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Forwards Request for Addl Info for Class 2 Plants That Ref Diable Canyon Tests on Natural Circulation,Born Mixing & Cooldown,Per Branch Technical Position Rsb 5-1, Design Requirements of RHR Sys
	ML20204J074
Person / Time
Site:	McGuire, Mcguire
Issue date:	03/23/1987
From:	Hood D; Office of Nuclear Reactor Regulation
To:	Tucker H; DUKE POWER CO.
References
	NUDOCS 8703270166
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' 'Mr. H. B. Tucker, Vice President

' itticar Production Deosrtment '

Duke Power Company 422 South Church Street

Charlotte, North Carolina 28242 ,

Dear Mr. Tucker:

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Subject:

Natural Circulatior., Boron Mixing and Cooldown Capabilities -

McGuire Nuclear Station, Units 1 and 2

References:

(1) WCAP 11086, "Diablo CanVon Units 1 & 2, Natural Circulation / Boron Mixing /Cooldown Test, Final Post Test Report," March 1986 (Proprietary).

(2) WCAP 11095, "Niteral Circulation, Baron Mixing and Cooldown Test, Final Post Test Report," March 1986,

- PG&E, Diablo Canyon (hon Proprietary).

s NPbBranchTechnicalPositionRSB5-1,"DesignRequirementsofResidualHeat Removal System" requires that a natural circulation test with suooorting a ~ analysis be conducted to demonstrate the ability to cooldown and deoressurize the plant and to demonstrate that boron mixing is sufficient under natural circulation conditions. As noted in Supolement 4 of the McGuire SER, you

! referenced tests to be conducted at Diablo Canyon to meet part of this recuirement. The Supplement also noted that Staff conclusions on the results of the Diablo Canyon test and supporting analysis to satisfy BTP RSB 5-1 re-quirements would be deferred until the Diablo Canyon results had been reviewed.

The Diablo Canyon tests (references 1 and 2) have now been completed; and the staff and its cont: actor, Brookhaven hational Laboratory (BNL), have reviewed and accepted those results. Enclosure 1 transmits the staff's SER on Diablo Canyon, including as an enclosure the BNL report. The SER includes a section entitled " Applicability to Other Plants." Natural circulation tests were also performed at North Anna 2, Farley 2, Salem and Sequoyah.

Thus, the staff is ,orepared to resume its review for McGuire. Additional in-formttion, identified in enclosure 2, is needed to this end. Where the e 8703270166 970323

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7./ ' s Mr. H. B. Tucker MAR 2 31987 infonnation was previously provided, reference to your previous response is acceptable. We request, however, that you indicate the current validity of such prior response and include any appropriate updating. Your. reply is re-quested within 60 days of this letter.

t Contact me at (301) 492-8961.if you have questions.

Sincerely,-

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.h Darl Hood, Project Manager -

PIIR Project Dirtetorate #4 Division of PWR Licensing-A

Enclosures:

(1) NRC letter on Diablo Canyon with 2 enclosures

-(2) Request for Additional Information cc: See next page eDISTRIBUTION:

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Mr. H. B. Tucker information was previously provided, reference to your previous response is acceptable. We request, however, that you indicate the current validity of such prior response and include any appropriate updating. Your reply is re-quested within 60 days of this letter.

Contact me at (301) 492-8961 if you have questions.

Sincerely, A R L- OC I Darl Hood, Project Manager PWR Project Directorate #4 Division of PWR Licensing-A

Enclosures:

(1) NRC letter on Diablo Canyon with 2 enclosures (2) Request for Additional Information cc: See next page t

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Mr. H. B. Tucker Duke Power Company McGuire Nuclear Station CC:

Mr. A.V. Carr, Esq. Dr. John M. Barry Duke Power Company Department of Environmental Health P. O. Box 33189 Mecklenburg County 422 South Church Street 1200 Blythe Boulevard Charlotte, North Carolina 28242 Charlotte, North Carolina 28203 County Manager of Mecklenburg County 720 East Fourth Street Charlotte, North Carolina 28202 Chairman, North Carolina Utilities Commission Mr. Robert Gill Dobbs Building Duke Power Company 430 North Salisbury Street Nuclear Production Department Raleigh, North Carolina 27602 P. O. Box 33189 Charlotte, North Carolina 28242 Mr. Dayne H. Brown, Chief Radiation Protection Branch J. Michael McGarry, III, Esq. Division of Facility Services Bishop, Liberman, Cook, Purcell Department of Human Resources and Reynolds 701 Barbour Drive 1200 Seventeenth Street, N.W. Raleigh, North Carolina 27603-2008 Washington, D. C. 20035 Senior Resident Inspector c/o U.S. Nuclear Regulatory Commission Route 4, Box 529 Hunterville, North Carolina 28078 Regional Administrator, Region II U.S. Nuclear Regulatory Commission, 101 Marietta Street, N.W., Suite 2900 Atlanta, Georgia 30323 L. L. Williams Area Manager, Mid-South Area ESSD Projects Westinghouse Electric Corporation MNC West Tower - Bay 239 P. O. Box 355 Pittsburgh, Pennsylvania 15230

4 EhCLOSURE 2 Reauest for Additional Information for Class 2 Plants that Reference Diablo Canyon Tests on Natural Circulation, Boron Mixino and Cooldown The staff requires that licensees / applicants referencing the Diablo Canyon test (WCAP 11086 & 11095) be able to demonstrate the thermal and hydraulic similarity of their plants with the Diablo Canyon olant which includes the general arrangement of piping and components, elevation heads, and the flow paths in the reactor core, downcomer, upper head, guide tubes, and flow nozzles between the downcomer region and upper head region. The comparison should include hydraulic resistance coefficients, predicted flow rates, and upper head temperatures. Major differences in the CVCS, RHRS, balance of plant systems, and safety-grade equipment utilized during natural circu-lation cooldown should be included. Each plant must also demonstrate that an adequate safety grade water supply is available for secondary makeuo during natural circulation cooldown without offsite power.

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UNITED STATES i~ ,. ( e NUCLEAR REGULATORY COMMISSION waswsNcTow.o c.rosss

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Docket kos. 50-?75 and M-323 Wr. J. D. Shiffer, Vice President Nuclear Power Generation c/o Nuclear Power Generation Licensinc Pacific Gas and Electric Company 77 Peale Street, Room 1451 San Francisco, California 94106

Dear Wr. Shiffer:

SUS.!ECT: DIABLO CANYON CONF 0DFANCE WITH RPANCH TECHNICAL POSITION RSS 5-1 REGAPDINr, NATURAL CIRCULATION, ROP 0f; MIXING, AND C00LDO F he have completed our review and evaluation of the Diablo Canyon Unit I natural-circulation, baron mixing, and cooldown test as described in WCAP-11095 (Non-Proprietary) and WCAF-11096 (Proprietaryl reports transritted to us by your sletter dated March 25, 1986. We were assisted in our effort by our consultant, Brookhaven National Laboratory (BNL), who parfomed a simulation cf the test utili. zing only safety grade equipment. Our evaluation is enclosed, inclucirc

, as an enclosure the BNL report.

As a result of our evaluation and the BNL evaluation we conclude that the besic ob.iectives of the test performed at Unit I have been met. We further conclude, that the test can be applied to Unit 2 and that both units meet the inter.t cf our Branch Technical Position PSE 5-1, " Design Requirements of Residual Heat Peroval System

for Class 2 plants. The evaluation of this matter is, there' ore, complete.

4 It is our understanding that other Westinghouse plants will rely on the Diable Canyon test and will reference it regarding RSB 5-1. As stated in our evaluation, because of certain differences between the Diablo Canyon units and other facilities.

further information will be required from utilities for those facilities in

' . order to .iustify applicatfor of the Diablo Canyon test to their plants. We have informed Westinghouse and the Westinghouse Owners Group of this position.

Sincerely, (Lerb

Hans Schierling, Senior Project Manacer Project Directorate #3 Division of PWR Licensino-A

Enclosure:

As stated cc: See ext n\ pa e nc0%4p

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Mr. J. D. Shiffer Diablo Canyon Pacific Gas and Electric Company cc:

NRC Resident Inspector Richard F. Locke, Esq. Diablo Canyon Nuclear Power Plant Pacific Gas & Electric Company. c/o U.S. Nuclear Regulatory Comnissien Post Office Box 7A42 P. O. Box 369 San Francisco, California 94120 Avila Beach, California 93424 Janice E. Kerr, Esq. Mr. Dick Blakenburg California Public Utilities Commission Editor & Co-Publisher 350 PcA111 ster Street South County Publishing Company San Francisco, California 94102 P. O. Box 460 Arroyo Grande, California 93420 Ms. Sandra A. Silver Bruce Norton, Esq.

660 Granite Creek Road c/o Richard F. Locke,'Esq.

Santa Cruz, Califernia 95065 Pacific Gas and Electric Corpany Pest Office Box 7442 San Francisco, California 94120 Mr. W. C. Gangloff Westinghouse Electric Corperation P. O. Box 355 Dr. R. R. Ferguson Pittsburgh, Pennsylvania 15230 Siera Club - Santa Lucia Chapter Rocky Canyon Star Route Creston, Californie 93432 Manager Editor San Luis Obispo County Telegran Tribune Chairman 1321 Johnson Avenue San Luis Obispo County Board of 1726 M Street, N.W. Supervisors Suite 1100 Room 220 Washington, DC 20036-4509 County Courthouse Annex San Luis Obispo, California 93401 Mr. Leland M. Gustafson, Manager i Federal Relations Director Pacific Gas and Electric Company Energy Facilities Sitino Division l

1726 M Street, N. W. Energy Resources Conservation and l Washington, DC 20036-4502 Development Cnmission 1516 9th Street i Sacramento, California 95814 Dian M. Grueneich, Esq. Ps. Jacquelyn Wheeler Edwin F. Lowry, Esc.

Grueneich & Lowry 2455 Leona Street San Luis Obispo, California 93400 345 Franklin Street San Francisco, California 94102 l

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Diablo Canyon ,

Pacific Gas & Electric Company Cc' Ps. Nancy Culver Ms. Laurie McDermott, Coordinator 192 Luneta Street Consumers Organized for Defense San Luis Obispo, California 934C; of Envir'onmental Safety 731 Pacific Street, Suite 42 San Luis Obispo California 93401 President California Public Utilities Commission Mr. Joseph 0. Ward, Chief California State Building Radiological Health Branch 350 McAllister Street State Department of Health San Francisco, California 941C2 Services 714 P Street, Office Building #8

" Sacramento, California 95814 Decional Administrator, Region V U.S. Nuclear Regulatory Commission 1450 Maria Lane Suite 210 Walnut Creek, California 94596 O

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g. n g ~ ENCLOSURE UNITED STATES

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NUCLEAR REGULATORY COMMISSION y .,- fj o E WeASHINGTON. D. C. 20555 s

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SAFETY EVALUATION REPOPT DIABLD CANYON UNIT 1 NATURAL CIRCULATION, FOR0h MIXING, AND COOLDOWN TEST

- DOEFET ND. 50-275 INTRODUCTION As part of the seismic evaluation of the postulated Hosgri earthquake in 197E, the licensee comitted in the Hosgri Report to perform a estural circulation, boron mixing, and cooldown test (Reference 1). Appendix J to the Hoscri Report '

provides the scenario and identification of systens and components that wculd be t

utilized for natural circulation cooldown to cold shutdown conditions following the postulated SSE. The staff addressed the test in Section 3.2.1 of its Safety Evaluation Report Supplement No. 7 in 1978 (Reference 2). The licenses conducted the test in March 1985 and provided the evaluation and results in a report (proprietary and non-proprietary version) by letter dated March 25,19f 6

' (Reference 3). The NRC staff has reviewed the report and was assisted in this effort by its consultant, Brookhaven National Laboratory (BNL). NRC sta" ard RNL met with the licensee and Westinghouse, its consultant, on November 21, 1986 to discuss the prelininary BNL evaluation (Reference 4).

This is the staff's evaluation of the test. The BNL evaluation and results of their studies are included in this evaluation as Enclosure 1.

Branch Technical Position RSB 5-1, " Design Requirements of the Residual Feat l

Removal (RPR) Systen", states that test programs for PWRs:

"shall include tests with supporting analysis to (a) confirm that adeounte 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 achieve such mixing, and (b1 l confirm that the cooldown urder natural circulation conditions can

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be achieved within the limits specified in the emergency operatine l

Drocedures. Comparison with performance of previously tested olants l of similar design may be substituted for these tests."

Therefore, as stated above, the licensee comitted to perfortn a natural circulation, boron mixing, and cooldown test at Diablo Canyon Unit 1.

! OBJECTIVES The obiectives of the test were to establish natural circulation conditions using core decay heat, confire that adequate mixing of borated water addad te the reactor coolant system (RCS) prior to cooldown can be achieved under natural circulation conditions, verify that the RCS can be horated to the enld shutdown concentration, maintain hot standby conditions under natural circulation f(

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conditions for at least 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months , determine if cooldown and depresstrization of the 905 from normal hot standby to. cold shutdown conditions can be accomplished using only safety-grade equipment, obtain reactor vessel head cooldown rates, and verify that adequate water volume is available in the condensate storace tank to. cool down the unit.

The acceptance criteria as stated in the test report (Reference 3) was as follows:

(1) The natural circulation evaluation was to verify that RCS natural circulation flow could be established, thereby pemittino boron mixing and RCS cooldown/depressurization to RHR system initiation conditions.

(?) The boron mixing evaluation was to demonstrate adequate boron mixing under natural circulation conditions when highly borated water at low -

temperatures and low flow rates f relative to RCS temperature and flow i rate) is injected into the RCS, and to evaluate the time delay associated with boron mixino under these conditions. The acceptance criterion for this phase of the test was that the RCS hot legs (Loops 1 and 4) indicate that the active portions of the RCS were borated such that the boron concentration had increased by 250 ppe or more.

(3) The acceptar: e criteria for the cooldown portion of tha test were to control plant cooldown under natural circulation conditions to be within Technical Specification limits, maintain temperature of all active portions of the RCS unifomly within 100'F of the core averace exit themocouple temperature, maintain the temperature of the steam generators and reactor vessel upper head to < 450*F when the core average exit themocouple temperature is 350*F, and assure that the RHR system is

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capable of cooling down the RCS to cold shutdown conditions.

(4) The acceptance criterion for the upper head bulk water temperature was that a 50'F subcooling margin be maintained during cooldown and depressurization. A 100'F difference between the core average exit temperature and the upper head bulk water temperature was imposed as an administrative limit.

! (5) The acceptance criterion for the depressurization portion of the test was that RCS oressure be reduced below RHR system initiation pressure 1 (390 psig).

, TEST

! The test was perfomed at Diablo Canyon Nuclear Power Plant Unit I on &cb 2F and 29, 1985. The reactor was tripped from 1001 power and the plant maintained at hot standbv. The reactor coolant pumps (RCPs) were operated for the first 3 4

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' hours and then tripped. Natural circulation flow was verified and the boron mixing part of the test was then initiated by injectino the contents of the boron injection tank (BIT). The system was maintained at hot standby under natural circulatinr. conditions for approximately 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months . Cooldowr at a rate

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of 20'F per hour was initiated using the atmospheric steam dumps (ASDs). The l

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RCS was then depressurized to RHR initiation conditions. The The time-for the RCS was combined conidown/depressurization steps was about 13 hours1.50463e-4 days 0.00361 hours 2.149471e-5 weeks 4.9465e-6 months .

then brought to a cold shutdown condition in abnut 4 1/2The hours utilizing the acceptance criteria pup svstem. A test chronoingy is included in Fnclosure 1.

for the test were met. The test was witnessed by NRC personnel.

It is noted that some non-safety grade systems and components were utilized durino the test. These included the letdown system, 3 control rod drive The mechanism (CD M fans, pressurizer heaters and volume control tank IVCT1.

use of the CRDM fans was required to maintain the CRDM temperatures within acceptable limits. However, in the event of loss of offsite power (LOOP) because of the SSE or for other reasons, the fans would not be available during the cooldown. This has a ma.ior impact on upper head cooling. The letdown system was used to prevent overfilling the pressurizer since RCD seal in,iection was maintained during the test. The safety-orade reactor vessel head vent could have been used as an alternate means of letdown but its use could have entailed potential discheroe of reactor coolant to the containment.

Contraction of the coolant volume during plant cooldown would also tend to r'iticate the effects of seal iniection. The safety arade refueling water storage tank (RWST) could have been used as an alternate to the VCT but the RWST contains high levels 'of dissolved oxygen and its use could have resulted in exceedino technical specification oxygen concentration limits which in turr could have resulted in excessive localized corrosion and consecuent increased radiation exposures to plant workers.

EVAlf!ATION In the event of an SSE, the operator would not have nortnal system capability for RCS pressure control. Pressure reduction could be achieved by the seismically qualified FORVs or, within thermal stress limits, by the auxiliary pressurizer spray. The pressurizer heeters are not seismically cualified,The but two of the four heater groups can be manually powered from vital buses.

charging pumps could probably be used to maintain or increase pressure, but this could result in pressurizer overfill. With regard to the delay in tripping the RCPs the licensee stated that this would ensure a more stable candition so that the test could be properly conducted. The delay in the PCP trip allowed PCS temperature to become more uniform, including some reduction in the upper head temperature. The delay also reduced the level of decay heat c

somewhat. As noted in Enclosure 1, this slichtly reduced the natural circulation flow and increased the boron mixing time. It also allowed the upper head temperature to become more uniform.

The Diablo Canyon Plant emergancy operating procedures (EOPs) are based on the Westinghouse Owners Group Emeroency Response Guidelines (ERGS), which assure the use of nnrmal operation systems. Peference 3 identifies the systems that would be normelly used for natural circulation cooldown. It also identifies alternate seismically cualified systems that could be utilized in the event the normal systems are incapacitated, and demonstrates how the necessary functions would be achieved. The effect of CRDP fan unavailatdlity is discussed belev.

In Reference 3, the licensee also comitted to develop alternative operational strategies to provide the operational guidance and technical basis to demonstrate that the Diablo Canyon plant can be taken from normal operating conditions tr cold shutdown using only seismically cualified systems.

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In support of the staff evaluation of the Diablo Canven Unit I test, the staff consultant Brookhaven National Laboratory (BNL) performed test simulation analyses as reported in a Technical Evaluation Report (TEP', included as Enclosure }. The RELAP5/ MODI code was utilized. The secuence of events assumed b," PM in the analysis differed snmewhat from the test. As noted in Enclosure 1.

the purpose of the BNL analysis was not to duplicate the test but to provide the information necessary to assess the impact of the use of non-safety grade equipment durino the test. Reasonably coed agreement between the test data anc analytical results were obtained for RCS natural circulation flow and temperature.

Since the BNL analysis did not assume utilization of the pressurizer heaters and the letdown system, it is difficult to compare RCS pressure test data and analytical results.

The laroest difference between the test and analytical results were obtained for reactor upper vessel head cooling time. The CRDM fans were operated durine the entire test, except for a 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months period. The use of the CRDF fans provided adequate cooldwn o' the upper head. The maximum temperature differential However, the CRDM fans are between the RCS and upper head temperature was 40*F.

not safety grade. Since the Piablo Canyon Plant is a T-HOT plant, the upper head temperature is near the RCS hog leg temperature during normal operatier.

because the bypass flow rate between the upper downtomer and the upper head is relatively low. As noted in Reference 5. for T-HOT plants without CRDV fan operation, a waiting period (soak time) is recuired before the RCS is depressurized to RHP entry conditions. This period is P hours for top hat upper support plate plants, which include the Diablo Canyon Plant. The BNL calculations, on the other hand, indicate a required waiting period of about 35 hours4.050926e-4 days 0.00972 hours 5.787037e-5 weeks 1.33175e-5 months . These calculations were done conservatively by dividing the upper head into a heat

- conduction nodes, with the upper head fluid assumed completely stagnant.

Conduction was the only mechanism assumed for cooldown, the heat loss from the dome to the containment environment was ignored, and the bypass fluid r.ixed only with the fluid in the bottom of the upper head. During the test all CRDV fars were turned off for about 100 minutes. The average upper head cooldown rate was estimated to be approximately 6*F per hour, which translates into about a 25 hour2.893519e-4 days 0.00694 hours 4.133598e-5 weeks 9.5125e-6 months hold period. However, the time period for the test without CRDM fans was too short to be conclusive.

Reference 3 states that 126,000 callons of water from the condensate storate

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tank (CST) was used as auxiliary feedwater (AFW) makeup for plant cooldown.

However, with the CRDM fans unavailable, the BNL calculations conservatively result in a 360,000 gallon secondary water makeup requirement. The Diablo Canyon CST has a volume of 400,000 gallons, of which 178,000 gallons are dedicated for AFWS supply. Additionally, 270,000 gallons of water are maintained in the fire water storage tank for AFWS supply. As stated in the FSAP (Reference 61 the fire water storace tank and the pipino between it and the CST are Seismic Cateenry I. The staff concludes that for the Diablo Canyon Plant a sufficiert assured water supply is available for plant cooldown via the steam generators even when the CPDF fans are not available.

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CONCLtISIONS Based on the Diablo Canyon Unit I test results (Peference 3) and their analyses reported in Enclosure 1. BNL concluded that:

1) The Diablo Canyon Unit I test demonstrated that adequate natural circulatior was established and the plant was capable of removing the decay heat by natural circulation using only safety-grade equioment.

2) Adeounte boron mixing was achieved during natural circulation in the main flow path of the RCS using only safety-grade equioment.

3) The effect of relatively unborated water entering the RCS from the upner head and pressurizer appears to be minimal as long as depressurization is conducted carefully to limit the size of possible void fomation.

Ai The pressure would rise and reach the PORV actuation pressure without letdown during the boron mixing period.

5) The test adeountely demonstrated that the RCS can be cooled to the RHR system initiation temperature while maintaining adequate subcooling durino natural circulation using only safety-grade eouipment.

6) The test demonstrated that the upper head could be cooled without void fomation when the CRDM fans were in operation.

7) The tests results indicate that the upper head cooldown rate without the

. CRD". fans is about 6'F per hour. This is higher than the conservative BNL calculation based only on conduction heat loss, which estimated a minimum rate of 3*F per hour.

81 The PCS pressure should be maintained about 1200 psie by means of either the pressurizer heaters (if available) or chargina during the conidowr.

period to prevent upper head voiding when the'CDDM fans are not in operation.

9) A sufficient supply of safety grade coolino water was available to support the proposed plant cooldown method even i# the CRDM fans were not available for the Diablo Canyon Plant.

10) Only one motor-driven AFW pump was sufficient to supply the necessary cooling water throughout the transient. ,

11) Sufficient ASD valve capacity was available to support the cooldown ever, when the co61down rate was assumed to be 50'F per hour.

12) The availability of the cressurizer heaters and letdown system, while not essential, would affect the operational procedures in a major wa_v. The strateqv to reduce the upper head cooling time by intentionally for-ino e void may be difficult to perfom without pressurizer heaters.

3P The RCS pressure would increase and stay high, and the PORV may be actuated periodically if the letdown system were not available, due te '

boron in.iection and the continuous in.iection of RCP seal flow. The operatinn of the auxiliary pressurizer spray normally requires leidowr to be in operation to prevent possible ther:ral stress on the spray nozzle.

References 1 and 3 contain single failure analyses demonstrating redundancy of safety grade systems that would be utilized following a seismic event. BNL has independently verified that adequate cooldown could be accorplished with feilure of one AFW pump or ASD. The Diablo Canyon Plant design provides a single RHR drop line with two inlet isolation velves in series. In response to a staff request to provide .iustification that the erobability of mechanical failure of either of the two valves is sufficiently low as to not merit consideration as a j single failure, the licensee stated that the combined failure coincident with the SSE is on the order of 10'9robability(of per year Reference valve O . ster-i The licensee has also indicated that failure of a power train or valve operator could be mitiaated by local operator action (Reference 1).

l The staff concludes, therefore, that based upon the licensee's submittals ar.d the RNL analysis, the Diablo Canyon Unit I natural circulation, boron mixing and cooldown test ad*Quately demonstrates that the Diablo Canyon Plant systems meet ,

the intent of PTP PSB 5-1 for a class ? plant. .

APPLICAPILITY TO OTFER PLAf;TS The Diablo Canyon Unit 3 test has been referenced by a number o' near-term-operatina-license (NTOL) plants and recently licensed Westinghouse plants.

Several of these plants have a limited safety grade supply for the AFW syster.

Also, some plants have different design upper vessel heads which contain much larger volumes of relatively staanant water. It is, there' ore, appropriate to perform more realistic calculations for upper head cooldown with only safety grade systems, in order to provide assurance that each plant in this category has a sufficient volume of safety grade water supply.. The staff has, therefore, requested additional information from Westinghouse with recard to the upper tead mixing phenomena, convection heat losses, and other pertinent items (Reference 7).

t If adequate infnrmation on these sub.iects is obtained, PNL could reanalyze upper head cooling in order to obtain more realistic cooldown times. The results of such reanalysis would be documented appropriately.. While the staff considers l natural circulation cooldown without voids as more desirable, cooldown with l voids may be acceptable provided it can be accomplished using only safety crede eouipment (including adequate instrumentation), approved procedures, and the operators have adequate trainino in the use of these procedures. If the use of I safety erade head vents is contemplated in order to vent the steam in the upoer head and/or enhance upper head mixing, due consideration should be given to the effect of this nperation on the integrity of the pressurizer relief tank and the effect of loss of its intecrity.

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h equiraments It is the Diablo intent Canynn UnitofI test a number of recent to demonstrate conformancelitersees i

and with the test NTO ng The staff recuires that licensees / applicants referencing the lic similaritv of PTP RSP 5-1. l demonstrate '

Diablo Canyon Unit 1 test bel forable secondary Ir addition tomakeuo demonstrate that an adequate safety orade water supply is availab e heed during natural circulation cocidown without offsite power.

Westinchnuse should provide the details of itsh estimation P hour hnid for the upper cooline time without the CRDF f ans.that the cooling ceriod should period estimated by Westinghouse.

In ordar to facilitate the staff's evaluationhich of identifies this matter, plant the BNL

included as Enclosure 1, includes a sensitivity analysis wlts to other Westinch parameters that may affect application of the test resu lts to these ~

plants, and provides estirates of the sensitivity in naturalo' the resuTab parameters.

circulation flow to these parameters in terms o' 1percent parametar.change l is subiettive circulatinn flow to a 10% change from the Diablo Ca and may vary from plant to plant.

DP1NCIPAL CONTRIBUTOPi l

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ENCLOSURES RFFERENCES (1) PG4E Report " Seismic Evaluation For Postulated 7.5M Hosgri Earthouake, Units 1 & 2, Diablo Canyon Site". Appendix J " Systems / Equipment For Achieving & Maintaining Hot Standby & Cold Shutdown of Diablo Canyon Units 1 & 2 Followino SSE", March 1978.

(24 U.S Nuclear Regulatory Commission, NUREG-0675, " Supplement No. 7 to the Safety Evaluation Report of the Diablo Canyon Nucler Power Station Units 1 and 2", May 1878.

(3) VCAD-11095 (Non-Proprietary 1, " Natural Circulation, Poron Mixine, and Cooldown Test - Final Post Test Report for Diablo Canyon Power Plant '

Units 1 and 2, March 1986", Letter DCL 85-078 from J. D. Shiffer (PGtE) to S. A. Varga (NRC) dated March 25, 1986.

WCAP-11096 (Proprietary) - same as above but proprietary (4) "INovember Meeting21,Summary -) Natural Circulation, Roron9,1986.

Mixing, and Cooldowr.

1986 ", H. Schierling (NPC1, dated December (Si Westinghouse Owners Group Emergency Response Guidelines, "Backgrcund Information for the Westinghouse Emer9ency Response Guidelines, ES-0.?,

Natural Circulation Cooldown."

(6) Diablo Canyon FSAR Update, Revision 1 September 1925.

(7) "Recuest for Additional Information Needed to Evaluate Reactor Vessel Upper Head Coolino Durino Natural Circulation Cooldown", Letter from C. P. Berlinger (NRC) to'D. Putterfield (WOG), January 15, let;,

ENCLOSURES

1. " Technical Evaluation Report for Diablo Canyon Natural Circulation, Boror Mirino, and Cooldown Test" J. H. Jo, r.. R. Perkins, and N. Cavlina, Brookhaven National Laboratory Technical Report A-3643, dated December 23, lop 6.

ENCLOSURE

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CHNICAL REPORT l

A B43 12-23-86 1

' TECHNICAL EVALUATION REPORT FOR DIABLO CANYON NATURAL CIRCULATION, BORON MIXING, AND C00LDOWN TEST

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J.H. J0, K.R. PERKINS, AND N. CAVLINA CONTAINMENT & SYSTEMS INTEGRATION GROUP l

DEPARTMENT OF NUCLEAR ENERGY, OR00KHAVEN NATIONAL LABORATORY ,

UPTON, NEW YORK 11973 ,

Prepared for the U $ Nuclear Re0ulatory Commission

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Office of Nuclear Reactor Regulation

. Conteact No DE.AC02 76CH00016 Y

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TECHNICAL EVALUATION REPORT FOR DIABLO CANYON NATURAL CIRCULATION, BORON MIXING, AND C00LDOWN TEST J.H. Jo and K.R. Perkins Containment A Systems Integration Group ,

N. Cay 11nat Department of Nuclear Energy Brookhaven National Laboratory Upton, New York 11973 December 1986 Prepared for U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Contract No. DE-ACO2-76CH00016 FIN A-3843 tlAEA Fellowship from the University of Zagreb, Yugoslavia

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TABLE OF CONTENTS Page LIST OF TABLES........................................................ v LIST OF FIGURES....................................................... vi

1. INTRODUCTION...................................................... 1-1

2. TEST DESCRIPTION.................................................. 2-1

3. SIMULATION OF THE TEST............................................ 3-1 3.1 Ge ne ral D e s c ri p t i on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 3.2 Natural Circulation.......................................... 3-2 3.3 Boron Mixing................................................. 3-5 3.4 Coo 1down..................................................... 3-6 3.5 Depressurization............................................. 3-8 4 REVIEW OF TEST RESULTS............................................ 4-1 4.1 Nat u ra l Ci rcul ati on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.2 Boron Mixing................................................. 4-2 4.3 Cooldown..................................................... 4-4 4.4 Depressurization............................................. 4-5 4.5 Reactor Vessel Upper Head Cooli ng. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5 4.5.1 Cooling with CRDM Fans Operating...................... 4-6 4.5.2 Cooli ng Without CRDM Fans Operati ng. . . . . . . . . . . . . . . . . . . 4-7 4.6 Cooling Water and Compressed Air Requi rement... . . .. .. . . . . . . . . 4-9 4.7 Effect of Non-Safety Grade Systems Used in the Test.......... 4-10

5. SENSITIVITY ANALYSIS.............................................. 5-1 5.1 Na tu ra l Ci rcul a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 5.2 Boron Mixing................................................. 5-1 5.3 R C S C o o l d ow n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2 5.4 Depressurization............................................. 5-3 5.5 Up p e r He a d C ool i n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3 5.6 Cooling Water................................................ 5-4

, 5.7 S u mma ry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4 l

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6. SUMHARY AND CONCLUSION............................................ 6-1

7. REFERENCES........................................................ 7-1 Appendix A - NATURAL CIRCULATION FL0W................................. A-1 4

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LIST OF TABLES Table Page 2.1 Chronol ogy of Ev ents and Operator Actions. . . . . . . . . . . . . . . . . . . . . . 2-2 3.1 Comparison of the RELAPS Estimated Steady State Conditions with the Plant Steady State.................................... 3-10 3.2 Sequence of Events f o r the Simulation. . . . . . . . . . . . .. . . . . . . . . . . . . 3-11 5.1 Summa ry of the Sens i t i vi ty Analysi s . . . . . . . . . . . . ... . . . . . . . . . . . . . 5-6 I

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LIST OF FIGURES Figure Pace 3.1 Noding vessel diagram........................................... 3-12 3.2 RCS f1ow........................................................ 3-13 3.3 Nat u ral ci rcul ati on fl ow rate vs . time. . . . . . . . . . . . . . . . . . . . . . . . . . 3-14 3.4 The calculated RCS temperature (20'F/hr cooldown)............... 3-15 -

3.5 RCS pressure and pressurizer level (20'F/hr cooldown)........... 3-16 3.6 RCS tempe ratu re f o r the t est . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17 3.7 Te st p re ssu re and pres su ri zer l e ve1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18 3.8 Bypass f1ow..................................................... 3-19 3.9 Bo ro n co nc e nt ra t i on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20 3.10 RCP pressure and pressurizer level (20'F/hr cooling)............ 3-21 3.11 Hot leg and saturation temperature of test and calculation -

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F / h r c o ol d ow n ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22 3.12 RCS temperature and saturation temperature with 50'F/hr coo 1down........................................................ 3-23 3.13 RCS pressure and pressur.izer level (50'F/hr cooldown). .. .. . . .. . . 3-24 3.14 . Atmosphe ri c stean dump val ve openi ng. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-25 3.15 Ac cumul at ed cool i ng wa t e r. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-25 4.1 Margin of subcooling in the upper head with CRDM fans in operation....................................................... 4-12 4.2 Upper heat temperature when heat loss is due to conduction only (2 5'F/ h r cool down of RCS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-13 4.3 Upper heat temperature and saturation temperature of RCS pressure with 20'F/hr coo 1down.................................. 4-14 O

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1. INTRODUCTION While cooling down under natural circulation conditions )n June 11, 1980, St. Lucie Unit 1 coolant flashing produced a void in the reactor vessel upper head and forced water into the pressurizer. The reactor was successfully brought to cold shutdown. Based on the NRC review of the event, a multi-plant action item (MPA B-66) was initiated which requires that all PWRs implement procedures and training programs to ensure the capability to deal with such events. In Generic Letter (GL) 81-21, dated May 5,1981', the licensees were r quired to provide an assessment of their facility procedures and training program including:

! 1. a demonstration (e.g., analysis and/or test) that controlled natural circulation cooldown from operating conditions to cold shutdown con-ditions, conducted in according with plant procedures, would not re-sult in reactor vessel voiding;

2. verification that supplies of " condensate-grade" auxiliary feedwater are sufficient to support plant cooldown methods. (Note: Branch

! Technical Position RSB 5-1 requires an adequate supply of auxiliary feedwater stored in safety grade systens.)

l 3. a description of plant training program and the provisions of emer-gency procedures (e.g., limited cooldown rate, response to rapid change in pressurizer level) that deal with prevention or mitigation of reactor vessel voiding.

It should be noted that at the time GL 81-21 was issued, procedures for natural circulation cooldown with upper head voids were not generally availa-ble. Since then, the Westinghouse Owners' Group has issued emergency response guidelines (ERGS) for natural circulation cooldown with voids. While the NtC staff considers natural circulation cooldown without voids as more desirable.

l cooldown with voids may be acceptable providing it can be accomplished using all safety grade equipment and approved procedures, and operators have ade-quate training in the use of these procedures.

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1-2 Additional requirements for pre-operational testing are set forth in the Standard Review Plan under RSB Branch Technical Position (BTP) 5-1. Tnis-

cssentially requires that a Class 2 plant demonstrate that it can be brough

, from hot standby to cold shutdown under the natural -circulation conditions using only systems and functions which are safety grade and with only onsite or offsite (not both) power available and assuming a single failure. '

RSB BTP 5-1 also requires that PWR pre-operational and initial startup test programs shall include tests with supporting analyses to (a) 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 achieve such mixing, and (b) confirm that the cooldown under natural circulation conditions ,can be achieved within the limits specified in the emergency operating procedures. Comparison with performance of previously tested plants of similar design may be substituted for these tests.

2 In response to these requirements licensees and vendors have submitted both individual and generic responses to MPA B-66 and they have conducted several boron mixing and natural circulation tests at representative commer-t cial plants. The objective of this project is to assist the NRC staff in j

evaluating data and supporting analyses obtained from the Boron Mixing and Natural Circulation Tests performed at San Onofre Unit 2 Diablo Canyon Unit l 1, and Palo Verde Unit 1. r

] The present report is primarily concerned with evaluation of t-ne data, analyses, and conclusions submitted by Westinghouse in WCAP-11086 "Diablo Can-yon 1 Naturrl Circulation / Boron Mixing /Cooldown Test Final Post Test Report,"

l in compliance with the design requirement of BTP RSB 5-1 for a Class 2 plant. ,

i The Diablo Canyon Power Plant is a 4-loop Westinghouse PWR. Separate reports .

will be issued for the comparison of the results of the test with the results

of previous analyses performed by utilities in their responses to MPA Iten '

8-66, " Natural Circulation Cooldown" for other Westinghouse plants, and for review of the emergency response guidelines for consistency with test find- -

ings. Similar reports will also be issued later for the evaluation of the natural circulation, boron mixing and cooldown tests performed at San one're I

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i 1-3 Unit 2 and Palo Verde for the CE Pre-Systen 80 and CE Systen 80 plants respec-tively.

Section 2 of the report summarizes the natural circulation, boron nixing and cooldown test performed at Diablo Canyon Power Plant. Section 3 describes the simulation of the test using the RELAPS/ MOD 1 Code to provide the analyti-cal basis for the review of test. The nodalization, boundary conditions, as-sumptions used for the calculation, and its results are discussed. In Section 4 the test results are reviewed on the basis of the simulation results. The test is divided into four stages for review: natural circulation, boron mix-ing, cooldown and depressurization. Section 5 presents the sensitivity analy-sis performed to facilitate the application of the test results to other West- ,

inghouse plants. Section 6 summarizes the conclusions and recommendations.

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2. TEST DESCRIPTION A natural circulation, boron mixing and cooldown test was conducted at Diablo Canyon Power Plant Unit 1 on March 28 and 29,1985.

The test began by manually initiating a turbine trip from 100% power at 2130 hour0.0247 days 0.592 hours 0.00352 weeks 8.10465e-4 months on March 28. The reactor was shutdown and the plant was maintained in hot standby condition. In about three hours, the natural circulation por-tion of the test was initiated by manually tripping all RCPs. After verifying the natural circulation condition in about 20 minutes, the boron mixing por-tion of the test was initiated by injecting the contents of the Boron Injec-tion Tank (BIT) into the RCS and was teminated in about 20 minutes. The flow rate into the reactor system was approximately 150 gpm. The system was main-tained at hot standby under natural circulation conditions for more than four hours. The cooldown/depressurization portion of the test was commenced by isolating letdown and cooling down with the atmospheric steam dump (ASD) valves. The cooldown rate was controlled at approximately 20*F/ hour. Tne cooldown/depressurization testing was continued for approximately thirteen hours until residual heat removal (RHR) initiation conditions (350'F, 403 psig) were achieved. The system was finally brought from RHR initiation con-ditions to cold shutdown conditions in the next four and a half hours by oper-ating the RHR system. The detailed chronology of the significant events and major' operator actions perfomed during the test il shown in Table 2.1.1 It is noted that some non-safety grade equipment and systems were used during the test because the operators of the plant did not want to risk damage to some of the equipment for the test. However, unavailability of these sys-tems (in strict adherence to the requirements of RSB Technical Position 5-1) cay have significant impact in the plant's perfomance under actual accident conditions. They were pressurizer heaters, letdown system, and control rod drive mechanism (CROM) fans. The impact of the potential unavailability of these systems will be assessed in detail in Section 4

2-2 Table 2.1 Chronology of Events and Operator Actions '

Tigi EVENT / ACTION HOT STAN0BY (FORCE 0 CIRCULATION) 2130: Plant operating at 100% power. Operators initiated the plant trip from 100% power by manually initiating a turbine trip.

2140: Reactor was shut down and plant was in hot standby conditions.

Operators were securing the plant secondary side. Relief valves on the

2 heaters had lifted. Operators were attempting to reseat the reliefs and waiting for the steam generator levels to return to 44% narrow range level.

2150: Operators have begun their Class 1 equipment alignment per Test Procedure 42.7.

l 2230: Operators have attempted to relatch the main turbine to minimize steam leakage on the secondary side.

2300: Steam generator levels were at 44% narrow range level.

2330: Main turbine was relatched. Vital power breaker for pressurizer heater 1-3 did not reenergize.

2400: Vital power breaker for pressurizer heater 1-3 had a blown fuse.

Pressurizer heater 1-3 was aligned to vital power.

0015: All Class 1 equipment was aligned. Total RCP seal injection flow was approximately 50 gpm.

HOT STAN09Y (NATURAL CIRCULATION AND BORON MIXING) 0028: Operators begin tripping the reactor coolant pumps.

0048: Natural circulation conditions have been verified.

0052: Contents of the Boron Injection Tank (BIT) injected into RCS. Flow rate was approxinately 150 gpm. .

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2-3 Table 2.1 (Continued)

,TEJ. EVENT / ACTION 0058: Power operated relief valve (PORV), PCV-456, opened to relieve excessive pressurizer pressure. PCV-456 actuated nine times from 005E to 0110 hours0.00127 days 0.0306 hours 1.818783e-4 weeks 4.1855e-5 months .

0111: Operators established letdown to lower the pressurizer level and minimize PORY actuation.

0113: Operators terminated 81T injection. RCS boron concentration increased from 890 ppm to 1195 ppm. Continued with the four hour at hot standby stabilization period. RCS temperature was steadily drifting downwards, due to operators trying to maintain the secondary side under hot conditions.

0200: Operators minimized steam loss on the secondary side by securing 50% of the condenser steam jet ejectors.

0415: Operators lowered pressurizer level by initiating letdown.

0440: Operators demonstrated that RCP seal injection flows can be controlled by manually throttling the isolation valve downstream of FCV-128 when using a centrifugal charging pump. After the demonstration, the reciprocating charging pump was placed in service. This would give operators better control of RCP seal injection flow during the remainder of the test, thereby minimizing RCP seal damage due to high seal injection flow.

0450: Plant has been at hot standby natural circulation conditions for greater than four hours. Operations set VCT makeup control system to provide 2000 ppm makeup to the Volume Control Tank (VCT). This simulated the charging pumps which were aligned to the Refueling Water Storage Tank (RWST).

f RCS COOLDOWN/DEPRES$URIZaTION TO RHR INITIATION CONDITIONS 1 .

0450: Operators isolated letdown and consnenced cooldown' Lsing the 10%

atmospheric steam dumps. Cooldown rate was approximately 20'F/ hour.

i 0533: Initiated letdown to lower pressurizer level and lower l primary / secondary system differential pressure.

i 0833: Isolated letdown.

l l 0845: Secured Control Rod Drive Mechanism (CROM) f an 1-1, 1

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i Table 2.1 (Coatinued) ,

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.T.lM. [yfN7/ACT10N 0957: Initiated letdown to lower pressurizer level. .

1319: All four loops TH0T less th'an 350*F. Plant in Mode 4 condition.

1356: Charging valve 8146 and auxiliar'y spray ' bypass salve 8148 opkn. No appreciable depressurization in the RCS observed.

1402: Closed charging valve 8141,. Depressurization ' rate was 8.0 psi / min. '

i 1515: : ,

Operators opened PORY PCV-456 to'depressu.112e the RCS and fiso isolatec >

letdown. 1

, t RCS COOLDOWN TO COLD SHUT 00WN y.?J40!TIONS -

1805: Operators initiated the RHR system. RHR pump was 1-2 placed in service, i

1831: The remaining CROM fans were secured./

2015: Operators re-energized the CROM fans t 3 $nly), .

2245: RCS temperature below 200*F. Plant in Mcde 5 concition.

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3. SIMULAT10ti 0F THE TEST

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3.1 _ General Description The natural circulation, boron mixing and cooldown test performed at

Dia'blo Canyon Power Plant was simulated using the RELAP5/M001 Code to provice the analytical basis for the test assessment. The RELAPS/M001 Code was selected for the simulation since it has been assessed by many organizations

, inQuding BNL. Its one-dimensional modeling of the reactor system was con-sidered adequate for this problem since all four loops were symmetric during the transient (test). It is also generally faster (in computing) than the TR/G PF1 code. This was an important consideration since the total test last-1 ed about 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months .

The Diablo Canyon Power Plant is a 4-loop Westinghouse PWR. All four 19ps including the steam generators were combined into a single loop since J.tey weri expected to be symmetric during the transient. Since the detailed modeling of nost parts of the RCS, other than upper head (Uti) region, was not expcted to be important and the transient was expected to be long and slow,

p an effort was made to-minimize the number of nodes used for the calculation in order to reduce the computing tine to an acceptable level. The final noding diagran used in the calculation is shown in Figure 3.1. Besides the main re-actor coolant systen (RCS), pressurizer and stean generators, the bypass flow

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from the Idowncomer to the UH to the upper plenum was modeled in detail. The boren injection and the RCP seal injection were also included in the nodel-

!> irg. Hest structures were utilized to represent the metal mass of the fuel, piping, steam jenarator tubes and other structures. The stean generator sec-i ondary model includes the oowncomer, boiler region, separator and steam done.

The modeling also included the primary and secondary relief valves. The heat loss throoh the piping and vessel wall was ignored since it was considered wry small compared to the decay heat. However, the ambient heat loss in the pressurizer was included in-the modeling to assess its effect on the depres-surifation rate. Simple control systems for the auxiliary feedwater (AFW) and atmospheric stean dump (ASD) valves were inplemented on the basis of level control and cooling rate, respectively. The power was provided by the Atis 5.1 e

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3-2 standard decay power table. The cooldown rate was set at 20'F/ hour as sin the test. /

e Since the pl.2nt was at full power when the test was initiated, the steady state for the hot full power condition was obtained for the simulation. The ; ,

steady state conditions were mainly based on the information available in the i

FSAR, augmentM by the information directly obtained from Pacific Gas and Electric (PG8E). Special attention' was paid to match the pressure drop and flow rate in the various regions of the RCS by adjusting the friction factors '

in the code input since this information would be important in the assessment of the natural circulation and cooling of the upper head. Table 3.1 presents the comparison between the actual plant data and the final steady state ob-tained by the calculation. The comparison indicates that the code sinulated th2 actual plant steady state very closely.

The sequence of events for the simulation is sumarized in Table 3.2.

This sequence of events did not exactly follow those of the test. The purpose of the calculation was not to duplicate the test, but to provide the basic in-formation to assess the impact of the deviation of the test procedures from thosci of the BTP RSB 5-1 guideline, such as the use of non-safety grade equip-ment during the test.

3.2 Natural Circulation The natural circulation phase of the calculation was simulated by trip-ping the reactor *and RCPs at time zero. The turbine stop valve (TSV) and main i fcedwater isolation valves (MFIV) were closed and the AFW was initiated at the same time as the reactor trip. In the test, the natural circulation was achieved in two stages. Initially the reactor was tripped from full power by a turbine trip to hot shutdown condition with the RCPs still running. The RCS tas maintained at this condition for several hours before the RCPs were -

- tripped and hot standby at natural circulation conditions was established.

This discrepancy would cause some differences between the test data and calcu-lated results as discussed later.

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3-3 Figure 3.2 compares the calculated RCS flow by RELAP5 and the pre-test prediction by PG&E. They are essentially identical. The decay heat used in the calculation and pre-test prediction were similar. The ANS decay heat was used for the calculation. It generally represents higher decay heat than in actual transients. Therefore, it was necessary to evaluate the effect of the decay heat on the natural circulation flow rate. The decay heat and natural circulation flow rate was expected to be related by (see Appendix A for the derivation),

W = KQI/3 where W = natural circulation flow rate 0 = decay heat K = a proportional constant.

This relationship indicates that the natural circulation flow rate is not

. very sensitive to the decay heat level. To confirm the above relationship, the steady state flow rate was plotted as a function of decay heat as shown in Figure 3.3 along with the results obtained by Westinghouse.2 They show essen-tially the same trend, indicating that the adequate natural circulation would be established to remove the decay heat throughout the anticipated transient.

I Figures 3.4 and 3.5 show the RCS temperature and pressure calculated by l BNL us'ing the RELAP5/ MOD 1 Code. As expected, the average coolant temperature dropped rapidly at the trip of reactor and pumps, and the pressure also expe-rienced a steep decline due to the shrinkage of the coolant. Once natural circulation was established, the temperature essentially remained constant as the secondary pressure and temperature held c-'atant at its PORV set pressure and its saturation temperature. The test data (Figure 3.6) showed slowly de-creasing temperature during this period. This appeared to be due to sone steam loss in the secondary side. This slight temperature drop during this period is not expected to have a significant effect on the rest of the l

l transient.

The test pressure (Figure 3.7) was different from the calculated pres-sure; the calculated pressure showed a steep decline in the beginning due to the shrinkage of the coolant while the test maintained its steady state

3-4 pressure after a short blip at the plant trip. This was due to the fact that '

the pressurizer heaters, which were not safety grade equipment, were used in the test durir.g this period, while they were assumed not available in the calculation. However, a similar pressure drop was also shown in the test when the pressurizer heaters were not available in the test briefly (between 24:00 and 24:30 hours, as shown in Figure 3.7). The calculated pressure and pressurizer level showed a slow increase after the initial drop because a small amount of R:P seal injection (20 gpn) was maintained in the ialculation as in the test. Letdown was assumed not available in the calculation since it was not safety graded equipment. This continuous injection of additional mass without letcown would eventually cause the opening of the PORV. Although the anbient heat loss in the pressurizer was modelled in the calculation, the pressure drtp due to the heat loss was not enough to compensate for the increase of the pressure due to injection of the RCP seal injection. It was estinated that the RCP seal injection would increase the pressurizer level about 10% each hour.

Westinghouse plants may be divided into two groups according to the mag-n9tude of the bypass flow: Th at and Tcold plants. For the Tcold plant, sufficient bypass flow exists to make the upper head fluid temperature essen-t.ially equal to the cold leg temperature. On the other hand, for the Tno plants ' (including Diablo Canyon), the bypass flow is much smaller. This results in the upper head temperature between the cold leg and the hot leg temperature. This type of plant poses some dif ficulty in cooling the upper head during the cooldown period and raises a possibility of void formation in the UH region. Thus a Thot plant requires a much more careful study on the coolability of the uppet head; this in turn requires an accurate estimation of the bypass flow rate during the natural circulation. The RELAP calculation of the bypass flow rate at natural circulation conditions differed substantially '

from the results obtained by Westinghouse.2 In the Westinghouse study, the bypass flow was reversed, flowing from the upper head to the upper downcomer -

during the natural circulation. The magnitude of the flow was reduced almost proportionately to the main flow from 60 lb/sec (0.15% of the mainflow) at design conditions to approximately 2-3 lb/sec (0.1-0.15% of the main flow) during the natural circulation. However, RNL's calculation using the RELAP/

tiODI Code showed that bypass flow never reversed and substantial bypass flow l

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3-5 was maintained despite rapidly decreasing main RCS flow; the flow was reduced from 70 lb/sec (0.2% of the main flow) during the forced flow to 14 lb/sec (1.0% of the main flow) during the natural circulation.

BNL's results appear to be qualitatively correct. The driving force exerted on the bypass flow is the gravity force created by the temperature in-duced density differences in the RCS loop. As shown in Figure 3.8, there are two buoyancy forces exerted on the flow path AEC acting in opposite direc-tions. One force is created by density differences in the flow path CDA (through steam generator) which forces flow from A to C. The other buoyancy force is created in the flow path ABC (through the core) which forces flow l

from C to A. In the specific geometry of the Diablo Canyon Power Plant the calculations indicate that the driving force of CDA surpassed that of ABC, thus resulting in the flow from A to C through E. The magnitude of this flow was difficult to confirm by independent calculation, however, since the re-sults are very sensitive to the calculated frictional losses. Based on the calculated bypass flow rate, the upper head fluid would be replaced completely every forty (40) minutes. This relatively large flow and short replacement time would enhance the mixing of fluid in the upper head, thereby promoting cooling and boron mixing in the upper head. However, the mixing of fluid within the upper head region may not be good considering the large amount of

! guide tube structures in it. The significance of this aspect will be dis-

cussed in more detail in Section 4.5.

l 3.3 Boron Mixinc After the natural circulation was established, the boron was injected. A total of 900 gallons of 21,000 ppm borated water was added to the RCS, using the boron injection tank (BIT). Figure 3.9 shows the boron concentrate calcu-lated by the code as well as the actual test result and the pre-test predic-tion. Also plotted are the calculated boron concentration in the upper head, when boron was mixed evenly in the upper head. Although the rate of increase of the boron concentration differs somewhat between both analyses and the test, all show a sufficiently rapid rise to insure the adequate mixing of boron in the main flow paths of RCS under natural circulation condition.

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Figure 3.9 shows that the increase of the boron concentration was slower

in the upper head than in the rest of the RCS. Nevertheless, it also reached the average bulk boron concentration in less than one hour. This was due to relatively large bypass flow fraction into the upper head. It should be noted that the boron concentration in the upper head calculated by the RELAP assumes complete mixing. However, the fluid in the upper head appears to be strati-fied with little or no mixing as discussed earlier. This suggests that there

, Ray be some unborated water in the upper head. A similar concern may be raised about the boron mixing in the pressurizer. This point will be further discussed in the next section. -

Both the test and the calculated pressure started increasing rapidly once the boron injection started. It eventually reached the PORV actuation pres-sure and opened the PORV. This was due to the injection of additional mass into the system without letdown. In the test, the letdown was initiated to minimize PORV actuation at the end of the boron injection period.

3.4 Cooldown The cooldown was initiated by opening the ASD valve at 12,000 seconds in the simulation. The base calculation was performed with a cooldown rate of 20*F/ hour and continuous R'CP seal injection as in the test. A simple propor-tional controller based on the rate of temperature drop was implemented in the calculation. The flow through the ASD valve was calibrated based on its ca-pacity at the normal operating pressure, which wts obtained from PG&E. The RCS temperature was approximately 570*F when the cooldown was commenced. Com-parison of the test temperature (Figure 3.6) with that of the calculation (Figure 3.4) show that the actual cooldown was very similar to the calculated ecoldown. The RCS temperature in the test was approximately 510'F when cool- -

3 down was commenced as discussed in the previous section on natural circula-tion.

The RCS pressure was more difficult to compare since the letdown was used

. in the test during most of the cooldown period to prevent the water-solid operation of the pressurizer due to continuous operation of the RCP seal

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injection. The RCS pressure obviously depends on the rate of letdown. The

. 3-7 pressure calculated with 20'F/ hour cooldown rate, RCP seal injection and r.c !

letdown (Fig. 3.5) renained at the PORV actuation pressure almost 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months .

This was because the volume created by shrinkage of the coolant due to cool-down was less than the increase of coolant volume due to RCP seal injection without letdown. This necessitated the periodic opening of the PORV. The pressure eventually began dropping later as the pressurizer continued to cool.

To assess the impact of the RCP injection, an additional calculation was performed without RCP injection. Figure 3.10 showed the gradual pressure de-crease as expected. It showed that pressurizer level was also gradually de-creasing and indicated that the pressurizer would eventually empty without further operator actions. In practice, the operators would try to maintain l

the pressurizer level by operating the charging and letdown systems when available. Figure 3.10 shows the pressure estimated when the pressurizer level was maintained at 50%. The RCS pressure still decreased due to the am-bient heat loss in the pressurizer. Figure 3.11 compares the calculated RCS coolant temperature with the saturation temperature corresponding to the RCS pressure and' indicates that more than 100*F of subcooling was maintained for the RCS during the cooldown period for both the test and the calculation.

Figures 3.12 and 3.13 give the results of another sensitivity calculation perforned with a cooldown rate of 50'F/ hour. As expected, the pressure de-l creased faster than the previous cases even with RCP seal injection since the l shrinkage of the coolant was more than the volume of the injected water. Dur-f ing this rapid cooldown, the bulk RCS temperature is adequately subcooled throughout the cooldown, as shown in Figure 3.12.

The upper head fluid temperature calculated by the code (20*F/ hour cool-down) is shown in Figure 3.4 The calculations indicate that upper head was cooled at about the same rate as the RCS and, thus, maintained the same margin of subcooling as the RCS. This was due to the fact that a substantial bypass flow was calculated by the code cooling the upper head and mixing with the upper head fluid. (Complete mixing was assuned in the upper head for the cal-culation but the expected effects of flow stratification are assessed in the next sectio 1.) Therefore, cooling of the upper head is expected to be sub-stantially less than that indicated by the calculation.

I

3-8 Other concerns during the cooldown were the capacity of the ASD valves to provide sufficient cooling to maintain the specified cooldown rate, especially during the latter stage of cooldown when the steam generator (SG) pressure was low, as well as the question of the adequacy of the supply of coolant water ;

available in the condensate storage tank (CST). Figure 3.14 shows that the .

fraction of ASD valve opening during the cooldown period remain less than 70*.

even near the end of the cooldown period with the high cooldown rate of 50'F/

hour. It was less than 50% open when the cooldown rate was 20'F/ hour. Figure 3.15 shows the accumulated AFW calculated by the code (not including an allow-ance for soak time). It should be noted that this represented a conservative (maximum) estimation of the required amount of water since higher decay heat was used in the calculation than in the test. The reactor system used about 120,000 gallons of cooling water until the end of cooldown (abnut 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months with 50*F/ hour cooldown). It was estimated that about 150,000 gallons of water would be needed until the end of cooldown with the 20'F/ hour cooldown rate (about 14 hours1.62037e-4 days 0.00389 hours 2.314815e-5 weeks 5.327e-6 months ). In the test, about 126,000 gallons of water was used for the entire test (about 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months ). These are well below the capacity of the CST. However, this did not account for the additional water required during the extended period of cooldown which might be needed to cool the upper head.

The calculation also showed that one motor-driven AFW train wis sufficient to supply the necessary cooling water throughout the transient.

3.5 Depressurization Since the pressure and temperature during the depressurization could be readily evaluated without the detailed calculation, the depressurization peri-od of the test was not simulated. Furthermore, there was no non safety grade equipment used during this period in the test.

An approximate equation has been developed to estimate the auxiliary spray water flow rate required to maintain a specified depressurization rate. ,

It was assumed that the pressurizer was at equilibrium state when the auxilia-ry spray was in operation. The rate equation is:

~6

- 3-9 yo ($2.) (dTsat)

W,p = T dt dp (3.1) pr -T sp where Wsp = spray flow rate, Tsp = spray water temperature, Tp r = pressurizer temperature, V = water volume at the pressurizer, p = density of water at the pressurizer.

The maximmn spray water flow rate required during the depressurization for Diablo Canyon to maintain 8 psi / min was estimated to be approximately 40 gpm at the end of depresst ization. This was less than the maximum flow rate of 55 gpm. The spray wat:r temperature was assumed to be 100*F and the pres-surizer level was assuned to de 60%.

Higher spray water temperature would decrease the depressurization rate and the PORV.may be needed at the end of the depressurization period. Note that it was assumed that letdown was to be unavailable. The operation of the auxiliary pressurizer sprayer normally requires letdown to be in operation in order to prevent the thermal stress which might be generated on the charging nozzles.

3-10 -

Table 3.1 Comparison of the RELAP5 Estimated Steady State Conditions with the Plant Steady State.*

Paraneters Plant RELAP5/M001 Power, MW 3338 3338 Pressure, psia 2252.8 2252.8 .

Hot Leg Temp., *F 608.8 612.1 Cold Leg Temp., *F 544.4 548.0 Coolant Flow,-lb/sec 36918 36678

Bypass Flew, lb/sec 77.3** 79.6 Ap Pump, psia 84.0 84.6 Pressurizer Level, % 60.0 61.7 Stean Pressure, psia 805.0 805.0 e Steam Temperature, *F 519.0 518.9 Steam Flow, lb/sec 4039 4035.8 SG Water Volune, ft3 7930 7068.0 Boron Concentration, ppm 890*** 890

The steady state conditions for the plant were taken from the FSARs unless otherwise stated.

- 0btained fron PG&E staff.

- - 0btained from the Diablo Canyon test report.1 4

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41 4 REVIEW OF TEST RESULTS 4.1 Natural Circulation The ' natural circulation was achieved in two stages in the test. The plant was tripped from full power by a turbine trip to hot standby conditions with the RCPs still running. The RCS was maintained at this condition for three hours before the RCPs were tripped and hot standby natural circulation was established. Under the accident conditions, the turbine trip and RCP trip would be anticipated to occur simultaneously. The delay in initiating natural circulation reduced the level of decay heat. This slightly reduced the natural circulation flow rate and boron mixing. The delay also allowed the
_ primary system to become more uniform in temperature including sone reduction in upper head temperature. This would tend to reduce the likelihood of void formation in the upper head during the natural circulation cooldown.
During the test, both the normal plant control systems and safety grade systems were used to accomplish the boron mixing and the cooldown goals. We would expect the plant procedures to follow an equivalent approach, i.e., the procedure would be as simple and direct as possible using the best available equipment. In.those cases where other than safety grade equipment was used it would be demonstrated how the necessary function could be achieved using only safety grade equipment. We believe the Westinghouse test report l achieved this goal.
The delay of natural circulation after the plant trip allows some add-itional cooling of the upper head. This aspect will be discussed in the re-view of the upper head cooling. During the natural circulation period (including the boron mixing period) in the test, the pressurizer heaters and letdown system were used (neither of which are safety grade). The pressurizer heaters were used to maintain the pressure after the plant trip. The unavail-ability of the pressurizer heaters would not affect the plant's ability to maintain natural circulation conditions since the natural circulation flow rate would not be affected by the RCS pressure during this period. Use of the pressurizer heaters, however, may necessitate the earlier use of the letdown.
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4-2 * ' ,
The unavailability of .the letdown would affect the system pres'sure more directly. Since the RCP seal ir.jection would be maintained throughout the natural circulation, this continuous injection of mass (combined with the re-quired boron injection later) without letdown wo0ld increase the pressure ~and Gventually open the PORVs. It was' estimated that the RCP injection would in-crease the pressurizer level about 10% each hour. Howe.ver, this would not directly affect the plant's ability to achieve natural circuiations.
Based on the above discussion and results from the previous section, it wasconcludedthatthetestincombinationwiththeanalysissuffkcientlyder-
~
onstrated the adequacy of natural circulation. Thus, the plant is capable of ,
removing decay heat by the natural circulation with only safety grade equip- ,
ment. ,
4.2 Boron Mixing -
Both the analysis and test results demonstrated that the rise of tne boron concentration in the main flow path of the RCS was sufficiently fast to ensure adequate boron mixing prior to cooldown under r.atural circulation con-ditions.
As discussed in Section 3.3, the RELAP calculations predict that a sub-stantial bypass flow into the upper head will occur' and the upper head boren concentration will approach that of the main RCS with adequate mixing of the upper head fluid. However, mixing of the fluid in the upper head does not appear to be adequate and the bypass fraction is uncertain. The fluid in sone parts of the upper head, especially in the upper region, has the potential to remain stratified considering the large amount of guide tube structures which ,
l impede mixing. Similarly, the fluid in the pressurizer may be isoltted from the rest of the RCS, if the sprayer is not. used. This suggests that the bornn mixing in the upper head and pressurizer sty be very slow, and the effect of
relatively unborated upper head and pressurizer water added to the RCS, par-ticularly during the upper head voiding (if it occurs), should be evaluated.
It is not required as part of the BTP RSB 5-1 to demonstrate the nixing of boron in the pressurizer.
4-3 The eJfect of the .unborated water entering the RCS would largely depene on the ratio of the flow rate of the incoming water from the upper head or pressurizer relative to the main coolant flow rate during the void formation.
In case of the St. Lucie event where void formation in the upper head was ob-s'erved ,3 it was conservatively estimated that the maximum flow rate from the upper head was less than 50 lb/see when the pressurizer level increased nost
rapidly during the depressuri ation.. This was less than 5% of the nain cool-
ant flow rate. An even smaller flow rate (about 10 lb/sec) was observed dur-
] ing the natural circulation /cooldown test performed at Palo Verde (a Combus-tion Engineering PWR) where the formation of a void was intentionally in-d6ced." A simple hand calculation based on the assumption that the upper head fluid is" in equilibrium during the depressurization indicates that the mass flow rate out of the upper head during void formation would be less than 15 lb/sec for the depressurization . rate of 10 psia / min. This means that the fluid leaving the vessel would have been diluted by about 15 ppm at the nost if no mixing took place at the upper head. However, this small amount of flow fromtheupper),endwouldmix,withthelargeamountoffluidintheupperple-num where /,elatively good mixing could be assumed. Furthermore, this fluid would go through the steam generators where there are thousands of stean gen-erator tubes of slightly different lengths, and large inlet and outlet plena,
~
and would further mix with the mai.n coolant before it entered the core region.
Similarly, the effect of the unborated water entering the RCS from the pressurizer would be negbigible during normal cooling /depressurization. But it may pose some problem during the rapid oscillation of the fluid between the
- pressurizer and the upper head if the emergency procedures do not specify the proper measures for depressurization. This kind of oscillation would occur
' only after the pressurizer wat first filled with water from voiding the upper head. This implies'that the water leaving the pressurizer is already mixed with the main coolant and the flow rate would be no more than the flow rate from the upper head as discussed above. The subsequent oscillations would not
~
, pose any further problem as 'far as boron mixing was concerned, because the fluid in the pressurizer (and upper head) would have been mixed with the RCS fluid during the initial phase of oscillation.
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.) N Another concern'during the boron mixing period of the natural circulation '
would be the RCS pressure increase due to the injection of additional mass in-to the system without letdown. It was observed during the Diablo Canyon test that the PORV actuation pressure was reached and a PORV was periodically opened to relieve the pressure. This behavior was reproduced in the calcula-tion as discussed in the previous section. In the test, letdown was initiated in order'to lowerithe pressurizer level and minimize PORV actuation at the end of the bcron injection.
It war concluded that:
1. Adequate boron mixing could be achieved during the natural circula ' '
\
tion in the main flow paths of the RCS using only safety grade equip 3 ment.
i
2. The ef fect of relatively unborated water entering the RCS from the upper head and pressurizer would be minimal.
3. The pressure would rise and may reach the PORV actuation pressure without letdown or venting through upper head vents during the boron nixing period. Operators should be prepared for this possibility.
4.3 Cooldown '
The Diablo Canyon test and the BNL analysis demonstrate that cooldown of the RCS to R.HR system initiating conditions can be accomplisNQ ,hile nai n-taining fthe required subcooling during the natural ci red r ice using only safety-grade equipment. Although the letdown system was L?.a c.. ~rg the test to prevent filling the pressurizer (and water solid operation) due to continu- '
ous RCP seal injection, use of the letdown system was not deemed to be essen-tial during cooldown. However, not using the letdown would maintain the RCS -
.prsssure high and actuate the PORV when the cooldown rate was low. Increasing the cooldown rate to 50*F/ hour would decrease the pressure throughout the cooldown period and would eliminate the need for PORV operation. Even in the case of the higher cooldown rate, the main RCS maintained the required nargin of subcooling. The ASD valve capacity was calculated to be sufficient to i
i i
4-5 maintain the high cooldown rate. Adequate amounts of cooling water was avail-able in the CST to cooldown the RCS. However, additional water may be needec to provide the additional cooldown period needed to cool the upper head.
Cooling of the upper head with and without the CRDM fans will be addressed in Section 4.5.
4.4 Depressurization The test demonstrated that the reactor coolant system could be depressur-ized to the RHR initiation pressure (400 psig) under the natural circulation conditions using the auxiliary spray and/or pressurizer PORVs. The test also demonstrated that the depressurization can progress to the end of the cooldown period without void formation in the upper head when the cooldown rate was 20*F/ hour ind the CRDM fans were available to cool the upper head. The fol-lowing sections (Section 4.5) ' indicate that the depressurization could pro-gress tc the end of cooldown without void formation even with a high cooldown rate of 50*F/ hour when the CRDM fans were available.
The Westinghouse Background Information for Energency Response Guideline FS-D.2 7 estimated that operators should wait about 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months after the beginning of cooldown for a Diablo Canyon type plant before pr.oceeding to depressurize if the CRDM fans are not available to provide additional tooling of the upper head. The BNL analysis of cooldown without CRDM fans will be discussed in Section 4.5.
4.5 Reactor Vessel Upper Head Coolinc As discussed earlier, a potential exists for void formation in the upper head during the cooldown/depressurization under natural circulation conditions since the upper head is relatively isolated from the rest of the RCS and its fluid temperature remains higher than the coolant temperature in the main flow l paths of the RCS. This will have a major importance to the plant's ability to bring it to cold shutdown conditions under the natural circulation condition.
Several factors influence the cooling of the upper head under natural circulation conditions. They include the following:
4 .
a) Heat removal f rom the upper head into the containment envi ronmer,t through the CRDM and the upper head dome when CRDM fans operate, b) Amount of bypass into the upper head, c) Heat conduction from upper head to upper plenum through the guide tube structures, d) Heat conduction down to the reactor vessel through the upper head dome.
Among these, availability of the CRDM fans appears to be the dominating factor. The CRDM fans, however, are not seismically qualified equipment and no credit can be taken for these under the RSB Technical Position 5-1 assump-tion. Therefore, the cooling of the upper head will be assessed with and without the CRDM fans.
4.5.1 Cooling with CRDM Fans Operating According to the Diablo Canyon FSAR,5 the three operating fans (out of 4) can remove 2.5*106 Btu / hour of heat from the upper head during nornal opera-tion. This translates into a cooldown rate of 32*F/ hour for the upper head fluid when the upper head fluid temperature is 600*F2 for a typical 4 loop Westinghouse plant. This cooldown rate was later reduced to 17'F/ hour accord-ing to revised Westinghouse estimate.6 Assuming the cooldown rate is approxi-mately proportional to the temperature difference between the upper head and the containment environment ( 100*F), then the cooling rate is given by; dT
'I T-100 dt
600-100 '
This equation indicates that it will take approximately twenty hours for ine upper head temperature to reach 350'F. and ten hours to reach 450*F. Ten hours is approximately the time to cool the main coolant to 350'F with 20*F/
hour cooldown rate. Figure 4.1 showed the margin of subcooling in the upper head when the CRDM fans were in operation for two different RCS cooldown rates (four hours of natural circulation prior to the cooldown was assumed). It was shown that more than 100'F of subcooling was available when the cooldown rate was 20*F/ hour. However, it was less than 50*F with 50*F/ hour cooldown rate.
l 1
4-7 To maintain the 50*F subcooling, the natural circulation prior to the cooldown should be increased to five hours for the 50'F/ hour cooldown.
Another concern for the upper head cooling is the degree of mixing. Even if excellent heat transfer occurs at the perimeter of the upper head, some hot spots may remain without good mixing of the fluid. However, since the cooling by the CRDM fans occurs in the upper part of the upper head region, good mix-ing is expected due to the natural convection (cold fluid above the hot fluid) within the upper head when the CRDM fans are in operation.
4.5.2 Cooling Without CRDt1 Fans Operatina Without the CRDM fans operating, the cooling of the upper head should de-pend on other mechanisms. Among the factors listed above, the second mecha-nism would be a major factor if sufficient bypass flow existed and it mixed well with the upper head fluid. As mentioned in Section 3.4, sufficient by-pass flow to cool the upper head is predicted assuming it is well nixed with the upper head fluid. With this assumption, the upper head fluid temperature calculated by the code decreased at about the same rate as the RCS cooling rate with some time lag. However, some part of the upper head, nay be strati-fied and its temperature may remain hot considering the large amount of guide tube structures and the lack of a free convection driving force. Under this circumstance, the only significant mechanism to cool the upper head would be the heat conduction through the guide tube structures and the upper head done wall down to the upper plenun region.
A simple calculation was performed to estimate the cooling rate of the upper head based on the conduction through these structures. The upper head was divided into four heat conduction nodes, and bypass flow was assumed to mix with the fluid at the bottom part of the upper head. The upper head ten-perature was 550*F when the cooling began and the cooling rate was 25'F/ hour.
Figure 4.2 shows the fluid temperature thus calculated at various locations in the upper head; node 1 represented the uppermost part of the upper head. It took approximately 43 hours4.976852e-4 days 0.0119 hours 7.109788e-5 weeks 1.63615e-5 months to reach 450*F after beginning the cooldown. The cooling time was not particularly sensitive to the RCS cooldown rate. The Westinghouse study estimated that the operator should wait about 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months to
4-8 ,
allow upper head cooling once the hot leg temperature reached 350*F.2 7n3 translated into approximately 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months after the beginning of the .:ooldown, which was about 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months shorter than the BNL estimation. It should be noted
-that several assumptions were made in the BNL calculation, which tend to make the result of the calculation conservative. Specifically, the upper head fluid was completely stagnant, conduction was the only mechanism for cooldown, the heat loss from the dome to the containment environment was ignored, and the bypass fluid mixed only with the fluid in the bottom of the upper head.
In the test, all the CRDM fans were temporarily turned off for about 100 minutes during the cooldown period to evaluate the effect of the CRDM fans and the ' average upper head cooldown rate was estimated to be approximately 6*F/
hour. This translated into about 25 hours2.893519e-4 days 0.00694 hours 4.133598e-5 weeks 9.5125e-6 months to cool the upper head by 150*F.
However, it is difficult to extrapolate this result since the time period for this test was short and several factors could influence the results for such a short test. Specifically, cooling from above will cause circulation within the UH region due to buoyancy effects.
Figure 4.3 compares the upper head temperature calculated by BNL with the saturation temperature corresponding to the RCS pressure with 20*F/ hour cool-down rate. It showed that the saturation temperature of the RCS pressure may
- go below the upper head temperature and thus a void may form during the cool-down operation even with a low cooling rate of 20'F/ hour without the CRDM fans in operation unless the RCS pressure was maintained by means of either the pressurizer heaters or charging. The pressure would decrease slowly due to RCS cooldown contraction and ambient heat loss in the pressurizer. The rate of pressure drop due to pressurizer heat loss was estimated to be approximate-ly 80 psia / hour.
, it is concluded that:
a) The test demonstrated that the reactor vessel upper head coolu,9 could be accomplished without void formation with 20*F/ hour cooldown of the RCS when the CRDM fans were in operation.
4-9 b) The test results indicated that the upper head cooldown rate was about 6*F/ hour for the Diablo Canyon plant. Note that this is slightly above the conservative (no upper head mixing) BNL calcula-tion, but it is considerably above the rate predicted for this type of plant in the Westinghouse Owner's Group estimate.2 c) The RCS pressure may go below the saturation pressure of the upper head and thus a void may form during the cooldown operation even with the recomended low cooldown rate of 20*F/ hour when the CRDM fans were not in operation.
'l 4.6 Cooling Water and Compressed Air Requirement Figure 3.15 shows the accumulated AFW calculated to be used during the cooldown operation. Approximately 120,000 gallons of auxiliary feedwater would be used until the end of cooldown when the cooldown rate was 50*F/ hour.
This included all the sensible heat of the system to bring the RCS from full power to the cold shutdown condition (including the water and metal struc-tures) and the initial eight hours of decay heat. However, the total cooldown operation may last as long as 50 hours5.787037e-4 days 0.0139 hours 8.267196e-5 weeks 1.9025e-5 months to allow time for upper head cooldown when the CRDM fans are not available as discussed in the previous section.
Accounting for the additional decay heat during this period, a total of 360,000 gallons of cooling water may be needed, based on the ANS limiting de-cay heat. This is less than total water available from the condensate storage tank (CST) and other seismic category I sources (a total of 1,170,000 gallons).
It was reported that 126,000 gallons of water was used during the test (during approximately 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months ) where the CROM fans were operating. This is fairly consistent with the 120,000 gallons calculated by RELAP but the test duration and decay heat are somewhat different.
Another concern during the natural circulation cooldown is adequate sup-ply of class I compressed air (or nitrogen gas) which is needed to operate the ASD valves. According to the PG&E staff,e eight bottles of class I air are installed to the two units at Diablo Canyon for this purpose and these are
4-10 expected to last about 18 hours2.083333e-4 days 0.005 hours 2.97619e-5 weeks 6.849e-6 months . Additionally, 35 bottles of air are stocked l Cn site at all times. This translates into additional 80 hours9.259259e-4 days 0.0222 hours 1.322751e-4 weeks 3.044e-5 months of supply, which is considerably more than the estimated cooling time even with the most conservative assumptions.
It is concluded that a sufficient supply of safety-grade cooling water and compressed air is available to support the proposed plant cooldown method for Diablo Canyon but other plants with less cooling water and air available than in Diablo Canyon may require a faster cooldown method.
4.7 Effect of Non-Safety Grade Systems Used in the Test During the test, several non-seismically qualified equipment and systens were used; they were the pressurizer heaters, letdown systems and CRDM fans.
The effect of unavailability of this equipment is summarized below.
a) Pressurizer Heaters The pressurizer heaters are a major part of the RCS pressure control sys-tem. They provide the ability to increase the pressure independently of the RCS water inventory and RCS water temperature. During hot standby conditions, the RCS pressure is expected to decrease due to the cooldown contraction of the RCS and the heat loss from the pressurizer as discussed in Section 3.4 It appears that during the cooldown without CRDM fans, the pressurizer heaters may be needed to maintain the RCS pressure above the saturation pressure of the upper head. Even with the CRDM fans in operation, should the pressure fall below the saturation pressure of the fluid temperature of any part of the RCS such as in the upper head, as happened at St. Lucie, the capability to ,
control the resultant void would be limited if the pressurizer heaters are not available. Without the pressurizer heaters, RCS pressure control can still be achieved by operating the safety grade charging system. However, maintaining the elevated pressure using the charging system would increase the pressurizer eater level and eventually cause water-solid operation of the pressurizer.
Operators should be instructed to prepare for these circumstances and appro-priate operating procedures should be included in the Emergency Operating Pro-cedures (EOP) including reduction of the cooldown rate. It should also be
4 11 mentioned that the strategy to cool the upper head more rapidly by intention-ally forming a void would be more difficult without the pressurizer heaters.
Some plants have upper head venting capability which could be used with charg-
' ing flow to form and vent a steam bubble in the upper head.
b) Letdown System The letdown system provides a direct means to reduce the water inven-to ry. It was used throughout the test to prevent overfilling of the pressur-izer (with resultant water-solid ope ration) since a substantial amount of RCP seal injection was maintained in the test. The RCP injection was estimated to increase the pressurizer level about 10% each hcur without letdown. The con-tinuous RCP seal injection without letdown may keep the RCS pressure high and actuate the PORV even during the cooldown if the cooldown rate was low as shown in Figure 3.5. Increasing the cooldown rate above 20*F/ hour would elim-inate the need for letdown or PORV operation.
Unavailability of the letdown system may also affect the depressurization procedure. The operation of the auxiliary pressurizer sprayer normally re-quires letdown in operation to prevent the thermal stress which might be gen-erated on the charging nozzles.
c) CRDM Fans The CRDM fans have a major impact on the cooling of the upper head. With CRDM fans operating, the reactor vessel upper head would be cooled at approxi-t mately 20*F per hour. Without them, the cooling time of the RCS would in-l crease by 20-30 hours and about 180,000-240,000 gallons of additional cooling water would be required. It would also increase the possibility of void formation during the cooldown/depressurization period.
l r
l l
e i I UPPER HEAD TEMPERATURE

SAT. TEMPERATURE OF RCS PRESSURE WITH 50 *F/hr COOLDOWN SAT. TEMPERATURE OF RCS PRESSURE F 700 - WITH 20*F/hr COOLDOWN -

J - - - - - - - - - - , - - ,
m s \
o s N
g 600 \ N -
@ \~s
a. s~~~s 3
s 500 - -
t g
400 5 10 15 TIME AFTER BEGINNING OF COOLDOWN, hr Figure 4.1 Margin of subcooling in the upper head with CRDM fans in operation.
9
-___ - _- _ _ _ _ _ _ _ _ _ _ - _ 4
4-13 I i a a 1

NODE 1 (UPPERMOST REGION)

--- NODE 2 600 - --- NODE 3 _
NODE 4 (BOTTOM REGION)
N\ ,'s
'\'
\
N
'N s U 500
\g _
g ,
N N'N s
[. \ \ N
'N, n:
w s s\ N s'%
w s N 400 - -
N.N 300 - _
(
l I I I I O 10 20 30 40 50 TIME AFTER BEGINNING OF COOLDOWN, hr Figure 4.2 Upper head temperature when loss is due to conduction only (25'F/hr cooldown of RCS).
l I
( -
\
4-14 1 I I I I I I

U P P E R H E A D TEM P.

600 SAT. TEMR OF RCS PRESS.-
u. ,
0 '
W -,
2 D ,N s H * %
4 Ns 2
W 's g 500 - 's s -
W N H %
N
. Ng I I 400 I I I I 10 20 30 40 50 60 f T MC ATER BEGINNING OF COOLDOWN, hr
Figure 4.3 Upper head temperature and saturation temocrature of RCS pressure with 20*F/hr cooldown.
5-1
5. SENSITIVITY ANALYSIS The results of the natural circulation and cooldown test performed at Diablo Canyon are expected to be refe'renced by other Westinghouse plants in determining their compliance with BTP RSB 5-1. To facilitate this applica-tion, the plant parameters which may affect application of the test results to other Westinghouse plants are identified and the sensitivity of the results to these parameters is estimated for each stage of the test. The results are suvearized in Table 5.1. The sensitivity listed is the expected change of the natural circulation conditions for each 10% change of the parameters unless otherwise mentioned.
5.1 Natural Circulation The parameters which affect the natural circulation f. low are:
I 1. Level of decay heat,
2. Relative elevation of the thermal center steam yenerators to the thermal center of core, and j 3. Coolant flow rate and total pressure drop across the loop during the normal operation.
Equations A.3 and A.6 in the Appendix A shows the approximate relation-ship between these parameters. Table 5.1 shows the sensitivity of the natural circulation flow to these parameters.
It expresses the percent change of natural circulation flow for the 10% change from the Diablo Canyon Plant con-dition. It indicates that the natural circulation flow rate is generally not sensitive to the variation of most plant conditions. Since the plant's abili-ty to cooldown and to mix boron in the main loop is not significantly affected by slight changes in the natural circulation flow rate', it is concluded that l these parameters do not have a major impact the plant's ability to cooldown and mix boron.
5.2 Boron Mixing l
l The plant's ability to mix boron prior to cooldown mainly depends on the injection rate of boron relative to the total inventory of water in the RCS, as shown in the following equation.
5-2 .
At = AC , ( 5.1 )
where At = time required to increase the boron concentration of the RCS oy at sec.
AC = required increase of boron concentration, ppm.
V = RCS volume, ft8 .
G = borated water injection rate, ft 3/sec.
C = concentration of the injected boron, ppn.
Since the time needed for boron injection is much less (order of I hour) than the available time prior to the initiation of cooldown (order of 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months ), minor variation in the boron injection time due to variation of the above parameters will not significantly affect the plant's ability to inject and mix boron. However, each plant should demonstrate that a seismically-qualified boron injection system is available (such as the BIT in the Diablo Canyon plant). It should also be demonstrated that the capacity of the boron source is large enough to sustain the specified flow.
5.3 RCS Cooldown The plant's ability to cool the RCS at a specified cooldown rate is de-termined by the capacity of the ASD valves to allow sufficient steam flow to account for the sensible heat and decay heat at the end of the cooldown period when the steam generator pressure is low, and the supply of sufficient cooling water. These are in turn affected by the total amount of water and structural material in the RCS, level of decay heat and the cooldown rate. Table 5.1 shows the sensitivity of the ASD valve opening and the required AFW sensitivi-ty to the parameters affecting the cooldown. (The required AFW amount in-cludes the additional amount of water required to remove the decay heat during the upper head cooldown period when the CRDM fans are not operating. This will be discussed in the next section.) The available capacity of the ASD -
valve and supply of cooling water for other plants should be compared to the required ASD valve opening and supply of AFW listed in Table 5.1 to determine their adequacy.
5-3 r
5.4 Depressurization The parameters affecting the depressurization rate are the water inven-
~
tory at the pressurizer, pressurizer auxiliary spray water temperature and sprayer flow rate according to Equation 3.1. Amount of the ambient heat loss will also affect the demand on the auxiliary pressurizer sprayer.
Table 5.1 summarizes the sensitivity of the depressurization rate to these parameters. If the desired depressurization rate is more than the maxi-num depressurization rate, manual operation of PORV would be needed to achieve the desired rate.
5.5 Upper Head Cooling The major parameters affecting the upper head cooling are:
1. Capacity of the CRDM fans when they are in operation,
2. The bypass flow rate to the upper head,
3. Upper head volume, 4 The upper head metal structure mass including the guide tubes, upper head dome and upper head plate.
Operation of the CRDM fans are the dominating factor to determine the up-per head cooling rate when they are in operation.
For the Diablo Canyon plant, the CRDM fans cooled the upper head at the rate of 17'F/ hour when the upper head temperature was 600*F with the CRDM fans. This time would be ap-proximately proportional to the inverse of the fan capacity. The capacity of the CRDM fans at Diablo Canyon was 82,0003 ft / min (with all four operating).
The bypass flow would have a major impact on the cooling of the upper head if it mixes well in the upper head. However, it was difficult to deter-cine the degree of mixing. The degree of mixing of the bypass flow in the up-per head remains a major question for the upper head cooling.
The upper head volume was expected to increase the cooling time roughly 7
in proportion of its size. The Westinghouse analysis of upper head cooling
5-4 indicates that there are three upper head support plate configurations which critically affect the cooldown rate. Diablo Canyon with a " top hat" support plate was estimated to require only about 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months to cool the upper head even cithout CROM fans available. Other plants with an " inverted top hat" design
-would require as much as 32 hours3.703704e-4 days 0.00889 hours 5.291005e-5 weeks 1.2176e-5 months to cool the upper head. This difference in the upper head cooling time was mainly due to the difference in the upper head volume for the different upper plant configurations.
The impact of the amount of the upper head metal structure to the upper head cooling time was more complex; while increasing the amount of ' guide tube structures, etc. would increase the sensible heat to be removed, it also in-creases the heat conduction down to the upper plate area. A simple calcula- ,
tion showed that 10% increase of the metal structure decreases the cooling time by about 4%. ,
5.6 Cooling Water The required amount of cooling water during the cooldown period was dis-cussed in Section 5.3. Additional cooling water would be needed to remove the decay heat if additional time is required to cooldown the upper head as dis-cussed in Section 4.1. For each additional hour, it was estimated that ap-proximately 5,000 gallons of additional cooling water would be needed. Decay heat level of 0.5% for the 3,300 MW plant during this period was assumed for the estimation. The required water should be linearly adjusted for different decay heat level.
5.7 Sumary i
Based on Table 5.1, the following plant and operating parameters will be required to apply the results of the natural circulation / boron mixing /
cooldown/depressurization test at Diablo Canyon to other Westinghouse plants.
l a) Total RCS volume, b) Upper head volume, c) Pressurizer water and vapor volume, ~
d) Steam generator secondary side water volume,
\
5-5 4
e) Total metal structure mass, f) Upper head metal structure (detailed geometry of the guide tubes and dome wall will be useful),
g) Elevation difference between the bottom of the core and top of U-tubes in the steam generators, h) Total pressure drop across the whole loop at 100% power, i) Pressure drop across the downcomer, core and SG U-tubes, respective-ly, j) Anbient heat loss for the entire RCS, k) Pressurizer ambient heat loss,
1) The coolant flow rate at 100% power, m) The bypass flow rate from the upper downcomer to the upper head at 100% power (and during the natural circulation if available),
n) Boron injection flow rate and concentration, o) Desired increase in boron concentration, p) Boron injection tank capacity, q) Planned cooldown rate, r) planned depressurization rate, s) Auxiliary pressurizer sprayer water temperature, t) Max. auxiliary pressurizer sprayer capacity, u) CRDM fan capacity and number of contrc1 rod drives, v) Auxiliary feedwater pump capacity, w) ' Atmospheric steam dump valve capacity, x) RHR initiation temperature and pressure, y) Condensate storage tank capacity and other water supply.
p l
l l
Tch12 5.1 Susanary cf the Sensitivity Analysis N.C. Condition Remark
! To Be Affected Plant Parameters , Base Condition Sensitivity Matural Circulation Flow 1600-1200 lb/sec 4
Decay Heat ANS 3.2% A l Steady State Coolant Flow 36,918 lb/sec 6.5% A 3 Steady State Pump ap 84 psia -3.1% A Elevation Change Between f
' Core and SG 58.3 ft 3.2% A Bypass Flow 13 lb/sec Steady State Hypass flow 77 lb/sec 10% B+
' ap Across the DC, Core,
SG 8.9/24.6/31.4 8 Boron injection Time Less than I hour 150 gpm -10% B+
Injection Flow Rate Boron Conc. of Inj. Flow 21,000 ppm -10% B+
! 300 ppm 10% B+ Y' j Desired Conc. Change
RCS Volume 12,080 ft 3 10% A J
I Boron Injection Tank Cap. 3000 gallon -- B+
1 Maxium ASD Valve Opening 70%
]
1 50*F/hr 5.5% B 2 Cooldown Rate Decay Heat AMS 4.5% C 4
! Total Water Volume (Primary A Secondary) 20,010 ft 3 41 8 i
3.08x106 lb 1.2% B I Total Metal Structure Capacity of ASDV 1.53x10' lb/hr at .
l 775 psig -10% B l
l l .
i l
i !
I
f Tchio 5.1 (Continued)-
N.C. Condition Base Condition Sensitivity Remark To Be Affected Plant Parameters Manium Required Aemiliary Pressurizer Sprayer 40 gym Finw Rate Depressurization Rate 8 psla/ min -101 8 100*F +25% 8 Spray Water Temperature Pressurizer Water Volume 900 ft 3 -101 8 Pressurizer Ambient Heat loss 130 kW -2% A Max. Aux. Pressurizer 8 Sprayer Capacity 55 gal / min --
Upper Head Cooling Time With CRDM Fans 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months Without CRDM Fans 43 hours4.976852e-4 days 0.0119 hours 7.109788e-5 weeks 1.63615e-5 months CRDM Fan Capacity 82,000 ft /3 min -101 8 13 lb/sec * ** y' Bypass Flow- ft 9 Upper Head Water Volume 471.7 ft3 61 ,
235,000 lb -4% 8 Upper Head Metal Structure (Guide Tehes and Dome Wall)
Cooling Water 360,000 gallon ANS 8% C Decay Heat 3 A Total System Water Vol. 20.010 ft 0.8%
Total Metal Structure 3.08x10 8 lb 0.3% A 43 hours4.976852e-4 days 0.0119 hours 7.109788e-5 weeks 1.63615e-5 months 61 C Upper Head Cooling Time Condensate Storage Tank 400,000 gallon -- B Capacity Other Water Supply -- -- 8 A - Results are not sensitive to these parameters.
B - Results are sensitive to these parameters.
B' - Results are not sensitive to these parameters, but these parameters can have major changes from plant to plant.
C - These parameters are estimated or assumed by the calculation.
- Difficult to determine without detailed calculation or uncertain.
- f or each 100*F increase of the sprayer water temperature.
6-1
6.
SUMMARY
AND CONCLUSION The natural circulation / boron mixing /cooldown test performed at Diablo Canyon in compliance with the design requirement of BTP PSB 5-1 for a class 2 plant was reviewed. Based on the test results and analyses, it was concluded that
1) The test sufficiently demonstrated that adequate natural circulation was established and the plant was capable of removing the decay heat by the natural circulation using only safety-grade equipment,
2) Adequate boron mixing could be achieved by the natural circulation in the main flow path of the RCS using only safety-grade equipment,
3) The effect of relatively unborated water entering the RCS from the upper head and pressurizer appears to be minimal as long as depres-surization is conducted carefully to limit the size _of possible void formation.
4) The pressure would rise and reach the PORV actuation pressure without letdown during the boron mixing period,
5) The test adequately demonstrated that it could cool the main RCS to the RHR System initiation temperature while maintaining adequate sub-cooling during the natural circulation using only safety-grade equip-ment.*
6) The test demonstrated that the upper head could be cooled without void femation when the CRDM fans were in ' operation,
7) The test results indicate that the upper head cooldown rate without the CRDM fans is about 6'F per hour. This is higher than the conser.
vative BNL calculation (accounting only for conduction heat loss) which estimated a mininum rate of 3'F/ hour.
6-2 -
8) The RCS pressure should be maintained above 1200 psia by means of -
either the pressurizer heaters (if available) or charging during the cooldown period to ivoid the void formation in the upper head when the CRDM fans were not in operation.
9) Sufficient supply of safety grade cooling water was available to sup-port the proposed plant cooldown method even if the CRDM fans were not available for the Diablo Canyon plant but the worst case require-ments (360,000 gallons) may not be available at all plants.
10) Only one motor-driven AFW pump was sufficient to supply the necessary cooling water throughout the transient.
11) Sufficient ASD valve capacity was available to support the cooldown even when the cooldown rate was 50'F/ hour.
12) The availability of the pressurizer heaters and letdown system, while not essential, would affect the operational procedures in a major way. The strategy to reduce the upper head cooling time by inten-tionally forming void may be difficult to perform without pressurizer heaters. ,
Some plants appear to have the capability to cont rol voiding by charging and venting through reactor vessel head vents.
13) The RCS pressure would increase and stay high, and the PORV may be actuated periodically if the letdown system was not available, due to the boron injection and the continuous injection of RCP seal flow.
The operation of the auxiliary pressurizer sprayer normally requires letdown to be in operation to prevent the possible thermal stress on the charging no221es. -
14) It is recommended that Westinghouse provide the details of its esti- ,
mation for the upper head cooling time without the CRDM fans. (The BNL analysis and the test data indicate that the cooling period should be substantially longer than the 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months estimated by Westing-house).
e 6-3
15) BNL concludes that the test demonstrates compliance with the require-ments of the OTP RSB 5-1 for Diabic Canyon.
l
{
l l
t
~
7-1
7. REFERENCES
1. "Diablo Canyon Units 1 and 2 Natural Circulation / Boron Mixing /Cooldown Test Final Post Test Report," WCAP 11086, March 1986.
2. " Forwards Description, Assumptions and Results of Study to Ascertain Po-tential for Void Formation in Westinghouse Designed NSSS's During Natural Circulations Cooldown/Depressurization Transients," Westinghouse OG-57, April 1981.
3. "Sumary of Meeting with FP&L and Combustion Engineering Regarding St.
Lucie Unit No.1 Cooldown on Natural Circulation," A Letter by Chris C.
Nelson, NRC, June 25, 1980, Docket No. 50-335.
4 "Palo Verde Trip Report - Boron Mixing and Natural Circulation Test," A
, Letter by C.Y. Liang and E.F. Branagan, Jr., NRC,7ebruary 26, 1986.
5. " Final Safety Analysis Report, Diablo Canyon .1 & 2," Pacific Gas and Electric Company, Docket No. 275/323, 1984
6. Private Comunication with A. Cheung of Westinghouse, October 1986.
7. "Backgroung Information for Westinghouse Owner's Group Emergency Response Guideline, EX-0.2, Natural Circul'ation Cooldown," HP-Rev.1. September 1, 1983.
8. Private Comunication with Tom Libf of PG&E, December 1986.
1 l
l
. , -- - ,- r -- - . .-.---,-- - - w-,- - -- - .. m-------, --- ,-- - -- _------,r_ -,
t .
A-1 Appendix A NATURAL CIRCULATION FLOW The single phase momentum equation states
'II
,g + h ( ov ) = - { - g,o - C fov2, 2
where C f=h+K.
The nomenclature is consistent with standard thermal / hydraulics notation.
For the steady state.
4( ov) a 0 at Therefore, 2
h ( ov2 ) = - h - g,o - C ov f
The equation above can be applied to the natural circulation condition since it is slow and thus can be assumed to be pseudo steady state.
Integrating over the loop 2
f h (ov 2) dr = - f h dz - fg edzg - fC ov f dz Since fh(ov)dz=0, 2
f $ dz - Appump '
F ~
A-2 and W = ova ,
Ap punp ~ b zodz - lC f dr = 0 .
Since W = constant for a (pseudo) steady state
Ap 0 (A*l) pump f p2)i
~f$98A)1~wf(O 2
(
For the natural circulatione, ap pump = 0.
~1I9'Alz i 9A2elev I8 cold - 8 hot)
, i -
W"2 C * (A.2)
T(C h )i {(Cf ,,3A'Il 9 AIelev ao F 0 EU nc
= Kg (Tcold - Th ot)
K lAT ,
where AZ,1,y = Elevation difference between the core and steam generator.
From the energy equation Odecay
Nnc # core
nc C
p (Thot - Tcold)
=W nc Cp AT '
o eliminating AT from the above two equations
N
nc 2 O decay . (A.3)
Or eliminating Wnc.
0 = KsaT 3 (A.4)
A-3 For the forced circulation with pump Equation (A.1) becomes s pump * "nc { (Cfh)g=W fc 2 F fc , ( A.5 )
since f (9,01)j << Appump Equation (A.3) indicates W nc and Equation (A.5) indicates F
fg a y ,
fC Fnc and Ff c are mainly functions of the geometry of the loop and weak functions of their respective velocities.
S'ince under steady state conditions the buoyancy force is balanced by the frictional resistance, ,
y 2 IC Wnc3 a ap ( A.f )
l u

ML20204J074

Text

Dear Mr. Tucker:

Subject:

References:

Enclosures:

Enclosures:

Dear Wr. Shiffer:

Enclosure:

SUMMARY

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