ML20118C118
| ML20118C118 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 10/05/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20118C115 | List: |
| References | |
| NUDOCS 9210090289 | |
| Download: ML20118C118 (6) | |
Text
,
kOOf00,k o
UNITED STATES n
.i
.i NUCLEAP. REGULATORY COMMISSION
[
WASHINGTON, D.C. 20066 S'FETY EVALUATION BY TPE OFFICE OF NUCLEAR REAC10R REGULATION RELATED TO AMENDMENT NOS.167 AND 147 TO FACILITY OPERATING LICENSE NOS. NPt., AND NPF-7 VIRGINIA ELECTRIC AW_ POWER COMPANY OLD DOMINION ELECTRIC COOPERATIVE NORTH ANNA PQWfjLSmi10M. UNITS NO 1 AND NO. 2 DOCKET N05, 50-337 AND 50-339
1.0 INTRODUCTION
By letter c'ated October 2,1989, and as supplemented by let' *s dated April 29 and August 24, 1992, the Virginia Elect.'ic and Power Compan.,,VEPCO) requested a change to the Technical Specifications (TS) for the North Anna Power l
Sta+'an, Units No. I and No. 2 (NA-l&2).
The proposed change requested approval for.
(1) deletion of settlement monitoring from most of the Category I structures from the NA-l&2, and (2) increased allowable differential settlement limits for certain structures.
VEPC0 proposed to delete from the TS, 47 out of the 51 markers located in the main plant area, and 5 out of the 17 markers located in the Service Water Reservoir (SWR) Trea.
The proposed TS l
chcnge would reduce the total number of settlement markers being monitored from 68 to 16.
l The April 29 and August 24, 1992, letters provided a ditional information d
l requested by the staff.
This additional information requested by the staff l
did not alter the propoced action or affect the staff's determination of no significant hazards consideration as noticed in the Federal Reaister. on l
Febreary 7, 1990 (55 FR 4283) l
2.0 BACKGROUND
l VEPC0 commenced a settlement monitoring program at NA-l&2 in May 1976.
In August 1976, VEPC0's survey of the Service Building (SB) in the main plant l
area indicated apparent settlement that would have caused excessiu stresse-in the four 24-inch diameter service water (SW) pipes buried beneath the l
footing of this structure. Therefort, in Oril 1977, VEPC0 took remedial l
action by cutting these SW pipes to rel X J the streses accumulated since the initial imposition of loading 11.1972, P,.
welded them. VEPC0 established a baseline for future surveys assuming zer, e:.erential settlement of pipes at that time.
9210090287 921005 PDR ADOCK 05000338 P
1 i
j 2
I lable J.7.5 of the NA-l&21S lists 51 settlement measurement points in the j
main plant arca, in October 1977, based on projected settlement of the strLctures, an allowable differential settlement of 0.03 foot (ft) between I
settlement point 117 located in the SB and point 113 locateo in the Main Steam Valve House (MSVH)/ Quench Spray Pump House (QSPH) was specified in the plant i
15 for settlement monitoring.
Subsequently, VEPC0 found that the measured differential settlement between the two points gradually approached the allowable va.lue.
Partly to alleviate this problem, VEPC0 performed a pipe stress analysis, and submitted a TS change request on March 10, 1988, to-7 increase the allowable differential settlement between points 117 and ll3R i
(which replaced point 113).
VEPCO alsc used improved surveying equipment, and performed additional surveys which indicated that the measured differential settlement between the above two points was, in fact, less than 75% of the allowable value.
Based on a review of the data obtained by VEPCO using the new surveying equipment, the NRC staff did not see any need to increase the i
allowable differential settlement between points 117 and ll3H, and rejected i
the IS change request by letter dated March 31, 1992.
Furthermore, the staff l
raised concerns about the accuracy of the measured settlemen u, and also about the pipe stress analysis.
As noted in the introduction, VEPC0 submitted a comprehensive report on h
October 2, 1989 for the rettlement monitoring program at North Anna.
in that report, it requested staff approval:
(1) to delete, from the NA-l&2 15, settlement monitoring requirements for most of the structures that are founded
~,
on rock or on concrete backfill over rock, and (2) to increase the allowable dif f erential settlcaent limits for certain structures.
In response to a staff request, VEPCO submitted in updated report containing the results of settlement monitoring surveys performed through April 992.
The review of VLPCO's request to delete the settlement monitoring for most of the Category I structures has bere completed. Also, VEPC0's r'c est to increase the allowable differential settlement limits of ceri.an structures has been reviewed and approved based on the es 'uation of stress calculations and comparison against allowahle stresses.
1-3.0 01EV1SJM The majority of the Category I structures et NA 151 are founded on rock, or on concrete backfill over rock.
Portions of the foundations of a few structures (e.g., the SB) are w ported on residual soil, structural fill over residual soil, or structurai fill over rock.
VEPCO reported in its October 1989 submittal that most e these structures are heavily-reinforced concrete shear-r wall-type <
ctures with thick mat foundations, and that no settlement-i related craaing of the walls for these structures had been observed.
1 Iwo structures,1,e., the NA-2 MSVH/QSPH (where settlement point ll3R is i
located) and the Aux 111ary Building, are partially rock-founded.
Both these buildings are heavily-reinforced concrete shear-wall-type structures and have thick mat foundations.
The western end of the SB from column 14 to column line 17-1/2 is founded on structural fill over either bedrock or a layer of A
t
-~
e, ee,
,-n,- e,w,-, w m e w - r
,w-
,,,, <.,,, _.. ~
,,_n,--.--,w
~.,. - - -.. -, - - - - - - - - - - -
residual soil.
In the area of column line 14 (where settlement point 111 is located), the foundation material is structural fill underlain by a layer of residual soil which, in turn, is underlain by bedrock. Again VEPC0 reported that no settlement-related cracks of shear walls had been observed.
In its report of October 2,1989, VEPC0 stated that from the beginning of the formal settlement monitoring program in 1976, its survey results were inaccurate due to two types of survey errors (i.e., random survey error and systematic survey error) caused by the following factors.
1.
Long survey loops from distant benchmarks were used from 1976 until 1979.
(Random errors are always present in measurements accumulated by the increased number of leveling setups involved in a long survey loop).
In 1979, VEPC0 established two rock-founded benchmarks closer to the main power block area, thus reducing the survey loop lengths considerably.
2.
Because of obstructions preventing access to survey markers, settlement readings on about 75% of the markers were taken by what are called, " side shots", which are not part of the main survey loop.
These are not as accurate as the " turning point shots" in which the marker point is a part of the continuous survey loop. While the elevation taken by a turning point shot is included in the calculation of the allowable survey closure error at the completion of the survey loop, the side shot elevation is not included in the closure error calculation, thus contributing to inaccurate results in the elevations of about 75% of the markers.
3.
Until 1987, VEPC0 used a wooden survey rod, called Yard Rod, which is not very accurate.
From 1987 onward, an Invar Rod, made of a highly temperature-resistant invar scale attached to an aluminum rod, was used.
This rod, which can be read accurately to 0.001 feet, has some problems-at certain locations of settlement markers because of the size of the rod. When obstructions or other access problems prevented the use of either the Yard Rod or the Invar Rod, VEPC0 utilized either a 5-foot section of a Philadelphia Rod or a folding Engineers Rule.
The use of the four different types of survey rods, each having certain shortcomings, introduced syste, tc errors in the survey data, and indicated apparent upward movemt c of certain markers.
As a result of a detailed evaluation of its surveying procedures, VEPC0 identified the above survey errors and corrected its results by using statistical procedures.
The NRC staff and its consultants visited the site in May.1989 when a surveying specialist performed an independent settlement
-measurement using his own equipment.
This independent survey verified the accuracy of VEPCO's differential settlement measurement between the two points, ll3R and 117, after VEPC0 had corrected its data to account for the effects of using different survey rods.
. Based on the results of a detailed evaluation and corrections of its settlement data, VEPC0 found that there has been very little settlement for most of the structures supported on rock or on concrete backfill over rock.
Therefore, VEPC0 proposed to delete from the plant TS all but six settlement
- markers, i.e., four markers, ll3(ll3R), 114, 116, and 117, located in the main plant area, and two markers, SM-17 and SM-18, located in the Service Water Reservoir (SWR) area.
In addition to these 6 markers, VEPC0 will continue to monitor 10 other markers located in the SW Pumphouse, SW Valve House, and SW Tie-in-Vault.
Based on a review of the updated, corrected settlement data pertaing to all the Category I structures in the main plant area submitted in April 29, 1992, the staff concurs with VEPC0's finding that the settlements of most of the structures h ave stabilized and are within the allowable limits.
As noted above, the staff had previously reviewed VEPCO's evaluation of underground pipe stresses due to differential settlement of Class I structures at NA-l&2.
The staff's review comments were forwarded to tf' licensee by letter dated May 8,1992, and basically questioned the adequacy of pipe models used for ASME Code stress calculations, including the adequacy of soil spriag constants, spacing of springs, and anchor stiffness values used.
By letter dated September 29, 1989, VEPC0 stated that a two-to ten-fold increase or decrease of the anchor translational stiffness values would result in a less than 1% change of critical pipe stress levels. Also, by letter dated May 21, 1992, VEPC0 indicated that with the use of a conservative rotational anchor stiffness (50% larger than that calculated), there is no significant sensitivity of critical pipe stress levels to a reasonable bounding increase or decrease in other parameters.
The reason for such insensitivity in the parametric studies is judged to be due to the fact that the prescribed vertical settlement and the slope at the SB end of the pipe model override the significance of the corresponding anchor stiffness.
VEPC0 also indicated that conservative estimates of soil spring stiffness and spacing of springs were utlized in the pipe stress calculations.
The staff finds VEPC0's response to be acceptable since conservative estimates of soil spring stiffness and spacing of springs were utilized in the pipe stru s calculations.
For the buried SW piping between the NA-2 MSVH and SB, these conservative estimates and a future differential settlement corresponding to the proposed revised TS limitation of 0.047 ft. would result in a critical stress of 44,176 psi.
This critical stress includes the residual stresses which existed prior to 1977 when portions of their pipes were cut and when a baseline for future settlement was established by assuming zero settlement at that time.
This critical stress is within the applicable Code-allowable stress of 45,000 psi and is, therefore, acceptable.
The margin of safety as defined in the TS provides assurance that the settlement of structures does not exceed the allowable settlement limit which, in turn, does not cause unacceptable pipe stress.
Since the basis for TS 3/4 4.7.12 is to maintain pipe stress within Code allowables, the staff I
, agrees that the mhrgin of safety it not significantly reduced and VEPC0's eval ' tion remains valid and therefore, the staff finds VEPCO's request to incr=&se the allowable differential settlement values of specified structures to be acceptable.
During the course of the re,iew of these matters, the staff discussed its concern with VEPC0 concerning the apparent lack of proper quality control in surveying procedures, following these discussions, VEPC0 submitted by letter dated August 24, 1992, a proposal agreeing to do the following.
1.
VEPC0 will submit to NRC, afte" the TS are changed, the results of the next 4 semi-annual surveys of the 16 markers (which VEPC0 will continue to monitor) so that NRC can review them and verify that proper quality control is being exercised in settlement monitoring of Category I structures.
The survey results will be submitted i.o later than 60 days after the completion of each semi-annual survey.
2.
VEPC0 will maintain all the attlement points deleted from the TS to the extent practicable and it will not initiate retions to physically remove existing settlement points. However, VEPC0 will not relocate any point that may be obscured or removed due to a future design change.
VEPC0 has stated, in this connection, that past experience indicates that design changes of such a nature seldom occur.
4.0
[yALQ61LQU Based on the information and settlement data submitted by the licensee, the staff finds acceptable the deletion from the NA-l&2 TS 3/4.7.12 the requirement to monitor the settlement of all Category I structures in the main plant area which have not exhibited any significant settlement since the inception of monitoring.
VEPC0 will continue to take measurements on four markers in the main plant area, and two markers in the SWR area, in addition to 10 markers in the SW Pumphouse, SW Valve House, and SW Tie-in Vault.
The TS change will reduce the total number of settlement markers being monitored from 68 to 16.
In addition it staff finds acceptable VEPCO's proposed T3 3/4.7.12 limitation of 0.047 ft f or allowable differential settlement pipe stress limits.
Since the basis for TS 3/4.7.12 is to maintain pipe stress within Code allowables, the VEPC0 evaluation is enveloped by applicable Code allowable stress values and n emains valid and is acceptable, finally, the staff finds acceptable the licensees proposed actions to demonstrate that the quality control procedures for the settlement monitoring program are adequate.
Therefore, based on all of the above, the staff finds the proposed changes to the TS changes to be acceptable.
5.0 STATE CONSULTATION
in accordance with the Commission's regulations, the Virginia State official was notified of the proposed issuance of the amendment.
The State official had no comment.
6.0 ENVIRONMENTAL CONSIDEPM1Q!i These amendments change a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes to the surveillance requirements.
The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure.
The Commission has previously issued a proposed finding that these amendments involve no significant hazards consideration and there has been no pubic comment on such find,ng (55 FR 4283). Accordingly, these amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact st.tement or environmental assessment need be prepared in connection with the issuance of the amendments.
7.0 G R U)1LQR The Commission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
A. Lee
~
R. Pichumant L. Engle Date: October 5, 1992 I
i
_