ML19253A663

Responds to IE Bulletin 79-12.Operating Procedures Reviewed to Ensure That Estimate of Critical Control Rod Pattern Is Made Prior to Each Approach to Critical & Control Rod Withdrawal Sequences Evaluated

ML19253A663
Person / Time
Site:	Oyster Creek
Issue date:	07/27/1979
From:	Ross D JERSEY CENTRAL POWER & LIGHT CO.
To:	Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
NUDOCS 7909100639
Download: ML19253A663 (7)

Text

P Jersey Central Power & Light Company N*.  ? Madison Avenue at Punch Bowl Road Mornstown, New Jersey 07960 (201)455-8200 July 27, 1979 Mr. Boyce H. Grier, Director Office of Inspection and Enforcement Region I 631 Park Avenue King of Prussia, Pennsylvania 19406

Subject:

Oyster Creek Nuclear Generating Station Docket No. 50-219 IE Bulletin No. 79-12 The purpose of this letter is to respond to the directives set forth in IE Bulletin No. 79-12.

Our responses to the specified actior. items in Bulletin No. 79-12 are given in Attachment No. 1.

( 4r Ve truly yours,

)

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Donald A. Ross, Manager Generating Stations-Nuclear cs Attachment cc: United States Nuclear Regulatory Commission Office of Inspection and Enforcement Division of Reactor Operations Inspection Washington, DC 20555

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79 Jersey Centra! Power & Ltght Cornpany rs a Member of the Generai Pubhc Ut: .cs System 9100g[/

Attachment No. 1 July 27, 1979 IE Bul'letin No. 79-12 Action #1:

Review and revise, as necessary, your operating procedures to ensure that an estimate of the critical rod pattern be made prior to each approach to cri tical . The method of estimating critical rod patterns should take into account all important reactivity variables (e.g., core xenon, moderator temperature,etc.).

Response

The operating procedures have been reviewed to ensure that an estimate of the critical control rod pattern which takes into account all important reactivity variables (core xenon, moderator tenperature and fuel depletion) is made prior to each approach to critical.

The assumptions used in the method (Plant Procedure 1001.2, " Estimated Critical Position") are:

1) Rod worths evaluated at moderator temperature required for startup (MPT). This value is approximately 170 F. currently.

2) The xenon distribution is uniform.

3) No steam voids in core.

The method is fairly accurate at low temperatures and xenon concentrations, but becomes increasingly inaccurate as temperature and xenon concentrations increase. Because of this, the method is not used for control rod densities of less than 50% (" checker board" pattern).

Because of the above inaccuracies, the procedure is not deemed reliable for all startup conditions. The following precaution is therefore included as part of the procedure.

"This procedure provides only a rough estimate of the critical control rod configuration and is intended only as a guide. The results calculated should not be relied upon by the operator performing the reactor startup.

He should anticipate criticality during any addition of positive reactivity."

Although ECP's are performed at Oyster Creek, they are not deemed as a necessary element in preventing short reactor periods.

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• Attachment No. 1 July 27, 1979 IE Bulletin No. 79-12 Action #3:

Review and evaluate your control rod withdrawal sequences to assure that they minimize the notch worth of individual control rods, especially those withdrawn immediately at the point of criticality. Your review should ensure that the following related criteria are also satisfied:

a) Special rod sequences should be considered for peak xenon conditions.

b) Provide cautions to the operators on situations which can result in high notch worth (e.g. first rod in a new group will usually exhibit high rod worth).

Response

As a result of the existing Estimated Critical Position prediction procedure, rod worth curves for the first six (6) groups of rods are developed using a 3D core simulator code under cold (no-voids) clean condition. If high rod worths are detected during this process, they may be reduced by altering the sequence. In any case, however, a copy of the rod worth curves is available in the control room for review and/or use by the licensed operators.

For the first six (6) groups of rods, the sequences are now, as they usually have been in the past, developed such that the first rod of a new configuration /

group is located in close proximity to an SRM detector. This ensures fast SRM response to the nearby reactivity changes and lessens the chance of withdrawing the higher worth rods through criticality.

After group six (6), where criticality is expected under high xenon conditions, procedures direct the operotor to use notch withdrawal except if a peripheral rod is being withdrawn (low worth).

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Attachment No. 1 July 27, 1979 IE Bulletin No. 79-12 Action #4:

Review and evaluate the operability of your " emergency rod in" switch to perform its function under prolonged severe use.

Response

The operability of the " emergency rod in" switch to perform its intended function under prolonged severe use has been reviewed and evaluated to be satisfactory for the following reasons:

1) After the switch failed at 0yster Creek on December 14, 1978, due to a bent mechanical switch stop tab, the switch was repaired so as to prevent future similar failures in the " emergency in" position. (The bent stop plate tab allowed switch overtravel and ontact reopening.)

The other function position of the switch is " notch override."

The mechanical stop plate with the bent tab was replaced. The two plates attached to the '. witch shaft which contain the dogs which contact the stop plate tab were repositioned such that the one used to stop switch motion fre the " emergency in" position contacts the base of the tab instead of its end. This has reduced the tendency to bend the tab when the switch is forcefully full stroked to the " emergency in" position.

2) A prerequisite has been added to Plant Procedure 201.1, " Approach to Critical," which requires that the switch be operability checked in the " emergency in" position prior to startup.

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Attachment No. 1 July 27, 1979 IE Bulletin No. 79-12 Action #5:

Provide a description of how your reactor operator training program covers the considerations above (i.e., items 1 through 3).

Response

After the Oyster Creek "short period" incident, a lecture was developed for the purpose of providing the licensed operators with practical knowledge necessary to avoid any future short-periods due te control rod manipulatior during reactor startups under any condition of reactor moderator temperatute and xenon concentration.

This lecture has been included in the licensed operator retraining program as a required topic for all licensed control room operators.

The lecture outline is as follows:

1) SRM Response a) Initial count rate vs. core keff. (yadan, temp.)

b) Time delays between reactivity addition and response (geometry)

2) Control Rod Worth a) Estimate of rod worth (rod position / rod location) b) Est iate of rod worth (configuration, e.g. first rod in group)

3) Subcritical Multiplication Review a) Time response vs. keff [ deva'op CRj(1-keffj) = CR f (1-kefff )]

b) ACR/Akeff vs. keff c) Examples

4) " Doubling Thumb-Rule" a) After four (4) doublings - very close to critical - several notches on high worth rod.

b) After three (3) doublings - reactor should not become critical on next rod un ass possibly a xenon concentration increase has effectively reduced the initial count rate. (Use incident as example.)

c) Conclude and stress that control rods should be notch withdrawn (between 06 and 24) after 3 doublings until critical and adding heat.

5) ECP a) Assumptions b) Limitations i) Decreasing accuracy with increasing temperature and xenon concentration ii) Not calculated past " checker board" pattern iii) Use only as a guide.

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Attachment No. 1 July 27,1979 IE Bulletin No. 79-12 Response to Action #5 continued:

6) Review the Oyster Creek "Short-Period" Incident.

7) Review of Plant Procedt.re 201.1, " Approach to Critical."

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