water, which produces steam to drive a
turbine. With nuclear reactions, the
process is different, but the operational
objective is essentially the same—to
heat water to produce steam to drive a
turbine. With a coal steam generator,
the activity of burning the pulverized
coal and producing steam is contained
within what is termed the “boiler
island.” Because of the strong similarity
to what happens in the coal plant, many
engineers and technicians in a nuclear
plant refer to the area where steam gen-
eration occurs as the “nuclear island.”
Within the nuclear reactor, equip-
ment must meet rigorous standards.
Since radiation is very high, the stan-
dards require all equipment materials
used in this phase of the process to with-
stand high levels of radiation for the life
of the plant. Therefore, materials such
as iron or steel are required rather than
aluminum. Also, temperatures in this
portion of the plant are elevated, which
will affect seal materials. The combina-
tion of severe environmental conditions
determines the various coatings and
paints allowed for use in this area,
which is known as “inside contain-
ment.”
To be suitable for service in a nuclear
reactor, an actuator must go through
specific testing by outside testing agen-
cies. The test reports produced must
then be reviewed not only by the actua-
tor suppliers, but by consulting engi-
neers and the utilities themselves. Many
documents also are reviewed by the
NRC.
The tests for actuators used “inside
containment” apply to all styles. The
plants use electric motor operators as
well as pneumatic cylinder actuators and
hydraulic cylinder actuators. All of these
actuators must be tested to ensure they
can withstand the environment for their
expected lives. Also, in addition to environmental concerns, the actuators must
be able to operate during extreme accident conditions such as an earthquake.
THE TESTING PROCESS
Testing for a motor operator is
described in a standard written by the
Institute of Electrical and Electronic
With O-ring sealed covers in place on this
electric actuator, the motor, limit and torque
switches, as well as other electrical
components, are housed within a pressure-tight enclosure. This keeps out dirt and
moisture and prevents breathing from
expansion and contraction of the internal air
caused by temperature changes.
Engineers (IEEE)—IEEE-382. This
standard describes the testing required
to produce a motor operator for use in a
U.S. nuclear reactor. An additional key
standard used is IEEE-344, which
describes standards for seismic testing.
The certification process puts the
actuator through a simulated life test
where it is aged seismically, environmentally and mechanically. In other
words, the actuator is exposed to the
normal radiation and temperatures it
would experience in over 40 years of
operating in a nuclear plant, and it is
stroked as many times as expected over
that time period. When it is shown to be
at the end of its expected life, it is then
put through the most rigorous test of all,
which is known as the loss of coolant
accident (LOCA) test.
The LOCA test simulates a Design
Basis Event (DBE), such as an earthquake, in which the severity would vary
depending on where the plant was
This cutaway illustration shows the internal
components of a typical electric actuator.
located. During the test, the actuator is
exposed multiple times to extreme
temperature variations up to 500° F
(260° C). It also might be sprayed with
various caustic materials and/or be
required to stroke to ensure it can
operate under those conditions. The
bottom line is that very few safety-related actuators have the integrity to
pass this sort of testing, but whether
the safety-related actuator is pneumatic or electric, it has to be able to survive the stipulated LOCA test requirements to serve inside containment.
Another issue regarding nuclear
actuators concerns the designation of
“active” with regards to safety systems.
When a situation occurs in which a safety system needs to be invoked, such as
ensuring coolant water is available for
the reactor, actuators required to operate to deliver that water are part of the
safety system. These actuators are then
classified as “active” as they may be
required to operate during or after a
DBE. Thus, even when equipment is not
within the inside containment area
itself, if it is considered active, it must
go through stringent testing to ensure
reliability.
The seismic and environmental tests
can take 12 to 18 months to complete.
Once the actuator is qualified to work
within this environment, another important step is taken.
Although actuators are used on
dampers in heating, ventilation and air
conditioning systems, many actuators
are operating valves inside the containment building. The valves have their
own standards because they are pressure containment devices. To operate in
those areas, the valves must carry what
is known as the “N” stamp, and the
tests of valves associated with the “N”
stamp are demanding and arduous.
Sometimes, the “N” stamp testing
terminology causes confusion as far as
how it relates to actuators. Actuators by
themselves have no “N” stamp. Instead,
they have a pedigree based on various
testing results over many years. Reviewing and evaluating the numerous test
reports is how a determination is made
as far as whether or not an actuator can
