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Non-nuclear energy

Plant life management and fuel performance

Fission and radiation protection

Within the European Union the safety of nuclear installations is a responsibility of Member States operating under harmonized safety standards and with coordinated research on safety issues at the European level

Many of the nuclear reactors currently operating in the EU are over 20 years old. In the EU a total of 65 reactor units were put in commercial operation before 1980 with a further 78 nuclear power plants commencing operation after this date. Looking at the future of all these plants, there is a natural drive to extend their lifetimes where this can be achieved safely bearing in mind the limits on plant life imposed by aging of non-replaceable components such as the reactor pressure vessel.

Operators and regulators also need screening criteria to exclude risk and methods to determine whether a plant with limited damage can be run safely for a limited time. For assessment of the structural integrity and remaining lifetime of a reactor pressure vessel, relevant and valid data on the properties of construction materials, and how they change with neutron irradiation, are necessary.

Therefore, the nuclear industry is increasingly interested in research activities aimed at better understanding and managing ageing phenomena. These phenomena include changes in mechanical and microstructural properties due to irradiation and corrosion.

The optimisation of the operational conditions of older reactors using, for example, appropriate prediction tools for evaluating the safety margins, and the decision process about plant life management involving, for example, replacement of equipment and prevention of corrosion are becoming key issues for those in charge of plant safety and performance at these facilities.

The following three key issues have been identified as priority research tasks:

  • Harsh environmental conditions impacting on structures and equipment. Research in preventive measures to ensure plant integrity is being performed in a number of networks, often run by the Joint Research Centre of the European Commission
  • The establishment of radiation zones when carrying out inspections, tests, maintenance, supervision and handling wastes. The reactor safety relevant aspects of radiation protection as well as on-line monitoring to ensure the integrity of components and structures are included here.
  • Establishing a safety culture. Aspects of technical and operational procedures are investigated in this area.

Irradiation embrittlement

Lifetime assessment of individual components and piping in nuclear power plant is a mandatory part of Periodic Safety Reports for nuclear power plants. Possible damage needs to be carefully monitored and evaluated to ensure continued safe and economical operation.

Nuclear power plants and associated fuel handling facilities are subject to the usual mechanisms of corrosion, thermal fatigue etc. that effect all industrial facilities. However they are also subject to ‘irradiation embrittlement’ - a process by which the neutron radiation experienced by some structural steel components in a nuclear plant affects the fracture toughness of the material.

Components that are close to the core, for example the central part of the reactor pressure vessel, accumulate a high neutron radiation dose and this irradiation is the most important life limiting component of the plant. The integrity of the primary pressure boundary of the nuclear reactor is essential to its safe operation. New evaluation methodologies have been developed to assess material properties and good progress has been made in techniques to investigate microstructure, which has given a much better understanding of damage mechanisms.

Surveillance programmes and in-service inspections of main components and structures allow early detection of changes in material properties due to a variety of aging mechanisms and prevent macroscopic damage and potential failures. Early maintenance action plans for failure prevention can include changes in operational parameters, more inspections and component replacement.

Mechanical testing of irradiated samples is performed to evaluate the behaviour of aged components and produce safety models for plant components, whilst non destructive testing techniques are a vital part of monitoring processes to ensuring the safe and economic operation of nuclear power plants.

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