This particular design has a very low burn-up indeed, but that's not because of the TRISO fuel. The Chinese HTR-PM helium cooled reactor, that went live 2 years ago, has a burnup of 90 GWd/ton of uranium, which is enriched at 8.5% U-235 [1]. That is an exceptionally high burn-up rate.
The eVinci reactor has an exceptionally low burn-up rate: about 4 GWd/ton, despite using a much more enriched fuel,at 19.75% ([2], but note that this is just an estimate, Westinghouse did not disclose the actual burnup). Why? That's the price you have to pay to have a micro-reactor. The square-cube law says that for such a reactor the surface of the core is very high compared to its volume, so the neutron economy is extremely poor. The only way to make it work is to use highly enriched uranium. Uranium enriched to more than 20% is considered weapons grade, so commercial reactors need to use fuel below that limit, and 19.75 is basically as high as you can go.
TRISO fuel is actually a miracle of science. It addresses many problems of the current generation reactor fuels. Fission results in transmutation. By the very nature of the fission process, you end up with fission products in burned fuel. Some of these products are gases (like Xeon) and they create pressure, and when you have hundreds of thousands of fuel elements some will burst, resulting in fission product discharge in the cooling water. Nasty stuff. The fuel in TRISO is encapsulated in some poppy-seed-sized granules, and it can withstand immense pressures, so this bursting scenario just does not happen. In addition to that, they can withstand immense temperatures as well, and they are surrounded by graphite that has an exceptional heat conductivity, and is also a very good moderator. From the point of view of reactivity control, graphite is actually the best moderator out there, ahead of hydrogen, deuterium and beryllium.
> is difficult to re-process
That's not a problem. You just don't reprocess it.
> it has quite low utilization of enriched uranium
This reactor will have low utilization, as discussed, but not because of the TRISO fuel.
That is an often repeated but incorrect thing. At the current rate of utilization, the proven reserves of uranium would last for one hundred years. What people miss is the fact that elevating a reserve to the status of “proven” (from the lower status of “probable”) costs money. Mining companies spend this money in order to get loans. Loan interest rates are better if the collateral is “proven” reserves. But the financing needs of these companies are limited by the business opportunity, so there is no incentive to “prove” more than 100 years worth of consumption. If the demand increases however, immediately you will see an increase of “proven” reserves. In the end we can extract unlimited amounts of uranium from seawater for only about 5x the current market price. It sounds like a lot, but it translates in less than one extra cent per kWh. The average retail price of 1kWh in the US is about 15 cents.
The eVinci reactor has an exceptionally low burn-up rate: about 4 GWd/ton, despite using a much more enriched fuel,at 19.75% ([2], but note that this is just an estimate, Westinghouse did not disclose the actual burnup). Why? That's the price you have to pay to have a micro-reactor. The square-cube law says that for such a reactor the surface of the core is very high compared to its volume, so the neutron economy is extremely poor. The only way to make it work is to use highly enriched uranium. Uranium enriched to more than 20% is considered weapons grade, so commercial reactors need to use fuel below that limit, and 19.75 is basically as high as you can go.
TRISO fuel is actually a miracle of science. It addresses many problems of the current generation reactor fuels. Fission results in transmutation. By the very nature of the fission process, you end up with fission products in burned fuel. Some of these products are gases (like Xeon) and they create pressure, and when you have hundreds of thousands of fuel elements some will burst, resulting in fission product discharge in the cooling water. Nasty stuff. The fuel in TRISO is encapsulated in some poppy-seed-sized granules, and it can withstand immense pressures, so this bursting scenario just does not happen. In addition to that, they can withstand immense temperatures as well, and they are surrounded by graphite that has an exceptional heat conductivity, and is also a very good moderator. From the point of view of reactivity control, graphite is actually the best moderator out there, ahead of hydrogen, deuterium and beryllium.
> is difficult to re-process
That's not a problem. You just don't reprocess it.
> it has quite low utilization of enriched uranium
This reactor will have low utilization, as discussed, but not because of the TRISO fuel.
[1]https://aris.iaea.org/PDF/HTR-PM.pdf
[2] https://www.osti.gov/servlets/purl/1501599