TAMPA — After five workers died at Big Bend Power Station, Tampa Electric vowed to immediately stop the kind of work that led to the fatal accident.No one would clean or do maintenance on a slag tank connected to a running boiler, the company promised — not before investigations into the accident had finished."We're not going to do it," CEO Gordon Gillette told the Tampa Bay Times in August, "until we understand what happened."A few weeks later, they did it anyway.In late August, workers at Big Bend drained water and hardened slag out of a tank onto the plant's floor while the boiler was running overhead, according to its workers union.It's a different procedure than the one that caused the fatal accident on June 29 — one that workers can perform at less risk, farther away from the tank. But both procedures involve opening the tank while a lava-like substance is building up above, and both fall in the broader category that the company vowed to stop.[ READ THE INVESTIGATION: Tampa Electric knew the procedure was dangerous. It sent workers in anyway. ][ MORE: TECO accounts for nearly half of Florida power plant deaths, data shows ]Tampa Electric confirmed that the August procedure took place and said the plant's general manager halted the work immediately upon learning about it."Since then, we have discussed this with the union, and we further clarified to our employees that our prohibition also includes releasing water from a slag tank with the boiler running," spokeswoman Cherie Jacobs said in an email.Union leaders say it is another example of the company taking risks with safety."No one has gotten hurt, but they could," business manager Doug Bowden said. "Especially considering what's just happened, we've said to them, 'You know what? Let's just stop all of it. Let's just turn it off for everything.'"Turning off and restarting a coal-fired boiler costs hundreds of thousands of dollars, experts say.Tampa Electric's safety record has come under intense scrutiny since the accident earlier this summer. Over the last two decades, more workers have died in Tampa Electric's power plants than in any other Florida utility's, the Times reported in July. No other utility has had more than three fatalities. Tampa Electric has had 10.Five of those were a result of the June accident at the Big Bend plant in Apollo Beach. They were burned to death by molten coal ash, called slag.At Big Bend and other coal-burning plants like it, slag accumulates inside a power-generating unit's boiler and drains through a man-sized hole into a tank below. The tank is usually filled with water, which cools the slag and turns it into harmless rock.On June 29, six workers — one Tampa Electric employee and five contractors — were tasked with removing a build-up of hardened slag at the bottom of a tank. They opened a door near the ground called the doghouse door, and, from just outside the tank, tried to break up the blockage with water blasters.The company kept the boiler running, hoping a second blockage — this one plugging the man-sized hole connecting the tank and the boiler — would keep the hot slag contained overhead. But about 20 minutes into the job, the plug burst and molten slag poured through the hole. It rushed out the doghouse door, covering the workers.Two workers died on the scene. Three died in the hospital. Only one survived.[ RELATED: Congresswoman to OSHA: 'Act swiftly' to fix rules after Tampa Electric power plant accident ]A Times investigation last month found a near-identical accident sent at least three Tampa Electric employees to the hospital in 1997. In the wake of that incident, a committee of workers and managers created new safety guidelines, which included not removing a blockage from the bottom of the tank while the boiler was running. The rules were followed for years, former and current employees told the newspaper, but eventually abandoned.The procedure in August had some notable differences from the one that caused the accidents in June and 1997. Jacobs, the Tampa Electric spokeswoman, pointed out that trained operators had opened the gate remotely from inside a protective booth.But Bowden said there was still a possibility someone could have gotten hurt. While dumping slag, he said, workers stand on platforms just a few feet away from the doghouse door."They should take the entire unit off line and make sure nobody is walking through that area," he added.Bowden has filed a grievance seeking to permanently ban slag tank work while the boiler is online.He said the company has already proposed possible solutions to the union's other concerns about the grievance process — speeding it up and allowing an independent safety expert to help resolve conflicts — but has not agreed to permanent rule changes for slag tank maintenance.Tampa Electric declined to comment on the talks.Last week, U.S. Rep. Kathy Castor, D-Tampa, sent a letter urging the Occupational Safety and Health Administration to tighten its rules to help prevent similar accidents. Specifically, she asked the agency to consider language to "protect workers from these slag tank maintenance-related injuries."OSHA is still investigating the June accident.Contact Kathleen McGrory at [email protected] or (727) 893-8330. Follow @kmcgrory.