PURPOSE: To improve an assured state at a welding part in the case that a plug is welded and into a hollow hole generated by pulling out a water and steam flowing pipe and a helium flowing pipe from a water steam pipe plate and a sodium pipe plate and then the hole is closed by the plug.

CONSTITUTION: A water steam flowing pipe 12 and a helium flowing pipe 11 are cut at a higher position than a lower surface of an upper sodium pipe plate 4, the cut pipes 12 and 11 are pulled upwardly over an upper steam pipe plate 8 and then removed from an upper sodium pipe plate 4 and an upper steam pipe plate 8. Then, a lower end of a plug 31 is abutted against the upper surface of the upper sodium pipe plate 4 so as to close the hollow hole 4a and a welding is carried out at 32, and an upper end of a plug 35 is fitted into the lower end of a hollow hole 8a of the upper steam pipe plate 8 and then a welding is carried out at 36.