4 ms·

This is a fast reactor. That is, a reactor in which the neutrons, instead of being moderated down to thermal energies, remain at high energy. The fission cros

This is a fast reactor. That is, a reactor in which the neutrons, instead of being moderated down to thermal energies, remain at high energy. The fission cross section for such energetic neutrons is much lower than for thermal neutrons. Therefore, there has to be a much greater density of fissionable material in the reactor core. The lack of a moderator also means rearrangement of the core in an accident is potentially much more dangerous. If the fuel itself rearranges to become more compact, say by melting and flowing, the reactivity could increase. This is not possible in (say) a light water reactor, where such a rearrangement would reduce reactivity. The nightmare scenario for any fast reactor, warned about by Edward Teller in 1967, is a rearrangement that causes the core to become supercritical on prompt neutrons alone (that is, on only the neutrons released promptly at the moment of fission, not on those + the delayed neutrons emitted by some fission products as they decay). A fast prompt supercritical configuration could potentially explode with great violence, greater than Chernobyl. An atomic bomb is a prompt fast supercritical system. I will want to see how the NRC does or does not license their design, a process that has just started. I will not be surprised if their approach ends up being unlicensable in the US because safety cannot be assured by analysis under accident conditions.

thecopy 2y ago
Based on your comment it sounds unreasonable to select this design. It must have some reason to exist?
- pfdietz 2y ago
  Fast reactors do have some attractive features. They have better neutron economy and work better with plutonium. They can achieve breeding ratios comfortably above 1 with the U-Pu system. They produce less actinide waste since the chance of neutron capture not causing fission is lower, and can more effectively destroy actinide waste. Sodium-cooled fast reactors will operate at higher temperature than LWRs, enabling the salt thermal storage scheme they propose to use. In large reactors, these features have not been enough to compensate for the disadvantages and sodium-cooled reactors have not been successful, coming in more expensive than light water reactors for a given power output. France, which had been developing fast reactors, has recently mothballed the effort with no plan to restart before 2050.
  - UltraSane 2y ago
    Russia has two of them, one 800MW and one 600MW, and plan to build a 1.2GW version.
    - pfdietz 2y ago
      Even the Russians admit their LWRs are cheaper.
- aidenn0 2y ago
  The main advantages of using a fast reactor are less nuclear waste and more energy for a given fuel input.
  - nine_k 2y ago
    Even burning some of the "nuclear waste" which is just nuclear fuel than needs refining.
- UltraSane 2y ago
  It can make more fuel than it uses. It needs enriched uranium at startup but then can convert regular uranium and/or thorium to usable fuel.
manvillej 2y ago
reddit had a nice list of the pros and cons: https://www.reddit.com/r/NuclearPower/comments/17k0wcc/natrium_reactor_breakdown/k75bj99/ https://www.reddit.com/r/NuclearPower/comments/17k0wcc/natri... I understand the risks around sodium, but the "passive natural circulation cooling" I don't understand. Is it more feasible with this design and why? " Pros: high temperature means we can use process-heat which is a much more efficient use of heat. fast spectrum neutrons means we can burn importantly troublesome parts of nuclear waste. fast spectrum is also better for breeding new fuel, significantly increasing how much energy we can extract from uranium/thorium. passive natural circulation cooling is much more feasible. Cons: fast spectrum is a little more complicated to control. fast reactors require high enrichment. inspection of the plant is very difficult with liquid metal. high temperature liquid metal doesn't play nicely with metal pipes. sodium burns in air and is explosive with water. we simply do not have nearly as much experience with sodium as we do water and that really cannot be understated. "
- nine_k 2y ago
  I suppose that "passive natural circulation cooling" means that plain convection of the coolant(s) is sufficient to cool the reactor, without involving pumps which could fail. Convection can't fail as long as there is coolant and no significant obstacles.
- UltraSane 2y ago
  Russia has been operating a sodium-cooled fast reactor for 45 years.
AnthonyMouse 2y ago
> If the fuel itself rearranges to become more compact, say by melting and flowing, the reactivity could increase. Wouldn't you just design the shape of the reactor so that if it got too hot for any reason, the shape it would melt into would result in a less compact geometry that would slow down rather than speed up the reaction?
- pfdietz 2y ago
  How do you do that in a way that's amenable to conclusive analytic demonstration? For example, how do you prevent melted fuel from flowing into the cooling channels that go through the core? About the only approach I'd be comfortable with would be dissolving the fuel in molten salt (probably chloride salt). This is not Natrium's approach. Melting of fuel is not a theoretical problem -- it has actually happened at two fast reactors in the US (EBR-1 and Fermi-1, the latter the reactor in the hyperbolically titled book "We Almost Lost Detroit"). No explosions occurred, but it's very troubling the fuel melted at all. The NRC will surely insist on analysis of the consequences of partial fuel melting accidents.
  - AnthonyMouse 2y ago
    To begin with you might start with a geometry which by design is already close to maximally compact, so that a geometry change would tend to go in one direction. > For example, how do you prevent melted fuel from flowing into the cooling channels that go through the core? Expect that to happen and use a geometry that doesn't cause the reaction to increase in speed if it does, e.g. because fuel flowing into the cooling channels would make the fuel less rather than more compact.
    - gpm 2y ago
      Slightly hesitant to jump in since pfdietz definitely knows more about this than I do... but... Cooling typically means things like maximizing surface area, minimizing the thickness of the object being cool, etc. Maximum neutron density presumably happens in a sphere, which is coincidentally the shape that minimizes surface area. The whole point of a nuclear reactor is that it heats up, and you can convert that heat into useful fuel. Presumably that means you need to carry quite a lot of heat away per volume. Presumably that means putting the fuel into a spherical shape really doesn't work that well.
credit_guy 2y ago
It's not quite like that. > An atomic bomb is a prompt fast supercritical system. Yes, it is, but more precisely it is a fast hypercritical system. The difference is this: in a supercritical system the ratio of neutrons in a new generation vs the old generation (generally denoted by k) is higher than one. For example 1.001 qualifies as supercritical. In a hypercritical configuration, k is higher than 2. In nature things generally evolve in a continuous way. To get to 2 you need to first get to 1 and 1.001, for example. For a bomb, it is very important to get to 2 extremely quickly, so that the assembly does not start the chain reaction while k is 1.001, or 1.1. To do that, you use either a gun or high explosives, to quickly reassemble a subcritical configuration into a hypercritical one, before a stray neutron has a chance to start a barely supercritical reaction (also known as a "fizzle"). With a reactor, there's no way to get an atomic bomb effect, because you go through the point of k=1.001. When you get there, there are plenty of neutrons around that this results in a "fizzle". Heat is produced and the core dilates and the ratio becomes low again. This results in "prompt excursions", each one lasting less than one second, each one some sort of a "bomb fizzle". These excursions are annoying, but they are not Hiroshima. > I will not be surprised if their approach ends up being unlicensable in the US because safety cannot be assured by analysis under accident conditions. What you are saying is a Catch-22. NRC can't approve a new design because it doesn't know how it performs under accident conditions, but then you can't know how something performs under accident conditions if you don't build it, and you don't build it if NRC does not approve it. The fact is, Russia has been operating the BN-800 sodium-cooled fast reactor for 10 years [1], and the smaller BN-600 for 45 years. So sodium-cooled fast reactors can work, and Russia likes them enough that it plans to build a bigger version, BN-1200. They don't explode. You could say "not yet", and sure thing, this means that the NRC needs to work hard to cover all the bases. But this is not impossible. [1] https://en.wikipedia.org/wiki/BN-800_reactor https://en.wikipedia.org/wiki/BN-800_reactor
- deleted 2y ago
  [deleted]
- pfdietz 2y ago
  You are assuming that as the core starts to expand, it doesn't expand in a way that increases k (for example, suppose liquid sodium is violently expelled from collapsing coolant channels; this could increase k). Can this be assured? Because if not, the yield could become very large. Granted, this doesn't seem likely, but can you assure it's impossible, in any possible accident, and any possible geometry of fuel and voids? > What you are saying is a Catch-22. NRC can't approve a new design because it doesn't know how it performs under accident conditions, but then you can't know how something performs under accident conditions if you don't build it, and you don't build it if NRC does not approve it. Even if you build it, and even if there is an accident, you don't know how it behaves under accident conditions. You just know how it behaved in that particular accident. It might be easier for developers if we could build thousands a fast reactors and let some of them explode to get some statistics on what works, but that's not how the NRC is arranged, and even that would not ensure all the catastrophic risk had been retired.

thecopy 2y ago

Based on your comment it sounds unreasonable to select this design. It must have some reason to exist?

pfdietz 2y ago
Fast reactors do have some attractive features. They have better neutron economy and work better with plutonium. They can achieve breeding ratios comfortably above 1 with the U-Pu system. They produce less actinide waste since the chance of neutron capture not causing fission is lower, and can more effectively destroy actinide waste. Sodium-cooled fast reactors will operate at higher temperature than LWRs, enabling the salt thermal storage scheme they propose to use. In large reactors, these features have not been enough to compensate for the disadvantages and sodium-cooled reactors have not been successful, coming in more expensive than light water reactors for a given power output. France, which had been developing fast reactors, has recently mothballed the effort with no plan to restart before 2050.
- UltraSane 2y ago
  Russia has two of them, one 800MW and one 600MW, and plan to build a 1.2GW version.
  - pfdietz 2y ago
    Even the Russians admit their LWRs are cheaper.
aidenn0 2y ago
The main advantages of using a fast reactor are less nuclear waste and more energy for a given fuel input.
- nine_k 2y ago
  Even burning some of the "nuclear waste" which is just nuclear fuel than needs refining.
UltraSane 2y ago
It can make more fuel than it uses. It needs enriched uranium at startup but then can convert regular uranium and/or thorium to usable fuel.

manvillej 2y ago

reddit had a nice list of the pros and cons: https://www.reddit.com/r/NuclearPower/comments/17k0wcc/natrium_reactor_breakdown/k75bj99/ https://www.reddit.com/r/NuclearPower/comments/17k0wcc/natri... I understand the risks around sodium, but the "passive natural circulation cooling" I don't understand. Is it more feasible with this design and why? " Pros: high temperature means we can use process-heat which is a much more efficient use of heat. fast spectrum neutrons means we can burn importantly troublesome parts of nuclear waste. fast spectrum is also better for breeding new fuel, significantly increasing how much energy we can extract from uranium/thorium. passive natural circulation cooling is much more feasible. Cons: fast spectrum is a little more complicated to control. fast reactors require high enrichment. inspection of the plant is very difficult with liquid metal. high temperature liquid metal doesn't play nicely with metal pipes. sodium burns in air and is explosive with water. we simply do not have nearly as much experience with sodium as we do water and that really cannot be understated. "

nine_k 2y ago
I suppose that "passive natural circulation cooling" means that plain convection of the coolant(s) is sufficient to cool the reactor, without involving pumps which could fail. Convection can't fail as long as there is coolant and no significant obstacles.
UltraSane 2y ago
Russia has been operating a sodium-cooled fast reactor for 45 years.

AnthonyMouse 2y ago

> If the fuel itself rearranges to become more compact, say by melting and flowing, the reactivity could increase. Wouldn't you just design the shape of the reactor so that if it got too hot for any reason, the shape it would melt into would result in a less compact geometry that would slow down rather than speed up the reaction?

pfdietz 2y ago
How do you do that in a way that's amenable to conclusive analytic demonstration? For example, how do you prevent melted fuel from flowing into the cooling channels that go through the core? About the only approach I'd be comfortable with would be dissolving the fuel in molten salt (probably chloride salt). This is not Natrium's approach. Melting of fuel is not a theoretical problem -- it has actually happened at two fast reactors in the US (EBR-1 and Fermi-1, the latter the reactor in the hyperbolically titled book "We Almost Lost Detroit"). No explosions occurred, but it's very troubling the fuel melted at all. The NRC will surely insist on analysis of the consequences of partial fuel melting accidents.
- AnthonyMouse 2y ago
  To begin with you might start with a geometry which by design is already close to maximally compact, so that a geometry change would tend to go in one direction. > For example, how do you prevent melted fuel from flowing into the cooling channels that go through the core? Expect that to happen and use a geometry that doesn't cause the reaction to increase in speed if it does, e.g. because fuel flowing into the cooling channels would make the fuel less rather than more compact.
  - gpm 2y ago
    Slightly hesitant to jump in since pfdietz definitely knows more about this than I do... but... Cooling typically means things like maximizing surface area, minimizing the thickness of the object being cool, etc. Maximum neutron density presumably happens in a sphere, which is coincidentally the shape that minimizes surface area. The whole point of a nuclear reactor is that it heats up, and you can convert that heat into useful fuel. Presumably that means you need to carry quite a lot of heat away per volume. Presumably that means putting the fuel into a spherical shape really doesn't work that well.

credit_guy 2y ago

It's not quite like that. > An atomic bomb is a prompt fast supercritical system. Yes, it is, but more precisely it is a fast hypercritical system. The difference is this: in a supercritical system the ratio of neutrons in a new generation vs the old generation (generally denoted by k) is higher than one. For example 1.001 qualifies as supercritical. In a hypercritical configuration, k is higher than 2. In nature things generally evolve in a continuous way. To get to 2 you need to first get to 1 and 1.001, for example. For a bomb, it is very important to get to 2 extremely quickly, so that the assembly does not start the chain reaction while k is 1.001, or 1.1. To do that, you use either a gun or high explosives, to quickly reassemble a subcritical configuration into a hypercritical one, before a stray neutron has a chance to start a barely supercritical reaction (also known as a "fizzle"). With a reactor, there's no way to get an atomic bomb effect, because you go through the point of k=1.001. When you get there, there are plenty of neutrons around that this results in a "fizzle". Heat is produced and the core dilates and the ratio becomes low again. This results in "prompt excursions", each one lasting less than one second, each one some sort of a "bomb fizzle". These excursions are annoying, but they are not Hiroshima. > I will not be surprised if their approach ends up being unlicensable in the US because safety cannot be assured by analysis under accident conditions. What you are saying is a Catch-22. NRC can't approve a new design because it doesn't know how it performs under accident conditions, but then you can't know how something performs under accident conditions if you don't build it, and you don't build it if NRC does not approve it. The fact is, Russia has been operating the BN-800 sodium-cooled fast reactor for 10 years [1], and the smaller BN-600 for 45 years. So sodium-cooled fast reactors can work, and Russia likes them enough that it plans to build a bigger version, BN-1200. They don't explode. You could say "not yet", and sure thing, this means that the NRC needs to work hard to cover all the bases. But this is not impossible. [1] https://en.wikipedia.org/wiki/BN-800_reactor https://en.wikipedia.org/wiki/BN-800_reactor

deleted 2y ago
[deleted]
pfdietz 2y ago
You are assuming that as the core starts to expand, it doesn't expand in a way that increases k (for example, suppose liquid sodium is violently expelled from collapsing coolant channels; this could increase k). Can this be assured? Because if not, the yield could become very large. Granted, this doesn't seem likely, but can you assure it's impossible, in any possible accident, and any possible geometry of fuel and voids? > What you are saying is a Catch-22. NRC can't approve a new design because it doesn't know how it performs under accident conditions, but then you can't know how something performs under accident conditions if you don't build it, and you don't build it if NRC does not approve it. Even if you build it, and even if there is an accident, you don't know how it behaves under accident conditions. You just know how it behaved in that particular accident. It might be easier for developers if we could build thousands a fast reactors and let some of them explode to get some statistics on what works, but that's not how the NRC is arranged, and even that would not ensure all the catastrophic risk had been retired.