Comment by credit_guy
14 days ago
> but the larger volume of TRISO fuel means the relative cost of dealing with it will be larger than their cost of dealing with spent LWR fuel
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.
The pressure of a HTGR is lower, but because the temperature is higher, more expensive materials are needed. A LWR pressure vessel is within the creep limit of ordinary steel; HTGR outlet temperature is well above that limit (and I suspect in accident conditions the temperature goes even higher for passive dissipation of decay heat). This especially bites in applications proposing to use that high temperature industrially, such as in thermochemical water splitting.
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.
I guess we shall see how China's HTR enterprise goes. The Germans had a lot of problems with the AVR and the THTR. Probably part or why they ditched nuclear altogether. They also tend to overengineer things and add too much unecessary complexity, which is exactly what they did with the THTR. I'm also quite skeptic of carbon moderated reactors, the graphite blocks tend to crack, the pebbles create dust which can block the coolant flow and the graphite reflectors also crack and get contaminated with Cs/Sr.
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...
> because the temperature is higher, more expensive materials are needed
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...
[2] https://en.wikipedia.org/wiki/Marine_grade_stainless
Well, requiring a material to withstand a higher temperature is an added constraint, so of course it's going to increase cost. Otherwise, the more exotic material would have been used in the lower temperature application too.
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.
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