PURPOSE: To safely remelt the solidified sodium in a piping by successively mounting a heat insulating material of a limited length from the end of the sodium when the sodium included in the above-mentioned piping solidifies.

CONSTITUTION: First the heat insulating material 3 in the solidified part is removed when the internal sodium solidifies. Electric power is then supplied from power feed equipment 4 to a preheating heater 2 of the solidified part. After the above-mentioned conditions are fulfilled, the heat insulating material 3' of 300 to 1,000mm length is mounted to the end of the solidified part. The quantity that the heat of the heater diffuses into the atm. decrease and the heat is eventually transferred to the piping and the solidified sodium in the piping when the heat insulating material is mounted; therefore, the piping 1 of the heat insulating material mounting part and the internal solidified sodium heat up gradually. A thermocouple 7 is removed when the attainment of the m. p. of the metal sodium is confirmed by the thermocouple 7. The next heat insulating material 3" is mounted in succession to the adjacent mounting part and from this point on, the above-mentioned operation is repeated until finally the restoration as shown in Figs. is made. The remelting operation is thereby completed over the entire area.