ML20093D141

Provides Addl Info Re Instrumentation Being Installed for Detection of Inadequate Core Cooling,Per NUREG-0737,Item II.F.2.Determination of Subcooling Margin Encl

ML20093D141
Person / Time
Site:	Byron, Braidwood, 05000000
Issue date:	07/06/1984
From:	Tramm T COMMONWEALTH EDISON CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
References
RTR-NUREG-0737, RTR-NUREG-737, TASK-2.F.2, TASK-TM 8924N, NUDOCS 8407160146
Download: ML20093D141 (6)

Text

a

[

N CommonwMith Edison

(

) One First National Plaza, Chicago, lilenog 1

Address Reply to. Post Office Box 767 (j] Chicago. lilinois 60690 i

July 6, 1984 Mr. Harold R.

Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555

Subject:

Byron Generating Station Units 1 & 2 Braidwood Generating Station Units 1 & 2 Subcooling Margin Monitor NRC Docket Nos. 50-454, 50-455, 50-456, and 50-457 Reference (a):

April 20, 1984 letter from B. J. Youngblood to D. L. Farrar.

Dear Mr. Denton:

This letter provides additional information regarding the instrumentation being installed at the Byron /Braidwood units for the detection of inadequate core cooling.

Specifically, this letter describes the manner in which the operator can determine the core coolant subcooling with single failures in the instrumentation systems.

Reference (a) requested that Commonwealth Edison document the manner in which the subcooling margin monitoring instrumentation would conform to the NUREG-0737, Item II.F.2 single failure criterion.

From discussions with the NRC staff, we understand that this refers to the following portion of Criterion 2 of Appendix B to NUREG-0737:

"No single failure within either the accident-monitoring instrumentation, its auxiliary supporting features or its power sources concurrent with the failure that are a condition or result of a specific accident should prevent the operator from being presented the information necessary for him to determine the safety status of the plant and to bring the plant to a safe condition and maintain it in a safe condition following that accident."

Attachment A to this letter describes the features of the instrumentation and operating procedures which address this requirement.

This description includes changes which have been made to address specific concerns identified during NRC meetings and conference calls.

A highly reliable system for operator determination of subcooling margin is being provided.

Sufficient redundancy and diversity is provided to support all emergency procedure requirements, even in the event of single failures in the installed instrumentation.

Human factors reviews have assured us that the instrumentation and procedures will be easy to use.

hkl 8407160146 840706 PDR ADOCK 05000454 A

PDR

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H. R. Denton July 6, 1984 Please address further questions regarding this matter to this office.

One signed original and fifteen copies of this letter and the attachment are provided for NRC review.

Very truly yours, f hr f.Ad%w T. R. Tramm Nuclear Licensing Administrator lm Attachment 8924N i.

ATTACHMENT A Determination of Subcooling Margin i

t Byron /Braidwood l

Instrumentation Subcooling is normally determined from two separate safety-related wide range reactor coolant pressure instrument channels and sixty-five core exit thermocouples.

Operating procedures govern the use of tnis instrumentation in the determination of subcooling.

Other installed instrumentation could be useful in particular emergency situations but no credit needs to be taken for that other equipment to assure an adequate determination of subcooling margin given single failures.

Each of the wide range reactor coolant pressure channels is powered from a separate ESF bus.

The pressure is displayed on control board indicators and can also be displayed on any of the various process computer output devices, including the CRT's which are mounted in the main control' boards.

Two digital thermocouple monitors are used for control board display of the temperatures measured by the 65 core exit thermocouples.

The process computer can also read each thermocouple.

The two thermo-couple monitor displays are each powered from a separate ESF bus.

The 65 thermocouples are separated into two groups, and each group is read by an individual thermocouple monitor display.

There are 33 thermocouples on one display unit and 32 on the other.

The thermocouples have been grouped so that either monitor can display representative temperatures across the entire core cross section.

Determination of Subcooling Margin 6

Normally, the plant process computer will compute the degrees of subcooling from saturation and output this number in digital form on the wide range SPDS ionic display.

The wide range SPDS iconic will be constantly displayed on one of the two CRT's mounted in the main control board.

At the reactor operator's option, the computed subcooling can also be displayed for trending in analogue form on a panel mounted strip chart recorder or on selectable graphic displays.

The Proteus process computer is designed to be highly reliable.

It can be powered from either of two AC sources or from a battery during short AC interruptions.

Multiple CPU's and redundant internal power supplies also contribute to high computer reliability.

These features are described in more detail in the response to FSAR question 31.15.

The availability of the process computer is expected to be at least 99%.

.~. - -

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., The minimum subcooling margin varies with reactor pressure because of the varying effect of pressure measurement error.

Allowances for instrument error are also necessary under adverse containment conditions.

The process computer will automatically determine the appropriate subcooling margin.

The color of the digital display will change if the measured subcooling does not meet the margin requirement.

The preestablished subcooling margin requirements are also available for reference by the operator as described below.

The emergency operating procedures specify the actions to be taken when the subcooling margin is determined to be inadequate (usually reinitiation of safety injection).

Alternate Subcooling Margin Determination In the unlikely event that the process computer display of subcooling is not available, the reactor operator will determine the subcooling manually.

The wide range pressure Indicators on the main control board are used in conjunction with adjusted saturation curves to determine the maximum allowable core exit temperature.

The curves.to be used are shown in the attached Figure 23, which is referred to as a job performance aid (JPA).

The saturation line has been adjusted to include allowances for instrumentation errors for both normal and adverse containment conditions.

The temperature determined from this figure can be compared directly to the average value of the ten highest functional thermocouples which is displayed on each of the two panel-mounted digital thermocouple monitors.

The temperature limits shown on Figure 23 contain allowance for instrument errors.

The curve on the right is simply the saturation curve for water.

This is provided for reference.

The middle curve is the ellowable subcooling margin for normal containment.

This curve is simply the saturation curve minus the degrees of subcooling error from Table A, column 1.

Table A was formerly incorporated into the operating procedures and the operators were required to make this determination under all conditions.

The left curve is the adverse containment curve, obtained by subtracting the values of Table A, column 2 from the saturation curve.

Using the wide range pressure, the operator reads across the cppropriate subcooling margin curve (adverse or normal).

He then reads down to obtain his maximum allowable T/C temperature for comparison with the value displayed on the thermocouple monitors.

The operator monitors the four containment pressure indicators at the MCB and the two containment radiation monitors to determine if the containment is at normal or adverse conditions.

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