When Walter Tamosaitis warned in 2011 that the Energy Department’s plans for a waste treatment plant at the former Hanford nuclear weapons complex were unsafe, he was demoted and put in a basement room with cardboard boxes and plywood for office furniture.

Tamosaitis had been leading a team of 100 scientists and engineers in designing a way to immobilize millions of gallons of highly toxic nuclear sludge as thick as peanut butter. The sludge, which could deliver a lethal dose of radiation to a nearby person within minutes, is stored in leaking underground tanks near the Columbia River in Washington state.

Two years later, Tamosaitis was fired after 44 years with San Francisco-based engineering firm URS, which was later acquired by Los Angeles-based AECOM. He filed a wrongful termination suit but encountered some initial legal setbacks, and it looked as if he had been blackballed from the industry.

But on Wednesday, Tamosaitis won a $4.1 million settlement from AECOM, among the largest known legal damages paid out to a whistleblower in the Energy Department’s vast nuclear waste cleanup program.

“It was something I lived with every minute of every day over the last five years,” Tamosaitis, 68, said in an interview. “Hopefully, I have sent a message to young engineers to keep their honesty, integrity and courage intact.”

AECOM spokesman Ed Mayer said the company reached its resolution with Tamosaitis “in order to avoid the cost and distraction of litigation relating to events that occurred over five years ago. The company strongly disagrees that it retaliated against him in any manner.”

Tamosaitis led the research into transforming the toxic and radioactive sludge into solid glass that could theoretically be buried safely for thousands of years. Over time, Tamosaitis said he began to worry that the technology for chemically mixing the sludge was flawed, potentially allowing explosive hydrogen gas to build up inside large tanks and clumps of plutonium to form that could start a spontaneous nuclear reaction.

His warnings, although disputed by his employer, were taken seriously by independent federal safety investigators and by senior Energy Department officials. Within months, department officials said the plant’s design and construction failed to meet federal safety standards.

In 2013, then-Energy Secretary Steven Chu ordered a halt to the construction of two massive processing facilities at Hanford: the pretreatment plant and the high-level vitrification plant that would turn the waste into glass. The suspension continues to this day amid doubts about whether the plants could operate as designed. So far, more than $13 billion has been spent on the project.