Members of the UC Berkeley Department of Nuclear Engineering participated in the Reddit.com Science AMA Series, responding to a large number of largely hostile questions. Lots of variations of “Can I still eat fish from the contaminated Pacific”. As typical with these AMA sessions the signal to noise ratio is low due to the uninformed questions and irrelevant branched threads of discussion by people who are more interested in politics. I “mined” the 1,447 comments for what I thought were fragments worth archiving.
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?”
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).
Regarding waste: Prof. Peterson was a member of Obama’s Blue Ribbon Commission on America’s Nuclear Future. I consider him one of the best-informed sources regarding Spent Nuclear Fuel (SNF) which the anti-nuclear lobby calls Nuclear Waste. It is not “waste” it is an extremely valuable source of carbon-free energy.
A: …Finland and Sweden have successfully sited and are building deep geologic repositories in granite, and France is very far along in developing its geologic repository in clay. The U.S. nuclear waste program is currently stopped and is in a state of disarray…
There are a wide range of opinions as water reactors (LWRs) is substantially more expensive than making new fuel from uranium, even if the plutonium is free. This is primarily because the plutonium must be handled as an oxide powder to make LWR fuel, and oxide powder is the most hazardous and difficult form to handle plutonium in. All of the Generation IV reactor technologies can use fuel forms that do not involve handling plutonium and minor actinides in the form of powders and that are much easier to fabricate using recycled material (e.g., metal, molten salt, sol-gel particles in either coated particle or vibropacked fuel forms).
In my personal opinion, the most sensible thing to do in the near term is to prioritize U.S. defense wastes for geologic disposal, and to use a combination of consolidated and on-site interim storage for most or all commercial spent fuel. Implementation of the Blue Ribbon Commission’s major recommendations, which include development of consolidated interim storage that would initially be prioritized to store fuel from shut down reactors, would put the U.S. on this path.
By using geologic disposal primarily for defense wastes first, and using primarily dry cask interim storage for commercial spent fuel, this will give a couple of decades for nuclear reactor technology to evolve further, and by then we will be in a better position to determine whether commercial spent fuel is a waste or a resource.
Nuclear innovation: Prof. Peterson replies
There are a number of factors which make innovation difficult in improving nuclear reactor technology, in particular the long operating life of nuclear power plants and their very large capital costs, which dissuade innovation. The trend toward designing larger and larger water-cooled reactors has increased these disincentives.
Given their lower capital cost and shorter construction times, innovation is much easier in small reactors. There will remain a role for large reactors, just as dinosaurs existed for millions of years alongside the new mammal species, but currently some of the most important policy issues for nuclear power involve creating an ecosystem where small reactors find customers. Smaller reactors, produced in larger numbers with most of the fabrication occurring in factories, would also use specialized manufacturing and skilled labor more efficiently. Imagine factories as being similar to airplanes, and the ability to keep more seats filled being really important to having low per-seat prices…
FHR (Fluoride Salt Cooled High Temperature Reactor), Where to take technical risk?
I will answer this question first indirectly, and then more directly.
A key question for innovation in developing new nuclear energy technology is where to take technical risk. SpaceX provides a good example of a highly successful risk management strategy. They focused on developing a highly reliable, relatively small rocket engine, that they tested in the Falcon 1, which uses an ancient rather than innovative fuel combination, kerosene and liquid oxygen. On the other hand, they chose to use aluminum-lithium alloy with friction stir welding for their fuel tanks, which is at the cutting edge of current technology. They have then used the approach of ganging together large numbers of these engines to create the Falcon 9, which is now successfully delivering cargo to the International Space Station.
Currently the most important barrier to deploying nuclear power is not the cost of the fuel, but instead is the capital cost of the plants, the need to assure that they can run with high reliability (which for current large reactor designs creates strong disincentives to innovate), and the relatively low electricity revenues one receives for producing base load power, particularly today in the U.S.
The primary reason that UCB, MIT, and UW, and the Chinese Academy of Sciences, are working on solid fuel, salt cooled reactor technology is because we have the ability to fabricate these fuels, and the technical difficulty of using molten salts is significantly lower when they do not have the very high activity levels associated with fluid fuels. The experience gained with component design, operation, and maintenance with clean salts makes it much easier to consider the subsequent use of liquid fuels, while gaining several key advantages from the ability to operate reactors at low pressure and deliver heat at higher temperature.
Q: Can I also ask what you think the safest way to transport the waste is?**
A: Per Peterson: There is a long record of safe transportation of nuclear waste, including spent fuel, world wide. The containers used to transport nuclear wastes are substantially more robust than those used to transport hazardous chemicals and fuels, which is why transportation accidents with chemicals generate significantly more risk.
This said, the transportation of nuclear wastes requires effective regulation, controls, and emergency response capabilities to be in place. The transportation system for the Waste Isolation Pilot Plant in New Mexico has logged over 12 million miles of safe transport, with none of the accidents involving the transportation trucks causing any release of radioactive materials.
One reason it is important to restore WIPP to service (it had an accident involving the release of radioactive material underground in late February, which had minimal surface consequence because the engineered safety systems to filter exhaust air were activated) is because the WIPP transportation system has developed a large base of practical experience and skilled personnel at the state and local levels who are familiar with how to manage nuclear waste transport. This provides a strong foundation for establishing a broader transportation system for commercial spent fuel and defense high level wastes in the future.
A commenter replied to Per’s hecklers, referring to WIPP:
Actually I work for this program and this is an understatement. Not only have there never been any accidents that caused a release of nuclear material, there have never been any accidents with a truck loaded with waste containers, ever. They’ve happened while empty, but never otherwise.
Per Peterson discussed the unpriced carbon emissions externality. Which I would say is effectively a tax on nuclear because nuclear produces nearly zero carbon energy in competition with coal and gas which do not pay their carbon externality costs. 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.