ML20246D550

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Safety Evaluation Accepting Util Submittals on Natural Circulation Cooldown for Unit
ML20246D550
Person / Time
Site: Beaver Valley
Issue date: 04/26/1989
From:
Office of Nuclear Reactor Regulation
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References
NUDOCS 8905100260
Download: ML20246D550 (5)
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[, g NUCLEAR REGULATORY COMMISSION 7,,

'E WASHINGTON, D. C. 20555 l

1 ENCLOSURE i q

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION K

RELATED TO NATURAL CIRCULATION COOLDOWN BEAVER VALLEY P0hdR STATION UNIT 2 DUQUE5NE LIGHT COMPANY {

DOCKET NO. 50-412 j l

1.0 INTRODUCTION

Branch Technical Position (BTP) RSB 5-1, " Design Requirements of the Residual Heat Removal (RHR) System," requires that test programs for pressurized water reactors (PWRs) include tests with supporting analysis to (1) confirm that ,

adequate mixing of borated water added prior to or during cooldown can be  !

achieved under natural circulation conditions and permit estimation of the times required to achie,'e such mixing, and (2) confirm that the cooldown under natural circulation conditions can be achieved within the limits specified in the emergency operating procedures. In addition, the plant is to be designed so that the reactor can be taken from normal operating conditions to cold shutdown using only safety-grade systems. A comparison of performance to that of previously tested plants of similar design may be substituted for these tests. >

Beaver Valley Power Station Unit 2 (BVPS-2) is classified as a Class 2 plant with regard to the implementation of the above BTP. i A natural circulation / boron mixing /cocidown test was performed at Diablo Canyon Unit 1 on March 28-29, 1985. By letter dated March 3, 1987, the staff informed the Diablo Canyon Unit 1 licensee that its systens meet the intent of BTP RSB 5-1 for a class 2 plant.

By letter dated May 11, 1987, the licensee for SVPS-2 submitted a Westinghouse l analysis to show the applicability of the Diablo Canyon cooldown test results to BVPS-2, rather than conduct such a test at its plant. The Westinghouse analysis, WCAP-11461, " Beaver Valley Unit 2, Natural Circulation Boron Mixing .

Evaluation Program Report," supports FSAR Section 5a, which relates to the capability of BVPS-? to successfully achieve cold shutdown conditions under the requirements of BTP RSB 5-1.

By letter dated January 3,1989, the licensee provided a list of FSAR ,

references and an analysis to support the estimated time to borate the RCS until cold shutdown conditions were reached. 4 The staff safety evaluation for Diablo Canyon Unit 1, with its associated Brookhaven National Laboratory technical evaluation report entitled " Technical Evaluation Report for Diablo Canyon Natural Circulation, Boron Mixing, and i Cooldown Test," identified the plant parameters that may affect application of J the test results to other plants. These parameters are the basis for the evaluation and are discussed below.

890S100260 890426 ~

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i- J 2.0 EVALUATION 2.1 Natural Circulation Diablo Canyon Unit 1 is rated at 3338 megawatts thermal (MWt) and has four  ;

loops in its reactor coolant system (RCS). BVPS-2 is rated at 265? MWt and l has three loops in its RCS. The general configuration of the piping and l components in each reactor coolant loop is the same for BVPS-2 and Diablo Canyon Unit 1. Both plants have the Model 51 steam generators and Model 93A reactor coolant pumps.

i The predicted natural circulation flow versus decay heat was compared for the I two plants. The comparison, on a normalized basis, used the methodology of '

the Westinghouse Owners Group " Emergency Response Guidelines" (ERG), Revision 1, September 1983. Included were the effects of elevation, hydraulic flow resistance, and decay heat levels, which varied at power levels ranging from 1 ,

to 6 percent of full power. The results were nearly identical and demonstrate that BVPS-2 should exhibit similar natural circulation performance as Diablo Canyon with all steam generators available. To support use of the methodology, the calculations were compared with Diablo Canyon RCS startup test data, flows and loop temperature differentials (delta Ts), for power levels ranging from 1 to 3 percent and two to four active loops. The test results showed that the natural circulation flow rates and delta Ts predicted for Diablo Canyon were conservative. On the basis of the licensee's evaluation, the staff finds that the ERG methodology will conservatively predict the BVPS-2 natural circulatfor.

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flow. '

The licensee used the same methodology to evaluate natural circulation with one of the three steam generators isolated. Reducing the number of active steam generators, at the same power level that was used with all steam generators available, results in a decrease in the predicted natural circulation flow and an increase in the predicted loop delta T. To support these results, the licensee provided a summary of steam generator isolation test data from Sequoyah 1, Salem 2, and McGuire 1. The staff finds this acceptable since it is supported by test data of plants with a similar design.

RCS Cooldows The capacity of the atmospheric steam dump (ASD) valves control cooling of the RCS at a specified rate, assuming a sufficient supply of auxiliary feedwater and a subcooled RCS. Steam flow through these valves removes the sensible heat and decay heat throughout the cooldown period. At the end of the cooldown period, when the steam generator pressure is low, the capacity of the valves is most limited. The energy to be removed is determined by the water inventory and the amount of structural material in the RCS, the level of decay heat, and the cooldown rate.

BVPS-2 has four safety-grade electrical / hydraulic valves available for cooldown. Each steam generator has a single ASD valve sized so that only two of three steam generators and their ASD valves are required for cooldown to

. l residual heat removal (RHR) initiation conditions. An additional, larger capacity, ASD valve (residual heat release valve) is located in a common header that is tostream of the main steam isolation valves. Power for the residual heat release valve is provided by one Class 1E bus. Power for the other three ASDs is provided by another Class IE bus. When power from the Class 1E bus powering the three individual ASDs is lost, the coumon residual heat release valve has adequate capacity to permit cooldown to RHR initiation conditions.

The staff finds that there is reasonable assurance that the ASD valves.have the capacity to cool down the RCS to the RHR initiation temperature in a reasonable time; therefore, the ASDs are acceptable.

Bypass Flow and Upper-Head Cooling Westinghouse plants may be divided into two groups according to the magnitude of the bypass flow: Thot and Tcold plants. Sufficient core bypass flow exists in the Tcold plants to make the temperature of the upper-head fluid essentially equal to the cold-leg temperature. On the other hand, for the Thot plants, which includes Diablo Canyon and BVPS-2, the bypass flow is much less, resulting in the upper-head temperature ranging between the cold-leg and hot-leg temperatures, and raises a possibility of void formation in the upper-head region.

A potential exists for void formation in the upper-head of the reactor vessel during the cooldown/depressurization under natural circulation conditions if the upper head is relatively isolated from the rest of the RCS, or its fluid temperature remains higher than the coolant temperature in the main flow paths of the RCS. Upper-head cooling under natural circulation conditions is influenced by core bypass flow and mixing in the upper head.

BVPS-2 has a safety-grade reactor vessel head letdown path to the pressurizer relief tank (PRT), which provides a means for venting gases from the vessel head region. Intermittent use of this path to vent any gases augments RCS pressure control during a natural circulation cooldown. The staff finds this acceptable.

The licensee's emergency operating procedures that provide instructions for cooldown of the RCS under natural circulation conditions are:

ES 0.2 Natural Circulation Cooldown l

ES 0.3 Natural Circulation Cooldown With Steam Void in Reactor Vessel-With Reactor Vessel Level Instrumentation System i (With RVLIS)

ES 0.4 Natural Circulation Cooldown With Steam Void in Reactor Vessel (Without RVLIS) l Although the staff considers natural circulation cooldown without voids desirable, the staff has reasonable assurance that there is no undue safety problem during this mode of operation, since the licensee has procedures that account for cooldown with voids.

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Boron Mixing The Diablo Canyon boron mixing test evaluation demonstrated adequate boron mi,ving under natural circulation conditions when highly borated water was injected into the RCS. The mixing effect created as the flow passes through the reactor coolant pumps and the _ steam generator tubes further diffuses the boron. The ability to achieve the proper shutdown margin, however, depends mainly on the injection rate of boron relative to the total inventory of water in the RCS. During the test, the required change in concentration of about 300 ppm was achieved in'less than I hour.

The boric acid tanks (BATS) and refueling water storage tank (RWST) supply the boron for BVPS-2. Because these tanks have boron concentrations

, significantly less than those of Diablo Canyon, a larger quantity of borated water needs to be added over a longer time to reach the desired concentration change. The licensee's calculated results showed that the time required to  !

borate the RCS to reach cold shutdown conditions without letdown is about

, 1.25 hours2.893519e-4 days <br />0.00694 hours <br />4.133598e-5 weeks <br />9.5125e-6 months <br />. On-the basis of this calculation, the staff finds that there'is reasonable assurance that sufficier,t time exists to achieve boron injection and mixing within the required shutdown margin. The staff further expects that there will be adequate boron mixing at BVPS-2 because its natural circu- i lation flow is similar to that of Diablo Canyon Unit 1. '

The boric acid is pumped from the BVPS-2 safety-grade BATS by safety-grade boric acid transfer and centrifugal charging pumps and injected to the RCS either through the normal charging line and reactor coolant pump (RCP) seals, or through redundant safety-grade injection lines. The RWST provides the backup supply of boric acid.

The injection of additional mass into the RCS without letdown has the potential to increase RCS pressure during the boron mixing period. However, if both the normal and excess letdown lines are unavailable for the RCS inventory control, letdown may be accomriished by means of the safety-grade reactor head vent letdown flow path, with throttling control, to the pressurin r relief tank. The staff finds this acceptable.

Depressurization The Diablo Canyon Unit I test demonstrated that the RCS could be depressurized from cooldown conditions to the RHR initiation pressure under natural circulation flow conditions using the pressurizer auxiliary spray and/or pressurizer power-operated relief valves (PORVs). However, at BVPS-2, q depressurization will be accomplished through the use of the normal i pressurizer spray valves or the pressurizer auxiliary spray systems. These i valves are not safety grade, and thus may not be available for the RCS j depressurization per BTP RSB 5-1. The pressurizer PORVs, however, which are i safety grade and Class-1E-operated, are available for depressurization. l

, Three safety or discharging (grade pressurizer venting) PORVs arerelief to the pressurizer provided, tank of whichisonly (PRT), one for required depressurization. The PORVs are included in the environmental qualification program which was reviewed and approved by the staff (see Supplement 5 of NUREG-1057). In addition, since these valves also meet the intent of NUREG-0737, item II.B.1, the staff concludes that the PORVs are properly protected to perform the depressurization of the RCS after cooldown.

At the end of the depressurization, when the RCS is at approximately 400 psig, the RHR system may be placed in service.

The staff finds there is reasonable assurance that adequate means are available for depressurization and reaching cold shutdown conditions.

, Cooling Water At Diablo Canyon, the condensate storage tank (CST) provides the auxiliary feedwater supply to the steam generators. At BVPS-2, auxiliary feedwater to the steam generators is provided by the safety-grade auxiliary feedwater system and the condensate is drawn from the primary plant demineralized water storage tank (PPWST). Additional sources of auxiliary feedwater for BVPS-2 are the demineralized water storage tank (DWST) and the seismic Category I service water system (SWS). If the water supply from the PPWST is not sufficient, the auxiliary feed pump suction is manually switched to the DWST. j Should this backup source be unavailable, the SWS will be cross-connected to the auxilicry feedwater system.

The technical spec';fication (TS) minimum capacity of the BVPS-2 PPWST is 200,000 gallons. The capacity of the DWST is 600,000 gallons; normally it contains about 593,000 gallons. Together, the DWST and SWS provide a large

' backup supply of cooling water at BVPS-2. Therefore, the staff concludes that there is reasonable assurance that sufficient cooling water inventory exists to meet the proposed plant cooldown method.

3.0 CONCLUSION

The staff assessed the capability of BVPS-2 to meet the guidance of RSB BTP 5-1, and has identified and evaluated the plant parameters that may

affect the application of the Diablo Canyon natural circulation test results l to BVPS-2.

l l On the basis of the licensee's submittals and the staff's evaluation above, the staff concludes that the licensee has demonstrated that the Diablo Canyon natural circulation tests are applicrble to BVPS-2 and that the plant complies

! with the guidance of BTP RSB 5-1.

l Principal Contributor: Don Katze

, Dated: April 26,1939 I

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