Bulletin 79-24: Frozen Lines
Accession No.
7908220114
SSINS No: 6820
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
September 27, 1979
IE Bulletin No. 79-24
FROZEN LINES
Summary:
On January 3, 1979, an unusual event occurred at Davis-Besse, Unit 1. The
event involved the freezing of the water in a portion of the high pressure
coolant injection (HPCI) system recirculation line that is common to both
high pressure coolant injection pumps. The line was not thawed until January
5, 1979, and the event was not reported to NRC until March 12, 1979. In
addition, over the past five years, there have been several events involving
frozen instrument and sampling lines.
Description of Circumstances:
The condition was discovered on January 3, 1979, while the reactor was
operating at approximately 96 percent power. During a monthly surveillance
test of the HPCI pumps, there was no flow through the recirculation line
from the pump discharge to the Borated Water Storage Tank (BWST).
Investigation revealed that water in a portion of the line exposed to the
outside weather was frozen. This portion of the line to the BWST is common
to both HPCI pumps. The line has redundant heat tracing (a heated wire to
prevent freezing); it is insulated; and it has a low-temperature alarm
system to warn of freezing conditions. Water in the line froze apparently
because of prolonged sub-freezing temperatures and a defect in the
insulation. There was no apparent malfunction of the heat tracing or
temperature alarm circuits, although the temperature sensing elements are
located in an area not exposed to the coldest temperature conditions.
This recirculation line serves two purposes: One is to provide a flow path
for surveillance testing, and the other is to provide a minimum flow path to
prevent possible damage to the pump in the event it is operated against a
closed discharge valve or a reactor coolant system pressure greater than the
maximum discharge pressure of the pump (approximately 1600 psig).
In the initial review of the event on January 3, 1979, the licensee
concluded that the pumps were still operable with the recirculation line
frozen. Therefore, no immediate action was taken to thaw the frozen section
of the line. On January 5, 1979, the line was thawed and the surveillance
test of the pumps was successfully completed.
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IE Bulletin No. 79-24 September 27, 1979
Page 2 of 2
Cause of Event:
This event is attributed to an inadequate design. The heat tracing provided
on this recirculation line was unable to prevent freezing when the line was
exposed to sub-freezing temperatures for a prolonged period of time. The
temperature sensors which control the heat tracing and which trigger the
low-temperature alarm were located in an area not exposed to the coldest
temperatures to which the piping is subjected.
Corrective Action:
To prevent recurrence, the licensee increased the temperature setting of the
thermostat for the heat tracing on the recirculation line. Further, a
temporary enclosure was built around the line and additional heat tracing
was installed.
After the line was thawed and blown down, surveillance testing to verify
pump operability was performed. An engineering evaluation was initiated by
the licensee to develop long-term corrective action.
Other Frozen Lines:
In addition to this event at Davis-Besse, there have been many events
involving frozen instrument and sampling lines. During the last five years,
facilities experiencing events of this kind include:
Arkansas Nuclear One Oconee
Davis-Besse Pilgrim
Duane Arnold Point Beach
Hatch Salem
Fitzpatrick Surry
Haddam Neck Trojan
Indian Point Zion
Millstone
Action To Be Taken:
All licensees and construction permit holders receiving this Bulletin for
action are requested to review their plants to determine that adequate
protective measures have been taken to assure that safety-related process,
instrument, and sampling lines do not freeze during extremely cold weather.
The results of these reviews should be reported to the Regional Director by
October 31, 1979.
Approved by GAO (ROO72); clearance expires 7/31/80. Approval was given under
a blanket clearance specifically for generic problems.
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DESIGNATED APPLICANTS FOR OPERATING
LICENSES
Salem 2
North Anna 2
Diablo Canyon 1 & 2
Sequoyah 1
McGuire 1
San Onofree 2
Summer
Watts Bar 1 & 2
Page Last Reviewed/Updated Tuesday, March 09, 2021