C. Ferrero

Study by microtomography of 3D porosity networks in irradiated beryllium

Abstract The analysis of gas release modes from irradiated materials during out-of-pile temperature transients and of the related microstructure evolution provides fundamental evidence for developing and validating models of the behaviour of gases in solids. Gas atoms are uniformly generated by nuclear reactions in the lattice. If temperature and/or gas concentration increase, gas atoms diffuse and precipitate into bubbles, then bubbles grow and coalesce until they form at grain boundaries porosity networks connected to the free surface. The formation of such percolation paths is related to the appearance of release bursts where most part of the gas inventory escapes. The morphology of percolation patterns determines the porosity limit necessary to start the percolation stage and finally affects the performance of the material in engineering applications. The gas percolation stage in a weakly irradiated beryllium sample, a 2 mm diameter pebble, has been examined by computer aided microtomography based on synchrotron radiation, with the aim to identify and qualitatively characterise the 3D morphology and topology of open porosity networks.

Modelling of swelling and tritium release in irradiated beryllium

Abstract The most important effects of neutron irradiation on beryllium are swelling, embrittlement and tritium retention. The helium produced by the 9Be(n, 2n)24He reaction is the cause of beryllium swelling and, at high neutron fluences, the main cause of tritium retention which is produced by simultaneous reactions. It was, therefore, decided to develop a computer code capable of describing the helium and tritium behaviour in beryllium. The approach used was to modify an existing code available for the modelling of fission gas behaviour in UO2 irradiated in fission reactors. A new model describing the trapping effects on tritium due to chemical reactions and capture in helium-bubbles has been included in the code. The resulting code ANFIBE (ANalysis of Fusion Irradiated BEryllium) allows the calculation of gas distribution, induced swelling and helium and tritium release from beryllium. Good agreement between experimental and calculated swelling and tritium release has been found.

Non destructive three dimensional analysis of the packing of a binary beryllium pebble bed

Abstract In the Helium Cooled Pebble Bed (HCPB) Blanket, studied within the European Fusion Technology Programme, beryllium in form of pebbles is used as neutron multiplier. The thermal–mechanical behaviour of a pebble bed strongly depends on the packing factor of the bed. In a binary pebble bed, in particular, a homogeneous distribution of small pebbles between the larger ones (infiltrated bed) has to be ensured in order to obtain a behaving homogeneously bed. Thus, a detailed non-destructive control of the pebble bed configuration can provide an important help in interpreting the pebble bed thermal mechanical characterisation test results. A three-dimensional (3-D) computer aided microtomography (CMT) experimental setup developed at the European Synchrotron Radiation Facility (ESRF) allowed to reconstruct 3-D images of the attenuation coefficient of a X-ray synchrotron radiation beam within a pebble bed without physically damaging it. By post-processing the acquired data, very useful quantitative informations were obtained (local and average void fraction in the sample, impurities and micro-cracks in the pebbles). In the present work, the micrographic technique and the first results of the analysis are presented and critically discussed in view of a future application for a medium scale HCPB Blanket mock-up.

ANFIBE : A comprehensive model for swelling and tritium release from neutron-irradiated beryllium-I: Theory and model capabilities

A mechanistic model for the description of helium swelling and tritium release in neutron-irradiated beryllium is presented. Initially aimed at predicting the mechanical stability and the tritium retention capacity of beryllium in a fusion reactor blanket, the ANFIBE code was finally extended to provide an exhaustive description of the properties of this material under fast neutron irradiation. In-solid diffusion and precipitation of helium and tritium, radiation re-solution, and bubble growth and coalescence in different structural domains of the material are taken into account and formulated in a compact rate equation system, enabling the evolution of swelling and release to be calculated under stationary and nonstationary irradiation and temperature conditions. A particular feature of the model is the treatment of the growth of gas bubbles and pores in the interactive compressive stress field created by the gas precipitated in cavities of different sizes and at different pressures, enabling a realistic and accurate calculation of the stress-sensitive intergranular-swelling components and of the related pore-venting effects. The salient physical hypotheses of the model are discussed, as well as the formalism adopted for the description of helium and tritium diffusion precipitation and swelling.

Helium induced swelling and tritium trapping mechanisms in irradiated beryllium: a comprehensive approach

Abstract Since beryllium is considered one of the best neutron multipliers for the blanket of a fusion reactor, several studies have been initiated to evaluate its behaviour under irradiation for both typical operating and accidental conditions. In the present work the effects of neutron irradiation on the mechanical properties of beryllium were studied with the aid of a computer code describing the most important mechanisms of helium and tritium retention and release. The analysis of irradiated beryllium was carried out following a mechanistic scheme, whereby the microscopic properties of the atomic lattice, the metallographic structure of the material and the geometrical design parameters of the specimens are considered. A model describing the trapping effects on tritium due to chemical reactions with beryllium oxide and capture in helium bubbles was also included in the code. The results of the numerical calculations referring to gas distribution, helium-induced swelling and tritium release from beryllium are compared with corresponding experiments. The good agreement between calculated and experimental data obtained so far constitutes a significant step in validation of the predictive capability of the code. Moreover, preliminary analyses were performed concerning a proposed EC water cooled ceramic blanket for the ITER fusion reactor and for the European BOT DEMO solid breeder blanket with the aim of providing an estimation of the operational and off-normal behaviour of this component.

ANFIBE: A Comprehensive Model for Swelling and Tritium Release from Neutron-Irradiated Beryllium - II: Comparison of Model Predictions with Experimental Results

A new computer code, called ANFIBE (ANalysis of Fusion Irradiated BEryllium), has been developed to describe the most important processes (diffusion, gas precipitation, bubble coalescence, helium-bubble trapping, chemical trapping, etc.) thought to affect gas behavior and swelling in beryllium during fast neutron irradiation. The new model allows the prediction of helium and tritium redistribution, induced swelling, and release. The relevant effects occurring in irradiated beryllium under steady or transient temperature conditions have been considered from a microscopic (lattice and subgranular volume elements), structural (metallographic features of the material), and geometrical (specimen design parameters) point of view. The main results of this validation work represent the second part of the presentation of this model. The relevant beryllium properties published in the literature are presented and critically examined. The performance of the code is assessed by comparing the code predictions with a large set of published experimental data on swelling and gas release in beryllium under fast neutron irradiation.

Study of the microstructure of neutron irradiated beryllium for the validation of the ANFIBE code

The behaviour of beryllium under fast neutron irradiation is a key issue of the helium cooled pebble bed tritium breeding blanket, due to the production of large quantities of helium and of a non-negligible amount of tritium. To optimise the design, a reliable prediction of swelling due to helium bubbles and of tritium inventory during normal and off-normal operation of a fusion power reactor is needed. The ANFIBE code (ANalysis of Fusion Irradiated BEryllium) is being developed to meet this need. The code has to be applied in a range of irradiation conditions where no experimental data are available, therefore a detailed gas kinetics model, and a specific and particularly careful validation strategy are needed. The validation procedure of the first version of the code was based on macroscopic data of swelling and tritium release. This approach is, however, incomplete, since a verification of the microscopic behaviour of the gas in the metal is necessary to obtain a reliable description of swelling. This paper discusses a general strategy for a thorough validation of the gas kinetics models in ANFIBE. The microstructure characterisation of weakly irradiated beryllium pebbles, with different visual examination techniques, is then presented as an example of the application of this strategy. In particular, the advantage of developing 3D techniques, such as X-ray microtomography, is demonstrated.