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.
The effect of a general state of stress on swelling has been considered in a theoretical void growth model. Angular-dependent loop growth has been incorporated into the model in order to include the effects of shear stress. It has been found possible to consider the effects of stress on void and loop growth as a relatively simple deformation process. We define the stress effect as follows: where (dϵi/dt)σ=0 is the strain rate associated with the isotropic swelling that would occur in stress-free conditions. For an applied stress having both hydrostatic and shear components, the hydrostatic component provides a driving force for isotropic volume swelling and the shear component causes a shape change. The total deformation process (stress-induced volume increase and shear) may be represented by the relation: where the σ's are applied tensile stresses and the i, j, K are the principal axes. K and ν are two material parameters that depend upon microstructure (dislocation density, void number density, and void size) but do not depend upon flux. K depends strongly on temperature whereas ν is independent of temperature. The extent to which plastic flow must accompany stress-assisted swelling is governed by the parameter ν. As ν approaches 1/2, the deformation process becomes pure shear. It is found that the value of ν is bounded and lies between -1/3 and +1/2. Thus, except under the application of purely hydrostatic stress, stress-assisted swelling cannot be completely shear-free. When transmission electron microscopy (TEM) data for neutron-irradiated solution-treated Type 316 stainless steel were used, the values of ν were found to lie between 0 and 0.5. For cold-worked steels, ν is expected to lie in the upper part of the range just given. The significance of the present results becomes evident in the analysis of fuel pin profiles. For isotropic swelling with no plastic flow, the fractional change in fuel pin diameter is 1/3 the fractional isotropic volume change. However, for stress-assisted swelling, the relation may be much different and depends upon the parameter ν. For a biaxial stress state of the type that occurs in pressurized cylindrical tube, for example, Δ D/D ≈ 0.7 when ν = 0.1, whereas ΔD/D≈ 1.4 ΔV/V when ν = 0.3.
Plans for remediation of the Hanford undergroundstorage tanks are currentlyundergoing reevaluation. As partof this process, many options are being considered for the Tank Waste RemediationSystem (TWRS). The "clean option" described here proposes an aggressive waste processing strategy to achieve the three majorobjectives: * greatly reduce the volume of high-level waste (HLW) to lessen demandson geologic repository space * decrease by several orders of magnitudethe amountof radioactivityand toxicity now in the waste tanksthat will be left permanentlyonsite as low-level solid waste (LLW) , accomplish the first two objectives without significantly increasingthe total amount of waste for disposal. RichardG. Cowan, Westinghouse Hanford Company; and N. G. (Penny) Colton, Pacific Northwest Laboratory. Furthermore,a group of nationaltechnical experts have also contributedand are acknowledged here. We are very grateful to the members of this groupfor the time and effort they dedicated to this task. We are particularlyindebtedto David O. Campbellfor his thorough review of the draft of this report.
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