China pebble-bed reactor passes “meltdown” test

New testing done at China's Shidaowan nuclear power plant has confirmed its ability to be naturally cooled down, an industry-first milestone for achieving commercial-scale inherent safety, according to researchers.

The Shidaowan plant, a demonstration high-temperature, gas-cooled reactor with a pebble-bed module (HTR-PM), went into commercial operation last December.

Shidaowan's twin 100-MW units house tiny uranium capsules encased in graphite shells about the size of billiard balls (dubbed "pebbles"), which make the energy density of the fuel much lower than in a traditional nuclear reactor with fuel rods. In the pebble design, the nuclear fission reaction occurs more slowly than in conventional reactors, but the fuel can withstand higher temperatures for longer and the heat resulting from the fission reaction is dispersed, enabling a passive cooling process.

The reactor doesn't rely on large volumes of water in the cooling process-instead, a small amount of helium gas, which can withstand much higher temperatures than water, is piped through the system to naturally cool it down. If the reactor starts to get too hot, its components automatically slow down the nuclear reaction and the system cools. This setup makes such a reactor "meltdown proof," in concept.

The study: Researchers at Tsinghua University in China performed two safety tests on the Shidaowan plant's reactor modules by shutting off active power supply to see if the decay heat could be removed passively. The responses of temperatures and nuclear power in each unit confirmed that they 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," according to findings published in the journal Joule.

The Shidaowan project is a collaboration involving Tsinghua University as a technical leader, responsible for research and development and main components and systems design; China Huaneng Group as owner and operator of the plant; and China National Nuclear Corporation as overseer of engineering and procurement and fuel manufacturing.

Other pebble beds: The pebble bed technology and design has previously been used in prototype reactors in China and Germany, but not a larger-scale plant like Shidaowan.

In the U.S., X-energy is working to deploy its pebble-fueled Xe-100 design-an 80-MWe high-temperature, gas-cooled reactor that can be scaled into a four-pack to make a 320-MWe power plant. X-energy's license request is under review by the Nuclear Regulatory Commission.

X-energy was one of two chosen for the Department of Energy's Advanced Reactor Demonstration Program in 2020. With $160 million in initial funding, ARDP set out to help bring two advanced reactors to commercial operation within seven years. (TerraPower's Natrium reactor is the other.)

X-energy has a joint development agreement with Dow to develop its first Xe-100 plant at the chemical company's plant in Seadrift, Texas. X-energy is also working with Energy Northwest under a joint development agreement to bring up to 12 of the Xe-100 small modular reactors to Washington state. That project is expected to be developed at a site adjacent to Energy Northwest's Columbia nuclear power plant.