The Role of Nondestructive Testing in Nuclear Power Plant Operations
Nuclear inspections require a special set of NDT practices to maintain public safety. The power generation industry is susceptible to large-scale incidents and the severe consequences that follow—in fact, according to experts, another major accident is likely to occur somewhere in the world within the next decade. However, regular inspections and stringent adherence to testing standards can significantly reduce or even eliminate the likelihood of a major accident.
Tube exchangers, turbines, or primary circuits are the source of many possible problems within a power plant—problems that can be avoided altogether if the flaws that cause them are detected early enough. For this, analysts need high-quality inspection instruments that are portable and can penetrate hard-to-reach areas easily and efficiently. Eddy current testing (ECT) and ultrasonic testing (UT) are two of the most effective nondestructive testing (NDT) methods for power plant maintenance.
Heat Exchanger Tubing Inspections
Since heat exchanger tubes undergo fluctuating expansions and contractions, flaws are likely to develop but easy to miss. By using eddy current equipment to inspect heat exchanger tubes, analysts can detect such potentially serious flaws as cracking or corrosion.
There are many heat exchanger tube testing solutions on the market, but one of the best incorporates a simultaneous injection box with multiplex frequencies combined. The multiplex frequency feature allows users to read defects ranging from surface cracks to underlying corrosion using four frequencies.
The injection feature also allows four frequencies, including eight pairs of coils. Additionally, it supports quick testing speeds and remote field testing (RFT). It can also be used to test most tube materials (including magnetic alloys) and accepts probes that can test a variety of surfaces.
Other ECT instruments offer combined tubing and surface functionality that can detect fretting wear within a rod cluster control assembly (RCCA). In particular, it can detect any deviations that may stem from rodlet vibrations within the guide cards during operations.
However, as most personnel within the power generation industry are aware, rodlet inspections can be hazardous, as the rodlets themselves are highly radioactive. What’s needed is a testing system that scans all of the rodlet clusters in two passes, allowing users to complete inspections more quickly and minimize their exposure to the rodlets. Quicker inspections are attainable via automated data analysis software that provides a comprehensive analysis of the materials.
Advanced ECT Probes
Analysts can obtain high-quality data with a single pass of a specialized ECT probe made especially for nuclear plant inspections. The probes have combined bobbin/array features that can detect all types of cracking or wear. Without combined bobbin/array features, analysts stand the risk of missing certain flaws.
Example: A level 1 analyst is testing tubing materials within a steam generator using bobbin probes as a field test. The analyst gathers the necessary data and delivers it to management. Although the results seem reliable, the manager decides to retest the tube anyway as an extra precaution, this time using a bobbin/array probe instead. This second test identified a previously overlooked, but significant, flaw in the form of circumferential cracking, allowing the analyst to provide solid recommendations for further maintenance and repairs.
Bobbin probes maintain high flexibility during testing but cannot read circumferential cracking. This problem occurs because current runs parallel to the coils, allowing the cracks to go undetected.
An array probe, on the other hand, comes with greater coverage per coil density, allowing for comprehensive coverage and a single-pass inspection feature that saves time. These encircling array probes are especially useful during external tubing inspections.
The combined bobbin and array features permit additional testing options and thorough coverage of an inspection area, leaving no flaw overlooked and minimizing the possibility of a serious accident. Plus, the accuracy probes with these features provide can save companies hundreds of thousands of dollars for each inspection by allowing inspectors to catch minute, early-stage flaws long before they develop into costlier, more hazardous problems.
Enhanced Ultrasonic Inspection Prowess
Phased array ultrasonic testing (PAUT) can foster the same efficient readings but on a volumetric level. The best UT instruments scrutinize flaws with hasty precision, able to scan items up to one million points per frame. However, such quality depends on the instrumentation. Further, the most innovative PAUT instruments should have pulsers that penetrate through thicker welds. Find a PAUT instrument from an experienced provider of UT equipment and probes.
UT can detect flaws on complex geometries and surfaces, depending on the right calibrations. UT products should provide simple setup times and easy calibration measures that shorten prep times. UT also provides a wide array of specialized scanners and probes to further customize the process.
Since corrosion can become a serious issue in nuclear plants, analysts need an instrument that can read corrosive anomalies within pipes or other unconventional surfaces. One method is an NDT paintbrush, where the two-axis coding provides a comprehensive scanning radius on flat and curved surfaces.
A paintbrush scanner is superior to conventional scanners because standard ones only come with an X-Y mapping system that cannot cover curved surfaces extensively. A paintbrush scanner has encoded wheels that follow the scanner position on-screen and allow the analyst to pinpoint unscanned areas. A quality paintbrush scanner will also include composite dedicated wheels that allow users to scan non-metallic surfaces.
Phased Array Detection Measures
For austenitic welds, PAUT provides more customization schemes. It’s also an ideal method for turbine blade inspections.
Example: A level 2 analyst inspects a turbine blade in a nuclear plant, focusing especially on the blade attachment serrations. The complex shape and geometry could be a challenge, but the analyst uses an instrument that accepts 2D matrix array probes and wedges to customize the wave beam, as well as a custom scanner that fosters an in-situ blade inspection. Armed with these tools, the analyst is able to complete a thorough inspection despite the blade’s geometric variations.
For tougher welds, PAUT offers low-frequency 2D matrix array to circumnavigate propagation issues caused by austenitic welds and stainless steel. With beam-steering and beam optimization, PAUT can inspect welds with high grain content with ease.
Dealing with fixed beam positions present other challenges, and can be particularly difficult to detect. If overlooked, such flaws can morph into serious problems and lead to structural damage within machinery or circuitry. With PAUT, analysts can adjust the wave beams accordingly to find subsurface indications that could jeopardize asset integrity.
2D matrix array probing is also ideal for detecting challenging anomalies, such as parallel flaws, that may go undetected when using other NDT methods.
Effective Nondestructive Testing for Nuclear Power Plants
The best NDT solutions for nuclear facilities incorporate both eddy current and ultrasonic testing methods. Both ECT and PAUT provide advanced surface, subsurface and volumetric flaw detection capabilities that other NDT methods typically cannot match. When it comes to nondestructive testing in a nuclear power plant, ECT and PAUT are among the most important approaches to include within regular maintenance schedules.
ECT and PAUT help plant operators avoid costly mishaps and accidents stemming from minor indications that later turn into larger issues. These methods also establish vital safety procedures that keep plants up to code, keeping regulators and heavy fines at bay. Most importantly, these measures help to ensure the continued safety of both power plant personnel and the public.
Zetec is a leading provider of eddy current and ultrasonic testing solutions, with decades of experience supporting power plants in their efforts to ensure safe work environments and efficient operations. Contact us today to learn more about our cutting-edge NDT products and services.