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Pebble Bed Advanced High Temperature Reactor Can Enable Lower Costs and Enable Deep Burn and Advanced Fuel Cycle Reactors

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The Pebble Bed Advanced High Temperature Reactor was covered here last year.

Brave New climate looks at the work on the Pebble Bed Advanced High Temperature Reactor to achieve lower costs (70% of the cost of existing light water reactors) and deep burn of nuclear fuel (ten times more efficient use of uranium).

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.

Per’s aim is to develop really compact nuclear units with very high power densities, based on mostly well-understood technology that is deployable on the time-scale of a decade or less. The driving aim is to get these units commercialized in the near term, and to bring down costs, thereby paving the way for later widespread commercial deployment of full Generation IV designs like the LFTR (liquid flouride thorium Reactor) and IFR (Integral Fast Reactor), which not only achieve high burnup (use up 15 times more of the uranium and leave less waste), but also completely close the fuel cycle.


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). The PB-AHTR has 4 to 6 times the power density of other nuclear fission reactor designs.

PB-AHTR uses conventional TRISO high temperature fuel in the form of pebbles slightly smaller than golf balls.

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
* the pebbles are inserted in the inlet pipes and rise up through the reactor module over time, and then are put back through 5 or 6 times. This allows for very high burnup — exceeding 50 %


* 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.
* 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



Lowering Reactor Costs

Per Peterson argues that fluoride-salt reactor technology (AHTR/LFTR) has a clear path to achieve substantially lower energy production costs than ALWRs. His expectation is that this evolutionary path will remain focused mainly on thermal-spectrum reactors, with efforts to push to higher temperatures and efficiency, and the introduction of thorium. Sodium-cooled, metal-fueled reactors are intrinsically bulkier and lower temperature/efficiency than AHTRs and LFTRs, but are not intrinsically more expensive than ALWRs.

At the Forum on Small and Medium Reactors (SMRs): Benefits and Challenges the Modular PB-AHTR Economics was discussed.

Lower energy costs than ALWRs
–  Primary loop components more compact than ALWRs (per MWth)
–  No stored energy source requiring a large-dry or pressure-suppression-type containment; reactor building volume 50% smaller than ABWR (per MWe)
–  Gas-Brayton power conversion 40% more efficient, turbine building 55% smaller than ABWR (per MWe)
•  Much lower construction cost than SFR/IFR
–  ORNL top down, apples-to-apples cost study concluded that the AHTR capital cost is 56% of the S-PRISM cost
–  Primary loop is much more compact (salt heat capacity is 4.5 times higher than sodium)
–  Low pressure containment (no sodium reaction)
–  Intrinsically higher temperature/power conversion efficiency
•  Much lower construction cost than MHRs
–  All components much smaller, operate at low pressure, compared to MHRs

Reactor Type  Reactor   Reactor and     Turbine     Ancillary   Total
              Power     Auxiliaries     Building    Structures  Building
              (MWe)     Volume          Volume      Volume      Volume
                        (m3/MWe)       (m3/MWe)    (m3/MWe)     (m3/MWe)
1970’s PWR     1000      129             161         46           336
ABWR           1380      211             252         23           486
ESBWR          1550      132             166         45           343
EPR            1600      228             107         87           422
GT-MHR          286      388               0         24           412
PBMR            170     1015               0        270          1285
Modular PB-AHTR 410      105             115         40           260



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