Bechtel is the best at replacing old steam generators like this one at San Onofre.

As things get older they need maintenance. Cars need new transmissions, roads need resurfacing, houses need painting. It's no different for nuclear power plants. Some of their components begin to wear out over time and must be replaced—especially the steam generators that transfer reactor heat to produce nonradioactive steam that drives the plant’s turbines.

A typical steam generator in a pressurized water reactor has more than 3,000 alloy tubes, through which radioactive water is pumped under high pressure. The tubes were expected to last for 40 years, but metallurgical reactions and chemicals in the water can shorten their lives. As individual tubes wear out, they’re plugged shut, but when about 15 percent of the tubes have corroded, it’s time to replace the generator.

Although swapping an old steam generator for a new one seems straightforward, the process is not exactly plug-and-play. Replacement generators can be more than 66 feet (20 meters) tall and weigh more than 770 tons (700 metric tons), making them difficult to lift into position and install. They aren’t exactly identical to the aging generators they replace, so they require new piping and connections. And the only way to gain access to the old generators in many big plants is to cut a hole in the wall of the containment building that houses the reactor, because the maintenance doors are too small and there is little room to maneuver equipment inside.