ML20085B563

Ao:On 720205,tubes Ruptured When Cooling Water in One of Four Containment Air Handling & Cooling Units Froze.Caused by Pneumatic Temp Transmitter Failure.Tubes Brazed or Plugged & Tube Bundles Ordered

ML20085B563
Person / Time
Site:	Palisades
Issue date:	02/15/1972
From:	Sewell R CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20085B566	List: ML20085B563
References
NUDOCS 8307080186
Download: ML20085B563 (3)

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s 2 General Of6ces 212 West Mich. gen Avenue, Jackson, Michigan 49201. Area Code 517 788 0550 February 15, 1972 Dr. Peter A. Morris, Director Re: Docket No 50-255 Division of Reactor Licensing License No DPR-20 United States Atomic Energy Commission Washington, DC 205!45

Dear Dr. Morris:

This letter is to apprise you of an abnormal occurrence that occurred at the Palisades Plant February 5, 1972. On that date, the cooling water in one of the four containment air handling and cooling units froze causing some tube ruptures to occur.

At the time the incident occurred, the plant was in the coldshutgowncondition. Outside ambient temperature was approxi-mately 15 F. The ventilation system was operating in the purge containment mode. A containment air handling and cooling unit high-water level alarn was annunciated for unit VHX-2. Investigation re-vealed that the source of the water was service water from tube ruptures in the VHX-2 containment air handling and cooling unit.

In the purge containment mode of operation, fresh air is drawn through the purge supply tempering unit by containment purge supply fan V-5 and discharged to the containment air cooler plenums 1 and 2. The flow from each of these two plenums splits with a por-tion of the flow going through the containment air cooler recirculation fans and being discharged high in the containment vessel. The re-mainder of the purge air supply is drawn (in a direction opposite the normal airflow direction through the plenum suction lines) through the two containment air handling and cooling units VHX-1 and VHX-2

[. by the suction of the purge exhaust fan V-35 The purge exhaust fan exhausts the air from containment to the main exhaust fans which dis-

. charge to the plant stack.

/h.- On February 5, the control valve that supplies heating steam g- to the purge supply tempering unit failed shut because of a failure in a pneumatic temperature transmitter. This allowed purge air at ambient temperatures to be drawn through the containment air handling and cooling units (VHX-1 and VHX-2). As a result, of this cold air being drawn through the two containment air handling and cooling units, some freezing of the service water in the tubes occurred in one unit (VHX-2) which caused tube ruptures to occur.

/7W 8307080186 720215 PDR ADOCK 05000255 / p

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1 Dr.PeterA.Ldris

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2 February 15, 1972 Freezing did not occur in the M -1 because of differences in airflow through the two units (M-1 and VHX-2) and/or differ-ences in the leakage characteristic through the modulating temperature control valves that regulate service water flow through these units.

This type of temperature control valve is not normally an effective shutoff valve.

The ruptured tubes were repaired by brazing where the rup-tures were accessible. Six tubes (each tube makes four passes through the cooling unit) contlined ruptures that could not be effectively repaired and these were plugged. Following completion of these re-pairs, the containment air handling and cooling unit was hydro tested to 92 psig (essentially shutoff head of both service water pumps).

No leakage existed. Service water pressure supplied to these heat exchangers under normal operating conditions is about 65 psig and the design base accident conditions are approximately 55 psig.

The plugging of six tubes represents a reduction of heat transfer area in the VHX-2 unit of 1.6 percent. Because of this reduction in heat transfer area, the manufacturer was requested to calculate the number of tubes that could be plugged as a function of service water temperature. The heat transfer capacity of the unit was held constant equal to that required under design base accident condi-tions and the full design fouling was assumed to exist. These calcu-lations revealed that the heat exchange capacity would be greater than or equal to the capacity required under design base accident conditions as long as the service water temperatures did not exceed 67 F (service water temperature assumed in the design base accident is 75 F).

A review was conducted of the capability to remove heat from the containment vessel during a design base accident assuming service water inlet temperatures of 67 F. From this review, it was concluded that the heat removal capability would exist that is in excess of that used in the analysis of the design base accident assuming the most limiting conditions of equipment failure. This review is conservative l because:

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1. The containment spray system and containment air cooling l system are completely redundant and self-sufficient with respect to heat removal capability.

2. The review assumed the containment spray system and one of the four containment air handling and cooling units were in-operable.

3 Even assuming that one of three containment air handling and cooling units in operation is VHX-2 and is operating at its design capacity, the other two operating containment air handling and cooling units will operate at 1.7 percent above their design capacities.

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. o Dr. Peter A. Morris O

3 e February 15, 1972 14 . The heat transfer capetbility analysis assumed heat transfer surface fouling factors to be th' full design value. The heat transfer surfaces are very clean.

To prevent recurrence of a similar incident, a temperature '

switch and alarm are being installed in the purge air supply to con-tainment. This alarm will function at abnormally low purge air supply temperatures above the freezing point of water to alert op-erators to a malfunction of the purge supply tempering unit.

Based on our review of this e.bnormal occurrence and the information presented above, we have concluded that returning the plant to service with six tubes plugged in containment air handling and cooling unit VHX-6 does not involve an unrevigwed safety question as long as service water inlet temperatures of 67 F are not exceeded.

Tube bundles (each containment air handling and cooling unit contains eight individual tube bundles) have been ordered to replace the bundles damaged during this abnormal occurrence. These replacement tube bun-dies will be installed priog to plant operation with service water inlet temperatureso that exceed 67 F. During 1971, intake temperatures did not reach 67 F until late August.

Yours very truly, Ralph B. Sewell (Signed)

RBS/dmb Ralph B. Sewell Nuclear Licensing Administrator CC: BHGrier, USAEC l

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