ML20155J411

Safety Evaluation Concluding That Licensee Has Demonstrated That Diablo Canyon Natural Circulation Tests Are Applicable to Millstone Unit 3

ML20155J411
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Site:	Millstone
Issue date:	10/18/1988
From:	Office of Nuclear Reactor Regulation
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ML20155J410	List: ML20155J406 ML20155J411
References
NUDOCS 8810260093
Download: ML20155J411 (8)

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/ , ' %, UNITED STATE 8 8 NUCLEAR REGULATORY COMMIS$10N 3 $ - WA.5MNUTON, D. C. 20804

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULAl_0N PELATING TO NATURAL CIRCULATION C00LOOWN MILT. STONE UNIT 3 h0RTHEAST UTILITIES

. DOCKET NO. 50-423

1.0 INTRODUCTION

BranchTechnicalPosition(BTP)RSB5-1,"DesignRequirementsoftheResidual Heat Reooval (RHR) System," requiret that test progr6ms for pressurized water I reactors (PWRs) include tests with supporting analysis to (1) confirm that adeauste mixing of borated water added prior to or during cooldown can be achieved under natural circulation cor.ditions and permit estimation of the times required to achieve such mixing, and (2) confirm that the cooldown under 4 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.

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Hillstone Unit 3 is classified as a Class 2 plant with regard to the implementation of the above BTP.

A natural circulation / boron mixing /cooldown test was performed at Diablo Canyon Unit 1 on March 28-29, 1985. On the basis of the Diablo Canyon tests and l submittals and the Brookhaven National Laboratory (BNL) technical evaluation report (TER) dated December 1986, the staff has concluded that the Diablo Canyen Unit 1 systems meet the intent of BTP RSB 5-1 for a class 2 plant. By letter deed Noverber 6,1987, the licensee for Millstone Unit 3 submitted a 8910260093 091018 PDR ADOCK 05000423 P PDC

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Westinghouse analysis to show the applicability of the Diablo Canyon cooldewn test results to Millstone Unit 3 rather than conduct such a test at the plant.

The licensee provided the analysis en'itled t "Millstone Unit 3, Natural Circulation System Comparison," B-12983, which evaluates the capability of Millstone Unit 3 to successfully achieve cold shutdown conditions under sb reauirements of BTP RSB 5-1. The report includes t:1e following:

1) A comparison of the Diablo Canyon plant and Millstone Unit 3 to demonstrate their similarity.

2) An evaluation of the applicability of the Diablo Canyon test results to H111 stone Unit 3.

By letter dated August 3, 1988, in response to NRC staff questions, the , ,

licensee provided a revised analysis.

The staff safety evaluation for Diablo Canyon Unit 1, with the attached BNL TET entitled "Technical Evaluation Report for Diablo Canyon Natural Circulation, Boron Mixing, and Cooldown Test," identified the plant parameters that may affect application of the test results to other plants. These parameters are the basis for our evaluation and are discussed in the following sections, f 2.0 EVALUATION Natural Circulation i

Diablo Canyon Unit 1 is rated at 3338 PWt and has four loops in its reactor coolant system (RCS). Millstone Unit 3 is rated at 3411 Mwt and also has a four-loop RCS. The licent2e has stated that the general configuration of the piping and components in each reactor coolant loop is the same in both M111 store Unit 3 and Diiblo Canyon Unit 1. Significant parameters governing natural circulation are hydraulic flow resistance and thermal driving head.

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. . 3 To demonstrafe similarity in design for natural circulation, these two parameters were compared.

Data from the Westinghouse report showed that the Millstone Unit 3 hydraulic resistance coefficients at normal flow conditions was slightly 1cwer than Diablo Canyon's. Thermal driving head, however, because of a difference in steam generator tube lengths, was 5-101 higher for Diablo Canyon. The report showed that the lower natural circulation driving head '

and the lower overali piping flow resistance for Millstone Unit 3 would

> decrease the natural circulation flow ratio to approximately 0.99 times that observed for Diablo Canyon. Therefore, the licensee concluded that the natural circulation loop flow rate for either plant would be nearly the same.

Differences in reactor power and decay heat levels between the two plants are not expected to alter this conclusion.

The staff questioned the applicability of flow resist .ce at normal flow conditions when significantly lower flows would exist during natural circulation. The revised report stated that the hydraulic resistance coefficient would slightly increase at lower flows but the expected flow ratio is expected to be valid for both normal and natural circulation. The staff finds this erplanation acceptable.

RCS Cooldown The plant's ability to cool the RCS at a specified coold in nu vssuming a

,fficient supply of auxiliary feedwater, is determined ty Oh - ..scity of the atmospheric steam dump valves (ADVs). Steam flow through these valves reinoves the sensible heat and decay heat throughout the cooldown period.

The end of the cooldown period, when the ste)m centrator pressure is low, provides the most limiting ccnditions for valve capacity. The energy to be renoved is determined by the water inventory and the amovat of structural l material in the RCS, and the level of decay heat. i l

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. 4 Millstone Unft 1 has four ADVs, one for each steam generator. These ADVs, because of a non-nsfety grade air supply, cannot be given credit for plant shatdown per BTP RSB 5-1. Four motor operated, safety grade main steam pressure relieving bypass valves (MSPitVs), each of ADV capacity, ensure a steam release path in the event any ADY is unavailable. The MSPRVs are powered frem Class 1E buses.

In the event of a single failure, three steam generators would be available for coollown. In response to FSAR Question 440.24, the licensee stated that two steam g.1erators are sufficient to to cool down the RCS to the RHR initiation temperature. Therefore we find that there is reasonable assurance that the MSPRVs have n.fficient capacity to perform an RCS cooldown to the RHR initiation temperature in a reasonable time and the MSPRV capacity is therefore acceptable. In adoition, the ADVs have handwheels for ran el ope'ation and thus are potentially operab'e in the event of an insufficient air supply, i

Bypass Flow and Upper-Head Cooling A potential exists for void formation in the upper-head of the reactor tiessel during the cooldown/depressurization of the RCS under natural circulai. ion I

conditions if the upper head is relatively isolated from the res*, of the kCS and its fluid temperature remains higher than the coolatt 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.

1 Westinghouse plants may be divided into wo groups according to the magnitude of the bypass flow: Thot and Teold plants. For the Tcold plants, such as Millstone Unit 3, iffic.ient bypass flow exists to make the temperature of the upper head fluid . ally e9.sa' to the cold-leg temperature. On the other hand, for the Tho' su , whi- s des Diablo Canyon, the bypass flow is e rcumstance results in upper head

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much smaller. Fs e temperature r.ing' V t w

eg and the hot-leg temperatures and

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raises a possiteitit -

ion in the upper-heed region.

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5 The licensee stated that the reactor vessel spray nozzle between the downcorer and the upper-head region for Millstone Unit 3 has a significantly larger flow area than that of Diablo Canyon. This circumstance allows better flow comunication and mixing in the upper head during natural circulation. The upper head volume for Millstone Unit 3 is larger than that of Diablo Canyon.

The NRC staff considers the upper head volume effect on cooling of Die upper head to be small compared to the contribution of flow through the spray nozzles. We would therefore expect a shorter cooling time for a Tcold plant compared with that of a Thot plant of the same size.

Boron Mixing The Diablo Canyon boron mixing test evaluation demonstrated adequate boren mixing under natural circulation conditions when highly borated water was injected into the RCS. Contributing to the diffusion of the boron is the mixing effect created as the flow passes through the reactor coolant pumps

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and the steam generator tubes. The plant's 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. The required concentration change of about 300 ppm for the test was achieved in less than 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.

Under normal operation at Millstone Unit 3, the boric acid solution is injected i

) into the RCS vie the charging and reactor coolant pump seal injection lines.

Upon loss of instrument air, charging flow control is possible by use of a f safety-related throttling flow path that bypasses the air operated charging flow control valva. A different thrattling valve, powered from a Class 1E bus can be used to throttle seal injection line flow.

1 In the event that both the nonna* and excess letdown lires are unavailable

! for the RCS inventory control, a safety grade reactor head vent letdown flow l path to the pressurizer relief tank is available.

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. 6 The source of boron for Millstone Unit 3 is the boric acid tanks (BAT 3) which have a boron concentration significantly less than that of Diablo Canyon.

Thus, addition of a larger quantity of borated water over a longer time will be required to reach the desired condntration change. The licensee calcula+ed that for Millstone Unit 3, approximately one hour is needed to achieve the same concentration change demonstrated in the test. On the basis of this calculation the staff finds that there is reasonable assurance that sufficient time exists for boron injection and mixing to achieve the required shutdown margin.

Depressurization -

The Diablo Canyon test demonstreted that the RCS could be depressurized from cooldown conditions to the RHR initiation pressure under natural circulation conditions using the pressurizer auxiliary spray and/or pressurizer power operated relief valves (PORVs).

At Millstone Unit 3 depressurization may be accomplished through the use of the pressurizer PORVs or the pressurizer auxiliary spray.' However, the pressurizer auxiliary sprsy is not safety grade, and thus is not available for the RCS depressurization per BTP RSB 5-1.

The licensee steted that either of the two PORVs is capable of providing the depressurization function, Each PORY has a safety-grade Class IE solenoid operated valve. In the event that a PORV fails open, the PORY block valves,

which are safety-grade, may be used to block the affected PORY flow path. At i

che end of the depressurization, the RCS is approximately at 400 psig. The RHR system may now be placed in service and the cooldown to cold shutdown condition cont:n'uei. <

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Cooling Water' The primary auxiliary feedwater supply to the steam generators is provided by the condensate storage tank (CST)a ' t Diablo Canyon, while Millstone Unit 3 uses the reismic category I demineralized water storage tank (DWST).

Alternate sources of auxiliary feedwater at Hillstone Unit 3 include the CST, service water system, and the domestic water system. Spoolpieces, maintained on site, must be added to connect the service water and domestic water systems to the auxiliary feedwater system.

The BNL TER estimated a 360,000-gallon auxiliary feed water requirement for Diablo Canyon on the basis of e 43-hour cooling time for the upper head.

This calculation was based on assumptions of no heat loss from the upper head to the containment and a limited amount of bypass fluid mixing with fluid in the Lottom of the uppar head. We would further conclude that had Diablo Canyon been a Tcold plant, and with heat transfer from the upper head to containment considered, the cooling requirement would have been eig,,if t:rtly less than 360,000 gallons.

The Millstone Unit 3 DWST has a capacity of 340,000 gallons with a technical specification (TS) uinimum capacity of 334,000 g:llons. In addition, the alternate or backup supply provides an essentially unlimited auxiliary feedwater supply. Since Millstone Unit 3 is a Tcold plant, we conclede 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 Millstone Unit 3 to meet the requirements of RSB BTP 5-1. We have identified and evaluated the plant paran ters that may affect application of the Diablo Canyon natural circulation test results to Millstone Unit 3. l l

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On the basis'of the licensee's submittals, and our evaluation as previously discussed, we conclude that the licensee has demonstrated that the Diablo Canyon natural circulation tests are applicable to Millstone Unit 3 and that they comply with the requiremerits of BTP RSB 5-1.

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