St. Vrain was helium cooled and graphite moderated and it only ran at 1430F, which still caused some hair-raising operational problems: https://www.nrc.gov/docs

St. Vrain was helium cooled and graphite moderated and it only ran at 1430F, which still caused some hair-raising operational problems: https://www.nrc.gov/docs/ML0403/ML040340070.pdf https://www.nrc.gov/docs/ML0403/ML040340070.pdf High temps are very good for efficiency, but are difficult for safety. Only China is building VHTRs right now. If anyone ever builds a supercritical water reactor then it would give you decent efficiency without the problems of exotic coolants.