Chinese nuclear reactor completes world-first passive cooling test

CNNC

CHINESE engineers have made a nuclear safety breakthrough by shutting off power to the cooling systems of two large-scale nuclear reactors and showing their design can’t meltdown because it passively cools itself.

Conventional nuclear reactors in use today rely on powered systems to remove excess heat. During an emergency, if the water or CO2 used as coolant can’t be pumped through the reactor then the heat given off by the radioactive fuel can build up and damage the reactor.

“Whether it be due to design flaws, equipment failure or human error, all the world’s major nuclear reactor accidents such as 3 Mile Island, Chernobyl, and Fukushima, have occurred because reactors have overheated causing meltdown, with a subsequent release of radioactive material to the environment,” says nuclear safety expert Geoff Gill.

Engineers in China have now published the results of a demonstration test they carried out last year at a commercial nuclear facility that show their reactor avoids this fate by using an inherently safe cooling system design that requires no external power or human intervention.

This sort of passive cooling has previously been shown to work using small test reactors no larger than 45 MW, but in August and September last year engineers at Tsinghua University led tests on a commercial-scale 200 MW high temperature helium-cooled pebble bed reactor plant in Shandong.

“During the tests, the active power supply was totally switched off to see if the decay heat can be removed passively,” the team writes in its paper published in the journal Joule. “The responses of nuclear power and temperatures within different reactor structures show that the reactors can be cooled down naturally without active intervention. The results of the tests manifest the existence of commercial-scale inherent safety for the first time.”

The fuel in the pebble-bed reactor is made up of thousands of tiny particles of uranium coated in carbon and ceramic materials shaped into a billiard-ball sized sphere. The power density is around 1/30th of that of a conventional pressurised water reactor in use today. This means the heat decay can be efficiently removed by passive heat transport mechanisms such as conduction, radiation and natural circulation.

The tests involved switching off the power to the primary helium circulator and feed-water pump and monitoring the temperatures of key components in the reactor for 50 hours. There was an initial spike in temperature to around 900^oC, well below the 1,620^oC safety threshold of the fuel particles, and then it dropped and plateaued until the end of the tests.

Gill said: “Any news of a reactor which will cool naturally without the need for electro-mechanical or human intervention will be of significant interest.”

The design involved in the test is one of six technologies known as Generation IV reactors being developed through an international group called the Generation IV International Forum (GIF).

He added: “For this reactor technology, it is a significant development, since as well as electricity production, the process heat from these reactors could be used in the petrochemical, metallurgy and hydrogen production industries.”

The Chinese researchers have said work is already underway to use the technology to provide low-carbon power and steam for China’s petrochemical industry.