ML20002D538
| ML20002D538 | |
| Person / Time | |
|---|---|
| Site: | Big Rock Point File:Consumers Energy icon.png |
| Issue date: | 07/12/1976 |
| From: | Cook R, Little W NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III) |
| To: | |
| Shared Package | |
| ML20002D537 | List: |
| References | |
| 50-155-76-14, NUDOCS 8101210317 | |
| Download: ML20002D538 (6) | |
See also: IR 05000155/1976014
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UNITED STATES NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND EMFORCl@ LENT-
REGION IIIr
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-Report of Operations Inspection
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IE Inspection' Report No. 050-155/76-14
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Licenece:
Consumers Power Company
212 West Michigan' Avenue
Jackson, Michigan' 49210
L icense No.-DPR-6~
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Big Rock Point: Nuclear Plant;
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Charlevoix,~ Michigan
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Type of Licensee:
Type of Inspectioc:
Special, Announced
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Date of-Inspection:
. June 18, 1976
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Principal. Inspector:
.R.
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Accompanying Inspectors: None
Other Accompanying Personnel: None
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Reviewed By:
W. S. Littic, Chief
7//.//M.
Nuclear Support Section
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SU} DIARY ~0F FINDINGS
Inspec t ion . Summ:sry
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Inspection of June 18, 1976, (76-14):
Examined at Battelle Memorial-
Institute (BMI) Columbus Laboratories metallographic samples' removed
from a plant code safety. valve nozzle. Reviewed topical report
pertinent to cracking found in safety valve nozzles and discussed
results with licensee and BMI personnel.
Enforcement Items
. None.
Licensee Action on Previously Identified Enforcement Items
None inspected.
Other Significant Items
A.
Systems and Components
None.
B.
Facility items (Plans and Procedures)
None inspected.
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C.
Managerial Items
None.
D.
Noncompliance Identified and Corrected by Licensee
None.
E.
Deviations
None.
F.
Status of Previously Identified Unresolved Items
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None inspected.
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Mn_.3nement_Tneervieg
A management interview was conducted with Mr. Noble, consumers Power
Technical Services Administrator and Dr. Berry, EMI Associate.Mancger,
at the completion of the inspection on June 18, 1976, The following
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items were discussed.
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A.
Comparison of nozzle material micro-structure uith steriotype
material was discussed. The licensee stated that metallographic
examinations would be performed _on material representative of
the nozzles presently install d.
(Paragraph 2, Report Details)
B.
.The susceptibility for failure of the installed nozzles being
related to material physical properties was discussed. The
licensee agreed to explore further the physical properties and
heat treat history of the presently installed nozzles to
establish the. propensity for unanticipated failure.
(Paragraph
2, Report Details)'
C.
The susceptibility of the installed nozzle material to stress
corrosion cracking was discussed. The licensee indicated that
constant strain rate tests may be considered contingent upon
the findings from Items A and B above.
(Paragraphs 2 and 4,
Report Details)
D.
The time required for crack nucleation and propogation in the
safety valve nozzles was discussed. The licensee stated that
a proposed periodic inspection schedule would be developed for
timely detection of possible cracking.
(Paragraphs 3 and 4,
Report Details)
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REPORT.DSTAILS.
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Persons Co'ntacted
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D. Noble, Consumers-Power. Technical Services-Administrator-
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.N. Boyd, BMIE Manager.of: Corrosion and-Electrochemicalfand
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Technology'Section'
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- W. Berry, BMI Associate Manager _of Corrosion'and Electrochemical
.and: Technology Section
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2.
General
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During inservice inspection'of_the' drum' code' safety valves to
determine the set, point pressures; theilicensee performed'aidye
penetrant _ test:(PPT) of the sealLwelded: area of the' safety-valve ~ -
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nozzles. The' DPT was performed because .the licensee:had ; pre-
viously. been informed that' corrosion crackin'g had. occurred 'in'
this area at anotherc facility. - Therefore, the licensee opted
to perform an- examination \\ of; their safety valve. nozzles.
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Th'e results of the examination revealedLinside diameter circum--
ferential cracking.in the= heat affected zone of.~the' nozzle. base-
material. . One nozzle showedievidence of-longitudinal' cracking-
in the nozzle ~ throat area about 1 to.2 inches below the seating
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surface.
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The nozzles were replaced in all the installed-codeDsafety valves:
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and the valvesLwere refurbishyp. This maintenance was cov'ered-
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during a previous inspection.-
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Two of the nozzles which were removed were sentito Battelle-
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Memorial Institute (BMI) Columbus. Laboratories, Columb'us, Ohio, for
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further metallographic examinations to establish'more: insight
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into the cause of the crack indications. Only"one of the; nozzles,a
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the one nozzle with longitudinal throat indications, was destruc-
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tively examined. An inspection was performed at BMI to review
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othe results of the metallographic examinations.
2.
Meta 11ography
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The' nozzle mmterial is classified as a 19-9DL staini s steel'
with an ASTM A4:8-71 designation. .The material composition of
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this high temperatare iron base alloy is 19Cr-9Ni-1.40Mo
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l.35W-0.40Cb-0.25T!..
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Metallographic examinations revealed that the circumferenef al'
cracks on the I. D. surface adjacent ~ to the weld -area wercapproxi-
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mately four inches in length and. penetrated.the base metal about 0.~115'
inch. The nozzle wall thickness at this location is 0.625 inch. A
relatively large number of small axially oriented . (longitudinal)'
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cracks were detected'in the throat area. .These cracks were-
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nominally '!/8 to 3/8 inch in length and the maximum depth of any of
these cracks metallographically examined was 0.029 inch.
During the metallographic examinations branches of an O. D. circum--
ferentially oriented crack adjacent to the seal weld were dis-
covered. The cut through the seal weld to. remove the nozzle
appears to have gone through .the O. D. crack ~ 1eaving only some of
the branch crccking in the nozzle base material. The deepest
branch crack depth examined is 0.040 inch. ilowever , it is believed
that the main crack was much deeper.
Intergranular attack was also
noted in the nozzle base material adjacent to the cut.
It is-
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speculated that the O. D. cracking may have been induced from
contained chlorides.
Photomicrographs and physical exmination of samples from the I. D.
cracks (seal weld and throat areas) revealed that the nozzle
material has been sensitized; probably through a stress relieving
process. To compare the degree of sensitization is quite sub-
jective, however, the lower portions of the nozzle in the welded
heat affected zone appeared to.be more sensitized than.that in the
nozzle throat area. The amount of sensitization does not appear to
be as dramatic as the sensitization that is experienced in 19-9DL
which has been solution annealed at 1850 F and aged at 1300 F for
four hours.
Hardness tests were performed across the nozzle cross section.
The
hardness was found to be approximately Re 19 to 20 with the surface
being slightly harder (Rc 29);-probably from cold working induced
during the fabrication. The nozzle material, 19-9DL, may tend to
stress crack if not stress relieved after fabrication. A typical
stress relief at 1200 F for four hours results in 'ardness of
nominally Rc 20 to 29 which is compatibic with the hardness of'the
rc=oved nozzle. However, the actual annealing process and th*
methodology for stress relieving is not completely known at this
time for either the nozzles removed or the nozzles installed.
There appears to be some similarity between the longitudinal cracks
and the circumferential crack. Both crack orientations show signs
of intergranular crack propogation.
Carbide percipitate was note:
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within the grains at the crack locations.
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.Because it'is known that 19-9DL stainless steel can exhibit' inter-
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granular . attack when heated ~ to sensitized 'at '120'0"F,- the cracks are
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classified as:intergranular stress - corrosion cracking. -The
cracks do show some semblence -to stress; corrosion . cracking experi-
enced -in Type ;304 stainless steel which has been sensitized and
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exposed.to:the operating environment of BoilingfWater Reactors.
It 'is ' surmised : that the installed:19-9DL nozzle- raterial should
exhibit similar. type crack propogation-andicorrosion'sensitivitic'- -
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as-Type 304 stainless steel.'
llowever,' some; vital physical
properties such as-impact test 'r'esults,:which would enhance a
more confident comparison to Type 304' stainless. steel are not-
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completely 'known for; the' installed: nozzle material. .Thc licensee
plans ~to procure information pertinent to the.19-9DLoinstalled
nozzle material to allow' performance comparisons.with other more
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commonly used-stainless steel alloys.
3.
Fractography-
A section of the I. D. circumferential crack was openedjfor'further-
examination. A light photomacrograph of one'of the fracture
surfaces and the sample were examinel. 'A dull-black or dark brown?
oxide extended from the I. D. surface'.to a depth of about~ 1/16
inch.
Progressing'toward the outside diameters a dark bluc.
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tarnished surface (band) existed adjacent to the dull bla'ck oxide.
A frosty metallic zone existed . adjacent to the dark 1 blue tarnish.
The observations suggest that . intermittent crack propogations -has
transpired. The~ black' oxide indicates early crack propogation and.
may have occurred one to-two years ago'
The blue tarnish indicates
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crack growth in the near past which may-have occurred nominally
three months ago.
The frosty metallic area, indicates crack growth
which probably occurred just prior to theLlast. shutdown.
Scanning electron microscope examinations-were' performed'and
confirmed the above observations.
4.
Crack Morphology
The physical cause of the intergranular stress corrosion cracking
is attributed to the sensitization of 19-?DL material"which make
the safety valve nozzles susceptible to this type of attack. The
sources of the stresses necessary to induce cracking ar,e probably
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from residual stress'from the seal welding process, surface. stresses
f rom fabrication of the nozzles, and hoop stresses. from operating
pressure. Thermal gradient stresses during.startup and/or shutdown
transients are believed to be the cause of-the strong indications
of intermittent crack propogation after what may be considered an
" incubation period".
A slowly applied strain. rate during transient
conditions'may also have~ contributed to the crack. morphology.
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