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Pebble bed reactors are a bad idea in general. They will be HUMONGOUS because they need a large surface to radiate away the heat for the passive safety, so the
by cyberax 2y ago
Pebble bed reactors are a bad idea in general.
They will be HUMONGOUS because they need a large surface to radiate away the heat for the passive safety, so they can't be easily put into a containment building.
A core of a PWR plant is _tiny_ for the amount of power it produces (around 3GWt!), just around 5 meters in diameter and 15 meters in height.
The pebble bed reactor in the article (HTR-PM) is around the same size, but it produces a mere 0.25 GWt.
Pebbles themselves are also problematic, they tend to swell, crack, and they can't be reprocessed using the current technologies. They MASSIVELY increase the amount of waste.
- Grosvenor 2y agoSeems like that would be fine for places where you have < 0.25GWt energy needs, and need a safe power source. Like remote installations/towns. Antarctic research stations, etc.
- cyberax 2y agoPebble bed reactors are indeed researched as a source for process heat (e.g. for steel or concrete production). But I really dislike that. If you just need 250MW of power, then just use electricity sourced from a regular PWR for heating. It'll be cheaper.
- Retric 2y agoThe maximum temperature from nuclear reactors is far below what you need for steel production because otherwise you’d melt the steel used to construct them. Waste heat can be useful for district heating systems because houses don’t need to reach high temperatures, but few designs give you access to even 300C and nothing currently hits 1,000+C.
- cyberax 2y agoSorry, you're right. Pebble bed reactors can provide helium at around 600C (reasonably up to 700C, but that's at the limits of material science).
- pfdietz 2y agoAnything at 300 C or below can be provided from renewable sources using industrial heat pumps. Heat is very storable, compared to electrical energy, so if the end user needs heat they could make the heat when renewable energy is cheap and store it, potentially for very long periods. Some district heating systems involve seasonal storage, essentially artificially charged geothermal. Nuclear thermal would be competing with otherwise curtailed renewable energy, which can be very cheap on the margin.
- shepherdjerred 2y agoIs GWt a common abbreviation for gigawatt? I first read that as gigawatt-tons which is a… confusing unit
- howenterprisey 2y agoGigawatt thermal, as opposed to gigawatt electric. Gigawatt thermal is the heat your power plant makes, whereas gigawatt electric is the electricity that the heat is used to generate. They're not the same because not all the heat can be converted into electricity, and the percent of heat that gets converted varies from power plant to power plant.
- shepherdjerred 2y agoOh that makes total sense! Thanks for explaining
- retzkek 2y agoThere is a similar unit used for tracking nuclear fuel “burnup,” which is how much energy it produced: GWd/t (or GWd/MTU).
- freeone3000 2y agoSo for a 3GW pebble bed reactor, we’re looking at a core the size of small house instead of a master bedroom? I don’t see a huge difference here; it’s the same amount of everything else (cooling, pumps, turbines, security) since it produces the same amount of heat/power.
- cyberax 2y ago> So for a 3GW pebble bed reactor, we’re looking at a core the size of small house instead of a master bedroom? No, we're looking at a core the size of a small residential tower. Probably around 30 meters in height.
- JojoFatsani 2y agoThat’s not too big man
- pfdietz 2y agoIt is, when you consider the problem of dealing with spent pebbles. These are not like the fuel rods of a LWR that occupy a fraction of a much smaller volume. The dry casks to store them would be immense.
- Etheryte 2y agoOne room worth of radioactive waste vs a residential tower worth of radioactive waste is a big difference.
- JumpCrisscross 2y ago> One room worth of radioactive waste vs a residential tower worth of radioactive waste is a big difference Most nuclear waste is stuff near the reactor, not the fuel per se. And nuclear waste isn't the Armageddon stuff it's portrayed as in mass media. If this works, waste volume won't be an issue.
- 2y ago
- pfdietz 2y agoThe other problem is mechanical abrasion of the pebbles, creating radioactive dust. One of Germany's PBRs had to filled with concrete after it was defueled, they couldn't decontaminate it enough to dismantle it.
- deleted 2y ago[deleted]
- credit_guy 2y agoThis reactor indeed has lower power density than a PWR, but not by a factor of 12. I compared it with NuScale's reactor, which is a PWR SMR. Details about both can be found in [1]. The HTR-PM reactor pressure vessel has a volume of about 640 m3, and yields 105 MWe, while NuScale has a volume of 101 m3 and yields 77 MWe. The power densities come to be 6.1 m3/MWe vs 1.3 m3/Mwe, and the ratio is 4.7x. Still, this is a good price to pay for getting a meltdown-proof reactor. > Pebbles themselves are also problematic, they tend to swell, crack, and they can't be reprocessed using the current technologies. It is simply not true that pebbles tend to swell and crack. Quite the opposite happens: fuel elements in the current generation PWRs tend to swell, crack and burst. This happens because some fission products and decay products are gasses, such as xenon, kripton, radon. They build up in time and create internal pressure. The same happens inside the fuel kernels in the pebbles used in this reactor, but those kernels are specially built to withstand much higher internal pressures. Here's a relevant quote from [2]: > As the pressure vessel size is reduced, the more efficient spherical geometry can be adopted, and the required wall thickness drops dramatically to the point that a 35 μm SiC layer can indefinitely contain gas pressures in excess of 100 MPa. This compares to the main reactor steel pressure vessel which may go as high as 20MPa or the Zircalloy cladding which can have pressures up to 10s of MPa in limited conditions – temperatures far below what the ceramic pressure vessel tolerate As for the reprocessing part, I think you are jumping the gun. There is no reprocessing done in the US, at all, for any type of fuel. Even where reprocessing happens, as in France, the benefit is quite reduced. One can extract some plutonium and unburned uranium, but in the end that will allow you to extract maybe 10-20% more energy from the original amount of natural uranium. It will not make you extract one hundred, or 10 times, or even just twice as much energy. Reprocessing is simply not a game changer. It is not clear at all if it makes economic sense to build the highly complex facilities that do reprocessing, for the limited benefit. [1] https://aris.iaea.org/publications/SMR_catalogue_2024.pdf https://aris.iaea.org/publications/SMR_catalogue_2024.pdf [2] https://www.usnc.com/triso/ https://www.usnc.com/triso/
- yeknoda 2y agoPWR pressure is on order 2-3x higher than an HTGR, so that’s a 2-3x thicker vessel. So good savings there
- pcl 2y agoPWR evidently means Pressurized Water Reactor.
- klysm 2y agoI’m not convinced the size is actually a problem - is that the most constrained resource here? What do we gain from decreasing power density?