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Environmental Fatigue Test for Structural Materials


Environmental fatigue of the metallic components in light water reactors has been the subject of extensive research and regulatory interest in Korea and abroad. Especially, it was one of the key domestic issues for the license renewal of operating reactors and licensing of advanced reactors during the early 2000s. To deal with the environmental fatigue issue domestically, a systematic test program has been initiated and is still underway. The materials tested were SA508 Gr.1a low alloy steels, 316LN stainless steels, cast stainless steels, and an Alloy 690 and 52M weld. Through tests and subsequent analysis, the mechanisms of reduced low cycle fatigue life have been investigated for those alloys. In addition, the effects of temperature, dissolved oxygen level, and dissolved hydrogen level on low cycle fatigue behaviors have been investigated. The most recent research is ongoing in the interests of evaluation of high frequency bending pipes for redefining design transients and incorporating the hold-time during the LCF test for representing the actual plant transients with cooperation with KHNP Central Research Institute (CRI) and Electric Power Research Institute (EPRI). 



Research Key Words and Materials

Environmentally-assisted fatigue (EAF)

Low cycle fatigue (LCF)

Pressurized-water reactor (PWR) environments

Operation plant transients

Effects of dissolved hydrogen (DH) and oxygen (DO)

Hydrogen induced cracking (HIC)

Austenitic stainless steel (ASS)

(316LN, 316 and 347 SS)

Ni-base alloy

(Alloy 690 and 52M weld)

Low alloy steel

(SA508 Gr.1a)



Related Project

(I) Environmental fatigue test considering plant operating conditions (KHNP CRI)

(II) Development of high frequency bending application technology on NPP pipes for environmental fatigue integrity (KHNP CRI)


*Related Project I

Project Title:

Environmental fatigue test considering plant operating conditions


Project Period:

2013.11.01 - 2016.06.30


Project Purpose:

(1) Characterize the influence of complex loading conditions (hold-time effects) on fatigue life of austenitic stainless steels in PWR environments

(2) The differing influence of surface roughness between air and water environments


Brief Description:

Many studies have been conducted to demonstrate the effect of light water reactor (LWR) coolant environments on fatigue life of materials. Typically, these environmentally-assisted fatigue (EAF) tests are conducted using polished cylindrical tensile specimens under strain control with fully-reversed loading (i.e., strain ratio of -1). However, there is a discrepancy between laboratory test data and plant operating experience regarding the effects of environment on fatigue; therefore, it is necessary to obtain fatigue test data with more plant-relevant transient conditions. This project will study the fatigue life of austenitic stainless steels in primary water reactor (PWR) environments while applying various loading conditions such as hold times or mixed waveforms. As time permits, additional work will be pursued to identify the differences in the effect of surface roughness between air and water environments.




Environment fatigue test system


Comparison of predicted and measured effects of environment [PVP2013-97500, 2013] (left),        

mixed wave test condition (hold-time) (right)



*Related Project II

Project Title:

Development of High Frequency Bending Application Technology on NPP Pipes for Environmental Fatigue Integrity


Project Period:

2012.10.01 - 2015.09.30


Project Purpose:

(1) Integrity of pressurizer surge line in nuclear power plant

(2) Environmental fatigue properties of stainless steel

(3) Bending applying local high-frequency induction heating


Brief Description:

Type 347 and 316 stainless steel piping of pressurizer surge line is vulnerable to environmental fatigue due to effect of thermal stratification. Especially, elbow and welding parts have very low margin of fatigue and leak-before-break (LBB) estimation. High frequency induction bending piping is alternate those elbow and welding parts in many other industries. This bending technique is expected to reduce welding parts, so it can satisfy regulation requirements related with environmental fatigue. To verify confidence of bended pipe, the fatigue properties should be investigated for the bending pipes. Both intrados and extrados of bending pipe material are compared with as-received material in fatigue properties. Environmental fatigue tests are conducted in both pressurized water reactor (PWR) coolant environment and room-temperature in air environment. It must be confirmed to satisfy environmental fatigue design curve of ASME Boiler and Pressure Vessel Code. 



High frequency induction heating bend (left) and LCF life of hot-bent 347 SS at several locations tested

 in PWR environments (right)



Accident Resistant Fuel Cladding Materials for LWR


  Advanced stainless steels have been developed as accident resistant fuel cladding materials for light water reactors (LWRs). Related accident tolerant fuel (ATF) R&Ds have recently become great interests by nuclear research institutes in the world since Fukushima accident in 2011 Japan where many candidate materials are being proposed to replace the current Zircaloy cladding. From design and manufacture to performance evaluation in PWR and accident-like oxidation environments, development of advanced austenitic stainless steels are ongoing in this laboratory with a close relation with other domestic research institutes.


Research Key Words and Materials


Advanced austenitic stainless steel (AASS)

LWR fuel cladding

Alloy design and manufacture

PWR corrosion behavior

High temperature steam oxidation behavior

Mechanical property assessment

Stress corrosion cracking (SCC) resistance

Irradiation resistance



Related Project

(I) Development of Metal-based Accident Resistant Fuel Cladding (ARFC) Materials for Light Water Reactors (KAIST-KUSTAR Institute)


*Related Project I

Project Title:

Development of Metal-based Accident Resistant Fuel Cladding (ARFC) Materials for Light Water Reactors 

Project Period:

2015.01.01 - 2019.12.31


Project Purpose:

(1) Optimize the composition of Fe-based stainless steels based on thermodynamic calculation

(2) Develop and optimize the thermo-mechanical processes (heat treatment, cold-working, grain-size control)

(3) Evaluate the long-term corrosion behaviors in LWR operating condition (360oC 190bar) and short-term steam oxidation resistance in postulated accident condition (up to 1200oC)


Brief Description:

  The importance of the accident resistant fuel (ARF) cladding to the overall safety of nuclear power plants has been widely recognized since Fukushima accident in 2011 Japan. Among several candidate materials for such purpose, SiC-based composite materials are extensively investigated. However, the major problems in fabrication and joining of SiC-based materials and associated cost have to be overcome before the field application of such materials. It may require major re-design of reactor core, which would be even greater obstacle. Nonetheless, as metallic materials, fabrication and joining of ARFC materials are much easier, and therefore the field application to the light water reactor fuel will be much faster, once the proper alloys are developed. In this laboratory, design and manufacture of advanced austenitic stainless steels have been investigated along with performance assessment in PWR normal operation and accident-like condition.


Manufacture process of AASS at KIMS (left) and high temperature steam TGA oxidation weight gain behavior of trial AASS as compared to various commercial alloys (right)





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