ML20115D277

Naesc Evaluation of Cooling Tower Basin Heatup Analysis W/Sprays & Fans Manually Controlled

ML20115D277
Person / Time
Site:	Seabrook
Issue date:	09/30/1992
From:	Belanger R NORTH ATLANTIC ENERGY SERVICE CORP. (NAESCO)
To:
Shared Package
ML20115D273	List: ML20115D266 ML20115D277 ML20115D286
References
NUDOCS 9210200325
Download: ML20115D277 (5)

Text

NORTH ATLANTIC ENERGY SERVICE CORP.

EVALUATION OF COOLING TOWER BASIN HEATUP-^ ANALYSIS WITH SPRAYS 2'3 FANS MANUALLY- CONTROLLED A'NALYSIS PERFORMED BY UNITED ENGINEERS AND CONSTRUCTORS AUGUST - SEPTEMBER 1992 EVALUATION PERFORMED BY RICHARD'R. BELANGER - NAESC s

P O O 43 PDR

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1.0 PURPOSE This evaluation determines the time available for manual actuation of the cooling tower sprays and fans during Post-LOCA cooldown and normal cooldown and provides an evaluation of tha acceptability of these times with respect to established regulatory guidance for requiring automatic rather than manual action. n

2.0 BACKGROUND

Technical Specification 3/4.7.5 requires that the cooling tower fans automatically actuate on a Tower Actuation (TA) signal. This implies a fully automatic cooling tower, requiring no manual actions. As currently configured, the spray bypass valves, SW-V-139 and 140, are normally open, dumping return flow to the basin. The fan control switches have been maintained in the " Pull-to-lock" position, defeating the automatic actuation of the fans.

During the winter months, automatic actuation of the cooling tower sprays could result in icing of the tower fill tile, degrading the heat removal capacitv of the cooling tower. Automatic actuation of the fans during the winter could damage the fans as a result of snow and ice loading on the fan blades. Manual actuation of the sprays and fans will allow the return water temperature to increase to the point (53*F) where tile icing will not occur during the most extreme winter environmental conditions, and allow the heat rejected from the tower to initiate melting of any snow or ice loading on the cooling tower fans.

Additionally, during extreme summer conditions, manual control of the cooling tower sprays and fans will preclude unnecessary heating of the return water due to heat transfer from the environment during low heat load operation of the cooling tower.

3.0 DISCUSSION The cooling tower functions as the backup ultimate heat sink, ' assuming the service water system heat loads following a seismic event which results in the collapse and greater than 95% blockage of the circulating water system tunnels.

The design basis for the cooling tower assumes that the seismic event which results in the collapse of the ocean tunnels also initiates a LOCA and a loss of offsite power. The design basis heat load for the tower therefore consists of the residual heat removal system heat rejection, containment building spray system heat rejection, diesel generator cooling system heat rejection, the cooling tower pumps, and other small loads imposed on the primary component cooling water system during the accident.

The cooling- tower ultimate heat sink was originally designed to support the operation of two units at the Seabrook site. The design basis for the tower therefore included the post-LOCA-heat loads from one unit and the cooldown loads from the second unit. With the cancellation of Seabrook Station Unit 2, the cooling tower now services only one unit. Therefore, significant performance margin exists in the design of the cooling tower with respect to its current duty requirements.

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1 To determine the acceptabi_lity of manual actuation of the cooling tower, basin heatup times for both Post LOCA cooldown and normal cooldown were determined and are comparea to existing guidance for manual action times. The limiting cooling tower basin temperature, which will maintain primary component cooling water-(PCCW) temperature within the design limits of equipment serviced was determined.

The cooling tower basin is then allowed to heatup to this temperature prior to initiation of sprays and fans, and the resulting time available for operator action to initiated cooling tower sprays and fans determined. These cases are discussed below.

3.1 POST-LOCA C00LDOWN Initially, during the injection phase of ECCS, no heat load is imposed on the cooling tower by the RHR or CBS heat exchangers. These heat loads are initially

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seen by the cooling tower foilowing the switchover from the. injection to the recirculation phase of post-LOCA cooldown. At this time, a significant heat load is imposed on the cooling tower.

The bulk temperature of the cooling twer basin water is currently limited to.

an initial temperature of 67.3*F by Technical Specification 3.7.5. This limit was chosen to ensure that the cooling tower basin temperature would be limited during the design basis event to ensure that the design limitations of the primary component cooling system are not exceeded. An analysis has been performed by United Engineers (Attachment A) which indicates that during periods of extreme environmental conditions, the coob ng tower basin average temperature could be allowed to increase to 80'F during a single train post-LOCA cooldown, or 87'F during a two - train cooldown, prior to initiati1g sprays and fans without exceeding equipment limitations (based on a limiti..g PCCW heat' exchanger outlet temperature of 126*F). This temperature increase would not adverscly affect the '

ability to remove the LOCA heat load. Based upon this analysis, the cooling tower initial basin temperature limit was increased to 70*F to minimize the potential for requiring basin recirculation with spray to reduce basin temperature during the summer months while maintaining adequate margin to allow for aperator action to initiate spray and fans.

If the tower is manually operated, assuming maximum ECCS flows and the minimum allowable RWST volume (ie minimizing the time to recirculation), and with the increased initial basin temperature, a minimum of.74 minutes will be available for-operator action to initiate cooling tower sprays and fans prior to reaching the limiting basin temperature; the interval between switchover to recirculation and spray activation is 51 minutes..This 74 minutes exceeds the 20 minutes stated in NUREG-0800, Section 6.3 as the basis for requiring automatic rather than manual _ actuation. Additionally, Emergency Operating Procedures d1iect the operator to contral cooling tower sprays and fans based upon PCCW heat exchanger outlet temperature. Initiation of cooling tower sprays early in ttie transient would significantly lengthen the heatup of the cooling tower basin, even without the fans. For the single train cooldown case, a total of 100. minutes is available for operator action prior to exceeding equipment design limitations.

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3.2 NORMAL C00LDOWN Although the post-LOCA cooldown is the design basis case for the cooling tower, a normal cooldown has also been evaluated with respect to the increased

, basin initial temperature and manual actuation of the spray and fans. The normal cooldown differs from the post-LOCA cooldown in that the initial heat

, load to the tower is higher in the normal cooldown case. This higher load results from normal plant loads which would be isolated in the post-LOCA

cooldown case remaining in service and therefore requiring cooling.

Initially in the normal cooldown: RCS sensible heat and decay heat is removed by the steam generators. The actual cooldown heat load is not provided to the cooling tower until the residual heat removal system is placed into service when the RCS temperature has been reduced to less than 350*F. This cooldown from normal operating temperature to RHR cut in temperature requires 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, i Therefore, the cooling tower heat load is constant during this initial four hour period, increasing at that time due to the decay heat being removed, and then

! gradually decreasing.

As in the post-LOCA cooldown case, the cocling tcwer basin is assumed to be at its maximum initial temperature and minimum volume. The tower basin temperature is again limited to a maximum temperature of 80'F prior to initiating ' spray and fans. This 80*F limit is conservative for this case as it is based on the larger heat loads experienced in the post-LOCA coc: Y n case. With the maximum normal cooldown heat load, and assuming a loss of m .;ite power which therefore adds the heat rejection from both diesel generators, greater than 106 minutes (Attachment B) is available for operator action to start the cooling tower spray _

and fans prior to reaching a cooling tower basin average temperature of 80'F.

Cooling tower spray and fan operation is governed by operat_ing procedures which require initiation of spray and fans based upon PCCW heat exchanger outlet temperature; thus this action will occur prior to reaching a basin average j temperature of 80'F, further limiting the peak PCCW system temperature.

3.3 OPERATOR ACTION

SUMMARY

. Manual control of the cooling tower is performed from the main control board l (MCB) . Following a Tower Actuatioh (TA) signal, automatic valves in the flowpath actuate to realign the service water system from the ocean to the cooling tower. At this time the spray header is bypassed. If a TA occurred, operators would utilize OS1216.01, " Degraded Ultimate Heat Sink", for guidance. Spray is initiated by repositioning a single valve ir each train from the MCB. Similarly, the cooling tower fan control _ switches are maintained in the " Pull-to-Lock" position to prevent automatic actuation of the fans. For Train A, a single control switch manipulation is required to start the fan Train B has two fans and therefore two control switches.

During the postulated cooling tower design basis event', operators are directed to monitor cooling tower performance at Step 10 of E-0, " Reactor Trip or Safety injection." This step directs the operator to monitor ultimate heat sink l l operation; if the cooling tower is the ultimate heat sink the operator is  !

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directed to initiate cooling tower spray and/or -fan operation based upon the combination of ambient wet _ bulb temperature and PCCW heat exchanger outlet temperature. There are also several alarms'in the main control room which will-

, alert the operator to the need to initiate spray and/or fan operation.

in the event of a loss of offsite power, if the fans are not initially l_oaded

by the emergency power _ sequencer (EPS), an additional operator-. action is required prior to starting the fans. The remote manual override (RMO) function of the EPS must be reset to allow manual loading of the fans onto-the diesel gen. ators.

This action is also accomplished at the MCB.

In summary, manual initiation of the cooling tower sprays and-fans requires' a-limited number of operator actions, all of which are performed at the main control board.

4.0 CONCLUSION

Based upon the above discussion, sufficient time for manual actions to initiate E

cooling -tower sprays- and fans exists in the_ event of a seismically induced

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4 circulating water tunnel collapse concurrent with a LOCA and loss of offsite

power. Additionally, a substantial period of tune exists to initiate manual spray E and fans in the normal cooldown case. Therefore, operation of the cooling tower with the sprays and fans manually controlled does not create a safety concern nor does it require immediate . operator action to mitigate the effects of an accident.

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