Session: CS-03-02 Environmental Fatigue Issues (Joint M&F)

Paper Number: 62187

Start Time: Tuesday, July 13, 2021, 08:00 PM

62187 - Design by Analysis, Codes, Standards and Rules on EAF - Applicability to Stainless Steels in PWR Primary Piping 

Evolution of common rooted codes and standards such as ASME III, RCC-M, KTA, PNAE, JSME and adoption of new rules on Environment Assisted Fatigue (EAF) have challenged designers and utility engineers with new unanswered questions on practical management of fatigue for long term operation (LTO). An international consensus on state of the art (SOAR) and best practises has been temporarily missed and being sought for. This follow-up paper to PVP2020-21501 further discusses on Codes, Standards and Rules for fatigue management in search of science based Design by Analysis approach for EAF. Our point of view reflects particular challenges in Finland, where continuous follow-up, verification and implementation of SOAR is targeted. Safety level provided by ASME III is a regulatory base line, but different codes can be, and have been used.

An EPR-1600 (Olkiluoto 3) is a design based on RCC-M and subjected to mandatory accounting of environmental fatigue by a Finnish Regulatory Guide in 2002. International discussion on applicable design curves and environmental effects has been lively during construction phase and may continue during the long operation period ahead. Anticipation on future evolution of SOAR and proactive measures in EAF are of interest. Non-stabilized low carbon stainless steels are used in primary loop of this reactor.

The VVER-440 units in Loviisa are already in the phase of LTO and consideration of further extension. A particular issue arises from adaptation of the current SOAR with the original design, which was according to PNAE-G-002-86 for primary piping and ASME III for main circulation pumps. The Regulatory Guide (YVL E.4 updated in 2020) recommends an approach endorsed by the US NRC (Regulatory Guide 1.207, rev.1, 2018), developed for non-stabilized stainless steels and interlinked with fatigue assessment according to ASME III. But the piping is made of type 321 stabilized stainless steel and designed accordingly, using a comprehensive Design by Analysis approach tuned for the steel type.

Research at VTT has revealed gaps in verified understanding of mechanisms contributing to environmental effects for stainless steels in PWR primary water environment. Also incompatibilities between applied EAF test methods versus standard procedures and original design rules versus new models, together with high scatter in international data raise questions on transferability of the lab data to components in plant.

Mechanism informed fatigue research based on valid data and code-compatible models are needed before an international consensus can be reached and quantitative EAF methodologies applied to stainless primary piping. Meanwhile, a realistic and safe EAF approach shall be adopted to ensure compatibility of new ‘add-on’ factors as integral parts of the codified ‘Design by Analysis’ procedure.

This paper discusses the different codes, standards, rules and assumptions by help of calculation examples.

Presenting Author: Petri Lemettinen Fortum Power and Heat Oy

Authors:

Jussi Solin VTT
Tommi Seppänen VTT
Petri Lemettinen Fortum Power and Heat Oy
Rami Vanninen TVO
Erkki Pulkkinen TVO