Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 86-106
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
December 16, 1986
Information Notice No. 86-106: FEEDWATER LINE BREAK
Addressees:
All nuclear power reactor facilities holding an operating license or a
construction permit.
Purpose:
This information notice is to alert addressees of a potentially generic
problem with feedwater pipe thinning and other problems related to this
event. Recipients are expected to review the information for applicability
to their facilities and consider actions, if appropriate, to preclude
similar problems suggestions contained in this occurring at their
facilities. However information notice do not constitute NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances:
On Tuesday, December 9, 1986, at 2:20 p.m., both units at the Surry Power
Station were operating at full power when the 18-inch suction line to the
main feedwater pump A for Unit 2 failed catastrophically. Eight workers who
were replacing thermal insulation on a nearby line were burned by flashing
feedwater. All were transported to area hospitals. Two workers were treated
and released. Four other workers subsequently died.
Units 1 and 2 are identical. In each unit, feedwater flows from a 24-inch
header to two 18-inch suction lines that each supply one of two main
feedwater pumps. At maximum load under normal conditions, feedwater flow
through each pump is 5 million lb/hr. Feedwater temperature, pressure, and
enthalpy are 370F, 450 psig, and 346 Btu/lb, respectively. At these
conditions the fluid is in the single phase, liquid only regime. That is,
the piping does not see a mixture of liquid and vapor.
The event was initiated by the main steam isolation valve on steam generator
C failing closed. Because of the increased pressure in steam generator C
that collapsed the voids in the water, the reactor tripped on low-low level
in that steam generator. A 2-by-4 foot section of the wall of the suction
line to the A Main feedwater pump was blown out and came to rest in an
overhead cable tray. The break was located in an elbow in the 18 inch line
about one foot from the 24-inch header. The lateral reactive force generated
by escaping
8612160250
.
IN 86-106
December 16, 1986
Page 2 of 3
feedwater completely severed the suction line. The free end whipped and came
to rest against the discharge line for the other pump.
Steam flashing from the break and condensing in control cabinets and in open
conduit piping apparently caused the fire suppression system to actuate,
resulting in release of halon and carbon dioxide in the emergency switchgear
room and in various cable tunnels and vaults and in the cable spreading
room. Because of the volume of water and steam being released, operators
isolated lines carrying high energy fluids to areas inundated by steam.
Steam generator water levels were maintained with the auxiliary feedwater
system, and system cooling was provided by actuating atmospheric dump valves
as necessary.
The primary system responded normally to the loss of load transient with a
partial loss of main feedwater. Primary coolant temperature was stabilized
at 520F and pressurizer level was recovered as it reached the low level
set point. Primary pressure decreased from 2235 to 2015 psig following the
reactor trip. By 2 a.m on the following day, reactor temperature had been
reduced to the point where the residual heat removal system could be put on
line. The unit reached cold shutdown that morning. During the recovery
effort, the operators and the plant performed as expected.
Discussion:
The pipe material is A-106B carbon steel and the elbow is 18-inch, extra
strong A-234 grade WPB carbon steel. Nominal wall thickness of the suction
piping is 0.500 inch. Measurements of the wall fragment demonstrated that
the wall had been generally eroded to about 0.25 inch and was one of the
causes of the failure. Preliminary examination of the 2-by-4 foot section of
pipe blown out during the event shows the thinning to be relatively uniform
except for some small localized areas. The thinnest areas are localized and
appear to be about 1/16 inch thick. Some corrosion pitting is present. A
preliminary microexamination indicated that the pipe surface near the
fracture had not been highly strained as with a high stress event, such as a
high pressure spike in the system.
It has not been determined at this time whether a pressure spike in the
system was a contributor to this event. There was no damage evident in the
hanger supports to the condensate system.
Inspection revealed a disabled check valve in the discharge piping of the A
main feedwater pump. This check valve was found with its seat displaced and
a hinge pin missing.
On December 10, the licensee shut down Unit 1 for examination of the
condition of feedwater piping. Inspection of the Unit 1 feedwater piping
shows wall thinning similar to but not as severe as that in Unit 2.
The NRC dispatched an augmented investigation team (AIT) to the site . The
AIT includes a metallurgist and a water hammer analyst.
.
IN 86-106
December 16, 1986
Page 3 of 3
The NRC will issue additional information as more inspection and analysis is
completed.
No specific action or written response is required by this information
notice. If you have questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Roger Woodruff, IE
(301) 492-7205
Vincent Panciera, Region II
(404) 331-5540
Attachment:
List of Recently Issued IE Information Notices
.
