Complex sentence: It has caused difficulties in reactors (e.g. USS Seawolf (SSN-575), Monju), although some sodium-cooled fast reactors have operated safely (notably the Superphénix).
Split into simpler sentences: It has caused difficulties in reactors (e.g. USS Seawolf (SSN-575), Monju). Although some sodium-cooled fast reactors have operated safely (notably the Superphénix).