Depending on the objective of the projects, they can be grouped mainly into research for fusion or next generation reactors, surveillance and testing, modelling and standardisation.
The promotion of nuclear fusion technology (NET) and the development of next generation nuclear power plants and necessary materials (including ARCHER, GETMAT, HELIMNET, MATTER and RAPHAEL) required fundamental research for material properties under different operating condition or with new cooling liquids including fatigue, creep or creep-fatigue tests.
The development of non-destructive monitoring techniques in some JRC Direct Actions [JRC19983] [JRC24554] [JRC26830] [JRC32454] as well as the evaluation of inspection techniques to detect material changes in the GRETE project are related to surveillance and testing. They are not related to a specific industry but are cross-cutting techniques that can also be used in the nuclear field.
The development of a specific creep-fatigue model which also could be applied to materials used in advanced gas-cooled reactors (AGRs) was part of the MATTER project, whereas multiaxial load was in the scope of some JRC Direct Actions [JRC13190] [JRC23875] [JRC28109] as well as in European Coal and Steel Community (now RFCS) funded research (7210-MC/109).
Standardisation and/or harmonisation was an objective of some projects and concerned different fields: fitness for service in the FITNET project; thermo-mechanical fatigue in the TMF-STANDARD project; and lifetime assessment for VVER nuclear power plants (NPPs) in the VERLIFE project.
2.2.1 Fusion and Next Generation Reactors
Research projects dealing with mechanical fatigue have partly been performed to promote nuclear fusion, e.g. projects of the Next European Torus (NET) network. Nevertheless, some results are also of relevance for nuclear fission because the investigated parameters, such as temperature are similar or close enough to the operating conditions of NPPs [JRC8609] [JRC12412] [JRC16627] and because materials have been tested that are also used for components in operating European NPPs, e.g. AISI 316 type stainless steel.
Further, projects from Euratom FP7 (ARCHER [ARCHER2015] [GETMAT2014] [Klenk2013], HELIMNET [HELIMNET2013] and MATTER [MATTER2016] [Utili2012]) were also intended for the development of new generation reactors operating at different conditions and using different coolants than actually running European NPPs (liquid metal cooling, supercritical water), which requires research in the field of new materials like ferritic/martensitic steels (e.g. P91) or oxide dispersion-strengthened (ODS) steels, which are not used in operating European NPPs. Although investigation of fatigue or creep-fatigue ageing mechanisms was one objective of the projects, most of the results are outside the scope of this synthesis report due to the material selection.
2.2.2 Surveillance and Testing
The non-destructive monitoring of the fatigue damage evolution is of high interest and has been investigated using different techniques. One of these techniques for the in-situ investigation of fatigue is the positron-lifetime measurement that has been investigated in different JRC Direct Actions [JRC19983] [JRC26830] [JRC32454]. Within a comparable context was a JRC Direct Action to improve the potential drop method for monitoring crack growth in real components subjected to creep-fatigue [JRC24554]. A finite element calibration technique was applied and the accuracy of the potential drop method under combined creep and fatigue conditions could be improved.
Non-destructive examination (NDE) techniques for piping and an evaluation of the different methods were also the subject of the GRETE project in Euratom FP5. This project included the preparation of low-cycle fatigue specimens made from different austenitic steel grades, together with destructive and non-destructive measurement on these specimens [EUR22282]. To monitor fatigue damage accumulation in austenitic steel piping, electromagnetic and ultrasonic techniques showed the most promising results within the project. For the application of these techniques on components in an industrial plant, more detailed information than normally available is required concerning fabrication of the material, production, and installation of the components.
Research projects dealing with multiaxial fatigue load have the potential to provide the foundation for more sophisticated and realistic models to predict fatigue life. Such projects include those funded by the RFCS (project 7210-MC/109 [EUR20050]) as well as JRC Direct Actions [JRC13190] [JRC23875] [JRC28109]. The materials investigated within some of these projects, however, are normally not used for passive components of the primary or secondary circuit in NPPs.
The project from Euratom FP7 was intended to perform research studies on the materials behaviour under GEN IV NPP operational conditions, to sustain high fast neutron fluxes and high temperatures, as well as to comply with innovative reactor coolants, and to find out criteria for the correct use of these materials in reactor applications. Besides the material characterisation a creep-fatigue model has been developed, which had been evaluated with grades of AISI 316 steel creep and fatigue tests [Utili2012]. It was shown that the model works well within a scatter band of 2 and that the time to creep rupture or time / cycles to failure in creep-fatigue could accurately be predicted even for low cycle fatigue (LCF) experiments. Other models to predict creep-fatigue like the linear life fraction rule (LLFR) are based on stress rupture curve and fatigue curve to calculate total creep life fraction and total fatigue life fraction (identical to the cumulative usage factor – CUF) consumed. The total damage is the sum of the two total life fractions and normally should be one. This simple approach does not contain any physical damage considerations but has been in use for a long time. Using the model from the MATTER project a separation of creep and fatigue damage is not necessary [Utili2012].
Test data from the project are hosted at the European Commission engineering materials database, MatDB. Although restricted access, some of the data are enabled for citation and hence data access requests can be submitted to the data owners.
Different damage approaches for life assessment in the creep-fatigue range have been evaluated in an EC-funded structural analysis project [EUR15761], where different life prediction methods were compared using data for the AISI 316 austenitic stainless steel.
The development and standardisation of a “fitness for service” (FFS) method in the thematic network was intended to be used in a wide range of industrial sectors including the nuclear industry [Hadley2008] [Janosch2005] [JRC33994] [JRC37165] [Kocak2006] [Taylor2003]. The procedure is intended to allow an analysis at any stage in the lifetime of a component, including design, fabrication, operation, life extension and failure investigation. It also should cover all major failure/damage modes, i.e. fracture, fatigue, creep and corrosion. Mechanical fatigue is one of the ageing mechanisms being considered for fracture assessments – either for assessments performed during the design phase or for postulated or detected cracks in components. In the case of postulated or detected cracks, a limitation of the maximum load during the lifetime or the prediction of the residual lifetime is of interest. The fatigue module of the FITNET FFS procedure comprises five different approaches to fatigue analysis. The first three are essentially fatigue damage assessment approaches, which assume that there is no pre-existing flaw in the structure and presenting different complexity of loading, whilst the last two are based on the assumption that a flaw exists [Janosch2005].
Harmonisation (and therefore standardisation) of VVER and PWR codes and procedures to assess lifetime of components and piping in VVER NPPs was also the main goal of the VERLIFE project [VERLIFE2003]. A methodology was proposed for the assessment of the lifetime of components and piping that takes into account the fatigue ageing mechanism. Guidance on the evaluation of indications is provided and design criteria are established.
Standardisation was the objective of the TMF-STANDARD project. Within the project, equipment and protocols for testing were developed to realise load profiles which could cause mechanical fatigue and thermal fatigue at the same time, so called thermo-mechanical fatigue (TMF). Although the majority of pre-normative research and development and validation testing have been carried out on Ni-based alloys using out-of-phase (Re = -1, f = 180°) and in-phase (Re = -1, f = 0°) TMF cycles it is neither restricted to a certain class of materials, nor to a specific type of TMF cycle [EUR22281] [JRC40879] [JRC40880] [JRC42967]. The testing of the Ni-based alloys has been performed at temperatures beyond the ones of operating European Pressurised Water Reactors or Boiling Water Reactors, namely 400 °C to 850 °C, but still in the operating range of Advanced Gas-cooled Reactors.
Test data from the TMF-STANDARD project are hosted at the European Commission engineering materials database, MatDB. Although restricted access, some of the data are enabled for citation and hence data access requests can be submitted to the data owners.
2.2.5 Further Research Outside the Nuclear Field
Further research projects (7210-MA/131, 7210-MA/823 and 7210-MA/951), funded by the RFCS and not directly linked to nuclear fission, handle the problem of fatigue design of welded stainless steel plate or tube and provision of design guidance [EUR19972]. The investigation of manufacturing techniques including welding techniques and post-weld improvement techniques in the RFCS project 7210-PR/303 [EUR22809] was intended to improve fatigue endurance of welds. Although not directly related to nuclear industry, the research results can be applied to operating European NPPs as the tested materials are similar or a generic approach is used.