A reduced order model for fission gas diffusion in columnar grains

Pizzocri D.,Di Gennaro M.,Barani T.,Silva F.A.B.,Zullo G.,Lorenzi S.,Cammi A. 한국원자력학회 2023 Nuclear Engineering and Technology Vol.55 No.11

In fast reactors, restructuring of the fuel micro-structure driven by high temperature and high temperature gradient can cause the formation of columnar grains. The non-spheroidal shape and the non-uniform temperature field in such columnar grains implies that standard models for fission gas diffusion can not be applied. To tackle this issue, we present a reduced order model for the fission gas diffusion process which is applicable in different geometries and with non-uniform temperature fields, maintaining a computational requirement in line with its application in fuel performance codes. This innovative application of reduced order models as meso-scale tools within fuel performance codes represents a first-of-a-kind achievement that can be extended beyond fission gas behaviour.

3D reconstruction of two-phase random heterogeneous material from 2D sections: An approach via genetic algorithms

Pizzocri, D.,Genoni, R.,Antonello, F.,Barani, T.,Cappia, F. Korean Nuclear Society 2021 Nuclear Engineering and Technology Vol.53 No.9

This paper introduces a method to reconstruct the three-dimensional (3D) microstructure of two-phase materials, e.g., porous materials such as highly irradiated nuclear fuel, from two-dimensional (2D) sections via a multi-objective optimization genetic algorithm. The optimization is based on the comparison between the reference and reconstructed 2D sections on specific target properties, i.e., 2D pore number, and mean value and standard deviation of the pore-size distribution. This represents a multi-objective fitness function subject to weaker hypotheses compared to state-of-the-art methods based on n-points correlations, allowing for a broader range of application. The effectiveness of the proposed method is demonstrated on synthetic data and compared with state-of-the-art methods adopting a fitness based on 2D correlations. The method here developed can be used as a cost-effective tool to reconstruct the pore structure in highly irradiated materials using 2D experimental data.

A surrogate model for the helium production rate in fast reactor MOX fuels

Pizzocri D.,Katsampiris M.G.,Luzzi L.,Magni A.,Zullo G. 한국원자력학회 2023 Nuclear Engineering and Technology Vol.55 No.8

Helium production in the nuclear fuel matrix during irradiation plays a critical role in the design and performance of Gen-IV reactor fuel, as it represents a life-limiting factor for the operation of fuel pins. In this work, a surrogate model for the helium production rate in fast reactor MOX fuels is developed, targeting its inclusion in engineering tools such as fuel performance codes. This surrogate model is based on synthetic datasets obtained via the SCIANTIX burnup module. Such datasets are generated using Latin hypercube sampling to cover the range of input parameters (e.g., fuel initial composition, fission rate density, and irradiation time) and exploiting the low computation requirement of the burnup module itself. The surrogate model is verified against the SCIANTIX burnup module results for helium production with satisfactory performance

Application of the SCIANTIX fission gas behaviour module to the integral pin performance in sodium fast reactor irradiation conditions

Magni A.,Pizzocri D.,Luzzi L.,Lainet M.,Michel B. 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.7

The sodium-cooled fast reactor is among the innovative nuclear technologies selected in the framework of the development of Generation IV concepts, allowing the irradiation of uranium-plutonium mixed oxide fuels (MOX). A fundamental step for the safety assessment of MOX-fuelled pins for fast reactor applications is the evaluation, by means of fuel performance codes, of the integral thermal-mechanical behaviour under irradiation, involving the fission gas behaviour and release in the fuel-cladding gap. This work is dedicated to the performance analysis of an inner-core fuel pin representative of the ASTRID sodium-cooled concept design, selected as case study for the benchmark between the GERMINAL and TRANSURANUS fuel performance codes. The focus is on fission gas-related mechanisms and integral outcomes as predicted by means of the SCIANTIX module (allowing the physics-based treatment of inert gas behaviour and release) coupled to both fuel performance codes. The benchmark activity involves the application of both GERMINAL and TRANSURANUS in their “pre-INSPYRE” versions, i.e., adopting the state-of-the-art recommended correlations available in the codes, compared with the “post-INSPYRE” code results, obtained by implementing novel models for MOX fuel properties and phenomena (SCIANTIX included) developed in the framework of the INSPYRE H2020 Project. The SCIANTIX modelling includes the consideration of burst releases of the fission gas stored at the grain boundaries occurring during power transients of shutdown and start-up, whose effect on a fast reactor fuel concept is analysed. A clear need to further extend and validate the SCIANTIX module for application to fast reactor MOX emerges from this work; nevertheless, the GERMINAL-TRANSURANUS benchmark on the ASTRID case study highlights the achieved code capabilities for fast reactor conditions and paves the way towards the proper application of fuel performance codes to safety evaluations on Generation IV reactor concepts.

Towards grain-scale modelling of the release of radioactive fission gas from oxide fuel. Part II: Coupling SCIANTIX with TRANSURANUS

Zullo G.,Pizzocri D.,Magni A.,Van Uffelen P.,Schubert A.,Luzzi L. 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.12

The behaviour of the fission gas plays an important role in the fuel rod performance. In a previous work, we presented a physics-based model describing intra- and inter-granular behaviour of radioactive fission gas. The model was implemented in SCIANTIX, a mesoscale module for fission gas behaviour, and assessed against the CONTACT 1 irradiation experiment. In this work, we present the multi-scale coupling between the TRANSURANUS fuel performance code and SCIANTIX, used as mechanistic module for stable and radioactive fission gas behaviour. We exploit the coupled code version to reproduce two integral irradiation experiments involving standard fuel rod segments in steady-state operation (CONTACT 1) and during successive power transients (HATAC C2). The simulation results demonstrate the predictive capabilities of the code coupling and contribute to the integral validation of the models implemented in SCIANTIX.

Towards grain-scale modelling of the release of radioactive fission gas from oxide fuel. Part I: SCIANTIX

Zullo G.,Pizzocri D.,Magni A.,Van Uffelen P.,Schubert A.,Luzzi L. 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.8

When assessing the radiological consequences of postulated accident scenarios, it is of primary interest to determine the amount of radioactive fission gas accumulated in the fuel rod free volume. The state-ofthe-art semi-empirical approach (ANS 5.4e2010) is reviewed and compared with a mechanistic approach to evaluate the release of radioactive fission gases. At the intra-granular level, the diffusiondecay equation is handled by a spectral diffusion algorithm. At the inter-granular level, a mechanistic description of the grain boundary is considered: bubble growth and coalescence are treated as interrelated phenomena, resulting in the grain-boundary venting as the onset for the release from the fuel pellets. The outcome is a kinetic description of the release of radioactive fission gases, of interest when assessing normal and off-normal conditions. We implement the model in SCIANTIX and reproduce the release of short-lived fission gases, during the CONTACT 1 experiments. The results show a satisfactory agreement with the measurement and with the state-of-the-art methodology, demonstrating the model soundness. A second work will follow, providing integral fuel rod analysis by coupling the code SCIANTIX with the thermo-mechanical code TRANSURANUS

On the use of spectral algorithms for the prediction of short-lived volatile fi ssion product release: Methodology for bounding numerical error

G. Zullo,D. Pizzocri,L. Luzzi 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.4

Recent developments on spectral diffusion algorithms, i.e., algorithms which exploit the projection of thesolution on the eigenfunctions of the Laplacian operator, demonstrated their effective applicability in fasttransient conditions. Nevertheless, the numerical error introduced by these algorithms, together with theuncertainties associated with model parameters, may impact the reliability of the predictions on shortlived volatile fission product release from nuclear fuel. In this work, we provide an upper bound on thenumerical error introduced by the presented spectral diffusion algorithm, in both constant and timevarying conditions, depending on the number of modes and on the time discretization. The definitionof this upper bound allows introducing a methodology to a priori bound the numerical error on shortlived volatile fission product retention.

On the intra-granular behaviour of a cocktail of inert gases in oxide nuclear fuel: Methodological recommendation for accelerated experimental investigation

M. Romano,D. Pizzocri,L. Luzzi 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.5

Besides recent progresses in the physics-based modelling of fission gas and helium behaviour, thescarcity of experimental data concerning their combined behaviour (i.e., cocktail) hinders further modeldevelopments. For this reason, in this work, we propose a modelling methodology aimed at providingrecommendations for accelerated experimental investigations. By exploring a wide range of annealingtemperatures and cocktail compositions with a physics-based modelling approach we identify the mostinteresting conditions to be targeted by future experiments. To corroborate the recommendations arisingfrom the proposed methodology, we include a sensitivity analysis quantifying the impact of the modelparameters on fission gas and helium release, in conditions representative of high and low burnup

Towards a physics-based description of intra-granular helium behaviour in oxide fuel for application in fuel performance codes

Cognini, L.,Cechet, A.,Barani, T.,Pizzocri, D.,Van Uffelen, P.,Luzzi, L. Korean Nuclear Society 2021 Nuclear Engineering and Technology Vol.53 No.2

In this work, we propose a new mechanistic model for the treatment of helium behaviour which includes the description of helium solubility in oxide fuel. The proposed model has been implemented in SCIANTIX and validated against annealing helium release experiments performed on small doped fuel samples. The overall agreement of the new model with the experimental data is satisfactory, and given the mechanistic formulation of the proposed model, it can be continuously and easily improved by directly including additional phenomena as related experimental data become available.

Physics-based modelling and validation of inter-granular helium behaviour in SCIANTIX

Giorgi R.,Cechet A.,Cognini L.,Magni A.,Pizzocri D.,Zullo G.,Schubert A.,Van Uffelen P.,Luzzi L. 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.7

In this work, we propose a new mechanistic model for the treatment of helium behaviour at the grain boundaries in oxide nuclear fuel. The model provides a rate-theory description of helium inter-granular behaviour, considering diffusion towards grain edges, trapping in lenticular bubbles, and thermal resolution. It is paired with a rate-theory description of helium intra-granular behaviour that includes diffusion towards grain boundaries, trapping in spherical bubbles, and thermal re-solution. The proposed model has been implemented in the meso-scale software designed for coupling with fuel performance codes SCIANTIX. It is validated against thermal desorption experiments performed on doped UO2 samples annealed at different temperatures. The overall agreement of the new model with the experimental data is improved, both in terms of integral helium release and of the helium release rate. By considering the contribution of helium at the grain boundaries in the new model, it is possible to represent the kinetics of helium release rate at high temperature. Given the uncertainties involved in the initial conditions for the inter-granular part of the model and the uncertainties associated to some model parameters for which limited lower-length scale information is available, such as the helium diffusivity at the grain boundaries, the results are complemented by a dedicated uncertainty analysis. This assessment demonstrates that the initial conditions, chosen in a reasonable range, have limited impact on the results, and confirms that it is possible to achieve satisfying results using sound values for the uncertain physical parameters.