Operating experience with the relevant ageing mechanisms

The following observations of BWR vessel and component service history are relevant to ageing degradation. These incidents offer a perspective on the design bases and their conservatism relative to operating parameters. It is particularly noteworthy that each has been resolved by qualified repair programs. Nozzle cracking, stub tube cracking, safe end cracking and closure stud cracking are all age related degradation mechanisms, which have been effectively managed.

Feedwater nozzle and control rod drive return line nozzles have experienced thermal fatigue cracking. Nuclear Regulatory Commission (NRC) findings and requirements with regard to this cracking are discussed in NUREG-0619 [4.8].

4.7.1. Feedwater Nozzle Cracking

GE conducted an extensive program to understand the feedwater nozzle cracking problem. The program report [4.13] concluded that cold feedwater leakage around the feedwater nozzle thermal sleeve must be avoided. Most GE and Japanese BWRs have installed new feedwater nozzle thermal sleeves and spargers.

NUREG-0619 [4.8] requires that surveillance be ongoing for all installations and clearly defines the frequency and extent of examinations which must be followed.

Potential feedwater nozzle cracking is a generic ageing degradation mechanism of current concern and is a plausible mechanism for evaluation with respect to life extension.

Leakage must be minimized because it can soon lead to thermal fatigue cracking.

4.7.2. CRD Penetrations (Stub Tube Cracking) 4.7.2.1 Stainless steel stub tube cracking

Several GE BWRs have experienced CRD stub tube stress corrosion cracking (SCC) since start of operation. All stub tubes in these three BWRs were fabricated from Type-304 furnace-sensitized stainless steel. Furnace-sensitization of Type 304 stainless steel was eliminated from later BWR vessels.

BWR/1 CRD housings have a close fit to the stub tube inside diameter (ID). Leakage (bottom head to stub tube weld defect) was stopped by rolling (expanding) the housing into the bottom head penetration.

CRD housings in the BWR/2 design have wider gaps between the stub tube and housing than do those in the BWR/1 plants. Leakage occurred in the stub tube just below the stub tube-to-housing weld. A total of 33 housings in a BWR/2 have been ID roll expanded into the bottom head penetration to stop leakage; some of these housings were later roll expanded again.

The BWR/3 design has a relatively large gap between the housing and the stub tube ID.

In the one BWR/3 plant that experienced CRD stub tube problems, three cracked stub tubes leaked; these leakages were stopped by installing sleeves on the stub tubes. Several stub tubes in this plant were later found (through ultrasonic testing) to be cracked. Although none of these stub tubes leaked, sleeves were installed over three of the largest cracks as a preventive measure.

CRD Stub tube repair programs

Several repair options stopped CRD penetration leakage. Roll expansion of the CRD housing into the bottom head penetration has been particularly successful when the gap between the housing and vessel penetration is small. Success also seems to be related to early detection and repair, which minimizes damage to the surfaces to be rolled. Mechanical sleeves are effective. At least two utilities are sponsoring and developing repair welding options.

Access is a major problem for both welding and machining operations because the penetrations are closely spaced and difficult to reach. However, based on the previous ability to install mechanical sleeves, it appears likely that there is sufficient access for making these repairs.

Rolled housings are not considered to be a permanent solution because the process causes cold-working of the stub tube. Although the amount of cold working is within permitted allowables, SCC cannot be ruled out in the long term. There could also be leakage after several thermal cycles, long term solutions, such as mechanical seals or housing and stub tube replacement, are under development. Activities are currently underway within ASME Section XI to develop a Code Case to permit the roll repair method as an alternative repair technique for long term operation.

Consequences of CRD stub tube cracking

Based on limited leakage area, safety evaluations have concluded that stub tube cracking does not limit vessel life or pose a safety concern. However, when cracking occurs, plant availability is impacted and examination and repair costs are significant.

All BWRs are equipped with features that prevent the ejection of a housing even in the event of total weld failure. This both restricts leakage and avoids withdrawal of the associated control blade. Internal pumps have equivalent features.

4.7.2.2 Alloy 182 Welds SCC

In 2002 through wall crack caused by SCC took place at the welds, which join the CRD stub tube and the bottom head in BWR/4 plant in Japan. This was attributed to high residual stresses in sensitized weld material (Alloy 182). The stub tube, which had the crack was replaced with new one. Alloy 82 was utilized as weld material. [4.14] The other 88 stub tubes of the plant were visually inspected with an underwater camera and no crack was found. METI required all Japanese BWR utilities to perform inspections of CRD stub tubes at the plants, which have similar stub tube design (material) and no prior inspection experience.

4.7.3. Safe End Cracking

GE BWR reactor pressure vessel safe ends are fabricated from low alloy steel, carbon steel, stainless steel, or Ni-Cr-Fe alloy. IGSCC has been observed in many stainless steel and Alloy 600 safe ends.

To date, there has been no IGSCC of low alloy steel safe ends; however, there have been instances of crack propagation into low alloy steel nozzle material from adjacent susceptible locations. EPRI Report NP-4443 [4.15] provides a comprehensive listing of safe end materials and safe-end-to-nozzle welding materials for many operating plants. Also, considerable data are presented in this document for recirculation inlet safe ends, recirculation outlet nozzle safe ends, core spray nozzle safe ends and jet pump instrumentation safe ends.

IGSCC in stainless steel was caused by sensitization from welding or furnace post weld heat treatment. Cracking was observed in creviced low carbon stainless steel and Alloy 600 safe ends having creviced geometry. Though not directly associated with the safe ends, there has been IGSCC cracking in Alloy 182 Ni-Cr-Fe nozzle butters, which were used by several vessel fabricators.

Fatigue cracking by thermal stratification has been observed in CRDRL safe ends. This is reduced by flow increase, which might prevent thermal stratification and fatigue. The same mechanism might apply to feedwater nozzle safe ends.

4.7.3.1 IGSCC in sensitized material

In 1975, GE began an ongoing research program to identify the cause of IGSCC and develop corrective measures. It has been found that IGSCC could occur when the following three conditions exist simultaneously: (1) a sensitized microstructure, (2) the sensitized material must be in contact with a corrosive environment (oxygenated BWR water), and (3) the material must be subjected to a tensile stress, which could include residual stresses from welding or other sources.

This led to a search for a material that would not become sensitized by welding. The material of choice (though not the only material identified) is Nuclear Grade Type-316 austenitic stainless steel, which has a carbon content limited to 0.02% (wt) maximum. It should be noted that the Nuclear Grade material has nitrogen added to compensate for the loss of strength caused by the low carbon content.

4.7.3.2 IGSCC in cold worked or creviced safe ends

Field experience during the past few years has shown that IGSCC can occur in stainless steel (including nuclear grade 316) if the steel is used in a cold worked or creviced condition.

Field experience also shows that Alloy 600 safe ends will suffer IGSCC if the design is creviced. The existence of cold work is not normally documented. It has been found to occur when base metal was ground near a weld or a weld repair. Several safe end-to-thermal sleeve designs used on GE vessels are creviced. These creviced safe ends were replaced in most BWR5 with noncreviced safe end designs to reduce SCC susceptibility.

4.7.3.3 IGSCC in Alloy 182 Nozzle Butter

On many GE RPVs, safe ends were replaced before plant startup to remove furnace sensitized material. The safe ends were replaced with non-sensitized stainless steel or Alloy 600. At some plants, the nozzle butter was removed and replaced with an Alloy 182 Ni-Cr-Fe butter. However, field experience has shown that Alloy 182 Ni-Cr-Fe nozzle butter also is susceptible to IGSCC cracking. Alloy 82 is resistant to IGSCC.

4.7.3.4 Consequences of safe end cracking

Cracking of nozzle safe ends has been detected and controlled before the pressure boundary has been compromised. In service inspection procedures has been effective, remedies have included repair and/or replacement of the affected materials. These remedies, while not inexpensive, indicate that IGSCC of nozzle safe ends can be managed. Nevertheless, this ageing degradation mechanism is an important generic consideration.

4.7.4. Closure stud cracking

Two RPV closure studs cracked at one BWR plant. The closure stud steel material is susceptible to IGSCC if moisture is present. However, except for brief periods such as upon removal of the top head for refueling the studs are kept dry. The cracking was caused by exposure of the studs in the preloaded condition to oxygenated water during refueling outages.