3 ms·

MSRs have some attractive features, but they also have significant drawbacks. The most pressing is that fissionable material is spread throughout the fluid, so

MSRs have some attractive features, but they also have significant drawbacks. The most pressing is that fissionable material is spread throughout the fluid, so fission and decay of fission products is occurring right up to the edge of the fluid. The walls and pipes containing the molten salt, and anything dipped into the salt, are exposed to unmoderated neutrons. One can shield using (say) graphite, but then damage to that (and soaking up of radioactive materials) become issues. The Molten Salt Reactor Experiment at Oak Ridge was near the end of its radiation exposure lifetime when the program ended. Contrast this to light water reactors. These are designed so that no lifetime component sees unmoderated neutrons. There's a thick barrier of water between the fissioning fuel and the reactor vessel wall and the support structures for the fuel bundles. The bundles themselves are exposed, but they are replaced for refueling and are not lifetime components.

cyberax 1y ago
To add to this, even with the shielding provided by water in light water reactors, the neutron exposure is _the_ limiting factor for the reactor vessel. The metric to look for is called "DPA" (displacements per atom), the number of neutron collisions that a material can tolerate before losing enough structural integrity to fall below the acceptable limits. The best modern reactor steels are at 150-180 DPA. And a lot of potentially cool reactors like TWR (travelling wave reactor) end up being logistically impossible because lifetime-limited components will be exposed to multiple hundreds of DPAs.
- jabl 1y ago
  Many old LWR's have had their reactor vessels heat treated during a maintenance break to undo some of that neutron radiation damage and extend the life of the reactor. Not sure whether it would be possible to do something similar to a liquid fueled reactor, including all the hot pipework. Maybe, but yet another cost. Notably some of the recent MSR projects propose replacing the entire reactor every now and then (Terrestrial or whatever they were called, not sure if they are still around).
  - HPsquared 1y ago
    I wonder if it's possible to run it hot enough for the radiation damage (basically a bunch of dislocations, right?) to just anneal itself out continuously, like how Wigner energy is dissipated in graphite when it's hot enough.
    - cyberax 1y ago
      No, this is fundamentally impossible with steel. Annealing works by making the material more "plastic", and this necessarily reduces its tensile strength. Which is the limiting factor for the vessel. You can make the vessel thicker to compensate, but then you can just make it thicker in the first place and skip annealing.
  - cyberax 1y ago
    Yes, it's called "annealing". Basically, the core is de-fueled and a huge electric resistive heater is put inside it. Then the entire vessel is heated to something like 600C, and kept there for several days. It helps the atoms displaced by neutron collisions to "snap back" into the correct places in the crystalline structure. But it can never restore the material completely, and over time the annealing breaks will have to be more and more frequent. It also can't be used for everything. Some pipes will experience large thermal stresses if annealed, and some components can't be heated properly due to complex geometry. As with everything in engineering, all problems can be solved with additional complexity. It's possible to design LFTR reactors to be more annealable, but it will likely make them impractically complex. There are also other issues with LFTRs. A significant part of the energy production will happen _inside_ the pipework carrying the molten salt, as delayed fission happens and daughter products decay. This will cause inevitable problems with the reactor power control. Modern light water reactors are engineering marvels. They are incredibly compact for the amount of power that they generate, and they are now designed with the anticipated 70-100 year operating lifetime. Getting LFTRs to the same level of maturity might be possible, but it'll require literally hundreds of billions (if not trillions) invested, just like with the classic nuclear.
    - pfdietz 1y ago
hunterpayne 1y ago
> One can shield using (say) graphite Oh dear god, no. Graphite is a very good moderator, it is in no way a shield. Those two properties are (sort of) opposites of each other. Lead makes the cheapest and best shield. Also, those parts that are exposed to neutron flux stay radioactive for about 10 years. So it shortens their lifetime in the reactor but the waste isn't a big issue.
- pfdietz 1y ago
  Lead is essentially useless as a shield for neutrons that are below the minimum excitation energy of a lead nucleus. Elastic neutron collisions with lead leave the neutron energy essentially unchanged.
  - hunterpayne 1y ago
    I assume this is why an alloy of lead is used in practice. Still doesn't change the fact that graphite is a moderator not a shielding material. Also, structural materials in reactors are usually invisible to neutrons and a sandwich of materials is often used. Different layers do different things. Usually, one layer of shielding and one layer of a material that isn't impacted (much) by neutron flux for structural strength. There is a rabbithole for almost all of these material choices, especially in nuclear. Not going down that rabbithole in a discussion targeted at folks who don't spend their lives working in nuclear doesn't make that person wrong. It makes them an effective communicator. PS Lead is a very very common shielding material in nuclear.
    - pfdietz 1y ago
      A moderator is a neutron shielding material, since it removes energy from the neutrons. That's what moderation is all about. Water is a much better moderator, but graphite still performs the function.
- rdtsc 1y ago
  > Oh dear god, no [...] Lead makes the cheapest and best shield. Oh my, definitely no :-) Do not use lead for neutron shielding. You're thinking gamma radiation but then we're talking apples vs oranges then. You want atoms comparable in size to neutrons, so something with plenty of hydrogen. Think water or PET (plastic) when you don't want water to "leak" when transporting a source. For thermal neutrons maybe PET impregnated with boron. Now neutrons may generate gamma when captured by hydrogen, then you may want some lead for secondary effects like that but I am not sure how strong those are.
nateglims 1y ago
I think it has a key advantage for China specifically though which is it consumes significantly less water and they have a lot of water poor territory. The oakridge experiment ended and not a lot of R&D has been done on salt reactors. It makes sense that China is still basically in research and testing phases for molten salts.

cyberax 1y ago

To add to this, even with the shielding provided by water in light water reactors, the neutron exposure is _the_ limiting factor for the reactor vessel. The metric to look for is called "DPA" (displacements per atom), the number of neutron collisions that a material can tolerate before losing enough structural integrity to fall below the acceptable limits. The best modern reactor steels are at 150-180 DPA. And a lot of potentially cool reactors like TWR (travelling wave reactor) end up being logistically impossible because lifetime-limited components will be exposed to multiple hundreds of DPAs.

jabl 1y ago
Many old LWR's have had their reactor vessels heat treated during a maintenance break to undo some of that neutron radiation damage and extend the life of the reactor. Not sure whether it would be possible to do something similar to a liquid fueled reactor, including all the hot pipework. Maybe, but yet another cost. Notably some of the recent MSR projects propose replacing the entire reactor every now and then (Terrestrial or whatever they were called, not sure if they are still around).
- HPsquared 1y ago
  I wonder if it's possible to run it hot enough for the radiation damage (basically a bunch of dislocations, right?) to just anneal itself out continuously, like how Wigner energy is dissipated in graphite when it's hot enough.
  - cyberax 1y ago
    No, this is fundamentally impossible with steel. Annealing works by making the material more "plastic", and this necessarily reduces its tensile strength. Which is the limiting factor for the vessel. You can make the vessel thicker to compensate, but then you can just make it thicker in the first place and skip annealing.
- cyberax 1y ago
  Yes, it's called "annealing". Basically, the core is de-fueled and a huge electric resistive heater is put inside it. Then the entire vessel is heated to something like 600C, and kept there for several days. It helps the atoms displaced by neutron collisions to "snap back" into the correct places in the crystalline structure. But it can never restore the material completely, and over time the annealing breaks will have to be more and more frequent. It also can't be used for everything. Some pipes will experience large thermal stresses if annealed, and some components can't be heated properly due to complex geometry. As with everything in engineering, all problems can be solved with additional complexity. It's possible to design LFTR reactors to be more annealable, but it will likely make them impractically complex. There are also other issues with LFTRs. A significant part of the energy production will happen _inside_ the pipework carrying the molten salt, as delayed fission happens and daughter products decay. This will cause inevitable problems with the reactor power control. Modern light water reactors are engineering marvels. They are incredibly compact for the amount of power that they generate, and they are now designed with the anticipated 70-100 year operating lifetime. Getting LFTRs to the same level of maturity might be possible, but it'll require literally hundreds of billions (if not trillions) invested, just like with the classic nuclear.
  - pfdietz 1y ago

hunterpayne 1y ago

> One can shield using (say) graphite Oh dear god, no. Graphite is a very good moderator, it is in no way a shield. Those two properties are (sort of) opposites of each other. Lead makes the cheapest and best shield. Also, those parts that are exposed to neutron flux stay radioactive for about 10 years. So it shortens their lifetime in the reactor but the waste isn't a big issue.

pfdietz 1y ago
Lead is essentially useless as a shield for neutrons that are below the minimum excitation energy of a lead nucleus. Elastic neutron collisions with lead leave the neutron energy essentially unchanged.
- hunterpayne 1y ago
  I assume this is why an alloy of lead is used in practice. Still doesn't change the fact that graphite is a moderator not a shielding material. Also, structural materials in reactors are usually invisible to neutrons and a sandwich of materials is often used. Different layers do different things. Usually, one layer of shielding and one layer of a material that isn't impacted (much) by neutron flux for structural strength. There is a rabbithole for almost all of these material choices, especially in nuclear. Not going down that rabbithole in a discussion targeted at folks who don't spend their lives working in nuclear doesn't make that person wrong. It makes them an effective communicator. PS Lead is a very very common shielding material in nuclear.
  - pfdietz 1y ago
    A moderator is a neutron shielding material, since it removes energy from the neutrons. That's what moderation is all about. Water is a much better moderator, but graphite still performs the function.
rdtsc 1y ago
> Oh dear god, no [...] Lead makes the cheapest and best shield. Oh my, definitely no :-) Do not use lead for neutron shielding. You're thinking gamma radiation but then we're talking apples vs oranges then. You want atoms comparable in size to neutrons, so something with plenty of hydrogen. Think water or PET (plastic) when you don't want water to "leak" when transporting a source. For thermal neutrons maybe PET impregnated with boron. Now neutrons may generate gamma when captured by hydrogen, then you may want some lead for secondary effects like that but I am not sure how strong those are.

nateglims 1y ago

I think it has a key advantage for China specifically though which is it consumes significantly less water and they have a lot of water poor territory. The oakridge experiment ended and not a lot of R&D has been done on salt reactors. It makes sense that China is still basically in research and testing phases for molten salts.