China’s Tsinghua University has achieved a groundbreaking milestone by demonstrating the inherent safety of the first operating commercial pebble-bed nuclear reactor. By shutting off the power and allowing the passive systems to maintain control of the reactor core, the university showcased the advanced safety features of this next-generation technology.

Older nuclear reactors, such as Pressurized Water Reactors (PWR), have a significant design drawback—they require active measures to shut down in an emergency, and their safety systems depend on an external power source to run coolant pumps. These systems can fail catastrophically if the power source is compromised, as seen in the Fukushima disaster of 2011. The plant, based on an outdated 1970s design, was hit by an earthquake and a tsunami that knocked out its backup diesel generators. The resulting chaos prevented emergency crews from intervening in time, leading to a hydrogen explosion and reactor core meltdown.

In contrast, Generation IV reactors like the Shidao Bay Nuclear Power Plant’s high-temperature gas-cooled (HTGR) pebble-bed reactor in Shandong Province, constructed by Tsinghua University in collaboration with China Huaneng Group and the China National Nuclear Company, are designed to be inherently safe. Engineers confidently assert that pebble-bed reactors are incapable of a meltdown.

Instead of using fuel rods cooled by circulating water inside a pressure vessel, a pebble-bed reactor utilizes a large, unpressurized hopper filled with spherical pebbles. These pebbles consist of layers of enriched uranium fuel, a carbon moderator, and a silicon carbide coating. The reactor uses helium gas as a coolant, which is chemically neutral and remains gaseous at all temperatures outside the cryogenic realm.

The design allows the pebbles to withstand temperatures up to 1,600 °C (3,000 °F), well above the reactor core’s maximum temperature. When the pumps are turned off, the heat from the pebbles causes the helium to circulate naturally. The high surface-to-volume ratio of the pebbles ensures that heat loss exceeds heat generation. Furthermore, the reactor is self-regulating: as the temperature rises, the fuel atoms move faster, causing Doppler broadening. This phenomenon leads to more fast neutrons being absorbed, reducing the number available to sustain the reaction and naturally smothering it as the temperature increases.

The concept of pebble-bed reactors is not new; it is based on the German AVR power plant, which operated safely for 21 years and successfully performed similar safety tests. However, the Shidao Bay reactor is the world’s first commercial pebble-bed reactor. During the recent test, power was shut off while the cores were running, allowing the passive safety features to take over.

In the Shidao Bay test, two pebble-bed reactors connected to a 210-MWe steam turbine moderated their own nuclear reactions, maintaining safe temperatures. The core shut down within minutes, and both reaction and temperature stabilized in about 35 hours. Importantly, there was no deterioration of the nuclear fuel.

This successful demonstration underscores the potential of pebble-bed reactors to provide safer, more reliable nuclear energy.

By Impact Lab