ML20057B473

Responds to GL 93-04, Rod Control Sys Failure & Withdrawal of Rod Control Cluster Assemblies. Concludes That Licensing Basis Satisfied GDC 25

ML20057B473
Person / Time
Site:	Farley
Issue date:	09/17/1993
From:	Dennis Morey SOUTHERN NUCLEAR OPERATING CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-93-04, GL-93-4, NUDOCS 9309220099
Download: ML20057B473 (5)

Text

Southern Nuclear Operatng Company 0

Post O'hce Bor 1295 Bamingham, Alabama 35201 Telephone (2C5) 668 5131 E.a o... uorey Southern Nudear Operating Companr

$eh?le?c$ the southern electric system September 17, 1993 10 CFR 50.54 (f)

Docket Nos. 50-348 50-364 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D. C. 20555 Joseph M. Farley Nuclear Plant 90-Day Response To NRC Generic Letter 93-04 Rod Control System Failure and Withdrawal of RCCAs Gentlemen:

Pursuant to the requirements of 10 CFR 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 required an initial response within 45 days which provided an assessment of whether the licensing basis for the Farley Nuclear Plant (FNP) was satisfied with regard to the requirements for system response to a single failure in the red control system (GDC 25). The generic letter also required a plan and schedule within 90 days for long-term resolution. Subsequent correspondence between the Westinghouse Owners Group (WOG) and the NRC resulted in scheduler relief for the requirement to determine compliance with the licensing basis.

Southern Nuclear Operating Company (SNC) letter dated August 4,1993 provided the initial (45-day) response. This response summarized the compensatory actions taken by FNP. It also provided a summary of the results of the generic safety analysis program conducted by the WOG and its applicability to FNP.

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Nuclear Regulatory Commission Page 2 Generic Letter 93-04 1,

SNC hereby submits its 90-day response to the generic letter. The attached response concludes that the licensing basis is satisfied for GDC 25. It also provides additional information for long-term clarification of this issue.

If you have any questions, please advise.

Respectfully submitted,  ;

SOUTHERN NUCLEAR OPERATING COMPANY  :

hh 77in Dave Morey i

MGE\sar:GL93-04. DOC i i

SWORN TO AND SUBSCRIBED BEFORE ME 1

Attachment THIS /7b DAY OFh8pn/m ,1993 J cci Mr. S. D. Ebneter / 1 Mr. T. A. Reed drym . , haetiT/' ' f Mr. M. J. Morgan V pt8y Public My Commission expires: 9-/f- #  ;

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ATTACilMENT JOSEPH M. FARLEY NUCLEAR PLAhT 90-DAY RESPONSE TO NRC GENERIC LETTER 93-04 ROD CONTROL SYSTEM FAILUkE AND WITIIDRAWAL OF RCCA'S Assessment of Licensing Basis Comoliance The purpose of this response is to provide an assessment of whether the licensing basis for the Farley Nuclear Plant (FNP) is still satisfied with regard to the requirements for system response ;o a single failure in the rod control system and to provide supporting discussion for this assessment in light of the information generated as a result of the Salem event (Required Response 1.(a) to Generic Letter 93-04).

The Westinghouse Owners Group (WOG) has undertaken the following initiatives to support the response to NRC Generic Letter 93-04: conducted rod control system testing in the Salem training center; examined the existing rod control system Failure Modes and Effects Analysis (FMEA); analyzed the worst-case asymmetric RCCA withdrawal combinations with three-dimensional analytical methods; and performed an equipment i survey of Westinghouse plants to determine the frequency and significance of control system circuit card failures.

After this extensive investigation, the WOG has concluded that GDC 25 continues to be met. However, the WOG also recognizes there are questions related to the interpretation of not caly the intent of GDC 25 but also the appropriate definition of the specified acceptable fuel design limit.

I Based on previous communications, the NRC has conservatively interpreted the GDC 25 fuel design limit to be the departure from nucleate boiling (DNB) design basis. The WOG believes that this is a conservative definition if applied to all events. The equipment survey ,

conducted by the WOG demonstrated the rate of card failures that could result in the j movement ofless than a whole group is on the order of 4 E-8 / critical reactor card hour.

This result indicates that the likelihood of a Salem-type event is extremely remote. With this in mind, a Condition III ( or IV) specified acceptable fuel design limit should be applicable.

The purpose of GDC 25 is to ensure that the appropriate limits (commensurate with 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 j within a given group would receive the same signals. The corrupted current orders j generated by the logic cabinet failures at Salem were transmitted identically to all eight I

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e Rod Control System Failure and Withdrawal of RCCAs Page 2 RCCAs in Shutdown Bank A (SBA). The fact that only one RCCA withdrew was due to a second unrelated effect. Had all the rods in SB A responded as predicted in the existing FMEA, all the rods would have withdrawn uniformly and been enveloped by the existing ,

Final Safety Analysis Report (FSAR) accident analyses. In addition, existing rod motion l surveillance requirements would detect the type of rod motion failure observed at Salem. l Thus, the requirement that one single failure not result in a specified acceptable fuel design limit being exceeded, in this case the DNB design basis, would remain satisfied.

Assessment of the Safety Significance of Potential Asymmetric Rod Motion in the Rod Control System i

Westinghouse has also performed a safety analysis using three-dimensional safety analysis f techniques to assist the WOG in its determination of the safety significance of an l uncontrolled asymmetric rod withdrawal. WCAP-13803, Revision 1, documented the ';

safety analysis program and concluded that the generic analysis and its plant-specific application demonstrate that DNB does not occur for a worst-case asymmetric rod j withdrawal for all affected Westinghouse plants. As such, the analysis program concluded '

that there is no safety significance for affected Westinghouse plants from a Salem-type rod withdrawal. j Southern Nuclear Operating Company (SNC) letter dated August 4,1993 provided the 45-day response to the generic letter for FNP-. The response provided a summary of the results of the generic safety analysis program conducted by the WOG and its applicability to FNP. i Long-Term Enhancements  ;

i While the assessment indicates that the licensing basis is currently satisfied, the WOG .

believes there are measures that can be taken by utilities to further reduce the probability  ;

of a Salem event. The WOG recommended modifications include a combination of rod control system logic cabinet changes (current order timing adjustments) and an additional plant test.

Presently, SNC plans to modify the FNP rod control system current order timing to  :

reduce the probability of an uncontrolled asymmetric rod withdrawal in the event of the {

failure identified at Salem. Once the current order timing adjustments are made, none of 'l the rods will move (with a high degree of certainty) if corrupted current orders are j present. However, based on additional evaluations and/or the results of pilot plant in situ l testing, SNC may elect to perform FNP-specific safety analyses to demonstrate that the i

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' Rod Control System Failure and Withdrawal of RCCAs Page 3 i consequences of an uncontrolled asymmetric rod withdrawal event would meet the DNB design basis using approved analytical methods, versus modifying the rod control system. 1 The planned rod control system enhancements will be implemented by the FNP Unit I thirteenth and Unit 2 tenth refueling outages in 1995. This schedule is contingent upon on  :

the successful demonstration of the timing adjustments at an operating plant and receipt of l the change notice from Westinghouse.

With regard to additional testing, FNP currently performs rod control system testing, including power cabinet current profile measurements, following each refueling. Upon  ;

receipt of new WOG testing recommendations, FNP will review applicable procedures to  !

ensure they conform to Westinghouse guidelines.  ;

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