ML18052B408
| ML18052B408 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 12/03/1987 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML18052B407 | List: |
| References | |
| NUDOCS 8712090246 | |
| Download: ML18052B408 (8) | |
Text
1.0 INTRODUCTION
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, 0. C. 20555 SAFETY EVALUATION REGARDING A POSTULATED FIRE IN THE CHARGING PUMP ROOM OR CORRIDOR CONSUMERS POWER PALISADES PLANT DOCKET NO. 50-255
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A fire in the charging pump room or 590 corridor at Palisades could result in the loss of all three charging pumps.
In this situation, the High Pressure Safety Injection (HPSI) pumps would be available to pump water into the reactor coolant system (RCS).
However, the shutoff head of the HPSI pumps, which is about 1213 psia, is much less than the normal operating pressure.
The staff expressed concern that the pressurizer could be emptied, prior to the reactor coolant system being depressurized below the shutoff head of the HPSI pumps.
In Reference 1, the licensee committed to make modifications and procedural changes to allow the alignment of the suction side of an HPSI pump to the discharge side of a containment spray pump (CSP) in the event of such a fire.
The licensee also provided analyses which demonstrated that the RCS could be depressurized below the shutoff head for this tandem pumping arrangement without interrupting natural circulation.
Appendix R of 10 CFR 50 requires one train of the systems necessary to achieve and maintain hot shutdown conditions to be protected against fire damage or an alternative safe, shutdown capability be provided.
To meet this requirement, the licensee proposes that the discharge side of CSP P-548 be aligned to the suction side of HPSI pump P-668 to provide make-up to the RCS if all three 8712090246 a71203.., Ir,,...
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that this can be done through a 4-inch line by opening the normally-closed valve CV-3070.
The modifications and procedures for doing this shall be in accordance with 10 CFR 50 Appendix R or an exemption must be requested.
2.0 EVALUATION The pump performance curves show that by aligning the HPSI pump in tandem with the CSP pump, the HPSI pump can provide water to the reactor coolant system when the RCS pressure is 1463 psia or less.
Since the normal RCS operating pressure is 2100 psi a, _the first aspect of a safe cool down is to remove the decay heat from the core while the RCS pressure is being reduced to 1463 psia.
For the purpose of this evaluation, the discussion of the initial cooldown is divided into two phases~ i.e., the removal of decay heat prior to cooling down, and the cooldown to safety injection at 1463 psi a.
This is followed by an evaluation of the cooldown to Shutdown Cooling System temperature and then the shutdown cooling.
2.1 Removal of Decay Heat In the event of a fire in the charging pump room or corridor, the reactor will be tripped and the steam flow to be turbine will be automatically stopped.
The bypass and Atmospheric Dump Valves may not be immediately available si;ce the operating procedures instruct the operators not to commence cooldown until make-up capability and pressurizer level control are available.
As a result, the pressure in the secondary side of the steam generators could increase to the average safety valve set point.
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- -~:::c ~.,_,_.;.~:.;;:... -=-Palisades "Techn1caT~:S:peEificatHfo"3. l. T states tHat when""'e°'ve~-t:~:-re~ctbr~
is in power operation, a minimum of 23 secondary system safetl(!\\*v:alve:s, shall be operable with their :lift settings between 985 psig (tlID psiigr)'f:
and 1025 (+/-1 percent) psi g~ *Taking the average of the high l iiwiit settings, the steam generator pressure could be as high as lOl0Ji1psiaJ_
Tll~
saturation temperature of the water in the steam generator at tllis pressure is 548°F.
With :natural 'circulation flow, the temperatiure 0;u* the:"*
cold leg water exiting the steam generator will also be about 9-i-SPF.
Based upon natural ci'rculati on cool down ca lcul at ions for Pali s:a<fes a,rmL other plants, the licensee has.e.stimated an upper limit for the, core delta T as 33.8°F, resulting in a hot leg temperature of 582°F"..
- Thfs, estimate is consistent with values that the staff expects during-- natuF-a:l circulation.
Based upon staff knowledge of natural circulation;\\
calculations, the staff concludes that it is unlikely that more than: an 40°F core delta T would occur during natural circulation.
At ~pressure of 1463 psia the saturation temperature of water is 593°F.
Thus, evero using the staff's conservative assessment of core delta T, there: would:-
stil l be about 5°F of subcooling at the core outlet when the RCS is depressurized to the shutoff head of the tandem pump arrangememr.
Therefore, the staff finds that as long as the pressure in the' r.eacto'r;-..
coolant system is at or above 1463 psi a and the pressurized wa.te:r level is:~.
maintained wi\\hin the pressurizer during the depressurization, adil of t.olie decay heat will be removed by single-phase, natural-circulatiore*f.low_
lthe--::7 next section evaluates whether or not the pressurizer inventor.wean be maintained within the pressurizer while the pressurizer is coo*lie:d: dow.~,to the injection point of the combined CSP and HPSI pumps.
4 2.2 Cooldown to S<:ifety*Injection When Palisades is operating at power, the RCS pressure is 2100 psia.
The saturation temperature of water at this pressure is 643°F.
To reduce the RCS pressure. down to 1463 psia, the pressure at which a combined CSP and HPSI pump could inject water into the RCS, the pressurizer has to be cooled down to about 593°F.
When the water in the RCS is cooled down, its volume decreases; thus, the water level in the pressurizer drops.
Also, the Palisades Technical
- Specifications allow up to 10 gpm leakage from the RCS.
This leakage would also decrease the pressurizer water level.
If the pressurizer was to empty, the pressurizer would no longer regulate RCS pressure and the primary loop pressure would be maintained at the saturation temperature of the hottest water in the primary.
Additionally, bubbles could be created in the RCS which could, if they became large enough, interrupt single-phase, natural-circulation flow.
According to* the licensee's submittal (Reference 1), there is about 202,000 pounds of steel in the Palisades pressurizer.
There is also about 29,000 pounds of water in it when the water is at its normal operating level.
The rate at which the pressurizer cools down depends on the heat capacities of these two materials and how the heat is removed from them.
The heat in the steel is primarily taken away by radiation to the surroundings in the containment.
The specific heat of steel is about 0.11 BTU/pound-°F; so to cool the pressurizer down from 643°F to 593°F, about 202,000 pound x 0.11 BTU/pound-°F x 50°F = 1,111,000 BTU's have to be removed.
- 1
- . ~,
5 The-heat in the water -cane be*-take'n-*away oy-*re'nftYVi"hg the water from the -
pressurizer.
Since the enthalpy of the water in the pressurizer under normal operating conditions is greater than 600 BTU/pound, more than 600 BTU/pound x 29,000 pounds= 17,400,000 BTU 1 s can be taken away by removing the water from the pressurizer.
Thus, in the initial cooldown, much more heat can be taken away from the pressurizer by removing water than can be taken away be radiating heat to the containment.
For the cooldown scenario being examined herein, when water is leaked from the RCS, the same amount is removed from the pressurizer.
Thus, the cooldown rate of the pressurizer *is a direct function of the RCS water leak rate; the higher the RCS water leak rate, the faster the pressurizer will initially cool down.
To perform the cooldown evaluation, the licensee wrote a computer program to calculate the pressurizer pressure versus water level.
From Palisades test data, the licensee determined that the radiative heat loss rate from the pressurizer is initially 150 kw and then decreases linearly with temperature down to 100°F.
In the licensee 1 s analysis, the 150 kw heat loss rate was assumed to be not only that due to radiation but also that due to an RCS leakage rate of 10 gpm and the contraction of water in the RCS~ caused by a drop of 4°F in the average water temperature.
Using these assumptions) the licensee calculated that the RCS would naturally cool to 1463 *psia pressure in about five hours and the water level,in the pressurizer would be about 6 percent.
A more conservative calculation that assumed a 125 kw initial pressurizer heat loss rate showed a longer cool down time which resulted in a lower pressurizer water level, but the water level was still maintained on scale.
6 S1hce ~he initial cool down rate of the press~rizer is primari)y due to the RCS water leak rate and since the Palisades test was obtained with essentially no leak rate, the staff has concluded that the licensee's calculated cooldown time is conservatively long.
Thus, the staff agrees with the licensee's conclusion that the pressurizer inventory will be maintained on scale while the RCS depressurizes to the shutoff head of the tandem pump arrangement.
Therefore, natural circulation will be maintained during the initial cooldown.
From St. Lucie cooldown data and a simple hand calculation, the licensee concluded that during the first five hours after the event, the reactor vessel head will not be hotter than the pressurizer; so it will not control the RCS pressure.
The staff agrees that during this time the reactor vessel head temperature will be less than the pressurizer temperature.
Specifically, the staff finds that the reactor vessel head temperature will be less than 593°F at five hours after the event.
Since the saturation pressure at this temperature is 1463 psia and since the combined CSP and HPSI pumps can inject water into this pressure, the staff concludes that the flashing of the upper head will not occur prior to safety injection.
2.3 Cooldown to Shutdown Cooling System Temperature
. After the RC~ pressure has been reduced to 1463 psi a, the combined CSP and HPSI pumps will begin injecting water into the RCS and will refill the pressurizer to its normal level with relatively cool water.
At this time, two Atmospheric Dump Valves (ADVs) would be operated to vent steam
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7 to cool down the**react6r~
Steam would also be going to the AuxiTfary _____ L.:...2-: ____,=...:.::..:....c.
Feedwater (AFW) turbine to drive the pump and to the Hogging Air Ejector.
The licensee 1 s calculations show that two AOVs have sufficient steam flow rate cap~city, even at low steam generator pressures, to provide a sufficient energy removal rate to cool the RCS hot leg temperature below 300°F within 62 hours7.175926e-4 days <br />0.0172 hours <br />1.025132e-4 weeks <br />2.3591e-5 months <br /> of the reactor trip.
The staff finds this acceptable.
2.4 Shutdown coolinq The licensee 1 s analyses show that, when the RCS temperature is below 300°F, the Shutdown Cooling (SOC) system can reduce this temperature at the rate of 40°F/hour, which because of thermal stresses~ is the maximum allowable cooling rate.
At this rate the SOC system could cool the RCS temperature down to the cold shutdown condition (210°F) in 2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.
However, the licensee has conservatively allowed ten hours for shutdown cooling.
Thus, the staff finds that the plant can be cooled to the cold shutdown condition in less than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
3.0 CONCLUSION
S The staff finds that in order to provide adequate core cooling after a fire in the charging pump room or in the 590 corridor at the Palisades plant, a modification and new emergency procedures are required.
The modification is to isolate from the fire area the control of the valves that are necessary to align the suction side of a HPSI pump to the discharge side of a CSP.
This
8 modification-is*-required to prevent l<rs-H1g~*c'ont'r*oT of these valves during such a fire.
New emergency proce~ures are required to get the discharge side of CSP P-548 aligned to the suction side of HPSI pump P-668 in this event_
The staff finds that the core can be adequately cooled by natural-circulation cooling during the extended, hot shutdown period while the RCS pressure is dropping to 1463 psia.
The staff also finds that, after the above changes are completed, the core can be adequately cooled during the cooldown phase while water is being injected into the primary coolant system by the combined containment spray pump and high pressure injection pump.
With the above required modifications and procedures implemented, the staff agrees with the licensee's conclusion that the Palisades plant can be safely cooled to the cold shutdown condition in less than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> after a fire in the charging pump room or in the 590 corridor.
- 4. 0 REFERENCE
- 1.
Letter to Director, NRR, USNRC, from K. W. Berry, Consumers Power, dated December 23, 1986
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