Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 7, 1990
Information Notice No. 90-76: FAILURE OF TURBINE OVERSPEED TRIP
MECHANISM BECAUSE OF INADEQUATE SPRING
TENSION
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees to the potential
failure of the overspeed trip mechanism on turbine-driven pumps as the
result of inadequate tension in the emergency trip spring. 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 March 17, 1990, and on August 14, 1990, during two separate monthly
surveillance tests of the Terry turbine-driven auxiliary feedwater pump at
Arkansas Nuclear One, actuation of the local manual trip lever on the
overspeed trip mechanism failed to close the turbine trip and throttle
valve. If a malfunction causes the turbine to overspeed, the overspeed trip
mechanism (shown in Figure 1) normally will rapidly close the turbine trip
and throttle valve, stopping steam flow to the turbine. Excessive turbine
speeds could damage the turbine and pump and overpressurize the auxiliary
feedwater system.
In response to an overspeed, a weight in the turbine shaft is forced outward
by the excessive centrifugal force, striking a tappet, which moves upward,
releasing the emergency head lever (see Figure 1). This lever normally
allows the emergency trip spring to move a rod connected to the turbine trip
and throttle valve trip mechanism. The movement of the connecting rod
should then disengage a trip hook lever on the trip and throttle valve
allowing the valve to close. However, during the two failed tests, the
emergency trip spring failed to move the connecting rod. During the first
of these tests, the plant personnel were able to actuate the trip mechanism
by tapping on the emergency trip spring holder, which produced an erratic
response, indicating that the spring force and the frictional forces were
evenly matched. Arkansas Nuclear One personnel lubricated the critical
mating surfaces and achieved good trip response.
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IN 90-76
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However, the second failure clearly indicated that insufficient spring force
was the root cause of the problem. The original emergency trip spring,
which had been in service approximately nine years, had stretched 1/8 inch
beyond its normal at rest position. This spring was replaced with a new one
of identical design. After this change, plant personnel successfully
completed a number of trip tests.
Discussion:
Inspection of the original emergency trip spring provided neither evidence
of a manufacturing defect nor that the spring had ever been stretched beyond
its elastic limit. The observed elongation was apparently the result of a
long-term relaxation process, which could develop on this or any other
similar trip mechanism. The tension in the spring, and thus the force
applied to the connecting rod, can be controlled by adjusting the position
of the trip spring holder on a threaded portion of the connecting rod. This
adjustment is made during turbine installation and normally is not changed
or checked thereafter. The Arkansas Nuclear One staff intends to revise the
maintenance procedures to specify adjustment of the trip spring holder
position in the event of any future problems attributable to insufficient
spring force.
The failure of overspeed trip mechanisms could result in severe damage to
turbines and pumps in boiling water reactor high-pressure coolant injection
and reactor core isolation cooling systems and in pressurized water reactor
auxiliary feedwater systems. The resulting overpressurization can also
damage other components. NRC Information Notice 90-45 discussed two
overspeed events involving turbine-driven auxiliary feedwater pumps. One
occurred at Rancho Seco, where the turbine, rated at 3600 rpm, reached 6020
rpm a few seconds after start. According to calculations, the auxiliary
feedwater system pressure reached 3850 psig, significantly exceeding the
system design pressure of 1325 psig. This overpressurization also affected
the motor-driven auxiliary feedwater train because the two trains were tied
together. The other overspeed event occurred at San Onofre, where the
turbine-driven auxiliary feedwater pump reached 5000 rpm and the auxiliary
feedwater system pressure was calculated to have reached 2420 psig.
The event at Arkansas Nuclear One revealed an additional failure process for
overspeed trip mechanisms. It serves as a reminder that the characteristics
of mechanical components, such as springs, are subject to change. Such
variability highlights the importance of periodic checking and adjustment,
inherent to adequate maintenance and testing practices.
Related Generic Communications:
1. NRC Information Notice 90-45, "Overspeed of the Turbine-Driven
Auxiliary Feedwater Pumps and Overpressurization of the Associated
Piping Systems", dated July 6, 1990.
2. NRC Information Notice 88-67, "PWR Auxiliary Feedwater Pump Turbine
Overspeed Trip Failure", dated August 22, 1988.
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IN 90-76
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3. NRC Information Notice 88-09, "Reduced Reliability of Steam-Driven
Auxiliary Feedwater Pumps Caused by Instability of Woodward PG-PL Type
Governors", dated March 18, 1988.
4. Information Notice No. 86-14, "PWR Auxiliary Feedwater Pump Turbine
Control Problems", dated March 10, 1986.
5. Information Notice No. 86-14, Supplement 1, "Overspeed Trips of AFW,
HPCI, and RCIC Turbines", dated December 17, 1986.
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: Thomas F. Stetka, Region IV
(817) 860-8247
Michael F. Runyan, Region IV
(817) 860-8142
Attachments:
1. Figure 1. Emergency Feedwater Pump Overspeed Trip Mechanism
2. List of Recently Issued NRC Information Notices
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