Information Notice No. 89-67: Loss of Residual Heat Removal Caused by Accumulator Nitrogen Injection

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                               September 13, 1989

Information Notice No. 89-67:  LOSS OF RESIDUAL HEAT REMOVAL CAUSED 
                                   BY ACCUMULATOR NITROGEN INJECTION  


All holders of operating licenses or construction permits for pressurized 
water reactors (PWRs).


This information notice is intended to alert addressees to potential problems 
resulting from the loss of residual heat removal (RHR) caused by the injection 
of nitrogen from an accumulator into the reactor coolant system (RCS).  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 do not 
constitute NRC requirements; therefore, no specific action or written response 
is required.

Description of Circumstances:

Salem Unit 1 lost both RHR pumps for about 50 minutes on May 20, 1989, as a 
result of an injection of nitrogen from an accumulator into the RCS and, sub-
sequently, into the RHR system.  This injection occurred while the licensee 
was conducting full-flow testing of the accumulator check valves.  The reactor 
was in Mode 5 (cold shutdown) after a recent refueling with the reactor head 
installed.  The RCS was filled to a cold calibrated pressurizer level of 
10 percent with air contained in the reactor vessel head and in the steam 
generator U-tubes.  All accumulators were filled to normal operating level and 
were pressurized to approximately 600 psig.  

At 9:25 a.m., while performing a post-maintenance full-flow test of the check 
valves for accumulator 13, the accumulator isolation valve remained open for 
about 70 seconds as a result of an operator error.  During this time, approxi-
mately 1800 cubic feet of nitrogen at about 62 psig entered the RCS.  As the 
nitrogen expanded into the RCS, the pressurizer level went off-scale high, and 
the reactor pressure rose from 14 psig to about 51 psig.  The operator, not 
realizing that nitrogen had been injected into the RCS, initiated pressurizer 
level restoration by draining the RCS to the refueling water storage tank 
(RWST).  At 9:35 a.m., the operator observed zero RHR flow and a reduction of 
the pump motor current from 44 amps to about 21 amps.  

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                                                            September 13, 1989 
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The operator, assuming that the pump was mechanically damaged, secured the 
pump and started the second RHR pump.  When this pump exhibited the same 
characteristics as the other pump, the operator realized that the pumps were 
gas bound and started venting the RHR system.  Venting was slow because of the 
size of the vent lines and an RHR system configuration that allowed air 
entrapment. Operators also had difficulty in locating one vent and drain 
valve; when it was located, an installed cap had to be removed.  Slow venting 
continued until 10:18 a.m. when the operators initiated filling of the RHR 
system, using gravity feed from the RWST.  At this time, the core exit 
temperature had increased to 122 F from a pre-event value of 92 F.  At 10:23 
a.m., the RHR system was filled, and RHR pump 11 was successfully started.  At 
10:37 a.m., RHR pump 12 was placed in service, and RHR pump 11 was secured to 
return the system to normal operation.  Because of an inadequate abnormal 
operating procedure and emergency classification guide, a 10 CFR 50.72 report 
for this loss of RHR pumps was not made until May 22, 1989.


An assessment of this loss-of-RHR-pump event at Salem Unit 1 identified the 
following concerns: 

1.   The operators appeared to believe that nitrogen injection into the RCS 
     was not possible during the full-flow test of the accumulator check 
     valves.  This mindset caused the operators to drain the RCS when the 
     pressurizer level kept increasing due to the expansion of injected 

2.   The abnormal operating procedures and the emergency classification guide 
     did not adequately address the potential for loss of RHR cooling.  The 
     symptom-oriented procedures did not address the reactor parameters during 
     a loss of shutdown cooling while in Modes 5 and 6.  Consequently, the 
     event was not reported immediately under 10 CFR 50.72.

3.   The operators were not adequately trained to differentiate gas binding of 
     the RHR pumps from mechanical damage.  This caused the operators to start 
     RHR pump 11 when they erroneously concluded that RHR pump 12 was mechani-
     cally damaged.

4.   Elevation differences in the RHR suction pipe, the difficulty in locating 
     one vent and drain valve, and the size of the vent lines caused the 
     venting operation to be extremely slow.  In addition, complete system 
     venting was not possible because gas was trapped in the high points of 
     the RHR system.
5.   The appropriateness of performing a full-flow test of the accumulator 
     check valves with fuel in the reactor vessel had not been fully assessed.  
     An event similar to the one described in this information notice would 
     not occur to this extent if this test had been conducted while the core 
     was off-loaded.  Furthermore, if this test had been performed with either 
     reduced accumulator nitrogen pressure or with the reactor vessel head 
     removed, the effects of nitrogen injection into the RCS would have been 

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                                                            Page 3 of 3 

It is important to note that if the accumulator check valve full-flow tests 
had been conducted when they are more commonly performed, during the shutdown 
prior to refueling, and an event similar to the one described in this 
information notice had occurred, the consequences could have been much more 
significant.  It is also important to note that operator training provided in 
response to NRC Generic Letter 88-17, "Loss of Decay Heat Removal," did assist 
the operators in restoring the RHR system once gas binding of the system was 

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 NRR project 

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

Technical Contacts:  Daniel Prochnow, NRR
                     (301) 492-1166

                     Warren Lyon, NRR
                     (301) 492-0891

Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment 
                                                            IN 89-67 
                                                            September 13, 1989
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                             NRC INFORMATION NOTICES
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to_________

89-66          Qualification Life of         9/11/89        All holders of OLs
               Solenoid Valves                              or CPs for nuclear
                                                            power reactors. 

88-46,         Licensee Report of            9/11/89        All holders of OLs
Supp. 4        Defective Refurbished                        or CPs for nuclear
               Circuit Breakers                             power reactors. 

89-65          Potential for Stress          9/8/89         All holders of OLs
               Corrosion Cracking in                        or CPs for PWRs. 
               Steam Generator Tube 
               Plugs Supplied by 
               Babcock and Wilcox 

89-64          Electrical Bus Bar Failures   9/7/89         All holders of OLs
                                                            or CPs for nuclear
                                                            power reactors. 

89-63          Possible Submergence of       9/5/89         All holders of OLs
               Electrical Circuits Located                  or CPs for nuclear
               Above the Flood Level Because                power reactors. 
               of Water Intrusion and Lack 
               of Drainage 

89-62          Malfunction of Borg-Warner    8/31/89        All holders of OLs
               Pressure Seal Bonnet Check                   or CPs for nuclear
               Valves Caused By Vertical                    power reactors. 
               Misalignment of Disk 

89-61          Failure of Borg-Warner Gate   8/30/89        All holders of OLs
               Valves to Close Against                      or CPs for nuclear
               Differential Pressure                        power reactors. 

88-48,         Licensee Report of Defective  8/22/89        All holders of OLs
Supp. 2        Refurbished Valves                           or CPs for nuclear
                                                            power reactors. 

89-60          Maintenance of Teletherapy    8/18/89        All NRC Medical 
               Units                                        Teletherapy 
OL = Operating License
CP = Construction Permit 

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