[en] A design study on the next generation advanced marine reactor MRX (Marine Reactor-x) has been developed in JAERI. The reactor is installed in the container which is filled with water. Thermal insulators are necessary for the reactor components, such as reactor vessel, pipings, etc. to reduce the heat loss and to secure the integrity of the components. A survey of the existing technology about the thermal insulating material and the entire process has been made. Design requirements of insulation are set up tentatively considering realization of In-servies Inspection and evaluating the capacity of decay heat removal in case of ship sinking. A simplified metal insulation (Millor Insulation) has been screened out and a conceptual design has been made. A study is also performed with a anti-corrosion method of the reactor vessel and the containments vessel. Outer wall of the reactor vessel to be contact with water should be protected by the arc welding of stainless steel. While coating the paint for BWR is appropriate for inner wall of the containment vessel. (author)