ML19319D239

Safety Evaluation of Proposed Change to Tech Specs Re Surveillance Requirement for Diesel Generator Automatic Sequencers.Requires Use of NRC Spec in Place of Util Proposed Tech Specs Change

ML19319D239
Person / Time
Site:	Crystal River, Saint Lucie
Issue date:	09/30/1977
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML19319D236	List: ML19319D235 ML19319D239
References
NUDOCS 8003130866
Download: ML19319D239 (7)

Text

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ENCLOSURE NO. l' 9

SAFETY EVALUATION OF THE PROPOSED. CHANGE TO THE TECHNICAL SPECIFICATIONS SURVEILLANCE OF AUTOMATIC SEOUENCE TIMER ST. LUCIE PLANT. UNIT 1

1.0 INTRODUCTION

The Florida Power & Light Company, in its letters of November 8, 1976, and May 31 and July 6, 1977 requested a cnange to Section A.8.1.1.2.C.6 of the Standard Technical Specifications for St. Lucie, Unit 1.

This particular scecification addresses the surveillance requirement for.the automatic sequencers, which initiate the timed sequence loading. cf the diesel generators. The licensee requested that the provision for verification of timer limits, which now requires that accuracy be maintained within - 10% of setpoint limits, be changed to 1 1 second of setpoint.

The licensee states that the reason for requesting the change to the Technical Specifications is because verificatf an of their timer's setpoint accuracy for time periods less than one second cannot be assured.

.2.0 EVAL JATION:

The licensee appears to have misunderstood the intent of the surveillance requirement. The present provision in the licensee's Technical Specifications for St. Lucie, Unit 1 is:

soos iso f(6 z.

. " Verifying that the automatic sequence timers are OPERAELE with each load sequence time within i10% of its required value".

The licensee's proposed change to the above position is:

" Verifying that the automatic sequence timers are OPERABLE with each load sequence time within 11 second of its required value".

The licensee has, in the past, interpreted these scecifications, f.e.,

1 0% of the required value as folicws - a load that is required to be 1

sequenced at 30 seconds would be acceptably sequenced so icng as the time of secuence was greater than 27 but less than 33 seconds, i.e.,

1 10% of its required value.

This interpretation is not that intended by the staff.

The cbjectives fcr diesei generator load sequencing are presented in Reculatory Guide ~1.9.

Position 4 cf the Regulatory Guide presents the voltace and frecuency values and relates tnem to the secuence time interval (delta-T).

The cojective of this provision c#

.s to the Regulatory Guide, i.e., verification of automatic sequencer tir..a assure an acequate time interval (delta-T) beween a;olication of 1 cads to permit diesel generator voltage and frequency values to recover to acceptable levels.

To ensure this, it is necessary to verify that each sequential setpoint

" interval" is not changed by more than 1 10%. Therefore, referring back to this~ example of a load sequenced at 30 seconds, it wouic be necessary to deter-mine the 310% of delta-T, by knowing-the load secuence time before and the lead sequence time after the 30 second setooint.

These time intervals, if t..ay are the typical,3 seconds between leads, would aliew a variation of _0.3 second; at each end of the 3 secono delta-T and not the three seconds as would be the cc in_:ne licensee's interpretation.

. However, the licensee's proposed change to the Technical Specifications is supported by preoperational load test results from diesel generators A and B (See Attachment No. 1) which demonstrate that the machine's voltage and frequency return to normal within acceptable limits as provided for in Regulatory Guide 1.9. -As a matter of fact, the frequency and voltage variations,as a result of sequential leading are recovered to accep-table levels for the worst case loading in less than 22 percent of a sequential time interval.

With.a timer setpoint of ; 1 second, successively sequenced loads c d

conceiva:1y be sequenced at one second apart for those loads having a secuence in:erval of three seconds.

The test results submitted by the licensee show the voltage decreasing to 92.4 percent of nominal (4160 volts) and returning to ncminal voltage in about 0.66 seconds for the largest sequenced load (575 kW) at three seconds after the diesel generator breaker closed on to the bus. The most. severe frequency decrease was to about 59.25 hert: anc returned to normal (60 hert:) in about 0.6 seconds.

These data, which were obtained during the preoperational testing, more than satisfy the requirements of Reaulatory guide 1.9 which allows the voltage and frequency to decrease, upon loading to 75 and 95 percent, respectively, of nominal and return to within 10 and 2 percent of normal within 40 percent of load sequence interval (three seconds). The above loading sequendes support the licensee's contention that verifying the sequence timers are operable within + 1 second of each load sequence time i

interval will not result in overloading of the diesel generators and satisfy the associated provisions of Regulatory Guide 1.9.

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1

3.0 CONCLUSION

S The licensee, through its interpretation of the Technical Specifica-tions, has been in violation of the provisions of the Technical Speci-fications.

No significant risk to public health and safety has been incurred because of the adequacy of the system. We find the licensee's proposed change to Section 4.8.1.1.2.C.6 of the Technial Specifications to be acceptable.

However, since the licensee's Technical Specifications is the NRC's Standard Technical Specifications, the staff requires that the following specificad:n be used in place of that proposed by the licensee:

" Verifying that the automatic sequence ticers are OPERABLE with the interval between each lead block within _t 1 second of its design interval."

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ENCLOSURE NO. 2 DIESEL GENERATOR LOAD SEQUENCING DESCRIPTION OF CIRCUMSTANCES:

The purpose of this circular is to aid licensees in interpreting one of the surveillance requirements of Tecnnical Specifications.

Apparently, the provision for verification of the accuracy of the automatic load sequence timers for diesel-generators has been misunderstood.

It is important to verify that the load sequence timer is not only operable but that the time interval for sequencing each load block is proper.

The objectives for diesel-generator load sequencing are presented in Regulatory Guide 1.9.

Position 4 of the Regulatory Guide presents voltage anc frequency values and relates tnair magnituce and duration to the load sequence time interval.

The objective of this provision of tne Regulatory Guide, i.e., verification of the automatic sequence timer, is to assure tnat the time ceriods between the application and acceleration of loads are adecuate to permit the voltage and frequency to recover to acceptable levels for each loac block energi:ed by the generator.

To ensure this, it is necessary to verify that each set-coin: has not cnanced or drifted by more than an allowable amount.

An examole of this point is as follows:

for a time intervai of three

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seconds between loads, an allowable sequence deviation of - 10% would allow

• 0.3 seconds for each setpoint.

The misinterpreta: Ton that must be avoided is as follows:

Let's assume that one load is to be energized after 15 seconds and tnat tne subsequent load is to be energized at IS seconds.

One should not assume that the 15 second point could be in error by as much as + 1.5 seconds.

Obviously the delta time interval is the criticaT value that must be available for load acceleration and must be maintained.

While this is the particular point that needs clarification it goes without saying that the timers must sequence loads within a total time period as indicated in the accident analyses.

RECCMMENDED ACTION SY LICENSEES:

It is recommended that licensees receiving this circular review their sequencer test methods and their previous test results to ensure that they have been in compliance with the design basis for diesel generator loading and that their Technical Specifications and the objectives of Position 4 of Regulatory Guide 1.9 are being satisfied.

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