United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 87-06: Loss of Suction to Low-Pressure Service Water System Pumps Resulting from Loss of Siphon

                                                         SSINS No.: 6835 
                                                         IN 87-06   
														     
                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555

                              January 30, 1987

Information Notice No. 87-06:   LOSS OF SUCTION TO LOW-PRESSURE SERVICE 
                                   WATER SYSTEM PUMPS RESULTING FROM LOSS 
                                   OF SIPHON 

Addressees: 

All nuclear power reactor facilities holding an operating license or a 
construction permit. 

Purpose: 

This notice is to alert recipients of a potentially significant problem 
involving the loss of suction to the low-pressure service water (LPSW) 
system pumps resulting from loss of siphon. It is expected that recipients 
will review this information for applicability to their facilities and 
consider actions, if appropriate, to preclude a similar problem occurring at 
their facilities. However, suggestions contained in this notice do not 
constitute NRC requirements; therefore, no specific action or written 
response is required. 

Description of Circumstances: (Refer to attached Figure) 

On October 2, 1986, Oconee Unit 1 was shut down because of a loss of LPSW 
system pump suction for Units 1 and 2. Unit 2 was already shut down for 
refueling. Unit 3 was also shut down because it was susceptible to a loss of 
LPSW pump suction. LPSW supplies cooling water for the decay heat removal 
system and other safety-related equipment. The loss of LPSW pump suction for 
Units 1 and 2 occurred during an electrical load shed surveillance test 
conducted on Unit 2. The load shed test involved shedding the Unit 2 
condenser circulating water (CCW) pump motors. The CCW system takes suction 
from Lake Keowee and supplies cooling water to the main condensers. In 
addition, Unit 2 CCW pumps normally provide suction to Units 1 and 2 LPSW 
pumps and this was the mode at the time of the load shed test. These LPSW 
system pumps take suction on the upstream side of the main condenser from a 
LPSW crossover line between Units 1 and 2 CCW. 

Each of the four CCW pump motors for each unit is capable of being powered 
from either of two emergency hydro-generators. However, the Oconee plant is 
designed to accommodate a loss (shedding) of the CCW pumps and still provide 
LPSW pump suction through a siphon arrangement. The siphon is necessary 
because of a high point in the CCW piping just downstream of the CCW pumps 
and upstream of the LPSW pump suction. This high point may be as much as 25 
feet above the level of Lake Keowee (depending upon lake level). The loss of 
suction to the 


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                                                       IN 87-06
                                                       January 30, 1987
                                                       Page 2 of 3

LPSW pumps was caused by a loss of this siphon. The CCW pump discharge 
flange is normally 9 feet below the surface of Lake Keowee when the lake is 
at full level. However, because of drought conditions, Lake Keowee level was 
about 6 feet below the flange at the time of the load shed test. (Technical 
Specifications permit plant operation with lake levels as low as 16 feet 
below this flange.) During operation at these reduced lake levels, water 
leakage at the flange had been observed. This flange was not originally 
designed to be leaktight. The amount of water leakage was insignificant 
during plant operation. However, with the CCW pumps off (shedded), air 
inleakage caused the high point in the CCW piping to drain and caused a loss 
of siphon flow in about 1 hour. 

Siphon flow, if initiated, could not be sustained in the system, as 
originally designed and built, during low lake level conditions because of 
air inleakage at the CCW pump discharge flange. It appears that previous 
surveillance tests were not of sufficient duration to determine that siphon 
flow was sustained. Since the large volume of water contained in the CCW 
lines provided LPSW flow for about an hour before the loss of LPSW suction, 
it appears that load shed testing personnel, in the past, may have been 
misled into thinking siphon flow had been sustained. 

The Oconee CCW system is designed to also provide suction and discharge 
(heat sink) for the cooling water pump for an emergency diesel generator 
(EDG) used in the standby shutdown facility (SSF). The SSF was designed to 
be an independent decay heat removal system. Analyses performed subsequent 
to the above load shed test showed that if siphon flow was lost in the CCW 
pipe, the CCW system could not provide an adequate heat sink for SSF 
operation to meet its design basis of 72 hours of operation. In addition, 
when the CCW pumps are not operating, the CCW system should provide 
emergency gravity-siphon CCW flow to the main condensers to recover 
condensate for decay heat removal following certain postulated events until 
the decay heat removal system is in operation. The gravity flow is possible 
because the CCW discharge from the main condenser is shifted to an alternate 
pipe that discharges downstream of Lake Keowee dam at an elevation well 
below the CCW intake. The siphon is required for the same reason as required 
by the LPSW. This feature of the CCW system also was disabled by the loss of 
siphon. 

The discharge flange on all CCW pumps has been modified to prevent air 
inleakage when the lake level is below the discharge flange. The LPSW pumps 
have been successfully tested for several hours with the CCW pumps off and 
the lake level below the discharge flange. The emergency CCW gravity-siphon 
flow to the main condensers and the EDG cooling water pump also have been 
successfully tested under the above conditions. In addition, the SSF cooling 
water pump has been modified to take a separate and independent suction from 
Lake Keowee. 

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                                                           IN 87-06
                                                           January 30, 1987
                                                           Page 3 of 3


No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 




                         Edward L. Jordan, Director
                         Division of Emergency Preparedness
                          and Engineering Response
                         Office of Inspection and Enforcement


Technical Contacts: H. Bailey, IE
                    (301) 492-9006

                    T. Peebles, RII
                    (404) 331-4196

                    C. Burger, RII
                    (404) 331-4198

Attachments:
1.   Oconee Plant, Partial Schematic of LPSW and CCW Systems 
2.   List of Recently Issued IE Information Notices
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