Abstract: Two cylindrical shells having different diameters and covered with a metallic heat-insulation are disposed respectively within the lower and upper portions of a pressure vessel, the upper shell being closed by a lid. A chamber filled with fluid in liquid phase up to the liquid-vapor interface and with gas above said level is formed by the shells between the vaporizable fluid and the internal vessel wall, a cold liquid seal being formed by the overlapping portion of the shells. Radial expansion of the shells is permitted by supporting means at the level of at least three through-passages and is absorbed by leak-tight means placed opposite to the through-passages.