United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-73: Loss of Shutdown Cooling During Disassembly of High Pressure Safety Injection System Check Valve

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                             November 21, 1991 


NRC INFORMATION NOTICE 91-73:  LOSS OF SHUTDOWN COOLING DURING DISASSEMBLY
                               OF HIGH PRESSURE SAFETY INJECTION SYSTEM 
                               CHECK VALVE


Addressees

All holders of operating licenses or construction permits for nuclear power 
reactors.  

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to an event where loss of reactor shutdown 
cooling (SDC) occurred because of inadequate reviews of work instructions 
that affected plant conditions and systems configured to establish and 
maintain SDC.  It is expected that recipients will review the information 
for applicability to their facilities and consider actions, as appropriate, 
to avoid similar problems.  However, suggestions contained in this 
information notice are not NRC requirements; therefore, no specific action 
or written response is required.

Description of Circumstances

On May 5, 1991, SDC for the Waterford Steam Electric Station, Unit 3, was 
lost for 19 minutes when the bonnet of check valve SI 512A (Attachment 1) in 
the high pressure safety injection (HPSI) system was opened.  This allowed 
air to enter the suction piping of the connecting SDC train that was in 
service.  During this period, the temperature of the reactor coolant 
increased from about 100�F to 110�F.

At the time of the event, the plant operators were refilling the reactor 
coolant system following refueling.  The reactor vessel head had been 
installed, but the containment equipment hatch was still open.  The "A" 
train of the SDC was aligned to fill the reactor coolant system from the 
refueling water storage pool, leaving the operating "B" SDC train as the 
only immediately available means to provide shutdown cooling.  When the 
vessel level reached the 19 foot elevation, the operators stopped filling 
the system so that maintenance personnel could remove the bonnet from check 
valve SI 512A to replace a bonnet seal.  Check valve SI 512A is a hot leg 
injection check valve in the "A" train of the HPSI system.  This valve is 
located at an elevation of 23 feet 10 inches and connects to a high point in 
the "B" train of the SDC system at the 23 foot elevation.  The opening of 
the check valve permitted air to enter the SDC suction piping, breaking the 
siphon that was holding the water in the high section of the SDC system and 
allowing water to run out of the affected SDC train and into the reactor 
vessel.  This raised the vessel level to 21.3 feet.  Since this level was 
below the level of 

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                                                       IN 91-73
                                                       November 21, 1991
                                                       Page 2 of 3


the high point in the "B" SDC train suction piping, the "B" low pressure 
safety injection (LPSI) pump became air bound (the LPSI pumps are also used 
for SDC pumping).  

After unsuccessfully attempting to vent this pump, the plant operators shut 
it down.  The licensee evacuated all unnecessary personnel from the 
containment and started the process of installing the containment equipment 
hatch.  The licensee recovered shutdown cooling within 20 minutes by 
realigning the "A" SDC train, venting the "A" LPSI pump (a standard 
precaution in the procedure for starting up an idle shutdown cooling loop), 
and placing the "A" LPSI pump into service.  Within a half hour of the start 
of the event, the licensee had closed the containment. 

Discussion 

The root cause of this event was a change in the work procedure that had not 
received adequate review.  

The licensee determined that check valve SI 512A needed repair about a month 
prior to the event.  The licensee's initial plans included the use of a 
freeze seal to isolate the valve with the reactor coolant level at 44 feet.  
A freeze seal was established in a horizontal section of piping 
(Attachment 1), using CO2, but it failed when plant personnel attempted to 
open the bonnet of the check valve.  Another freeze seal was established 
later using liquid nitrogen, but it too failed.  The licensee found that the 
freeze seals failed because this section of pipe had a slight slope, 
allowing a small amount of air to be trapped at the elevation of the freeze 
seal, which was about an inch above the piping's vent path.  The path left 
by the trapped air permitted water to leak through and melt the seal. 

The licensee then changed the procedure to accomplish the repair without the
use of a freeze seal at a lower reactor coolant level of 19 feet.  The 
change was discussed and concurred in by the Maintenance, Operations and 
Planning, and Scheduling Departments.  However, the licensee made the change 
without reviewing isometric drawings and system configurations that would 
have shown that opening the HPSI check valve would break the suction of 
operating LPSI pump "B".  The licensee did not perform an adequate review 
and did not obtain the required approvals.  Some special controls that would 
have been applied at a level lower than 18 feet (previously defined by the 
licensee as reduced inventory requiring special controls) were not applied 
at this level. 

Careful review and performance of work procedures, including changes to 
previously approved work procedures, are particularly important during 
operations which involve opening a partially drained reactor coolant system 
when SDC is required.  The NRC has discussed loss of SDC in a number of 
generic communications including Information Notice 87-23, "Loss of Decay 
Heat Removal During Low Reactor Coolant Level Operation," May 27, 1987, and 
Generic Letter 88-17, "Loss of Decay Heat Removal," October 14, 1988.  
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                                                       IN 91-73
                                                       November 21, 1991
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This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate Office of 
Nuclear Reactor Regulation (NRR) project manager.  




                                   Charles E. Rossi, Director 
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation


Technical contacts:  Donald C. Kirkpatrick, NRR
                     (301) 492-1176

                     Thomas F. Westerman, RIV
                     (817) 860-8145


Attachments:  
1.  Waterford Unit 3 Partial Safety Injection 
      & Shutdown Cooling Systems 
2.  List of Recently Issued NRC Information Notices

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Page Last Reviewed/Updated Tuesday, November 12, 2013