Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
November 6, 1987
Page 2 of 3
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
