Arthur T. Motta

RADIATION EFFECTS IN CRYSTALLINE CERAMICS FOR THE IMMOBILIZATION OF HIGH-LEVEL NUCLEAR WASTE AND PLUTONIUM

This review provides a comprehensive evaluation of the state-of-knowledge of radiation effects in crystalline ceramics that may be used for the immobilization of high-level nuclear waste and plutonium. The current understanding of radiation damage processes, defect generation, microstructure development, theoretical methods, and experimental methods are reviewed. Fundamental scientific and technological issues that offer opportunities for research are identified. The most important issue is the need for an understanding of the radiation-induced structural changes at the atomic, microscopic, and macroscopic levels, and the effect of these changes on the release rates of radionuclides during corrosion. {copyright} {ital 1998 Materials Research Society.}

A thermal spike model of grain growth under irradiation

The experimental study of grain growth in nanocrystalline metallic foils under ion irradiation showed the existence of a low-temperature regime (below about 0.15–0.22Tm), where grain growth is independent of the irradiation temperature, and a thermally assisted regime where grain growth is enhanced with increasing irradiation temperature. A model is proposed to describe grain growth under irradiation in the temperature-independent regime, based on the direct impact of the thermal spikes on grain boundaries. In the model, grain-boundary migration occurs by atomic jumps, within the thermal spikes, biased by the local grain-boundary curvature driving. The jumps in the spike are calculated based on Vineyard’s analysis of thermal spikes and activated processes using a spherical geometry for the spike. The model incorporates cascade structure features such as subcascade formation, and the probability of subcascades occurring at grain boundaries. This results in a power law expression relating the average grain ...

Corrosion of Zirconium Alloys

When zirconium alloys are used in water-cooled reactors, they are subjected to waterside corrosion. The emphasis of this chapter is on uniform oxidation and hydrogen embrittlement. The twin processes of oxidation and hydriding are described comprehensively in terms of their underlying mechanisms.

Precipitate evolution in the Zircaloy-4 oxide layer

A study by transmission electron microscopy has been made of the incorporation and oxidation of intermetallic precipitates Zr(Cr,Fe)2 into the uniform oxide layer of Zircaloy-4. Oxide thicknesses of 1, 4 and 14 μm were studied. The main results are: 1. (i) most of the precipitates are incorporated unoxidized into the oxide layer and are later oxidized, 2. (ii) iron concentration evolves significantly during oxidation, especially before the transition, generally segregating to the precipitate-matrix interface, occasionally precipitating as metallic iron, before being dissolved in the matrix. In order to understand the behaviour of fuel cladding in a reactor environment, the same approach was then realized on ion irradiated samples. In this case, the studied oxide thickness was 1 μm. The Zr(Fe,Cr)2 precipitates made amorphous by ion irradiation appear to behave as the original intermetallic precipitates. However, the iron redissolution seems to be delayed in the ion irradiated precipitates. The behavior of the precipitates during oxidation is discussed in the light of previous work and possible consequences for the oxidation process are considered.

Amorphization of intermetallic compounds under irradiation — A review

Abstract This is a review of the field of irradiation-induced amorphization of intermetallic compounds. It includes an update of recent experimental results using in-situ particle irradiation showing the effects of dose rate, temperature, crystal orientation, electron energy and the presence of stacking faults. The review describes amorphization by ion, electron and neutron irradiation in the context of a kinetic description, where the rate-limiting step is the accumulation of enough radiation damage in the lattice opposed by thermal annealing. Stability criteria, thermodynamic or otherwise, are combined with kinetics of radiation damage and annealing to provide an overall description of the amorphization process, and of the experimentally measured critical dose and critical temperature of amorphization. From the experimental observations, it is proposed that irradiation-induced amorphization in intermetallic compounds is an entropy-driven transformation, caused by the need of the material to maintain short-range order while accommodating the random ballistic motions of the atoms caused by irradiation.

Failure of Zircaloy cladding under transverse plane-strain deformation

Experiments have been performed to examine the ductility of Zircaloy 4 cladding tubes under conditions of near plane-strain deformation in the hoop direction (transverse to the tube axis) at temperatures of 25 and 300°C and at strain rates of 10 3 and 10 2 s 1 . To conduct these experiments, a specimen configuration was designed in which near plane-strain deformation is achieved, and a test methodology was established to determine two failure conditions: the limit strain at the onset of localized necking and the fracture strain. Experiments performed on cold-worked stress relieved material using the transverse plane-strain specimen geometry indicate major differences in failure behavior from that observed in uniaxial tension, although both test conditions result in failure by a localized necking process. The experimental results also indicate that while plane-strain fracture strains increase with temperature between 25 and 300°C, at a given temperature they are insensitive to strain rate. The limit strains at localized necking also increase with temperature but only at the high 10 2 s 1 strain rate. Finally, the failure data indicate a strong sensitivity to surface flaws, as predicted by localized necking theory.

The influence of hydride blisters on the fracture of Zircaloy-4

Abstract The fracture behavior under near plane-strain deformation conditions of Zircaloy-4 sheet containing solid hydride blisters of various depths has been examined at 25 and 300 °C. The study was based on material with either model ‘blisters’ having diameters of 2 and 3 mm or a continuous layer of hydride; in all cases, the substrate material contained discrete hydride precipitates. The fracture strains decrease rapidly with increasing hydride blister/layer depth to levels of about 100 μm deep, and then remain roughly constant. For a given blister depth, the material is significantly more ductile at 300 °C than at room temperature although measurable ductility is retained even at 25 °C and for large blister depths. The material is somewhat more ductile if the hydride is in the form of a blister than in the form of a continuous layer (rim). The hydride blisters/layers are brittle at all temperatures, and crack shortly after yielding of the ductile substrate. Consequently, both experimental evidence and analytical modeling indicate that fracture of the sheet is controlled by the crack growth resistance of the substrate at 25 °C. At elevated temperatures, the hydride particles within the substrate are quite ductile, inhibit crack growth, and failure eventually occurs due to a shear instability.

A ballistic mixing model for the amorphization of precipitates in Zircaloy under neutron irradiation

A model is proposed for the crystalline-to-amorphous transformation (amorphization) of Zr(Cr, Fe)2 precipitates in Zircaloy under neutron irradiation. The model is based on the observations that a “duplex” structure forms upon neutron irradiation: an amorphous layer starts at the precipitate-matrix interface that moves into the precipitate until the precipitate is completely amorphous. A depletion of Fe from the amorphous layer is observed, and the thickness of the amorphous layer is directly proportional to fluence. This last feature cannot be accounted for by models in which the rate controlling step for amorphization is diffusion-controlled. The rate-controlling step for amorphization is a departure from stoichiometry induced by ballistic mixing across the crystallineamorphous interface. This explains the fact that amorphization starts at the interface and gives the correct linear dependence of amorphous layer thickness with fluence. It is shown that the amorphization front velocity observed experimentally can be reproduced with the present model.

In situ transmission electron microscopy and ion irradiation of ferritic materials

The intermediate voltage electron microscope‐tandem user facility in the Electron Microscopy Center at Argonne National Laboratory is described. The primary purpose of this facility is electron microscopy with in situ ion irradiation at controlled sample temperatures. To illustrate its capabilities and advantages a few results of two outside user projects are presented. The motion of dislocation loops formed during ion irradiation is illustrated in video data that reveals a striking reduction of motion in Fe‐8%Cr over that in pure Fe. The development of extended defect structure is then shown to depend on this motion and the influence of nearby surfaces in the transmission electron microscopy thin samples. In a second project, the damage microstructure is followed to high dose (200 dpa) in an oxide dispersion strengthened ferritic alloy at 500°C, and found to be qualitatively similar to that observed in the same alloy neutron irradiated at 420°C. Microsc. Res. Tech., 2009.

Effect of irradiation on the precipitate stability in Zr alloys

Abstract Zirconium alloys undergo structural changes under various types of irradiation. This is particularly the case for intermetallic precipitates such as Zr2(Fe,Ni), Zr(Fe,Cr)2 in Zircaloys and Zr(Fe,V)2 in Zr-Fe-V alloy: under irradiation those phases are subject to a crystalline-to-amorphous transformation. Experimental results obtained with electron and ion irradiations are presented. With both types of irradiations, the dose to amorphization increases with the irradiation temperature and diverges when a critical temperature is reached. The relative stabilities of the four types of precipitates studied appear to be reversed for low and high irradiation temperatures.