Efficient operation of the liquid nitrogen supply station for the cryogenic equipment of the pilot plant for tritium and deuterium separation

Full text: At liquid nitrogen temperature the materials lose their elastic properties and become brittle. Protecting the personnel working with liquid nitrogen becomes difficult and to avoid accidents special equipment is used as helmets, gloves, goggles, special footwear, etc. The liquid nitrogen can destroy insulation of electrical cables, and so short circuits and electrocution can occur. Objects made of carbon steel (like pipes, props, containers, pillars, metal roofs, etc) when cooled by a sufficient amount of cryogenic liquid can break down to minimal mechanical stresses. Exceedingly dangerous is liquid nitrogen entered and retained into carbon steel ducts carrying pressured gas (even et low pressure values), since their cooling at extremely low temperatures can provoke their explosion. Resulting pieces and fragments are dangerous for both personnel and equipment around. The gas components of atmosphere (oxygen, nitrogen, argon) have critical parameters which allow liquefaction only at high pressure. For this reason they are called 'permanent gases'. Consequently, transformed in cryogenic liquids in closed precincts these gases will get totally vaporised when the precincts are not properly thermally isolated. The resulting raised pressure can lead to precinct destruction. For instance such event happens when an amount of liquid nitrogen is isolated into an external pipe between two closed taps while a relief valve is not in place. In such conditions isolation of liquid nitrogen ducts is absolutely necessary. This report reviews various solutions for obtaining an efficient isolation. All thermal isolations aim at reducing the heat transfer. In cryogenics the heat transfer from environment to the fluid in liquid phase is an important factor affecting the efficiency and yield of the liquefaction system. Choosing the type of isolation depends essentially on the specific application. The factors which must be considered are the cooling power, weight, the complexity of the application and the costs. The cryogenic isolations may be of the following five types: isolations in high vacuum; vacuum multi-layer isolations (MLI); vacuumed powder isolations; isolations with homogeneous materials (cellular glasswool, poli-isocyanure foams); systems of isolation made of composite materials. The heat penetrates the system by radiation, conduction and heating by Joule effect in the afferent lines of sensors. From technical reasons such thermal infiltrations cannot be neither avoided or diminished. Assuming that in conditions of advanced vacuum the thermal conduction in gas is negligible, the addition of heat by thermal radiation should be calculated in order to determine the degree of thermal isolation achieved by MLI. Experimental investigation are to be conducted to find the most efficient screening methods in cryogenic systems. The MLI solution is frequently used and reducing the heating by thermal radiation at the walls of vacuumed coldbox, operated at temperature lower than 80 K was the target of our work. At the same time a mathematical modelling of the MLI was developed depending on the number of layers and the types of heat transfer