Many politicians and tech companies are now pushing to build new nuclear power plants to meet the world's rapidly growing energy demands. Small reactors — known as Small Modular Reactors, or SMRs — are gaining momentum.

They could be built in less time and come online faster than conventional nuclear power plants. Originally developed to power submarines and aircraft carriers, SMRs largely work the same way as larger reactors.

They contain a nuclear reactor fueled by uranium, which releases enormous amounts of heat through nuclear fission. That heat turns water into steam, and the steam pressure drives turbines that generate electricity.

Companies and startups are working on a wide variety of SMR designs and concepts, but the underlying principle is similar across all of them.

One advantage over conventional nuclear stations is that mini reactors require a maximum of two hectares (around five acres) of land — roughly the size of two soccer fields. By comparison, conventional plants require up to 280 soccer fields, according to the industry. 

Why use a modular design for mini nuclear plants? 

What makes SMRs particularly attractive is their modular construction. Individual components can be mass-produced as prefabricated modules and pre-assembled off-site — much like a prefab house that simply needs to be put together once it arrives at its location.

Manufacturers promise an extremely short construction time of between one and a half to six years. Large reactors in the US can take anywhere from seven to 10 years to build.

Proponents also argue that modular units could deliver low-emission power to remote regions, where grid infrastructure is often not developed enough to handle the enormous energy output of a large conventional plant. 

Those advantages are also shared by renewable energy sources like solar panels and wind turbines — without the risks that come with nuclear power. 

Why less radiation can mean greater risk 

The output of mini nuclear reactors is significantly lower than that of conventional nuclear power plants. Depending on the design, SMRs can deliver between 10 and around 200 megawatts of power, while a conventional plant generates between 1,000 and 1,600 MW.

To produce the same amount of energy as the roughly 400 high-capacity reactors currently operating worldwide, tens of thousands of small reactors would need to be built.

SMRs could offer some safety advantages: they contain less radioactive material than large reactors and would be distributed across multiple sites. This means that an accident or military strike resulting in a meltdown would not be as catastrophic as a worst-case scenario at a large reactor. Nevertheless, the consequences of an SMR accident could still be severe.

Germany's Federal Office for Radiation Protection points out that despite lower radiation levels per reactor, the overall risk posed by mini reactors could be many times higher. Replacing the output of existing nuclear plants with large numbers of smaller ones would require an enormous number of reactors worldwide — and that increases the likelihood that at least one of them will eventually suffer a serious incident.

Nuclear accidents and radiation leaks are extremely rare — but the consequences can be catastrophic. The last major nuclear accident occurred in 2011 at the Fukushima power plant in Japan, following a tsunami. Parts of the region have been contaminated for generations to come, and nearly 170,000 people were forced to flee their homes. 

Extreme efficiency and the dream of radioactive recycling 

Part of the hype around some new mini reactors rests on the hope of extracting significantly more energy from the same amount of uranium. Some SMR models — known as "fast reactors" — could theoretically yield 60 to 70 times more energy from uranium than conventional technology, according to the International Atomic Energy Agency (IAEA).

But the technology is still in the early stages of development and has so far played only a limited role in IAEA projections. In practice, it has barely been proven in SMRs at all. Only two SMR facilities are currently in operation worldwide — one in China and one in Russia.

Work is also underway on recycling SMR fuel rods, but that process remains largely untested. Most SMR projects continue to rely on the same conventional technology used in traditional reactors.

And the problem of radioactive waste remains unsolved — for both conventional and fast reactors. The risks this poses are already visible: at the Asse facility in Lower Saxony, Germany, a temporary storage site built in the 1970s, hundreds of barrels of intermediate-level radioactive waste are now rusting and beginning to leak. Germany has yet to establish a permanent disposal site.

In fact, no permanent repository for spent fuel or highly radioactive reactor waste is currently in operation anywhere in the world. After 20 years of construction, the Onkalo facility in Finland is set to become the first when it opens in 2026. 

Another problem for nuclear power is growing water scarcity. Recent heat and drought have already forced European nations to curb nuclear output due to for cooling due to low river levels.

Because SMRs are mainly just smaller, simplified versions conventional reactors, they still use water to run and for cooling. While individual units use less water than traditional plants, clustering units could increase overall use, warn experts. Researchers are investigating the use of alternative coolants like molten salts and helium gas for newer designs.

When would SMRs make a climate impact — and at what cost? 

Enthusiasm for small reactors runs high in the nuclear community. But what may look promising at first glance is, in many respects, difficult or impossible to back up with solid evidence.

Calculating the costs of a technology still in development is no easy task. Nevertheless, the German Institute for Economic Research estimates that electricity from new mini nuclear plants — even accounting for economies of scale, including construction and manufacturing — will cost at least twice as much per megawatt-hour as the already very affordable electricity generated by solar or wind power. In a worst-case scenario, the cost could be nearly eight times higher.

Adding up all the announcements, plans and strategies of major nations, a few hundred SMRs could come online in China, the US, Europe and India by 2050. But according to Germany's Federal Office for Nuclear Safety and Radiation Protection, citing figures from the nuclear industry, production would only become economically viable at a threshold of 3,000 reactors.

The international community has committed to limiting global warmingto well below 2 degrees Celsius (3.6 degrees Fahrenheit) by the end of the century. Meeting that target requires large parts of the global economy to become climate-neutral by 2050. The EU aims to reach zero emissions by then; Germany is even targeting net negative CO2 emissions by 2050.

That means climate-friendly nuclear technology would arrive too late to meaningfully limit global warming — and its advantages would be significantly diminished as a result.

What would remain: the radiation risk, and the question of what to do with the radioactive waste. 

This article was originally published in German.