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Pyrochlores have long been considered as host phases for long-term immobilization of radioactive waste nuclides that would undergo α-decay for hundreds of thousands of years. This work utilizes ion-beam irradiations to examine the combined effects of radiation damage and He accumulation on bubble formation in Gd2Ti2O7 over relevant waste-form timescales. Helium bubbles are not observed in pre-damaged Gd2Ti2O7 implanted with 2 × 1016 He/cm2, even after post-implantation irradiations with 7 MeV Au3+ at 300, 500, and 700 K. However, He bubbles with average diameters of 1.5 nm and 2.1 nm are observed in pre-damaged (amorphous) Gd2Ti2O7 and pristine Gd2Ti2O7, respectively, after implantation of 2 × 1017 He/cm2. The critical He concentration for bubble nucleation in Gd2Ti2O7 is estimated to be 6 at.% He.

International audience; The formation of He bubbles in Ferritic/Martensitic steel HT9 is investigated through the use of in situ Transmission Electron Microscopy coupled with He implantation and heavy ion irradiation. Of particular interest is the effect of increasing He appm/dpa ratio on the formation and growth of the bubbles, as well as the effect of the sequential order of ion irradiation i.e. Hepre-implantation followed by heavy-ion irradiation versus true dual-beam irradiation. The role of He is discussed.

Development of He bubbles at grain boundaries (GBs) may lead materials to serious embrittlement, but the nucleation mechanism of such bubbles is not well understood. In the present paper, we analyzed the stability of various He–vacancy clusters, which are precursors of He bubbles, at several kinds of GBs of α-Fe using a set of empirical potentials. The results clearly indicated the preferred nucleation of He bubbles at GBs. We also found that the dissociation energy of vacancies from He–vacancy clusters at GBs is generally less than that for the intragranular counterparts, and that the He-to-vacancy ratio in equilibrium for various GB cases becomes larger in comparison with the intragranular case. The results obtained here are invaluable in establishing a kinetic model of He bubble nucleation at GBs.

Theories of fission-fragment-driven re-solution of fission-gas atoms from intragranular bubbles in irradiated UO2 nuclear fuel are reviewed. Two mechanisms of re-solution are generally accepted: the heterogeneous process destroys entire bubbles in the path of fission fragments and returns the gas to the solid as individual atoms; the homogeneous process re-solves fission-gas atoms singly by scattering collisions with fission fragments and uranium recoils whose paths intersect the bubbles. Coupling of these two re-solution models with the bubble nucleation analogs determines the size and number density of the intragranular bubble population. Two approaches are reviewed: the single-size theory, in which all bubbles are accorded one size, and the bubble distribution theory, which seeks to determine the variation of bubble number density with size.

The theory of homogeneous nucleation is used to determine the relationship between the diffusion kinetics of helium and the density of helium bubbles both in the matrix and dislocations. The flux of helium atoms reaching a dislocation line is taken to be the effective helium generation rate for the dislocation. In this calculation the effect of the presence of sinks in the matrix is taken into account. Furthermore, the calculation determines the effect of continuously changing flux of helium atoms to the dislocation core due to changes in the sink strength in the grain interior as a function of irradiation time and helium generation rate. The bubble spacing on a dislocation line is then calculated in terms of diatomic nucleation using the condition of nucleation saturation.

Models of the behaviour of the fission gas in fast reactor fuel are used to analyse recent transmission electron microscope observations of gas bubble distributions. The single knock-on and complete bubble destruction mechanisms of re-solution are considered and compared. The complete bubble destruction model is found to provide a better description of the experimental observations, and the effectiveness of the re-solution process is estimated. Information about bubble nucleation kinetics is also derived.

Zr-tritides were investigated by TEM for aging times up to almost 3 y. After ∼ 3 weeks 3He bubbles of ∼1 nm in diameter were detected. The bubble density soon reached a value of ∼ 5 × 1023 m−3 and then apparently remained constant. Using strong diffraction contrast the interstitial loop and dislocation damage accompanying bubble formation could be imaged. Loops, network dislocations and internal boundaries were found to be heavily decorated with tiny bubbles. After ∼ 2 y an interconnected system of He filled channels evolved from these structures. The observed microstructural evolution is discussed in the light of bubble nucleation and growth models. From the bubble densities, energies of He diffusion or He dissociation from He clusters, respectively, of ∼ 1 eV are derived. Under the present conditions, bubbles can grow by loop punching or by the diffusional transfer of self-interstitials (SIA's) from the bubble surface to dislocations attached to them. Under high pressure, bubbles at dislocations could be unstable against elongation along the dislocations. The evolving system of channels presumably is responsible for 3He loss in the “state of accelerated 3 He release”.

Bubble formation in an Fe-12 at% Cr alloy has been studied after 100 keV He+ implantation and 300 keV Fe+ irradiation of the preimplanted specimens. Influence of the implantation/irradiation temperature on bubble radius and density has been quantitatively measured. An analysis of the implantation data in terms of an existing model for bubble nucleation has been presented. A microstructural evidence for the release of He from the bubbles under the influence of displacement cascades is shown. This allows a qualitative understanding of some of the data under Fe+ irradiation.

The blisters produced by 200 keV D+ irradiation of Cu at 120 K and subsequent heating to room temperature are found to be of two distinct types: small semi-spherical blisters ( ~ 7 /gmm across) and large blister flakes ( ~ 30 /gmm across). A simple method has been developed to remove blister flakes enabling direct observation of the exposed underside of the flakes by scanning electron microscopy. The small semi-spherical blisters, which form before the more extensive blister flakes, have a consistently deeper plane of fracture than the flakes. To explain the different depths of fracture two alternative models are proposed. Two mechanisms in particular are likely to be important. Compressional stress may inhibit bubble nucleation and early growth near the depth region around the maxima in the damage and gas deposition profiles. It is proposed that in the later stages of the irradiation shear introduced by differential expansion, caused by a combination of radiation induced swelling and localised heating plays a central role in fracture.

Although recent experimental and theoretical studies of fission gas release and gas-driven fuel swelling have greatly improved our understanding of these phenomena, some of the important mechanisms require further analysis. In this paper we consider and compare different ways of modelling gas bubble nucleation and the fission-induced re-solution of gas atoms from bubbles within a general chemical rate theory description of intragranular fission gas behaviour in nuclear fuels.

With the help of several experimental methods we have shown that it is possible to determine from moment to moment the balance of helium in irradiated and annealed BeO. The specific heat at very low temperatures (near the λ-point of helium) establishes the mass of helium which has been precipitated; chromatography gives the amount of helium desorbed during the anneal and electron microscopy determines the radius and mean separation of helium bubbles in BeO. In this way the process of diffusion and precipitation of helium in BeO can be followed accurately. Precipitation of helium begins during irradiation; bubble nucleation begins from 600 °C and peaks at 1200 °C. The diffusion coefficient of helium in BeO satisfies the relation: D = 2.5 × 10−3exp (−A/kT), where A = 3.45 eV between 1080 and 1400 °C.