ML19305E139
| ML19305E139 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 04/04/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19305E133 | List: |
| References | |
| NUDOCS 8004230017 | |
| Download: ML19305E139 (17) | |
Text
.
'q,i SAFETY EVALUATION REPORT RELATED TO POINT EEACH UNIT I STEAM GENERATOR TUBE DEGRADATION DUE-TO DEEP CREVICE CORROSION April 4, 1980 8 0 0 4 '2'd y,M
INTRODUCTION In accordance with the Confirmatory Order dated November 30, 1979, Point Beach Unit 1 was shutdown on February 29, 1980 for steam generator hydrostatic testing and eddy current inspection after having completed the authorized operating period of sixty (50) effective full power days (EFPD's) since the restart subsequent to the October 1979 steam generator inspection.
The evaluation herein provides an update of the SER issued in support of the Confirmatory Order to reflect the operating experience at Unit i since the Order was issued, and the results of the steam generator inspection obtained during the February 29, 1979 outage.
The background information and results of two consecutive inspections (August and October,1979) as discussed in the November 30, 1979 SER are incorporated into this evaluation by reference.
BACKGROUND CONFIRMATORY ORDER DATED NOVEMBER 30, 1979 Inservice inspections of the Point Beach Unit 1 steam generators performed during the August and October 1979 outages indicated extensive general intergranular attack (IGA) and stress corrosion cracking on the external surfaces of the steam generator tubes within the thickness of the tubesheet (generally referred to as " deep crevice corrosion").
In view of these findings and of the apparent high rate at which this corrosion phenomenon was developing, the licensee agreed to certain conditions to assure safe operation of Unit 1 for a period of sixty (60) effective full power days.
This commitment was formalized by a Confirmatory Order dated November 30, 1979, amending the Operating License to include, in part, the folicwing conditions:
1.
a) Hydrostatic testing to be performed within 30 EFPD's.
b) Hydrostatic testing and eddy current inspection within 60 EFPD's.
Submittal of the proposed eddy current inspection program for NRC staff review.
Eddy current inspection results also to be submitted, with no resumption of power until the Director, Office of Nuclear Reactor Regulation determines in writing that the results ara accept-able.
2.
More restrictive limits on primary to secondary steam generator leakage.
3.
More restrictive limits on primary coolant activity.
4.
Unit i not to be operated with more than 18% of tubes plugged in either of the steam generators.
While not covered under terms of the Confirmatory Order, the licensee imolemented additional measures in an attempt to retard further tube degradation.
These measures included 1) a crevice flushing program to remove harmful chemicals from the tubesheet crevices, 2) reduced operating temperature and pressure, 3) continued close surveillance of feedwater chemistry and condenser tube leakage, and 4) sludge lancing to be performed within 12 months of the return to power.
. CE:E T5
'.T OR ABOVE TUEE5HEET The Safety Evaluation issued in support of the November 30, 1979 Confirmatory Order refle::ed thestaff's understanding that the extensive degradation observed during the Au;;s: and October 1979 inspections involved general ir,tergranular attack and cra: kin; within the tubesheet crevices, exclusively.
Subsequent to the Confirmatory Orcer, ha.cever, the staff became aware of five (5) tubes with defect indications at or above the tubesheet which had not been addressed in the November 30 SER.
In resp:qse to our request, the licensee submitted by letter dated December 21, 1971 ad:itional details regarding the defects in these five tubes ardan evaluation of : heir sicnificance.
The licensee reviewed the single frequency eddy current tes: results since 1975 for the subject five tubes and compared the signals of these pas: inspections to the same frequency signal obtained during the multi-frequency ins:ection in October 1979.
This comparison showed that the signals have not charged through three er four inspections since 1975.
On the basis of this review the licensee concluded that the defects observed in October 1979 at or above the tubesheet have remained essentially unchanged since at least 1975 and occurred as a result of earlier thinning or cracking rather than to the intergranular attack pher:me on currently being experienced in the tubesheet crevice area and which i
was or.l
- irst observed in November,1977.
In resp:nse to our request, the licensee submitted by letter dated December 21, 197? addi-ional details re;arding the defects in these five tubes and an evaluation of : heir significance.
Eased unca our review of this submittal and a subsequent conference call with the licensee on December 22, 1979, we concluded that (1) the eddy current indications at or above the tubesheet, which were observed during the October 1979 inspection, are old defects, possibly due to wastage or stress corrosion cracking, which were active mechanisms in 1975 and earlier, (2) these indications are not related to the active phenomenon of general intergranular attack and cracking currently beir.; experienced in the tubesheet crevices, and (3) the staff conclusions set forth in the November 30, 1979 SER remained val 1d and that the unit could continue to be safety operated under terms of the Confirmatory Order.
Nonetheless, we have continued our investication into the significance of the defects found at or at:ve the tubesheet, particularly with regards to eddy current capabilities to ce:e:t these defects and their safety significance.
This matter is addressed in further detail in this evaluation.
OPEPATI Z EXPERIENCE SUBSEOUENT TO THE CONFIRMATORY ORDER Followin; the issuance of the Confirmatory Order, Point Beach Unit I was returned tc :c,.er en December 1, 1979.
On December 11, 1979, Unit I experienced a rapid increase in primary to secondary leak rate, to 260 gpd, and was forced to shutdown un:er terms of the Confirmatory Order.
The source of the leak was identified as one leaking tube and two leaking plugs in steam generator B.
Although not required by si:V the Technical Specifications or the Confirmatory Order, the licensee performed cultifrequency eddy current examinations in both the A and B steam generators.
A to:al :# approximately 1900 tubes were inspected. The inspection bounded all areas o' ;revi:usly observed dee: crevice corrosion by at least one row and column of
- bes.
Tne inspection b:undaries were expanded whe new indications were observed
. r.ei' the boundary.
A set of randomly selected tubes outside the boundaries were als: 'ns?scted.
Representatives from the NRC staff and consultants were at the site :n Decem:er 16, 1979 to observe the inspection in progress.
As a result of tris ir.s:Ection, twenty (20) tubes were plugged in steam generator A and fifteen (15) tu:es were plugged in steam generator B.
None of the observed indications cccurred at or above the top of the tubesheet.
The inspection program and results were f:rmally documented in Licensee Event Report 79-02LOIT-0 dated December 22, 1979.
prior to resuming power operation, 2000 psid primary to secondary and 800 psid secondary to primary hydrostatic tests were performed.
No tube failures or addi-tional leakage resulted from these tests.
Based u: n our review of the December 11 tube leak occur ence and the inspection results we concluded that the conclusions reached in the lovember 30, 1979, SER remained valid and that the operating restrictions imposet by the Confirmatory Order c:ntinued to provide adequate assurance of safe operation, point Beach Unit 1 was returned to power on December 22, 1979 and operated to the ccnole: ion of its authorized 60 EFPD operating period (on February 24,1980) wi th only a very minor, but equivalent to a constant 30 gpd primary to secondary leak.
This rcas within the trace amount of equivalent leakage normally experienced at this #.
!GRCH 1950 INSPECTION RESULTS FIELD E:DY CURRENT TESTING s
The eddy current testing (ECT) program implemented during the March 1980 steam generat:r inspection was submitted for NRC staff review by letter dated February 26, IMO.
This program was modified to incorporate NRC staff comments.
ECT of 1035 cf the tubes in regions of previously observed deep crevice corrosion activity (including the kidney shaped central bundle region) was performed within boundaries bounding previously observed defects by at least one tube row and column.
Where defects were observed to u;:ur at the boundary, the inspection was expanded to bound these defectives by one tube row and column. An additional 3% random sample was inspected on the cold leg side and also among tubes on the hot leg side in areas not being 100% inspected.
Representatives of the NRC staff were on site during the inspection to ncnitor the inspection as it proceeded, and to facilitate timly cecisions from NRC/NRR regarding the need for additional inspection or tube pullin; for laboratory examination.
Ehifrequency eddy current testing (ECT) conducted in accordance with the approved progra revealed 18 defect indications on the hot leg side in steam generator A anc 24 cefect indications on the hot leg side in stean generator B.
In addition,3 tubes in S.G. B and 6 tubes in S.G. A were found with undefinable indications within the tubesheet.
On March 31, a hydrostatic test conducted after the ECT inspection revealed two tubes leaking at approximately 2 drips / minute.and two wet plugs in S.G. E.
Following plugging of these tubes and repair of the wet plugs a second hydro:es: revealed another leaking tube in S.G. B which was plugged.
Table I surari:es the ECT indicated defect depths in the two steam generators.
Table II su Tarizes the elevation of the defect indications above the lower, primary surface of :ne tubesheet.which is about 23 inches thick.
Some cefects affected several intnes of tube length and one tube had indications running from the tube expansion at the :rimary surface of the tubesheet to approximately one inch below the upper, se::.dary tubesheet surface.
The elevations indicated in Table II are the highest e:eva:i:ns rsac"ed by each defect.
j TA.'LE I ECT INDICATED DEFECT DEPTHS l DEFECT DEPTH I!;
NUMBER OF TUBES PERCENT OF TUBE WALL S.G. A S.G. B 90 to 100 5
3 S0 to 89 7
7 70 to 79 2
7 60 to 69 3
3 2
50 to 59 40 to 49 1
2 t
TABLE II ELEVATION OF ECT DEFECT INDICATIONS DISTANCE A50VE THE NUMSER OF TUBES FRIMARY TUBESHEET SURFACE (INCHES)
S.G. A S.G. B 0-4 1
i 5-9 2
10-14 2
2 15-19 8
6 20-21 8
12 1
1 1/2" AB0VE SE'. )NDARY T.S. SURFACE I
P I'o defective tubes were discovered outside of the central bundle region on the hot leg side nor any.there on the cold leg side of either steam generator.
Tables I and II in Appendix I provide a tube by tube evaluation of ECT indicated defect depths and elevations and results of re-evaluations of ECT tapes from previous inspections for each defective tube.
Study of these tables reveals that 15 tubes in steam generator A and 4 tubes in steam generator B had the same ECT indications but were overlooked in either the December or the December and October 1979 inspections.
All of the tubes with defect indications were plugged except those that were removed for laboratory examination.
All the ECT indications were of small amplitude and indicate very small volume defects.
TUBE PULLING A!!D LABORATORY EXAMINATIONS In their February 26, 1980 submittal the licensee committed to remove a tube from the Unit 1 steam generators if one was found with an eddy current testing indicated defect at or above the top of the tubesheet, such as were observed in five tubes during the October 1979 inspection.
The primary interest in removing this type of tube was two fold:
(1) to determine if the intergranular attack occurring within the tubesheet crevices is resulting in tube degradation at or above the upper secondary surface of the tubesheet and (2) to correlate field ECT with laboratory examination of the defects.
As indicated in Table II one tube was discovered in steam generator B with an indication approximately 1/2" above the top of the tubesheet.
This was tube R19-C37 and the indication was 58% deep.
In accordance with their commit-ment, this tube was removed from the steam generator for laboratory examination.
in addition, the NRC (after a review of the ECT results) required removal of two other tubes for laboratory examination.
These were tubes R30-C41 which had a 47%
indication approximately 21" above the primary face of the tubesheet and tube R26-C53 which had a 855 indication approximately 18" above the primary face of the tubesheet.
Removal of these tubes was intended to provide additional data regarding the extent and magnitude of IGA and the accuracy of ECT.
The tube removal procedures extended the outage time approximately six days and resulted in approximately an additional 155 manrem exposure.
LABORATORY RADIOGRAPHY AfiD EDDY-CURRENT TESTING Radiography and ECT were performed on all three of the removed tube specimens by Westinghouse at their Pittsburgh R&D facility.
As a result of the pulling process the original 22.1/2" length of tube R30-C41 within the tubesheet was elongated to approximately 24-3/4".
This measurement was based on the ring left on the tube at the top of the tubesheet.
Radiography of the removed tube revealed many defect indications in the region up to 23-1/4" from the tube end.
Many ECT indications existed up to 23-1/2" from the tube end.
No radiographic or ECT ina'ications existed at or above the ring marking the top of the tubesheet.
The laboratory ECT examination indicated an approximately 70 to 80% defect based on Based on the elongation caused in the tube removal proc to approximately 21.3" from the tube end in the unstrained tube.
The field ECT indicated a 475 defect at 400 KHZ approximately 21" from the tube end.
- ield evaluation of :ne defect based on the multi-frequency signal estimated the defect dep h in the sare 705 to 80% range as obtained in the laboratory * (at 400 KHZ) in the absence of tubesheet interference effects.
Defect depths are reported based on the single frequency signal when possible since it is the technique currently a?; roved by the A5ME Code.
The p0liing of tube R25-C53 elongated the original 22.5" of tube in the tubesheet crevice to approximately 25-7/16".
Radiography of the removed tube revealed many defec: indica:icns in the recion up to approximately 19.8" from the tube end as well as a single defect 25" above the tube end.
Eddy current testing revealed many defect indications up to 19.8" from the tube end.
Edcy current testing also raealed two 905 defects located approximately 7/16" and 2-7/16" below the tubesheet ring.
No radiographic or ECT indications existed at or above the ring marking the top of the tubesheet.
None of the above laboratory ECT indications for tube R26-C53 were specifically.
identified in the field.
Some of the indicated defects may have been introduced or nade worse during the tube pulling operation.
" Squirrel" indications (minor disturbances in the ECT sicnal of underterminable origin) were observed in the field over -he full length of tute within the tubesheet.
It was not possible to verify through laboratory ECT the 86% ECT indication observed in the field 18" above the tube end, since this corresponded to one of the locations where the tube broke during oulling.
However, this field ECT indication will be compared with the results of the fractography analysis of the fracture surface as part of a detailed report which the licensee has connitted to submit by April 30, 1980.
Tube Rii-C37 was of particular interest because of the field ECT indication of a 535 defect located approximately 1/2" above the tubesheet.
Unfortunately, when the tube was examined there was no ring clearly indicating the top of the tubesheet as there was on the other two tubes which were removed.
Since the section of tube within the tubesheet experiencesa different load and elongation during the removal process than the section of tube above the tubesheet, the exact location of the top o' :ne tubesheet relative to the tube cannot be directly quantified.
Radiography and ECT of the removed tube revealed many defect indications in the region up to 23.75" from the tube end.
Radiography also showed crack like indica-tions approximately 24-3/8" above the tube end and ECT indicated ar. approximate 60% defect 24 -1/2" above the tube end.
No ECT indications were observed above the 605 indication.
Although the 60% laboratory ECT indication corresponds well with the 58% field ECT indicatiog its elevation cannot be directly correlated to the field indications because the location of the top of the tubesheet is not identifiable.
Calculations based on strains in the other tubes which were removed indicate that this defect would have been inside the tubesheet.
Nonetheless, it is the defect with the highest elevation in the tube, its depth corresponds well to the field ECT depth and it could be the defect of interest given the non-uniform straining of the tubes during rem:vai.
Metalicerachic Examinations Matii'.:gra; hic examination consisted primarily cf photomicrographs (PM) to determine at what elevation IGA existed in the tubes.
. F;- ure ?.33-Cal F'O.cere prepared for sections centered on the top of the tubesheet anc a;;r:xi :3 ely 0.35" belcw and 0.45" above the top of the tubesheet.
In each of tress "e;icrs F".s of 50 and 200 pov;er magnification were made.
The 200 power PMs were :en:ered on the region in the 50 power photcmicrographs indicating the greatest surfi:e irregularities.
For the section of tube below the top of the tubesheet the PMs sncwed shallow grain boundary separation on the order of 0.0025" maximum.
At the to; cf the tubesheet, shallori surface separation was observed affecting grain boun: aries to just over 0.001" ir, depth.
Similarly above the top of the tubesheet surface separation of the grain boundaries was observed to a depth of approximately 0.001 inches.
Extensive general IGA as is occurring deeper in the tubesheet crevice was ro: cbserved in any of these regions.
Photcmicrographs were also prepared for tube R26-C53. Again the PMs were centered about the top of the tubesheet and approximately 0.4" below and 0.2" above the top of the tubesheet.
The section below the top of the tubesheet showed shallow srain boun:ar/ sepa ation penetrating approximately 0.002" maximum.
The regi:n centered about the to? of the tubeshe showed no grain boundary separation although sore surface irregularities penetrating less than 0.001" existed.
Above the top cf the tubesheet some areas of grain boundary separation penetrating approximately 0.003" were observed.
Extensive general IGA as is occurring deeper in tr.e tubesheet crevice was not observed in any of these regions.
Five photomicrographs were made of tube R15-C39.
One was centered on the 60%
defect described earlier while the other four were centered approximately 1-5/8" and 3/4" below and 1" and 1-3/4" above the-defect.
The two sections below the defect showed IGA penetrating to depths of nearly 0.004".
Photographs of the tube surface at the defect show a crack running less than approximately 1/2" longitudinally then turning ar,d running less than approximately 1/4" circumferentially.
Photo-nicrcgraphs of a section made through the defect show a crack penetrating approximately 0.017" surrounded by localized IGA.
The longitudinal section made for the PM may not have included the deepest section of the crack.
Section D above the defect indicates one localized area of grain boundary separation approximately 0.001" deep and section E above the defect shows no grain boundary separation but some shallow surface irregularities less than 0.001" in depth.
PROPOSED CONDITIONS FOR CONTINUED OPERATION The licenses has proposed the following conditions to allow continued operation of Poin: Beach Unit 1.
1.
Within 90 EFPD, a 2,000 psid primary-to-secondary hydrostatic test and a 800 psid secondary-to-primary hydrostatic test will be performed.
An eddy current exa'.ination consisting of about 1,000 tubes in the central region of the hot leg in each steam generator and 35 of the remaining tubes outside this area will be performed.
2.
Primary coolant activity for Point Beach Unit I will be limited in accordance with :he provisions of Sections 3.4.8 and 4.4.8 of the Standard Technical 5:e:ifications for Westinghouse Pressurized Water Reactors, Revision 2, July 1979, rather than Technical Specification 15,3.1.C.
3.
Cl:se surveillance of primary-to-secondary leakage will be continued and the rea:::r will be snutcown for tube plugging on confirmation of any of the following
- .:ttions:
. Primary-to-secondary leakage of 150 gpd (0.1 gpm) in either steam generator;-
i.
Any primary-to-secondary leakage in excess of 250 gpd (0.17 gpm) in either s: sam generator; or An upward trend (average over a three-day period) in primary-to-secondary leakage in either steam generator in excess of 15 gpd (0.01 gpm) per day, when measured primary-to-secondary leakage is above 150 spd in that steam generator.
.e reactor will be shutdown, any leaking steam generator tubes plugged, and an eddy current examination as described in Item 1., above, will be performed i# leakage due to crevice corrosion in either steam generator exceeds the limits s:sted in Technical Specifications 15.3.1.D.
5.
Unit I will be operated at a reactor coolant pressure of 2,000 psia with the associated parameters (i.e., overtemperature tai and low pressurizer pressure trip; Dint) with the limits indicated in the Safety Evaluation Report appended t: your letter of January 3,1980.
Or return to p:wer operation, the licensee proposes to continue the follow program to assist in retarding further tube degradation:
Unit 1 will be operated at a reduced reactor coolant system hot leg tempera a.
b.
Continue close surveillance of feedwater chemistry conditions and condenser
-tube leakage.
Perform sludge lancing within nine months of returning to power.
c.
E'," L.A i0::
ECT T-RCGRAM, RESULTS, A!!D CAPABILITIES Merbers of the t;RC staff and their consultant from Oak Ridge National Laboratory cn site during the inspection to review the testing and evaluation techniques.
Ei:/ current testing examinations were conducted in accordance with the program
- r
- p; sed in the licensee's February 26, 1980 submittal and approved, with comment, b/
- he tiRC.
vicusly observed and was expanded in any areas where new i The random inspection of peripheral hot leg tubes and cold leg tubes revealed no des: crevice corrosion.
Therefore, the inspection performed is adequate to ensure that the great majority of tubes with deep crevice corrosion have been removed from' servi:s by plugging.
Tne " arch 1980 ECT results show a marked reduction in the number of dica e: defects compared to the August and October 1979 inspections.
fifteen of the 24 ECT indicated defects in steam generator B and 6 of-the 18 ECTIn addit in:i:1:ed defects in steam generator A were shun to exist previously through re-exi+.5-ion of the ECT tapes from previous inspections.
Thus, the number of new defe: s discovered in this inspection is smaller than the raw data indicates.
i 3:e::icn results suggest that some of the remedial actions taken.by the licensee The f:'.::. ".; the October 1979 inspection, particularly the lower temperature operation, 92; s " :s : e time of the December 1979 outage.:s su:c9eding in retarding the I
.g.
As discussed in our November 30, 1979 SER the accuracy of the eddy current technique is so e.nat diminished in t.e tutesheet region u.. : ann:t be fully relied upon to detect every tube degraded by deep crevice corrosien.
Inis appears to be oarticularly true for tubes subject to general IGA, but which do not contain cracks.
Partially through wall cracks of significance are generally detectable, even in the tubesheet region, wi th ECT.
As experience has shown, however, very small volume defects which in turn produce very small amplitude ECT signals may be easily overlooked (as was the case with the 19 tubes above).
Our evaluadon of the safety significance of IGA and stress corrosion cracking occurring within the thickness of the tubesheet is discussed in our November 30, 1979 SER which is incorporated into this SER by reference.
With regard to the tubes observed during the October and March inspections to contain defects at or slightly above the top of the tubesheet, we have concluced that multifre-quency ECT can detect defects of a significant size to threaten tube integrity during normal or postulated accident conditions.
All of the defects discovered at or above the top of the tubesheet are small amplitude, smail volume defects.
Assuming the defects at or above the tubesheet to be wall thinning (wastage related), rough estimates!
of the size of the defects were made by the staff based on comparison with the ECT signatures from the ASME Coce calibration standard.
These estimates show that if these defects are wastage related, the volumes of these defects are very small compared to what is necessary to burst or collapse the tube under postulated accident conditions,.
as determined by independent tests sponsored by NRC (NUREG/CR-0718).
In the case of tube R19-C37 which exhioited a field ECT indication of 585 approximately 1/2 inch above the tubesheet, the laboratory examination indicates that the defect indication observed in the field is most likely a crack.
NRC sponsored burst and collapse tests (NUREG/CR-0713) have been performed on free standing tubes with EDM notches (simulating a crack) of up to 85-90% (through wall) in depth.
The results indicate the lower bound burst strength to exceed the maximum primary to secondary pressure differentials during normal operation or postulated accidents for notches (cracks) ranging to about 1 inch in length.
It should be noted that the burst strength of a tube containing a crack defect slightly above or below the top of the tubesheet is considerably higher than for free standing tubes, because of the re-straint against radial expansion of the tube provided by the tubesheet.
The above tests indicated a collapse failure to be a much less limiting failure made than a burst failure mode for free standing tubes during postulated accidents.
Cracks of sufficient size to cause a burst or collapse failure under postulated accidents are considered by the staff to be well within the detectable capability of the multi-frequency eddy current technicue, regardless of the location of the crack relative to tha too of the tubeshset.
Tube Removal and Laboratorv Exam Laboratory radiography and ECT confirm the position taken by the staff that general IGA may not be detectable in the crevice of the tubesheet until it is severe enough for preferential crack growth to occur.
Detection of defects below the top of the tubesheet by laboratory examinations is due partly to increased capability of ECT without the influence of the tubesheet and partly to the creation of new or the opening of old defects durin; the removal prccess.
Laboratory radiography and ECT confirmed the absence of defects above the tubesheet in tubes R30-C41 and R26-C53.
Unfortunately the top of the tubesheet could not ce identified on tube R19-C37.
. However, assuming that the upper most defect detected in the tube is the defect which was identified by field ECT, there is a good correlation between the laboratory and m7 field ECT.
More importantly, the defect which was detected was small enough so as not to jeopardize tube integrity.
Primary-to-secondary and secondary-to-primary hydrostatic tests conducted on March 6 revealed one tube (R23-C44) which exhibited
'.,"9!
a slight leak at a rate of 3 drips per minute, and one wet plug in a previously d
plugged tube (R23-CEO) both in S.G. B.
No tube ruptures occurred.
The defect iSF found by ECT just above the tubesheet in tube R19-C37 in S.G. B withstood the id i simulated accident pressure differentials.
This provides additional support to our
$0*
previously stated conclusion that multifreauency ECT can detect defects at or above the top surface of the tubesheet which 'would je'opardize tube integrity durino ar 3(
normal operating or postulated accident conditions.
~ OM The staff wants to emphasize that as inspection techniques with increased cacabilities,
, 20Vf such as multifrequency ECT, are developed, that many small volume defects which
. ect previously went undetected will now be found.
These defects must be evaluated in iO8f the context of the magnitude of defects which jeopardize tube integrity during normal 3ri or postulated accident conditions.
As inspection techniques become more capable, abE correspondingly more discriminate criteria must be established.
Many plants which 23 V have not been inspected with multifreciuency ECT are going to show new defects when 0C multifrequency inspections are performed.
These results must be dealt with rationally and requirements for tube inspection, plugging, and removal must be carefully applied.
tint METALL0 GRAPHIC EXAMINATIONS 237 Members of the NRC staff and their consultant frcm Brookhaven National Laboratory met met with representatives from WEPCO and their Westinghouse consultants in Pittsburgh 7T on March 28, 1980 to review results of the metallographic examinations.
Review of
-u71 the photomicrographs described earlier revealed no general IGA similar to that occurring:
'33k) within the tubesheet crevice above the top of the tubesheet in tubes R26-C53 or R30-
. Our C41.
Shallow grain boundary separation on the order of two grains or less existed
- arin on all photomicrographs of these tubes.
Shallow grain boundary dissolution of this iI nature can result from several mechanisms including previous operating environments p1nin or tube pickling during manufacturing.
This grain boundary separation is much less y n1 severe than that occurring within the tubesheet. The staff has concluded that the span shallow grain boundary dissolution at and above the top of the tubesheet is not
- se f significant in terms of tube integrity.
Metallographic examination of tube R19-C37
' fee revealed stress corrosion cracking and shallow IGA of the tube near the top of the ab tubesheet.
Re-evaluation of past ECT tapes showed that this defect existed as far in b back as 1976 but was overlooked using single frequency ECT. The nature of the crack
- Chn is similar to that of stress corrosion cracks which occurred during previous operating 132.
periods. The staff believes that this is an old defect which has not significantly changed since 1976.
CONCLUSI0"5
[^".
Based on the information presented above the staff has reached the following con-clusions:
uam i
38 1)
The inspection and tube plugging performed has been adequate to ensure the
-i great majority of defective tubes have been removed from service.
- en
' th'
- 2) Multiple frequency eddy current testing used to perform the inspection is capable of detecting defects near the tubesheet and tube support plate interfaces which would jeopardize integrity of the tube during normal operation or postulated accident conditions.
. 3)
Hydr:stati: tests simulating postulated accident conditions performed prior tc returnir.g to operaticn will identify any significant defects overlooked during ECT examination.
4)
Intergranular attack at and above the top of the tubesheet as observed in the removed tube samples is extremely shallow and poses no threat to tube integrity at or above the top of the tubesheet.
5)
Based on 'tne number of new defects, the rate of deep cr'evice corrosion appears to nave decreased.
7
- 6) A maximum 90 effective full power day operating period, prior to the next ECT inspection as proposed by the licensee, will provide adequate assurance that a large number of tubes will not simultaneously reach a point of incipient failure.
7)
Remedial actions proposed by the licensee will continue to mitigate the effects of postulated accidents and retard the rate of corrosion.
The staff has determined that the following conditions should be required for continued operation:
1)
Within 90 effecting full power days from the date of this order, a 2,000 psid primary-to-secondary hydrostatic test and 800 psid secondary-to-primary hydrostatic ;
test shall be performed.
Also during this plant outage, an eddy current examina-tion shall be performed on tubes in each steam generator.
The program shall require such examinations of about 1000 tubes in the central region of the hot leg, three (3) percent of all hot leg tubes outside this central region and 3%
of the cold leg tubes.
The Central region shall encompass all areas where deep crevice corrosion has previously been observed.
2)
Primary coolant activity for Point Beach Nuclear Plant Unit 1 will be limited in accordance with the provisions of Sections 3.4.8 and 4.4.8 of the Standard Technical Specifications for Westinghouse Pressurized Water Reactors, Revision 2, July 1979, rather than Technical Specification 15.3.1.C appended to License OPR-24.
3)
Close surveillance of primary to secondary leakage will be continued and the reactor will be shut down for tube plugging on detection and confirmation of any of the following conditions:
a)
Sudden primary to secondary leakage of 150 gpd (0.1 gpm) in either steam generator; b) Any primary to secondary leakage in excess of 250 gpd (0.17 gpm) in either steam generator; or c) An upward trend in primary to secondary leakage in excess of 15 gpd (0.01 gpm) per day, when measured primary to secondary leakage is above 150 gpd.
. 2 s
sactor.till :s s.- dr.:n, any leaking steam gen =cator tubes plugged, ir."
l' id:j cu" s. i.1-ina.i0n parfer. ed if any of :ne 'Ioll ain; c:ncitiCr.s
.n e..-. s. 1.. :...
a......
I'
'. r.firmati:n of :r' ary to seccndary leakage in either steam generator
'r. excess cf 5:0 ;:d (0.35 gpm); or, 0;
Any two identified leaking tubes in any 20 calendar day period.
This eddj turrsn
- r ;rsr. will be as described in item 1.
5.
The "R". Staff will be ;rovided with a summary of the results of the eddy current exa-ination perfor ec under items 1 and 4 above.
This summary will include a pho agraph of the ubesheet of each steam generator which will verify the lo:ation of tubes which have been plugged.
The licensee will no resume operation after the eddy current examinations rsc'. ired to be performed in accordance with condition 1 or 4 until the Director Jffi:e of Nuclear Reac:or Regulaticn has determined in writing that the resQlts
. r,...- -..... l.:
...g
,,... s..e These :: ditions are si-iiar to these in the Ncvember 30, 1979 Order except that ta a;: :ved c;erating period has been lengthened from 60 to 90 effective full power da/i> ard ne shutdcwn to :srform hydrostatic tests are being required prior to the end of the ?] day period.
These conditions differ from the licensees proposal in that ths pri ary to secondary Isak rate limits and requirements for ECT examination are
- s c:-servative.
Or ths tasis of our re/iss. and evaluation, we conclude that continued safe operation of ::f.: Esach Uni: 1 ray is permitted within the stated terms of the Confirmatory Grisr.
A?PENDIX !
TA5LE 1 P01liT BEACH =1 ' A' S/G i
i l
M.F.
M.F.
de =
Dec.
Oct.
R C
1950 1979 1979 i
12 l 19 l 80%
( AF.E SAME l19-21" ATE
- cn a n ceR251 NoR651 N.C.
i i
7 22 29:/96';
SAME fiDD/SAME l 12" ATE /17" ATE ; R251 N.C.
12" ATE /17" ATE l
1 i
R551 I
t g
13 !22 65*;
SAME NDD l
12-17" ATE
,R251 N.C.
R551 10 l 23 415 i
NDD 40" A4t i.s251 R551 i
7 ! 24 83'i I
RAYBE(?)
NDD l
17"-20" ATE NDD R551 R251 5 1 2d 795 l
MAYBE(?)
NDD I
l 17"-21" ATE NDD R551 l
R251 i
i 25
~5 t 59::
Squirrels NDD l
l12"-20" ATE R351 l
i R851 i
20 l 45 '
85, l
SAME SAME l
l21" ATE lR251 N.C.
R851 9 l49 90L NDD l
21" ATE R251 17 ! 50 85'l NDD 19" ATE R251 19 i 50 '
97f; NDD i
11" ATE R251 I
2C ! 50 t 97'l I
NDD t
i
{ll" ATE fR251 l21" ATE lMAYBE(?)
87?:
NDD NDD R951
[
- 151 lR l
j 12 ' 51 83' NDD
- 17" ATE Ri 51 il 53
83 MAYBE(?)
13" ATE Squirrels 4
g :.. i :.1
~
l
. I
P0lNT BEACH #1 'A' S/G l
M.F.
M.F.
j Dec.
Oct.
Tube =
R C
1920 1973 1979 i
t l 15 55l 505 l15" ATE 8
27 Squirrels SAME 15-20" ATE R251 N.C.
15 23 l5:uirrels I
No 321'" ATE Squirrels R251 l28 34 Squirrels SAME 18-21" ATE R251 N.C.
28 35 Squirrels SAME
,i
,17" ATE
- 251 N.C.
R
! 20 41 91 %
NDO
-19" ATE R351 25 43 73%
SAME Very S.V.
17" ATE N.D.D.
,R351 R751 11 46 $quirrels SAME p2"-21" ATE R351 29 52 $quirrels SAME 44" ATE R151 N.C.
i t
k i
I i
l 9
l I
i
APPEi.DlXi TABLE Il B S/G INLET POINT BEACH =1 l
I M.F.
M.F.
S.F.
Tube =
Dec.
Oct.
Aug.
R C
195:-
1979 1979 1979 i
i 18 26 7:
(AME Changed fiDD 13" ATE R151 No R651 R551 I
change.
l: 13 l 26 7 3 '.
l SAME SAME NDD I
l l
21" ATE lR151 N.C.
!R651 N.C.
R551 13 33 71 '.
SAME Changed tiDD 20" ATE R151 ti. C.
R651 R552 6
l24 91 ',
SAME SAME NDD 11" ATE R151 N.C.
R$51 N.C.
R552 20 35 63?:
SAME l
Changed NDD 21" ATE R151 N.C.
lR351 R552 8
37 S ?.
NDD 5" ATE P,151
- 19 37 535 SAME SAME flDD i
1/2" ATS 53!,
R351 N.C.
R552 R151 N.C.
i
' 10 41 70':
SAME NDD l21" ATE R251 N.C.
R751 R651 4
30 41 47' SAME Soc.e Change NDD 21" ATE R251 N.C.
R751 R651/R151 30 42 425.
SAME Changed NDD 21" ATE R251 N.C.
R751 R151 22 46 7 6 ?,
SAME NDD 15" ATE R251 N.C.
R351 24 48 845 Changed NDD 12" ATE R251 R351 R652 l
30 43.
35?;
SAME SAME NDD I21" ATE R251 N.C.
R951 N.C.
R$52 i
25 l 49 i Saa Chanced NDD I
l5" ATE R251 R351 E652 i
g tu
- t i 9 9 ;,( ? )
SAME NDD ji6" ATE R251 N.C.
iR351 R652
,f54"fFulllength 5 5 '.
23 sor.e are neaR351 i
R251 i.
l l
1 B S/G TNLET POINT BEACH *1 I
Tube =
i M.F.
M.F.
S.F.
l De:.
Oct.
Aug.
R C
r.
ivi:
1979 1979 l
93 oi e r, u
09:.1 i.
ni-I 1
7 oc-,
c.,.
c..-
.c. :
- 21" ATE R25i la or
/.
NDD R,, a_ l e
- c.
n.:
21
! 63 l 625 SAME l
NDD i
l
- 21" ATE
- R351 R1051 12 l 67
655 NDO l
l21" ATE R351 R1051 I
2 72 '
925 SAME NDD
!To; of Roll R351 N.C.
R1051 26 53[
555 (New) l j)c ni:
- ' 30 I
43, Squirrels SAME SAME
!21" ATE R251 R751 i
i
?
l 26 53 l Squirrels NDD l Full T.S.
R251 25 55 Squirrels NDD Full T.S.
R251 I
22 63 'S:uirrels SAME SAME 21'" ATE R251 R1051
, 22 64 'Scuirrels SAME No 2d" ATE R351 Squirrels RIO51 25 55 !i..,,,_
(New)'
745 kw i
e 1
i i
t.
h h
l i
e t
l 8
i i
!,