First Fool-Proof Nuke? Chinese Reactor Demonstrates Loss-of-Coolant Safety

Most feared accident at a nuclear plant is the LOCA (Loss of Coolant Accident) – it’s what happened at Three Mile Island, and at Fukushima.

New Chinese design tested in real life demonstration and passed – cooling down completely on its own, so we’re told, after shut down of cooling system.

Could this be the first “fool proof” reactor? Waiting for more trials with bigger fools.

New Scientist (Paywall):

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.

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 Zhe Dong 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 Mamdouh El-Shanawany, 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.
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Researchers say still more info needed, but sure is encouraging if true.
That said, this is a different design than many of the “small modular” reactors being build in the US and elsewhere.

Interesting Engineering:

Conventional nuclear reactors use fuel rods that are energy-dense, containing large amounts of uranium with smaller amounts of graphite. In the HTR-PM reactor design, the fuel rod is inverted, and a large amount of graphite is used within which uranium is encased. This makes the energy density of the fuel much lower, almost like pebbles in a larger body of water. 

The approach has two major advantages. The first is that the nuclear fission reaction occurs much slower than a conventional reactor and can withstand a higher temperature for much longer. The second advantage is that the excess heat generated through the process is dispersed over a larger fuel area and can be cooled using passive or non-energy-consuming methods such as conduction and convection. 

The approach has previously been demonstrated in prototype reactors built in Germany and China but a full-scale HTR-PM reactor had yet to be attempted. 

Success did not come easily either. The Institute of Nuclear and New Energy Technology began constructing the commercial-scale HTR-PM at a facility in Shandong in 2016, with expectations that the site would be ready for testing a year later. 

The reactor began commercial operation only in December 2023. To demonstrate that it could cool itself down without an external source, the team shut down both modules when it was running at full power and began tracking temperature movements inside the reactor. 

As expected, the reactors cooled down naturally and reached a stable temperature 35 hours after they were shut down. 

The technology’s drawback is that it cannot be retrofitted onto existing nuclear reactors. To build a future where nuclear reactors are meltdown-proof, the nuclear energy industry will have to build HTR-PM reactors first.