Pebble Bed Advanced High Temperature Reactor

The Pebble Bed Advanced High Temperature Reactor (PB-AHTR) is a liquid salt cooled, high temperature reactor design developed at UC Berkeley in collaboration with Oak Ridge National Laboratory and other national labs.

PB-AHTR reactor system schematic.

The annular Pebble Bed Advanced High Temperature Reactor (PB-AHTR) design has a nominal thermal power output of 900 MWth (and electrical output of 410 MWe). The PB-AHTR differs from conventional helium-cooled HTRs because its liquid salt coolant enables operation with a core power density of 20 to 30 MWth/m3, compared to the 4.8 to 6.0 MWth/m3 typical of modular helium reactors (MHRs).1 The PB-AHTR delivers heat with a core outlet temperature of 704oC, achieving 46% thermal efficiency with a multi-reheat helium Brayton (gas-turbine) cycle. The low-pressure, chemically inert liquid-salt coolant, with its high heat capacity and capability for natural circulation heat transfer, provides: (1) robust safety (including fully passive decay-heat removal) and (2) improved economics with passive safety systems that allow higher power densities and longer-term scaling to large reactor sizes [>1000 MW(e)] for central station applications.

PB-AHTR primary, intermediate, and power conversion systems

PB-AHTR uses conventional TRISO high temperature fuel in the form of pebbles slightly smaller than golf balls. The baseline PB-AHTR design uses the well understood beryllium-based salt flibe(7Li2BeF4) as its primary coolant, and flinak (LiF-NaF-KF) as its intermediate coolant. Metallic structures and components like the reactor vessel are constructed using Alloy 800H, a ASME Section III code qualified material, with Hastelloy N cladding for high corrosion resistance. The coolant loop of the ORNL Molten Salt Reactor Experiment 2 operated with clean fluoride salt, like the PB-AHTR, for over 26,000 hours without any detectable corrosion to Hastelloy N samples that were studied after the reactor shut down 3. The major components in the reactor core are fabricated from graphite, which is chemically inert to fluoride salts.

PB-AHTR fuel pebble

The PB-AHTR combines together technologies derived from earlier reactor designs to create a new high-temperature reactor design with a unique combination of features:

Like modern MHRs, the baseline PB-AHTR uses a conventional low-enriched uranium fuel cycle. But the PB-AHTR technology also supports advanced fuel cycle options: