ML20057B341

Forwards Final Response to GL 93-04, Rod Control Sys Failure & Withdrawal of Rod Control Cluster Assemblies

ML20057B341
Person / Time
Site:	Wolf Creek
Issue date:	09/17/1993
From:	Rhodes F WOLF CREEK NUCLEAR OPERATING CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
ET-93-0107, ET-93-107, GL-93-04, GL-93-4, NUDOCS 9309210217
Download: ML20057B341 (5)

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i WOLF CREEK ' NUCLEAR OPERATING CORPORATION Fom:st T. Rhodes Wce Preso nt Engineenng September 17, 1993 ET 93-0107 I

U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station PI-137 Washington, D. C. 20555

Reference:

NRC letter dated June 21, 1993, " Rod Control System Failure and Withdrawal of Rod Control.

Cluster Assen61ies, (Generic Letter 93-04) "

Subject:

Docket No. 50-482: Response To NRC Generic Letter 93-04 Gentlemen:

1 This letter provides Wolf Creek Nuclear Operating Corporation's (WCNOC)  ;

final response to the reference. Pursuant to the requirements of 10 CFR l

50. 54 (f) , the .NRC issued Generic Letter 93-04, " Rod Control System Failure and Withdrawal of Rod Control Cluster Assemblies," on .

June 21, 1993.

The generic letter requires each addressee to provide an assessment of whether or not the licensing basis for its facility is still satisfied with regard to the requirements for system response to a single failure in the Rod Control System (GDC 25 or equivalent). If the assessment (Required Response 1.(a)) indicates that the licensing basis is not satisfied, then the licensee must describe compensatory short-term actions consistent with the guidelines contained in the generic letter, and provide a plan and schedule for long-term resolution (Required Response 1.(b)). Subsequent correspondence between the Westinghouse Owners Group and the NRC resulted in scheduler relief for Required Response 1. (a) (NRC Letter to Mr. Roger Newton dated July 26, 1993). In accordance with the granted schedular relief, WCNOC's letter to the NRC dated August 5, 1993 (ET 93-0094) provided the compensatory actions l taken by WCNOC in response to the Salem Rod Control System Failure Event. It also provided a summary of the preliminary results of the  ;

generic safety analysis program conducted . by the Westinghouse Owners  ;

Group and its applicability to the Wolf Creek Generating Station.

Attached is WCNOC's final response to Required Response 1.(a). The attached response concludes that the licensing basis is satisfied for GDC 25, and also provides additional information for long-term I

clarification of this issue. j PO. Box 411/ Burhngton. KS 66839 / Phone: (316) 364-8831 l 9309210217 93091~/ P An Equal Oppo" unity Employer M/F/HC/ VET '

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ET 93-0107 Page 2 of 2 If you have any questions concerning this matter, please contact me at *

(316) 364-3831 extension 4 002 or Mr. Kevin J. Moles at extension 4565. j Very truly yours,

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y 4-l Forrest T. Rhodes l Vice President Engineering FTR/jra ,

Attachment ,

i cc: W. D. Johnson (NRC), w/a ,

J. L. Milhoan (NRC), w/a )

G. A. Pick (NRC), w/a W. D. Reckley (NRC), w/a J

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STATE OF KANSAS )

) SS COUNTY OF COFFEY )

Forrest T. Rhodes, of lawful age, being first duly sworn upon oath says that he is Vice President Engineering of Wolf Creek Nuclear Operating Corporation; that he has read the foregoing document and knows the content thereof; that he has executed that same for and on behalf of said Corporation with full power and authority to do so; and that the facts therein stated are true and correct to the best of his knowledge, information and belief.

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. Forrest T. Rhodes

i. ;g , Vice President

/, Engineering

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...... g SUBSCRIBED and sworn to before me this IT dayofbE , 1993.

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. kttachment to ET 93-0107 Page I of 2 FINAL RESPONSE TO NRC GENERIC LETTER 93-04 1

ljpsessment of Licensino Basis Comoliance l

The purpose of this discussion is to provide an assessment of whether the licensing basis for the Wolf Creek Generating Station (WCGS) is still satisfied with regard to the requirements for system response to a ,

single failure in the Rod Control System and to provide supporting l discussion for this assessment in light of the information generated as  !

a result of the Salem event (Required Response 1. (a) ) ,

The Westinghouse Owners Group (WOG) has undertaken the following ,

3 initiatives to support the response to NRC Generic Letter 93-04: l conducting Rod Control System testing in the Salem training center, examining the existing Rod Control System Failure Modes and Effects ,

Analysis (FMEA), analyzing the worst-case asymmetric Rod Cluster Control Assetrbly (RCCA) withdrawal combinations with three-dimensional analytical methods, and performing an equipment survey of Westinghouse  ;

plants to determine the frequency and significance of control system circuit card failures.

After this extensive investigction, the WOG has concluded that GDC 25 continues to be met, but also recognizes that there are questions as to the interpretation of not only the intent of GDC 25, but also the appropriate definition of the specified acceptable fuel design limit as j well.

Based on previous communications, the NRC has conservatively interpreted the GDC 25 fuel design limit to be the DNB design basis. The WOG ,

believes that this is a conservative definition if applied to all i events. The equipment survey conducted by the WOG demonstrated that the failure rate of card failures that could result in the movement of less  !

than a whole group is on the order of 4*E-8 card failures / critical card hour. This would indicate that the likelihood of a Salem-type event is extremely remote. With this in mind, the WOG would propose that a i Condition III (or IV) specified acceptable fuel design limit would be f applicable. j l

Based on the WOG's understanding of GDC 25, the purpose of this ['

criterion is to ensure that the appropriate limits (commensurate . with the probability of occurrence) are not violated for a " worst-case" stand  !

alone single failure. The test program conducted at the Salem training  !

center demonstrated that all the rods within a given group would receive l the same signals. The corrupted current orders generated by the logic j cabinet failures at Salem were transmitted identically to all B RCCAs in Shutdown Bank A (SBA) The fact that only one RCCA withdrew in the plant was due to a second unrelated failure. Had all the rods in SBA responded, as predicted in the existing FMEA, all the rods would have withdrawn uniformly and have been enveloped by the existing Updated Safety Analysis Report (USAR) accident analyses, In addition, existing rod motion surveillance requirements would detect the type of rod motion fallure observed at Salem. Thus, the requirement that gna single

, failure not result in a specified acceptable fuel design limit- being exceeded, in this case the DNB design basis, would remain satisfied.

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. Attachment to ET 93-0107 Page 2 of 2 i

Assessment of the Safety Sionificance of Potential Asymmetric Rod Motion in the Rod Control System WCNOC's August 5, 1993 letter to the NRC provided a summary of the i preliminary results of the generic safety analysis program conducted by the WOG and its applicability to WCGS. Westinghouse completed the safety analysis using three-dimensional safety analysis techniques to assist the WOG in its determination of the safety significance of an j uncontrolled asymmetric rod withdrawal. WCAP-13803, Revision 1, documented the safety analysis program and concluded that the generic analysis and their plant-specific application demonstrate that DNB does not occur for a worst-case asymmetric rod withdrawal for all affected Westinghouse plants. As such, the analysis program concluded that there is no safety significance for affected Westinghouse plants for a Salem-type rod withdrawal.

Lona-term Enhancements .

While the assessment indicates that the licensing basis is currently  !

I satisfied, the WOG believes that there are measures that can be taken by utilities to clarify compliance with GDC 25. Those recommended +

modifications include a combination of Rod Control System logic cabinet-changes (current order timing adjustments) and an additional plant J

surveillance, or USAR safety analyses analyzing asymmetric rod withdrawal and an additional plant surveillance.

The WOG proposals include implementing a new current order surveillance '

(such as current order traces from each group following each refueling '

outage) to ensure detectability of rod control failures. WCNOC has been performing this . type of surveillance since beginning commercial  ;

operation. The WOG also proposed that Westinghouse plants modify the t Rod Control System - cuirent order timing to prevent any uncontrolled' asymmetric rod withdrawal in the event of the failure identified at Salem. If corrupted current orders are present, none of the rods will move (with a high degree 'of certainty) once the current order timing adjustments are made. WCNOC will implement this proposed modification, i

The WOG proposal to modify current order timing will be implemented i prior to startup from the Fall 1994 refueling outage. -This schedule _is i based on the future successful demonstration of the timing adjustments -!

at an operating plant and receipt of the official technical bulletin from Westinghouse. The basis for allowing this time period to implement the current order timing changes is that existing . rod motion surveillance tests provide assurance that the f ailure scenarios of an uncontrolled asymmetric rod withdrawal will be detected, and the analysis program performed and documented in. WCAP-138 03, Revision 1,

concluded that there was no safety significance for affected

• I Westinghouse plants for a Salem-type rod withdrawal, l

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