Temperature and Fluence Limits for a Type 316 Stainless-Steel Controlled Thermonuclear Reactor First Wall

Abstract

Results of a series of neutron irradiation experiments conducted on annealed and 20% cold-worked Type 316 stainless steel in a high-flux mixed-spectrum fission reactor to simulate a controlled thermonuclear reactor (CTR) first wall displacement per atom (dpa) and helium production are reviewed. Using previously suggested criteria of a maximum of 10% volume increase and a minimum of 0.5% uniform strain in a uniaxial tensile test, estimates of temperature and fluence limits for this alloy are made. The large amounts of helium produced by irradiation in the mixed-spectrum fission reactor caused significantly more swelling than occurred in fast reactor irradiations (low helium-generation rates). Cold working effectively suppressed swelling up to 550 to 600°C. Using a criterion of 10% swelling and limited data on the fluence dependence of swelling, a first wall life of 16.5 (MW yr)/m² (at 530°C) for 20% cold-worked Type 316 stainless steel is estimated. Embrittlement may be the property that limits first wall life. At 350°C acceptable ductility was retained in the cold-worked steel to very high damage levels (49 dpa, 3320 appm helium), and it appears that the 0.5% uniform strain criterion will not be limiting. At higher temperatures, however, this is not the situation. At 650°C the uniform and total plastic strain were zero in samples irradiated to 61 dpa and 4140 appm helium. At 575°C, 0.5% uniform strain was retained in the cold-worked material to relatively high damage levels; however, the fractures were intergranular. The creep-rupture life at 550 and 45 000 psi was reduced by 5 × 10⁴ compared to the unirradiated property. Generally greater embrittlement in the solution-annealed material suggest that cold-worked material would be preferred for CTR first wall structures. The marked reduction in ductility and rupture life and intergranular tensile fractures suggest that stress will have to be maintained at very low levels to prevent fracture. The loss of ductility indicates reductions in fatigue life that must be investigated.