Are Small Modular Reactors Safe? The Risks Explained

Tech companies want small modular reactors to power AI data centers. Vendors call them clean, cheap, and safe. This page checks that claim against the record, so far.

• Are small modular reactors safe?
• What is a small modular reactor?
• The safety claims
• The real risks
• How many are actually running?
• What regulators actually require
• The bottom line for communities

Are small modular reactors safe?

On paper, many small modular reactor (SMR) designs are built to be safer than today's big plants. But that safety is mostly untested in the real world. Almost none are running anywhere. And independent researchers have flagged real, open risks around waste, fuel, and water.

So the honest answer is: safer by design, but not yet proven safe by experience. A reactor that has never run at commercial scale cannot claim a real-world safety record. Bold claims from vendors are not the same as decades of operating data.

This matters now because AI companies want SMRs to power thirsty nuclear data centers. That plan puts new reactors near communities that never asked for them. Below, we walk through the claims and the catches in plain terms.

What is a small modular reactor?

A small modular reactor is a nuclear reactor that makes 300 megawatts of electricity or less. That is much smaller than a normal plant, which often makes over 1,000 megawatts. The word "modular" means the parts are meant to be built in a factory, then shipped to a site and assembled.

Traditional reactors are giant, custom, one-off builds. Each one is designed and poured on site over many years. SMRs try to flip that model. The pitch is to mass-produce standard units like an assembly line, cut costs, and add power in smaller chunks as needed.

SMRs also use different cooling methods. Some use water, like most plants today. Others use molten salt, liquid sodium, or helium gas. Many run on a stronger fuel called HALEU, which we cover below. The designs vary a lot, so "SMR" is really a category, not one machine.

Because they are smaller, vendors say SMRs can sit closer to where power is used. That includes remote towns, factories, and data centers. That flexibility is a selling point. It is also part of why safety and siting questions matter so much.

The safety claims

The main safety pitch is "passive cooling": SMRs are designed to cool themselves without pumps, power, or human action. In many designs, the reactor is smaller and holds less heat. The International Atomic Energy Agency notes that in some high-temperature designs, heat can passively escape to the surroundings even if active cooling fails.

The 2011 Fukushima disaster happened when pumps lost power and cores overheated. SMR vendors say their designs avoid that trap. Companies like NuScale say the core can shut down and cool on its own using gravity and natural circulation. The idea is fewer moving parts that can fail.

The U.S. Nuclear Regulatory Commission has reviewed and approved SMR designs on paper. It certified NuScale's design in 2023, the first SMR design cleared in the United States. It approved an uprated NuScale module in 2025. Regulators say the safety case is credible.

Smaller cores also hold less radioactive material overall. In theory, that means a smaller possible release if something goes wrong. These are real engineering ideas, not marketing spin. But note who is making them: vendors selling reactors and agencies reviewing paperwork, not plants with a long track record.

The real risks

The biggest catch is waste. A 2022 study in the journal PNAS, led by Lindsay Krall with Allison Macfarlane and Rodney Ewing, found that many SMR designs may produce more radioactive waste per unit of power, not less. The team studied designs from Toshiba, NuScale, and Terrestrial Energy.

Their finding was stark. Water-, salt-, and sodium-cooled SMRs could increase the volume of nuclear waste needing management by factors of 2 to 30 compared with a big reactor. The waste can also be more chemically reactive and harder to store. Smaller cores leak more neutrons into surrounding parts, making them radioactive too.

Fuel is a second worry. Many SMRs need HALEU, uranium enriched to between 5 and 20 percent. That is stronger than normal reactor fuel. The IAEA warns that higher enrichment and many scattered units can raise proliferation concerns, meaning the fuel or know-how is closer to weapons-grade than today's plants.

Water is a third issue. Water-cooled SMRs still need large amounts of water to run and cool. That competes with drinking supplies and farms, especially in dry regions. Some designs use less, but not all. If a reactor sits next to a data center, both draw on the same local resources.

There is also a fuel supply catch. For years, the main commercial source of HALEU has been Russia. The United States is now trying to build its own supply. Until it does, a fuel that is meant to power "energy independence" may depend on a rival state.

Waste storage is a long-term burden, not a one-time cost. The United States still has no permanent site for spent nuclear fuel. So waste piles up on site, stored in casks, for decades. More reactors in more places means more of these small storage sites to guard and track.

Decommissioning is the other end of the story. Every reactor must one day be shut down, taken apart, and cleaned up. That work is slow, expensive, and creates its own radioactive waste. A canceled or bankrupt vendor can leave that bill and that mess with the local community.

Spreading many small reactors around also raises security questions. Each site needs guards, fencing, and cyber protection against hacking. A reactor bolted onto a private data center may not get the same public oversight as a large utility plant. More sites can mean more targets, not fewer.

The deepest risk is simple: these designs are unproven at scale. No U.S. SMR has run commercially. Cost and timeline claims keep slipping. NuScale's first planned U.S. project, meant to power Idaho-area utilities, was canceled in 2023 after costs rose. Companies like Oklo and X-energy have plans and licenses, but not running reactors. See our page on how much a small modular reactor costs for the money side.

That Idaho project is worth a closer look. NuScale worked for years with a group of Utah public utilities on the Carbon Free Power Project. It planned six 77-megawatt modules at Idaho National Laboratory. In 2021, NuScale said it could deliver power for about $58 per megawatt-hour.

By early 2023, that estimate had jumped to around $89 per megawatt-hour. Not enough utilities signed up to buy the power. In November 2023, the two sides canceled the project before a single reactor was built. It was billed as the nation's first SMR plant, and it never happened.

Passive safety claims also deserve honest scrutiny. Before 2011, Fukushima's operators believed their plant was safe too. A tsunami knocked out power and cooling, and the cores overheated. Real events can expose gaps that paper reviews and computer models miss.

How many are actually running?

Very few SMRs are running anywhere in the world, and almost none in the United States. Despite years of hype and billions in funding, real operating units can be counted on one hand. Most projects are still designs, licenses, or construction sites.

Russia has the Akademik Lomonosov, a floating plant with two small reactors that started supplying power in 2020. China runs the HTR-PM, a high-temperature gas reactor rated around 210 megawatts that went commercial in 2023. These are the main SMR-class units with real operating time.

China's Linglong One, also called the ACP100, is a 125-megawatt land-based SMR expected to start commercial operation in 2026. If it does, it would be the world's first land-based commercial SMR. That is a notable milestone, and it shows how early this whole field still is.

In the United States, the count of operating commercial SMRs is zero. Oklo aims to complete its first Aurora reactor in Idaho around 2027. X-energy is working toward projects with a chemical plant in Texas and utilities elsewhere. These are goals, not power on the grid today. The nuclear companies powering AI are still mostly selling a future, not a running plant.

What regulators actually require

Regulators do not just take a vendor's word that a reactor is safe. In the United States, the NRC reviews the design, the site, and the operator in separate steps. A company must win approval before it builds and again before it loads fuel.

There are two main licensing paths. One path splits approval into a construction permit first, then a separate operating license later. The other path, a combined license, bundles building and running into one review. A reactor design can also be certified once, then reused at more sites.

These reviews take years and cost millions of dollars. NuScale's uprated module, for example, took the NRC many months of review before its 2025 approval. That slow pace is partly the point. Caution is a feature of nuclear safety, not a bug.

But approval on paper is still not the same as safe operation over decades. A certified design has passed a review, not a real-world stress test. And a rushed race to power AI could push regulators and lawmakers to speed things up. Faster approvals can mean less time to catch problems.

The bottom line for communities

If a company proposes an SMR near your town, treat the safety pitch with healthy skepticism. The designs may be genuinely safer than old reactors. But "safer on paper" is not the same as a proven track record, and independent studies raise real waste and fuel concerns.

Ask hard questions. How much water will it use? Where does the waste go, and for how long? Who pays if costs balloon or the project is canceled, as NuScale's Idaho plan was? Is this reactor really for the town, or mainly for a data center's power bill?

Living near any large power or data project changes a place. Our guide on living near a data center covers what that can mean day to day. SMRs are early technology, and communities should not be the test site without full facts and a real say.

Want to see where these projects are landing? Check our data center map to find sites near you. And if you want to push back on a plan in your area, our fighting back guide shows how residents are organizing right now.