Binary Collision and Molecular Dynamics Simulation of Fe-Ni-Cr Alloys at Supercritical Water Condition

Abstract

Fe-Ni-Cr alloys are commonly used as pressure vessel (in-core) materials for nuclear reactors and have been classified as candidate materials for Supercritical Water-Cooled Reactors (SCWR). In service, the in-core materials are exposed to harsh environments: intense neutron irradiation, mechanical and thermal stresses, and aggressive corrosion prone environment which all contribute to the components’ deterioration. For better understanding of the mechanisms responsible for degradation of the Fe-Ni-Cr alloys (SS304, SS308, SS309 and SS316) under high neutron irradiation dose, pressure and temperature conditions as pertains in SCWR conditions, these alloys were examined using Binary collision and molecular dynamics simulations using (SRIM-TRIM code and LAMMPS, VMD codes) respectively. The neutron irradiation damage assessments were conducted under irradiation doses of 30 dpa (thermal neutron spectrum) and 150 dpa (fast neutron spectrum). The results indicated that more defects were generated in the fast neutron spectrum SCWR than in the thermal neutron spectrum, and the depth of penetration of neutron in the fast spectrum was (32.3 μm) about three times that of the thermal spectrum (~ 11.3 μm). The work revealed that there was a marginal difference of 97.18 % of the neutron energy loss in SS308 compared to 97.14 % in SS316 and SS309. The evaluation of mechanical deterioration revealed that Young’s Modulus, Ultimate Tensile Strength and the Breaking/Fracture Strength decreased with increasing temperature. The SS308 and SS304 two materials had very high ultimate tensile strengths and breaking strengths even at the temperature of 500 ºC. By linking the neutron damage assessment and the mechanical evaluation, SS304 and SS308 could be considered in the design of the SCWR pressure vessel and couplings since the SS308 was found to be least damaged by the neutron irradiation whiles SS304 had high breaking strength. However, further research is recommended on the two Fe-Ni-Cr alloys SS308 and SS304 on hydrogen embrittlement, swelling, creep, as well as corrosion studies upon interactions with supercritical water environment; an extensive testing and evaluation program is required to assess the corrosion effects on the material properties of these two materials.