January 1998	SIB-96-138, Rev. 1
	BWR Reactor Internals Cracking Evaluations
Introduction
Since the discovery of core shroud cracking at Brunswick, Unit 1 in the Summer of 1993, Structural Integrity Associates (SI) has been deeply involved in BWR reactor internals cracking issues. In the Summer of 1994, driven by the extensive core shroud cracking discovered at Dresden, Unit 3 and Quad Cities, Unit 1, coupled with less severe cracking at other facilities, the BWR Owner's Group formed the Vessel & Internals Project (BWRVIP) to generically address the core shroud cracking issue, as well as potential cracking in other internal components. The BWRVIP has to date generically addressed a number of reactor internal components. In the future, it is planned that other internal components will be generically addressed by the BWRVIP.
With the knowledge that BWR reactor internals are susceptible to intergrannular stress corrosion cracking, and the attention given to such possibilities by the BWRVIP, the BWR owners are increasing inspection activity inside the reactor vessel, and have discovered cracking in a number of components. SI's involvement on the subject can be categorized into four areas:
?	core shroud cracking evaluations,
?	core spray system cracking evaluations,
?	jet pump assembly cracking evaluations,
?	other internals cracking evaluations, and,
?	support to the BWRVIP.
Each of these five areas will be discussed in more detail in the following paragraphs.
Core Shroud Cracking Evaluations
In the Summer of 1993, while in the process of performing an in-vessel visual inspection (IVVI) of the core shroud at Brunswick, Unit 1, Carolina Power & Light Company personnel discovered extensive cracking at weld H3 (see Figure 1). GE Nuclear Energy (GE) was contracted to evaluate the degraded condition, and SI was retained to provide third party review of GE's evaluations.

When it was discovered that the cracking was more extensive and deeper than originally anticipated, GE was contracted to develop a repair for welds H2 and H3, with SI again providing a third party review function. A "two weld repair" was implemented in December of 1993, and the Brunswick unit returned to power in the Spring of 1994.
Subsequently, SI has supported a number of other utilities in resolving the core shroud cracking issue, both in the U.S. and overseas. The complexion of SI's services has changed somewhat since 1993, but the foundation is based upon the experience at Brunswick. Today, SI provides core shroud cracking services in three areas:
? preoutage planning,
? flaw evaluations, and,
? third party review of repairs.
SI's pre-outage planning services have evolved dramatically from initially just trying to understand the phenomena and predict susceptibility, to most recently providing support for the installation of core shroud repairs, either contingency or required. SI has developed repair Technical Bid Specifications, or consulted in their generation, and reviewed proposals submitted by repair vendors to utilities.
With the core shroud flaw evaluation methodologies fully developed, SI has had extensive involvement in review of flaw evaluations performed by GE, as well as performance of flaw evaluations for cracked shrouds. Both limit load and linear elastic fracture mechanics techniques are utilized, with the challenge being to utilize generic methodologies if found acceptable, and more refined techniques, utilizing finite element analysis methods, if so dictated by the nature of the inspection results.
After the "two weld repair" was implemented at Brunswick, Unit 1, a number of repair vendors embarked upon concepts which could repair all shroud circumferential welds. To date, both GE and MPR Associates/Framatome Technologies have implemented core shroud repairs. And because of SI's in-depth knowledge of core shroud cracking issues, SI has been contracted as the third party reviewer for most of the repairs implemented to date, including those supplied by both of the aforementioned vendors. SI's role as third party reviewer encompasses the complete product delivered by the vendors, and varies from utility to utility. For some utilities, SI has only provided review of design documentation, such as Design Specifications, seismic analyses, design calculation packages, and Stress Reports. For other utilities, SI has reviewed selected material requirements, and fabrication/installation drawings, procedures, and specifications, and has performed as-built reconciliations. And finally, a number of utilities have contracted SI for a complete review of all of the vendor's activities.
Since the Summer of 1993, SI has been involved with the core shroud cracking issue at Brunswick, Units 1 and 2, Chin-Shan, Units 1 and 2 (Taiwan), Cofrentes (Spain), Dresden, Units 2 and 3, FitzPatrick, Grand Gulf, Hatch, Units 1 and 2, LaSalle, Units 1 and 2, Millstone, Unit 1, Monticello, Nine Mile Point, Unit 1, Oyster Creek, Peach Bottom, Units 2 and 3, Pilgrim, Quad Cities, Units 1 and 2, Santa Maria de Garoņa (Spain), Susquehanna, Units 1 and 2, and Vermont Yankee. In addition, SI performed a study for the Japanese Owner's Group on the repair concepts offered by three vendors; namely, the two mentioned above and Siemens. SI's involvement with core shroud cracking issues has been extensive, and allows SI the ability to fully support any shroud cracking issue identified in the future.
Core Spray System Cracking Evaluations
Incidences of cracking in the reactor internal core spray system (piping and spargers) have been reported in the U.S. since 1978, with a number of plants to date reporting cracking of some fashion (see Figure 2 for a core spray piping layout). With more attention given to the subject of BWR reactor internals cracking issues since the formation of the BWRVIP, and more sophisticated visual and ultrasonic inspection techniques available, the frequency of observed cracking has increased.

Like the core shroud cracking issue, SI's involvement with reactor internal core spray system cracking is again in three areas:
? preoutage planning,
? flaw evaluations, and,
? repair development and third party
reviews.
The service offered by SI prior to plant outages is a screening criteria to be used for core spray system inspections. Since stress information is typically not available to a utility for internal core spray systems, stress analyses are performed, and allowable flaw sizes at each core spray system weld location are determined. The utility then has the ability to evaluate any identified indications on a real-time basis using the screening criteria, hence minimizing an increase in outage schedule.
SI's role in evaluation of any discovered indications has been as a third party reviewer of evaluations performed by other vendors, as well as performance of flaw evaluations for cracked core spray piping. Since utilities are currently evaluating various alternatives for core spray system inspections, it is envisioned that SI will have an in-depth involvement with flaw evaluations in the future, utilizing more refined analytical techniques.
Repairs implemented to date consist of two types. The first has been in place for a number of years, and was developed to repair tee box cracking. The repair is performed by welding brackets to the tee box and adjacent core spray piping header arms. With an increase in the discovery of cracking at other locations, mechanical brackets have been developed which clamp onto the existing core spray piping or spargers, and provide a mechanical load transfer. SI has provided a third party review function to utilities implementing core spray system repairs, and recently completed a feasibility study for weld overlay repair of degraded piping. To date, the third party review activities have been focused only on the analytical tasks undertaken by the repair vendors.
Since the middle of 1995, SI has been involved with core spray system cracking issues at Brunswick, Unit 2, Dresden, Units 2 and 3, FitzPatrick, Millstone, Unit 1, Peach Bottom, Unit 3, and Vermont Yankee. Because of this recent involvement, and our involvement with the BWRVIP on this subject (as discussed below), SI is well suited to provide detailed support in this area.
Jet Pump Assembly Cracking Evaluations
Cracking in jet pump assembly components has been observed for a number of years, primarily in hold down beams, sensing lines, and riser brace leaves (see Figure 3). Recently, riser pipe cracking has been reported at three plants in the U.S., and one overseas. Unlike other reactor internal components, jet pump assembly degradation due to flow-induced vibration is of a major concern, as well as gaps at the restrainer bracket/mixer intersection.

	
	Figure 3. Jet Pump Assembly
To date, SI's involvement has focused on three components. SI has designed a riser brace repair for Dresden, Unit 3, performed qualitative set screw gap evaluations for Peach Bottom, Units 2 and 3, and Dresden, Units 2 and 3, performed quantitative set screw gap analyses for Pilgrim and Cooper, and performed third party review of the riser pipe cracking at Hatch, Unit 1 and Peach Bottom, Unit 3.
Other Internals Cracking Evaluations
Since the end of 1994, SI has also been involved with the cracking of other BWR reactor internal components. SI has evaluated access hole cover cracking at Pilgrim, top guide cracking at Oyster Creek, steam dryer drain channel cracking at Susquehanna, Units 1 and 2, and provided an independent review of the work performed by GE to determine the structural capability of the core plate hold down bolts at Nine Mile Point, Unit 1.
Support to the BWRVIP
In the Summer of 1994, the BWR Owner's Group formed the BWRVIP to address reactor vessel and internals cracking issues. The objective of the BWRVIP was four fold:
? Generic resolution of reactor vessel and
internals integrity and operability issues.
? Development of generic, cost-effective
strategies.
? Focal point for regulatory interface.
? Information sharing.
Five subcommittees were formed to meet the objectives, and dealt with mitigation, inspection, assessment, repair, and integration. The initial focus of the BWRVIP was on the core shroud cracking issue. Inspection, 
assessment, and repair generic documents were developed, and were subsequently reviewed by SI. Since that time, SI's involvement with BWRVIP activities has escalated, with participation on various "teams" in a consulting role, development of BWRVIP guidelines and other documentation, and third party review of activities undertaken by other vendors. Currently, SI is developing criteria for addressing core shroud vertical weld cracking and for evaluating shroud cracking utilizing finite element analysis techniques, performing a study of crack growth rates in nickel base and low alloy steel materials, assisting in the development of a hydrogen water chemistry technical basis document, and providing consulting services to the underwater welding team.
If you would like information regarding cracking of BWR reactor internals or SI's capabilities in this area, please contact:
	Marcos Herrera or Dick Mattson
	Structural Integrity Associates, Inc.
	3315 Almaden Expressway, Suite 24
	San Jose, California 95118-1557 USA
	Phone: (408) 978-8200 ? Fax: (408) 978-8964
	e-mail: info@structint.com