ML20085C206
| ML20085C206 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 08/26/1991 |
| From: | Withers B WOLF CREEK NUCLEAR OPERATING CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| WM-91-0020, WM-91-20, NUDOCS 9109030184 | |
| Download: ML20085C206 (3) | |
Text
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W8LF CREEK
' NUCLEAR OPERAllNG CORPORATION Bart D. Withers ere *re ew August 26. 1991 che rmuwe onw VM 91-0020 U. S. Nuclear Regulatory Commission ATIN:
Document Control Desk Mail Station P1-137 Washington, D. C.
20555 Subject.:
Docket No. $0-482:
Inspection of Hafnium Control Rods Gentlemen:
Wolf Creek Generating Station (VCGS) was the second Westinghouse plant to begin operation using hafnium control rods.
Section 4.2.3.1(10) of WCGS's SSER 2 states,
'the Nuclear Regulatory Commission (NRC) staff believes that a minimal surveillance program consisting of a visual inspection of representative rods should be carried out at the first two plants to have the new hafnitun control rods.*
License Condition 2.C(14) of the WCGS Operating License requires the performance of a visual inspection of a.
sample of hafnium control rods during one of the first five refueling
- outages, and that a summary of the results of this inspection be submitted to the NRC.
In later communications with HRC personnel, a commitment was made to submit eddy current inspection results along with the required visual examinrtion results of the control rods.
This letter is being submitted to fulfill the requirements of WCGS License Condition 2.C(14) and the NRC commitment.
If you have any questions concerning this matter, please call me or Mr. H. K. Chernoff of my staff.
Very truly yours.
__ w Bart D. Withers President and Chief Executive Officer BDW/aem cc L. L. Gandrum (NRC)
A. T. Howell (NRC)
R. D. Martin (HRC)
D. V. Pickett (NRC) l l
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t Attechmznt to VH 91 0020 Page 1 i. 2 HAFNibA CONTROL ROD INSPECTION RESUI,TS Wolf Creek Generating Station (WCGS) is a 4 loop pressurized water nuclear reactor operated by Wolf Creek Nuclear Operating Corporation (WCH00).
The I
reactor has 53 full length rod cluster control assemblies (RCCAs) which are moved in symmetrically located groups, or banks, to provide reactivity control and shutdown of the core.
The RCCAs at WCGS utilize hafnium absorber material hermetically sealed in stainless steel cladding.
Two types of eddy current testing (encircling coil testing and profilometry visual inspection were performed during the examination of testing) and a RCCAs at WCGS during Refuel 3.
Inspections were performed on $5 RCCAs (entire cycle 3 core plus two unirradiated spares).
Encirclin coil testing e
was performed for the detection of cladding indications i.e. - wear, cracks, inner diameter (ID) indications, ebrupt geometrical changes such as dents or
- ridging, and cladding wall outer diameter (OD) wear cross-sectional area i
loss.
Profilometry testing was performed to measure clad swe111rg,
- ovality, OD wear, and circumferential extent of indications.
Visual observation, with use of binoculars, of the RCCAs was conducted from the operating deck of the refueling floor while the RCCAs were being handled with the SCCA change tool to check for gross defects, including extensive bowint of the rodlete.
Although no defects were apparent by the visual inspection, the re6ults of the eddy current testing identified two unanticipated findings.
l The first finding was determined to be excessive guide card wear caused by control rod vibration at the guide tube which was demonstrated by thinning of the control rod clad.
At least nine of the control rod rodlets experienced through-clad wear and three rodlets had between 502 and 53I cladding cross-sectional area loss.
A minimum of'50! cladding cross-sectional area at any axial location was established as the acceptance criteria for continued operation of control rods.
The second finding was localized, non-uniform swelling on most of the control rods.
A hot cell examination performed _by the vendor (Westinghouse letter 89-KE*-G-0038 dated August 9, 1989, to S. K. Clark, WCNOC from D.
J.
Petrarca, Westinghouse) confirmed that the cause of this swelling was L
hafnium hydriding, which occurs when hydrogen from the reactor coolant diffuses through the stainless steel cladding and reacts with the hafnium i
absorber.
The normal nominal control rod diameter is 0.381 inches.
After swelling no control rod diameters exceeded 0.410 inches.
This corresponds to the maximum theoretical control rod diameter which could be caused by hafnium hydriding.
Thia was independently verified with a ring gauge.
l
l Attachment to VH 91-0020 Page 2 of 2 An evaluation has been conducted to address potential safety concerns which would result from future operation with hafnium control rods.
Areas addressed by this evaluation included the potential for control rod swelling preventing RCCA insertion or dela)ing drop time, and potential loss of control rod structural integrity due to cladding loss from guide card wear or swelling.
This evaluation concluded that RCCA insertion capability will not be degraded.
Hafnium hydriding is a self-limiting process which cannot produce sufficient control rod swelling to prevent RCCA insertion.
Evaluation of the effect of increased control rod diameters on RCCA drop time concluded that RCCA drop times will remain within Technical Specification limits for at least three eighteen month cycles.
The potential for impact on control rod structural integrity due to swell induced cladding cross-section area loss or guide card wear is being addressed by periodic control rod inspections with appropriate acceptance criteria.
These inspections resulted in the replacement of two RCCAs during Refuel 4.
An amendment to WCGS Technical Speciff ations has been received which will allow the use of silver-indium.cadmini, control rods, hafnium control rods, or a mixture of both.
This change supplements WCNOC's options for future replacement of hafnium control rods that are in operation.
Pending further operating experience and control rod examination hafnium will remain a viable option as a neutron absorbing material in control rods at WCGS.
__