United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 87-57: Loss of Emergency Boration Capability Due to Nitrogen Gas Intrusion

                                                      SSINS No.:  6835
                                                         IN 87-57

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

                                November 6, 1987 


Information Notice No. 87-57:  LOSS OF EMERGENCY BORATION CAPABILITY
                                   DUE TO NITROGEN GAS INTRUSION


Addressees:

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

Purpose:

This information notice is being provided to alert addressees to potentially 
significant problems resulting from air/gas intrusion into fluid systems.  The
event described is also an example of fault propagation between units with 
shared systems.  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:

On May 28, 1987, with Turkey Point Units 3 and 4 in Mode 6 (refueling) and 
Mode 5 (cold shutdown), respectively, a loss of all boric acid flowpaths to 
both units occurred.  Nitrogen gas entered the Unit 4 boric acid pumps 
resulting in these pumps being gasbound and inoperable.  At Turkey Point, the 
boric acid system (which is part of the chemical, volume and control system) 
consists of four boric acid transfer pumps (two per unit) and three boric acid 
storage tanks.  Each unit normally has one boric acid transfer pump aligned to 
take suction from a boric acid storage tank, injecting boric acid into the 
charging pump suction header.  The second boric acid transfer pump is normally 
aligned to circulate boric acid solution through the boric acid storage tanks.
The boric acid system design allows for various system alignments, including 
interconnecting Units 3 and 4.  During this event, a Unit 3 boric acid 
transfer pump and a boric acid storage tank were out of service.  Loss of the 
boric acid system resulted in the licensee being unable to borate or emergency 
borate the reactor coolant system to ensure maintenance of the required 
shutdown margin.  

Nitrogen entered the boric acid system through a failed Unit 4 boric acid 
transfer pump mechanical seal.  To provide cooling for this seal, the seal is 
provided an accumulator tank partially filled with demineralized water.  The 
accumulator is given a 40-psi nitrogen overpressure to preclude leakage of the 
boric acid 


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                                                    IN 87-57
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across the seal faces (i.e., in the event of seal failure, the differential 
pressure will ensure that demineralized water flows into the boric acid 
system).  The design of this accumulator provided a continuous nitrogen supply 
from a 3000-gallon liquid nitrogen storage tank, through a 40-psi pressure 
regulator, and isolation valves that were normally open.  Apparently, as 
demineralized water entered the boric acid system through the failed seal, 
additional nitrogen was automatically supplied to the accumulator to maintain 
pressure.  The falling water level then allowed the nitrogen cover gas to 
enter the boric acid system through the failed seal.  

Based on previous operating experience, the licensee initially assumed that 
the loss of boric acid flow to Unit 4 was due to plugging of the piping caused 
by inadequate heat tracing and boric acid crystallization.  While 
troubleshooting efforts were being conducted to locate the source of the boric 
acid blockage, the Unit 3 boric acid system was cross connected to Unit 4 to 
provide a temporary boric acid flowpath.  This allowed nitrogen gas intrusion 
into the Unit 3 boric acid system and gas binding of the available Unit 3 
boric acid transfer pump, resulting in the loss of both normal and emergency 
boration flowpaths to both units on May 28 and June 3, 1987.  

On June 3, 1987, when the licensee observed that the water level in an accumu-
lator for a Unit 4 boric acid transfer pump could not be maintained within the 
sight glass, the cause of the problem was properly diagnosed as gas binding of 
the pumps as a result of nitrogen intrusion.  The licensee restored a boric 
acid flowpath by isolating the affected pump and venting the boric acid 
system.  

One of the root causes of this event appears to be the design of the accumula-
tor cover gas system.  A continuous supply of nitrogen allowed the 
uncontrolled intrusion of nitrogen gas into the Unit 4 boric acid system 
through the failed boric acid transfer pump mechanical seal.  To preclude 
recurrence of this, the licensee intends to (1) lock closed the nitrogen 
supply valves to the pump seal water accumulators, and (2) add nitrogen only 
in a batch method, as required to maintain seal pressure.  The inability of 
the operators to recognize the symptoms of gas binding in fluid systems 
permitted nitrogen gas intrusion into the Unit 3 boric acid system when Units 
3 and 4 boric acid systems were interconnected.  In response, the licensee is 
providing additional operator training, upgraded procedures, and emphasizing 
management control of system configurations and compliance with procedures.  

Discussion:

The event described above is intended to be illustrative of the potential for 
system inoperability resulting from air/gas intrusion.  Since 1981, more than 
90 licensee event reports (LERs) have been submitted to the NRC pertaining to 
events involving air/gas intrusion.  The most significant events of those 
reported have involved:  (1) the loss of reactor coolant makeup capability, 
(2) the loss of essential service water flow, (3) the inoperability of emer-
gency diesel generators (loss of cooling flow), (4) the loss of shutdown 
cooling, and (5) water hammer.  System or component failures resulting from 
air/gas intrusion can be significant.  

                                                    IN 87-57
                                                            November 6, 1987
                                                            Page 3 of 3


A previous event involving gas intrusion at McGuire Unit 1 is discussed in IE 
Information Notice 82-19, "Loss of High Head Safety Injection Emergency 
Boration and Reactor Coolant Makeup Capability."  Additional events are dis-
cussed in Information Notice No. 83-77, "Air/Gas Entrainment Events Resulting 
in System Failures."  Air/gas intrusion concerns associated with the residual 
heat removal system of pressurized water reactors are addressed in Generic 
Letter 87-12, "Loss of Residual Heat Removal (RHR) While the Reactor Coolant 
System (RCS) is Partially Filled."

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 
office. 




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


Technical Contact:  S. D. Stadler, RII
                    (404) 331-5599


Attachment:  List of Recently Issued NRC Information Notices

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