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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