Per Peterson: key requirements for new reactor designs, what about thorium?


More from the Science AMA Series with members of the UC Berkeley Department of Nuclear Engineering. Aside from “Can I still eat fish from the contaminated Pacific” the dominant questions seemed to be variations of “is thorium the answer?”. I was surprised – I thought obsessing on the LFTR concept was a niche nerdy issue. The slant no doubt illustrates that the reddit crowd doesn’t represent an Anglo cross section.

 I guess I’ll start things off. What type of reactors should we be building? I know a big deal a few years ago was made about liquid flouride thorium reactors. Is that the way of the future, or are there superior alternatives?

Prof. Per Peterson replies (emphasis mine):

I do not think that we have the basis to determine or select the best coolant or fuel type to use in future reactors. But there are some attributes which we do need to make sure are used in future reactors.

The first is to use passive safety systems, which do not require electrical power or external cooling sources to function to remove decay heat after reactors shut down, as is the case with the AP-1000 and ESBWR designs, and with all of the light water reactor SMRs now being developed in the U.S.

The benefits of passive safety go well beyond the significant reduction in the number of systems and components needed in reactors and the reduced maintenance requirements. Passive safety systems also greatly simplify the physical protection of reactors, because passive equipment does not require routine inspections the way pumps and motors do, and thus can be placed in locations that are difficult to gain access to rapidly.

The second is to further increase the use of modular fabrication and construction methods in nuclear plants, in particular to use steel-plate/concrete composite construction methods that are quite similar to those developed for modern ship construction. The AP-1000 is the most advanced design in the use of this type of modularization, and the ability to use computer aided manufacturing in the fabrication of these modules makes the manufacturing infrastructure much more flexible. In the longer term, one should be able to design a new reactor building, transfer the design to a module factory over the internet, and have the modules show up at a construction site, so the buildings are, in essence, 3-D printed.

The final attribute that will be important for new reactors will be to make them smaller, and to develop a regulatory framework and business models that work for multi-module power plants. While there will likely always be a market for large reactors, creating an ecosystem that includes customers for smaller reactors (inland locations served only by rail, installations needing reliable power even if fuel supplies are interrupted, mature electricity markets that need to add new capacity in small increments).

On thorium, a question:

Hello! What do you think is the most important advantage that thorium has over uranium as a “fuel?”

Prof. Per Peterson’s reply

The thorium fuel cycle has clearly attractive features, if it can be developed successfully. I think that most of the skepticism about thorium emerges from questions about the path to develop the necessary reactor and fuel cycle technology, versus open fuel cycles (uranium from seawater) and closed, fast-spectrum uranium cycles.

The most attractive element of the thorium fuel cycle is the ability to operate sustainably using thermal-spectrum neutrons. This allows the design of reactor core structures that use high-temperature ceramic materials like graphite, which have substantial thermal inertia and cannot melt. Because these ceramic materials also provide significant moderation, it is difficult to use them in fast-spectrum reactors and thus the most plausible fast-spectrum reactor designs need to use metallic structural materials in their cores.

So thorium reactors are compatible with higher intrinsic safety (cores which do not suffer structural damage even if greatly overheated) and that can deliver heat at higher temperature, which enables more efficient and flexible power conversion.

Molten fluoride salts are compatible with these high-temperature structural materials, and given their very high boiling temperatures make excellent, low pressure heat transfer fluids. In the near term, the largest benefits in using fluoride salts come from the low pressure and high temperature heat they can produce. This can be achieved with solid fuel, which is simpler to work with and to obtain regulatory approvals.

But molten salt technologies also have significant challenges. One of the most important is managing the much larger amounts of tritium that these reactors produce, compared to light water cooled reactors (the quantities are closer to what heavy-water reactors, such as the CANDU, produce, but methods to control and recovery of tritium are much different for molten salts than for heavy water, and key elements remain to be demonstrated).

To repeat a critical point “…largest benefits in using fluoride salts come from the low pressure and high temperature heat they can produce. This can be achieved with solid fuel…”. This summarizes why Prof. Peterson’s lab is focused upon developing the PB-AHTR design, which will also prove out many materials and technologies required subsequently to implement the more challenging Liquid Fuel molten salt reactor concept (such as LFTR).

New nuclear designs have a severe first-mover DIS-advantage


More from the Science AMA Series with members of the UC Berkeley Department of Nuclear Engineering.

Prof. Per Peterson first discussed the unpriced carbon emissions externality. Which I would say is effectively a tax on nuclear because it competes directly with coal and gas.

Next Per raised a very important issue: how the NRC gatekeeping sets up a strong incentive to free-ride on NRC rulings.

But there is another important market failure that affects nuclear energy and is not widely recognized, which is the fact that industry cannot get patents for decisions that the U.S. Nuclear Regulatory Commission makes. For example, there are major regulatory questions that will affect the cost and commercial competitiveness of multi-module SMR plants, such as how many staff will be required in their control rooms. Once the first SMR vendor invests and takes the risk to perform licensing, all other vendors can free-ride on the resulting USNRC decision. This is the principal reason that government subsidies to encourage first movers, such as cost sharing or agreements to purchase power or other services (e.g., irradiation) make societal sense.

Is this being discussed in the USgov? I’ve never seen a word about it. This is another example of the sub-optimal result we get from wasting billions on energy farming production subsidies, while rationing a few millions for nuclear R&D. Even America has very limited funds – and needs to spend them very carefully.

What do you think of Bill Gates’s TerraPower’s TWR design?


More from Science AMA Series with members of the UC Berkeley Department of Nuclear Engineering. Here’s the question What do you think of Bill Gates’s TerraPower’s TWR design? Do you think it could be a viable element in a carbon-neutral energy future?

And Prof. Rachel Slaybaugh’s reply:

The TWR is a large sodium-cooled fast breeder reactor. Things about it that make it attractive are that it:

  • gets much more energy out of the mined resources than typical reactors (enhancing sustainability)
  • can establish a fleet of reactors that don’t require fuel enrichment or fuel reprocessing (reducing fuel costs and proliferation concerns). The initial plant requires enriched uranium, but its follow-ons do not.
  • has strong safety characterisitics. The low-pressure liquid metal coolant can naturally circulate and dump heat to atmosphere indefinitely without any power whatsoever.

It also has some drawbacks. Most notably designing materials that will be able to withstand the amount of radiation required. Another challenge is that the plant is large and low-leakage. To get the Traveling Wave going, the plant has to conserve as many neutrons as possible. Large fast reactors have some inherent issues with stability, so TerraPower probably has to do some tricky stuff to keep the plant safe. It’s not impossible, but it’s probably difficult.

For the future? If they can overcome the challenges I think it could certainly be part of a low carbon future.

Why are SMR (Small Modular Reactors) so important?

Just a quick note on the captioned topic. I am completely confident that SMR's are the future, though the range of power production will not always be limited to “small”, and the nuclear design will certainly not be limited to today's PWR (pressurized-water-reactor) technology. I wrote this note today in reply to the following comment:

It would not solve the waste problem which the IFR and LFTR probably would solve.

There isn't a “waste problem” because there is no technical issue with unburnt fuel, there is a political problem. If uranium wasn't so cheap the economics would have driven greater reprocessing.

It's important not to confuse the IFR or LFTR contributions with the concept of “mass manufacturing”. Remove the “S” and you have “MR” or Manufactured Reactor which is what is significant.

It isn't SMR-PWR vs. IFR/LFTR, it is volume manufacturing and the safety, quality and cost control that goes with the process-control that is important. When affordable, reliable power becomes a hot political issue – then I think that both fast reactors and thorium reactors will have their opportunities to compete. And both will be manufactured in quantity, where safety will be inherent in both the engineering and the process, not in ridiculously costly inspections.

So when you think of SMR don't think narrowly of current technology – which is constrained by what can be shoved through sclerotic regulators like NRC. Think instead a range of sizes of fast, high-temperature or thermal reactors.

It's also important to keep in mind that what the OECD countries do does not really matter that much w/r/t global warming. It is what the fast-developing countries like China, Brazil, Indonesia, Pakistan, or Uganda do. Those countries need cheap, reliable electricity that they can deploy without first creating a safety/technical culture and the associated infrastructure. One or two gigawatt mega-reactors are not appropriate and will not be adopted in those markets. At the right price 25 to 250 MW reactors that can be buried and refueled in 10 or 30 years – these just might be adopted by countries that don't give a damn about global warming. Let us hope…

We can also hope for a new politics where Bill Gates would have been able to build Terrapower in the USA instead of being forced to go to China. Frankly I think that will not happen – England's reforms would not have happened without the New World to generate the innovation. We don't know where the new models for US/EU will come from or what they will be like. But they might originate in Chile, Shanghai or Estonia.