1,720,989 research outputs found
Isotropic softening model for fuel cracking in BISON
No full textIn oxide nuclear fuel, the temperature gradient from the centerline to the radial edge of the pellet induces cracking due to thermal stress. We present a model that represents the effect of cracking as an isotropic softening of the material. The model considers a reduction of the elastic constants as a function of the number of cracks present in the fuel. This pragmatic approach aims to represent the average effect of cracking on the fuel stresses without attempting to explicitly describe the crack pattern or localization. Albeit simplistic, this approach has definite advantages in terms of computational expense and numerical convergence behavior for the mechanical analysis. It is also consistent with the uncertainties inherent in modeling the fuel cracking process, and suitable for engineering calculations aimed at representing the global fuel element behavior. The applied scaling of the elastic constants conserves the principal strain values and redistributes the principal stresses isotropically. Moreover, the model allows for the effect of an evolving number of cracks. In particular, an empirical correlation for the number of cracks as a function of the rod average linear heat rate is developed. Although the basic concept of the model was known, in this work we revisit and improve the original formulation, and implement the model in the BISON fuel performance code. Application in BISON is demonstrated through simulations of an idealized single fuel pellet irradiation and two integral fuel rod experiments. Results showcase the impact on calculated fuel stresses, the coupling to fuel creep, and the effect on cladding strains during pellet-cladding mechanical interaction
An effective numerical algorithm for intra-granular fission gas release during non-equilibrium trapping and resolution
No full textFission gas release and gaseous swelling in nuclear fuel are driven by the transport of fission gas from within the fuel grains to grain boundaries (intra-granular fission gas release). The process involves gas atom diffusion in conjunction with trapping in and resolution from intra-granular bubbles, and is described mathematically by a system of two partial differential equations (PDE). Under the assumption of equilibrium between trapping and resolution (quasi-stationary approximation) the system can be reduced to a single diffusion equation with an effective diffusion coefficient. Numerical solutions used in engineering fuel performance calculations invariably rely on this simplification. First, we investigate the validity of the quasi-stationary approximation compared to the solution of the general system of PDEs. Results demonstrate that the approximation is valid under most conditions of practical interest, but is inadequate to describe intra-granular fission gas release during rapid transients to relatively high temperatures such as postulated reactivity-initiated accidents (RIA). Then, we develop a novel numerical algorithm for the solution of the general PDE system in time-varying conditions. We verify the PolyPole-2 algorithm against a reference finite difference solution for a large number of randomly generated
operation histories including prototypical RIA transients. Results demonstrate that PolyPole-2 captures the solution of the general system with a high accuracy and a low computational cost. The PolyPole-2 algorithm overcomes the quasi-stationary approximation and the concept of an effective diffusion coefficient for the solution of the intra-granular fission gas release problem in nuclear fuel analysis
Analysis of transient fission gas behaviour in oxide fuel using BISON and TRANSURANUS
Full text linkThe modelling of fission gas behaviour is a crucial aspect of nuclear fuel performance analysis in view of the related effects on the thermo-mechanical performance of the fuel rod, which can be particularly significant during transients. In particular, experimental observations indicate that substantial fission gas release (FGR) can occur on a small time scale during transients (burst release). To accurately reproduce the rapid kinetics of the burst release process in fuel performance calculations, a model that accounts for non-diffusional mechanisms such as fuel micro-cracking is needed. In this work, we present and assess a model for transient fission gas behaviour in oxide fuel, which is applied as an extension of conventional diffusion-based models to introduce the burst release effect. The concept and governing equations of the model are presented, and the sensitivity of results to the newly introduced parameters is evaluated through an analytic sensitivity analysis. The model is assessed for application to integral fuel rod analysis by implementation in two structurally different fuel performance codes: BISON (multi-dimensional finite element code) and TRANSURANUS (1.5D code). Model assessment is based on the analysis of 19 light water reactor fuel rod irradiation experiments from the OECD/NEA IFPE (International Fuel Performance Experiments) database, all of which are simulated with both codes. The results point out an improvement in both the quantitative predictions of integral fuel rod FGR and the qualitative representation of the FGR kinetics with the transient model relative to the canonical, purely diffusion-based models of the codes. The overall quantitative improvement of the integral FGR predictions in the two codes is comparable. Moreover, calculated radial profiles of xenon concentration after irradiation are investigated and compared to experimental data, illustrating the underlying representation of the physical mechanisms of burst release
Modelling of Burst Release in Oxide Fuel and Application to the Transuranus Code
No full textThe substantial release of fission gas during temperature transients (burst release) can be critical during operational reactor transients and (design-basis) accidents. A purely diffusion-based model cannot explain the rapid kinetics of the process. In this work, we present a model for transient fission gas release in oxide fuel. This model arises from experimental observations relative to both in-reactor irradiation and post-irradiation annealing of UO2 fuel. In particular, micrographs demonstrate the presence of grain-face separations (micro-cracks) in transient-tested fuel, thus indicating that a basic mechanism of burst release is micro-cracking. The presented model extends a previously developed diffusion-based model, introducing the effect of micro-cracking. This is interpreted as a reduction of the grain-face gas inventory and storing capacity during transients. The process is modelled through an empirical temperature-dependent function based on the experimentally observed characteristics of gas release during both heating and cooling transients. The model also includes an irradiation-induced micro-crack healing process, which gradually restores the original grain-face gas storing capacity. This process is described by means of an empirical burnup-dependent function. The resulting extended model (diffusion-based fission gas behaviour together with transient release) is overall semi-empirical, but the burst release capability notably preserves the continuity in time and space as well as the consistent coupling of the calculated fission gas release and swelling. The new model was originally implemented in the fuel performance code BISON. In this work, we implemented the model in the TRANSURANUS fuel performance code, and applied it to the simulation of some light water reactors fuel rod irradiation experiments of the OECD/NEA International Fuel Performance Experiments database. The results point out a representation of the kinetics of burst release consistent with experimental evidence
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
PolyPole-1: An accurate numerical algorithm for intra-granular fission gas release
No full textThe transport of fission gas from within the fuel grains to the grain boundaries (intra-granular fission gas release) is a fundamental controlling mechanism of fission gas release and gaseous swelling in nuclear fuel. Hence, accurate numerical solution of the corresponding mathematical problem needs to be included in fission gas behaviour models used in fuel performance codes. Under the assumption of equilibrium between trapping and resolution, the process can be described mathematically by a single diffusion equation for the gas atom concentration in a grain. In this paper, we propose a new numerical algorithm (PolyPole-1) to efficiently solve the fission gas diffusion equation in time-varying conditions. The PolyPole-1 algorithm is based on the analytic modal solution of the diffusion equation for constant conditions, combined with polynomial corrective terms that embody the information on the deviation from constant conditions. The new algorithm is verified by comparing the results to a finite difference solution over a large number of randomly generated operation histories. Furthermore, comparison to state-of-the-art algorithms used in fuel performance codes demonstrates that the accuracy of PolyPole-1 is superior to other algorithms, with similar computational effort. Finally, the concept of PolyPole-1 may be extended to the solution of the general problem of intra-granular fission gas diffusion during non-equilibrium trapping and resolution, which will be the subject of future work
An investigation of FeCrAl cladding behavior under normal operating and loss of coolant conditions
No full textIron-chromium-aluminum (FeCrAl) alloys are candidates to be used as nuclear fuel cladding for increased accident tolerance. An analysis of the response of FeCrAl under normal operating and loss of coolant conditions has been performed using fuel performance modeling. In particular, recent information on FeCrAl material properties and phenomena from separate effects tests has been implemented in the BISON fuel performance code and analyses of integral fuel rod behavior with FeCrAl cladding have been performed. BISON simulations included both light water reactor normal operation and loss-of-coolant accidental transients. In order to model fuel rod behavior during accidents, a cladding failure criterion is desirable. For FeCrAl alloys, a failure criterion is developed using recent burst experiments under loss of coolant like conditions. The added material models are utilized to perform comparative studies with Zircaloy-4 under normal operating conditions and oxidizing and non-oxidizing out-of-pile loss of coolant conditions. The results indicate that for all conditions studied, FeCrAl behaves similarly to Zircaloy-4 with the exception of improved oxidation performance. Further experiments are required to confirm these observations
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
