The effects of very high neutron fluences on tensile properties and swelling of 300-series austenitic stainless steel were evaluated by destructive examination of several EBR-II thimbles that had accumulated fluences approaching 1.2 x 1023 n/cm2 (En > 0.1 MeV). Irradiation temperatures ranged from 370 to 470 C (698 to 878 F). Immersion density measurements on control or safety rod thimbles 3D1, 5C3, 5A3, and 3A1 indicate that swelling increases with neutron fluence. Maximum measured volume changes are about 11 percent at 1.1 x 1023 n/cm2 (En > 0.1 MeV) and temperatures near 420 C (788 F). No indications of saturation were observed; in fact, swelling rates increase with increasing fluence over the whole range of fluences investigated. Anomalous swelling behavior was observed in control rod thimble 5A3. In this component, which may have been subjected to mechanical constraint, swelling gradients were found to be much lower than anticipated on the basis of the corresponding gradients in irradiation conditions. This behavior may be the result of an effect of stress on swelling not previously encountered. Tensile property changes are similar to those classically observed. When irradiation and test temperatures are equivalent, yield strength (0.2 percent offset) increases rapidly at low fluences and becomes fluence-independent at high fluence levels (>7 × 1022 n/cm2, En > 0.1 MeV). Uniform elongation correspondingly decreases with increasing fluence and appears to saturate near 0.5 percent at higher fluence levels. This transition in fluence dependence of the properties is associated with a transition in fracture mechanism. The transition occurs from the usual homogeneous plastic dimpling fracture at low fluences to an extremely heterogeneous channel fracture at high fluence levels.

No abstract

In order to determine mechanical behavior under various simulated reactor transient events, internally pressurized specimens of fast flux-irradiated 20 percent cold-worked Type 316 stainless steel fuel pin cladding were rapidly heated until they burst. Tests were conducted at heating rates of 10F°/s and 200°F/s with pressures of 2500 to 14 300 psi (17.2 to 98.6 MPa), resulting in failure temperatures from 1000 to 2000°F (811 to 1366 K). The specimens were taken from subassemblies irradiated in the Experimental Breeder Reactor-II at temperatures from 700 to 1300°F (644 to 978 K). Peak burnup and fluence levels ranged from 28 000 to 50 000 megawatt days per metric ton metal (MWd/MTM) and 2.5 to 4.0 × 1022 neutrons (n)/cm2 (E > 0.1 MeV), respectively. Irradiation degraded both the failure strain and failure strength, when the transient test conditions resulted in intergranular fracture; intergranular fracture occurred above 1000 to 1200°F (811 to 922 K), depending on strain rate. Below these temperatures the fracture mode is transgranular, so that the failure strength is not reduced. A correlation based on the ratio of irradiated material failure strain and failure strength was developed to describe the effects of irradiation on the mechanical properties of the cladding. The strain ratio correlation indicates that the failure strain does not further decrease beyond a fluence of 2 × 1022 n/cm2. The mechanical properties of cladding which had contained fuel during irradiation were degraded more than were the properties of unfueled material.

Tension tests were performed on annealed Type 304 stainless steel specimens sectioned from Experimental Breeder Reactor-II (EBR-II) duct thimbles. The specimens had accumulated neutron fluences of up to 10.3 × 1022 neutrons (n)/cm2, E > 0.1 MeV (10.3 × 1026 n/m2, E ×16 fJ). Irradiation temperatures for the material tested ranged from 700 to 750°F (644 to 672 K). The tests were performed at temperatures of room temperature to 1400°F (1033 K), employing strain rates of 2 × 10-3 to 2/min (3.3 × 10-5 to 3.3 × 10-2/s). At test temperatures below 800°F (700 K), the strength increased with fluence until about 7 × 1022 n/cm2 (7 × 1026 n/m2), beyond which no further increase was observed. The elongation decreased with fluence, reaching levels as low as 0.5 percent uniform and 1.3 percent total elongation at 700°F (644 K). High-fluence failures below 800°F (700 K) occurred by transgranular channel fracture. At test temperatures above 1000°F (811 K), elongations were reduced to very low levels at high fluences (average total elongation (TE) = 0.03 percent). Extensive intergranular fracture at the high fluences produced the low ductility and resulted in failure of the specimens before reaching the strength characteristic of the true hardness of the material.

No abstract