Multilayer insulation for cryogenic equipment of nuclear facilities

It is well known the fact that the most important problem in cryogenic systems is the heat leak and this study considers the major heat leak of the system, which do to the thermal radiation. The leakage of heat through thermal radiation in cryogenic apparatus can be reduced substantially by introducing multilayer insulations or buffer screens; otherwise load thermal radiation will be rapidly directed to the cryogenic liquid. The systematic studies are difficult because thermal conductivity of different materials depends strongly on temperature, so that each experiment must be carried out in conditions close to the real ones, involving the existence of the chamber with liquid helium and all related installations. This, together with the need to have room temperature before operation must be met for a time sufficiently long to achieve the necessary balance measurements, so to reach the condition to perform measurements in less time possible. Also, the cooling process should be resumed for each version of the geometry, composition and structure of layers to be tested what involves a major consumer of liquid helium lost through evaporation, resulting in very high costs of experimentation. The input parameters are boundary temperatures, number of layers, pressure values, layer emissivity and spacer conductivity. Both emissivity and conductivity are functions of temperature. The results based on a model have to be compared with the experimental data in order to obtain the best fit for the emissivity and spacer thermal conductivity dependence on temperature. From engineering and technological point of view an important parameter is also the layer density. (authors)