ML17308A446

Technical Evaluation Rept for Natural Circulation,Boron Mixing & Cooldown at St Lucie Unit 2.

ML17308A446
Person / Time
Site:	Saint Lucie
Issue date:	03/31/1988
From:	Jo J, Perkins K BROOKHAVEN NATIONAL LABORATORY
To:	Office of Nuclear Reactor Regulation
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ML17221A730	List: ML17308A446
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CON-FIN-A-3843 NUDOCS 8804050126
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ENCLOSURE 2 TECHNICAL EVALUATION REPORT FOR THE NATURAL CIRCULATION, BORON HIXING AND COOLDONN AT ST; LUCIE UNIT 2 J.H. Jo and K.R. Perkins Containment 8 Systems Integration Group

'Department of Nuclear Energy Brookhaven Nati onal Laboratory Upton, New York 11973 March 1988 Prepared for U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Washington, D.C. 20555 Contract No. DE-AC02-76CH00016 FIN A-3843

ABSTRACT Thi s report revi ews the conformance of the St. Luci e Uni t 2 nucl ear pl ant operation and equipment with the requirements of Reactor Safety Branch Tech-nical position 5-1 regarding boron mixing and cooldown under natural circula-tion conditions. The utility has referenced the San Onofre tests as demon-strating the St. Lucie capability.

The BNL review indicates that the plants are sufficiently similar to allow St. Luci e Unit 2 to take credit for the San Onofre test results. How-ever, the technical specifications for commitments of water in the Unit 2 con-densate storage tank to the auxiliary feedwater system do not allocate a suf-ficient volume of water to accomplish cooldown under the worst case condi-tions.

TABLE OF CONTENTS Page A BSTRACT................................................. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ill ACKNOWLEDGMENTS....................................................... V11 1 INTRODUCTION.....,....,.....,......,.........,....,,..............

2. CONCLUSIONS FROM THE EVALUATION OF THE SONGS NATURAL CIRCULATION COOLDOWN TEST AND

SUMMARY

OF SENSITIVITY STUDY.................... 2-1 2.1 Conclusions From the Review of the SONGS Natural Circulation C ooldown Test.................................... 2-1 2.2 Summary of Sensitivity Analysis for a CE Pre-Syst 80 PWR 2-4 2.2.1 Natural Circulation... ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 2-4 2.2.2 Boron Mixing.......... 2-5 2.2.3 RCS Cooldown.......... 2-5 2.2.4 Depressuri zati on...... 2-5 2.2.5 Upper Head Cooling.... 2-6 2.2.6 Cooling Water......... ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 2-7

3. APPLICABILITY OF THE SONGS TEST TO ST. LUCIE UNIT 2............... 3-1 3.1 Natural Ci rcul ati on. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 3-1 3.2 Boron Injection..... ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 3-2 3.3 Cooldown............ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ J ~ ~ ~ ~ ~ ~ ~ 3-2 3.4 Depressuri zation,... ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 3-3 3.5 Upper Head Cooling.. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ l ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 3-4 3.6 Supply of Condensate Water ~ ~ ~ ~ ~ ~ t~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 3-4 4~ CONCLUS I ONS ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 4-1 5 REFERENCES........................................................ 5-1

ACKNOWLEDGMENTS This work was performed for the Planning, Program and Management Support Branch, NRC/NRR. Mr. B.L. Grenier is the Project Manager and D. Katze and R.C. Jones, GEST/RSB, are the Technical Monitors. C.-Y. Liang, DEST/RSB, and E. Branagan DRPEP/RPB, have al so provided considerable assistance to the project.

The authors .are also grateful to Ms. S. Flippen for her excellent typing of the manuscri pt.

1-1

l. INTRODUCTION While cooling down under natural circulation conditions on June ll, 1980, the St. Lucie Unit 1 primary system coolant flashed and produced a void in the reactor vessel upper head, which forced water into the pressurizer. At the time of the event there was concern that the void formation would interrupt natural circulation and inhibit decay heat removal. However, the voiding was controlled by depressuri zati on and eventually the reactor was brought to shut-down cooling system (SCS) entry conditions. Based on the NRC review of the event, a multi -plant action item (NPA B-66) was initiated which requires that all pressuri zed water reactors (PWRs) implement procedures and training pro-grams to ensure the capability to deal with such events. In Generic Letter (GL) 81-21, dated tray 5, 1981, the licensees were required 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 accordance wNh 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. The Reactor Sys-tems Branch Technical Position (RSB 5-1) requires an adequate supply of auxiliary feedwater stored in safety grade systems.

3. a description of the plant training .program and the provisions of emergency 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, Combustion Engineering (C-E) has issued an analysis~ sup-porting natural circulation cooldown with voids and subsequent testing at Palo Verde has demonstrated cooldown and depressuri zation with void formation.

While the NRC staff considers natural circulation cooldown without voids as more desirable, cooldown with voids may be acceptable providing it can be

1-2 accomplished using only safety-grade equipment and approved procedures, and operators have adequate training in the use of these procedures.

Additional requirements for pre-operational testing are set forth in the Standard Review Plan under RSB 5-1. This technical position essentially requires that a Class 2* plant demonstrate that it can be brought from hot standby to cold shutdown under the natural circulation conditions using only systems which are safety grade and with only onsite or offsite (not both) power available and assuming a single failure.

RSB 5-1 also requires that PWR pre-operational and initial startup test programs shall include tests with supporting analyses to (a) confirm that adequate mi xing 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 mi xing, and (b) confirm that 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 desi gn may be subshituted for these tests.

In response to these requirements, Florida Power and Light referenced the natural circulation cooldown and boron mixing test which was to be conducted at Unit 2 of the San Onofre Nuclear Generating Station (SONGS) as being appli-cable to St. Luci e Unit 2. The SONGS natural circulation test was performed on July 27, 1983 and the results were presented in CEN-259, "An Evaluation of the Natural Circulation Cooldown Test Performed at the San Onofre Nuclear Gen-erating Station." 3 BNL reviewed the test report and issued a Technical Evalu-ation Report (TER). 4 The specific items addressed by the SONGS natural circulation cooldown test included a demonstration of the ability to mi x boron under natural ci rcu-lation,, an evaluation of reactor vessel upper head cooldown rates with and requirements

)1 1<<1 <<1 PPP for plant heat removal capability for compliance with its posi-tion. The classification was based on the date when CP (construction permit) or PDA (preliminary design approval) applications were docketed and/or an OL (operating license) was issued. Recommended implementation for a Class 2 plant i s specified in the 'position letter. St. Lucie Unit 2 is a Class 2 plant.

1-3 without operation of the control element drive mechanism cooling fans, an assessment of the adequacy of seismic Category I condensate supply and an evaluation of the adequacy of the safety grade nitrogen supply for the atmo-spheric dump valves. A subsequent tests at San Onofre addressed the perfor-mance of the residual heat removal (RHR) system with only natural circulation in the primary system.

Section 2 of this report summarizes the conclusions derived from the BNL evaluation of the SONGS natural circulation cooldown test., The sensitivity analysis which was performed as a part of the SONGS test evaluation to facili-tate the application of the test results to other C-E Pre-System 80 plants is also summarized in this section. In Section 3, the applicability of the SONGS test results to St. Lucie Unit 2 is discussed.

2-1

2. CONCLUSIONS FROM THE EVALUATION OF THE SONGS NATURAL CIRCULATION COOLDOl)N TEST AND SUHt1ARY OF SENSITIVITY STUDY In compliance with the requirements of RSB 5-1 for a Class 2 plant, SCE (Southern Cali fornia Edison), performed a natural circulation cool down/boron mixing test at the San Onofre Nuclear Generating Station and submitted the test report CEN-259, "An Evaluation of Natural Circulation Cooldown, Test Per-formed at the San Onofre Nuclear Generating Station." s BNL evaluated the data, analyses and conclusions cited in the report and issued a Technical Evaluation Report (TER). " The conclusions from the TER are summarized in this section. Also included is the summary of the BNL sensitivity study which was performed as a part of the SONGS test evaluation is also included.

2.1 Conclusions From the Review of the SONGS Natural Circulation Cooldown Test The Natural Circulation Cooldown Test and the Remote Initiation of Shut-down Cooling Test which were performed at %NGS Units 2 and 3 to demonstrate their compliance with the design requirement of RSB 5-1 for a Class 2 plant were reviewed. The BNL Technical Evaluation Report4 concluded that:

1) The test sufficiently demonstrated that adequate natural circulation was established and the plant was capable of removing the decay heat by natural circulation using only safety-grade equipment.

. 2) Adequate boron mixing could be achieved in less than one hour by the natural circulation within the main flow path of the RCS using only safety-grade equipment.

3) Relatively unborated water enteri ng the RCS from the upper head and pressurizer will not have a significant effect on criticality as l'ong as depressurization is conducted carefully to limit the size of pos-sible void formation.

4) Boron injection may be conducted prior to cooldown without filling up the pressurizer even when letdown is not available. However, it

2-2 appears desirable to allow the pressurizer level to decrease during the natural circulation prior to boron injection. Operation of the auxiliary spray may be necessary to maintain pressure control during boron injection.

5) The test adequately demonstrated that it could cool the main flow paths of the RCS to the SCS initiation temperature while maintaining adequate subcooling during the natural ci rculation using only safety-grade equipment.

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

7) The test demonstrated that the RCS could be depressuri zed to the SCS initiation pressure under natural ci rcul ation using the auxi1 i ary ciallyy spray when the letdown was available. However, if the letdown is not available the pressurizer level would increase by about 405 and it may be necessary to use the head Went valve to depressurize, espe-near the end of the depressuri zation period.

8) The estimated cooling time for the upper head without the CEDE fans varied from 15.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> (C-E analysis) to about 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> (BNL). The difference appears to be due to assumptions regarding the uniformity of the upper head fluid temperature.

9) The pressurizer level would have to be maintained at 605 or higher to maintain the RCS pressure above the saturation pressure corresponding to the upper head fluid temperature during the prolonged period of upper head cooling.

10) Sufficient supply of safety grade cooling water was available to sup-port the proposed plant cooldown method even if the CEDN fans were not available for SONGS.

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

2-3

12) Suffi ci ent atmospheric dump val ve (ADV) capacity was avail abl e to support the cooldown even when the cooldown rate was'0'F/hour. How-ever, it may be necessary to lower the cooldown rate if one of two ADVs is not available.

13) sirablee Additional supply of safety-grade nitrogen gas for the ADVs is de-for the prolonged cooldown period. However, the test demonstrated that ADVs could be manually operated via manual hand-wheels in the event the nitrogen supply should become depleted.

14) The strategy for pressure control should be very carefully planned when pressurizer heaters and letdown are not available. Both of the available safety-grade pressure control systems (charging and surizerr auxiliary spray) require injection of additional water into the system. Without letdown this may result in overfilling of the pres-(and water-solid operation). Occasional use of the head vent valve may be preferable to extended auxiliary spray operation.

ingg

15) The SONGS 2 natural circulation cooldown test, combined with the supporting analysis, demonstrated that the plant meets the RSB 5-1 requirements for a Class 2 plant with respect to the natural ci rcula-tion, boron mixing, safety-grade condensate water supply and capabi 1-ity to operate the ADVs. However, the test did not provide any si g-nificant information -applicable to the estimation of upper head cool-time without the CEDtl fans operating. The estimated time for the upper head cooling by C-E appeared to be optimistic.

16) The SONGS 3 test demonstrated that the SCS could be remotely i niti-ated from the control room under natural circulation conditions and that the plant could be controlled and cooled under natural ci rcula-tion using only one train of SCS. However, it would take consider-ably longer to achieve cold shutdown under natural circulation than demonstrated in the test where cold shutdown was achieved with the RCPs and CEDM fans in operation. Since the SCS has an unlimited heat sink no safety concerns are expected to arise due to the prolonged SCS operation.

2-4 2.2 Summar of Sensitivity Analysis for a C-E Pre-System 80 PWR The results of the natural circulation and cooldown test performed at SONGS are referenced by other Pre-System 80 C-E plants to establish compliance with RSB 5-1. To facilitate comparisons to other plants, the parameters which appear to affect application of the test results to other C-E Pre-System 80 plants were identified and a sensitivity study was performed as part of the San Onofre test evaluation." The results of the sensitivity study are summar-ized in Table 2.1. The tabulated .sensitivity provides the estimated change of the natural circulation conditions for each 10$ change of the plant parameters from the base condition unless otherwise mentioned.

2.2.1 Natural Circulation The major parameters which affect the natural circulation flow are:

1. Level of decay heat,

2. Elevation change between the top of the steam generator 0-tubes and bottom of the core, and

3. Friction in the loop, which is monitored as the total pressure drop across the loop as a function of flow.

Natural circulation i s long and slow, and can be considered to be pseudo steady state. Table 2.1 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 SONGS condition. It indicates that the natural circulation flow rate is generally not sensitive to the expected vari-ation of most plant conditions. Since the ability to cool down and to mi x boron in the main loop of St. Luci e is not significantly affected by slight changes in the natural circulation flow rate, it is concluded that these parameters do not have a major impact on the plant's ability to cool down and mi x boron.

2-5 2.2.2 Boron Ni xing The ability to mi x boron prior to cooldown and the time required to achieve this mixing under the natural circulation conditions in St. Luci e depend on the configuration of the RCS, the injection rate of boron relative to the total inventory of water in the 'RCS, and the required increase in the boron concentration.

Since the time needed for boron injection is much less (order of 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />) than the available time prior to the initiation of cooldown (order of 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />) as demonstrated by the SONGS test, minor variations in the boron injection time due to possible variation of these plant parameters would not si gnifi-cantly affect the plant's ability to inject and mi x boron.

2.2.3 cientt RCS Cooldown The plant's ability to cool the RCS at a specified cooldown rate under the RSB 5-1 scenario is determined by the capacity of the ADVs to allow suffi-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, as'ell as 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 2.1 shows the sensitivity of the ADV opening and the required AFH sensitivity to the parameters affecting the cooldown. (The re-quired AFW amount includes the additional amount of water required to remove the decay heat during the upper head cooldown period when the CEDtl cooling fans are not in operation.) The available capacity of the ADV and supply of cooling water for other plants should be compared to the required ADV opening and supply of AFW listed in Table 2.1 to determine thei r adequacy.

.2.4 The parameters affecting the depressurization rate are the water inven-tory at the pressurizer, pressurizer auxiliary spray water temperature and spray flow rate. The amount of the ambient heat loss will also affect the demand on the auxiliary pressurizer sprayer.

2-6 Table 2.1 summari zes the sensitivity of the depressur i zation rate to these parameters. If the desired depressuri zati on rate is more than the capacity of the auxiliary pressuri zer spray, manual operation of the head vent valve (or a power operated relief valve (PORV) if it is available) would be needed to achieve the desired rate.

2.2.5 Up er Head Coolin The major parameters affecting the upper head cooling are:

1) Whether the CEDH fans are in oper ation,

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 CEDH fans is the dominating factor to determine the up-per head cooling rate when they are in operation. It was shown during the SONGS test that the upper head would be cooled to the saturation temperature of the SCS entry pressure within two hours of completion of the main RCS cool-down when the CEDN fans were in operation.

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 is difficult to deter-mine the degree of mixing when the fans are not in operation. The degree of mixing of the bypass flow in the upper head remains a major question affecting the rate of the upper head cooling.

The impact of the amount of the upper head water and metal structure to the upper head cooling time was complex. While increasing the amount of guide tube structures, etc. would increase the sensible heat to be removed, it would also increases the heat conduction down to the upper plate- area, A simple calculation shows that a 10$ increase of the metal structure decreases the

2-7 cooling time by about 4$ . An increase of the upper head water volume was expected to increase the cooling time roughly in proportion to its size when the upper head volume varied slightly from that of SONGS. However, for the C-E System 80 plants which have much larger upper head volume a separate analysis would be needed.

2.2.6 Cooling Water The required amount of cooling water during the cooldown period was dis-cussed in Section 2.2..3. Additional cooling water would be needed to remove the decay heat if additional time is required to cool down the upper head.

For each additional hour, it was estimated that approximately 6,000 gallons of additional cooling water would be needed. A decay heat level of 0.55 for a 3,390 HW plant during this period was estimated.

4'able 2.1 Summary of the Sensitivity Analysis ditionn Natural Circulation Con-To Be Affected Plant Parameters Base Condition Sensitivity Remark Natural Circulation Flow 1300-1100 lb/sec Decay Heat ANS Standard 3. 3X Steady State Coolant Flow 4l,l ll 1 b/sec 6.6$

Steady State Pump zp 96.7 psia 3%3)I Elevation Change Between Core and Steam Gener- 65.0 ft 3.2X ator (SG)

Bypass Flow Under the 12 lb/sec Natural Circulation Steady State Bypass Flow 493 lb/sec 10%

zp Across the Vessel/SG 43.4/47.0 Boron Injection Time Less than 1 hour I

Injection Flow Rate 88 gpm -10$ B+

CO Boron Conc. of Inj. Flow 4,000 ppm -105 B+

Desi red Conc. Change 230 ppm 105 B+

RCS Volume 9,900 ft s 10$ A Maximum ADV Opening 90$

Cooldown Rate 50'F/hr 5.5$ B-Decay Heat ANS Standard 4.5$ B,C Total Water Volume (Primary 8 Secondary) 17,800 fthm 4$

Total Hetal Structure 4.9xl0s lb 1.2%

Capacity of ADVs 1.51x10s lb/hr at 900 psi a -105

Table 2.1 (Continued)

Natural Circulation Con-dition To Be Affected Plant Parameters Base Condition Sensi ti vi ty Remark Auxiliary Pressurizer Spray Flow Rate 47 gpm Depressuri zation Rate 10 psia/min -10$

'Spray Water Temperature** 100'F +25%

Pressurizer Water Volume 900 fthm -10$

Pressurizer Ambient Heat Loss 130 kW Max. Aux. Pressurizer Spray C'apacity 88 gal/min Upper Head Cooling Time Without CEDM Fans 38 hours Bypass Flow 12 lb/sec Upper Head Water Volume 650 10$ B Upper Head Metal Structure ft'55,000 lb -4$ B,C (Guide Tubes and Dome Wall)

Cooling Water 350,000 gallon Decay Heat ANS Standard 8$ B,C Total System Water Vol. 17,800 fthm 0.8$ A Total Metal Structure 4.9xl0e lb 0.3$ A Upper Head Cooling Time 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> 6$ C Total Available Safety Grade 344,000 gallon 8+

Cooling Water A - Results are not sensitive to these parameters.

B - Results are sensitive to these parameters.

8+ - 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.

• • - For each 100'F increase of the sprayer water temperature.

3- 1

3. APPLICABILITY OF THE SONGS TEST TO ST. LUCIE UNIT 2 The Combustion Engineering plants can be divided essentially into two categories with respect to the requirements of RSB 5-1, Pre-System 80 and Sys-tem 80. The most significant distinction between these two types is the rela-tive size of the reactor vessel upper head: the volume of the RVUH of .the Pre-System 80 design is approximately half of that of the System 80 design.

SONGS Units 2 and 3, and St. Lucie 2 belong to the Pre-System 80 design.

Accordingly C-E stated that the procedures that would be employed by St. Lucie 2 to meet RSB 5-1 would be identical to that employed by San Onofre. Specif-ically, following plant cooldown to the shutdown cooling system (SCS) initi a-tion temperature (350'F), the system pressure would be maintained relatively high for a fifteen hour hold period until the RVUH had cooled sufficiently to depressuri ze the RCS and place the plant on shutdown cooling without forming a steam bubble in the upper head.

The areas of specific concern during the natural circulation cooldown as previously discussed included boron mixing, reactor vessel upper head cool-down, seismic Category I condensate supply, and nitrogen capacity for ADV con-trol. These items will be discussed for each stage of natural circulation, boron mixing, cooldown 'and depressuri zation.

3.1 Natural Ci rcul ati on Applicability of the SONGS test to St. Lucie Unit 2 depends on similarity in design between these two plants. St. Lucie Unit 2 and SONGS Unit 2 have very similar RCS configurations.s The diameters, piping lengths and config-uration of the hot and cold legs of these plants are identical. The sizes of the St. Lucie vessel (4,615 ft ) and steam generator (1,627 ft ) are slightly smaller than those of SONGS (4,957 ft and 1,830 ft , respectively), but the

'configurations are similar . The rated thermal power of St. Lucie is about 755 of that of SONGS, and the steady state coolant flow rate of St. Lucie is about 55 less than that of SONGS. Since the configurations of the RCS and coolant flow rate of both plants are similar, and the natural circulation flow rate, the aT (temperature differential across the core), and consequently the plant's ability to remove the decay heat under natural circulation are not

3-2 sensitive to the level of decay heat as indicated by the sensitivity study, the conclusions of the SONGS test regarding the natural circulation will apply to St. Lucie Unit 2. That is, adequate natural circulation will be estab-lished to remove the decay heat by the natural circulation using only safety-grade equi pment.

3.2 Boron Injection Since the configurations of the RCS of both plants are very similar as discussed above, and the required time for boron mixing (order of one hour) is much less than the available time prior to the initiation of cooldown (about four hours) as demonstrated by the SONGS test, boron mixing at St. Lucie will be similar to that at SONGS and the conclusions obtained at SONGS will apply to St. Lucie Unit 2. They are:

a) Adequate boron mixing wi 11 be achieved within the available time prior to cooldown by the natural circulation in the main flow path of the RCS usi ng only safety-grade eq6ipment.

b) The effect of relatively unborated water entering the RCS from the upper head and pressurizer will not have a significant effect on criticality as long as depressuri zation is conducted carefully to limit the size of possible void formation.

c) Boron injection may be conducted prior to cooldown without filling up the pressurizer even when letdown is not available. However, it appears desirable to allow the pressurizer level to decrease during the natural circulation prior to boron injection, Operation of the auxiliary spray may be necessary to maintain pressure control during boron injection.

quiredd 3.3 Cooldown The SONGS test and the BNL analysis demonstrated that cooldown of the RCS to the SCS entry temperature could be accomplished while maintaining the re-subcooling during the natural circulation using only safety-grade

3-3 equipment at SONGS Unit 2. Since the configurations of the RCS and steam gen-eratorss of St. Lucie are similar to those of SONGS, the same conclusion ap-plies to St . Lucie. As in SONGS, the pressuri zer level would have to be main-tained at least at 605 to prevent rapid depressuri zation and keep the required margin of subcooling in the upper head in case the CEDM fans and pressurizer heaters are not available.

Since the total RCS water and metal mass of St. Lucie Unit 2 is about 105 less, and the decay heat at St. Lucie is about 24$ less than those of SONGS, the total steam flow required to cool down the RCS at 50'F/hour and remove the decay heat is estimated to be about 16$ less at St. Lucie. Table 2.1 shows that the maximum estimated opening of ADVs at SONGS during the cooldown was about 90$ . Since the capacity of the ADVs at St. Lucie is about 73$ of that of SONGS, it may be difficult to sustain the 50'F/hour cooldown rate at the end of the cooldown period at St. Lucie when the steam generator pressure is low. BNL estimates that the maximum cooldown rate at the end of cooldown when the ADVs are fully open, would be about 45'F/hour. It should be noted that this estimation was based on a conservative %ssumpti on of maximum decay heat.

With respect to the supply of nitrogen to the ADVs, the St. Lucie Unit 2 has DC motor operated valves capable of being powered from safety-grade vital buses. Therefore, operation of ADVs will be available at all times from the control room. It should also be noted that the ADVs at St. Lucie Unit 2 are equipped with manual handwheels as in SONGS and thus manual 'local control is possible in case operation from the control room is not possible. The opera-tion of ADVs in manual control was demonstrated during the SONGS test.

3.4 Depressuri zation Since the capacity of the auxiliary pressurizer spray and the size of the pressuri zer of SONGS Unit 2 and St. Lucie Unit 2 are identical and configura-tions of the rest of the RCS are similar, the same conclusions obtained at SONGS Unit 2 will apply to St. Lucie Unit. 2: The RCS can be depressuri zed to the SCS initiation pressure under natural circulation conditions usi ng the auxiliary spray. It may also be necessary to use the PORVs during the

3-4 depressurization period to prevent filling up pressurizer if letdown is not avail abl e.

3.5 Upper Head Cooling Since the configurations of the RCS and size of the upper head of St.

Lucie Unit 2 and SONGS Unit 2 are similar, the RVUH cooldown under natural circulation conditions with CEDE cooling fans in operation and with CEDN cool-ing fans secured would be similar for both plants., When the CEDN cooling fans are in operation, the upper head can be cooled without void formation and depressurization can proceed within two hours of completion of RCS cooldown.

With CEDN fans secured, the'stimated cooling time for the upper head ranges from 15.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> by C-E to about 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> by BNL. The FSAR of St. Lucie Unit 2~ estimated the upper head cooling time to be 25.7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />. The differences in these estimations are due to different assumptions regarding mixing in the upper head. More studies would be needed for a better prediction of the upper head cooling. As in SONGS, controlling the pressure to maintain the margin of subcooling in the upper head during the prolonged period of upper head cool-down may be difficult without the pressuri zer heaters, and a strategy to allow formation of steam bubbles while cooling the upper head fluid may have to be considered.

3.6 Supply of Condensate Water It was estimated that about 350,000 gallons of auxiliary feedwater would be needed at SONGS, which included the additional cooling water to remove the decay heat during the prolonged upper head cooldown period when CEDN cooling fans were not available. Since St. Lucie Unit 2 has about 10$ less water and metal mass in the RCS and about 25% less decay heat than SONGS Unit 2, the estimated cooling water requi rement at St. Lucie Unit 2 is reduced to 276,000 gallons assuming the same upper head cooling time. The FSAR of St. Lucie Unit 2 (Appendix 5.2.8) estimates that it required about 270,500 gallons of condensate water in 25.7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> to cool the RCS and the vessel upper head using very conservative assumptions regarding fluid mixing in the upper head.

3-5 The condensate storage tank (CST) of St. Lucie Unit 2 contains about 400,000 gallons of safety grade cooling water; of this, 154,000* gallons are reserved for exclusive use of St. Lucie Unit 2, another 154,000* gallons are reserved for St. Lucie Unit 1 in the (highly unlikely) event. that a vertical tornado missile ruptures the St . Lucie Unit 1 CST (the CST of St . Lucie Unit 1 is not considered to have a full protection from a vertical tornado missile),

and the remaining usable portion of the CST volume is provided for the Unit 2 secondary system makeup during normal plant operations (St. Lucie Unit 2 FSARs Sections 5.4.7.5, 9.2.6.2 and 10.4.9, and SER~ Section 9.2.6).

Thus, the present technical specifications at St. Lucie do not appear to allocate enough auxiliary feedwater from the CST to accomplish cooldown to cold shutdown conditions under the strict RSB 5-1 scenario. However, wi th no tornado warnings in effect, St. Luci e Unit 2 appears to have a sufficient sup-ply of safety grade cooling water to last the prolonged upper head cooling period.

'MP 9.2.6.2 of

<<11 FSAR,s Section y p 5.4.3 of SER~ and Item 3.7.1.3 of Technical Speci-fication of St. Lucie Unit 2; i.e., 149,600 gallons for Unit 2 and 125,0QQ gallons for Unit 1. This allocation was based on the estimated requirement of condensate water to compl ete normal shutdown cool ing (FSAR~ Section 10.4.9.3).

t~

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4. CONCLUSIONS The applicability of the SONGS natural circulation cooldown test to St.

Luci e Unit 2 was assessed. Differences between the two plants were identi-fied. Where differences existed, the sensitivity study conducted for San Onofre was used to approximate the expected behavior at St. Lucie. Based on this review it was concluded that:

1) Adequate natural circulation can be established and the plant is capable of removing the decay 'neat by the natural circulation using only safety-grade equipment.

2) Adequate boron mixing can be achieved within the available time prior to cooldown by the natural circulation in the main flow path of the RCS using only safety-grade equipment.

3) Relatively unborated water entering the RCS from the upper head and pressurizer will not have a si gniffcant effect on criticality as long as depressuri zation is conducted carefully to limit the size of pos-sible void formation.

4) Boron injection may be conducted prior to cooldown without filling up the pressurizer even when letdown is not available. However, it appears desirable to allow the pressurizer level to decrease during the natural circulation prior to boron injection. Operation of the auxiliary spray may be necessary to maintain pressure control during boron injection.

5) Cooldown of the main RCS to the SCS initiation temperature can be accomplished while maintaining adequate subcooling during the natural circulation using only safety-grade equipment.

6) The upper head can be cooled without void formation when the CEDti cooling fans are in operation.

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7) The RCS can be depressuri zed to the SCS initiation pressure under natural circulation using the auxiliary spray if the letdown system is available. However, if the letdown is not available the pressur-izer level would increase substantially and it may be necessary to use the PORVs to depressuri ze, especially near the end of the depres-surization period.

8) The estimated cooling time for the upper head without the CEDN fans varied from 15.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> by C-E analysis to about 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> by BNL. The difference was due to assumptions regarding the uniformity of the upper head fluid temperature.

9) The pressurizer level would have to be maintained at 60/ or higher to maintain the RCS pressure above the saturation pressure corresponding to the upper head fluid temperature during the prolonged period of upper head cooling.

10) A sufficient supply of safety grade cooling water is available to last the prolonged upper head cooling period even if the CEDH fans were not available as long as the CST water does not have to be shared with Unit l.

11) Only one motor-driven AFM pump is sufficient to supply the necessary cooling water throughout the postulated transient.

12) Sufficient ADV capacity would be available during most of the cool-down period with the cooldown rate of 50'F/hour. However, it may be necessary to lower the cooldown rate at the end of the cooldown peri-od under maximum decay heat conditions. Further reduction of the cooldown rate might be needed if one of two ADVs would not be availa-ble.

13) Operation of the ADVs would be available at all times from the con-trol room since the ADVs are DC motor operated valves capable of being powered from safety grade vital buses. The ADVs could also be

4-'3 operated via manual handwheels in the event operation from the con-trol room is not possible.

14) The strategy for pressure control should be very carefully planned when the pressurizer heaters and the letdown system are not availa-ble. Both of the available safety-grade pressure control systems (charging and auxiliary spray) require injection of additional water into the system. Without letdown thi s may result in overfilling of the pressurizer (and water-solid operation). Occasional use of the PORVs may be preferable to extended auxiliary spray operation.

15) The capability to cooldown to cold shutdown conditions, once SCS entry conditions are reached, has not been evaluated but a test at San Onofre indicated that SCS cooling would be prolonged unless the reactor coolant pumps (RCPs) are available to circulate water through the steam generators. Since the SCS has an unlimited heat sink no safety concerns are expected to arise due to the prolonged SCS opera-tiatedd tion.

16) The NRC requirement to demonstrate that the SCS can be remotely ini-from the control room has not been addressed at St . Luci e.

In summary, it i.s concluded that the St. Lucie Unit 2 plant is in compli-ance with the requirements of RSB 5-1 with the exception of demonstrating re-mote initiation of the SCS, if the CST water does not have to be shared with Unit 1.

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5. REFERENCES

1. "Natural Circulation Cooldown Reanalysis for CESSAR-F Computer Simulation of a Natural Circulation Cooldown," Letter from A.E. Sherer, C-E, to D.G.

Eisenhut, NRC, LD-83-074, August 12, 1983.

2. "An Evaluation of the Natural Circulation Cooldown Test Performed at the

,Palo Verde Nuclear Generating Station," Arizona Nuclear Power Project, Revision 0, February 1987.

3. "An Evaluation of the Natural Circulation Cooldown Test Performed at the San Onofre Nuclear Generating Station," Combustion Engineering, CEN-259, January 1984.

4. "Technical Evaluation Report for the Natural Circulation, Boron Hixing and Cooldown Test Performed at San Onofre Nuclear Generating Station," Brook-haven National Laboratory, Technical Report A-4843, October 1987.

5. "Remote Initiation of Shutdown Cooling Test Performed September 16-17, 1985 at the San Onofre Nuclear Generating Station Unit 3," Letter Report from N.O. Hedford, SCE, to G.O. Knighton, December 27, 1985, Docket No.

50-362,

6. "Final Safety Analysis Report for St. Lucie Plant, Unit 2," Docket No. 50-389, Amendment 5, August 1981.

7. "Safety Evaluation Report Related to the Operation of St. Lucie Plant, Unit No. 2," Docket No. 50-389, NUREG-0843, October 1981.

8. "Technical Specification for St . Lucie Unit 2," Appendix A to Operating License No. NPF-16, Docket No. 50-389.

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