The basic idea behind all nuclear power plants is the same: Convert the heat created by nuclear fission into electricity. There are several ways to do this, but in each case it involves a delicate balancing act between safety and efficiency. A nuclear reactor works best when the core is really hot, but if it gets too hot it will cause a meltdown and the environment will get poisoned and people may die and it will take billions of dollars to clean up the mess.

The last time this happened was less than a decade ago, when a massive earthquake followed by a series of tsunamis caused a meltdown at the Fukushima Daiichi power plant in Japan. But a new generation of reactors coming online in the next few years aim to make these kinds of disasters a thing of the past. Not only will these reactors be smaller and more efficient than current nuclear power plants, but their designers claim they’ll be virtually meltdown-proof. Their secret? Millions of submillimeter-sized grains of uranium individually wrapped in protective shells. It’s called Triso fuel, and it’s like a radioactive gobstopper.

Triso— that’s a TRIstructural ISOtropic particle, if you must know—is made from a mixture of low enriched uranium and oxygen, and is surrounded by three alternating layers made from graphite and a ceramic called silicon carbide. Each particle is smaller than a poppy seed, but its layered shell can protect the uranium inside from melting even under the most extreme conditions that could occur in a reactor.

Paul Demkowicz is the director of the Advanced Gas Reactor Field Development and Qualification Program at Idaho National Laboratory, and a large part of his job is simulating worst-case scenarios for next-generation nuclear reactors. For the past few years, Demkowicz and his colleagues have been running qualification tests on Triso fuel that involve putting them in a reactor and cranking the temperature. Most nuclear reactors today operate well below 1,000 degrees Fahrenheit, and even the next generation high-temperature reactors will top out at about 2,000 degrees. But during the INL tests, Demkowicz demonstrated Triso could withstand reactor temperatures over 3,200 degrees Fahrenheit. Out of 300,000 particles, not a single Triso coating failed during the two-week long test.

“In the new reactor designs, it’s basically impossible to exceed these temperatures, because the reactor kind of shuts down as it reaches these high temperatures,” says Demkowicz. “So if you take these reactor designs and combine them with a fuel that can handle the heat, you essentially have an accident-proof reactor.”

In a conventional nuclear reactor, the main line of defense against a meltdown is the fuel control rod, which power plant operators use to control the fission rate in the core. If things get too hot, they push more rods into the core so the fission rate—and temperature—goes down. Every operating nuclear reactor in the world is also ensconced in a massive containment structure designed to prevent radioactive material from escaping just in case something goes wrong.

But with Triso fuel, these safety features are redundant, since each particle is effectively wrapped in a control rod. This opens the door for small reactor designs that wouldn’t have been possible before. “Now you don’t have to go build this large containment vessel that costs hundreds of millions of dollars for a reactor, because the fuel carries its own containment,” says Joel Duling, the president of the Nuclear Operations Group at BWXT, a company that makes Triso fuel and nuclear reactors. “So you can have a reactor that fits in a cargo container and still has all the safety features of a traditional commercial reactor.”