United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 79-24: Frozen Lines

                                                            Accession No.
                                                            SSINS No: 6820

                                UNITED STATES
                           WASHINGTON, D.C. 20555
                             September 27, 1979

                                                      IE Bulletin No. 79-24 



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. 

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

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. 


Salem 2
North Anna 2
Diablo Canyon 1 & 2
Sequoyah 1
McGuire 1
San Onofree 2
Watts Bar 1 & 2
Page Last Reviewed/Updated Friday, May 22, 2015