Advanced structural analysis for the UK nuclear renaissance

Nuclear power reactors contain large steel pressure vessels and high-pressure pipework which must be carefully designed and regularly inspected when they are service to guarantee safety. When a reactor is operating, these systems are loaded not just by internal pressure, but also by thermal stresses which arise from temperature gradients, and by residual stresses which are 'locked-in' during construction. Thermal and residual stresses are often termed 'secondary' stresses and they are generally more difficult to measure and predict than the stresses which result from directly applied forces. Often, this means that parts which are in fact safe are pre-emptively taken out of service due to secondary stress concerns, incurring large costs in addition to plant downtime.

In this project, new techniques will be developed to accurately predict how complex and multi-axial secondary stresses in components behave as they are further stressed in-service. This will require the development of a generalised mathematical framework to describe multi-axial stress relaxation, along with new computational methods to enable the analysis of complicated real-world structures. The predictive accuracy of the new analysis techniques will be tested in a series of experiments using neutron and synchrotron diffraction to observe how residual stresses deep inside metallic components change as they are subjected to changing external loads. The analysis techniques developed during this project will be integrated with existing structural integrity assessment procedures, allowing them to be readily used in industry, and leading to cheaper and more reliable power plants.

Firstly, the outcomes of this research will benefit organisations involved in the design, construction, operation and maintenance of nuclear reactors in the UK and worldwide. Prominent examples include EDF Energy plc, AMEC plc, and the Ministry of Defense. The results generated and the techniques developed during this project will allow such organisations to make more accurate safety assessments of the high-dependability structures necessary for this type of power generation. This applies to both the initial design of such structures, and to subsequent assessments later in the life of the reactor. The availability of more accurate assessment methods will reduce both the capital and operational costs of nuclear power generation to the operator and, ultimately, to energy consumers. This is important in the context of the 2008 Climate Change Act, under which the Government of the UK has a duty to reduce greenhouse gas emissions: nuclear generation is one means of providing low carbon base load electricity supply, and lowering the associated costs will help to keep this important option open as the Government seeks to rebalance the nation's energy mix.

Other companies and organisations whose business involves the analysis of structures which contain thermal and residual stresses will also benefit from this work. These include, for example, companies engaged in the manufacture of welded or forged aerospace components, the construction and installation of welded offshore structures, and the use of large chemical reaction vessels and high-pressure pipework. These organisations will have available a more accurate means of analysing the integrity of their products, allowing them to design better, cheaper goods and to avoid unnecessary pre-emptive component replacement. Hence, the outcomes of this research will make such organisations more efficient and more economically competitive.

In addition to any direct material benefits, this work will profoundly change the current scientific understanding of secondary stress relaxation. It will consolidate and extend two key concepts used to describe the behaviour of residual and thermal stresses (eigenstrain and elastic follow-up) which will allow a complete formulation of stress relaxation behaviour under general boundary conditions to be developed for the first time. The modelling techniques that will be enabled by this and developed during the project will deliver new insights into the behaviour of materials undergoing failure processes including plasticity, stable fracture and creep. Ultimately, this will allow researchers to better understand the seemingly uncharacteristic behaviours that materials can exhibit under the complex and multiaxial conditions which exist in real structures. The later stages of the project will focus on applying the techniques developed to the assessment of structural integrity at the macro-scale. However, these techniques will be completely general and will consequently have wide-ranging scientific implications including, for example, in understanding the micro-mechanics of multi-phase polycrystalline materials. Further details of this are outlined in the Case for Support.