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Next-Gen Nuclear Reactor Design


sevenperforce
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On suggestion by @CastleKSide, I decided to start a thread on nuclear reactor design, particularly with a focus on near-term next-generation reactors, but generally open. This will split the discussion on the SpaceX thread about startups working in modular reactors.

Tagging possibly-interested parties @mikegarrison @KSK @JoeSchmuckatelli @RCgothic @CatastrophicFailure @Rakaydos @AHHans

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The flip side of the coin is where are said reactors going to be built and who is going to build them.  The US has made an economic disaster out of nuclear power design, and that isn't uncommon.  France seems to be the leader, although Finland has recently built 4 (almost done the fifth) in direct response to global warming.  Presumably China and India would be ideal places to build such things, but getting the things built to spec and maintaining budget would be crucial.

The economic disaster that is the US nuclear power industry is such that *maybe* they can get regulations in place and have the old guard retire/go out of business/die and build the next-next-generation of nuclear reactors (although by then wind, solar, and batteries will be likely entrenched).  But if they finally stop advancing you might be safe in planning a nuclear reactor.

Edited by wumpus
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There was a brief period when South Africa was looking like it was going to be a worldwide leader in advanced reactor design, back when Advanced meant pebble bed designs. But their teams ran into problems, I dont remember if they were technical or political, and they lost their lead.

Also relevant: Megapower. NASA's Kilopower's bigger, earth bound (for now) brother.

https://www.lanl.gov/discover/publications/1663/2019-february/_assets/docs/1663-33-Megapower.pdf

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43 minutes ago, Rakaydos said:

technical or political

They are always both.  That was also when the Green and other Progressive / youth led organizations were virulently anti-nuclear...  Even if not often persuasive, they were certainly loud. 

Put it all together and just getting money and talent was and is half the battle. 

 

The major - and to me somewhat surprising (and pleasantly so) - difference is that the modern Green /Environmentalists see Global Warming as the ultimate enemy, and that nuclear can be a solution.  As religiously anti - nuke as so many were, to hear even grudging acknowledgement that nuclear plants are acceptable is a sea-change. 

Edited by JoeSchmuckatelli
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15 minutes ago, sevenperforce said:

I would like to get a better idea of the scaleability characteristics of nuclear. Is bigger better/more efficient? Is there a lower limit?

Kilopower is a small reactor NASA designed for use on spacecraft... BEFORE Starship was on the horizon to make cheap lift a reality. It's less efficient that Megapower, (Which fits in a shipping container) which is less efficient than utility-scale nuclear, but modular nuclear has the benefits of eventually reaching  economies of scale.

Edited by Rakaydos
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55 minutes ago, Rakaydos said:

Kilopower is a small reactor NASA designed for use on spacecraft... BEFORE Starship was on the horizon to make cheap lift a reality. It's less efficient that Megapower, (Which fits in a shipping container) which is less efficient than utility-scale nuclear, but modular nuclear has the benefits of eventually reaching  economies of scale.

Is it like thermonuclear, where efficiency continues to go up no matter how big you get, or is there a power level where you basically max out your benefits?

Modular-distributed makes a lot of sense for a lot of reasons, but I find myself wondering whether we could ever see the entire eastern seaboard running on one offshore megareactor.

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6 minutes ago, sevenperforce said:

Is it like thermonuclear, where efficiency continues to go up no matter how big you get, or is there a power level where you basically max out your benefits?

Modular-distributed makes a lot of sense for a lot of reasons, but I find myself wondering whether we could ever see the entire eastern seaboard running on one offshore megareactor.

A single reactor is a single point of failure- sabotage, incompetency, enemy actors, political winds, anything that takes it out, takes it ALL out. this also applied to the distribution network to get the power where it needs to go, which isnt 100% efficent either. I dont know the first thing about what the actual numbers are, but at some point, distribution losses and grid failures have to be a larger concern than the "bulk discount" of a larger reactor.

A bigger reactor also takes longer to design, build and bring online. That's another advantage to small modular reactors.

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As already mentioned may main issue with many new reactor design is that they use graphite as the moderator. And while graphite will not burn in normal circumstances, a nuclear reactor with a failed cooling system will get hot enough to ignite graphite if it comes into contact with air. (It will also react with water if the amount you pour onto it is insufficient to cool it down fast enough.) As a case in point please remember that a large amount of the radioactive contamination from the Chernobyl disaster was released because the graphite moderator was on fire, or the Windscale fire. So every new reactor design that plans to use graphite as moderator needs to address this issue in a satisfactory fashion. And, no, "we won't let oxygen get to our reactor core" is IMHO not satisfactory!

AFAIK the German pebble-bed reactors - e.g. the THTR-300 - had a claimed security feature that at high temperatures the nuclear reaction is self limiting - although more through Doppler broadening than thermal expansion AFAIK - with the reactor core being able to withstand temperatures where the cooling trough radiation will keep the temperature stable. But I'm sure that didn't include oxygen (i.e. normal air) getting into the mix...

Edited by AHHans
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2 hours ago, sevenperforce said:

I would like to get a better idea of the scaleability characteristics of nuclear. Is bigger better/more efficient? Is there a lower limit?

Hmmm... Considering that the German Wikipedia claims that TIGRA reactors of 20 kW power were (are?) on offer, I'd say that there's only a financial lower limit on a power reactor size. I.e. building any kind of nuclear reactor will involve significant costs so that doing so just to generate a few tens of kW of electricity or heat just isn't worth it.

I don't think nuclear will ever be "scalable" in the sense that you have one design that you can just scale up or down to match the power required for your application. But it is scalable in the sense that you can get reactors from tens of kW to a few GW.

Thermodynamic efficiency mostly depends on the input temperature of the thermodynamic cycle. (Because the lower temperature is usually close to surrounding air / water temperature.) But with nuclear the fuel is so cheap that the thermodynamic efficiency doesn't matter much. Nuclear power plants usually run at a low thermodynamic efficiency compared to e.g. coal power plants. My guess is that this is in order to increase the margin of safety: a leak in a pressure vessel in a nuclear PP is a much bigger problem than in a coal PP. I think the main reason why nuclear PPs are relatively large is because with the current designs it doesn't cost much more to build a 4 GWthermal reactor than to build a 0.5 GWthermal reactor.

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36 minutes ago, AHHans said:

doesn't cost much more

...and the amount of required paperwork is the same. 

I've never really thought about scalable nuclear in the sense of private / base uses (ala gas generators)... But my 'industrial production' presumption is that given the host nation's insatiable desire for power, the largest plant that meets the intersection of affordable and relative safety is what gets built. 

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5 hours ago, sevenperforce said:

I would like to get a better idea of the scaleability characteristics of nuclear. Is bigger better/more efficient? Is there a lower limit?

A confounding factor is the cost of shipping in fossil fuel. It's the reason the US military and Russian towns in the Arctic are looking at miniature nuclear that wouldn't make sense in a more on-the-grid locale.

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4 hours ago, AHHans said:

As already mentioned may main issue with many new reactor design is that they use graphite as the moderator.

Is there anything you can use in place of graphite with the same design?

4 hours ago, AHHans said:

And while graphite will not burn in normal circumstances, a nuclear reactor with a failed cooling system will get hot enough to ignite graphite if it comes into contact with air. (It will also react with water if the amount you pour onto it is insufficient to cool it down fast enough.) As a case in point please remember that a large amount of the radioactive contamination from the Chernobyl disaster was released because the graphite moderator was on fire, or the Windscale fire. So every new reactor design that plans to use graphite as moderator needs to address this issue in a satisfactory fashion. And, no, "we won't let oxygen get to our reactor core" is IMHO not satisfactory!

That's one of the things we all love about molten salt reactors -- they are SUPPOSED to be melty, and if they get too hot they simply melt through the freeze plug and are dumped. It's also fairly low-pressure.

One of the challenges in a molten salt reactor is that the salt is the coolant and so the salt has to be pumped around in a loop, which is very challenging to get right. Would it be possible to have a helium-cooled molten core reactor? 

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7 hours ago, sevenperforce said:

One of the challenges in a molten salt reactor is that the salt is the coolant and so the salt has to be pumped around in a loop, which is very challenging to get right. Would it be possible to have a helium-cooled molten core reactor? 

Well, probably yes, but it wouldn't be easy. Helium is favored because it's immune to neutron activation, but it would consistently pick up the molten fuel and deposit it down the cooling tract. Unless you physically separate the two, which would incur an efficiency loss.

I find it quite notable that neither Afrikantov nor Hydropress/Dolezhal are giving molten salts the time of day.

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13 hours ago, JoeSchmuckatelli said:

...and the amount of required paperwork is the same. 

Indeed!

11 hours ago, sevenperforce said:

Is there anything you can use in place of graphite with the same design?

Not that I know of. [...] O.K. I wasn't aware of how good a moderator carbon actually is, it's second only to heavy water (deuterium oxide). Other materials - e.g. pure oxygen, pure deuterium, or carbon-dioxide - would have similarly good scattering to capture ratios but lack in density.

 

11 hours ago, sevenperforce said:

That's one of the things we all love about molten salt reactors -- they are SUPPOSED to be melty, and if they get too hot they simply melt through the freeze plug and are dumped. It's also fairly low-pressure.

Yes, which is why MSRs are one of the few designs where I believe the "inherent safety" claims of their proponents. Another one would be an accelerator driven design: i.e. a subcritical core that is kept running by constantly adding neutrons from a spallation source.

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17 hours ago, Rakaydos said:

A single reactor is a single point of failure- sabotage, incompetency, enemy actors, political winds, anything that takes it out, takes it ALL out. this also applied to the distribution network to get the power where it needs to go, which isnt 100% efficent either. I dont know the first thing about what the actual numbers are, but at some point, distribution losses and grid failures have to be a larger concern than the "bulk discount" of a larger reactor.

A bigger reactor also takes longer to design, build and bring online. That's another advantage to small modular reactors.

I think one major benefit of an smaller reactor if the lower thermal energy in the core.  As I understand an reactor on an submarine you can just drop the control rods and its off. Even if its not actively cooled nothing bad happens. 
At Fukushima their problem was that they did not have cooling water to cool the shut down reactor who still needed water to cool down the core who was huge, hot and got extra heat from rest radiation. 
This makes the smaller reactor safer, and as you say if one reactor has to be shut down its not an huge deal. Most nuclear plants has multiple reactors anyway but it tend to be 3-4. 

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11 hours ago, DDE said:

Helium is favored because it's immune to neutron activation, but it would consistently pick up the molten fuel and deposit it down the cooling tract. Unless you physically separate the two, which would incur an efficiency loss.

Efficiency loss due to physical separation hasn’t stopped molten salt reactors before; there are lead-cooled salt reactors, for example.

And I would have to imagine that there would be a geometry solution to salt deposition, right? Helium is not known for being particularly dense….

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Whenever I think about next-generation nuclear reactor designs, I’m reminded of the Cecil Kelley criticality incident. There was an unfortunate excursion in a plutonium reprocessing lab that claimed the life of one of the technicians when an extraordinarily high level of dissolved plutonium was introduced to a vat in the lab. The operator turned on the vat to mix it, not realizing how high the plutonium levels were, and the plutonium-enriched layer of fluid at the top was pulled down into a vortex, creating a critical mass:

791px-Plutonium_Vortex.jpg

One way to make an “inherently safe” reactor would be to use a liquid fuel solution that could have its criticality controlled by geometry through rotation, thus requiring a constant source of energy to maintain operation. Lose power, and the fluid spins down quickly and can no longer maintain critical geometry. 

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Thanks for making this thread @sevenperforce

Im a big big fan of nuclear power, for multiple reasons. I honestly think it is the only viable way to get off of a petrochem energy economy, due to articles like https://spectrum.ieee.org/solar-energy-isnt-always-as-green-as-you-think as well as the major study that google conducted some years back. There was another IEEE article about wind turbines that shaped my views as well. Essentially the argument is the crucial requirement for leaving fossil fuels is finding a way smelt steel, since we do not have a way to do that from electricity. Without steel production wind turbines cease to be a viable baseline energy generation tech, and solar will never be able to augment that enough until we develop a viable pure electric steel-making process. I view the stifling of nuclear research under the mountains of regulations a large mistake, and tales about how the US molten salt project ended and the loss of data that followed actually make my blood boil. (Like seriously if you end a program make sure the data you paid for gets archived. what government)

I tend to be skeptical of small scale nuclear reactors tho. Economy of scale is a hard thing to run against esp when your tech needs as much shielding as fission reactors do. Any body got interesting articles on revolutionary shielding? maybe meta-materials could spark something in that area, stopping particles with smarts instead of shear mass.  I wish there was more research going into that honestly. Would not even have the dangers associated with experimental reactors, but I bet the general public would not make the distinction.

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15 hours ago, sevenperforce said:

Whenever I think about next-generation nuclear reactor designs, I’m reminded of the Cecil Kelley criticality incident.

The last section of the Wikipedia article about that incident made me think that it should be mandatory for employers to pay for life insurance for their workers.  So that dependents are cared for in case something happens, and to motivate the employers to increase workplace safety. ;)

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5 hours ago, CastleKSide said:

Thanks for making this thread @sevenperforce

Im a big big fan of nuclear power, for multiple reasons. I honestly think it is the only viable way to get off of a petrochem energy economy, due to articles like https://spectrum.ieee.org/solar-energy-isnt-always-as-green-as-you-think as well as the major study that google conducted some years back. 

Selected excerps:

Quote

 

> Most manufacturers recycle this waste to make more polysilicon. Capturing silicon from silicon tetrachloride requires less energy than obtaining it from raw silica, so recycling this waste can save manufacturers money. But the reprocessing equipment can cost tens of millions of dollars. So some operations have just thrown away the by-product.

> But accidents do happen and are more likely in places that have limited experience manufacturing semiconductors or that have lax environmental regulations.

> there is a newer approach: thin-film solar-cell technology. The thin-film varieties will likely grow in market share over the next decade, because they can be just as efficient as silicon-based solar cells and yet cheaper to manufacture, as they use less energy and material. (...) Moving to thin-film solar cells eliminates many of the environmental and safety hazards from manufacturing, because there's no need for certain problematic chemicals—no hydrofluoric acid, no hydrochloric acid.

> Making solar cells requires a lot of energy. Fortunately, because these cells generate electricity, they pay back the original investment of energy; most do so after just two years of operation, and some companies report payback times as short as six months. 

> the amount of water used to produce, install, and operate photovoltaic panels is significantly lower than that needed to cool thermoelectric fossil- and fissile-power plants.


 

I'm not sure how you got "Solar is unviable" from this article.

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On 10/20/2021 at 1:22 AM, DDE said:

Well, probably yes, but it wouldn't be easy. Helium is favored because it's immune to neutron activation, but it would consistently pick up the molten fuel and deposit it down the cooling tract. Unless you physically separate the two, which would incur an efficiency loss.

I find it quite notable that neither Afrikantov nor Hydropress/Dolezhal are giving molten salts the time of day.

Two fluid Thorium MSR's use helium to capture gaseous reaction products from the fuel salt.  They inject some helium into the hot side of the loop, then pull off the helium which has picked up any xenon, and cesium.  This off gas is then processed to reclaim the helium and to hold the radioactive products until enough half lives have passed

 

 

https://world-nuclear.org/information-library/current-and-future-generation/molten-salt-reactors.aspx

 

 

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