Above Image: Assuring the quality and integrity of welds in nuclear steel grade pipework is important for safe and long-term operation
ASSURING THE QUALITY AND INTEGRITY of welds in nuclear grade steel pipework is critically important for safe and reliable long-term operation of nuclear reactor systems. Failure of a weld within a nuclear power plant can lead to a loss of coolant accident (LOCA) and possibly damage to the reactor core. Repairing faulty welds is very expensive, even in new-build plants. EDF has estimated that repairing 53 welds at its Flamanville 3 EPR construction site in France will cost around €1.5 billion and will have to be undertaken robotically (see Weld Repairs at Flamanville 3, NEI January 2020).
Long term irradiation can result in localised embrittlement of steel or welds in the reactor pressure vessel, heat exchangers and primary and secondary cooling circuits. These problems become more complex in the forthcoming next generation of small and advanced modular reactors (SMRs and AMRs) which may use revolutionary cooling fluids such as molten salts and liquid lead, may operate under much higher temperatures, greater pressures and may see accelerated steel creep- fatigue and radiation damage from the higher neutron fluxes in fast reactors.
In 2019 the UK government launched a research project led by EDF to establish high temperature AMR Structural Integrity Codes and Standards (EASICS).
Since 2009, EDF has operated the UK’s fleet of high- temperature Advanced Gas Cooled (AGR) reactors, constructed between 1965 and 1988. Building on EDF’s experience operating high temperature reactors, the EASICS project will contribute to developing new engineering codes and standards for SMRs and AMRs.
There are currently no widely accepted high- temperature design codes available, because there are known material qualification knowledge gaps. EASICS is critically important for new materials and fabrication methods developed for SMR and AMR manufacture, since qualification by computer simulation, using the new EASICS design codes, is likely to be the only way that British manufacturers can qualify reactor components in the UK. Even so, EDF says that there are shortfalls in the available codes and standards, which lack high temperature materials test data. Such test data is needed to verify and validate the simulation code results.
SMR stakeholders may have to accept a patchwork of different technical approaches to regulatory assessment, perhaps combining some physical testing in laboratories with much greater reliance on computer simulation.
The key purpose of EASICS is to enable SMR and AMR designs to undergo Generic Design Assessment (GDA) by UK nuclear regulators — an essential step if these next-generation reactors are to be deployed in the UK. A licensed design is also essential if a reactor is to be marketed internationally. In 2019 Rolls-Royce secured an initial £18 million tranche of funding under Prime Minister Boris Johnson’s Conservative government to develop a 440MWe SMR. This would be matched by a further £18 million raised from the private sector (UK Confirms Support for Rolls Royce Led SMR, NEI, November 2019). Both the UK Department for Business, Energy and Industrial Strategy and the independent Office for Nuclear Regulation have emphasised the importance of digitised data management for new reactor designs. BEIS’s Advanced Manufacturing and Materials (AM&M) Phase 2b R&D programme, launched in January 2020, focuses on development of modular design technology, reduced cost and the ability to meet regulatory standards.
R&D funded under the AM&M programme must be capable of meeting ONR Nuclear Site Licence Condition 17 and enhance the use of digital information to support regulation. This concerns management systems and quality management arrangements, which are increasingly digital. Not surprisingly, the programme stresses the importance of developing new digital techniques that support design and quality assurance activities under Licence Condition 17.
Radiographic tracking
At present, testing and assurance of reactor weld quality relies on manual x-ray radiography records and paper- based record keeping, with limited ability to digitally interrogate data or provide digital regulatory assurance.
Radiography of reactor welds is essentially an analogue process rather than digital. Welded joints in nuclear plant pipework are x-ray radiographed to detect potential abnormalities or defects in the welds. A radiographic film provides a traceable permanent visual record of the weld quality, but the process is time-consuming, expensive and prone to operator error. Strong data record-keeping is needed to correlate each weld with its individual test radiograph result. Errors can occur, either by mismatching a radiographic test result with the wrong pipe weld or by
fraudulently substituting a successful radiographic test result for a defective weld. Strong data management assurance is critical for preventing both.
Furthermore, the process of radiographic testing can substantially affect site productivity and cause construction delays. Radiography of pipework by skilled industrial radiographers usually requires a large area of the reactor construction site to be isolated and the evening construction shift workforce to be evacuated to prevent any unnecessary radiation exposure. The industrial radiographers work in isolation, under pressure to complete testing as quickly as possible so that site construction can resume. Radiography is often undertaken overnight to minimise construction delays, but some disruption to the evening construction shift teams inevitably occurs.
Similar problems will also apply to the future manufacture of SMRs and AMRs in a factory environment. The commercial manufacture of SMR cores in an advanced Industry 4.0 factory strongly depends upon near- continuous operation, minimising production line shutdowns.
In order to benefit from economies of scale, SMR factories may be configured to produce 10 units per year (one every five weeks). Even using robotic technology, this is an astonishingly fast production rate. By comparison the British Royal Navy’s PWR2 small modular reactors deployed on Vanguard Class nuclear submarines were manufactured by Rolls-Royce at rate of about one every four years. With a very tight five-week production schedule, commercial SMR core assembly factories operating on a continuous basis could be significantly interrupted by the radiographic requirements necessary for testing and validation of the welds in SMR and AMR reactor cores.
Cost savings with BE-tag
The French national standards organisation AFNOR (Association Française de Normalisation) published a completely new best practice standard in October 2019 that should significantly improve the data integrity and quality assurance of nuclear reactor welds. AFNOR Standard NF A09-283:2019 Traceability and Security of Radiographic Testing provides a strong system of data management controls for radiographic inspections.
Although the standard currently applies only in France, it is likely to be widely adopted and utilised across all French nuclear builds internationally.
The standard defines the minimum technical requirements of a system providing reliability, security and traceability of radiographic testing. The standard also provides recommendations for implementation and is applicable to radiographic testing performed on X-ray films, digital radiography with rewritable media and digital radiography without media.
French start-up company BEWEIS has been closely associated with the development of the new AFNOR standard. After developing digital weapon-tracking technology for French special forces in the 2010s, BEWEIS adapted its military technology for the nuclear sector to provide digital quality assurance for reactor welds. BEWEIS’s BE-tag technology is a hardware and software solution that combines radiographic film with RFID tracking and software record keeping. The system provides very powerful quality assurance of radiographic testing of nuclear reactor welds, with real-time inspection, monitoring and defect management, and is easy to use.
The BE-tag system allows NDT radiographers, manufacturers, site construction supervisors and senior managers to view weld radiography data with different levels of access and oversight as needed across a secure shared database. It is autonomous and can operate across a local WiFi network, with strong encryption, providing real-time status updates for example to tablet computers, without manual inputs required by NDT operators.
Operator error or forgery of results is impossible due to the combination of radiography test identification systems deployed, some of which are hidden from the NDT radiographer.
Digital data management gives nuclear regulators full access to understand and interrogate all welding test data that may be required, including its full digital history.
The system has been piloted at different kinds of high-hazard industrial and nuclear facilities in Europe. Pilot studies have shown that BE-tag delivers 25-30% productivity improvements in NDT assurance by simplifying and speeding-up the radiographic test procedure, and has also delivered 70% reduction in staff supervisory costs for NDT. Furthermore, in the nuclear pilot study no weld identification and tracking errors occurred, with 6500 welds inspected and 20,000 films generated.
Another major benefit of BE-tag lies in the economic costs avoided thanks to real-time weld inspection tracking and the BE-tag anti-fraud system. It eliminates the need for complex and expensive repairs caused by late weld-defect identification and rectification repairs, and also there are no re-inspection costs. For example, early deployment of BE-tag could have potentially reduced EDF’s €1.5 billion weld rectification costs at Flamanville 3. Such rectification costs would be significantly lower because detection and repair can be completed at an early stage of construction, rather than afterwards. The BE-tag hardware has recently been incorporated into the NDT requirements for the ITER International Experimental Fusion Reactor in France.
Delivering the UK’s Nuclear Sector Deal goals
The nuclear sector faces substantial pressure to reduce costs, particularly for new builds. In 2018 the UK government set a target in its Nuclear Sector Deal for a 30% reduction in the cost of new build projects by 2030.
The Deal noted that achieving this will depend on joint action by the nuclear industry and government to address the main drivers of reactor construction costs, including through advanced construction techniques. The government’s view was that innovation in manufacturing processes, using digital models and virtual reality to improve construction methods, can deliver significant reductions in costs and programme schedule.
BEWEIS’s digital and anti-fraud BE-tag technology has the potential to help drive substantial cost savings both for large scale EPR and small scale SMR/AMR reactor projects in the UK and abroad.
Author information: Ian Jackson is a Nuclear Consultant
