Seattle firefighters are training for a transit tunnel or highway disaster, by battling smoke and extreme heat inside a pipe at an unfinished nuclear-power plant.

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SATSOP, Grays Harbor County — Firefighters are training in 700-degree heat this week, inside an unused nuclear-plant pipe.

As new light-rail tunnels extend from Seattle’s Westlake Station to Husky Stadium and out to Northgate, and as the region prepares for the late-2015 opening of the new Highway 99 tunnel, first responders are preparing for the possibility of a construction accident, or that a car could crash and ignite a fuel tank.

Tunnel fires are notoriously tough to fight. Local tunnels are equipped with ventilation fans, but Seattle firefighters are training to fight blazes as if the systems fail to work.

On Wednesday, they practiced how to react if a conveyor belt on a tunnel machine sparks or the wiring fries during tunnel boring.

Unlike in a house fire, heat in a tunnel is confined and reflected by the tunnel walls and can exceed 1,000 degrees. Smoke will rise, drift overhead toward an entrance, then descend — resulting in a pocket of cleaner air with darkness on either side. Survivors might be found there, but time is running out.

“The heat starts to bank, from the top down,” says Battalion Chief Mike Nakamichi, sweat evaporating off his arms as he pulled off his heavy jacket.

Firefighters said that in a tunnel, water must be targeted at the base of the flames; excessive water would turn to steam and worsen visibility.

Crews also tried out new four-hour breathing gear, in which they rebreathe their air, filtered, instead of using oxygen tanks. Seattle fire officials said the longer packs would be useful for prolonged rescues where firefighters are helping many victims or dealing with toxic fumes even after temperatures are controlled.

Firefighters had to find and drag bundles that simulated victims. They were checked after each session for high blood pressure and heat-related ailments. A few were near exhaustion, especially after the tunnel had baked an hour or more.

“The firefighters are getting beat up before they get to the fire,” said Seattle Battalion Chief Scott Yurczyk.

A flaming stack of wood pallets drove temperatures high enough to cause spalling — when moisture within the concrete walls will explode, like popcorn, and send concrete shards flying. Here at Satsop, pieces flew off, causing a crack like a shotgun blast.

The Highway 99 tunnel will have not only sprinklers and vents, but the interior will be coated with a 1.2-inch-thick calcium shield designed to withstand a gasoline fire. During construction, the builders will supply their own emergency crews instead of relying on city firefighters.

Firefighters from Grays Harbor County, Ventura County, Calif., and New York City are training alongside 50 to 60 Seattle firefighters this week.

There are many ways for a tunnel fire to start, said FDNY Capt. Larry Tompkins, who has fought transit-tunnel fires in New York. In some cases, equipment ignited, or people working or living in subways have left debris piles that caught fire, he said.

The Washington Public Power Supply System tried to build five nuclear plants in the 1970s and early 1980s but only one is producing energy, at Hanford.

The tunnel in Wednesday’s exercise, 12 feet in diameter, was meant to carry hot water from the reactor to the cooling tower.

Managers at Satsop Development Park hope the unique site draws fire teams from around the country, boosting the local economy.

Firefighters are also training in heavy rescue at a reactor building, where they descend by rope or stairs, to lift pillars and steel off simulated victims, if a transit station were to ever collapse.

Worldwide, there have been at least 188 fires in highway, transit and freight-rail tunnels, according to Promat, a maker of fireproof materials.

Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom.