being a most significant contributor to the overall cost and manufacturing time of primary nuclear plant; this against a backdrop of the industry striving to drive down the cost of nuclear power generation in order to ensure viability with other forms of power generation. Applying the HIP process to LAS materials presents particular challenges due to the propensity for oxygen pick-up during the powder manufacturing stage, or in subsequent filling and processing operations. The potential for oxide formation on powder particles presents a risk to the material properties being adversely affected, particularly a material’s fracture toughness, which is critical to the structural integrity of nuclear pressure vessels. Previously, Rolls-Royce has shown it is possible to achieve enhanced tensile properties compared to wrought equivalent material, and to meet the specified Charpy impact toughness requirements. However, under certain conditions, i.e. relatively high oxygen levels in the HIP powder, the Charpy impact toughness was found to be 66% of typical wrought material at room temperature. This paper presents further material testing work conducted by Rolls-Royce to assess any improvement in material property results when the potential for oxygen ingress in the process is reduced. This paper also presents the latest HIP vessel demonstrator work that Rolls-Royce has conducted to assess the viability of the technology to achieve vessel geometries. Of note is the production of a Small Vessel Demonstrator, which, to the best of Rolls-Royce’s knowledge, is the first of a kind HIPed, TSEB welded, LAS, high integrity pressure vessel with integral cladding. Achieving integral cladding by the HIP process has the potential to provide significant cost and time savings by deleting time consuming fusion techniques and machining operations. A Large Vessel Demonstrator section has also been manufactured and successfully EB welded. The paper concludes that although meeting material property specification requirements, further work is required to improve the Charpy impact toughness of HIPed LAS material to achieve similar values to the forged equivalent. Although some improvement in toughness was observed at sub-zero temperatures by reducing the potential for oxygen pick-up in the powder filling process, improvements might be best achieved by focusing on reducing the initial oxygen content of the powder stock, i.e. striving for oxygen levels far lower than 200ppm. It also concludes, that although achieving large-scale vessel geometry is considered feasible, further development work is required in order to achieve near net/net shape vessel geometry to incorporate integral cladding and/or minimising general planar machining operations. This is in the field of preventing geometric imperfections by ensuring a high powder packing density by focusing on the HIP can powder filling process and the powder morphology and particle size distribution, and also possibly can design/manufacture. EB welding has, in the main, been successful, but further work is required to optimise the EB parameters, particularly with beam retraction to prevent the generation of defects.