Design and choice of materials for fast reactors with sodium heat-transfer agent

Abstract

INTRODUCTIONMaterials for nuclear power plants with sodium heat-transfer agent should provide operation at sodium temperatures up to 600~ for 30 years. In addition to the high temperature and high mechanical loads created by the pressure of the heat-transfer agent, the material is subjected to variable thermal stresses due to abrupt temperature jumps in service and irradiation by fast neutrons with an energy E>0.1 MeV with a fluence of up to 2x 1023cm -2 (for some units). Such materials should be corrosion resistant in the heat-transfer medium, i.e., sodium, water, and steam [1].In the present paper, we consider the main problems and methods of their solution in design of structural materials for fast reactors with sodium heat-transfer agent and the main service characteristics of the recommended steels. MATERIALS FOR SHELLS AND PIPELINESThe most important problems in design and choice of materials for welded structures for base shells and pipelines of fast reactors with sodium heat-transfer medium are connected with the need to provide(1) a sufficiently high level of mechanical properties in long-term operation at high temperature and under irradiation;(2) compatibility of various materials with the liquid metal that provides the heat transfer with allowance for the processes of decarburization, carburization, and nitriding; (3) long-term resistance to thermal loads (basically 2 x 105 h); (4) resistance to local fracture in the near-weld zone; (5) ease of manufacture in metallurgical production and welding.Taking into account the large sizes of the structures and the respective semiproducts (forgings, sheets, pipes) and the need for a considerable welding volume in assembly, we chose an austenitic steel as a base structural material for shell structures and pipelines. Pearlitic steels are not suitable for 389 the purpose because of the difficulties of their welding and heat treatment.Due to the low pressure of the liquid-metal, heat-transfer medium, the mechanical stresses in the shell of the reactor, pipelines, and other units are comparatively low. However, the good thermal conductivity of the liquid metal and the elevated service temperature determine the high level of stationary and cyclic thermal stresses caused by abrupt temperature changes in the case of emergency protection and other transient service regimes. The number of heat cycles can amount to 103 . Thermal fatigue is the most important factor to be taken into account in the evaluation of the strength of the structure.On the basis of experimental and theoretical data, we created a method for computing the structural strength with allowance for thermocycling loads. The performed computations have shown that austenitic steel with a yield strength g0.2 = 100-200 MPa under the operational conditions is suitable for the base equipment of fast reactors [2].We evaluated the working capacity of reactor units with allowance for the resistance of the metal to thermal fatigue by summing the damage [1 -3], i.e.,where N is the number of cycles before the appear...