Welding of fuel element Zr-4 end plug to tantalum tube

The high performance gadolinium-containing fuel element of AC-600 reactor needs to complete the in-reactor irradiation test before it is put into formal use, which provides a reliable basis for the core design.

The core temperature of fuel element pellets is one of the main parameters, which is the key data in the analysis of the interaction between fuel element pellets and cladding, the release of fission gas, the thermal performance of oxide fuel, irradiation creep and elongation. In the test of irradiation, the tungsten rhenium thermocouple is inserted into the tantalum tube. With the tantalum tube inserted into the center of the component hollow core, the core temperature of the core is directly measured. On the one hand, the tantalum tube protects the Tungsten-Rhenium thermocouple, and on the other hand, after welding with the Zr-4 end plug, it can avoid the fission gas escaping from the fuel cladding.

Tantalum is resistant to corrosion by various liquid metal coolants and has sufficient strength to eliminate any irradiation damage due to the annealing effect under irradiation. Under long-term operating conditions in the reactor, the fuel Zr-4 cladding directly contacts the PbBi alloy. The technical requirements have been solved and adopted in the in-reactor foraging test of the plate-type fuel element. However, there is a lack of relevant data on the welding technology of tantalum tube and Zr-4 cladding and the corrosion performance of the welding seam in the molten PbBi alloy, and there is no research report in China. This has become one of the key technologies that must be solved in the measurement of core temperature of fuel element. The thin wall tantalum tube and Zr-4 material are welded with two different metals. The corrosion behavior of the welding seam in the molten PbBi alloy can realize the temperature measurement of the fuel element center.