Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 16, 1990
Information Notice No. 90-53: POTENTIAL FAILURES OF AUXILIARY STEAM
PIPING AND THE POSSIBLE EFFECTS ON THE
OPERABILITY OF VITAL EQUIPMENT
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees to potential
problems resulting from unanalyzed failures of auxiliary steam lines. 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:
During the winter of 1989 at the Millstone Nuclear Power Station, personnel
of the Northeast Nuclear Energy Company (the licensee) encountered
difficulty in maintaining room temperatures at various locations in the
plant. To correct this problem, the licensee initiated a plant design
change to install new heating coils. The safety analysis performed for the
design change included analyzing the effects of a failure of the house
heating steam line. Upon completing this analysis, the licensee decided to
examine other piping used for house heating steam. During this
investigation, the licensee discovered that a failure of the house heating
steam system could degrade equipment located in areas classified as "mild
environments." The licensee identified that the areas that were vulnerable
to a house heating steam line break included the Unit 1 switchgear room,
lower and upper HVAC rooms, and the control room. Further investigation
revealed five general areas within Millstone 2 that had not been analyzed
for an auxiliary steam system steam line break. These areas included
control room air conditioning, A & B emergency diesel generator (EDG) rooms,
EDG tank room, and Auxiliary Building 1406. A break in Building 1406 could
affect the fans that cool vital switchgear and the room adjacent to the
spent fuel pool. On May 25, 1990, the licensee identified two more areas in
the Unit 2 auxiliary building that had not been analyzed for an auxiliary
steam line break.
In response to these findings at Millstone, personnel at the Haddam Neck
Generating Station investigated their piping configuration to determine if a
similar concern existed. They determined that a break of a heating steam
line could affect the operation of safety-related 480-volt switchgear.
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IN 90-53
August 16, 1990
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Discussion:
The house heating steam system is classified as a high-energy line break
(HELB) system at Millstone. Because this system is not seismic and house
heating steam lines are present in many areas of the plant, a seismic event
could result in the degradation of safety related equipment classified for
mild environments. The licensee determined that this condition could
potentially affect safety related structures and systems needed to (a)
shutdown the reactor and maintain it in a safe condition and (b) remove
residual heat. The licensee identified the cause of the oversight to be an
incorrect conclusion drawn from the HELB study performed in 1973. This
report addressed an auxiliary steam system line break and its non-relevance
on safety-related equipment, stating that the environmental conditions
following any break would be of no consequence to any shutdown methods.
House heating steam systems (or other auxiliary steam line systems) are not
considered typical high-energy line systems. As such, they may have been
overlooked or improperly analyzed when failure modes and effects and single
failure analyses were performed as part of the HELB safety assessment.
Millstone Units 1 and 2, and Haddam Neck have three different nuclear steam
supply systems and three different architect/engineer firms. Therefore, the
oversight which occurred at these plants may have also occurred at other
sites.
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
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: P. Baranowsky, NRR
(301) 492-1156
A. Mattson, NRR
(301) 492-1174
R. Architzel, NRR
(301) 492-0804
Attachment: List of Recently Issued NRC Information Notices
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