Abstract: A heat pipe stablized specimen container for irradiation of specimens at substantially constant temperature within a liquid metal cooled fast reactor comprises a heat pipe containing a vaporizable substance such as sodium. The container is of double-walled construction with the gap filled with argon and at top of the container a volume of argon is trapped within a cavity of the liquid metal level within the container in such a way that retention of argon in this zone is not dependent on sealing welds in the structure of the container, the argon blanket in this zone affording thermal insulation at the top of the container and also around part of the heat pipe, viz an adiabatic section of the latter.