Tsinghua University in China has showcased the unparalleled safety of the first operational commercial pebble-bed nuclear reactor.
Traditional nuclear reactors, such as Pressurized Water Reactors (PWR), have inherent safety challenges. In emergencies, these reactors need active measures for shutdown and a reliable power source to operate coolant pumps. These systems, however, can sometimes fail, as seen in the 2011 Fukushima disaster. The outdated 1970s plant was hit by an earthquake and tsunami, which destroyed its backup generators and prevented emergency crews from reaching the site. This led to a catastrophic chemical explosion and reactor core meltdown.
The Shidao Bay Nuclear Power Plant in Shandong Province represents a new generation of reactors, known as Generation IV. Constructed by Tsinghua University, China Huaneng Group, and the China National Nuclear Company, this high-temperature gas-cooled (HTGR) pebble-bed reactor is designed to be inherently safe, with engineers asserting that these reactors cannot melt down.
Unlike traditional reactors that use fuel rods cooled by water, a pebble-bed reactor utilizes spherical pebbles composed of enriched uranium fuel, carbon as a moderator, and a silicon carbide coating. Helium gas, which is chemically inert and remains gaseous under a wide range of temperatures, serves as the coolant. The pebbles can endure temperatures up to 1,600 °C (3,000 °F), while the reactor core’s design ensures it cannot exceed these temperatures. When the pumps are turned off, the heat from the pebbles causes the helium to circulate naturally, and the high surface-to-volume ratio of the pebbles ensures that heat loss surpasses heat generation.
The reactor is also self-regulating. As the reactor temperature rises, the atoms in the fuel move faster, leading to Doppler broadening, where neutrons move at different speeds. This results in the fuel absorbing more fast neutrons, thus reducing the number available to sustain the reaction, effectively smothering the reaction as temperatures increase.
All in all, a pebble-bed reactor cannot experience a meltdown.
The concept, though not new, draws from the German AVR power plant, which operated for 21 years and performed similar safety tests. However, while the AVR was experimental, the Shidao Bay reactor is the world’s first commercial pebble-bed reactor. During the test, the power was shut off, allowing the reactor’s passive safety features to take over. The two pebble-bed reactors connected to a 210-MWe steam turbine successfully moderated their nuclear reactions and maintained safe temperatures, with the core shutting down within minutes and stabilizing within 35 hours.
Crucially, there was no deterioration of the nuclear fuel.
The test was documented in Joule.
Source: Tsinghua University