Information Notice No. 90-64: Potential for Common-Mode Failure of High Pressure Safety Injection Pumps or Release of Reactor Coolant OutsideContainment During a Loss-Of-Coolant Accident
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 4, 1990
Information Notice No. 90-64: POTENTIAL FOR COMMON-MODE FAILURE OF HIGH
PRESSURE SAFETY INJECTION PUMPS OR
RELEASE OF REACTOR COOLANT OUTSIDE
CONTAINMENT DURING A LOSS-OF-COOLANT
All holders of operating licenses or construction permits for
This information notice is being provided to alert addressees to the
potential for the common-mode failure of the high-head safety injection
pumps (charging pumps) due to gas binding or the release of reactor coolant
outside of contain-ment during a loss of coolant accident (LOCA), depending
upon the manner in which the vent line isolation valves have been installed.
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.
On July 9, 1990, with the plant in cold shutdown in preparation for
refueling operations, the Haddam Neck licensee had the Volume Control Tank
(VCT) discharge and vent flow paths isolated in the Chemical and Volume
Control System (CVCS). The charging pumps were aligned to the refueling
water storage tank (RWST), with a train of the residual heat removal (RHR)
system in operation. The Haddam Neck licensee subsequently discovered that
water was draining from the VCT through the high-point vent isolation valves
(see Figure 1). The 3/4-inch vent line, containing two ASCO
solenoid-operated isolation valves, provides for continuous venting of
hydrogen from the high points of the charging pump suction piping to the
VCT, during normal CVCS letdown opera-tions. Further investigation by the
licensee revealed that the ASCO valves would not isolate in the reverse flow
direction because the valve disc unseats with a differential pressure across
the valve seat of 15-30 psid. The licensee indicated, as a result of
discussions held with ASCO, that no ASCO-manufactured solenoid-operated
valves were designed to provide isolation in the reverse flow direction.
Initiation of a safety injection actuation signal (SIAS) causes the normal
discharge path from the VCT to isolate by automatic closure of the VCT
discharge isolation valves. The suction of the charging pumps is then
October 4, 1990
Page 2 of 3
from the VCT to the RWST. The ASCO vent line isolation valves are shut
automatically upon closure of the VCT discharge isolation valves and
manually de-energized. With the valves in the closed position, a
differential pressure in the reverse flow direction could allow gases in the
VCT to pass through the high-point vent line and cause gas binding of the
pumps. Haddam Neck responded to this concern by adding two check valves as
shown in Figure 1.
At Haddam Neck, the licensee identified a possible common-mode failure to
both high-point vent isolation valves in the suction vent line that connects
the charging pumps to the VCT. The common mode failure occurs if there is
leakage flow through both valves after they have been shut. From their
discussions with the valve manufacturer (ASCO), the licensee learned that
these valves are designed to provide isolation in one flow direction only.
The ASCO valves are identified as solenoid-operated, two-way, 1/2 inch
isolation valves with a maximum pressure rating of 400 psig and are
commerical grade. The model numbers of the valves are 6109R and L8211D89.
The installed configuration of the ASCO valves at Haddam Neck was such that
failure of these valves to isolate during a LOCA (i.e., during the ECCS
injection phase) could drain down the VCT and allow hydrogen gas in the VCT
(and any gas that had accumulated in the vent line) to be transported to the
suction of the charging pumps by way of the high-point vent charging pump
A second scenario, was identified by the licensee involving the
recirculation phase of a small-break LOCA, when the RHR pumps supply reactor
coolant from the sump to the suction header of the charging pumps. If the
ASCO valves are installed in the reverse direction (i.e., to isolate venting
from the VCT to the suction of the charging pumps), then the discharge
pressure from the RHR pumps would be enough to increase pressure in the vent
line to the ASCO isolation valves so as to lift the valve discs off of their
seats and pressurize the VCT in excess of the 75 psig VCT relief setpoint.
This would allow a release of reactor coolant outside containment.
In addition, if the two ASCO vent line valves are installed in opposite
directions, a single failure of a vent line isolation valve could lead to
either gas binding or a release of reactor coolant outside of containment.
Licensees with similar vent lines and ECCS pump arrangements may wish to
review the above information for applicability to their plants.
The effectiveness of the Haddam Neck high point vent configuration (as shown
in Figure 1) was not evaluated in conjunction with the development of this
information notice. It is important that any plant modifications made to
alleviate hydrogen buildup concerns do not introduce other ways for gas to
be ingested into the charging pump suction or other safety concerns.
October 4, 1990
Page 3 of 3
Information notice 88-23, "Potential for Gas Binding of High Pressure Safety
Injection Pumps During A Loss-Of-Coolant Accident" (including Supplements 1
and 2) also describes situations that could result in gas binding of high
pressure safety injection pumps.
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 NRC regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: John Thompson, NRR
Attachments: 1. Figure 1, Haddam Neck High Point Vent Line
2. List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Friday, May 22, 2015