Nuclear power plants are running on safety margins that are too pessimistic because fracture tests use small lab specimens that underestimate how much punishment large reactor components can actually take. This matters because the UK plans to get 25% of its electricity from nuclear by 2050, but existing reactors—currently supplying 15% of low-carbon power—are scheduled to shut by 2028, except Sizewell B, which may run until 2035 or beyond. Without better failure predictions, operators cannot safely extend reactor lifetimes to bridge the gap before new plants like Hinkley Point C come online. The current defect assessment procedure, R6, is the nuclear industry standard, but it relies on fracture toughness values from standard small-scale specimens that are far lower than what large components actually exhibit. This leads to underestimated critical crack sizes and overly conservative safety margins. If this research succeeds, it will deliver an analytical method to design bespoke laboratory specimens whose crack-tip stress fields match those in real large-scale components. That would allow engineers to determine tolerable crack sizes more accurately, enabling safer life extension of ageing reactors and supporting the UK’s nuclear energy targets without compromising safety. The method will be validated through finite element modelling, fracture testing, and large-scale component data, and integrated directly into R6, which is managed by EDF Energy and overseen by the Office for Nuclear Regulation.
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The Government aims to supply 25% (24 GW) of electricity by 2050 from nuclear energy and have committed to approve up to eight new sites for nuclear fission reactors by 2030. All existing power plants, currently generating 15% (5.5 GW) of low-carbon electricity, are expected to be shut down by 2028, except Sizewell B, which is designed to operate till 2035. Its operator, EDF Energy, is expecting to extend the life of Sizewell B by an additional 20 years. Nuclear capacity will remain below current level even when Hinkley Point C comes online. It becomes almost inevitable that some indications are required on how the current reactors, through save service life extension, will assist Government’s aim before new nuclear power can contribute to the UK’s energy supply. For instance, small modular reactors are considered as a potential contributor to 24 GW ambition. To maximise life of ageing nuclear power plants while ensuring safe operation, more accurate safety margins must be determined. Failure prediction of large-scale structural components according to the current defect assessment procedures is too pessimistic. The reason for this is that material resistance to fracture is measured from standard small-scale laboratory specimens and the obtained values, which are then used in failure assessment, are much lower than those observed in large-scale components. Due to low fracture toughness, the predicted critical crack size in a component is underestimated. Crack-tip condition in terms of stress field at both scales is the key in this context. There has been considerable effort worldwide on the development of approaches for assessing crack-tip condition effects on fracture. However, the treatment of crack-tip condition effects remains at empirical level. The aim of this proposal is to develop and validate for the first time an analytical method for the selection of fracture specimens with non-standard dimensions. These bespoke specimens tested under laboratory conditions would produce crack-tip stress fields that closely correlate with those seen in large-scale components of operational nuclear power plants. Achievement of this aim will facilitate more accurate determination of tolerable crack size and thus less conservative safety margin. An extensive finite element modelling programme, fracture toughness testing of bespoke design specimens and experimental data obtained from large-scale component tests will be used to validate the proposed stress field matching model. The development and validation of this method will be a major improvement to defect assessment procedure R6. The procedure is the nuclear industry standard in the UK for demonstration of safely acceptable defect size in high-integrity components of power plants and small modular reactors. R6 has continuously evolved since the first issue in 1976, enabling safe operation of nuclear power plants. The development of the procedure is managed by EDF Energy with the support from R6 panel consisting of member companies and thus direct beneficiaries: Jacobs Clean Energy, Rolls-Royce Submarines Limited, TWI Limited, Frazer-Nash Consultancy Limited, Engineering Analysis Services Limited, and the Nuclear Research and Consultancy Group. The Panel is observed by Office for Nuclear Regulation. Overall, the outcomes of the proposed research project will make a positive contribution towards maintaining the UK’s position as a leader in structural integrity technology.
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