Many years ago I witnessed an APU wash on a Tristar which was performed on the stop cycle.The wash was performed with a chemical solution of 10% Rochem and 90% water - unfortunately the person who mixed the solution and filled the pressure pot was on leave and his relief filled the pot with 100% Rochem.There was a loud explosion accompanied by a large fireball and the turbines exited the jetpipe at high velocity, absolutely totalling the APU.Spectacular but expensive!