Between spring and summer, in that brief window when neither heat nor air conditioning is on, is maintenance and refueling season at the nation's nuclear power plants.

Every 18-24 months, thousands of craft workers and engineers descend on a reactor, shut it down, replace some of the fuel rods and shuffle the rest around, and hammer out a punchlist of maintenance and repairs that are all but impossible to perform while the reactor is running.

Wilkes-Barre, Pa., Local 163 Business Manager John Olejnik used to work outages at the twin reactor Susquehanna Steam Electric Station in Salem Township, and now that he runs the local, those outages are a huge source of man-hours.

But even before the COVID-19 pandemic struck the U.S., nuclear refueling outages were a petri dish for the transfer of all manner of illness, Olejnik said.

"We called it power plant flu," he said.

And it wasn't just the tight workspaces common to all construction sites. It was the requirements of the nuclear shutdown itself — the timing, the security, the quality control — that made it, in his words, "a breeding ground."

"Something as simple as the security line at the beginning and the end of the shift, getting X-rayed, bomb sniffed, you'd rest your hand on a scanner to match it with your ID, and everyone is touching that scanner," he said. "Every outage, everyone got sick."

This spring, 32 of the nation's 96 nuclear reactors had outages scheduled just as the novel coronavirus spread wildly out of control. Nearly all of the electrical work was done by IBEW members, representing millions of man-hours. In recent years, for Olejnik's local, those 30 days represent about one-third of his annual man-hours and he is far from alone.

The work is planned nearly a year in advance. Nuclear Regulatory Commission rules require every job, down to the torque specification on every bolt, to be planned out and written in advance. Everything is written in mountains of paper that might as well be stone: maintenance schedules, life cycles, the fuel itself. Nothing is flexible. Nothing can be put off or the lights go out. Emergency rooms go dark. Home offices, home schools? All impossible.

"If these reactors aren't up and running by the projected end date, they have to start thinking about Plan B, which could include scheduled rolling brown outs, especially considering the high power demand that comes with the warm weather," said Construction and Maintenance Department Director Mike Richard. "There is no more essential work than keeping nuclear plants safe and running. It's a huge challenge and the dangers are real."

And yet, Richard said, reports from plant owners, contractors and his conversations with business managers tell him a story of craftsmen and craftswomen rising to the occasion under enormous pressure.

Two of Exelon's four Illinois plants, for example, recorded their fastest-ever refueling outages.

But there were flare-ups at nearly every job, Richard said, and work had to be delayed at some sites, including the Enrico Fermi Nuclear Generating Station in Newport, Mich.

There was an outbreak in a unit known as a torus, a large steel doughnut-shaped water containment vessel that had painters and electricians in close quarters on walkways with a restricted air flow. It forced a work slowdown, impacting members of Toledo, Ohio, Local 8, said Utility Department International Representative Mark MacNichols, a career nuclear worker.

"That could have happened anywhere; it happened in the torus, but it could have been cleaning the tubes and condensers or installing a power box or just pulling wire when it isn't possible to be six feet apart," he said. "The hard thing with nuclear is keeping people safe requires changes to punchlists that took a year to write and cannot be changed without approval from federal regulators."

At the onset of the outbreak, there was concern that the outage workforce, which includes a substantial number of travelers who go from outage to outage during the high seasons, would not be able to get from place to place in a quarantine. And if they did get to the site and tested negative, no one quite knew where they would stay and how they would stay virus-free.

The Fermi workers, for example, were put up at the Motor City Hotel and Casino in Detroit, vacant at the time, Richard said. Everyone drove in their own cars, but had a certain window to get from the hotel to the plant to make sure no one exposed themselves to someone outside the quarantine.

"The utility has timed out how long it should take to get there. You are outside the window, you are pulled and can't work," Richard said. "If you need to get gas, you better let people know. They were very serious about avoiding another outbreak."

But IBEW members responded in the way they always do, Richard said. "Our members showed up, did the job with professionalism and kept the lights on. And they did it in the face of enormous challenges and personal risk. I couldn't be any prouder to be a member of this great union."