Leslie Dewan, cofounder and chief executive officer of Transatomic Power.

Nuclear energy startup Transatomic Power has backed away from bold claims for its advanced reactor technology after an informal review by MIT professors highlighted serious errors in the company’s calculations, MIT Technology Review has learned.

The Cambridge, Massachusetts-based company, founded in 2011 by a pair of MIT students in the Department of Nuclear Science and Engineering, asserted that its molten-salt reactor design could run on spent nuclear fuel from conventional reactors and generate energy far more efficiently than they do. In a white paper published in March 2014, the company proclaimed its reactor “can generate up to 75 times more electricity per ton of mined uranium than a light-water reactor.”

Those lofty claims helped it raise millions in venture capital, secure a series of glowing media profiles (including one in this publication), and draw a rock-star lineup of technical advisors. But in a paper on its site dated November 2016, the company downgraded “75 times” to “more than twice.” In addition, it now specifies that the design “does not reduce existing stockpiles of spent nuclear fuel” or use them as its fuel source. The promise of recycling nuclear waste, which poses tricky storage and proliferation challenges, was a key initial attraction of the company and captured considerable attention.

“In early 2016, we realized there was a problem with our initial analysis and started working to correct the error,” cofounder Leslie Dewan said in an e-mail response to an inquiry from MIT Technology Review.

The dramatic revisions followed an analysis in late 2015 by Kord Smith, a professor of nuclear science and engineering at MIT and an expert in the physics of nuclear reactors.

At that point, there were growing doubts in the field about the company’s claims and at least some worries that any inflated claims could tarnish the reputation of MIT’s nuclear department, which has been closely associated with the company. Transatomic also has a three-year research agreement with the department, according to earlier press releases.

In reviewing the company’s white paper, Smith noticed immediate red flags. He relayed his concerns to his department head and the company, and he subsequently conducted an informal review with two other professors.

“I said this is obviously incorrect based on basic physics,” Smith says. He asked the company to run a test, which ended up confirming that “their claims were completely untrue,” Smith says.

He notes that promising to increase the reactor’s fuel efficiency by 75 times is the rough equivalent of saying that, in a single step, you'd developed a car that could get 2,500 miles per gallon.

Ultimately, the company redid its analysis and produced and posted a new white paper.

To be sure, it would still be a notable accomplishment if the company could build a reactor that improved fuel energy efficiency over conventional reactors by a factor more than two. In addition, the new white paper notes the reactor could reduce waste by 53 percent compared to light-water reactors. The viability of the latter finding was recently verified by Oak Ridge National Laboratory.

But the analysis found that the reactor couldn’t sustain a fission chain reaction using spent fuel long enough for it to be a feasible option, as previously hoped, Dewan said in a subsequent phone interview.