The Exxon Nuclear Company, Inc., acting as a subcontractor to EG &G Idaho Inc., Idaho National Engineering Laboratory, Idaho Falls, Idaho, conducted a laser beam welding study to attach internal stainless steel thermocouples into stainless steel upper end caps in nuclear fuel rods.The fuel rods and thermocouples are used to test simulated loss -of-coolant -accident (LOCA) conditions in a pressurized water reactor (LOFT reactor, Idaho National Laboratory).The objective of this study was to determine the feasibility of laser welding a single 0.063 inch diameter stainless steel (304) sheathed thermocouple into a stainless steel (316) upper end cap for nuclear fuel rods.A laser beam was selected because of the extremely high energy input in unit volume that can be achieved allowing local fusion of a small area irrespective of the difference in material thickness to be joined.A special weld fixture was designed and fabricated to hold the end cap and the thermocouple with angular and rotational adjustment under the laser beam.A commercial pulsed laser and energy control system was used to make the welds.Process tests were performed and evaluated. Results indicated that successful welds could be made using the laser system.A process qualification was written specifying laser energy, spot size and weld overlay.This program was successfully concluded by welding seventeen (17) centerline fuel rodsfor the LOFT reactor.