What goes around comes around: The revival of Toshiba’s 4S

Today, commercial microreactors are common in the marketplace of nuclear ideas. Dozens of companies are vying for their designs to reach scaled deployment to meet surging energy demand.

However, the term "microreactor" didn't appear in Nuclear News until 2019, when the Department of Defense popularized it (in a nuclear context) in the early days of what would become Project Pele. Even before then, however, all the way back in 2005, Toshiba was developing the 4S (Super-Safe, Small, and Simple), a 30-MWt, pool-type reactor designed for remote locations with small grids. Once sealed and delivered, the reactor would run for 30 years with no refueling. If the word "microreactor" had been in use then, the 4S would certainly have been categorized as such.

While the 4S was by no means the first reactor designed to come in at less than 50 MWe, it was one of the first to have essentially the same sales pitch as today's wave of what we now call microreactors.

Perhaps it should come as no surprise, then, that a start-up has decided to try to revive the 4S design. That company is Zap Energy, which announced last week that it is "starting with the 4S concept and revitalizing it with modern design tools as a base for our first fission product design."

There is one more unusual twist to this already unusual story: Zap Energy, up until now, has been solely focused on developing fusion power.

A brief history of 4S: The early stages of Toshiba's plans for the 4S were discussed at the American Nuclear Society's 2004 Winter Meeting. Toshiba aimed to deploy its reactor in the remote village of Galena, Alaska. The reactor would provide all of the village's power needs and much of its heating needs. Galena, NN explained, had no electrical grid or roadway connection, so the 4S would have been a massive boon to a community otherwise dependent on diesel imported via a river that was only accessible during the summer months.

To bring this vision to reality, Galena's local government partnered with the Department of Energy for preliminary siting studies. The DOE, in turn, partnered with the University of Alaska and Idaho National Laboratory. The Alaskan state legislature also committed $500,000 for the development of several white papers on the project as other small communities in the state showed interest in replicating this first project in Galena.

Starting in 2007, Toshiba submitted a series of technical reports to the Nuclear Regulatory Commission and continued its preliminary work with Galena. Plans were made to gain design certification; but as deadlines were delayed and targets were missed, the project's momentum waned. In 2011, Galena ultimately decided to pursue a new fossil-fired power plant. Two years later, the NRC reviews ceased without any review documents, and the 4S largely entered the dustbin of history as a paper reactor.

Back to Zap: Until last week, the public only knew Zap Energy as a fusion developer pursuing the development of Z-pinch fusion plasma technology. In 2023, the company was awarded $5 million in funding from the DOE's Advanced Research Projects Agency-Energy (ARPA-E) as it continued to work toward developing its vision of modular fusion power plants. At the time, the company claimed that its technology could yield a "seriously cheap" power source, because it "confines and compresses plasma without costly and complex magnetic coils."

However, that singular focus changed when, on April 29, Zap announced that it had found a new CEO in Zabrina Johal and was transitioning to a new strategy that aimed to "combine near-term fission deployment with the long-term breakthrough potential of fusion."

For Zap, this strategy is "grounded in a simple observation: fission and fusion are not separate industries, but deeply connected disciplines that share materials, engineering challenges, supply chains, and system architectures." By pursuing fission first and fusion second, the company hopes to "exploit deep technology overlap . . . particularly in liquid metals, neutron environments, and high-power-density design, to speed progress across both."

The company maintains that its core mission "remains the commercialization of fusion," but that urgent energy needs demand more rapid commercialization through fission.

Some in the industry have expressed varying degrees of skepticism and enthusiasm for the development. Commonwealth Fusion Systems CEO Bob Mumgaard called the move "a dramatic departure" and added that "blending fission and fusion is a bad idea." He continued, "Saying fission and fusion both involve neutrons glosses over deep differences in physics and engineering," before pointing out that fission is already a "crowded space, with incumbents that are years ahead."

Nevertheless, Zap maintains that significant synergies between fission and fusion will speed their development across the board. Moreover, the company argues in a new white paper that their decision to revitalize the 4S reactor concept "reduces the number of engineering details that need solving." While no specific timetables for development or deployment are laid out, the white paper notes that the next steps for fission will be "evaluating how new manufacturing technologies, AI-enhanced methods, and the knowledge gained in recent years can enhance the [4S] design and reduce costs."

The white paper concludes that, taken together, Zap's dual-track fission and fusion plans now position it as a "platform company for nuclear energy deployment: delivering clean, firm power to a grid that needs it now, while building the commercial foundation for fusion deployment and closing the fission nuclear fuel cycle in the decades ahead."