10272020Tue
Last updateTue, 27 Oct 2020 7pm

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Actuation in Nuclear Power Plants

THE TESTING PROCESS

vmfall11_nuclear_repairMuch 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. Testing for a motor operator is described in a standard written by the Institute of Electrical and Electronic 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 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 be used for an application.

vmfall11_nuclear_hydraulic_actuatorThis 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.

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.

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 technology can only be used in applications that do not require active testing.

vmfall11_nuclear_chartEnvironmental qualification parameters.

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