Advanced Nuclear Welding Technologies: a Global Opportunity for the UK
Mahmoud Mostafavi (Lecturer) and Harry Coules (Lecturer and EPSRC Fellow), Solid Mechanics Research Group.
Welding is a vitally important technique in the nuclear industry. For example, a typical nuclear power plant could have hundreds of kilometres of steel pipework, with over 100,000 welded pipe sections and with the requirement for every single weld to be produced to a very high quality standard. As well as pipework, welding is also used for joining in components such as boilers, pressure vessels, reactor support structures and steam generators. In addition to nuclear power stations and other nuclear industry plants involving vessels and pipework, welding is also an essential operation in situations such as the manufacturing of nuclear fuel rods and production of metal containers for storing nuclear wastes.
Despite decades of experience of welding in the nuclear industry, there is still much opportunity to develop improved welding methods and technologies and for the UK to become a global leader in this area. Building additional skills and experience in nuclear welding will help the UK deliver our own domestic nuclear programmes, but also help grow our export potential and become an international partner of choice for other countries and nuclear industry organisations.
One example of an issue which presents scientific and engineering challenges for nuclear welding is residual stress. The heat from welding can result in microscopic changes in the nature of the material in the vicinity of the weld; complex microstructures can form when the weld metal solidifies and cools to ambient temperature. Also, mechanical stresses can become “locked in” to the material; these locked in stresses are said to be residual, because they are separate from any other sources of stress such as high operating temperature or mechanical loading. It is essential to develop an understanding of residual stresses in order to properly assess the structural integrity and safe operation of welded components.
Another engineering challenge arises because welding is intrinsically a multi-variable process. Arc welding is the most common technique in the nuclear industry, but others are available; some of the process parameters that can be varied include weld speed, heat input and post-weld heat treatments. Optimising a welding process is not a straightforward task; computer modelling can be used to help with this as well as production and assessment of test welds in a laboratory environment.
Developing a better understanding of advanced welding and joining technologies for the nuclear industry has been identified as an important issue by UK Government (Department of Business, Energy and Industrial Strategy, BEIS). Frazer-Nash Consultancy (FNC) has recently been awarded a major research contract by BEIS to consider such issues; the Solid Mechanics Research Group (SMRG) at the University of Bristol is one of FNC’s partners on this project, along with Cammell Laird, Nuclear AMRC and VEQTER Ltd (an SME which was created as a spin-out company from University of Bristol in 2004).
In this project SMRG will carry out modelling and experimental work on the issues of residual stress and fracture toughness of welds. This builds on extensive experience that SMRG has in these areas, including another two ongoing projects relating to the nuclear industry:
- Advanced Structural Analysis for the UK Nuclear Renaissance
- ATLAS+; Advanced Structural Integrity Assessment Tools for Safe Long Term Operation
The FNC-led project will last for least two years; SMRG has been able to recruit two new researchers to work on this project, hence helping to build the UK skill base in nuclear welding research. Successful delivery of this project will help develop UK expertise in respect of reducing the cost of nuclear new build, as well as enhancing UK supply chain competitiveness in the international nuclear market.
Towards a Better Understanding of Stress Corrosion Cracking in Industrially Important Materials
Unai de Francisco, SMRG PhD Student
In many industrial settings an important issue is to make use of materials which are strong but light. Sometimes lighter materials can be more expensive, but the extra material cost can be justified due to the better performance from having lower weight. My research activities are focused on developing a better understanding of the behaviour of a category of strong-but-light materials: aluminium alloys.
High strength aluminium alloys have many applications, including construction of aircraft structural components. One of the challenges for the use of these alloys is that, when they are exposed to a moist environment, they can experience stress corrosion cracking (SCC). SCC occurs in materials due to the combined effects of tensile stress and a corrosive environment (such as moist air). The environment that can promote SCC in an alloy is often one which is only mildly corrosive to the metal. As such, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure.
SCC in aluminium alloys occurs via hydrogen embrittlement, where water vapour in the environment dissociates at the alloy surface and produces hydrogen. This hydrogen is absorbed into the alloy, causing it to become embrittled. This phenomenon has been investigated extensively in the past, but the main mechanisms of SCC in high strength aluminium alloys are still not fully understood. This is what my PhD research is about.
My objective is to visualise the very early stages of SCC of high strength aluminium alloys in moist environments. Currently I’m using 4-point bend rigs to load specimens at a fixed displacement and leaving them in an environmental chamber to attempt to observe the early stages of SCC at a microstructural scale. This will be done by periodically visualising the specimens under a 3D optical Alicona microscope, which can successfully characterise the depth of corrosion pits in the specimen surface.
The later objectives of my project include:
• Using combined scanning electron microscopy and electron back scattering diffraction to analyse the SCC. This may be helpful in identifying the crystallographic orientations of the grains which contribute to fracture and aid the understanding of the mechanisms behind the SCC.
• Use atomic force microscopy to visualise SCC at a very high resolution, to detect the cracking initiation at a very small scale.
• Develop a model which accurately depicts the behaviour of SCC cracking at a microstructural scale. This can aid the understanding of the main mechanisms and the prognosis of aluminium alloy components.
Image of a fracture surface obtained with the 3D Alicona microscope:
Before-and-after images showing the emergence of corrosion pits after a sample is exposed to a humid environment: