ML20203N518
| ML20203N518 | |
| Person / Time | |
|---|---|
| Site: | Harris  |
| Issue date: | 04/18/1986 |
| From: | Watson R CAROLINA POWER & LIGHT CO. |
| To: | Grace J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| References | |
| CON-NRC-439, REF-PT21-86-157-000 227, HO-860285(O), PT21-86-157-000, NUDOCS 8605050464 | |
| Download: ML20203N518 (6) | |
Text
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CP&L Carohna Power & Light Company n,m m onw.,a.m.m
. U i.i(',22 A 9 p g SHEARON HARRIS NUCLEAR PROJECT P. O. BOX 165 New Hill, North Carolina 27562 l
File Number: SHF/10-13510 Letter Number: HO-860285(0)
NRC-439 Dr. J. Nelson Crace United States Nuclear Regulatory Commission Region II 101 Marietta Street, Northwest (Suite 2900)
Atlanta, Georgia 30323 CAROLINA POWER & LICHT COMPANY SHEARON HARRIS NUCLEAR POWER PLANT 1986 - 900,000 KW - Unit 1 MAIN FEEDWATER ISOLATION VALVES, ITEM 227
Dear Dr. Grace:
Attached is an interim report on the subject item, which was deemed reportable per the provisions of 10CFR50.55(e) and 10CFR, Part 21 on March 20, 1986.
CP&L is pursuing this matter, and it is currently projected that corrective action and submission of the final report wil.1 be accomplished by April 29, 1986.
Thank you for your consideration in this matter.
Yours very truly, I
A.IWatson
~
R Vice President Harris Nuclear Project RAW /ajj Attachment cc: Messrs. C. Maxwell (NRC-SHNPP)
J. M. Taylor (NRC) 8605050464 860418 PDR ADOCK 05000400 3
PDR l
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CAROLINA POWER & LICHT COMPANY SHEARON HARRIS NUCLEAR POWER PLANT UNIT 1 INTERIM REPORT MAIN FEEDWATER ISOLATION VALVES ITEM 227 APRIL 11, 1986 REPORTABLE UNDER 10CFR50.55(E) & 10CFR21 I.
NBI-MS-863470/2-Os5
n
SUBJECT:
10CFR50.55(e) and 10CFR21 reportable item Shearon Harris Nuclear Power Plant Unit 1 main feedwater isolation valve operator piston rod tailures.
ITEM:
Main feedwater isolation valves 2FW-V28SAB-1 and 2FW-V27SAB-1 SUPPLIED BY:
Borg-Warner Fluid Controls NATURE OF DEFICIENCY:
During stroke testing of the above referenced feedwater valves the operator piston rods failed (snapped in two).
DATE PROBLEMS f
IDENTIFIED:
2FW-V28SAB-1 August 8, 1985 2FW-V27SAB-1 March 17, 1986 t
DATE PROBLEM REPORTED:
On March 20, 1986, CP&L (Mr. K. V. Hate') notified the NRC (Mr.
P. Fredrickson) that this was reportable per the provisions of 10CFR50.55(e) and 10CFR21.
SOURCE OF PROBLEM:
Examination of the piston rod and hub on valve 2FW-V28SAB-1 indicated several areas of concern:
1.
Ultimate Tensile Strength Three tensile specimens were removed from the mid-radius of the 4" diameter section of stem and tested for tensile strength.
The ultimate 1
tensile strength of each sample was approximately 138 ksi. This value is less than the 160-180 ksi ultimate tensile strength assumed by Borg-Warner for this material.
Also, Charpy V-notch tests were performed on the material.
Charpy impact values for the test specimens ranged from 16-20 ft-lbs.
A brittle fracture appearance was observed for each specimen.
2.
Radius at Stem-to-Hub Junction The actual radius at this junction could not be measured since the fracture occurred right at the radius.
However, the radius was measured at the t
point where the stem steps from a 3"
to a 4"
diameter.
This radius was found to be 0.008".
Assuming the radii are comparable, the stem-to-hub junction becomes subject to a
NBI-MS-863470/3-0S$
SOURCE OF PROBLEM:
(Cont'd) significant stress concentration.
According to Stress Concentration Factors (Peterson, 1974),
the stress concentration factor based on netsection
- stress, for this geometric configuration and radius can be in the range of 7-10.'
Application of this factor results in a situation in which this area of the stem is overstressed during operation of the valve.
It was also noted the Peterson data is for a stepped shaft.
As such, it represents a conservatively low estimate of stress concentration for this stem configuration.
The design radius for, the ' stem-to-hub junction per Borg-Warner drawing is 0.015" - 0.030".
Even with a radius of 0.030",
however, the stress l
concentration factor remains greater than 3.
j 3.
Ilub Thickness Measurements taken of the hub show the thickness is not uniform.
The minimum thickness measured from the upper hub surface to the lower is 0.63";
the maximum is 0.72".
In addition, the distance from the upper surface to the lower surface of j
the raised center area on the bottom of the hub is 0.85".
Using these dimensions and the expected operating loads on the stem, CP&L computed the expected stress levels in the stem utilizing the flat plate model (Roark,-Sth Ed.,
Case IL) for the stem, considering this a better representation of stem configuration than.the ' stepped shaft model.
Assuming a total load on the stem / piston rod of 250,000 lbs (corresponding to 2515 psi hydraulic pressure and 0
psi ~ nitrogen pressure),
we computed the stresses listed below for' the l
indicated thicknesses.
We also computed the 3
stresses for a
total load of 320,000 lbs 1
(3200 psi hydraulic pressure, 0 psi nitrogen pressure).
This case was considered because Borg-Warner had stated the piston rod / stem is 4
designed to withstand the forces generated by this 3200 psi differential.
llub Stress (psi)
Stress (psi) i Thickness (in.)
(250,000 lb load)
(320,000 lb load) 0.63 184,000 236,000-0.72 141,000 180,000 r
0.85 101,000 130,000 NBI-MS-863470/4-0S5
SOURCE OF PROBLEM: (Cont'd)
These figures indicate a strong possibility the stem hub area is overstressed during valve operation.
Also, it should be noted the above stress values do not account for the stress concentration due to the sharp radius at the stem-to-hub junction.
4.
Crack Indication on the Piston Head Examination of the piston rod showed a crack had developed in the 10" section of the rod head approximately 1/8" from the junction with the 4" diameter stem.
The crack circles the 4" stem for approximately 270*.
The Metallurgy unit analyzed a wedge shape piece from the piston rod head and perform a detailed analysis of the crack.
It was determined that the crack was-in a nonstructural seal weld that had been machined too thin.
5.
Coupling Deformation Dimensional data taken from the coupling indicates the shoulder of the coupling, which holds the stem
- hub, was deformed upwards approximately 0.010" - 0.015".
SAFETY IMPLICATIONS:
Failure of these piston rods would result in loss of control of the main feedwater isolation valves.
REASON DEFICIENCY IS REPORTABLE:
This condition would eliminate the ability to isolate the feedwater pipelines entering containment.
CORRECTIVE ACTION:
Borg-Warner has redesigned, manuf actured and supplied replacement piston rods and couplings.
The replacement piston rod material is ASTM A304 Cr. 4340H, heat treated to a tensile strength of 180-200 ksi.
In addition the radii of the piston rod groove has been increased to 0.090 - 0.095 - inches.
The groove diameter has been increased from 3.000/3.040 inches to 3.250 inches and the groove width has been enlarged to 0.640 - 0.650 inches.
The rod hub thickness is now 0.840 0.850.
The groove and hub surfaces are shot peened to an intensity of 0.012 - 0.016.
Analysis of the modified piston rod l
end has shown that the resultant stresses are within j
allowable limits.
Borg-Warner is confident that this
)
r.ew design has eliminated the repeat of the type of failure noted above.
l l
NBI-MS-863470/5-OSS
_J
CORRECTIVE ACTION:
(Cont'd)
Replacement with these new piston rods and couplings is currently underway for the two subject valves and valve 2FW-V26SAB-1 (which has yet to fail).
Completion is estimated by April 29, 1986.
NBI-MS-863470/6-OSS