Abstract: According to the K13 catheter boron crystallization event in the E13 thermocouple column of the RIC system happened in a nuclear power plant,a sealing design was adopted. The sealing of K13 catheter had been simulated from the aspects of welding groove,welding bead,welding process,and shape of plug. The performances of joint were evaluated by nondestructive test,hydraulic test,macroscopic observation and microstructure of weld. The welded joint is free of defects,such as incomplete fusion and cracks,and the final sealing and welding process were determined. The practical application shows that the sealing and welding process are easy to be implemented,and the sealing ability is good. At the same time,it avoids the complicated processes produced by pumping out and inserting a large number of thermocouples,and it can be better controlled in the construction period and risk.