What happens when the fallout from the nation’s largest industrial disaster goes nuclear?

In 2009, App Thacker was hired to run a dredge along the Emory River in eastern Tennessee. Picture an industrialized fleet modeled after Huck Finn’s raft: Nicknamed Adelyn, Kylee, and Shirley, the blue, flat-bottomed boats used mechanical arms called cutterheads to dig up riverbeds and siphon the excavated sediment into shoreline canals.

The largest dredge, a two-story behemoth called the Sandpiper, had pipes wide enough to swallow a push lawnmower. Smaller dredges like Thacker’s scuttled behind it, scooping up excess muck like fish skimming a whale’s corpse. They all had the same directive: Remove the thick grey sludge that clogged the Emory.

The sludge was coal ash, the waste leftover when coal is burned to generate electricity. Twelve years ago this month, more than a billion gallons of wet ash burst from a holding pond monitored by the region’s major utility, the Tennessee Valley Authority, or TVA.

Thacker, a heavy machinery operator with Knoxville’s 917 union, became one of hundreds of people that TVA contractors hired to clean up the spill. For about four years, Thacker spent every afternoon driving 35 miles from his home to arrive in time for his 5 p.m. shift, just as the makeshift overhead lights illuminating the canals of ash flicked on.

A dredging machine removes sediment and coal ash from the Emory River as a tug boat pushes a barge of debris in 2009. AP Photo / Wade PayneDredging at night was hard work. The pump inside the dredge clogged repeatedly, so Thacker took off his shirt and entered water up to his armpits to remove rocks, tree limbs, tires, and other debris, sometimes in below-freezing temperatures. Soon, ringworm-like sores crested along his arms, interwoven with his fading red and blue tattoos.

Thacker’s supervisors gave him a cream for the skin lesions, and he began wearing long black cow-birthing gloves while he unclogged pumps. While Thacker knew that the water was contaminated — that was the point of the dredging — he felt relatively safe. After all, TVA was one of the oldest and most respected employers in the state, with a sterling reputation for worker safety.

Then, one night, the dredging stopped.

Sometime between December 2009 and January 2010, roughly halfway through the final, 500-foot-wide section of the Emory designated for cleanup, operators turned off the pumps that sucked the ash from the river. For a multi-billion dollar remediation project, this order was unprecedented.

The dredges had been operating 24/7 in an effort to clean up the disaster area as quickly as possible, removing roughly 3,000 cubic yards of material — almost enough to fill an Olympic-sized swimming pool — each day. But official reports from TVA show that the dredging of the Emory encountered unusually high levels of contamination: Sediment samples showed that mercury levels were three times higher in the river than they were in coal ash from the holding pond that caused the disaster.

Then there was the nuclear waste. According to a 2011 TVA report, the river-bound coal ash and sediments contained higher radiation levels than the holding pond. Half of the river sediment samples taken contained cesium-137, a highly soluble compound with a 30-year half-life that can contaminate large bodies of water for well over a generation.

It’s best known as the predominant source of radiation in the fallout of the 1986 Chernobyl nuclear disaster. It can cause burns, acute radiation sickness, and death. Exposure also increases the risk for cancer.