Swelling and Tensile Property Evaluations of High-Fluence EBR-II Thimbles

Abstract

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.

Low temperature neutron irradiation effects on microstructure and tensile properties of molybdenum

“…Wechsler [36] suggested that the low temperature irradiation embrittlement is likely associated with changes in plastic properties, particularly with inhomogeneous plastic deformation rather than changes in inherent fracture processes. Crack formation from coarse slip steps and the intersection of coarse slip bands was suggested to cause 'channel fracture' [37,38]. Cracks may also initiate at grain boundaries due to local stress concentration caused by dislocation pile-up in a defect-free channel [5].…”

Swelling and creep observed in AISI 304 fuel pin cladding from three MOX fuel assemblies irradiated in EBR-II

“…10 is derived from a 304 stainless duct which contained a control rod in EBR-II Row 3 [2,31] where the temperature rise was somewhat stronger than that of a typical PNL-3 fuel pin. The second example in Fig.…”

“…When safety and control rod ducts constructed from annealed AISI 304 were examined from EBR-II it was discovered that swelling levels approaching and sometimes passing double digit levels had already occurred in these components [2]. Subsequent experiments utilizing this steel confirmed its tendency toward early onset of swelling and rather large amounts of swelling [3][4][5].…”

Overview of Intergranular Fracture of Neutron Irradiated Austenitic Stainless Steels

“…The appearance of channel fractures was correlated with the onset of highly localized deformation of dislocation channeling and by implication the development of a high density of shearable defects of radiation damage [29]. A development of the fracture appearance of A304 with increasing neutron fluence is shown in Figure 10 [30,31].…”

“…The specimens were loaded at 230-650°C in the fracture toughness test using a standard test rate; at 230-540 °C, tests showed mixed transgranular fracture including islands of channel fracture; at 650 °C, ductile IG fracture was observed [32]. [30]. Note the channel fracture at ~54 dpa.…”