Information Notice No. 86-106: FEEDWATER LINE BREAK

 
                                                          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 

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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. 

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                                                       IN 86-106 
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                                                       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
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