This week (October 18–22, 2021) is Nuclear Science Week, an international, week-long celebration of nuclear science. According to ASNT’s 2020 Materials Evaluation readership survey, 12% of the members serve the nuclear industry, and it is tied for the fourth-largest industry represented. NDT inspectors working in nuclear power plants represent a small percentage of the membership, but have a big impact on creating a safer world.

The following are excerpts from “Introduction of NDT Methods and Techniques in Power Plants” by Terry Haigler, which was published in Materials Evaluation, Volume 78, Issue 10 in October 2020. These excerpts have been lightly edited for ASNT Pulse. Access the full article in the NDT Library at ndtlibrary.asnt.org.

Introduction of NDT Methods and Techniques in Power Plants

Nondestructive testing (NDT) is widely used as part of routine maintenance, asset management, and fitness-for-service assessments in the power generation industry. There are many NDT methods and techniques that are used for power plant inspections, depending on defect type and location, materials, components, component access, failure mechanisms, and code requirements. To better understand these variables, it’s important to understand the basic design and operation of power plants.

There are many ways to generate electricity. The goal for a power plant is to generate enough energy to turn the generator that creates the power we use at our homes and offices. The generator is a rotating machine that converts mechanical energy to electrical energy using coiled wires that spin inside a magnetic field, which causes an electrical current to flow through the wires. At a basic level, the primary difference between power plants is how they create the energy to spin the generator. Power plants use fuel to create high-pressure steam to turn a large turbine that’s connected to the generator. The primary fuels used for power generation are fossil fuels, such as coal or gas, or nuclear fission.

Nuclear Power

In a nuclear power plant, many of the components are similar to those in a fossil-fueled plant, except that the boiler is replaced by a nuclear steam supply system (NSSS). At the heart of this system is a nuclear reactor that creates energy by the nuclear fission (splitting) of atoms, where coal and gas and heat recovery steam generator systems use the combustion of fuel to create the energy. The necessary components within the NSSS are determined based on one of two reactor designs: a boiling water reactor (BWR) or a pressurized water reactor (PWR). Inside a BWR, the reactor vessel has a steam/water mixture that moves upward through the core, absorbing heat along the way. The steam leaves the top of the core and enters the steam piping, which directs the steam to the turbine, making it spin along with the attached generator. A PWR differs from a BWR in that steam is produced in the steam generator (secondary side) rather than in the reactor vessel. A device called a pressurizer keeps the water that’s flowing through the reactor vessel under very high pressure (more than 0.007 MPa [2200 lb/in.²]) to prevent it from boiling (US Nuclear Regulatory Commission, n.d.).

In both designs, there are multiple redundant safety-related systems that require routine inspections along with the primary reactor components. Additional safety structures like the containment vessels also require specialized inspections. Unlike fossil-fueled plants and gas turbines, nuclear power plants are heavily regulated by the American Society of Mechanical Engineers (ASME) and the Nuclear Regulatory Commission (NRC) with very specific guidelines on when and how the plants are inspected, along with which methods and techniques are to be used.

Liquid Penetrant Testing

Liquid penetrant testing (PT) is more prominently used in the nuclear power industry and for turbines, since most of the components are designed using nonferromagnetic materials (like many nonmagnetic stainless steels), eliminating the magnetic particle testing MT option. PT inspections are used to locate surface-breaking discontinuities by relying on capillary action. PT is also used in coal/gas plants but is dependent upon surface preparation and component surface conditions.

References

US Nuclear Regulatory Commission, n.d., Reactor Concepts Manual, NRC Technical Training Center, retrieved at https://www.nrc.gov/docs/ML0230/ML023020519.pdf.

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Haley Cowans is Educational Materials Editor at ASNT, hcowans@asnt.org.

Access to papers in the NDT Library is free to members.

Excerpted from, “Introduction of NDT Methods and Techniques in Power Plants” by Terry Haigler, Materials Evaluation, Volume 78, Issue 10 in October 2020.
DOI: https://doi.org/10.32548/2020.me-04154