PSI - Issue 16

Peter Trampus / Procedia Structural Integrity 16 (2019) 161–168

164

Peter Trampus / Structural Integrity Procedia 00 (2019) 000 – 000

4

4.1. Austenitic welds

Intergranular stress corrosion cracking (IGSCC) has been found extensively in stainless steel piping in Boling Water Reactors (BWRs) and it was initially only detected by leakage. The inspection qualification process, introduced later, has seen subsequent improvement in over the last few years. The results of these inspections have a direct impact on plant availability and cost by the need for mitigation activities and augmented ISI.

4.2. Dissimilar metal welds

Similar problems have emerged from the inspection of dissimilar metal welds (DMWs), where NDE techniques simply missed flaws, which turned into a leakage in the subsequent operation period. In the last decade the steam generators (SGs) in VVER-440 units have shown DMW problems. The advances in the use and deployment of phased array ultrasonic testing (PA UT) are significant, and procedures including this technology tend to perform better than those relying on conventional UT.

4.3. Cast austenitic stainless steel

There exists a concern of possible thermal embrittlement and thermal fatigue cracks. But the coarse-grained anisotropic structure of cast material makes it difficult to inspect reliably. To overcome the metallurgical characteristics, low-frequency transmitter-receiver longitudinal technique with synthetic aperture focusing technique (SAFT) was employed. Recently, PA UT is extensively adopted to improve the inspection results.

4.4. Steam generator tubing

Several tubing degradation mechanisms have been encountered, each of them with differing characteristics of the eddy current (ET) signals. If optimized or advanced probes (e.g. with motorized rotating coils) are being used and an adequate data analysis algorithms and logistics are applied, these mechanisms can be identified, their effects can be sized and a trend in degradation can be established. Some of the plants adopt multi – array ET technique for SG tube inspection, which provides more accurate sizing result and reduces overall inspection time.

4.5. Reactor pressure vessel head penetrations

The Pressurized Water Reactor (PWR) Reactor Pressure Vessels (RPV) head penetrations have emerged as a problem area of considerable significance in recent decades. Boric acid corrosion initiated by primary water stress corrosion cracking (PWSCC) on the penetration tubes led to significant decrease in structural integrity. Due to high radiation level, and complicated geometrical and clearance conditions, modular tools with complicated sensor carriers, have been developed and sophisticated UT technique (e.g. time-of-flight-diffraction, TOFD) applied successfully. Drastic damage, which has occurred, demonstrates the importance of properly interpreting the results of ISI and taking appropriate corrective action. The RPV head penetrations in VVER-440s differ from PWR design. The problems here did not jeopardize the vessel head integrity, however, due to deformation of the cladding tube, it could lead to stuck of control rods. Monitoring of the VVER penetration require advanced UT technique.

4.6. Reactor internals

In the bolt and weld areas of reactor internals flaws have occurred. In view of the high radiation level and the minimal chances for a repair due to the severe access conditions, replacement appears to be the only alternative for effectively fixing the problem. However, as the loads and flaws growth rate are generally small, a certain degree of defective areas can be tolerated providing assurance by evaluations. Both tasks impose a large responsibility for the ISI. The most representative example here is the baffle bolts SCC.