Tieteestä–On science
reactors were made of stainless steels. Very quickly Zr-alloys, because of their low thermal neutron capture cross-section and good corrosion properties in high-temperature water, became commonly used for the fuel cladding and structural components of fuel elements. The most common structural materials of nuclear reactors are low-alloy steels (pressure vessels and piping) and stainless steels (piping, reactor internals, pumps, valves, etc.), Ni-base alloys (steam generators, bolts/pins, dissimilar metal welds, etc.) and titanium (seawater condensers). The selection of structural materials is based on functional requirements (temperature, pressure, water chemistry), design codes and standards as well as technological aspects such as manufacturing experience and capability. Other important criteria are lifespan, inspectability, repairability and the cost of the material and its processing. The requirements stipulated in nuclear codes take the implementation of various aspects into consideration, and they therefore play a key role in materials selection. The codes and standards on which design, construction and plant operation are based include detailed materials and quality requirements, which have become more stringent as experience has increased.
Requirements and research The reliability requirements of materials and components are at their highest in the case of pressurised reactor coolant systems, including the first isolation valve. The second-highest safety class covers all components that are connected with the primary coolant, e.g., the steam generator. Lower safety class components are typically separated by barriers, such as heat exchangers or valves, from higher safety class systems. Engineers involved in materials science in the nuclear industry are very familiar with how
thoroughly the materials aspect is covered by the codes, standards and good industrial practices. There are more than 10 quality requirements associated with the fabrication of components for a large modern pressurised water reactor (PWR), for which material quality controls such as chemical analysis, tensile tests, impact tests, hardness and metallographic examinations have to be performed. Requirements related to material toughness values or critical flaw size are directly related to the safety assessment of a component. Therefore, testing has to be performed by qualified personnel using authorised procedures and facilities. Fully documented traceability of materials, processes and procedures is necessary to enable monitoring by independent inspectors. The non-destructive examination (NDE) of welded structures is important for structural integrity assessments in the design phase, during fabrication and in-service inspections and especially for maintenance or repair actions. Good cooperation between welding engineers and NDE inspectors is crucial in this. Commonly used NDE techniques include the liquid penetrant, magnetic particle, radiography, ultrasonic and eddy current techniques, which have specific sensitivities with respect to their ability to detect defect size and type, but also with regard to the type of material and microstructure being examined. At present, there is a general trend toward fully automated testing and evaluation using data acquisition and processing techniques. The qualification of NDE inspections is also becoming more common. The selection and behaviour of the structural materials for the reactor core and near-core region has to be considered based on the thermal neutron spectrum and the water chemistry of the coolant. The basic principles of operation are similar for all light water reactors, but the materials selection and AALTO UNIVERSITY MAGAZINE 05 \ 31