How do inspectors ensure the safety of older nuclear reactors?
Japanese regulators granted a 10-year extension to an aging reactor at the Fukushima Daiichi nuclear plant despite observing damaged components, according to a report in the New York Times. Inspectors spent just three days looking around, which is apparently very brief. What do inspectors look for in a graying reactor?
Rust, cracks, and pits, among other things. Nuclear reactors have thousands of parts—valves, pumps, pipes, turbines, etc. Inspectors monitor all these bits and pieces, but the greatest concerns in older plants are the reactor vessel, which houses the core, and the containment structure, which is the last line of defense between the fissioning uranium and the public. Radiation and ordinary weathering processes can undermine these components, so inspectors have to routinely measure the thickness of their walls and check for signs of corrosion or cracks in order to ensure that the barriers remain leak-proof.
Sixty years ago, when commercial nuclear reactors were in development, engineers couldn't say for sure how long a reactor vessel might last. Its 6- to 8-inch-thick steel walls are bombarded with radiation, and go through extreme heating and cooling cycles whenever the reactor is restarted—which happens every 18 or 24 months during the refueling process. (If one of those walls becomes brittle, it's necessary to shut down the reactor permanently, since replacing a reactor vessel requires dismantling half of the plant. Researchers are experimenting with technologies to repair damaged parts of the wall, but none of these methods has been deployed in a commercial reactor.)
To test the soundness of the vessels over time, nuclear engineers keep metal samples of the same type and thickness as the vessel wall itself inside the reactor during operation. They remove them periodically and examine them for wear. The assumption is that the samples are suffering the same effects as the reactor vessel. The current consensus is that, under normal operating conditions, a vessel can last for at least 60 years.
While early nuclear scientists were acutely concerned about the reactor vessel, most had confidence in the hardiness of the containment structures. After all, the thick metal and concrete walls outside the reactor are, in theory, insulated from the harsh conditions of a nuclear reactor. They are designed to withstand earthquakes and other acts of nature, and aren't exposed to a regular barrage of radiation. By the late 1980s, however, inspectors began to notice flaws. More than one-quarter of the containment systems at the 104 nuclear reactors operating in the United States have now shown some form of degradation, which can be caused by freeze-thaw cycles, erosion, and even plant matter growing through the concrete. Unlike reactor vessels, however, it's economically feasible to repair damaged containment structures.
There are several ways to monitor a containment structure. Inspectors conduct visual examinations, either with the naked eye or a magnifying glass, in search of rust or pitting. They can apply a liquid to the wall, wipe off surface fluid, and see if any managed to penetrate. Some engineers create a magnetic field in the wall, then spread a thin layer of iron particles. If the characteristic magnetic field pattern is disrupted, it suggests the presence of cracks. Sound waves, radiography, electrical resistance, and compression tests can also indicate flaws.
In the United States, the 1954 Atomic Energy Act (PDF) initially limited nuclear plant licenses to 40 years. As the first generation of nuclear reactors approached and surpassed the middle of their 40-year lifespan, the Nuclear Regulatory Commission published procedures in 1995 to offer license extensions. Today, most expiration dates have been put off by two decades, and there's talk of extending the limits up to 80 total years of operation. The possibility has split the scientific community, largely because of potentially undetectable effects of aging.
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Explainer thanks Scott Burnell and Prema Chandrathil of the Nuclear Regulatory Commission, John Lee of the University of Michigan, and Akira Tokuhiro of the University of Idaho.