[4B5] Ultrasonic monitoring of high-cycle thermal fatigue (HCTF)
L Clarkson and F Cegla
Imperial College London, UK
The 1998 release of radioactive steam at the Civaux 1 nuclear power plant (NPP) stands as a prominent example of high-cycle thermal fatigue (HCTF) failure. The root cause was a crack in a pipe elbow downstream of a tee-junction, where fluids with differing thermal and hydraulic properties mixed. Similar HCTF-related cracks have been identified as recently as 2023 in the Penly 2 and Cattenom 3 reactors, underscoring the ongoing challenge thermal fatigue poses to NPP safety and reliability. This highlights the urgent need for non-destructive evaluation (NDE) methods and real-time, non-invasive monitoring tools capable of tracking and assessing HCTF progression.
Previous studies have shown that components such as pipes are particularly vulnerable to HCTF under thermal transients exceeding 50°C at frequencies between 1 Hz and 10 Hz, conditions frequently found in fluid mixing zones. These transients induce cyclic stresses that can lead to material degradation and cracking. Compounding the challenge, the low thermal conductivity of austenitic stainless steels acts as a thermal low-pass filter, making it difficult for conventional temperature sensors to detect rapid surface temperature changes through the material.
This study demonstrates the use of inverse thermal modelling (ITM) to assess internal temperature distributions in 304 stainless steel subjected to rapid (<1 minute), high-temperature (>250°C) transients. ITM overcomes the limitations of conventional methods by leveraging the temperature-dependent speed of ultrasound waves, which propagate through the component and enable detection of temperature fluctuations across its entire thickness. The paper presents monitored ultrasonic data from several SS304 specimens, including C-scan ultrasonic shear wave velocity and attenuation maps before and after the cycling. The data shows that evidence of cracking can be seen in the ultrasonic signatures.
Previous studies have shown that components such as pipes are particularly vulnerable to HCTF under thermal transients exceeding 50°C at frequencies between 1 Hz and 10 Hz, conditions frequently found in fluid mixing zones. These transients induce cyclic stresses that can lead to material degradation and cracking. Compounding the challenge, the low thermal conductivity of austenitic stainless steels acts as a thermal low-pass filter, making it difficult for conventional temperature sensors to detect rapid surface temperature changes through the material.
This study demonstrates the use of inverse thermal modelling (ITM) to assess internal temperature distributions in 304 stainless steel subjected to rapid (<1 minute), high-temperature (>250°C) transients. ITM overcomes the limitations of conventional methods by leveraging the temperature-dependent speed of ultrasound waves, which propagate through the component and enable detection of temperature fluctuations across its entire thickness. The paper presents monitored ultrasonic data from several SS304 specimens, including C-scan ultrasonic shear wave velocity and attenuation maps before and after the cycling. The data shows that evidence of cracking can be seen in the ultrasonic signatures.
