Part 21 Report - 1996-900

 
 

ACCESSION #: 9607260276



          Westinghouse

          Energy    NUCLEAR SAFETY ADVISORY LETTER

          Systems

          Business

ESBU      Unit



THIS IS A NOTIFICATION OF A RECENTLY IDENTIFIED POTENTIAL SAFETY ISSUE

PERTAINING TO BASIC COMPONENTS SUPPLIED BY WESTINGHOUSE.  THIS

INFORMATION IS BEING PROVIDED TO YOU SO THAT A REVIEW OF THIS ISSUE

CAN

BE CONDUCTED BY YOU TO DETERMINE IF ANY ACTION IS REQUIRED.



                                  P.O. Box 355, Pittsburgh, PA 15230-0355



Subject: Reactor Coolant Pump (RCP) Support Column Tilt Issue



                                                     Number: NSAL-94-025



Basic Component: RCP Supports                         Date: Nov. 10, 1994



Plants: See Page 2, Table 1



Substantial Safety Hazard or Failure to Comply Pursuant to 10 CFR

21.21(a)                                               Yes []    No []



Transfer of Information Pursuant to 10 CFR 21.21(b)    Yes []



Advisory Information Pursuant to 10 CFR 21.21(c)(2)    Yes []



Reference:



SUMMARY



The issue concerns a situation whereby certain reactor coolant pump (RCP)

support columns may be titled beyond design parameters.  Specifically,

some three and four loop plants have a loop piping layout in which the

crossover leg piping interferes with the front inside support column of

the RCP.  The other two columns on the RCP are oriented to be vertical in

the normal full power condition.  This condition has been recognized by

the support designer and changes have been made to the column layout to

accommodate the interference between the column and the crossover leg.

The change consists of moving the base of the one RCP column toward the

reactor pressure vessel approximately 6-12 inches so that titled column

no longer interferes with the crossover leg piping.  The required tilt is

from approximately 2 to 5 degrees depending on the column length and

amount of movement.  This condition was apparently reconciled during the

design phase and was not considered a significant issue.  Upon further

review it has been determined that while the actual change in the value

of the vertical stiffness is small, the potential impact on the thermal

expansion of the system, and thus the loop loadings, may be significant.



Based upon the analyses performed, it has been determined that while some

of the affected plants will have completely acceptable results from their

specific RCP column tilt, other plants may have difficulty in meeting

certain margin values for such specific evaluations as Leak-Before-Break.

While such a situation may technically place a given plant outside its

licensing basis, it has been determined that adequate margins remain such

that this situation does not pose a challenge to the reactor coolant

system (RCS) pressure boundary and therefore, does not represent a

substantial safety hazard or failure to comply per the definitions

provided in 10 CPR Part 21.21 (a).



Additional information, if required, may be obtained from the originator.

Telephone 412-374-5036.



Originator(s):

               J. S. Galembush          H. A. Sepp, Manager

                                        Strategic Licensing Issues



NSAL/94-025                                                  Sheet 1 of 6



                    TABLE 1 PLANT APPLICABILITY LIST



Based upon design information held by Westinghouse, the following plants

are affected by the RCP Column Tilt issue:



     J. M.  Farley 1 & 2

     V. C.  Summer

     Shearon Harris

     Vogtle 1 & 2

     Seabrook

     Wolf Creek

     Callaway

     Comanche Peak 1 & 2

     Sequoyah 1 & 2

     Watts Bar 1 & 2

     Sizewell B

     Kori 3 & 4

     Yonggwong 1 & 2

     Ohi 1 & 2

     Takahama 1

     Maansham 1 & 2



NSAL/94-025                                                  Sheet 2 of 6



                          TECHNICAL DESCRIPTION



ISSUE DESCRIPTION



As described in the "Summary" section, the issue concerns a situation

whereby certain reactor coolant pump (RCP) support columns may be tilted

beyond design parameters.  Specifically, some three and four loop plants

have a loop piping layout in which the crossover leg piping interferes

with the front inside support column of the RCP.  The other two columns

on be RCP are oriented to be vertical in the normal full power condition.

This condition has been recognized by the support designer and changes

have been made to the column layout to accommodate the interference

between the column and the crossover leg.  The change consists of moving

the base of the one RCP column toward the reactor pressure vessel

approximately 6-12 inches so that the tilted column no longer interferes

with the crossover leg piping.  The required tilt is from approximately 2

to 5 degrees depending on the column length and the amount of movement.

This condition was apparently reconciled in an informal manner during the

design phase and was not considered a significant issue.  Upon further

review it has been determined that while the actual change in the value

of the vertical stiffness is small, the potential impact on the thermal

expansion of the system, and thus the loop loading&, may be significant.



Based upon the analyses performed for this evaluation, it has been

determined that while some of the affected plants will have completely

acceptable results with their specific RCP column tilt, other plants may

have difficulty in meeting certain margin values for such specific

evaluations as Leak-Before-Break (LBB).  While such a situation may

technically place a given plant outside its licensing basis, it has been

determined that adequate margins remain, such that this situation does

not pose a challenge to the reactor coolant system (RCS) pressure

boundary.



TECHNICAL EVALUATION



Analyses have been performed to evaluate the impact of the RCP column

tilt on the plant with the most severe tilt.  A plant with a column tilt

associated with a 12 inch movement of the column base from the front

inside pump column was chosen for this evaluation.



The additional loadings associated with column tilt originate from the

use of a support stiffness that has been rotated from vertical by a few

degrees.  This routed stiffness at one of the three column locations acts

to restrain and rotate the RCP.  For a loop piping system thermal

expansion of approximately 1.9 inches, the vertical displacement

associated with the rotation of an RCP column will vary from about 10

mils for a nearly vertical column to about 130 mils for an upper bound

tilted column.  This difference in displacements at the three RCP columns

is what causes a rotation in the pump not previously accounted for.  This

source of additional loading varies from plant to plant based on the

primary equipment support design.  There are plants without pin-ended

columns that will not have this type of additional loading.  That are a

number of plants that have support details not of Westinghouse design.

Many of the Westinghouse designed supports have a tilt that is

approximately half of the that analyzed as a part of this evaluation.



When this enhancement was made to the thermal analysis, differed system

loadings resulted.  The bending moment at the RCP outlet nozzle for the

test case increased by more than 100%.  The RCP column loads also

changed.  The tilted column went from a compression load to a tension

load.  The change in column loading met applicable Code allowable limits.

The loop LBB evaluation was of particular concern for system load

changes.  The LBB evaluation is preformed with the goal of achieving

certain margins required by the



NSAL/94-025                                                  Sheet 3 of 6



NRC.  The new loadings were reviewed for all 12 weld locations in the

primary loop for the test plant, and acceptable margins were maintained.



The test case loop evaluation for column tilt loads was successful.  The

column tilt for the test plant is approximately twice (12 inches versus 6

or 6.5 inches) that of most other plants.  Unfortunately, it has been

determined that due to the overwhelming plant specific nature of both RCS

loops and the RCP column design, the test case configuration is not

generic nor necessarily enveloping, but it is representative of a typical

Westinghouse configuration.  Every one of the plants affected by the RCP

column tilt issue has a plant-specific loop analysis that has a unique

set of margins.  Because loadings local to the RCP can be quite different

due to the inclusion of column tilt, those plants that have small margins

in this area may be unable to meet NRC mandated LBB margins.  Those

plants with small LBB margins in the cold leg may require additional

analysis.  As mentioned, there are conservative NRC mandated margins that

are part of the licensing associated with LBB.  Even if the licensing

margins we not satisfied, one can argue that the LBB conclusions are

still valid with smaller margins and that the LBB design basis continues

to apply.



A second test plant was reviewed, which has an LBB critical point

(location, of lowest margin) at the RCP outlet nozzle weld.  This test

plant also has a significant column tilt (approximately 10 inches out-of-

plumb).  Using an estimated increase in bending moment, based on the

first test case results, the critical location for the second case does

not meet NRC required LBB margins.  This evaluation is preliminary

because the increase in loading was estimated.  This second cue is

unusual because the critical location is at the RCP outlet nozzle.  It is

possible that more refined modeling and analysis could be performed that

would change the loading and the location of the critical weld for the

second test case plant.



The inclusion of RCP column tilt has been shown to change loop loadings

by large percentages in the area of the RCP.  A review of the increased

loadings on our first test plant yielded acceptable results in all areas

reviewed.  For the first test case evaluation, there were significant

margins available in the areas where loadings increased.  Good margins in

the RCP area are fairly typical for Westinghouse plants, but there may be

plants where margins are slim.  If sped& modeling techniques are the

cause of low margins, additional analysis may reduce some of the,

potential conservatism that led to the low margins.  For those plants

with low margin around the RCP, structural analysis may need to be

revisited to assure compliance with the licensing basis, but as has

already been mentioned, there is not a pressure boundary integrity

concern due to the RCP column tilt issue.



ASSESSMENT OF SAFETY SIGNIFICANCE



From information available to date, plants that incorporate an RCP column

tilt in their design may be operating in an unanalyzed condition in that

the actual tilt at hot conditions may be greater than the tilt called out

in the design drawings thereby potentially affecting the structural

reliability of the RCP supports.  More importantly, regardless of my tilt

condition described in the design drawings, there may be no analysis

available to substantiate the acceptability of such an "as built" tilt in

any affected plant design.  At hot conditions the plant thermal analysis

may be impacted in the following areas:



     - Primary equipment support evaluation

     - Primary piping fatigue evaluation

     - Primary piping leak-before-break (LBB) evaluation

     - Auxiliary line thermal anchor movement analysis



The above list is tied to any change to the plant thermal analysis.  The

seismic and LOCA analyses are expected to be less significantly impacted

by a tilted column stiffness, but must also be included in the above



NSAL/94-025                                                  Sheet 4 of 6



to determine exactly what magnitude of column tilt is indeed significant.

It is noteworthy that the RCP column design is determined by the seismic

and LOCA loads, therefore, even though the thermal portion of the load is

changing, the total load the column may be experiencing represents a much

smaller percent change.



The various loadings performed in a system evaluation have been reviewed

and qualitatively assessed for the RCP column tilt situation.  Due to the

small angle of tilt associated with the RCP column tilt issue, the

loadings for deadweight, OBE seismic, SSE seismic, and LOCA will change

very little from the design basis analysis numbers that assumed no column

tilt.  This judgement is based on the fact that a stiffness matrix

rotated by a few degrees is expected to offer the same kind of restraint

to a deadweight or a dynamic loading as the original stiffness.  The

deadweight loading will load the column with an additional small shear

load because there is no lateral movement.  The seismic and LOCA loading

conditions will involve some small lateral movement but the additional

lateral restraint is likely to generate a somewhat smaller seismic

response and potentially, slightly higher LOCA loadings.



The thermal condition represents the loading condition most likely to be

impacted by the column tilt.  Because a normal plant heat-up and cooldown

moves an RCP by more than 1.5 inches in a lateral direction, there is a

more significant restraining load generated by the rotated stiffness as

well as a heretofore unaccounted for rotation of the RCP centerline due

to the 1.5 inch deflection.  Both of these effects will likely increase

the column loadings and the adjacent piping and pump nozzle loadings.



The thermal loadings factor into the ASME Section III Code fatigue

analysis and into the leak-before-break (LBB) evaluation.  The fatigue

analysis has a stress limit for which there is usually good margin and a

usage factor which, for the crossover and cold leg, there is good margin.

The LBB evaluation is performed to satisfy certain NRC mandated margins.

Even though the mandated margins may not be satisfied, it is the

Westinghouse position that the LBB conclusions will still be valid with

smaller margin such that the LBB design basis continues to apply.

However, such a situation may technically place a given plant outside its

licensing basis.



Primary equipment nozzle leads for the various loading conditions are

compared to a set of allowable nozzle loads.  This comparison is made for

both a load case, such as thermal, as well as for a load combination like

normal or upset.  There is typically sufficient margin in the nozzles to

accommodate a large increase in the thermal case loading.  There is also

margin between the Code limit and pressure boundary failure.  There are

not expected to be load magnitudes large enough to challenge pressure

boundary or shutdown requirements.



In summary, the effects of RCP support column tilt have the potential to

change the loadings on the loop piping, the primary equipment nozzles,

and the primary equipment supports and embedments.  Even with the loading

changes it is judged that pressure boundary integrity will be maintained

thus allowing continued safe plant operation.



NRC AWARENESS/REPORTING CONSIDERATIONS



Westinghouse, submitted to the NRC an Interim Report of an Evaluation of

a Deviation or Failure to Comply Pursuant to 10CFR21.21(a)(2) on

September 19, 1994.  This action was required since the Westinghouse

evaluation of this issue would exceed the initial 60 day evaluation

period allowed in 10 CFR part 21.  Based upon our analyses and

evaluation, this issue does not represent a substantial safety hazard or

failure to comply pursuant to 10 CFR 21.21(a).



NSAL/94-025                                                  Sheet 5 of 6



RECOMMENDED ACTIONS



While it ha been determined that a substantial safety hazard does not

exist, it is prudent that utilities review their plant specific drawings

to verify the status of the RCP columns.



NSAL/94-025                                                  Sheet 6 of 6



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