Chandra had attempted to use the newly installed submersion heater unit (borrowed from CP3) using the existing SCR but continued to experience the "SCR SHORTED" trip.  This SCR had failed when used with the original submersion heater and may have been damaged then.  So, I borrowed a "virgin" SCR from CP6's regeneration control panel and installed it in CP4's control panel this evening.  We will review the schematic and SCR manual etc. and consult Richard M. before are next regeneration attempt so as to "put our heads together" in an effort to understand why these SCR units are failing.