Wolfgang Hoffelner

NUCLEAR ENERGY MATERIALS PREDICTION: APPLICATION OF THE MULTI-SCALE MODELLING PARADIGM

The safe and reliable performance of fusion and fission plants depends on the choice of suitable materials and an assessment of long-term materials degradation. These materials are degraded by their exposure to extreme conditions; it is necessary, therefore, to address the issue of long-term damage evolution of materials under service exposure in advanced plants. The empirical approach to the study of structural materials and fuels is reaching its limit when used to define and extrapolate new materials, new environments, or new operating conditions due to a lack of knowledge of the basic principles and mechanisms present. Materials designed for future Gen IV systems require significant innovation for the new environments that the materials will be exposed to. Thus, it is a challenge to understand the materials more precisely and to go far beyond the current empirical design methodology. Breakthrough technology is being achieved with the incorporation in design codes of a fundamental understanding of the properties of materials. This paper discusses the multi-scale, multicode computations and multi-dimensional modelling undertaken to understand the mechanical properties of these materials. Such an approach is envisaged to probe beyond currently possible approaches to become a predictive tool in estimating the mechanical properties and lifetimes of materials.

Discrete dislocation dynamics simulations of dislocation interactions with Y2O3 particles in PM2000 single crystals

In oxide dispersion strengthened steels the interactions between dispersoids and dislocations determine the materials plasticity. Using three-dimensional Discrete Dislocation Dynamics simulations, the effect of Y2O3 dispersoids on the motion of dislocations in BCC single crystal PM2000, a commercial alloy candidate for gas-cooled reactors, has been studied. The dispersoid distribution used in this model has been derived from experimental TEM observations of PM2000. As screw dislocations are predominant in the studied material, the behaviour of a single screw dislocation under shear loading in a distribution of spherical Y2O3 dispersoids is studied. The critical resolved shear stress, the minimum value of the external stress needed to move the dislocation through the obstacle field, is found to be only slightly lower than the experimentally determined value revealing that dispersoids are the main hardening contributions.

Criticality of relaxation in dislocation systems.

Relaxation processes of dislocation systems are studied by two-dimensional dynamical simulations. In order to capture generic features, three physically different scenarios were studied and power-law decays found for various physical quantities. Our main finding is that all these are the consequence of the underlying scaling property of the dislocation velocity distribution. Scaling is found to break down at some cutoff time increasing with system size. The absence of intrinsic relaxation time indicates that criticality is ubiquitous in all states studied. These features are reminiscent of glassy systems and can be attributed to the inherent quenched disorder in the position of the slip planes.

Abnormal subgrain growth in a dislocation-based model of recovery

Simulation of subgrain growth during recovery is carried out using two-dimensional discrete dislocation dynamics on a hexagonal crystal lattice having three symmetric slip planes. To account for elevated temperature (i) dislocation climb was allowed and (ii) a Langevin type thermal noise was added to the force acting on the dislocations. During the simulation, a random ensemble of dislocations develops into a subgrain structure and power-law type growth kinetics are observed. The growth exponent is found to be independent of the climb mobility, but dependent on the temperature introduced by the thermal noise. The in-depth statistical analysis of the subgrain structure shows that the coarsening is abnormal, i.e. larger cells grow faster than the small ones, while the average misorientation between the adjacent subgrains remains nearly constant. During the coarsening Holts relation is found not to be fulfilled, such that the average subgrain size is not proportional to the average dislocation spacing. These findings are consistent with recent high precision experiments on recovery.

Condition Monitoring of High Temperature Components With Sub-Sized Samples

Advanced nuclear plants are designed for long-term operation in quite demanding environments. Limited operation experience with the materials used in such plants necessitate a reliable assessment of damage and residual life of components. Non-destructive condition monitoring of damage is difficult, if not impossible for many materials. Periodic investigation of small samples taken from well defined locations in the plant could provide an attractive tool for damage assessments. This paper will discuss possibilities of using very small samples taken from plant locations for complementary condition monitoring. Techniques such as micro/nano-indentation, micropillar compression, micro bending, small punch and thin strip testing can be used for the determination of local mechanical properties. Advanced preparation techniques such as focused ion beam (FIB) allow the preparation of samples from these small volumes for micro-structural analyses with transmission electron microscope (TEM) and advanced X-ray synchrotron techniques. Modeling techniques (e.g. dislocation dynamics DD) can provide a quantitative link between microstructure and mechanical properties. Using examples from ferritic oxide dispersion strengthened materials the DD approach is highlighted to understand component life assessments.

Microcharacterization of Damage in Materials for Advanced Nuclear Fission Plants

Miniature and sub-miniature samples were used for determination of mechanical properties of materials for advanced fission plants. Results from indentation and focused ion beam prepared micro-samples, punch tests and thin strip (irradiation) creep tests are shown. The results allow conclusions concerning materials damage. Irradiation damage profiles were determined with indentation. Results from micro-pillar tests showed a good agreement with results from conventional samples in case of oxide dispersion strengthened steels. Thin strip irradiation creep experiments revealed a negligible influence of dispersoid size/distribution on creep rates. Punch tests of fibre reinforced materials showed consistent results which still need quantitative analysis.

Materials Databases and Knowledge Management for Advanced Nuclear Technologies

International collaborations like the Generation IV initiative have the aim to create the technical basis for design and operation of advanced nuclear plants. Materials data shall be created in joint international materials projects. Data will be aggregated in databases like the “Generation IV materials handbook”. Mechanical data, but also microstructural information and information concerning materials production shall be included. This information will be used to create or amend code rules, to provide a basis for life-time analysis, damage assessments and for safety analyses. Such considerations need not only raw materials data but also tools for data analysis and evaluation. Multiscale modeling, establishing constitutive equations, development of advanced life-time prediction methods, quantitative correlation of mechanical properties with microstructure, quantification of environmental effects, tools for non destructive evaluation and condition based monitoring etc. are important analysis techniques needed for safe design and operation of advanced plants. These needs led the author to ask the question if current databases could not be enlarged by data evaluation and methods tools which could even end some day in the availability of web-based design codes and safety analyses. The database could also be used as a web-based discussion and development space. It could become then a powerful tool for knowledge management The paper will discuss this concept on basis of some examples.Copyright