SUMMARY OF THE SPECIAL SESSION ON THOUGHTS FROM END-USERS – Chaired by Larry Nelson

This year a second special session was conducted during the annual meeting. It was held after lunch on Thursday afternoon.

Data → Mechanisms → End-User Applications

At the last ICG-EAC meeting in 2018 a special session was held commemorating the 40th anniversary of the Group. This session consisted of several presentations which focused on describing the importance of the Group to the Scientist, Engineer, Utility and Regulator. Following the presentations, two panels were formed, one made up of utility members and the second from regulator members. The session and especially the panel discussions were very well received by the attendees as per the survey. Therefore, the ICG-EAC Leadership decided to hold a similar session during the 2019 annual meeting with an expanded scope. The panel format was kept for 2019 except that the single panel consisted of a mixture of members from the regulator, utility and vendor groups. The ICG-EAC Administrator, Larry Nelson, was asked by the Group Leadership team to organize this session.

After a few introductory remarks, Larry began the session by asking three of the panel members to each make a 15 minute presentation addressing two different questions. The first panelist was Pål Efsing (Ringhals, Sweden) and he was given the following questions to address:

  1. The applicability or disconnects of laboratory SCC growth rate data to the plant – how representative are our tests?
  2. What role do transient conditions have in plant components: load following, start up and shutdown, scrams, chemistry transients, etc.?

Pål began his presentation by discussing the overall strategy for addressing EAC at his utility. It is important to understand the degradation mechanisms and that is where the work of the ICG-EAC is very beneficial. This information is used to develop and optimize activities associated with aging management of a structure or component. This may be addressed by more frequent inspections, replacement of the component or visual monitoring. It is important to understand the mechanism of degradation in order to minimize the key drivers. The data that are generated by the various laboratories must be carefully screened. Unfiltered data are for the most part not useful and might lead to incorrect conclusions. Although lab data show an accelerated effect if a plant load-follows, the French experience show very limited additional down-time for their plants. Pål concluded his presentation by saying that it is important to be conservative, but the plant operator must take a realistic view on when a crack is really active or not.

The second panelist was Ian de Curières (IRSN, France) and he was asked to address the following two questions:

  1. Do accelerated tests for initiation provide relevant results? May their dependencies be trusted?
  2. Given the broad spectrum in microstructures and stresses in components, should we target the upper 95% or 90% or 70% or 50% of the SCC behavior?

Ian began by asking himself a series of questions regarding initiation. Although initiation testing in the lab must be accelerated, it is valuable in determining the various dependences for cacking. The goal should be to describe the cracking mechanism and to use this to make predictions. Good knowledge of the dependencies and the surface condition of the plant components is necessary. This will eventually lead to model development, which can be used for prediction. Ian then described the emperical model for crack initiation developed by Peter Scott. This defines the minimum time for the first crack to appear. From this point, statistical evaluation based on Monte-Carlo simulation may be performed. He concluded his remarks by saying that initiation tests are very important, but must be technically relevant to plant conditions. Accelerated testing is relevant, provided the conditions are well defined and do not introduce biases. Sufficient data must be generated to allow statistical reliability.

The third and final panelist to make a presentation was Mike Wright (CNL, Canada). Mike was given the following two questions, which he was asked to address from a vendor prospective:

  1. How can we best differentiate between cause and effect in SCC, especially in characterization studies?
  2. What are the lessons for future plant designs?

Mike began by saying that EAC is multi-disciplinary. It requires a range of experts to completely understand and predict it. However, one should beware of the “Outsider”, since they may look at things totally backward based on their experience. Mike gave an example from his own experience. In the ideal world one would be able to reconcile performance of a material in the lab with its performance in the field, but the world is not ideal. One must be careful in doing experiments; just because it can be seen doesn’t mean that it’s important. One must keep in mind that dependencies can be overlapping and one should consider continuum behavior for most variables. Mike concluded his talk by listing several lessons that have been learned in the LWR community and that should be taught to the molten-salt and liquid-metal developers:

  • The health of protective/passive films must be managed proactively.
  • Understanding the effects of coolant chemistry, including all additives and plausible impurities is key.
  • Protective/passive films can be compromised by mechanical straining (fatigue stresses) by material inhomogeneity and irradiation effects.
  • Testing to explore vulnerabilities needs patience and good techniques.

After the presentations, the panel consisting of Peter Chou (USA), Thierry Couvant (France), Ian de Curières (France), Pål Efsing (Sweden), Katsuhiko Kumagai (Japan), Sam Nouraei (UK), and Mike Wright (Canada) was asked to answer questions from the floor.

There were many questions and the panelists did an excellent job of addressing them. The discussion seemed to generate more questions than answers, although a question from the floor regarding “leak before break” implementation by the regulators certainly generated a lot of discussion and the interpretation is, to a large extent, very dependent on country. Larry finally brought the hour-long panel discussion to an end.

At the end of this special session a “Challenge of the year” was launched.

Thierry Couvant (EdF, France) presented a challenge to the Group to make small steps towards Baffle-Former bolt IASCC prediction. He proposed an approach for bridging research with industrial issues. Models need to be improved and key gaps identified and ranked. All ICG-EAC members are asked to contact Thierry (thierry.couvant@edf.fr) if they might be able to contribute in any way to address this challenge (results may be presented during the 2020 meeting).

After some discussion about Thierry’s challenge, Larry asked the attendees to thank the panel and he brought the End-User session to a close.