TLAA106ENVIRONMENTALLY-ASSISTED FATIGUE
The impact of reactor coolant environment may result in increased component fatigue usage factors beyond those determined in air, for example by TLAA101. Where the operating conditions may affect component fatigue, it is necessary to recalculate the predictedcumulative usage factor (CUF).An overview of approaches by different countries (US, France, Finland, Japan and Germany) is given in [1]. In many countries (USA[2], Japan [3-4], Hungary [5-6], Czech Republic [7]), an environmentally-adjusted cumulative usage factor (CUFen) is used for assessment of the impact of the reactor coolant environment which accounts for the effects of strain rate and chemistry among others. In Switzerland, this approach is applied to loads which were not specified in the design [8-9]. In other countries (e.g. Germany[10]),environmental fatigue is addressed by a simplified approach in which so-called “levels of attention” are defined for original CUFs (from TLAA 101), i.e. it is demonstrated that the CUF at end of life does not exceed a value such as 0.4for austenitic materials and ferritic materials.In these cases, no additional measures are required.In some other countries, the fatigue curve is modified to (partially) incorporate environmental fatigue, for example Finland[11] and France[12].Therefore, depending on the approach, either CUFen or CUF is the analysis parameter.For the CUFen analysis, the assumed parameter values used to account for the reactor water environment, such as the dissolved oxygen content, is verified to be applicable for the plant for the intended period of operation. For example, changes in parameter values due to changes in water chemistry control are appropriately considered to ensure that the assumed values are either accurate or conservative to the actual operating conditions.
For evaluations of environmentally assisted fatigue, the TLAA is found acceptable under (i), (ii), or (iii) based on the following:
- Acceptance by (i):
In this case, the number of cycles for each transient is determined for the intended period of operation based on (see TLAA101):
a)A baseline cycle count for the transient that is determined from a review of the operating experience for the plant.
b)A projection of the expected number of cycles from the end of the baseline cycle count to the end of the intendedperiod of operation.
c)The sum of these two counts provides the projected cycle count for the intendedperiod of operationfor the transient.
If the original TLAA accounts for the environmental effects on fatigue using CUFen, and the projected number of cycles for each transient in the TLAA is less than or equal to the number of cycles used in the existinganalysis, then the CUFenanalysis is still valid and acceptable for the intendedperiod of operationaccording to (i).
If in the original TLAA (from TLAA 101)the projected number of cycles for each transient in the TLAA is less than or equal to the number of cycles associated in the existing analysis with a CUF equal to or lower than the applicable “level of attention” (eg. from German approach [10]) then the analysis is still valid and acceptable for the intendedperiod of operationaccording to (i).
- Acceptance by (ii):
As with case (i), the projected number of cycles for the intendedperiod of operationis determined for each transient in the TLAA, and the projected CUFen value for the intendedperiod of operationis evaluated using these projected numbers of cycles. If the projected CUFen value meets the regulatory limit or criterion, then the TLAA is acceptable for the intended operating period according to (ii).
When the TLAA does not account for the environmental effects on fatigue, the projected CUF value for the intended operating period is evaluated using these projected numbers of cycles. If the projected CUF is equal to or lower than the applicable “level of attention,” then the TLAA is acceptable for the intended operating period according to (ii).
- Acceptance by (iii):
In this case AMP101 is used to manage the CUF value by monitoring transient occurrences, periodically updating the CUF / CUFen value, as applicable, and ensuring that the corrective actions are taken prior to the CUF / CUFen value exceeding the regulatory limit, criterion or applicable “level of attention.” AMP101 provides for corrective actions, including include repair or replacement of components, design modifications, revision of operating procedures, and more rigorous analysis to demonstrate that the design code limit will not be exceeded during the intendedperiod of operation.
References
[1]F.H.E. de Haan – de Wilde, M.H.C. Hannink, F.J. Blom, Overview of international implementation of environmental fatigue, PVP2013-97695, July 14-18, 2013, Paris. (update is prepared for ASME-PVP2017)
[2]NUREG 1801, Generic Ageing Lessons Learned (GALL) Report. Final report, Rev.2, December 2010. (section X.M1 for environmental fatigue guidelines)
[3]JSME Code for Nuclear Power Generation Facilities – Environmental Fatigue Evaluation Method for Nuclear Power Plants, JSME S NF1-2009, 2009, Japan Society Mechanical Engineering.
[4]Nuclear Power Generation Facilities, Environmental Fatigue Evaluation, JNES-SS report: JNES-SS-1005, March 2011.
[5]MSZ 27003 Rules for Construction of Nuclear Facility Components, Hungarian Standardization Institution, 2013.
[6]PNAE G-7-002-87 ‘Standards for Strength Calculation of Equipment and Pipelines of Nuclear Power Plants’.
[7]CZECH ASSOCIATION OF MECHANICAL ENGINEERS, Annex H of Section III - Strength assessment of equipment and piping of nuclear power plant type WWER, NTD_AME_Section_III_2013, No. 1.
[8]EIDGENÖSSISCHES NUKLEARSICHERHEITSINSPEKTORAT (ENSI), Alterungs-überwachung, ENSI-B01/d, Richtlinie für die schweizerischen Kernanlagen, August 2011.
[9]EIDGENÖSSISCHES NUKLEARSICHERHEITSINSPEKTORAT (ENSI), Alterungs-überwachung, ENSI-B01/d, Erläuterungsbericht zur Richtlinie, August 2011.
[10]KTA 3201.2 (2013-11) Komponenten des Primärkreises von Leichtwasserreaktoren, Teil 2: Auslegung, Konstruktion und Berechnung.
[11]Courtin, S., A. Lefrançois, J.Le Duff, A. le Pécheur, Environmentally assisted fatigue assessment considering an alternative method to the ASME Code Case N-792. PVP2012-78088, Toronto, July 2012.
[12]Courtin, S., Metais, T., Triay, M., Meister, E., Marie, S., Modifications of the 2016 edition of the RCC-M code to account for environmentally assisted fatigue. PVP2016-63127, Vancouver, July 2016.
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