The answer depends on how large the reactor is, and this then leads to the max permitted wall thickness for thermal stress considerations. There may also be other metallurgical considerations, depending on what reaction is proceeding in the reactor. I assume you are applying this to supercritical oxidation based on the 600C design temp.
The new creep resistant alloys P91, P92, P112 are some ferritic alloys that can be used for the pressure resistant boundary, and one might clad the interior with a stainless steel if the reaction requires such properities. There may also be hydrogen damage or other effects that need to be dealt with depending on the reactions.
One big issue is finding a foundry that can fabricate the vessel properly, given all the special metallurgical requirements of these new alloys. The shop will need to hire a metallurgist to confirm the shp procedures will retain the high allowable stresses that the code assumes the material has retained, but which can be degraded by improper fabrication procedures. The best bet is to have a large boiler mfr ( Alstom, BW, Fowster Wheeler, MItsui Babcock, etc) either fab it or review the fab procedure.