Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
November 1, 1989
Information Notice No. 89-73: POTENTIAL OVERPRESSURIZATION OF LOW
PRESSURE SYSTEMS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees of the potential for
overpressurization of low pressure systems during testing of valves
connecting these low pressure systems to higher pressure 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 September 5, 1989, the McGuire Unit 2 reactor was being returned to
service following a refueling outage. The reactor coolant system was at
140 F and 325 psig (Mode 5). The B-train of the residual heat removal
system (RHRS) was in service. Plant personnel were performing a required
stroke test procedure on the A-train containment spray pump suction valve
(NS18A). The function of valve NS18A is to open during the recirculation
phase of a loss-of-coolant accident (LOCA) to permit the A-train containment
spray pump to take suction from the containment sump. The plant has two
containment sump suction lines, each of which serves a containment spray
pump and an RHR pump for each of the two independent engineered safety
feature trains. The NS18A valve is interlocked so that it will not open
unless the refueling water storage tank is isolated (by closing valve NS20A)
from the containment spray system and the containment sump isolation valve
is opened. The containment sump was kept isolated for the duration of the
test by bypassing the valve interlock in accordance with procedures. Also
in accordance with procedure, valve NS20A was closed to isolate the
refueling water storage tank. See attachment 1.
The procedure for stroke testing the NS18A valve had undergone the normal
reviews and approvals by utility personnel. The plant staff did not realize
that opening the NS18A valve would pressurize the containment spray system
with reactor system pressure from the operating RHR loop nor did the test
procedure contain an adequate caution. The suction piping of the A and B
RHR trains is normally cross-connected and was cross-connected at the time
the NS18A valve was cycled. The containment spray system is designed for
220 psig, whereas the RHR system is
8910260022
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IN 89-73
November 1, 1989
Page 2 of 2
designed for 450 psig. When the NS18A valve was stroked open, the
containment spray system was pressurized thus causing the relief valve
upstream of the containment spray pump to open and discharge to the
pressurizer relief tank. The capacity of the relief valve was not
sufficient to prevent overpressurization of the containment spray system,
which caused failure of the containment spray heat exchanger bottom flange
gasket. The containment spray discharge header was isolated so that
containment spray did not take place. Operators quickly responded to the
loss of reactor coolant by closing the NS18A valve. An estimated 2200
gallons of reactor coolant was lost through the failed containment spray
heat exchanger during the approximately 1.5 minutes the NS18A valve was
open. Licensee personnel inspected the containment spray system piping not
including the heat exchanger room. Therefore, the licensee did not detect
the failed heat exchanger gasket. After the NS18A valve was closed, the
containment spray system was aligned to take suction from the refueling
water storage tank by opening the NS20A valve in accordance with procedures.
With the NS20A valve opened, water from the refueling water storage tank
began to flow into the containment spray system and onto the floor of the
auxiliary building through the failed bottom flange gasket of the spray heat
exchanger. The leak continued for about 1 hour before being detected and
isolated. The licensee estimates that a combined total of 10,000 to 15,000
gallons of reactor coolant water and water from the refueling water storage
tank leaked through the failed gasket.
The licensee revised the surveillance testing procedures for the containment
spray suction valve to caution that stroke testing should not be performed
with the associated RHR train pressurized above 100 psig. Testing
procedures for valves in other interconnecting lines were examined to ensure
that they contained proper precautions.
This information notice requires no specific action or written response. If
you have any questions about this matter, 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: Caudle Julian, RII
(404) 242-5585
Sammy Diab, NRR
(301) 492-1075
Walton Jensen, NRR
(301) 492-1190
Attachments:
1. Figure - McGuire Unit 2 RHR and Containment Spray Systems
2. List of Recently Issued NRC Information Notices
