Cladding of nuclear components, in particular large vessels, is applied to prevent the corrosion of the vessel substrate by the aggressive nature of the operating environment. The vessel substrate material is typically a low-alloy steel such as SA508 Grade 3 Class 1, which is a commercial-grade pressure vessel steel used across the nuclear sector. This substrate material can be covered or ‘clad’ with a less corrosive, more inert material such as a nickel-based alloy or a grade of stainless steel.

The current cladding technique is weld-based using wire or strip, fused to the substrate via the welding process. This is an expensive, time-consuming and materially wasteful process — to attain the necessary inert chemistry, several weld passes are made to build up the clad thickness. After each pass, a run of NDE is required to ensure that the clad is not only fully bonded to the substrate, or its previous clad layer, but also that there are no excessive defects such as cracks or high levels of porosity. The final clad layer is then machined back to attain the chemistry content and to provide a good-quality surface finish with a geometrically sound profile. In a typical cladding process, up to 60% of the laid-down clad could be machined away, making the current process wasteful.

The ideal cladding technique would lay down material onto the substrate in a single pass and would not require post-clad machining. The cost and time-savings associated with such a technique would be significant. One solution is to adopt an additive manufacturing process, known as diode laser powder deposition (DLPD). Cladding by DLPD is a deposition welding process in which a layer of metal powder (the clad) is deposited on the parent (the substrate) material. The two materials are fused by the energy provided by the laser and a metallurgical bond occurs. Figures 12.7 and 12.8 show the deposition of clad during proving trials.

This cladding technique offers high precision, a high level of automation, a robust and repeatable process and a clad-substrate interface with low dilution, thus attaining the necessary chemistry with a thin layer of clad deposit. This cladding technique is significantly quicker (see Table 12.1) than the existing weld-based technique and combined with a single-pass potential and the possibility of including an in-line NDE/process feedback system as a part of the cladding process, cladding of large vessels could be reduced from weeks to hours.