Manufacturing of molten salt reactor heat exchangers using inner hole welding

Inner hole welding was applied to weld UNS N10003 nickel-based alloy tubes (phi 13.72*1.65 mm) to heat exchanger tubesheet in the Thorium Molten Salt Reactor for the first time. Welding defects were eliminated after the welding technology optimization after a great deal of trials and errors. And proper inner hole welding procedure and good butt joints were obtained. Welding procedure qualification shows that the strength of the welded joints at room temperature, 650 degrees C and 700 degrees C was higher than the qualified values of the base metal. After the confirmation of exact welding conditions on full-size mockup, the first batch of two heat exchangers of Thorium Molten Salt Reactor was fabricated by using inner hole welding technics in the world. The tube bundles containing 58 tubes (116 joints) were welded to tubesheet successfully. Penetration and radiation inspection revealed all the welds had no defects and were 100% qualified. Finally the completed unit passed 0.85 MPa hydrostatic testing. The successful development and application of inner hole welding process also establish a good foundation for the manufacture of high-temperature heat exchangers of other reactors besides Molten Salt Reactor.