Pebblebed

Pebble bed refers to a class of high-temperature gas-cooled nuclear reactors in which the core consists of a large number of spherical fuel elements, called pebbles. Each pebble typically contains TRISO-coated fuel particles embedded in a graphite matrix. The reactor is cooled by a flow of an inert gas, usually helium, at elevated temperature. The pebbles form a porous bed that allows heat transfer while permitting continuous fueling and refueling without large outages.

In operation, the core is a moving or partially fixed bed of pebbles. Pebbles can flow slowly under gravity, allowing fresh fuel to be added at one location and spent fuel to exit at another. Helium is circulated through the bed, transferring heat to a secondary system that drives a turbine or supplies process heat. The design aims for high outlet temperatures, enabling efficient electricity generation and industrial heat applications, including potential direct Brayton-cycle operation.

Safety and performance characteristics are among the defining features. The fuel particles are embedded in graphite and sealed by multiple layers (TRISO), providing strong containment even under high temperature. The reactor typically exhibits a negative temperature coefficient, and its inherent safety features reduce the likelihood of severe core damage in many scenarios. High-temperature operation also supports efficient power conversion and potential cogeneration, though reactor behavior depends on specific design choices and fuel cycle management.

History and notable projects include early work in Germany with the AVR experiment, which explored pebble-bed concepts in the second half of the 20th century. More recent efforts include South Africa’s Pebble Bed Modular Reactor program and China’s HTR-10 and HTR-PM demonstrations, which use pebble-bed technology to pursue modular, high-temperature reactors for electricity and process heat.