Justification of the Use of Liquid Nitrogen to Make Ice-Plugs in Carbon-Manganese Steel Pipes for Maintenance Purposes in Nuclear Plants

During maintenance operations in nuclear power plants, it is sometimes necessary to isolate temporarily sections of pipes. When no other mechanical device is available (floodgates, valves), the freeze sealing technique can be used. With a heat-exchanger surrounding a portion of the pipe and filled with liquid Nitrogen (−196°C), the water contained in the pipe is frozen, resulting in an ice-plug. During these operations, stresses of thermal origin are generated in the pipes, due to the thermal amplitude between the initial temperature of the pipes (40°C), and the coolant (−196°C). This is why it is necessary to study the resistance of the structure, in particular towards brittle fracture in case of the presence of a small defect in the material. The main purpose of this study, is to evaluate the maximum size of a defect acceptable, without leading to rupture of the structure while freezing. In order to reach this aim, a freezing test has been performed, and the measuresd temperatures on the surface of the tube have been used to conduct thermal and mechanical calculations. In places of maximum load, different defects have been postulated, and their nociviy has been calculated with RSEM simplified methods. This study takes place in the file concerning the justification of the cryogenic technique towards the French Nuclear Safety Authority. The evaluation of the maximum admissible defect is the first step of the demonstration, the second step will consist in studying the possible extensions to other configurations that can be met in a nuclear plant (materials, diameter, thickness), and to make a second ice-plug test in a Carbon-Manganese pipe where the maximum admissible defect has been machined, in order to show that it is resistant to the freezing with liquid Nitrogen.