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.

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 704^oC, 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 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.

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:

• Modular helium reactors (PBMR): TRISO pebble fuel, nuclear-grade graphite; high-temperature metallic and carbon composite structural materials; helium Brayton power conversion.
• Sodium fast reactors (S-PRISM/EBR-II): Pool-configuration reactor vessel; reactor building seismic base isolation; direct reactor auxiliary cooling system (DRACS) for passive decay heat removal.
• Light water reactors (AP-1000/ESBWR): Integral effects test scaling and best-estimate safety code validation methods; modern computer aided design, manufacturing, and modular construction technologies.
• Molten salt reactors (MSRE/MSBR): Liquid salt pumps, heat exchangers, corrosion resistant alloys; liquid salt corrosion test and thermophysical property data base.

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:

• Deep burn fuel cycle: the PB-AHTR can use deep burn TRISO fuels to destroy plutonium and other transuranics from commercial spent fuel
• Once-through seed-blanket fuel cycle: the PB-AHTR can operate with a low-enriched uranium seed and thorium blanket fuel cycle that can reduce uranium consumption and waste generation while maintaining once-through operation.
• Closed thorium fuel cycle: the PB-AHTR can operate with a closed thorium based fuel cycle with greatly reduced production of plutonium and other transuranics. Achievable conversion ratios are being studied now.
• Liquid fluoride thorium reactors: The PB-AHTR provides technology that can be applied to future deployment of molten salt reactors using sustainable closed thorium fuel cycles.⁴
• Fission/fusion hybrid reactors (LIFE): The PB-AHTR provides technology that can be applied for the future deployment of fission/fusion hybrid reactors that would operate sustainably without enrichment or reprocessing of their fission fuel.⁵

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