L-2011-273, Information Regarding Anticipated Transients Without Scram (ATWS) Provided in Support of the Extended Power Uprate License Amendment Request

Information Regarding Anticipated Transients Without Scram (ATWS) Provided in Support of the Extended Power Uprate License Amendment Request

ML11207A455
Person / Time
Site:	Saint Lucie
Issue date:	07/22/2011
From:	Richard Anderson Florida Power & Light Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
L-2011-273
Download: ML11207A455 (7)

Text

0 Florida Power & Light Company, 6501 S. Ocean Drive, Jensen Beach, FL 34957 FPL July 22, 2011 L-2011-273 10 CFR 50.90 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 Re: St. Lucie Plant Unit 2 Docket No. 50-389 Renewed Facility Operating License No. NPF-16 Information Regarding Anticipated Transients Without Scram (ATWS) Provided in Support of the Extended Power Uprate License Amendment Request

References:

(1) R. L. Anderson (FPL) to U.S. Nuclear Regulatory Commission (L-2011-021),

"License Amendment Request for Extended Power Uprate," dated February 25, 2011 (Accession No. ML110730116).

By letter L-2011-021 dated February 25, 2011 [Reference 1], Florida Power & Light Company (FPL) requested to amend Renewed Facility Operating License No. NPF-16 and revise the St. Lucie Unit 2 Technical Specifications (TS). The proposed amendment will increase the unit's licensed core thermal power level from 2700 megawatts thermal (MWt) to 3020 MWt and revise the Renewed Facility Operating License and TS to support operation at this increased core thermal power level. This represents an approximate increase of 11.85% and is therefore considered an Extended Power Uprate (EPU).

During the course of their review and as discussed in the July 12, 2011 public meeting, NRC staff in the Reactor Systems Branch informally requested information related to Anticipated Transients Without Scram (ATWS) to support their review of the EPU LAR.

The requested information is documented in Attachment 1 to this letter.

In accordance with 10 CFR 50.91 (b)(1), a copy of this letter is being forwarded to the designated State of Florida official.

This submittal does not alter the significant hazards consideration or environmental assessment previously submitted by FPL letter L-2011-021 [Reference 1].

This submittal contains no new commitments and no revisions to existing commitments.

an FPL Group companyAc

L-2011-273 Page 2 of 2 Should you have any questions regarding this submittal, please contact Mr. Christopher Wasik, St. Lucie Extended Power Uprate LAR Project Manager, at 772-467-7138.

I declare under penalty of perjury that the foregoing is true and correct to the best of my knowledge.

Executed on luyX ' .Oif, Very truly Y, Richard L. Anderso Site Vice President St. Lucie Plant Attachment cc: Mr. William Passetti, Florida Department of Health

L-2011-273 Attachment 1 Page 1 of 5 Response to Request for Information The following information is provided by Florida Power & Light in response to the U. S. Nuclear Regulatory Commission's (NRC) informal request regarding anticipated transients without scram (ATWS). This information was requested to support the extended power uprate (EPU) license amendment request (LAR) for St. Lucie Unit 2 that was submitted to the NRC by FPL via letter (L-2011-021) dated February 25, 2011, Accession Number ML110730116.

SRXB Request Regarding ATWS (Paraphrased by FPL)

LAR Attachment 5, Section 2.8.5.7 states that ATWS DSS, DTT and AFAS and setpoints are not impacted by EPU. Provide justification for this determination, including the basis for ensuring that peak RCS pressure remains below 3200 psig.

Response

As addressed in the LAR, 10 CFR 50.62 specifies the design requirements with which St. Lucie Unit 2 complies. These requirements were imposed to reduce the probability of a severe ATWS event, which is defined by the NRC as the occurrence of an anticipated transient in conjunction with a failure of the reactor protection system (RPS) to trip the plant resulting in a reactor coolant system (RCS) overpressurization exceeding 3200 psig. No additional analyses are required by 10 CFR 50.62.

The limiting ATWS events are the loss of load (LOL) and the loss of main feedwater (LOFW).

For the St. Lucie Unit 2 (PSL2) class of plants, Reference 1 demonstrated that a diverse scram system (DSS) with a 2450 psia trip setpoint and a 2-second response time would maintain the peak RCS pressure to <3200 psig for the limiting anticipated operational occurrences (AOOs).

The pressure turn-around is dominated by the reactor trip initiated by DSS with minimal contribution from the moderator temperature coefficient. The DSS setpoint of 2450 psia is set such that it is above the RPS high pressurizer pressure trip (HPPT) setpoint and below the pressurizer safety valves (PSV) as-left setpoint.

PSL2 also complies with the requirements for a diverse turbine trip (DTT) and a diverse auxiliary feedwater actuation system (DAFAS). However, as stated in Reference 1, the addition of a DTT and a DAFAS provides an insignificant reduction of ATWS risk if a DSS is installed, and the installation of the DSS alone meets the reliability goals of the ATWS rule.

Although no explicit ATWS analyses have been performed for St. Lucie Unit 2 at EPU conditions, the EPU loss of condenser vacuum (LOCV) and feed line break (FLB) scenarios presented in the LAR provide adequate justification that LOL and LOFW scenarios with ATWS considerations would continue to meet the criteria as presented in References 1 and 2. It is important to note that the FLB analysis is a postulated accident, not an anticipated transient/AOO, and is only utilized as a conservative representation of the LOFW transient. Both the current and EPU LOCV and FLB analyses applied more conservative inputs and

L-2011-273 Attachment 1 Page 2 of 5 assumptions with respect to the RCS overpressurization scenarios than were required in the ATWS analyses, as described in Table 1. The EPU analyses clearly demonstrate that the SL2 DSS will be effective in limiting peak RCS pressures to <3200 psig for these limiting ATWS scenarios, as described below.

Loss of Load

1. The ATWS LOL analysis (References 1 and 3) assumes instantaneous termination of all feedwater flow and steam flow to the condenser, and delays reactor trip until the DSS setpoint of 2450 psia is reached. The peak RCS pressure was -2600 psia.

2. The EPU LOCV analysis (LAR Attachment 5, Section 2.8.5.2.1) assumes the same instantaneous termination of feedwater and steam flows, and the reactor trips on the RPS HPPT at a setpoint of 2415 psia. The peak RCS pressure was 2669 psia.

Loss of Feedwater

1. The ATWS LOFW (References 1 and 2) assumes instantaneous termination of all feedwater flow, and delays reactor trip until the DSS setpoint of 2450 psia is reached.

2. EPU FLB (LAR Attachment 5, Section 2.8.5.2.4) assumes the same instantaneous termination of feedwater; but in addition, the 0.21 ft 2 break depletes the steam generator inventory more quickly than the simple LOFW (LAR Attachment 5, Figure 2.8.5.2.4-4),

forcing a degradation in heat transfer and a rapid RCS heatup. The steam generator low level trip is ignored and the reactor trips on RPS HPPT (2460 psia setpoint).

The RPS HPPT and the PSVs as-left setpoints have not changed for the EPU. With the DSS setpoint also unchanged at 2450 psia, it can be judged from the results of the current LOCV and FLB analyses that the peak pressure of ATWS LOL and LOFW at EPU conditions would remain

<3200 psig.

This is furher substantiated from the EPU LOCV and FLB results by a conservative adjustment to account for delaying the reactor trip from the RPS HPPT trip to the DSS trip. With the application of the conservative adjustment, the peak RCS pressure for ATWS LOL and LOFW at EPU conditions is seen to remain much less than 3200 psig, as described below:

EPU LOCV Adjustment - If the EPU LOCV trip is delayed from the RPS HPPT to the DSS trip (i.e., delayed until 2450 psia with an additional 0.85 second response time as per Table 1), then the peak RCS pressure would increase by 107 psi, from 2669 to 2776 psia.1 1 Reference 3, Section 2.8.5.2.1, Table 2.8.5.2.1-2 provides the HPPT setpoint of 2415 psia at 16.30 seconds, while the PSVs open at a pressure setpoint of 2575 at 18.195 seconds. Using these data, the rate of pressurization is about 84 psi/sec. If the trip is delayed to 2450 psia with 0.85 seconds additional response, then the peak pressure would increase by 107 psia [= 84*0.85 + (2450-2415)].

L-2011-273 Attachment 1 Page 3 of 5 EPU FLB Adjustment - If the EPU FLB trip is changed from the RPS HPPT to the DSS trip, (i.e., tripped at 2450 with an additional 0.6 second response time as per Table 1),

2 then the peak RCS pressure would increase 43 psi, from 2704 to 2747 psia.

Conclusion Based on the extrapolation of the EPU LOCV and FLB results to account for a reactor delay associated with waiting for a DSS trip while ignoring the RPS HPPT trip, it is concluded that the DSS trip set at 2450 psia will maintain the peak RCS pressure during the limiting ATWS events to <3200 psig for the EPU.

References

1. CE-NPSD-354 Task-494, Rev. 0, "Functional Design Specification for the Diverse Scram System for Compliance with the ATWS Rule 10CFR50.62."

2. CENPD-263-P, Rev. 0, "ATWS Early Verification Response to NRC Letter of February 15, 1979, for Combustion Engineering NSSS's."

3. CENPD-158, through Rev. 1, "ATWS Analyses, Analysis of Anticipated Transients without Reactor Scram in Combustion Engineering NSSS's."

2 Reference 3, Section 2.8.5.2.4, Table 2.8.5.2.4-2 provides the HPPT setpoint of 2460 psia at 31.04 seconds, while the PSVs open at a pressure setpoint of 2575 at 32.66 seconds. Using these data, the rate of pressurization is about 71 psi/sec. Ifthe trip remains at 2460 psia (conservative with respect to 2450 psia) with 0.6 seconds additional response, then the peak pressure would increase 43 psia

[=71"0.6].

L-2011-273 Attachment 1 Page 4 of 5 Table 1 - Input parameters for LOCV, FLB and ATWS EPU LOCV EPU FLB Parameter value value ATWS value Units Comments 2710 (LOFW) Per References 1 & 2, the LOFW ATWS analyses assumed 2710 MW NSSS NSSS Power 3044.2 3044.2 MWt power for the 2560 MW Class of plants, whereas the LOL ATWS analyses of 2560 (LOL) References 1 &4 assumed 2560 MW for the 2560 MW Class.

RCS Volume Nominal Nominal Nominal Lower temperature is conservative for overpressure, because it lowers SG Full Power Cold 532 532 546 oF pressure which delays MSSV opening. ATWS used a nominal value. EPU used a Leg Temperature nminimum value including uncertainties. Therefore, the EPU value is more adverse than ATWS required.

Reactor Vessel Minimum Minimum Nominal The difference between the minimum and nominal RCS flow rates has a negligible Flow impact on the peak RCS pressure for the ATWS events.

Pressurizer 2180 2180 2250 psia Lower initial PZR pressure setpoint for the EPU delays the HPPT trip, therefore Pressure 1resulting in a higher RCS pressure.

Pressurizer Water 782 (LOFW) ft3 Higher initial PZR level for the EPU causes a faster rate of pressurization once Volume 932 927 769 (LOL) heatup begins, therefore resulting in a higher RCS pressure.

Total PORV Relief 302,670 (LOFW) ibm Unavailable PORVs makes the overpressurization for the EPU more adverse than Capacity 303,800 (LOL) I TWS required.

Pressurizer Safety 2575 2575 2500 psia Delayed opening of the PSVs makes the overpressurization for the EPU more Valve Setpoint 27psa dverse than ATWS; required.

Total Pressurizer (LOFW)

Safety Valve Rated 636,546 636,546 592,130 (LO I)

Ibm/hr Capacity 990,000 (LOL)

Pressurizer PressurieCro Unavailable spray flow makes the overpressurization for the EPU more adverse Pressure Control Not credited Not credited Operating han ATWS required.

System______________

Pressurizer Level Not credited Not credited Operating Unavailable letdown flow makes the overpressurization for the EPU slightly more Control System N dverse than ATWS.

L-2011-273 Attachment 1 Page 5 of 5 EPU LOCV EPU FLB Parameter value value ATWS value Units Comments Steam Generator Pressure 707 707 Lower SG pressure is conservative for overpressure, because it delays MSSV 850 psia opening. ATWS used a nominal value. EPU used a minimum value with uncertainties. Therefore, the EPU value is more adverse than ATWS required.

Minimum SG water level (inventory) is conservative as it absorbs less energy from SG Water Level Minimum Minimum Nominal [he primary system during the event and results in a faster rate of RCS pressurization.

High Pressurizer Pressure Trip 2415 (RPS) 2460 (RPS) 2450 (DSS) psia Setpoint DSS response time is slightly greater than the RPS HPPT.

High Pressurizer For EPU LOCV: Table 2.8.5.2.1-2 (Reactor trip time-HPPT Setpoint reached)

Pressure Trip 1.15 1.4 2 sec Response Time For EPU FLB: Table 2.8.5.2.4-2 (Reactor trip time-HPPT Setpoint reached +

conservative delay (0.99-0.74)]

AFAS Setpoints N/A N/A N/A FW does not impact peak RCS pressure since no AFW flow enters the SGs prior to the time of peak RCS pressure.

Peak RCS Pressure ATWS LOL pressure is comparable to the LOCV and FLB pressure at pre-EPU 2669 2704 -2600 psia power.