ML20055B272

Safety Evaluation Supporting Amend 79 to License DPR-65

ML20055B272
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Site:	Millstone
Issue date:	07/02/1982
From:	Office of Nuclear Reactor Regulation
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NUDOCS 8207210123
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I SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO. 79 TO FACILITY OPERATING LICENSE NO. OPR-65 NORTHEAST NUCLEAR ENERGY COMPANY, ET AL.

MILLSTONE NUCLEAR POWER STATION, UNIT N0. 2 DOCKET NO. 50-336 Introduction By application dated June 18, 1982, Northeast Nuclear Energy Company (NNECO or the licensee) requested an amendment to the Technical Specifications (TS) appended to Facility Operating License No. OPR-65 for the Millstone Nuclear

- Power Station, Unit No. 2 (Millstone-2 or the facility).

The amendment requested reduction of the design reactor coolant system (RCS) flow by two (2) percent as a result of observed flow reduction for the current'(Cycle 5) operation. This flow reduction resulted from plugging a total of 704 steam generator (SG) tubes in both SGs last outage (December 1981 to March 1982). The SG tubes were plugged because of extensive corrosion induced pitting indications in the central tubes between the tube sheet and the first support plate. For more information on the SG tube degradation, the plugging criteria and special requirements for operation, see our safety evaluation (SE) supporting Amendment No. 73, issued March 5, 1982.

l- Backoround NNEC0 states that plugging the 704 SG tubes has resulted in an approximate

! 3 percent reduction in RCS flow. The average recorded flows for the last five' cycles are: -

Cycle 1 402,800 gpm Cycle 2 393,300 gpm Cycle 3 390,700 gpm Cycle 4 394,000 gpm Cycle 5' 384,600 gpm h

Between Cycles 1 and 2, 800 SG tubes were taken out of service (pb . :?

' for a preventive maintenance " rim cut". This preventive maintenance. aso resulted in an approximate 3 percent reduction in four pump RC'S flow. The l

I remaining RCS flow values are, as expected, within measurement accuracy.

The current TS consistently uses 370,000 gpm as the minimum RCS flow for l ,

thermai margin safety limit determinations. When the measurement uncer-

' tainty of 13,000 gpm (3.5 percent) is subtracted from the Cycle 5 flow

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rate, the RCS flow value is about 371,600 gpm (above the required TS flow).

Since the TS specified trip point is 91.7 percent of 370,000 gpm, conditions are acceptable for normal operation. However, for an underfrequency con-dition external to Millstone-2 (approximately 59 Hz), NNEC0 expects the reactor coolant pumps (RCPs) to reduce flow to the 91.7 percent."RCS Flow-Low" trip setpoint. Since the delta pressure (aPJinstrumentation has a

+ 4 percent " noise" (instrumentation swing) this would negate NNEC'S commitments to the Northeast Power Coordina,ingt Council to remain avail-able during such a grid underfrequency condition.

Discussion The NNECO application is to reduce the RCS flow, less measurement uncer-tainties, from 370,000 to 362,600 gpm, a 2 percent reduction. The low RCS flow trip setpoint would remain at the 91.7 percent of the reduced design flow value (362,600 gpm). To offset the 2 percent reduced flow trip set-point,NNEC0proposestodecreasethetotalplanar(Fx})andtotalintegrated radial (F,!) peaking factors by 2 percent. They reference past safety analyses and our safety evaluations as justification; in that a 2 percent change in RCS flow is equivalent to a one percent change in power for DNS analyses.

The greatest impact would be for those transients during which ONB considera-tions are paramount such as the four pump loss of flow and seized rotor events.

NNEC0 has reviewed these transients.

NNECO completed an evaluation of the impact of the reduced RCS flow trip setpoint on the large and small break LOCA analyses. They conclude that peak clad temperature (PCT) would not increase more than 1000F following a large break LOCA. This increase in PCT is due to a slight increase in fuel stored energy due to the reduction in RCS flow. For the small break LOCA, the licensee states that this increased initial fuel stored energy will not affect the results of the analysis of record. The increased fuel stored energy would be removed from the fuel rod during the lengthy flow coastdown prior to the uncovery of the core when the fuel rod heatup transient begins.

NNECO concludes that the proposed TS changes only affect the larga break LOCA analysis and that the 1000F increase in PCT conservatively bounds these changes. The proposed TS changes are for the remainder of Cycle 5 operation only.

Evaluation At the onset of our review, we questioned NNECO in regards to alternate means of reaching the desired goal of assuring plant availability following' an underfrequency condition of the grid. The load shedding devices are located at each substation throughout the system. Even if the frequency setpoints for load shedding were increased at a number of substations, lack of control for neighboring systems could still allow the frequency to decrease below 59 Kz. In addition, changing such setpoints would be inconsistent with the northeast grid's operating practices.

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titlEC0 has been looking at possible instrumentation changes to reduce the

+ 4 percent " noise" in the RCS flow AP system. They have concluded that Tittle improvement, if any is possible, can be made during plant operation.

Looking at the available options, we conclude that the proposed TS change is a desirable way to insure that a minor electrical system underfrequency condition is not made worse by a potential trip (possibly unnecessary) of Millstone-2. The specific areas of review are as follows, e DNB Reduction Approximation tir4ECO's reference of Cycles 4 and 5 Safety Analysis (dated June 3, 1980 and November 17,1981) and our resultant SE (dated October 6, 1980 and March 5, 1982) has been used to approximate the RCS flow reduction in core power level terms. For Cycle 5 (the current cycle) the THINC-I code in conjunction with the W-3 correlation was.used to equate a 4 percent measured RCS flow uncertainty to a 2 percent equivalent power uncertainty. Based on our previously approved 2-to-1 RCS flow-to-power conversion using acceptable codes, we find fif4ECO's more conservative 2 percent reduction in total planar and total integrated radial peaking factors to offset the 2 percent reduction in RCS flow acceptable.

e Transient and Accidents flNECO indentified the four RCP loss-of-flow and the seized rotor events as the most likely.affected events. Since startup from the Cycle 3 refueling out-age.(May 1979), the 4-channel rei.ctor protection system (RPS) trip that provides a fast response from tsa four RCP loss-of-flow event is the RCP speed sensing signal (RCPSSS). See oyr SE dated May 12, 1979. The proposed TS change has no effect on the RCPSSS trip setpoint. Therefore, the limiting Dr4B transient is not changed from the Cycle 5 review.

In the seized RCP rotor event, the RCS flow-low'RPS trip provides the needed reactor reponse on sensing low AP. Here, as in other non-LOCA transients, offsetting the reduction in RCS flow with the conservative 2 percent reduction in allowable local power peaking factors is acceptable.

e Small Break LOCA We agree that the 2 percent reduction in RCS flow will have a negligible effect on the results accepted in the Cycle 5 reload SE (March 5, 1982).

This is due to the long tt1e from break until uncovery of the core; enough time that the additional stored energy in the fuel (from the 2 percent flow reduction) has been dissipated.

e Large Break LOCA Again, because of the additional stored energy in the fuel, in the form of about 2 percent higher fuel temperature, the PCT will be higher. The

Cycle 5 calculated PCT is 2045cF. A direct proportion increase would be 420F to 2087oF. We would expect the calculated increase to be between 25 and 50oF. We find UNECO's bounding value.of 1000F conservative and conclude that the. limits of 10 CFR 50.46 (22000F) will not be exceeded.

However, we.believe it is desirable to have an actual approved code calcu-lated. PCT available at all times. NMECO has agreed to submit such results by November 1, 1982.

Technical Specification Changes The specific TS changes in the NNECO application are (1) correct the design gpm) on Pages 2-2, 2-4 and 3/4 2-14 flowvalue(from}70,000[to362,600 and (2) reduce Fxy and F 2 percent on Pages 3/4 2-6, 3/4 2-9 and in /

Figure 3.2-3 (Page 3/4 2-8). Based on the above evaluation, we find these TS changes acceptable.

Environmental Consideration We have determined that the amendment does not authorize a change in ef fluent types or total amounts nor an increase in. power level and will not result in any significant environmental impact. Having made this determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact and, cursuant to 10 CFR $51.5(d)(4), that an environmental impact statement or negative declaration and environ-mental impact appraisal need not be prepared in connection with the issuance of this amendment.

Conclusion We have concluded, based on the considerations discussed above, that:

(1) because the amendment does not involve a significant increase in the probability or consequences of an accident previously evaluated, does not create the possibility of an accident of a type different from any evaluated previously, and does not involve a significant reduction in a margin of safety, the amendment does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public. .

Date:

Principal Contributor:

E. L. Conner

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