ML20136F590
| ML20136F590 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 05/12/1995 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20136C539 | List:
|
| References | |
| FOIA-96-485 NUDOCS 9703170009 | |
| Download: ML20136F590 (8) | |
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UNITED STATES t
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NUCLEAR REGULATORY COMMISSION
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f WASHINGTON, D.C. 20066 4001 e%
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ATTACHMENT 1 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO REVISIONS IN THE TECHNICAL SPECIFICATIONS FOR SURVEILLANCE REQUIREMENTS ON THE REQUIRED MINIMUM SHUTDOWN COOLING FLORIDA POWER & LIGHT COMPANY ST. LUCIE UNIT 2 DOCKET NO. 50-389
1.0 INTRODUCTION
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By'1etter dated February 27, 1995 (Ref 1), the Florida Power & Light Company (FPL) requested changes to the Technical Specifications for the St. Lucie Unit 2 plant. A provision is to be added to the existing Technical Specifications (TSs) to allow the 3,000 gpa required minimum reactor coolant flow rate via the shutdown cooling system to be reduced, under certain conditions, to 1,850 gpe. The revision will accommodate isolation of a Shutdown Cooling System (SDCS)-to-Reactor Coolant System (RCS) injection line for maintenance during MODE 6 conditions, and considers single failure criteria.
The statement of required surveillance will be revised to clarify that the specified reactor coolant flow rate is the total flow rate to the reactor pressure vessel. The changes are described below:
1)
Surveillance Requirements (SR) 4.9.8.1 and 4.9.8.2 specify that, "At 1 east one shutdown cooling loop shall be verified to be in operation and circulating reactor coolant at a flow rate of greater than or equal to i
3,000 gpa at least once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />." The surveillance will be restated as follows:
"At least once per 12 hour1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />s:
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At least one shutdown cooling loop shall be verified to be in operation.
i 9703170009 970301 PDR FOIA BINDER 96-485 PDR
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b.
The total flow rate of reactor coolant to the reactor pressure vessel shall be verified to be greater than or
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equal to 3,000 gpm."
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The specified minimum flow rate will be modified by adding the following i
footnote:
i "The reactor coolant flow rate requirement may be reduced to 1,850 gpa
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if the following conditions are satisfied before the reduced requirement l
is implemented: the reactor has been determined to have been l_
subcritical for at least 125 hours0.00145 days <br />0.0347 hours <br />2.066799e-4 weeks <br />4.75625e-5 months <br />, the maximum RCS temperature is s 117 j'
'F, and t,he temperature of CCW to the shutdown cooling heat exchanger is l
s 87
'F."
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Bases section B 3/4.9.8 will be revised by adding a summary statement of 4
the reasons for the specific plant conditions under which the footnote 1
may be applied. The sunniary statement is as follows: "The footnote providing for a minimum reactor coolant flow rate of 21,850 gpm
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considers one of the two RCS injection points for a SDCS train to be isolated. The specified parameters include 50 gpm for flow measurement uncertainty, and 3 'F. uncertainty for RCS and CCW temperature measurements. The conditions of minimum shutdown time, maximum RCS temperature, and maximum temperature of CCW to the shutdown cooling heat exchanger are initial conditions specified to assure that a reduction in flow rate from 3,000 gpm to 1,800 gpm will not result in a temperature transient exceeding 140 *F during conditions when the RCS water level is at an elevation 2 29.5 feet."
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2.0 BACKGROUND
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The plant design provides'two independent SDCS loops, each with a Low Pressure Safety Injection (LPSI) pump and two injection lines. During MODE 6 operation, an OPERABLE shutdown cooling loop must be capable of circulating 3
reactor coolant at a flow rate 2 3,000 gpm to satisfy the specified i
surveillances. To support certain maintenance activities on the low pressure
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3 l5 safety injection headers, which are the same headers used to circulate reactor coolant during SDCS operation, one of the two RCS injection lines for the
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affected train must be isolated.
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With the water level less than 23 feet above the top of the reactor pressure j
vessel flange, two independent shutdown cooling loops must be OPERABLE j
pursuant to LCO 3/4.9.8.2, and the 3,000 gpm minimum capacity to satisfy the j,
SR also applies. The SDCS pump flow through a single injection line header I'
. path can provide a. flow rate of approximately 2,000 gpm. Since this single i
RCS injection point in the SDCS train is incapable of achieving it 3,000 gpm, f
this train must be declared inoperable. Therefore, maintenance that requires isolation of one of the two injection flowpaths for either SDCS train is l
4 prohibited, and is limited to the plant condition where only one OPERABLE l
shutdown cooling loop is required e.g., the RCS water level is greater than 23 i
feet above the reactor pressure flange (TS 3/4.9.8.1). For this reason, analyses were performed by the licensee to verify the adequacy of one SDCS train to satisfy the Bases for the existing LCO when the flow rate capacity of j
that train is limited to less than 3,000 gpm.
j 3.0 EVALUATION t
l The licensee addressed the MODE 6 operability requirements for shutdown j
cooling loops as described in Basis Section 3/4.9.8 which states:
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j (1) sufficient cooling capacity is available to remove decay heat and j
maintain the water in the reactor pressure vessel below 140 'F as j
required in the REFUELING MODE, and (2) sufficient coolant circulation is maintained through the reactor 4
core to minimize the effects of a boron dilution. incident and l
prevent boron stratification.
In addition, the requirement to have two shutdown cooling loops OPERABLE when
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there is less than 23 feet of water above the reactor pressure vessel flange with irradiated fuel in the core ensures that a single failure of the i
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4 operatina shutdown cooling loop will not result in a complete loss of decay heat removal capability.
An evaluation was performed by the licensee to show the ability to satisfy the above bases for TS 3/4.9.8. This considered a MODE 6 plant configuration where the water level is less than 23 feet above the reactor pressure vessel flange, the alternate SDCS train is inoperable, and the operating shutdown cooling loop is circulating reactor coolant through only one SDCS-to-RCS cold leg penetration. The licensee stated that the specified conditions for which the reduced flow rate will be permitted are considered to be conservative, and are compatible with the existing bases for the affected Limiting Conditions for Operation (LCO). The calculations were performed by Ouzo Brown Boveri-
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Combustion Engineering (ABB-CE) and were confirmed by FPL. The principal assumptions were as follows:
a.
Initial maximum RCS temperature 120 *F.
b.
Initial component cooling water (CCW) temperature 90 "F.
c.
Decay heat fractions per NRC Branch Technical Position (BTP) ASB 9-2 (Standard Review Plan 9.2.5).
d.
Shutdown cooling system (SDCS) Injection Flow 1,800 gpm (Minimum Guaranteed).
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RCS Level 29.5' (Tht: is 4 feet below the reactor pressure vessel flange and corresponds to RCS Mid-loop operation).
The licensee made analyses for the effects of the reduction in flow rate for shutdown cooling which addressed: (1) decay heat removal, (2) boron dilution, (3) boron stratification, and (4) guaranteed minimum flow. The results are discussed below.
3.1 Decay Heat Removal The analysis for decay heat removal was to find if sufficient cooling capacity is available to remove decay heat and maintain the water in the reactor pressure vessel below 140 *F a required in the Mode 6 refueling mode, i
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The analysis was based on a RCS flow rate of 1,800 gpm, and a reduced inventory corresponding to an RCS water level of 29.5 feet (mid loop). The
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primary system masses and volumes were conservatively calculated assuming that j
the steam generator nozzle dans are installed. The decay heat fraction was 1
taken from the NRC BTP ASB 9-2 formulation. This was based on an operating history of 16,000 effective full power operating hours (EFPH}.
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Based on these inputs, the resultant peak RCS temperature was shown to be less
!l than 140 *F and therefore acceptable when the time after shutdown is at least J
5 days (120 hrs).
i-FPL further determined that the proposed minimum shutdown time of 125 hours0.00145 days <br />0.0347 hours <br />2.066799e-4 weeks <br />4.75625e-5 months <br /> s
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i would provide. additional conservatism and extend applicability of the analysis j
to a 24 month fuel cycle (assumed 16,5000 EFPH).
3.2 Boron Dilution i
j The analysis for boron dilution was to find if sufficient coolant circulation is maintained through the reactor core to minimize the effects of a boron dilution incident.
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Calculations performed by the licensee show that at a flow rate of 1,800 gpm l
and an inventory corresponding to mid-lop operation, a transit time of 17 minutes is required for one cycle of RCS fluid circulation.
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The Updated Final Safety Analysis Report (UFSAR) documents the Inadvertent j
j Boron Dilution event for MODE 6 conditions. The UFSAR analysis shows that, for the assumed condition of 3 charging pumps running, a boron dilution alarm would be required at no less than 58 minutes, and the minimum time to
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criticality would be more than 88 minutes. These times correspond to more than 3 and 5 times the shutdown cooling loop transit time (17 minutes),
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Therefore, the flow rate of 1,800 gpm is considered adequate to provide sufficient mixing and a timely indication of a gradual, uniform decrease in s
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RCS boron concentration. Also, it is concluded that the operator has
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sufficient time to recognize the event and isolate the dilution source prior l
to significant loss of shutdown margin.
bI 3.3 Boron Stratification
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t The results of an analysis for boron dilution have indicated that sufficient f
coolant circulation is maintained through the reactor core to prevent boron stratification.
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The licensee stated that, for the range of temperatures under consideration, boron precipitation is considered to be unlikely unless a very highly j
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concentrated boron source (much greater than 3.5 weight percent boric acid) l_l i
inserts large amounts of boron into the RCS.
LCO 3.1.2.7 limits the maximum 4
concentratior,in the boric acid makeup tanks to 3.5 wt. % (6,119 ppm boron)-
f thus, assurance is provided that boron in the RCS will not achieve r
concentration at which precipitation would occur. Boron precipitation caused by dilution with cold water is also not expected to occur for the boron il concentrations required during MODE 6 refueling operations.
The licensee stated that, assuming the RCS is well mixed prior to entering the reduced flow configuration, stratification is not considered credible at the proposed flow rate without the occurrence of boron precipitation. Diffusion in a stagnant region is the only other mechanism for stratification and, based j
on an assessment of the diffusion coefficients, would need greater that four months to develop (well beyond the time scale of interest).
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Based on the above, it is concluded that boron stratification would not occur for the conditions assumed in this analysis.
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3.4 Guaranteed Minimum Flow The minimum flow rate assumed in the analysis was 1,800 gpm. The flow rate j
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corresponding to one LPSI pump injecting reactor coolant into the RCS through one line of maxiium resistance is calculated by the licensee to be approximately 2,000 gpm. Therefore, since this value is greater than 1,800 gpe, there is assurance that a single point injection will satisfy the value of flow rate assumed in the analysis.
4.0 CONCLUSION
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Based on the staff evaluation in Section 3.0 above, the staff concludes that the licensee proposed revision to the Technical Specifications for the St.
j Lucie Unit 2 pla,nt to allow a reduction in the required minimum shutdown cooling flow rate under certain conditions during operation MODE 6 are acceptable.
The staff has concluded, based on the considerations discussed above that: (1) there is reasonable assurance that the health and safety of the public will f
not be endangered by operation by operation in the proposed manner, and (2) such activities will be conducted in compliance with the Commission's regulations, and issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
5.0 REFERENCE 1.
Letter from D. A. Sager, Florida Power & Light Company (FPL), to USNRC, February 27, 1995.
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ATTACHMENT 2 SYSTEMATIC ASSESSMENT OF LICENSEE PERFORMANCE 3
FACILITY NAME St. Lucie Unit 2 SUPMARY OF REVIEW Florida Power & Light Company (FPL) submitted changes to the Technical Specifications.for the St. Lucie Unit 2 plant. These changes are a result of
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proposed modifications to revise surveillance requirements to allow a reduction in the required minimum shutdown cooling flow rate under certain conditions during operation MODE 6.
The Reactor Systems Branch (SRXB) has reviewed the proposed changes within the SRXB review scope and concludes that the licensee proposed changes are acceptable.
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NARRATIVE DISCUSSION OF LICENSEE PERFORMANCE - SAFETY ASSESSMENT /0VALITY VERIFICATION The licensee showed an understanding of the technical issues from a safety standpoint and used acceptable approaches to resolve them.
AUTHOR:
H. Baluk.iian DATE:
4/25/95
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