Some of China's new reactors are pebble-bed reactors, which I just think are super-cool: https://en.wikipedia.org/wiki/Pebble-bed_reactor
Unlike in traditional LWRs, the fuel is integrated with a solid moderator. All that graphite goes along with the spent fuel. The volume of the spent fuel is therefore much larger than the volume of spent fuel from a LWR. Storage casks will be proportionally more expensive.
TRISO fuel is also more expensive to manufacture than traditional LWR fuel.
[1] https://en.wikipedia.org/wiki/Waste_Isolation_Pilot_Plant
Ah, so the money a utility spends on its spent fuel is just imaginary? It's not actually being spent? And if more is spent (as TRISO fuel would require) that's all financial figments too? What one learns.
I will add that any graphite moderated reactor will have larger carbon-14 production than a LWR, due to neutron capture on carbon-13 (as well as (n,p) reactions on nitrogen-14 impurities in the graphite.) In the west this renders "spent" graphite into intermediate nuclear waste, even ignoring fission products in the fuel particles. It also means that one cannot just burn off the carbon and release the filtered CO2 in any process that reprocesses TRISO fuel.
On the plus side, TRISO fuel doesn't have the same issue with zirconium availability that traditional LWR fuel has.
Yes. But the main cost of a reactor comes from the pressure vessel that the reactor is in. A LWR works at a pressure of about 160 bar (i.e. 160 times higher than the atmospheric pressure). The pressure inside a helium cooled reactor is about 50 bar. The cherry on the cake is that helium cooled reactors have much higher thermal efficiency (40% vs 30%). They can in principle even be used to produce hydrogen, in a much more efficient way than water electrolysis.
In the US there is the Xe-100 design. But I doubt it will be commercial in less than 15 years. The Kairos design also uses TRISO pebbles, it's a molten salt design. That solves the pressure vessel problem even better. Still 15 years out probably.
Also, I understand the passive safety of HTGRs is achieved by reducing the core thermal power density (and hence power density of decay heat). So for a given power, that core and pressure vessel will be much larger than in a LWR. If I'm reading a reference properly the ratio of power densities here is more than a factor of ten, which will more than balance the lower pressure and higher thermal efficiency. Material requirements for a pressure vessel scale as pressure x volume.
Is that a fact or an educated guess? Nuclear reactors do not use ordinary steel, they use nuclear grade steel. I don't know much about steel, but with a bit of googling I found out that the 316L steel (on of the 2 most common nuclear grade steels, the other being 304L) has much higher creep temperature than ordinary steel. This is the steel used in making the pressure vessels of PWRs and BWRs.
The steel used in the HTR-10 (the precursor of the 2 HTR-PM rectors that China recently hooked to the grid) has a composition [1] that seems to me to be almost identical to the 316L [2]. The HTR-10 has an average helium temperature at the outlet of 700°C.
Do you have some more concrete sources of information that would indicate the steel in HTGR is different and significantly more expensive than the steel in PWRs?
[1] https://www-pub.iaea.org/MTCD/publications/PDF/te_1382_web/T...
But that's all secondary to the observation you ignored, that the apples-to-apples comparison you're trying to make ignores power density, and when that's included HTGRs require much heavier pressure vessels.
This reasoning sounds good in theory, but practice and theory sometimes differ. The steel in reactors is subject to neutron embrittlement. In order to avoid that, you need special steel. Which happens to also withstand much higher temperatures. So no, it's not an additional constraint.
> that the apples-to-apples comparison you're trying to make ignores power density.
The strange thing is that the second part of your argument had nothing to do with TRISO. Not sure why you brought it up. You said that this is a tradeoff of passive safety. If you want passive safety, all things being equal, you need to accept lower power density.
However, all things are not equal. When you use TRISO as a fuel, that can withstand absolutely huge temperatures, in excess of 2000°C. Since radiative cooling is governed by the Sefen-Boltzman equation (i.e. the radiated power is proportional to the 4th power of the temperature), if the pebbles reach high temperatures, they get rid of a lot of heat by radiation, which is then absorbed by the steel pressure vessel and conducted outside.
Additionally, when you use TRISO in the form of pebbles, you recirculate the pebbles. You take them out and let them "rest", and then put them back later, so you limit the decay heat.
Anyway, enough theory, let's look at actual numbers. The American designs Xe-100 and NuScale are perfect for our comparison. They both have a thermal output of 200 MW. NuScale is a PWR and has a lower efficiency, so the electrical output is 60 MW, while the Xe-100 produces 82.5 MWe. NuScale's reactor pressure vessel has a volume of 129 m3, and Xe-100 has 390 m3, so larger by a factor of 3. If we only care about the electricity output, then Xe-100 needs 4.73 m3/MWe, while NuScale 2.15 m3/MWe, the ratio is only 2.15.
However, because the Xe-100 has to withstand much lower pressures, the mass of the pressure vessel is 274 tons, vs 260 tons for NuScale. Per MWe, the Xe-100 vessel is actually 23% lighter.
https://en.wikipedia.org/wiki/Pebble-bed_reactor
I saw the 4500 US$/kW cost figure for the HTR-PM. For the record, the AP1000 cost the Chinese around $2,000/kW in overnight capital costs (for the Sanmen and Haiyang plants). In the US the AP1000 construction costs are currently estimated around $6,800/kW and $4,500/kW for the following installed 10th unit.
https://web.mit.edu/kshirvan/www/research/ANP193%20TR%20CANE...
[0]: https://www.spiegel.de/spiegel/vorab/juelich-entsorgung-radi...