ML19259C843

Responds to IE Bulletin 79-12 Re Short Period Scrams at BWR Facilities.Describes Means of Preventing Addl Scrams. Responds to Item 1-5 Re Critical Prediction of Rod Pattern Estimates & Emergency Rod in Switch

ML19259C843
Person / Time
Site:	Cooper
Issue date:	07/11/1979
From:	Pilant J NEBRASKA PUBLIC POWER DISTRICT
To:	Seyfrit K NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
References
NUDOCS 7908160006
Download: ML19259C843 (3)

Text

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Nebraska Public Power District

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1 July 11, 1979  !

i Mr. Karl 1. Seyfrit, Director U.S. Nuclear Regulatory Comm.'.ssion Office of Inspection and Enfo.rcement Region IV 611 Ryan Plaza Suite 1000 Arlington, Texas 76011

Subject:

Response to IE Bulletin No. 79-12

Reference:

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1) IE Bulletin 79-12, "Short Period Scrats at BWR Facilities," May 31, 1979

2) Letter, R. E. Engel (GE) to D. F. Eisenhut (NRC),

"NRC Concerns Regarding Worths of Dropped Control Rods During Reactor Startup With High Xenon,"

April 11, 1978

Dear Mr. Seyfrit:

l This letter is written in response to IE Bulletin 79-12. i' I

Reference 1 describes three separate incidents of fast period scrams '

which were caused by the continuous withdrawal of control rods near the point of criticality. While NPPD believes these events have little safety significance (see Reference 2), we agree the additional scrans ,

are highly undesirable from a reporting and operatienal viewpoint. '

Therefore, we have implemented , so.ution which will, 1) prevent the continuous withdrawal of control rods in the regions of high worth for rod groups 3 and 4, and 2) force the withdrawal of rod groups 3 and 4 in 3 a notchwise manner such that the individual notch worths will be sig- '

afficantly reduced. Our solution utilizes the Banked Position With-i drawal Sequence (BPWS) for rod groups 3 and 4; beyond the 50% rod l density point, the Group Notch Rod Sequence Control System (RSCS) and {

Reduced Notch Worth Procedure (RNWP) were already being utilized to l reduce individual notch worths. We have modified our control rod j withdrawal procedures and the RUM to force notch withdrawal of rod I groups 3 and 4 between notch positions 04 and 12. We are confident that these actions represent the most positive solution to the problem that (

can be effected. Our response to items 1 through 5 of Reference 1 is as follows: f i

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e Mr. K. V. Eeyfrit July 11, 1979 Page 2.

Item 1 NPPD cannot agree that a critical prediction represents a solution to the problem for the following reasons:

a. At present, it is technically impossible to make an accurate crit-icality prediction in the hot Xenon recovery situation; even the initial cold criticality predittion at the beginning of cycle can differ by as many as 20 rods from the actual critical rod pattern configuration.

b. Estimatine a critical rod pattern would not prevent short periods; the conditions that exist for high notch worths would still be present with or without a critical prediction.

c. To give the operator a critical prediction that is known to be inaccurate .'.s extremely counterproductive, it would induce a false sense o'. security in the operator and could potentially cause the situation rather than avoiding it.

Item 2 Since inaccuracies ex st in all critical rod pattern estimates, NFPD believes it more fruitful to develop a course of co rective action that linics rod worthn (since this was the cause of the events) in potential problem areas. Therefore, we have implemented the BPWS method of rod withdrawt1 for rod groups 3 and 4; groups 3 and 4 will be netched be-tween positions 04 to 12. Beyond 50% rod density, RSCS forces notchwise withdrawal of control rods. Additionally, we are utilizing RNWP for beyond 50% rod density to prevent fast period scrams. With the util-ization of BPWS for groups 3 and 4 plus RSCS and FC3H' controlling beyond 50% rod density, we feel that we have taken action that covers all situations including high Xenon startups. We have also added procedurai steps requiring the monitoring of all SRM's to assure selection of the most significant data.

Item 3 Implementation of BPWS minimizes the notch worths of individual rods for rod groups 3 and 4. RSCS and RNWP already accomplish that for beyond 50% rod density. BPWS, RSCS and RNUP cover all situt. ions and thus there is no need to consider special rod sequences for peak Xenon con-ditions. Cautions have been provided the operators on situations which can result in high notch worths (e.g., first rod on a new group, high Xenon startups, etc.)

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Mr. K. V. Seyfrit July 11, 1979 Page 3.

Item 4 The "etergency rod in" switch was inspected and found to be in good condition. A procedural step has been added to require testing it on each startup.

Item 5 The effects of coderator temperature, Xenon concentration and core exposure en reactivity (and therefore control rod worths) are thoroughly discussed in our operator licensing and operator requalification training programs. Procedures have been revised to incorporate the BPWS method and to include cautions for the approach to critical. Our training program strongly emphasizes the need to understand and carefully follow our procedures. The operators are advised to exercise caution and seek the advice of the Reactor Engineer when unusual situations are encountered.

If you have any questions regarding this response, please corcact me.

Sincerely,

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J. .. Pilant Director of Licensing and Quality Assurance JMP:LCL:cg

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