Following the code

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Portrait of Guidotti

The nuclear industry still relies on software that Tim Guidotti introduced four decades ago

During high school, Tim Guidotti lived at Scott Air Force Base in southern Illinois, where his father was stationed. His plan was to attend Oregon State University, as his father had, but first he wanted to establish Oregon residency. After high school, Guidotti headed to Waldport and took a job at a fishing resort and marina for six months.

“I took the time to consider different majors. At first, I thought I would go into fisheries and wildlife,” said Guidotti, B.S. nuclear engineering ’78, M.S. mechanical engineering ’80. He had befriended some people at the coast who worked in fisheries, but the more he learned from them about what to expect after joining the workforce, the more doubts crept in about whether the lifestyle would agree with him.

“So, I pulled out the course catalogue again and went through it like I was shopping for a career,” Guidotti said. “That’s how I first learned about nuclear engineering, which was a pretty new field at the time. I didn’t really give the choice much thought."

The arbitrary decision turned out well for Guidotti and for the nuclear power industry.

In 1979, while he was finishing up his master’s degree, Guidotti became an associate engineer at the Battelle Pacific Northwest National Laboratory in Richland, Washington. There, he and his colleagues boosted the power and flexibility of the COBRA computer code. COBRA had been developed a decade earlier to predict temperatures within a reactor core. Guidotti’s team expanded that ability and enabled the software to analyze more complex reactor problems, particularly the behavior of reactor cooling systems during simulated loss-of-coolant accidents, or LOCAs — which can be catastrophic. A LOCA played a major role in the 2011 Fukushima Daiichi nuclear disaster.

But these computer simulations, which were mandated by the Nuclear Regulatory Commission, were time-consuming and costly, and the nuclear power industry had no choice but to foot the bill.

I remember one simulation that cost $30,000 just for the computer time,” Guidotti said. “They were run on a mainframe computer located more than a thousand miles away in Los Alamos, New Mexico, and they could take days to complete."

Nuclear plant operators, fuel-processing facilities, and other businesses that were required to conduct these periodic safety evaluations were eager to find more efficient, economical options.

“The trajectory of high-level computing was clearly toward smaller, faster, and more affordable machines,” Guidotti said.

He saw an opportunity, and in 1984, Guidotti founded Numerical Applications in Richland, Washington. Building on the foundation of COBRA, the company developed GOTHIC software, which soon gained a reputation for its ability to quickly and faithfully model coolant flow, heat transfer, and other thermal-hydraulic processes in reactors and reactor subsystems under conditions ranging from normal operations to serious accidents. Engineers and nuclear plant operators now had a powerful new tool that provided information that could be used to identify potential vulnerabilities and to optimize system design. In addition, GOTHIC simulations ran quickly and efficiently on relatively inexpensive minicomputers — the forerunners of desktop systems.

Numerical Applications continually adjusted GOTHIC to keep up with the changing needs of the industry. Forty years after its inception, the software is still recognized internationally as the gold standard for resolving difficult safety analysis issues.

GOTHIC users include numerous utilities, nuclear plant vendors, research laboratories, and regulatory agencies, which rely on the software to perform a range of safety analyses at existing power plants. But it’s also used to certify the design of advanced systems, such as reactors cooled by sodium or molten salt and small modular plants.

“I think GOTHIC became so popular because it could be used to solve so many different types of problems and scenarios,” Guidotti said.

GOTHIC’s versatility was on display when nuclear safety organizations and researchers used it to evaluate the Fukushima nuclear disaster, which culminated in a hydrogen-air explosion that literally blew the roof off one of the containment buildings. The software determined where the hydrogen migrated to within the building, tracked the pressure buildup, and modeled the conflagration as it ignited. The findings also indicated that a filtered venting system may have prevented the explosion. Filtered venting systems, which were already in use at some reactors, allow plant operators to release pressure from a reactor containment building without releasing radioactive materials into the environment. Since Fukushima, GOTHIC has been used to evaluate more of these important systems at other nuclear plants, while several countries have stepped up efforts to install them retroactively.

GOTHIC wasn’t done, though. Out of curiosity, a staff member at Numerical Applications wanted to know if the software could model the lethal tsunami that had inundated Japan’s coast and swamped the nuclear plant.

“It worked,” Guidotti said. “GOTHIC simulated the formation of the tsunami based on inputs for the amount of water that was displaced by the earthquake. It’s not what the software was designed for, but it demonstrates its computational power for solving many types of complex dynamic problems."

In 2011, Guidotti sold Numerical Applications to Zachry Nuclear Engineering in Stonington, Connecticut, and stayed on to assist with the transition. He retired in 2016. For six weeks. “I kept getting calls from former clients asking for help,” he said.

So, he established NUSA Applications, where he serves as CEO and principal engineer for a small staff that provides technical consulting to the nuclear power industry.

“We’ve been given some very high-level opportunities to help nuclear fuel vendors and to work on advanced reactor designs,” Guidotti said. “It’s fun — challenging and fun. The future for nuclear is very bright, and there’s a lot of interest in new designs. I guess retirement will just have to wait a while.”

Photo By: Kim Fetrow

Dec. 22, 2023