Before we tackle environmentally-assisted fatigue (EAF), what is fatigue?
In ASME Boiler & Pressure Vessel Section III Division 1, Subsection NB (in short, ASME III NB) fatigue refers to the damage caused by systems and components as a result of cyclic loads.
The qualification of the primary coolant system of light water reactors (ASME III Class 1 components) must address the fatigue damage caused by pressure and thermal cycling of the equipment and piping during their design life.
How is fatigue life analysed and qualified for 40 years, 60 years, or more of service?
The fatigue analysis and qualification is achieved by following a procedure outlined in ASME III Div.1 Subsection NB for Class 1 piping and components. This procedure can be outlined in five key steps:
STEP-1 TRANSIENTS: Developing the thermal and pressure transients, in the form of time-dependent functions Pi(t) and Ti(t). For each transient “i” has a corresponding number of cycles ni. For ASME III plants, this information, the transients Pi(t), Ti(t) and their cycles, is provided in the ASME III Design Report.
STEP-2 STRESS INTENSITIES: These pressure and temperature transients Pi(t) and Ti(t) are then applied to a heat transfer and finite element model of the component, to obtain the resulting stress intensities.
STEP-3 FATIGUE CURVE (S,N): The stress intensities are entered into the design fatigue curve (S,N) provided in ASME III Appendix I, to determine – for each transient “i” – the number of cycles Ni permitted by the ASME III Code.
STEP-4 INDIVIDUAL USAGE FACTORS UFi: The usage factor UFi for each transient “i” is nothing else than the ratio of the predicted number of cycles ni of transient “i” specified in STEP-1, divided by the number of cycles Ni permitted by Step-3, in other words UFi = ni/Ni. This ratio can be viewed as the fatigue damage imparted by these pressure and temperature transients on each component.
STEP-5 CUMULATIVE USAGE FACTOR CUF: In this final step, all the individual usage factors UFi calculated for each transient Pi(t) Ti(t) are added together to obtain the cumulative usage factor for a component, which labelled CUF = Σ UFi. This CUF must be kept at or below 1.0 for the full design life of each component:
CUF = Σ UFi ≤ 1.0
How was the fatigue curve (S,N) developed?
This is not an academic question, as it lies at the heart of the EAF challenge. The fatigue curves were developed in the 1960’s by cycling smooth bar specimen, until a fatigue crack developed in the specimen. These tests were conducted in laboratories, around the world, in a laboratory environment, which means in air at ambient temperature. Then, an adjustment factor, a knock-down factor, was applied to the experimental data. The adjustment factors amounted to reducing the cycles to fatigue failure by a factor 20, and the stresses by a factor 2. The purpose of these adjustments was to account for statistical variation of the failure data, the surface finish of a smooth bar compared to an actual component, and the size effect of actual component which would contain more flaws than a smooth bar specimen. The smooth bar specimen failures, knocked-down by a factor of 20 on cyles and 2 on stress (whichever governs) were published as the ASME III Appendix I design fatigue curves that we used to analyse and qualify the Class reactor coolant systems.
Isn’t a factor of 20 in the ASME III Appendix I design fatigue curve enough?
For the effects mentioned (statistical variability, surface finish, size) a factor of 20 on the test failures is quite sufficient. However, there is one aspect that was not fully accounted for by the factor 20: the effect of a corrosive environment. We know that fatigue when combined with corrosion will accelerate the propagation of cracks. This question, the effect of a corrosive environment on the fatigue life of a component in the reactor coolant system came to a head when plants studied the possibility of extending the service life beyond the original 40 years.
Were the effects of a corrosive environment on fatigue life missed in the ASME III NB Code?
No. This matter, the effect of corrosion on fatigue life, was not overlooked in the ASME III Code. In fact, ASME III NB-3121 states “It should be noted that the tests on which the design fatigue curves (Figs. I-9.0) are based did not include tests in the presence of corrosive environments which might accelerate fatigue failure.”
What new data was developed to quantify EAF?
The NRC sponsored a research and test program, conducted primarily at Argonne National Laboratory, to quantify the damage caused by primary water (reactor coolant) on primary coolant systems. For example, the University of Tokyo had started this research in 1991, this was followed by studies such as NUREG/CR-5999 (1993) and NUREG/CR-6260 (1995), EPRI TR-105759 (1995), NUREG/CR-6583 (1997), NUREG/CR-5704 (1998), NUREG/CR-6674 (2000), EPRI TR-1003079 (2001), WRC Bulletin 487 (2000), EPRI 1012017, and European initiatives NUGENIA and CORDIS. This work, and other, was then nicely compiled in a single report NURG/CR-6909.
The results, published in NUREG/CR-6909 indicate that the EAF caused the components to crack earlier, as illustrated in Figure 1, and therefore additional penalties would have to apply to account for EAF.
Figure 1 – Experimental Results of the Effects of reactor coolant environment on the fatigue
life of low-alloy steels (Cr-Mo steels) and austenitic stainless steels (Source NUREG/CR-6909)
How is the fatigue qualification procedure modified by EAF?
The final decision on how best to correct the fatigue life of a component in contact with the reactor coolant system, to account for EAF, was published in Reg. Guide 1.207, in March 2007. It endorsed the NURG/CR-6909 approach which departs from the ASME III Appendix I fatigue curves in two fundamental ways: (1) The margin applied to the in-air (S,N) curves would be 12 on cycles rather than 20, and (2) the CUF would now be calculated in the form
CUF = Σ (UFi × Fen,i)
Where Fen.i is an environmental correction (penalty) factor which depends on several parameters, such as the metal and its sulfur content, the temperature, the oxygen content, and the strain rate during each particular transient “i”. the penalty factor Fen,I could be quite high depending on the metal, the coolant, and the pressure-temperature transient.
What is the current regulatory guidance and expectation on EAF?
As we mentioned, the current NRC expectation is that EAF would be evaluated following RG 1.207 which refers back to NUREG/CR-6909. This is the culmination of years of regulatory communications, including GSI‐78 “Monitoring of Fatigue Transient Limits for reactor coolant system”; GSI‐166 “Adequacy of Fatigue Life of Metal Components”; SECY‐95‐245 “Completion of the fatigue action plan for current license basis”; GSI‐190 “Fatigue Evaluation of Metal Components for 60‐year Plant Life”.
Will these changes be introduced into the ASME III Code?
The effects of EAF have already been introduced into ASME III through the following Code Cases:
- N-761: Fatigue Design Curves for Light Water Reactor (LWR) Environments Section III, Division 1. This Code case is based on shifting the design (S,N) curves to the left. This approach is now replaced by the preferred CC N-792 Fen-based approach.
- N-779 Alternative Rules for Simplified Elastic-Plastic Analysis Class 1 Section III, Division 1.
- N-792 Fatigue Evaluations Including Environmental Effects Section III, Division 1. This approach espouses the NUREG/CR-6909 and RG 1.207 method based on Fen.
Two other ASME III initiatives which are still in development are:
- ASME III project 10-293: Procedure to Determine Strain Rate for Use with the Environmental Design Fatigue Curve Method and the Environmental Fatigue Correction Factor (Fen).
- ASME III project 15-352: Fatigue Evaluations Using Flaw Tolerance Methods to Consider Environmental Effects Section III, Division 1.
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