When I visited California earlier this month, Tom Blees and I paid a visit to Prof Per Peterson and Prof Jasmina Vujic at the Nuclear Engineering Department of UC Berkeley. After chatting over lunch, Per took us on a personal tour of his lab, which was quite an experience. Per’s research focuses on development of a high-temperature reactor with an incredibly high power density. Why? In short, it’s about the money. Per’s argument — and a quite persasive one — is that if the costs of advanced reactors can be brought way down, below that of pressurised and boiling water reactors (PWRs and BWRs), then their scaled-up deployment is highly likely. The following post owes a lot to Per’s insights on this critical issue.

Currently, one the most frequently cited criticism of nuclear energy, especially with reference to Europe or North America, involves economics. High construction costs for Advanced Light Water Reactors (ALWRs) have emerged as the number one issue limiting near-term deployment, and it now appears that the $18.5 billion in loan guarantees now available will fund no more than 2 or 3 new plants. The major area of anti-nuclear emphasis today is on preventing an expansion of this loan guarantee volume to the $50 to $100 billion level that the nuclear industry believes could be productively used in the near term. Even with loan guarantees, cited nuclear construction prices in the US remain high enough that nuclear remains marginally competitive and most utilities are slowing down their plans for new nuclear construction. Really, nuclear is getting nowhere very fast in the US at present, despite its great promise. AREVA France is now facing similar issues. China, happily, is not.

The main issue with Generation IV reactors such as the IFR or LFTR is the general expectation that they will be more expensive than ALWRs — at least in the early stages of deployment. Increasing the cost of new nuclear construction can hardly be viewed as a winning strategy these days.

For instance, a lot of design work was done by GE on the S-PRISM, after Department of Energy support ended, to bring down the cost. But it still needs to be updated to take into account new construction technologies and requirements (including aircraft crash). It would be very helpful to be able to argue convincingly that IFR technology will be less expensive than ALWRs. If this could be shown to be the case, one could also expect more substantive commercial interest and investment, such as a willingness to cost-share the Design Certification and to construct a prototype reactor outside the federal appropriations process (for example, under loan guarantees with some federal contract for procuring fuel irradiation services for transmutation fuel development and demonstration). Members of SCGI are working behind the scenes on these key issues, and progress is being made, but it’s naturally a protracted process.

Per 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. IFR is more mature than AHTR and LFTR, so the big question is what will be the most practical route to commercial demonstration. IFR will be a tough sell, though, if the general perception remains that it is more expensive than ALWRs. This is a complex topic, which I will endeavour to do more justice to in later posts in the IFR FaD series.

So, back to Per’s lab. He has various engineering models set up to test movement of TRISO pebble fuel through a fluoride salt coolant, whereby 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 %, high power density due to the heat capacity of the liquid salt, and high temperatures thanks to the durability of the pebbles. This is a big (potential) advantage over the current Pebble Bed Modular Reactor technology (PBMR), because in that design, the gas coolant has a very low power density. He’s flipped the problem on its head. The reactor also has various inherent safety design features, such as control rods that sink naturally in response to elevated coolant temperature, thereby passively regulating reactivity. Very safe!

His testbed lab units use analogue fluids, including water and oils, and synthetic pebbles made from a nylon-like material. The model core of the reactor stood about 2 metres tall, and I asked what the power output of a full-sized 4 m tall (2 m wide) reactor core unit would be. Try 400+ MWe. Wow…

I’ll end this post with something a little more technical, if the above wasn’t already too techie for you (apologies to some BNC readers). Below I reproduce Per’s summary of the PB-AHTR, which he wrote up late last year in response to some prompting from me and others on the IFRG (a nuclear energy mailing list the Per and I, and many others from SCGI, are part of). It’s a terrific summary of Per’s research, for those with an engineering background or nuclear science predilection. For those who lack either, the core message is this:

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 commercialised 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 and IFR, which not only achieve high burnup, but also completely close the fuel cycle.

Here’s Per’s summary:

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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(7Li 2 BeF 4 ) 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:

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

References

1. P. Bardet, E. Blandford, M. Fratoni, A. Niquille, E. Greenspan, and P.F. Peterson, “Design, Analysis and Development of the Modular PB-AHTR,” 2008 International Congress on Advances in Nuclear Power Plants (ICAPP ’08), Anaheim, CA, June 8-12, 2008.

2. “MSRE Systems and Components Performance” Oak Ridge National Laboratory, ORNL-TM- 3039, June 1973.

3. “The Development Status of Molten-Salt Breeder Reactors,” Oak Ridge National Laboratory, ORNL-4812, pp. 200-201, pp.207-211, August 1972.

4. Energy From Thorium

5. Laser Inertial Fission/Fusion Energy (LIFE)