Nightmare technological challenges at such high temperatures. A typical "steamer" spends one month in three offline for maintenance of the steam plant. These temperatures would be much higher.

USA Nuclear plants used to have the same one in three downtime ratio, until President Carter ordered them to build a 2nd set of steam handling equipment, so that the alternate set could be used while one set was being repaired. 

The only thing worse is liquid sodium breeder reactors.

Solar and Wind power are cheaper, especially when combined with Pumped Storage Hydroelectric power storage. (used by utilities since 1892, fully mature storage technology) 

Solar generation costs continue.

https://c1cleantechnicacom-wpengine.netdna-ssl.com/files/2016/07/Lazard-Solar-Wind-Prices-LCOE-3-1.png

Most new nuclear power projects in the USA are ending up in bankruptcy.

https://www.dominionenergy.com/about-us/making-energy/renewables/water/bath-county-pumped-storage-station