Environmentally Assisted Fatigue (EAF) Rules and Screening Options in the Context of Fatigue Design Rules Within German Nuclear Safety Standards
Authors: Reese Sven H. , Rudolph Jürgen 
Published: November 2015

Consideration of Environmentally Assisted Fatigue (EAF) has been introduced to the German Regulatory Framework with the November 2013 revision of the German Safety Standard KTA 3201.2. Therefore, so called threshold of attention values by means of the cumulative usage factor have been included in the German KTA Program of Standards for the design of primary circuit components (KTA 3201.2: components of the reactor coolant pressure boundary of light water reactors, part 2: design and analysis) and for secondary side piping components (KTA 3211.2: pressure and activity retaining components of systems outside the primary circuit; part 2: design and analysis). Possible measures are linked to the KTA 3201.4 in-service inspection and operational monitoring regulations. Additionally, consideration of EAF, respectively the introduction of threshold values (temperature, strain rate and amplitude) is currently being discussed in the revision process of KTA 3204 for reactor pressure vessel internals. Note that threshold of attention values refer to the cumulative usage factor CUF and limit (threshold) values (temperature, strain rate and amplitude) refer to the activation of the penalty factor Fen.

In contrast to international procedures (like e.g. US-NRC Regulatory Guide 1.207, ASME Code CC N-792), EAF has to be considered for German plants below the design life of 40 years as well. More precisely, in the framework of ASME EAF has to be taken into account only for plant life extension, license-renewals or new builds while in Germany an evaluation is compulsory for all operating NPPs. Based on this fact an engineering approach is needed which is realized by introducing so called threshold of attention values. If the cumulative usage factor is higher than the threshold of attention value, additional measures have to be accomplished by the operator. According to the regulatory framework these measures are additional calculations (e. g. Fen calculation), NDT including fracture mechanical calculations or real component testing.

The calculation of the threshold of attention values is based on representative specified and operational measured temperature transients. Based on this information a comprehensive component specific picture of operational loading can be drawn. According to the latest revision of NUREG/CR-6909 (currently Rev. 1, draft for comments) the Fen formula comprises dedicated threshold values for the relevant temperatures, strain rates and strain amplitudes. Having these boundary conditions in mind, detailed plant transient information can be analyzed according to its relevance. Based on the comprehensive set of information threshold of attention values for KTA 3201.2 / 3211.2 were determined to be 0.4, both for ferritic and austenitic materials of BWR and PWR components. In the framework of KTA 3204, technical evaluation yields to the assumption, that Fen threshold values are likely not to be exceeded for pressurized water reactor internals.

In the framework of the paper the technical background for the evaluation of threshold of attention values in the context of EAF will be described in detail.