Mitigating Environmentally Assissted Cracking Through Optimisation of Surface Condition

Periodic Reporting for period 3 - MEACTOS (Mitigating Environmentally Assissted Cracking Through Optimisation of Surface Condition)

Berichtszeitraum: 2020-09-01 bis 2022-02-28

Environmentally Assisted Cracking (EAC) is one of the major failure modes occurring in light water reactors (LWRs), affecting our safe, reliable and economic nuclear energy production. The condition of surfaces exposed to the primary coolant plays a main role on the susceptibility of components to EAC. However, many national and international guidelines and standards do not address surface condition of critical components in NPPs.
The goal of MEACTOS project is to improve the safety and reliability of Generation II and III nuclear power plants (NPPs) by improving the resistance of critical locations, including welds, to environmentally-assisted cracking (EAC) through the application of optimized surface machining and improved surface treatments.
The resulting quantitative knowledge and understanding of the effect of improved surface conditions on EAC initiation resistance will be used to formulate EAC mitigation strategies that are based on optimization of component surface condition
MEACTOS will help to improve safe and reliable economic nuclear energy production in Europe.

The initial activity of the project was to organize a workshop to establish the initial state of the art with which to outline the research strategy to be developed. Among the decisions adopted in this workshop are:
- Research strategy: two experimental campaigns with up to 5 environments where chosen materials has experienced cracking in service, these environments tested with different acceleration factors for interaction material/environment, the first phase using high factors to establish a threshold stress to initiate cracking by EAC, the second phase at constant load, to verify crack initiation times at stresses around the stress threshold
- Materials to be used: A 812 as a representative of plants in operation to verify the applicability of advanced machining techniques as mitigation procedures, 316L as a candidate material to be used in the construction of new power plants.
- Analysis procedures: use of CERT tests at different strain rates (decreasing), determination of the critical stress for each strain rate, the threshold stress for a material/environment tested would be defined as the asymptote of critical stress versus deformation rate curve representation.
To summarize the test results of the screening phase, the trend of the observed results seems to indicate that advanced machining slightly improves the resistance to the initiation of cracks for Alloy 182, compared to traditional machining and to the reference.
The initially peened specimens showed a lower stress threshold and hence lower resistance to EAC but the additional specimens showed the highest stress threshold and hence highest resistance to EAC. Hence, shot peening can potentially be of benefit to improve EAC initiation but in this respect the quality of the peening procedure is of utmost importance.
A general outcome of MEACTOS - though expected to some degree – is that high surface hardness can be associated with low critical stress and therefore high EAC susceptibility. Another general result is that different surface treatments generally show different critical stress levels but deviation between surface treatments was not that significant.
Assessing the impact of new advanced surface finishing methods was one of the main goals of MEACTOS. Since critical stress data for STI- and SAM-surfaces show only little difference it can be further concluded, that advanced surface machining methods have nearly the same impact on EAC initiation behaviour than standard methods, i.e. they are not inferior. In combination with benefits like higher cutting speed and less pollution by lubricants, advanced surface machining methods are therefore a promising alternative to standard procedures.
Roughness values can support conclusions towards EAC initiation that are basically drawn by means of hardness evaluation.
The overall conclusion that has to be drawn from the discussed results is that surface hardness (and if applicable also surface roughness) has to be limited to the already defined national thresholds to keep critical stress for EAC initiation above a certain limit – preferably yield stress - and avoid EAC in practice. The outcome of MEACTOS is therefore a comprehensive scientific evaluation of some existing best-practice knowledge that is already defined in a few national nuclear codes.
To date, a total number of 46 MEACTOS-related contributions have been presented at meetings and conferences and 10 journal papers have been published. Most journal papers are available open access.
A total number of 162 datasets have been uploaded to MatDB. All datasets were very carefully peer reviewed and corrected if needed.

The project is aligned with the expected impacts detailed in the NFRP1-2016-2017 Work Programme:
- To help industrial stakeholders to develop efficient solutions in response to the new requirements of the amended Nuclear Safety Directive.
The overall conclusion that has to be drawn from the discussed results is that surface hardness (and if applicable also surface roughness) has to be limited to the already defined national thresholds to keep critical stress for EAC initiation above a certain limit – preferably yield stress - and avoid EAC in practice. The outcome of MEACTOS is therefore a comprehensive scientific evaluation of some existing best-practice knowledge that is already defined in a few national nuclear codes.
- Reinforcement of the safety features of the Generation –II and –III EU nuclear reactor fleet.
Assessing the impact of new advanced surface finishing methods was one of the main goals of MEACTOS. Since critical stress data for standard industrial surface finish and advanced machined surfaces using supercritical CO2 show only little difference it can be further concluded, that advanced surface machining methods have nearly the same impact on EAC initiation behaviour than standard methods, i.e. they are not inferior. In combination with benefits like higher cutting speed and less pollution by lubricants, advanced surface machining methods are therefore a promising alternative to standard procedures.
- Improve the market profile of EU-based reactor designs and strengthen the competitiveness of the EU nuclear sector through promoting an excellent level of safety in response to market requirements and trends.
For the European Union, the world's largest energy importer, reducing dependence on fossil fuels is not just a fashionable technological trend, it is a strategic necessity.
The results of MEACTOS project has been aligned with the recommendations in the NUGENIA roadmap for Technical Area 4 - Integrity Assessment of Systems, Structures and Components for Gen II & III reactors.
• “Establishing guidelines and rules for the manufacturing conditions in view of crucial SCC parameters on material performance”. (Those collected in WP 8 “Harmonization & guidelines)
• “Developtment of improved mitigation techniques for inter-granular stress corrosion cracking (IGSCC): e.g. modified water chemistries or development of other new techniques”. (Advanced Machining using supercritical CO2)
• “Development and testing of accelerated stress corrosion cracking (SCC) methods for life time prediction”. (Those used in WP6, tasks 6.1 & 6.2)
• “Development of empirical/semi-empirical models for prediction of stress corrosion cracking (SCC) initiation in LWR and super critical water

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Periodic Reporting for period 3 - MEACTOS (Mitigating Environmentally Assissted Cracking Through Optimisation of Surface Condition)

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