PURPOSE: To prevent high pressure gas from entering a gap, to reduce heat input, and to prevent the occurrence of natural convection, by a method wherein a piston seal ring, sealing an annular gap formed between the outer drum of a pump and the hole part of an installation bed, is situated in the lower end position of the annular gap.

CONSTITUTION: A flexible support device 13, additionally serving as a cover body for an annular gap 5, is located between the outer periphery of the upper end of a pump outer drum 1A and an upper surface 2a of a roof slab. A position ring seal 14, sealing the annular gap 5, is situated in a position in the vicinity of the lower end of the annular gap 5, e.g., a lower end surface 2b of the roof slab. Thus, by means of the piston seal ring 14, high temperature gas is prevented from entering the annular gap 5 and heat input is decreased, and as a result, through prevention of the occurrence of natural convection, the pump outer drum 1A can be prevented from deformation due to thermal expansion. Further, since the piston seal ring 14 is only situated, structure is simplified, and since adhesion of liquid metal steam is decreased, washing during checking can be facilitated.