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Nukes – Part 6

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John Benson's picture
Senior Consultant Microgrid Labs

PROFESSIONAL EXPERIENCE: Microgrid Labs, Inc. Advisor: 2014 to Present Developed product plans, conceptual and preliminary designs for projects, performed industry surveys and developed...

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  • Jan 6, 2022

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I will start this Nukes Part by disagreeing with myself. In Nukes Part 4 I said: “A class of reactors is generally called advanced reactors. They do not use water for cooling, heat transfer, and reactivity control, but instead use some other fluid.

In spite of claims by the above companies that the above designs are intrinsically safe, I don’t buy it. I’m not going to review any advanced reactor designs in future Nukes, unless they are able to define a true breakthrough design that I can believe.

I just spent several hours researching TerraPower, and feel like they have the resources to pull off a successful design, certification and first project, and this post will review this innovative design.

Matt Chester's picture
Matt Chester on Jan 6, 2022

 I’m not going to review any advanced reactor designs in future Nukes, unless they are able to define a true breakthrough design that I can believe.

Quite understandable-- the rate of media-hyped 'breakthroughs' in all sorts of energy tech is enough to make you jaded, but when the real breakthroughs come we'll be ready. Thanks for your review and insights!

John Benson's picture
John Benson on Jan 6, 2022

Thanks for the positive comments. I believe that this is a real breakthrough, and TerraPower seems to have all of the pieces required to make it happen.



Bob Meinetz's picture
Bob Meinetz on Jan 6, 2022

John, I thought I had found an island of agreement with the old John Benson. Now I find there's a new, improved version who has renounced his former self, and I think the new JB is going way out on a limb. Here's why:

The idea of a traveling wave reactor has been around since the 1950s. Gates has been working on his incarnation since 2010, and he remains years (decades?) away from building a successful prototype.

The traveling wave idea was rejected by GE nuclear engineers in the 1960s-70s because, like many other designs, it made unrealistic assumptions about the predictability of high-flux environments inside reactor cores. Inside those cores, new elements are created every moment - some isotopes are stable, others aren't. Even when columns of fuel pellets are separated by zirconium cladding and a few inches of water, it took years of experience to understand how positive void coefficients, and other local irregularities in a nuclear reaction, could upset the stability of the whole reactor in moments, possibly leading to disaster. Now, we're supposed to assume a solid mass of highly-reactive nuclear fuel will remain stable for decades?

If Gates's idea is successful, it
• won't be ready for deployment until it's too late to make any meaningful contribution to the fight against climate change
• will be too complicated (too expensive)
• no one, including myself, will find the idea of leaving nuclear waste entombed in concrete a few meters below ground is an acceptable means of disposal
• even the most durable steels are vulnerable to corrosion due to the high temperatures of molten salt reactors. Though there have been improvements, substantial problems remain.

SMRs remain the best solution for development now:
1) The physics of pressurized-water reactor (PWR)  technology are well-understood
2) The best passively-safe design (NuScale) operates at reduced temperatures and pressures, increasing reactor stability and safety
3) Small size and assembly-line manufacture will make them affordable for developing countries and remote locations.

John Benson's picture
John Benson on Jan 7, 2022

Hi Bob:

I'm still a strong believer in SMRs. However I believe that TWR also have some strong attributes.

As long as the Nuclear Navy stays afloat, we will have a very similar problem as you described above for old TWRs, except worse - cores from old submarines are small enough to be less of a challenge to transport, and there are already hundreds of those setting around.

I'll grant you one thing - my (other) friends from California would have a fit if someone suggested permanently burying an TWR in our state, but I believe that there are work-arounds for potential issues, and Wyoming is not California.

Thanks for the comments.



Peter Farley's picture
Peter Farley on Jan 18, 2022

The nuclear Navy has about 8-10 GW thermal of old cores to store and the money and inbuilt security apparatus to look after it. For TWRs to make a useful dent in US energy needs you would need 300-1,000 GW thermal and build a huge security apparatus

Peter Farley's picture
Peter Farley on Jan 9, 2022


 Despite the many disagreements we have about nuclear in general, I think your comments about these new fangled reactors are correct. It seems to me that if small reactors are to succeed they will be low cost small PWRs perhaps with some thermal storage to provide more output flexibility or they will be linked with renewables to provide more stable power for hydrogen electrolysis or other forms of power to transport fuels, low temperature process heat or district heating. They won't be "truckable", they might be able to fit on a barge but the Russian experience suggests there is a long way to go to get the costs anywhere near where they need to be. The focus should be on a ground up method of achieving as safe as possible operation at as little cost as possible  

Bob Meinetz's picture
Bob Meinetz on Jan 10, 2022

Peter, nice to find common ground with you on this topic. PWR technology has half a century of collected experience to work from, and safety as always, is #1.

Cost is still a question mark, but the engineers at NuScale are well aware of that, and are working to create a manufacturing and distribution dynamic that will be ultimately safer, more flexible, and cheaper than any nuclear alternative.

Peter Farley's picture
Peter Farley on Jan 18, 2022

Bob Nuscale is not inspiring. Their cost per MW so far seems to be higher than an AP 1000 and in the last seven years they have pushed out their start date by six. The idea of returning 700-800 ton cores to a factory for refueling is extremely difficult. Just look how long it takes to move a so called super load around the country these days. Then it is a security nightmare. a target visible from space and vulnerable to a shoulder launched anti tank missile to create a dirty bomb. In France they move reactor fuel rods around in unmarked trucks, because a partial reload for a conventional reactor is about 20 tonnes.

It seems to me that the Chinese approach of the ACP 100 is the most likely to succeed but it still faces formidable cost challenges

Bob Meinetz's picture
Bob Meinetz on Jan 18, 2022

Peter, the latest estimate I've seen for price of NuScale's SMR is $5B/GW; AP1000 has a "list" price of $6.3B/GW.

A lot depends on where they will be built and where they will be installed, however. China is building 2GW plants for 1/4 the cost of the U.S., due to onerous U.S. regulatory requirements and an overtaxed Nuclear Regulatory Commission (the main reason NuScale has had to push out their start date).

The main selling point of NuScale's SMR, however, is purchasers can start small and build big, as financing becomes available. A one-module plant would be capable of delivering 77MW of clean generation to remote locations; at the quoted price that's $385M for a state-of-the-art nuke plant, powering its own grid.

I don't know the exact figure, but the weight of a NuScale fuel assembly is probably an order of magnitude smaller than that of an AP 1000 - no comparison.

"Then it is a security nightmare shoulder...launched anti-tank missile...etc. etc. etc." Peter, your imagination is running away with you. If anyone felt it necessary to launch a satellite-based missile at a NuScale module, cleanup would cost less than the missile would. The danger to residents is nonexistent; you're imagining Armageddon. If you spent a little time learning about radiation you'd sleep better.
For example: if you flew to Japan and spent the afternoon visiting Fukushima City, you would receive thousands of times more radiation from the plane flight. Really.

Peter Farley's picture
Peter Farley on Jan 18, 2022

Sorry Bob I didn't explain myself very well.

1. The way I understand Nuscale works is that a number of reactors feed a common steam header and then a decent size steam turbine so if you start small with one reactor per turbine you have a much more expensive system in the end.

2. In 2018 they lodged their licence application, expecting an answer in 2021 which they received. It is design and financing which are the major causes of delay. The disclosed price for the 860MW Utah plant in 2020 was $6.1bn, about $7,100/ MW. higher than the AP 1000 list but agreed considerably cheaper than AP1000 actual. Apparently it has now been scaled back 460MW, 6 modules but I can't find any new budget but it is a reasonably safe assumption that the price per MW will rise

2. In a normal PWR as I understand it only 1/3rd of the fuel rods are changed at a time so about 50-80 tonnes which can easily be split over 3-5 conventional tractor trailers. This equivalent to a 350MW reactor. The Nuscale "sealed" reactor module is around 700 tons for 77 MW so the freight task is huge. I didn't mean a foreign government would use their spy satellites, I meant that any civilian or militia organisation with access to photos form the latest high res weather satellites could easily track the load and lay in ambush for it. If a Javelin can punch through 12" of tank armor it can punch through the side of the "truckable" reactor vessel

 Again I am not worried about radiation if everything goes to plan. I agree worries about radiation from reactors is vastly overrated   

Bob Meinetz's picture
Bob Meinetz on Jan 18, 2022

"The way I understand Nuscale works is that a number of reactors feed a common steam header and then a decent size steam turbine so if you start small with one reactor per turbine you have a much more expensive system in the end."

True, including costs of real estate and "balance of plant" operations (steam turbines, security, etc.) the first reactor will be more expensive.

"I meant that any civilian or militia organisation with access to photos form the latest high res weather satellites could easily track the load and lay in ambush for it. If a Javelin can punch through 12" of tank armor it can punch through the side of the "truckable" reactor vessel..."

Do you think a militia organization can walk into the corner gun shop and buy an FGM-148 Javelin missile - a state-of-the-art, fire-and forget weapon capable of bringing down any of the thousands of commercial aircraft that take off and land every day in the U.S.? 

I can assure you that U.S. Homeland Security has a record of where every single FGM-148 missile is, has been, and will be - that tracking these weapons is among the highest of the department's security priorities. If anything, you should worry about being on a commercial flight that's hit by one (but you shouldn't worry about that, either).

Michael Keller's picture
Michael Keller on Jan 19, 2022

The NUCALE design is not sealed in the sense of a transportable package. A vessel containing the fuel is moved underwater to an area where the fuel assemblies are removed and put into racks for eventually placement in transportable casks. Using casks is the same approach used by conventional reactors.


The NUSALE reactor is not readily transportable on a run-of-mill truck. While the components are smaller than those of large conventional reactors, transport requires specialized equipment for oversized, heavy loads. Also requires several shipments, not just one.

Michael Keller's picture
Michael Keller on Jan 19, 2022

The NUSCALE plants are designed to be passively fail-safe and that puts a limit on the power output of the reactor. While good from a public safety standpoint, economic competitiveness quickly becomes a major problem. The much larger BWRX is inherently more competitive, but attaining a passively fail-safe design is more complex. That being said, the BWRX design has difficulty competing with natural gas combined cycle plants that are relatively inexpensive to build, have outputs 3 times that of the BWRX, and can technically flexibly deal with the grid fluctuations caused intermittent green energy. That latter factor extends to production costs associated with the renewable intermittent issue.

Michael Keller's picture
Michael Keller on Jan 19, 2022

The BWRX is a boiling water reactor that relies on natural circulation of water thru the core and production of steam directed to a steam turbine. Conventional BWR’s use recirculating pumps.

Also, conventional BWRs have high outputs which means if active cooling is lost, the fuel melts. That is what happened at Fukushima. The BWRX does not have that problem

Michael Keller's picture
Michael Keller on Jan 17, 2022

I agree, a small version of a light water reactor would be easier to deploy, relative to most of the advanced reactor concepts. The  GE/Hitachi 300 megawatt boiling water reactor is pretty straightforward and intrinsically more economical than the NUSCALE approach which requires 4 or so reactors to match. That being said, I doubt the designs will be able to compete with renewable energy and natural gas. Smaller inevitably results in higher production costs, particularly when considering the very high capital costs to build nuclear plants. The financial risk remains quite high relative to other forms of generation.

Roger Arnold's picture
Roger Arnold on Jan 9, 2022

Bob, a traveling wave reactor was TerraPower's initial vision, but as development progressed, they abandoned that idea. The design they're now lined up behind is a molten sodium fast reactor. They call it "Natrium". The technology is described at A key feature is that it provides for integrated heat storage in molten chloride salt. The heat store isolates power generation from the reactor itself, and supports load following.


The Natrium reactor design is conservative, and draws on past experience with molten sodium fast reactors. That's a big reason it's been able to negotiate the NRC licensing process more quickly than rival designs. It differs from previously built sodium fast reactors -- and circumvents most of the problems they encountered -- in that it is not a breeder reactor. It does breed and then burn some plutonium internally, but is only 4x more fuel efficient than a 3rd generation LWR. I believe it produces spent fuel waste similar to a conventional LWR, but 75% less per gigawatt-hour. 


In an ideal world, the Natrium design would not be my first choice. I think some of the other 4th generation design candidates have much better long-term potential. I'd like to see some of the designs that offer 200x improvement in fuel efficiency and zero long-term radioactive waste succeed. But Natrium would be a big step-up over present LWR designs in cost, performance, and intrinsic safety. If its conservative approach can get it approved and into the field more quickly, then I'm all for it.

Bob Meinetz's picture
Bob Meinetz on Jan 10, 2022

Thanks Roger, I was unaware Terrapower had abandoned the TWR - long overdue!

"But Natrium would be a big step-up over present LWR designs in cost, performance, and intrinsic safety."

I have yet to understand how. The concept of placing the reactor in a pit, with no containment structure is, in my opinion, courting disaster. That it's only 345 MW is irrelevant - should sodium leak and catch fire (not that water could ever leak into a subterranean pit</sarcasm>) the resulting radioactive plume would be virtually unstoppable.

Liquid sodium is a more efficient carrier of heat, but locating the reactor underground is an accident waiting to happen. More importantly (and notwithstanding predictions for delivery later this decade), a revolutionary approach like this won't be approved until >2040 - that's too late.

Peter Farley's picture
Peter Farley on Jan 18, 2022

The whole metallurgy cycle of this reactor is difficult to believe.

 . high heat flux means high corrosion,

 . high temperature high density fluids mean fast erosion particularly on the salt side

 . three different chemistries at each interface sets up inter-metallic corrosion

 . high radiation flux means high rates of embrittlement.

 . As Bob says a sodium leak anywhere into air or water is a potential bomb. 

 . Nobody has demonstrated molten salt storage lasting more than 6-8 years let alone in a radioactive environment

I am a great fan of supercritical CO2 in theory, but really? Supercritical fluids are tricky beasts, the thermo-mechanical loads on those tiny turbines are immense, surely we should try them out for 5-6 years at scale with gas heating or even solar thermal first to make sure they work reliably. That means to go from 25 MW now to a 300 MW turbine and then build up sufficient confidence that you can bet a multibillion dollar investment on it means that a commitment to even design a nuclear system with SCCO2 isn't even possible before 2030 which means 2040 before commercial operation, if all the metallurgy problems are ever solved     

Michael Keller's picture
Michael Keller on Jan 17, 2022

No sodium reactor has ever been licensed by the NRC. Fast reactors have rather fundamental safety issues because the core is so compact. Events that mechanically distort the core can produce huge bursts of energy while being very difficult demonstrate that the core remains subcritical. That situation  immediately runs into a licensing buzz saw.

From an operational standpoint, sodium reactors have a dismal history. Sodium, air and water are more or less enemies and that leads to an army of complications, including abysmal reliability. Unclear why a utility would want to invest in such a power plant as the ability to turn a profits looks to be really low.

Peter Farley's picture
Peter Farley on Jan 9, 2022

Lets assume the Natrium reactor is as good as they claim, they still have to build a molten sodium circulation system that will last 60 years. No-one has got remotely close to that yet. Similarly molten salt storage at Crescent Dunes and Ivanpah has also been far less reliable than hoped. Even the fuel rod shuffling system has to be an order of magnitude more robust than the fuel rod loader in a conventional reactor 

Then we have three heat exchange phases with different materials, fuel rod/sodium, sodium/salt and salt/water. Because of the different materials and higher temperatures erosion/corrosion of the heat exchanger itself is likely to be faster and in the first two phases radiation embrittlement is a further factor, so the chances of high reliability of each one of these systems is doubtful. All three in series being reliable is quite unlikely

Then it still has a steam turbine which at full power will be around 42-45% efficient but at part load on a hot day as low as 34-35%. Overall it will need about 80% as much cooling water per MWh as a conventional reactor. Increasingly water stressed plant is going to make suitable inland sites more and more difficult to find. On the coast just build a large conventional reactor with seawater cooling and a strong transmission link

As Bob Meinetz repeatedly and correctly observes, fuel cost is the least of the costs of nuclear reactors so the Natrium reactors fuel efficiency is extremely unlikely to offset all the other risks.  

Roger Arnold's picture
Roger Arnold on Jan 10, 2022

Peter, those are good comments. You're right that there are three levels of heat exchange. However it's likely, I believe, that the third level will not be salt/water, but rather salt/supercritical CO2. For thermal power generation, SCO2 Brayton cycle are coming on fast. Higher thermal efficiency, superior throttling efficiency, much smaller size, and lower cost.


I'm also not so sure about the 60-year reliability requirement. Setting that as a hard requirement, and the effort required to prove it, is a big part of what has made nuclear power so outrageously expensive. We've treated nuclear plant operation as if "any failure will be the end of the world". No, it won't. That's well proven by now. So if something fails, fine. Shut down, fix or replace it, and go on. Make sure that the overall design facilitates that, and that radiation exposure from any failure will remain within reasonable safety limits. But don't be paranoid about breaking anything. Expect failures, design the system so the results won't be total catastrophe, analyze any that happen, and fix the problem in the next design rev.


People need to relearn what "failsafe" actually means. The cost of guaranteeing perfection before doing anything is never doing anything.

Michael Keller's picture
Michael Keller on Jan 17, 2022

The SCO2 cycle remains in the land of struggling to reach commercial deployment, as it has for 50 years. The required heat exchangers (technically regenerative designs) struggle to efficiently and effectively transfer heat. The turbo-compressors are faced with a working fluid that is peculiar, particularly when attempting to re-compress SCO2.

Also, while the reactor’s working fluid may be really hot, that does not mean the energy can be readily transferred to some form of turbine/generator. Steam turbines are typically limited to around 1000 degrees Fahrenheit by material considerations.

Peter’s observation on molten salt is spot-on. The stuff readily turns into a brick if it cools off and re-liquifying is not operationally straightforward, even when using electric heaters. The stuff can self-weld and the affected piping/valves may have to be cutout - really unhelpful for a highly radioactive reactor. Should note the neutrons associated with fast reactors are very high energy and that causes unusually high  radioactive activation of materials in and around the reactor. Again, really unhelpful from an operations and maintenance perspective.

Roger Arnold's picture
Roger Arnold on Jan 18, 2022

The SCO2 cycle remains in the land of struggling to reach commercial deployment, as it has for 50 years.

For 50 years? Well, you've had hands-on experience and know a lot more about power turbines than I do, so I'll take your word. However the earliest reference I could find to SCO2 Brayton cycle turbines was a paper by Steven Wright from May of 2011. That was when he was still working at Sandia National Labs, heading a program researching the potential of SCO2 turbines in power generation. I believe it was soon after that paper was published that Wright left Sandia to found a startup company with the intent of commercializing the technology. The company was located in Arvada Colorado, very close to where I grew up. I don't know what has happened to Wright's company, but DOE still has programs for SCO2 turbines, and the web page descriptions suggest great promise. 

Regarding sodium fast reactors, you're of course right that none have ever been licensed by the NRC -- for commercial operation. However several have been built and operated within the US going back to 1950. The Integral Fast Reactor, based one the earlier EBR2, was sodium-cooled, and was close to completion when it was abruptly canceled by Bill Clinton in 1994 for political reasons. 

The only point of all that is that DOE and the NRC do have experience and knowledge of sodium fast reactors. I was speculating that that might account, at least in part, for DOE's willingness to sign a cooperation agreement and commit funding for Terrapower's demonstration project to be built in Kemmerer, Wyoming, and the NRC's willingness to grant a license for reactor construction at the site.

I was somewhat confused, however, by a Forbes article that jumped between reporting on Terrapower and reporting on NuScale. It's actually NuScale that currently furthest along toward design certification by the NRC. NuScale's design is a simplified, passively safe SMR version of an otherwise fairly conventional PWR. The construction license for Terrapower's demonstration project in Wyoming was more about site suitability than it was about design certification.

Michael Keller's picture
Michael Keller on Jan 19, 2022

Early on (1950’s) scientists and engineers recognized that supercritical CO2 has a lot of advantages, chiefly much smaller turbo machinery than using steam. However, trying to move the concept from the laboratory into the real world has been a long struggle.

I think the BWRX design (see earlier comments on this posting) is more likely to win. The BWRX design is a scaled down version of a natural circulation design that has already been licensed by the NRC. The NRC has reviewed the BWRX design and there are no show stoppers- licensing looks to be easily completed. The folks in Canada (Ontario) are moving quickly on building a BWRX.

As many developers have found out to their dismay (Westinghouse, NUSCALE) a passively fail-safe design does not mean the cost to obtain a license is inexpensive.    

All the sodium fast reactors and fluid fueled reactors are novel and have never been licensed. The historical record points towards massive licensing problems, particularly cost. In my opinion, the financial risk is huge with little likelihood of the machines being profitable.

Michael Keller's picture
Michael Keller on Jan 19, 2022

FYI. Earliest record I found was a 1948 patent by Sulzer Brothers for supercritical CO2 power cycle. Was part of patent searches conducted by my firm in conjunction with a patent pending advanced supercritical cycle.

Michael Keller's picture
Michael Keller on Jan 17, 2022

Heat exchangers inevitably leak. Just a real world fact.

John Benson's picture
John Benson on Jan 19, 2022

Hi guys: 

I checked in a few days ago, and decided you were having a good conversation, so I didn't pop in.

However this morning, I thought it was a good time to add a few comments.

I believe either TerraPower still believes that a TWR is an important part of their future, or they need to do some serious work on their website. Go to the page linked below.

Having said the above, I noted that TerraPower is a bit secretive about their future plans, so it may be that they do not intend use a TWR in their initial prototype. In either case, I mainly believe they have a strong potential future because:

- depth of engineering expertise

- depth of regulatory expertise at the leadership level

- very deep pockets

I agree that they need to deal with a complex design, but it seems to be well thought out.


John Benson's picture
Thank John for the Post!
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