It's that the pressure of water either in liquid or supercritical form gets too high.
Material issues for fast and/or high temperature reactors are not trivial but don't look insurmountable. 1980s literature seemed to think fast reactors could be longer-lived than LWRs, LWRs look longer lived today than they did back then.
Material issues for fast and/or high temperature reactors are not trivial but don't look insurmountable. 1980s literature seemed to think fast reactors could be longer-lived than LWRs, LWRs look longer lived today than they did back then.