> As the temperature rises, neutron absorption increases, reducing fission and thus temperature.
Negative fuel temperature coefficient is not an unusual feature.
The real question is whether the heat removal system of the reactor as a whole is sufficient to remove the decay heat to keep the fuel within the limits.
I remember talking to an engineer at the old GE nuclear research facility in San Jose. He said you can design reactors to be cooled by natural convection.
> He said you can design reactors to be cooled by natural convection.
That's the point of the Westinghouse AP1000; the containment (steel liner) and protection from the outside world (concrete wall) are separated, allowing the liner to cool by convection and water dripping from above. Admittedly you need to top up the water tank at the top, but that is less of a task than trying to push water into the containment.
It just makes them larger. And it makes the building containing them larger. And this makes them more expensive.
NuScale's reactor was originally motivated by the desire to make it safer by using natural convection. But it ends up requiring 1/3rd more labor hours to build a NPP using their reactors than it does to build a conventional large reactor power plant.
The reactor vessel is humongous, so the natural convective cooling can carry away the decay heat. The pebbles themselves can tolerate extremely high temperatures (literally glowing white-hot) without burning.
Can pebble beds have a cooldown pan similar to a LFTR, where a plug melts and the "pebbles" fall and spread into a pan where they won't stay critical because they are too separated / unconcentrated?
Because the real problem with solid rods is that they ... are solid rods, and if they start "overreacting" you can't split up the rods, unlike a pile of pebbles/spheres.
The unique "melt plug" safety story of LFTRs is mostly a fairy tale.
Modern PWRs also have this safety feature, if a core melts down, the molten mass will be contained in a core catcher. Where it'll be mixed with inert material that can provide enough surface area and thermal mass to prevent further fuel mass migration.
The biggest problem in the core catcher design was to make sure that the molten fuel lava spreads out enough for the passive cooling to stop it from melting through concrete.
Pebble bed reactors will have a similar problem. You can "drain" pebble beds somewhere, but then you need to make sure that this "somewhere" can conduct away the decay heat without melting.
A failure that results in the plug melting will mean that the reactor is beyond salvaging. It's essentially the same situation as with PWRs equipped with core catchers.
> can't have a pool of cooling liquid
What "liquid"? Water?
Do you realize what's going to happen if molten salt drops into water? First, there's going to be a steam explosion that will atomize the fuel and spread it through the whole containment building (because the water vapor can't be contained in a reasonable volume).
Then the water will boil away from decay heat, and the fuel lava will continue chewing through your reactor building.
That's why LFTR reactors with "melt plugs" will essentially use the same approach as PWRs: spread the molten fuel across sacrificial concrete cladding.
Gee, I dunno, maybe any of a thousand different materials/chemical reactions to absorb heat? Yeah, I didn't say water. Maybe you just have the dump pool be a bunch of molten thorium or even solidfied thorium salt and that also plummets the neutron economy as the neutrons get absorbed by thorium as the hot uranium salt melts the thorium salts. Maybe you keep the thorium salts liquid.
But fundamentally since the nuclear fuel is a liquid, the cooling pool is very wide and shallow, separating the fuel well past sustained nuclear fission. This is the problem with solid rods, they go runaway, the fuel is a solid rod, you can't separate apart the fuel without shoving some moderator into it. Yeah I don't know the viscosity of Uranium tetrafluoride, maybe that's a problem, but I doubt it.
Why does the plug only melt past some point of no return? The plug can melt at whatever temperature point is desired. A "meltdown" can just be part of the usual fuel flow and recirculation.
The difference is that the liquid spread because, you know it is a liquid. In liquid form? Rather than a solid form or some semi-solid uranium lava. See the difference? What am I missing here?
In the dump pool, it will cool into solid salts. Then you just need to reheat it to recover the fuel, and pass it back through the salt reprocessing systems.
Plus I thought LFTRs have some mechanism for self-moderation by expansion of the fluid when it gets hot, separating the fuel apart and reducing neutron economy.
You seem stuck in the limitations of solid fuel rods, solid fuel reactors, and their inherent inconvenience. Yeah, the liquid is 650 degrees or something like that, but it's still liquid and you can do things with liquids that you can't do with solid rods
> Gee, I dunno, maybe any of a thousand different materials/chemical reactions to absorb heat?
Well, I definitely don't know which reaction can absorb on the order of 2GWh of residual decay heat within the first 2 days.
I'm assuming a 3GWt reactor, something that at least can be competitive with PWRs.
To give you some perspective, this amount of energy is enough to vaporize more than 3000 tons of water. More than an Olympic swimming pool.
> Maybe you just have the dump pool be a bunch of molten thorium or even solidfied thorium salt and that also plummets the neutron economy as the neutrons get absorbed by thorium as the hot uranium salt melts the thorium salts. Maybe you keep the thorium salts liquid.
Sigh. It's not the fission that is a problem. Fission will be quenched by all the neutron poisons. Even in Chernobyl or Fukushima the fission stopped immediately after the accident.
It's the decay heat that has to be conducted away.
> Why does the plug only melt past some point of no return? The plug can melt at whatever temperature point is desired. A "meltdown" can just be part of the usual fuel flow and recirculation.
If normal recirculation works, then there's no problem with supplying cooling water. The touted advantage of molten salt reactors is their passive safety, they are supposed to fail safe even if EVERYTHING fails.
> You seem stuck in the limitations of solid fuel rods, solid fuel reactors, and their inherent inconvenience. Yeah, the liquid is 650 degrees or something like that, but it's still liquid and you can do things with liquids that you can't do with solid rods
Yeah, because I actually worked in the nuclear power industry.
> Well, I definitely don't know which reaction can absorb on the order of 2GWh of residual decay heat within the first 2 days.
The sales pitch for salt-cooled reactors is the lack of any coolant that would become pressurized hot gas in an accident. Heat can stay in salt or in other low vapor pressure materials.
The problem with LWRs is the water goes to steam in accidents, and this steam must be contained. This drives the size of the containment building, and the containment building is costly.
An alternative for LWRs would be to filter and vent the steam instead of trying to contain it. This would allow small quantities of radioactivity to escape (including all the noble gas fission products), but the filtering can actually be quite good, reducing emissions by many orders of magnitude. Second generation filtered containment venting systems can filter iodine as well as cesium and strontium. If Fukushima had had such systems the impact would have been far lower.
> The sales pitch for salt-cooled reactors is the lack of any coolant that would become pressurized hot gas in an accident. Heat can stay in salt or in other low vapor pressure materials.
Sodium-cooled reactors and the upcoming lead-cooled reactor also have this property. It turns out to not be such a huge advantage, we have plenty of experience working with pressurized water.
> The problem with LWRs is the water goes to steam in accidents, and this steam must be contained. This drives the size of the containment building, and the containment building is costly.
No, it's really not a problem. The loop doesn't suddenly loose compression if something bad happens. If there's electric power, there's more than enough time to slowly cool down the reactor.
And a containment building (that also protects against external threats like an airplane ramming into the reactor) has more than enough volume if the primary loop is de-pressurized and the water flashes into steam.
> An alternative for LWRs would be to filter and vent the steam instead of trying to contain it.
The water in the primary loop is clean. It's constantly purified by filtration through ion exchange resins. Once the activated oxygen decays (in ~1 hour) you can swim in it (although I wouldn't drink it).
PWRs (actually, all thermal power plants) have areas where steam can be dumped. If you watched "Chernobyl" series, the ridiculous scene with divers was supposed to happen inside such an area ("barboter pool").
Modern PWRs are also designed to do that safely. There's plenty of capacity to condense all the water from the primary loop after the loss-of-cooling. Of course, after that the fuel will melt down, and chew through the reactor vessel.
The filtering system you linked is not strictly necessary for modern PWR designs. They will still be safe in case of an accident with total loss of cooling, but the containment building will be hopelessly contaminated internally. This filtering system can allow the steam to be vented into the atmosphere, perhaps giving more time to fix the emergency cooling systems.
> No, it's really not a problem. The loop doesn't suddenly loose compression if something bad happens. If there's electric power, there's more than enough time to slowly cool down the reactor.
It can in design basis accidents, for example a complete break of a main circulation pipe leading the loss of coolant (LOCA) into the containment. The emergency cooling system would then operate by spraying water into the core that would evaporate into steam that would go right out of the reactor vessel. The containment has to be sized for such an accident.
As an example of such an accident, consider what would have happened at Davis-Besse had the erosion of the lid of the reactor vessel progressed to an actual perforation. As it was, the steel was removed in an area down to the inner stainless steel liner, a liner that was never intended to be load bearing against the internal pressure.
> And a containment building (that also protects against external threats like an airplane ramming into the reactor) has more than enough volume if the primary loop is de-pressurized and the water flashes into steam.
Right, it does. That's why it's so big and expensive, with so much internal volume. If it didn't have to, it could be made much smaller. The airplane requirement doesn't change this; it's easier to make a smaller containment building resistant to aircraft impact than a larger one.
> The water in the primary loop is clean. It's constantly purified by filtration through ion exchange resins. Once the activated oxygen decays (in ~1 hour) you can swim in it (although I wouldn't drink it).
That's true in normal operation, where you might have some small number of fuel rods with cracks or perforations (but even that is getting pretty uncommon these days). It would not be true in a design basis accident, where some or all of the fuel may have partially or completely melted, and where cladding will have been compromised by high temperature reaction with steam. The design must assume essentially all the volatile fission products have gone into the water. At TMI, fission products carried in the water (and also noble gases) raised radiation levels in the containment building to 800 rem/h during the accident.
> The filtering system you linked is not strictly necessary for modern PWR designs.
I offered up the possibility that such systems could replace the large volume containment of modern systems (or at least reduce its size and cost). Sure, they're not obviously necessary if you have a large volume containment already (although some countries ended up requiring them anyway since some accident scenarios do involve venting, as happened at Fukushima, which admittedly had pre-modern designs.)
