A large-scale nuclear power station in China is the first in the world to be completely impervious to dangerous meltdowns, even during a full loss of external power. The design can’t be adapted to existing nuclear reactors around the world, but could be a blueprint for future ones.
All modern nuclear power plants rely on powered cooling mechanisms to take excess heat away from reactors or, in the event of an emergency, human intervention to shut the plant down. Water or liquid carbon dioxide are often used as coolants, but these typically rely on external power supplies to function.
If these systems fail, then the reactors can become too hot and lead to explosions or overheating, causing the plant to literally melt from the excess heat. This was one factor in the Fukushima nuclear accident in Japan in 2011, where a loss of both standard and emergency power systems led to a meltdown.
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A relatively new kind of reactor design, called a pebble-bed reactor (PBR), has the advantage of being passively safe, which means that if power for cooling systems is lost, then the reactor can safely shut down by itself. Rather than use highly energy-dense fuel rods like many other reactor designs, PBRs use a large number of low-energy-density “pebbles” as fuel, which contain a small amount of uranium surrounded by graphite. This can help slow the nuclear reaction and withstand high temperatures.
This lower energy density means any excess heat will be spread out over all of the pebbles, and so will be easier to transport away using natural cooling processes like conduction and convection, says at Tsinghua University in China.
While small working prototype reactors have been built in Germany and China, no full-scale PBRs have been shown to work and be passively safe – until now. Dong and his colleagues have demonstrated that the system works with a full-scale nuclear plant, the High-Temperature Gas-Cooled Reactor Pebble-Bed Module (HTR-PM) in Shandong.
“Up to now, every commercial reactor except HTR-PM has had an emergency core cooling system,” says Dong. “However, due to the inherent safety, there is no emergency core cooling system in the HTR-PM plant.”
To test this, which became commercially operational in December 2023, Dong and his team switched off both modules of HTR-PM as they were operating at full power, then measured and tracked how the temperature of different parts of the plant went down afterwards. They found that HTR-PM naturally cooled and reached a stable temperature within 35 hours after the power was removed.
It is rare to be able to test a working power plant fully by removing its cooling power supply, says , formerly at the International Atomic Energy Agency (IAEA). Because the emergency cooling system doesn’t depend on any complicated technology, it is very safe, he says.
Other countries might want to look into adapting this technology for their own future reactors, but we will first need more extensive measurements about the plant as it cools down, such as pressure and higher-resolution readings, says El-Shanawany.
Joule
