Much testing and precise quality control goes into manufacturing nuclear actuators. Here, a
highly trained quality control inspector is performing a quality test on two control packages for
a nuclear pneumatic linear actuator.
be used for an application.
When a qualified actuator with the
appropriate pedigree is mated to an
“N” stamp valve, the integral unit must
pass one more important test. This test
is to assure the “N” stamp valve combined with the approved attached actuator has sufficient integrity to withstand
difficult seismic requirements. The test
is governed by another standard known
as QME- 1. Although it is essentially a
valve test, the actuator is an important
component so it is part of the testing.
technology can only be used in applications that do not require active testing.
Because motor operators are very simple with just a motor, gear set, and limit
and torque switches within robust cast
iron and steel housing, some advanced
options are available that can assist the
nuclear industry in certain applications.
For example, several major actuator
manufacturers offer a thrust monitoring
component so the nuclear plant can
WHEN AND WHERE
EQUIPMENT CAN BE USED
As explained, only the most rugged
actuators manufactured today are suitable for use inside a nuclear reactor.
When equipment is not part of the safety system and it is outside of the reactor,
standard industrial equipment can be
used that takes advantage of some of
today’s newer technologies.
An example is smart actuators. The
modern electric actuator used in many
process plants today consists of an integral motor control center with a digital
control module. That configuration
allows remote monitoring of valve and
actuator data useful for maintenance
and diagnostics. Unfortunately, the technology of today does not allow these
sophisticated electronics to reside within
an actuator or to pass the environmental
and seismic testing required for inside
the reactor. Actuators with advanced
monitor valve thrust requirements and
actuator thrust outputs. While that
capability is a standard item with a
smart actuator, it’s not as simple with
the nuclear actuator. The typical way
this capability is achieved in a nuclear
actuator is by measuring the deflection
of the motor shaft via the spring pack
on the end of the motor.
Another example of a nuclear option
becoming very popular is the torque
limiter. When sizing an actuator for a
valve in a nuclear application, the criteria are different than for most other
applications. The valve factors used to
calculate the thrust required are very
high so sufficient safety margins are
ensured. The safety margins are set by
the industry and monitored very closely.
Another factor that comes into play is
power voltage. The actuators must be
sized to work on reduced voltages that
might be available during accident conditions. When a motor is running on
reduced voltage, it produces reduced
torque. The combination of these challenges means an actuator might be much
larger for a valve than normally used.
Motor operators are also very powerful and have a reserve torque and thrust
capability. The industry calls them stall
torques, and they can more than double
the rated output torque of an actuator.
Because of the demands, the associated
valve must have a stronger superstructure as well as a larger stem than what
might be expected. The larger stem then
might require a larger thrust, and the
exact balance becomes a challenge for
The torque limiter is a brake that will
reduce the force put on the valve during
abnormal operation so that the valve
does not need as large a structure to
hold the actuator. Another option used
for high-speed valve applications is
thrust compensators, which are essentially disc spring packs that allow for a
gradual loading of the actuator as the
This self-contained, linear hydraulic actuator is
designed for a nuclear plant’s main steam
isolation valve or main feedwater isolation
valve. It uses an electric motor to turn the
hydraulic pump, which generates the fluid
power pressure and flow to extend and retract
the main hydraulic cylinder. It is coupled to the
isolation valve to safely shut down the plant.
Various nuclear plant applications use
pneumatic piston operators. In many