Southern California Edison Company, et al.; San Onofre Nuclear Generating Station, Units 2 and 3; Issuance of Director's Decision Under 10 CFR 2.206

NUCLEAR REGULATORY COMMISSION

[Docket Nos. 50-361 and 50-362]

Southern California Edison Company, et al.; San Onofre Nuclear

Generating Station, Units 2 and 3; Issuance of Director's Decision

Under 10 CFR 2.206

Notice is hereby given that the Director, Office of Nuclear Reactor

Regulation, has acted on a Petition for action under 10 CFR 2.206

received from Mr. Stephen Dwyer dated April 25, 1997, for the San

Onofre Nuclear Generating Station (SONGS), Units 2 and 3.

The Petition requests that the Commission shut down the San Onofre

Nuclear Generating Station pending a retrofitting of the steam

generators. As a basis for the request, the Petitioner asserts that the

ability of the steam generators to withstand a major seismic event is

seriously compromised by the degraded eggcrate supports discovered in

the SONGS Unit 3 steam generators.

The Director of the Office of Nuclear Reactor Regulation has

determined that the request should be denied for the reasons stated in

the ``Director's Decision Under 10 CFR 2.206'' (DD-98-06), the complete

text of which follows this notice and which is available for public

inspection at the Commission's Public Document Room, The Gelman

Building, 2120 L Street, N.W., Washington, D.C. 20555-0001, and at the

Local Public Document Room located at the Main Library, University of

California, P.O. Box 19557, Irvine, California 92713.

Dated at Rockville, Maryland, this 11th day of June 1998.

For the Nuclear Regulatory Commission.

Samuel J. Collins,

Director, Office of Nuclear Reactor Regulation.

Director's Decision Under 10 CFR 2.206

I. Introduction

By e-mail dated April 25, 1997, Stephen Dwyer (Petitioner)

requested that the Nuclear Regulatory Commission (NRC) take action with

regard to San Onofre Nuclear Generating Station (SONGS) regarding his

concerns about the ability of the SONGS steam generators to withstand a

major seismic event.\1\ Specifically, the Petitioner stated that the

ability of the SONGS steam generators to withstand a major seismic

event is seriously compromised by the degradation observed in the SONGS

Unit 3 steam generator internal tube supports (eggcrate supports)

during its 1997 refueling outage. The Petitioner requested an

investigation to determine if Unit 2 has experienced degradation

similar to that found in Unit 3 and also stated that further seismic

analysis should be performed for the SONGS steam generators and that a

retrofitting upgrade of the steam generator supports could be

accomplished at this time. On June 26, 1997, the NRC staff acknowledged

receipt of the Petition as a request pursuant to Section 2.206 of Title

10 of the Code of Federal Regulations (10 CFR 2.206) and informed the

Petitioner that there was insufficient evidence to conclude that

immediate action was warranted. Notice of the receipt of the Petition

indicating that a final decision with respect to the requested action

would be forthcoming within a reasonable time was published in the

Federal Register on July 3, 1997 (62 FR 36085).

---------------------------------------------------------------------------

\1\ The Petitioner sought to add this concern to his Petition

dated September 22, 1996, wherein he requested the NRC to shut down

the SONGS facility ``as soon as possible'' pending a complete review

of the seismic design of the SONGS units based on information

gathered from the Landers and Northridge earthquakes. By letter

dated June 26, 1997, the NRC advised the Petitioner that his e-mail

request dated April 25, 1997, concerning the ability of the SONGS

steam generators to withstand a major seismic event, would be

treated as a separate 10 CFR 2.206 Petition. The Director's Decision

(DD-97-23) issued by the NRC on September 19, 1997, denied the

Petitioner's September 22, 1996, request to shut down the SONGS

units, providing a detailed discussion of the adequacy of the

seismic licensing basis for the SONGS facility.

---------------------------------------------------------------------------

My Decision in this matter follows.

II. Discussion

A. Request for an Investigation to Determine if SONGS Unit 2 Has

Experienced Eggcrate Degradation Similar to Unit 3

1. Background

The SONGS units utilize Combustion Engineering Model 3410

recirculating steam generators. This model of steam generator contains

9,350 Inconel 600 (ASME Material Specification SB-163) U-tubes with a

nominal diameter and wall thickness of 0.75 and 0.048 inch,

respectively. Secondary side tube support structures consist of seven

horizontal full eggcrate supports, three horizontal partial eggcrate

supports, and upper bundle supports (i.e., two batwing diagonal

supports and seven vertical supports). The materials used for

fabrication of the steam generator vessels and internals (including

tube supports) are low-alloy and carbon steels, respectively. Figure 1

is a simplified cross-sectional diagram of the SONGS steam generators

that clearly displays the 10 eggcrate support levels, and Figure 2 is a

three-dimensional representation of the steam generators that gives

additional structural detail.

The eggcrate supports consist of 1- and 2-inch carbon steel strips

interlocked perpendicular to each other as shown in Figure 3. The

eggcrate supports limit lateral motion of the tubes and, at the same

time, allow free flow of fluid around the tubes.

During the 1997 refueling outage for SONGS Unit 3, the licensee

discovered that portions of the eggcrate supports had experienced

degradation, ranging from minor wastage of the eggcrate material to

severe thinning in localized areas. The significant degradation

observed during this refueling outage was confined mainly to the

periphery locations of the eggcrate supports. The secondary sides of

the steam generators in both units were inspected during their 1997

refueling outages and during their 1998 mid-cycle outages and, as

discussed below, significant degradation was limited to the periphery

locations of the SONGS Unit 3 eggcrate supports.

The licensee has extensively researched the cause of the eggcrate

degradation and has concluded that the degradation was caused by a form

of flow accelerated corrosion (FAC), a general term describing

processes that use assistance from fluid flow to remove the protective

oxide layer from base material. Removal of the protective oxide layer

exposes the base material to the fluid environment, allowing further

material removal through corrision and/or erosion processes. The carbon

steel

[[Page 33969]]

eggcrate material utilized in the SONGS steam generators can be

susceptible to FAC in the presence of sufficiently high fluid

velocities.

The licensee concluded that the FAC occurred during recent

operation of Unit 3 primarily as a result of steam generator secondary

side increased fluid velocities caused by the buildup of deposits on

the steam generator tubes. This buildup of deposits on the tubes

significantly reduced the available flow area within the tube bundle

causing flow diversion to the periphery of the tube bundle. The flow

diversion to the periphery was also affected by the increased steam

quality of the fluid within the tube bundle. The buildup of deposits on

the tubes changed the heat transfer characteristics of the tubes

causing the steam quality to increase in the central region of the

steam generators. This resulted in an increase of the flow resistance

in the central portions of the steam generator, forcing more flow to

the peripheral regions, with resulting higher velocities. The resulting

large velocity gradients at the periphery initiated vortices which

further elevated local velocities that were capable of dislodging the

protective oxide layer of the eggcrate material and initiating erosive

FAC.

The chemical cleaning of the SONGS Units 2 and 3 steam generators

during the 1997 refueling outages removed the deposit buildup and

restored fluid flow to their original design values (i.e., nominal

conditions). The licensee stated in its October 17, 1997, letter that

with the flow area restored to nominal conditions, the high fluid

velocities that lead to FAC would no longer exist, thus stabilizing

eggcrate support degradation. The licensee has also made changes to the

chemistry control program for the secondary system at SONGS Units 2 and

3 to reduce the feedwater iron transport. This is expected to prevent

the level of deposit buildup observed in the steam generators before

chemical cleaning was done in 1997. The staff concurs with the

licensee's evaluation that FAC was caused by deposit buildup on the

steam generator tubes and that removal of the deposits should restore

the steam generator secondary fluid flow to within nominal design

values, thereby eliminating continued significant eggcrate degradation.

To confirm that FAC has been stopped by the chemical cleaning of the

steam generators, and to assure that no significant degradation of the

eggcrate support structure goes undetected, the licensee has committed

to conduct periodic inspections of the secondary side of the steam

generators in both units during future outages. The licensee will

conduct periodic inspections of the secondary side of the steam

generators to check the level of deposit buildup on the tubes and to

verify that future degradation of the eggcrate, if any, remains within

the assumptions used in the analysis to demonstrate continued

operability of the steam generators (discussed later in this Decision).

2. Description of the Eggcrate Inspections

The SONGS licensee inspected the steam generator secondary side

support structures, which include the eggcrate supports, in both SONGS

units during their 1997 refueling outages and during their 1998 mid-

cycle outages. The results of these inspections are contained in the

licensee's letters dated May 16, 1997, and June 5, 1997 (SONGS Unit 2

and Unit 3 refueling outage inspections results, respectively), and

letters dated March 10, 1998, and April 15, 1998 (SONGS Unit 2 and Unit

3 1998 mid-cycle outages, respectively).

The objective of the inspections for both units was to provide

video documentation of all areas in which indications of support bar

degradation was suspected and to verify that other areas did not

exhibit these same characteristics. The extent and results of these

video inspections are summarized below.

The inspection of the secondary side of each steam generator was

divided into six areas: (1) general inspection, (2) inner tube bundle,

(3) batwings and vertical straps, (4) eggcrate periphery, (5) eggcrate

interior (blowdown lane),and (6) stay cylinder. Each of these areas was

inspected to the extent necessary to understand, with a high degree of

confidence, the amount of degradation present. The majority of these

areas did not exhibit any significant degradation and therefore the

design function of the support structures was not adversely impacted.

The general inspections were performed in the steam generators from

the top of the moisture separator can deck and included the general

area, U-bend, and annulus regions. The areas inspected included I-

beams, I-beam to shroud attachments, drains, vertical supports,

batwings and the batwing hoop, and baffle anti-rotational keys. These

inspections identified no significant degradation in either unit in

these areas.

The inner tube bundle consists of that area between the outer or

peripheral tubes to the inner tubes of the stay cylinder. The inner

bundle inspections were performed in both steam generators from the can

deck. A small camera was dropped down in between the tubes in a number

of different locations to assess the general material condition of the

eggcrates away from the periphery area. For the steam generators in

both units, the inspections indicated that the inner bundle did not

exhibit the degraded characteristics of the periphery eggcrates found

in the Unit 3 steam generators during the 1997 refueling outage.

No indications of thinning were detected during the inspections of

the interior batwing and vertical strips on either unit.

Comprehensive peripheral eggcrate inspections were performed in

both steam generators in the two units from the can deck. This included

the lattice bars and tube to lattice bar interfaces at each eggcrate.

The area near the periphery of the eggcrate supports in the Unit 3

steam generators experienced the maximum thinning, as shown in Figure 3

and discussed above. As stated earlier, minor isolated instances of

thinning were observed in the peripheral eggcrate locations in the

SONGS Unit 2 steam generators, but overall the thinning was

considerably less than that observed on SONGS Unit 3.

Inspections of the blowdown lane eggcrates were performed in the

steam generators through the 6-inch handhole at the secondary face of

the tubesheet from the handhole to the stay cylinder. This included the

lattice bars and the eggcrate rings. The inspection scope was to sample

the eggcrate area nearest the tubes on both the hot- and cold-leg sides

of the blowdown lane. Minor amounts of eggcrate degradation were found

in the steam generators of both units, with the Unit 3 steam generators

exhibiting the larger amount of degradation in this area.

For the inspection of the overall condition of the eggcrates and

ring in the stay cylinder, a support plate inspection device was used.

Little or no degradation was found in this area in either unit.

3. Summary of SONGS Unit 2 Eggcrate Inspection

The licensee's initial assessment of the Unit 2 stream generator

eggcrate supports, conducted after the degradation issue was identified

in the SONGS Unit 3 steam generators, was reported in its letter dated

May 16, 1997. The licensee concluded that the Unit 2 eggcrate supports

were in very good to excellent overall condition, based on the limited

video examinations of the eggcrates performed in support of the

chemical cleaning process. Although the licensee considered operation

for the normal period of operation between refueling intervals to be

acceptable on the basis of this limited examination,

[[Page 33970]]

the licensee conservatively performed a more extensive video

examination of the eggcrates during a mid-cycle outage that began on

January 24, 1998. As reported in its March 10, 1998, letter, the

licensee observed minor isolated instances of thinning in the periphery

areas of the eggcrate supports, but overall the thinning was

considerably less than that observed on SONGS Unit 3.

The NRC reviewed the program established by the licensee to conduct

the video examinations of the eggcrate supports during the SONGS Unit 2

mid-cycle outage and reported its findings in Inspection Report 50-361/

98-10; 50-362/98-01, dated May 29, 1998. This program was similar to

the licensee's program for inspecting the Unit 3 eggcrate supports

during its mid-cycle outage. The primary difference between the

inspection programs for the two units was that a larger portion of the

Unit 3 eggcrate structures was inspected. The staff concluded in its

inspection report that the scope of the SONGS Unit 2 secondary side

visual inspections was satisfactory and the results supportive of the

licensee's conclusion that no steam generator tubes needed to be

removed from service due to insufficient support from any secondary

side support structures, which includes the eggcrate support

structures.

4. Actions Taken as a Result of Observed Eggcrate Degradation

Following the secondary side inspection activities conducted during

the SONGS Unit 3 1997 refueling outage and 1998 mid-cycle outage, the

licensee plugged and stabilized (by insertion of a steel cable inside

the subject tube) some Unit 3 steam generator tubes as a precautionary

measure due to the degradation observed in certain eggcrate supports.

No tubes in the Unit 2 steam generators were removed from service. Once

the tube is removed from service in the above described manner, support

from the eggcrate structures is no longer needed. The criterion

established by the licensee for removing tubes from service is

described in detail below.

B. Concern About the Seismic Adequacy of the SONGS Steam Generators

The Petitioner asserts that the degradation of the steam

generators, eggcrate supports could seriously weaken the supports and

make the steam generators vulnerable to seismic events.

In its letter of May 16, 1997, the licensee committed to perform an

evaluation of the effect of the degraded eggcrates on steam generator

tube integrity in the SONGS Unit 3 steam generators before return to

power from the Unit 3 1997 refueling outage. This initial evaluation

was provided by the licensee in its letter of June 5, 1997, and

included the effects of a postulated design-basis earthquake. The

licensee submitted the final version of the degraded eggcrate support

evaluation for SONGS Unit 3 on October 17, 1997. As stated in the

previous section, the amount of eggcrate support degradation observed

in SONGS Unit 2 was considerably less than that observed in Unit 3.

Therefore, the staff concludes that demonstrating the ability of the

SONGS Unit 3 steam generators to withstand a design basis seismic event

will demonstrate the adequacy of the Unit 2 steam generators as well.

The staff's review of the seismic adequacy of the SONGS Unit 3

generators is detailed below.

1. Methodology and Acceptance Criteria

The Petitioner did not specifically request the staff to evaluate

the eggcrate supports assuming other design loads concurrent with

earthquake loads. However, to provide additional conservatism, and to

conform with General Design Criterion (GDC) 2 of 10 CFR Part 50,

Appendix A, the licensee, in its October 17, 1997, letter, evaluated

the ability of the eggcrate supports to perform their intended safety

function assuming the most limiting combination of load conditions.

GDC 2 requires, in part, that the design bases for structures,

systems, and components important to safety reflect appropriate

combinations of the effects of normal and accident conditions with the

effects of natural phenomena such as earthquakes. The earthquake for

which these plant features are designed is defined as the safe-shutdown

earthquake (SSE).\2\ The Petitioner's concerns on the adequacy of the

seismic design of the SONGS units, based on information gathered from

the Landers and Northridge earthquakes, were addressed previously by

the staff in DD-97-23 (see footnote 1).

---------------------------------------------------------------------------

\2\ The SSE is defined, in part, as ``that earthquake which is

based upon an evaluation of the maximum earthquake potential

considering the regional and local geology and seismology and

specific characteristics of local subsurface material. It is that

earthquake which produces the maximum vibratory ground motion for

which certain structures, systems, and components are designed to

remain functional.'' See 10 CFR Part 100, Appendix A, Section

III.(c),

---------------------------------------------------------------------------

Appendix A of Standard Review Plan,\3\ (SRP) Section 3.9.3,

``[American Society of Mechanical Engineers] ASME Code Class 1, 2, and

3 Components, Component Supports, and Core Support Structures,''

delineates acceptable design limits and appropriate combinations of

loadings associated with normal operation, postulated accidents, and

specified seismic events for the design of Seismic Category I fluid

system components (i.e., water- and steam-containing components). This

appendix also provides that necessary plant features important to

safety meet the appropriate design limits specified in Section III of

the ASME Boiler and Pressure Vessel Code (ASME Code) when the component

is subjected to concurrent loadings associated with the normal plant

condition, the vibratory motion of the SSE, and the dynamic system

loadings associated with the faulted plant condition. Faulted plant

conditions are those operating conditions associated with postulated

events of extremely low probability, such as loss-of-coolant accidents

(LOCAs) or main streamline break (MSLB) accidents. The design limits

and loading combinations utilized by the licensee in the October 17,

1997, evaluation of individual steam generator tubes are the same

design limits and loading combinations that were reviewed and approved

by the staff at the time of plant licensing. This evaluation is

contained in Chapter 3 of NUREG-0712.\4\ Therefore, the staff finds

acceptable the licensee's use of these design limits and loading

combinations in evaluating the impact of the degraded eggcrate supports

on individual steam generator tubes.

---------------------------------------------------------------------------

\3\ The Standard Review Plan (SRP) is published as NUREG-0800,

and is used as guidance for the Office of Nuclear Reactor Regulation

staff responsible for the review of applications to construct and

operate nuclear power plants.

\4\ NUREG-0712, ``Safety Evaluation Report related to the

Operation of San Onofre Nuclear Generating Station, Units 2 and 3,''

Chapter 3, February 1981.

---------------------------------------------------------------------------

The evaluation of the potential for lateral movement of the entire

steam generator tube bundle (whole bundle evaluation) was not

explicitly addressed during the staff's review performed at the time of

plant licensing. Also, the ASME Code does not provide specific design

limits for the whole bundle evaluation. The whole bundle evaluation

contained in the October 17, 1997, letter performed by the licensee to

verify that the structural integrity of the eggcrate is maintained to

ensure that it does not shift in a way that could damage the tubes.

This is not an ASME Code evaluation; however, ASME Code techniques were

used by the licensee to generate and assess the results. The staff has

reviewed the specific ASME Code techniques utilized by the licensee,

and concludes that they provide conservative results, and are,

therefore, acceptable for the whole bundle evaluation.

[[Page 33971]]

Furthermore, the loading combinations used in the licensee's whole

bundle evaluation are the same loading combinations used in the

individual tube evaluations, and are the same loading combinations that

were reviewed and approved at the time of plant licensing.

2. Degraded Eggcrate Support Assumptions

The staff reviewed the assumptions used in the licensee's October

17, 1997, evaluation regarding the amount of eggcrate support judged to

be available, and verified that these assumptions were supported by the

results of the licensee's inspections.

For the individual steam generator tube analysis, the licensee

calculated the maximum loads that could occur assuming that adequate

support was not available at two consecutive eggcrate levels (see

Figure 1). The staff finds this assumption conservative and acceptable

because the licensee has removed from service all tubes where two

consecutive eggcrate levels were found degraded to the point where

adequate support could not be assured.

For the whole bundle analysis, the licensee used the inspection

results to sort the eggcrates into categories based on a conservative

estimate of the remaining thickness of the eggcrate lattice bars. The

staff reviewed the sorting criteria used by the licensee, and concludes

that the material strength assumptions established by the licensee for

the degraded eggcrate supports are conservative, and appropriate for

evaluating the ability of the eggcrate structures to perform their

intended function.

The visual inspections performed by the licensee during the 1998

mid-cycle outages for both units confirmed the appropriateness of these

assumptions pertaining to the amount of eggcrate support degradation

used in the licensee's evaluation.

3. Evaluation Results

Using the above described methodology and assumptions, the licensee

determines that the peak calculated loads on the individual steam

generator tubes would remain below the allowable design limits approved

by NUREG-0712 during and following a postulated design basis

earthquake.

The results of the licensee's whole bundle evaluation confirmed

that the eggcrate structure will provide sufficient support to ensure

that the tube bundle will not impact the eggcrate support ring during

and following a postulated design basis earthquake.

The staff finds these results acceptable, and as detailed above,

also finds acceptable the methodology and assumptions used by the

licensee in the generation of these results. The staff concludes,

therefore, that the amount of degradation observed in the eggcrate

supports will not prevent the SONGS Units 2 and 3 steam generators from

performing their intended safety functions.\5\

---------------------------------------------------------------------------

\5\ Since the amount of support degradation in SONGS Unit 2 was

observed to be considerably less than that observed in Unit 3, the

NRC staff concludes that the licensee's October 17, 1997, evaluation

of SONGS Unit 3 steam generator structural integrity and the staff's

review of that evaluation support the adequacy of SONGS Unit 2 steam

generators to withstand a design basis event and perform their

intended safety function.

---------------------------------------------------------------------------

4. Confirmatory Actions

The licensee's 1998 mid-cycle inspection of the SONGS Unit 3 steam

generators confirmed that the condition of the Unit 3 eggcrate internal

supports remained within the analytical assumptions used in the

licensee's evaluation contained in its October 17, 1997, letter and

also supported the licensee's contention that the phenomenon (FAC) that

led to the degradation of the eggcrates had been arrested by the

chemical cleaning of the steam generators.

Furthermore, the licensee has committed in its letters to the NRC

(April 15, 1998, for Unit 2 and October 17, 1997, for Unit 3) to

inspect the eggcrate supports during future outages to assure that

their condition remains within the analytical assumptions used in the

licensee's evaluation. These inspections will continue to be conducted

until it is established that further inspections are not required.

In summary, on the basis of the video inspection results for the

steam generators in both units, and the staff's review of the detailed

evaluations performed by the licensee, the staff concludes that the

SONGS steam generators are fully capable of performing their intended

safety function during and following a postulated SSE, and no

retrofitting upgrade of the steam generators is required.

III. Conclusion

As explained above, there is no evidence of significant degradation

of the SONGS Unit 2 steam generator eggcrate supports, and the

extensive analyses demonstrate the ability of the steam generators in

both SONGS units to perform their intended safety function.

Accordingly, the Petitioner's requested action, pursuant to Section

2.206, is denied.

A copy of this Decision will be filed with the Secretary of the

Commission for the Commission to review in accordance with 10 CFR

2.206(c) of the Commission's regulations. As provided by this

regulation, the Decision will constitute the final action of the

Commission 25 days after issuance, unless the Commission, on its own

motion, institutes a review of the Decision within that time.

Dated at Rockville, Maryland, this 11th day of June 1998.

For the Nuclear Regulatory Commission.

Original signed by

Samuel J. Collins,

Director, Office of Nuclear Reactor Regulation.

Attachments: Figures (3)

BILLING CODE 7590-01-M

[[Page 33972]]

[GRAPHIC] [TIFF OMITTED] TN22JN98.015

[[Page 33973]]

[GRAPHIC] [TIFF OMITTED] TN22JN98.016

[[Page 33974]]

[GRAPHIC] [TIFF OMITTED] TN22JN98.017

[FR Doc. 98-16539 Filed 6-19-98; 8:45 am]

BILLING CODE 7590-01-C