The pebble bed reactor, explained
A pebble bed reactor replaces the fuel rods of a conventional plant with hundreds of thousands of graphite spheres, each about the size of a billiard ball. The idea dates back decades, it has a mixed history, and since 2023 it has a commercial plant in operation.
How a pebble bed reactor works
A pebble bed reactor is a kind of high-temperature gas-cooled reactor (HTGR). The U.S. EIA explains that HTGR designs use graphite as a moderator and helium gas as a coolant, and that their very high operating temperatures could make them suitable for industrial processes that need high heat.
In a pebble bed, the fuel, the moderator and the first fission-product barrier are all in one object: the pebble. Thousands of pebbles are gathered in a vessel to form the core. Helium circulates through the spaces between them. Wikipedia notes that helium does not easily absorb neutrons or impurities, so compared with water it is both more efficient and less likely to become radioactive.
Continuous refuelling. In most stationary designs, fuel replacement is continuous. Pebbles move through the core, are checked after each pass and are reused until spent, at which point they are removed and replaced with fresh ones. The reactor does not need to shut down to refuel.
TRISO fuel: the heart of the pebble
Each pebble holds thousands of TRISO (tri-structural isotropic) particles. The U.S. Department of Energy describes each particle as a uranium, carbon and oxygen fuel kernel encapsulated by three layers of carbon- and ceramic-based materials. The particles are about the size of a poppy seed and can be fabricated into cylindrical pellets or billiard ball-sized spheres called "pebbles".
DOE's headline claim is strong: TRISO particles cannot melt in a commercial high-temperature reactor, and each particle acts as its own containment system. TRISO is also more resistant to neutron irradiation, corrosion, oxidation and high temperatures than traditional reactor fuels.
HTR-PM: the first commercial pebble bed plant
China's HTR-PM at Shidao Bay in Shandong Province is the first commercial pebble-bed plant. World Nuclear News reported in December 2023 that the world's first modular high temperature gas-cooled reactor nuclear power plant has entered commercial operation.
In safety demonstration tests in 2023, with the normal energy transfer stopped, Tsinghua University said the reactors cooled down naturally without intervention of humans or emergency core cooling systems.
Lessons from early pebble bed plants
The history is not all positive, and any honest account should include it. Wikipedia summarises the key plants:
- AVR, Germany. The experimental AVR first reached criticality in 1966 and ran for 21 years. In one safety test, all the control rods were removed and coolant flow was halted, and the fuel remained undamaged. Decommissioning, however, revealed a cracked neutron reflector and heavy contamination.
- THTR-300, Germany. The full-scale thorium plant suffered technical difficulties and was closed after about four years of operation. A 1986 incident released part of the radioactive inventory.
- PBMR, South Africa. The Pebble Bed Modular Reactor project was never completed, and its test facility was placed in care and maintenance.
- China. China licensed the AVR technology in 2004, built the HTR-10 prototype and developed it into HTR-PM.
The common criticisms are that graphite can burn if the vessel is breached and air gets in, that pebbles create graphite dust, and that graphite pebbles are hard to reprocess. For the wider debate on small reactors, see disadvantages of small modular reactors. For other reactor types, see small modular reactors and CANDU reactor.
Who is developing pebble bed reactors now
Beyond China, X-energy is developing the Xe-100, a pebble-bed high-temperature gas-cooled reactor, and its own TRISO-X fuel. Amazon has invested in X-energy, and the two companies aim to bring more than 5 gigawatts online in the United States by 2039. Many HTGR fuels, including TRISO for several U.S. designs, rely on HALEU. See HALEU vs natural uranium for why fuel supply matters.
While reactors move through licensing, USP&E supplies thermal power backed by 24+ years in power, 150+ power projects in 45+ countries and 2,000+ MW in stock. To discuss a site, talk to our team.
Frequently asked questions
What is a pebble bed reactor?
A gas-cooled, graphite-moderated reactor whose fuel is a bed of spherical graphite pebbles, each holding thousands of tiny TRISO fuel particles. Helium flows between the pebbles to carry heat away. It is a type of high-temperature gas-cooled reactor (HTGR).
Is there a pebble bed reactor in operation?
Yes. China's HTR-PM demonstration plant at Shidao Bay, with two 250 MWt pebble-bed reactor modules driving a single 210 MWe turbine, entered commercial operation in December 2023.
How big is a fuel pebble?
In HTR-PM, each pebble is 60 mm in diameter and contains 7 g of fuel enriched to 8.5%, in about 12,000 coated fuel particles dispersed in a graphite matrix. DOE describes the pebbles as billiard ball-sized.
Why are pebble bed reactors considered safe?
TRISO fuel retains fission products at very high temperatures, and the core can shed decay heat passively. In 2023 tests at HTR-PM, the reactors cooled down naturally without human intervention or emergency core cooling systems.
What are the problems with pebble bed reactors?
Graphite can burn if air gets in, pebbles create graphite dust, used pebbles are hard to reprocess, and early plants had operational problems. Germany's THTR-300 closed after a few years following technical difficulties and a 1986 release incident.