ML19343B809

Submits Info Re Postulated Uncontrolled Boron Dilution at Cold Shutdown,In Response to NRC .Boron Dilution Event for Cold Shutdown Recalculated Based on Return to Criticality 15 Minutes After Wave Front Reaches Core Inlet

ML19343B809
Person / Time
Site:	Point Beach
Issue date:	12/23/1980
From:	Fay C WISCONSIN ELECTRIC POWER CO.
To:	Clark R, Harold Denton Office of Nuclear Reactor Regulation
References
TAC-43514, TAC-43515, NUDOCS 8012300586
Download: ML19343B809 (5)

Text

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WlSCORSin Electnc a cc?.mr 231 W. MICHIGAN, P.o. BOX 2046. MILWAUKEE, WI 53201 December 23, 1980 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U. S. NUCLEAR REGULATORY COMMISSION Washington, D. C.

20555 Attention: Mr. Robert A. Clark, Chief Operating Reactors Branch #3 Gentlemen:

DOCKET NOS. 50-266 AND 50-301 POSTULATED UNCONTROLLED BORON DILUTION AT COLD SHUTDOWN POINT BEACH NUCLEAR PLANT, UNITS 1 AND 2 The following is provided in response to your letter of September 30, 1980, which expressed concern that.nder two plant conditions inadvertent boron dilution was not conservatively considered in the Point Beach Final Facility Description and Safety Analysis Report. The plant conditions of concern were:

1.

Cold shutdown with the reactor coolant system drained to the " half-pipe" level (reduced Reactor Coolant System

[RCS] volume), and 2.

Cold shutdown with no reactor coolant pumps running (an effective reduced RCS volume).

The boron dilution event for the cold shutdown condition has been recalculated. This calculation used conservative RCS volumes (i.e., reduced or effectively reduced RCS volumes for the applicable primary system flowpath).

Mixing of the diluting water (boron free) and the RCS water was assumed to take place in the vessel downcomer and proceed in a " wave front" fashion through the rest of the RCS. The result of this calculation determines the boron concentration at the core inlet as a function of time from which con-l servative (high) boron dilution rates were derived. The required shutdown l

margin is calculated based on a return to criticality fifteen minutes after l

the wave front reaches the core inlet.

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The attached Figure 1 presents the results of this analysis for-l the most limiting cases. This figure shows the shutdown margin required for l

a given charging rate and boron concentration to remain subcritical for fifteen minutes after the wave front reaches the core inlet. Sufficient time

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Mr. Harold R. Denton December 23, 1980 (fifteen minutes before criticality) is maintained so long as the shutdown margin is to the left of the curve for a given boron concentration and flow ra te.

Figure 2 presents the same results only plotted with different axes.

shis figure shows the allowable charging flow rate for a given boron concer.-

tration and shutdown margin. Should the charging flow rate be greater the.n the allowable charging flow rate, the time period the reactor remains sub-critical after the wave front reaches the core inlet will be less than fifteen minutes.

Therefore, keeping the charging flow rate less than the allowible charging flow rate ensures the operator has sufficient time to termine.te the event.

The time period of fifteen minutes after the wave front reaches the core inlet until a return to criticality occurs is considered an adequate time period for the operator to terminate the event because the operator can determine that a dilution is in progress from the following information:

A.

Source Range Neutron Flux 1.

Hf gh flux at shutdown alarm set at 1/2 decade above shutdown count rate 2.

Audible count rate indication for significant changes in flux level 3.

Periodic surveillance of the source range meters normally performed by the operator B.

Indication of Chemical and Volume Control System (CVCS) i status and Reactor Makeup Water System (RMWS) status 1.

Indication of boric acid and blended (total) flow rate, or indication of boric acid and clean makeup flow rate including audible indication from totalizers 2.

CVCS valve position status lights 3.

RMWS " running" status light l

The present operating procedures require that upon determination that unintentional dilution is in progress, the operator should immediately open the charging pumo suction lines to the RWST and then close the suction lines to the Volume Control Tank (VCT). These procedures will be reviewed to determine if additional information should be included in the procedures to ensure that a proper shutdown margin or an allowable charging flow rate is maintained.

Technical Specification 15.1.g(2) requires that at least a one percent shutdown margin be maintained for the plant conditions in question.

For most of each operating cycle, maintaining a one percent shutdown margin results in a sufficiently low boron concentration that a charging flow rate

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Mr. Harold R. Denton December 23, 1980 higher than the charging flow rate limit for a single charging pump would be required to cause criticality within fifteen minutes. Along with the procedure review described in the preceding paragraph, we shall also determine whether an addition to the Technical Specification is necessary to ensure proper shutdown margin for the conditions described in your

[

letter.

I We would be pleased to respond to any questions concerning the material provided herein.

Very truly yours, t

C. W. Fay, Director.

Nuclear Power Cepartment Attachments Copy to: NRC Resident Inspector 4

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