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International audience; This paper presents the state-of-the-art knowledge about the micro-mechanical modelling of the fuel behavior under irradiation with normal and off normal operating conditions. Modelling of fundamental processes can provide key insights in the behavior of the material. Such models target specific phenomena due to the limits of computational resources and scope of theory, necessitating a multiscale approach. This work follows a multiscale paradigm building up in spatio-temporal scale. The micro-mechanical modelling studied addresses all the loading conditions encountered in the reactor with elasticity, plasticity, creep and fracture behavior. Atomistic-scale modelling review reveals mechanisms and physical parameters for elasticity of fresh and irradiated fuel, rupture, dislocation gliding and internal stresses induced by pressurized bubble with fission gases. Simulation techniques proposed at this scale are Density Functional Theory, Molecular Dynamic with empirical potential, Dislocation Dynamics and Phase Field Crystal methodology.

This work introduces a meshed debonding model intended for use with tristructural isotropic (TRISO) fuel particles. The ability to better examine the potential effects of interface debonding betwee...

BISON is a nuclear fuel performance application built using the Multiphysics Object-Oriented Simulation Environment (MOOSE) finite element library. One of its major goals is to have a great amount ...

A new gap conductance model is proposed in this study as a combination of Toptan’s model and the Ross-Stoute model. A variance-based sensitivity analysis is performed to understand how simulation r...

In a light water nuclear reactor environment, zirconium-alloy cladding undergoes corrosion that introduces hydrogen into the cladding. Hydrogen being slightly soluble in zirconium means that the terminal solid solubility of hydrogen is quickly reached. This leads to hydrogen precipitation into crystals of zirconium hydride. These hydrides decrease the ductility of the cladding, which increases the chance of cladding failure, especially during dry storage or transportation events, depending on the temperature and stress conditions. In this work, we tested the existing hydrogen migration and redistribution model implemented in the Bison fuel performance code by comparing Bison predictions of hydrogen distribution in various temperature profiles against historical published experimental data. Using 1D simulations of hydrogen evolution in zirconium-alloy, we demonstrated the accuracy of the existing model in the case of a sharp temperature gradient profile. We also performed a sensitivity analysis to investigate the impact of the model parameters on Bison predictions of hydrogen migration and redistribution to understand the discrepancies in the case of a low temperature gradient profile. In both cases, the Bison prediction of hydrogen distribution is accurate in the regions where the temperatures are higher. In these areas, the relative error on average does not exceed 1%. On the other hand, the error grows increasingly higher in the cold regions where hydrides are more likely to form and a steep gradient in the hydrogen concentration profile develops. The potential for the uncertainties in the model parameters to explain this behavior is investigated through the sensitivity analysis, including the impact of a clamping factor that artificially reduces the maximum allowable volume fraction of hydrides. This study also provides a literature review on hydrogen migration and redistribution experiments and a systematic comparison of the relative experimental data.

We demonstrate a global sensitivity and uncertainty analysis approach to quantify the impact of uncertainty in the hydrogen migration and redistribution models implemented in the U.S. Department of Energy Office of Nuclear Energy fuel performance code Bison. In this study, we provide a brief description of the physical phenomena studied and the sensitivity analysis methods used. To identify the key parameters related to the hydrogen migration and redistribution model in Bison, we study the impact of the variance of the model parameters on the amount of hydrides formed near the outer surface of the nuclear fuel cladding, where hydrides are more likely to form, under the normal operation conditions of a light water reactor. To quantify the impact of the input variance of the parameters on the output variations, we compute the variance-based indices (Sobol indices) and the Pearson correlation coefficients. The results of this work show that the activation energy for the terminal solid solubility of hydride precipitation, the hydrogen heat of transport and the activation energy for hydrogen diffusivity are the key parameters. An optimized set of these parameters was then determined as an attempt to increase the accuracy of Bison predictions by decreasing the root mean square error of the predictions versus experimental results, using a basin-hopping optimization framework.

Hydrogen absorbed into zirconium alloy nuclear fuel cladding as a result of the waterside corrosion reaction can affect the properties of nuclear fuel, principally through the precipitation of brittle hydride particles. Multiple phenomena are involved in this overall process: after hydrogen pickup degradation of mechanical properties is controlled by hydrogen transport, hydride precipitation and dissolution kinetics and the formation of specific mesoscale hydride microstructures. The precipitation of hydrides especially affects cladding ductility and fracture toughness, but can also affect other phenomena, including via stress-induced hydride reorientation. These processes can affect cladding performance both during normal operation and during extended dry storage, as hydride morphology can be modified during the preparatory vacuum drying processes. We review the processes of hydrogen transport, hydride precipitation and dissolution and formation of mesoscale hydride microstructures, and highlight where more research is needed, both from an experimental and from a modeling point of view.

Losses of carbon (C) and nitrogen (N) from the decomposition of woody debris in harvested forest ecosystems is of interest in national C accounting, regional water quality assessment as well as for managing long-term localised forest fertility. The main commercial forest type in Ireland is Sitka spruce dominated plantations and the woody debris from harvesting is routinely gathered into windrows in reforested stands. This study examined woody decomposition losses via two pathways namely respiration to the atmosphere and rainwater leaching to soil water. A chronosequence of forest stands was sampled, ranging in age (of the harvest debris) from three to fifteen years. Respiratory C loss from windrows was found to be 11.5 t C ha−1 year−1 three years after reforestation. This rate had dropped by 29% to 8.1 t C ha−1 yr−1 twelve years later. There was a strong seasonal trend in this efflux and the main driver of this variation was soil temperature, however moisture content and drying/rewetting cycles also played a role. Leached losses of C followed a similar trend to respired losses, but their rate was consistently less than 1% of respiration. Total dissolved N fluxes in woody debris leachate followed a different pattern, where more was retained by the system than lost. Thus, windrowed woody debris acted as a nutrient reservoir for N but as a steady source of C.

Meeting the reporting requirements of the Kyoto Protocol has focused attention on the potential of forests in sustainably sequestering carbon (C) to mitigate the effects of rising levels of atmospheric CO2. Much uncertainty remains concerning the ultimate effect of management on such sequestration effects. The management of woody debris (WD) and other deadwood stocks is an example of a management intervention with the scope of affecting the source-sink dynamics of forest C. Windrowing is the most commonly employed approach to the management of post-harvest WD. This study investigated the quantities of windrowed deadwood C across a chronosequence of reforested commercial Sitka spruce stands in Ireland and how its decomposition rate affected its contribution to forest C sequestration. The C stocks in windrowed WD ranged from 25 to 8 t C ha−1 at the 4- and 16-year-old stands, respectively. Losses due to the decomposition of these stocks ranged from 5.15 t C ha−1 yr−1 at the youngest site (4 years old) to 0.68 t C ha−1 yr−1 at the oldest site (16 years old). Using a visual decay-class categorization of WD components and an assessment of wood density, decay rate constants were estimated for logs, branches, and stumps (the main WD constituents of windrows) as 0.037, 0.038, and 0.044, respectively. These results, derived from stand stock evaluations, were placed into context with data previously published from the same chronosequence that characterized the day-to-day fluxes to or from this pool. This comparison indicated that though only a very small quantity of C was lost in dissolved leachate form, the most significant pathway for loss was respiratory and ranged from 16 to 8 t C ha−1 yr−1 at the 9- and 16-year-old sites. These estimates were many times greater in extent than estimates made using a density-loss approach, the difference indicating that fragmentation and weathering play a large role in woody decomposition in intensively managed forests.

Il contributo si concentra sulle dinamiche dell'economia ai tempi della pandemia e nel postpandemia con particolare attenzione alle forme nuove di aggressione delle organizzazioni mafiose su diversi mercati. In particolare partendo innanzitutto da una criticità che va assumendo il carattere originario di un circolo vizioso: il sovraindebitamento delle famiglie e delle imprese. Inoltre ci si concentra sull’indicazione di tre moltiplicatori del dirty money criminale e alle nuove modalità di acquisizione di patrimoni nonché alle nuove forme di riciclaggio. L’attenzione è dedicata: a) al mercato parallelo del credito illegale, mediante l’esercizio abusivo del credito e l’usura. Preoccupa che, anche prima della pandemia, gli affari delle mafie si siano organizzati addirittura producendo false denunce di usura: un meccanismo che ha consentito una enorme truffa a danno del Fondo (ex art. 14 l. n. 108/1996). Un altro segmento attenzionato b) è l’aggressione ai patrimoni privati di cittadini e imprese mediante l’acquisto di immobili presso le aste giudiziarie e i fallimenti aziendali; infine, c) l’acquisizione di proprietà di imprese e di immobili attraverso prestiti usurai supportati da patti commissori.

The physics-based fission gas behaviour model available in the BISON fuel performance code provides satisfactory predictive capabilities for application to light-water reactor conditions. In this work, we present a model extension for application to fast reactor (U,Pu)O 2 fuel. In particular, we detail the introduction of a lower bound to the number density of grain-face bubbles, representing a limit to the coalescence process once extensive bubble interconnection is achieved. This new feature is tested first against an experimental database for UO 2 -LWR, and secondly is validated against integral irradiation experiments for fast reactor (U,Pu)O 2 fuel rods irradiated in the FFTF (Fast Flux Test Facility) and in the JOYO reactors. The comparisons of BISON results with the experimental data are satisfactory and demonstrate an improvement compared to the standard version of the code.

We propose a model describing the HBS formation and the progressive intra-granular xenon depletion in UO2. The HBS formation is modeled employing the Kolmogorov-Johnson-Mehl-Avrami (KJMA) formalism for phase transformations, which has been fitted to experimental data on the restructured volumetric fraction as a function of the local effective burnup. To this end, we employed available experimental data and novel data extracted in this work. The HBS formation model is coupled to a description of the intra-granular fission gas behavior, allowing to estimate the evolution of the retained xenon in order to consistently compute fission gas retention and its effect on the fuel matrix swelling. The satisfactory agreement of the model predictions to experimental data and state-of-the-art models’ results, in terms of both xenon depletion and fuel matrix swelling as a function of the local burnup, paves the way to the inclusion of the model in fuel performance codes.

This paper describes the results of a systematic sensitivity analysis for silver diffusional release from Tristructural Isotropic (TRISO) fuel particles in the AGR-1 base irradiation test and high temperature safety test database. The BISON code was used to model the diffusional release of silver through the coated particle for a wide range of the irradiation and the high temperature conditions conducted in the AGR-1 tests. The current BISON results agree well with other computations and measurements. We then conducted the systematic sensitivity analysis to identify the impact of Ag diffusion coefficient in the TRISO layer materials on the prediction of Ag release. Sobol indices were computed to indicate the contribution of each input, and the scatter plots were analyzed to investigate the correlations between diffusivity parameters and predicted Ag release. Informed by the Sobol sensitivity analysis, a calibrated set of diffusion coefficients were obtained which gave better predictions. The result of this best-estimate calibration indicates a Ag diffusion pre-factor in the SiC from 3.85 × 10−10 to 3.19 × 10−9 m2/s and Ag activation energy in SiC between 178 and 185 kJ/mole. The outputs are expected to provide the systematic approach for better understanding of Ag transport inside TRISO particle.

Phosphorus (P) availability in calcareous forest soils is commonly low compared to siliceous soils. The main reason for this is that phosphate ions tend to precipitate with calcium (Ca). Weathering of calcareous rocks and the potential of microorganisms to dissolve calcareous parent material is not fully understood. Therefore, we examined microbial carbonate dissolution and the abundance of phosphorus-solubilizing bacteria in temperate forest soils with contrasting calcareous parent materials. We incubated soil extracts with weathered parent materials (i.e., dolomite and limestone) from two calcareous forest soils differing in P content and determined the rates of P and Ca solubilization. In addition, we determined the abundance of phosphorus-solubilizing bacteria (PSB). We found that the net Ca solubilization rate ranged from 8.8 to 511.1 nmol m−2 d−1 across both soils and depths. Calcium dissolution rates were negatively related to pH and positively related to the concentration of organic acids. The gross P solubilization rates were on average 63.6% higher from dolomite (P-poor soil) than from limestone (P-rich soil). The abundance of soil PSB ranged from 3.8 % at the limestone site (P-rich soil) to 24.4 % at the dolomite site (P-poor soil). The higher abundance of PSB in the soil derived from dolomite is in line with the high Ca and P solubilization rates at this site, indicating that PSB abundance is related to rock weathering rates from calcareous soils. Pseudomonadales and Enterobacteriales were by far the two most abundant bacterial orders in the PSB community of both soils and soil depths. In conclusion, this study shows, first, that weathering of calcareous bedrocks is strongly affected by the activity of soil microorganisms, and second, that there is likely a selective pressure in P-poor soils towards a higher abundance of PSB.

The near-term Accident Tolerant Fuel (ATF) concepts consist of minor modifications of the standard UO2-Zr fuel rod materials to provide enhanced accident tolerance. Uranium dioxide fuel with chromia dopant, developed by AREVA, is of interest in view of its enhanced fission gas retention. The predictive capability of the BISON fuel performance code for chromia-doped UO2 fuel is assessed through simulation of the Halden IFA-677.1 fuel rod experiment. A sensitivity analysis is conducted to interpret the deviation of code’s predictions from the experimental data. A power ramp test is also modeled with BISON and calculations are compared to the experimental database. Finally, a large-break LOCA (LBLOCA) case is simulated with BISON, leading to an initial assessment of the enhanced safety associated with chromia-doped fuel compared to the standard UO2 fuel in LWRs.

The description of intra-granular fission gas behaviour is a fundamental part of any model for the prediction of fission gas release and swelling in nuclear fuel. In this work we present a model describing the evolution of intra-granular fission gas bubbles in terms of bubble number density and average size, coupled to gas release to grain boundaries. The model considers the fundamental processes of single gas atom diffusion, gas bubble nucleation, re-solution and gas atom trapping at bubbles. The model is derived from a detailed cluster dynamics formulation, yet it consists of only three differential equations in its final form; hence, it can be efficiently applied in engineering fuel performance codes while retaining a physical basis. We discuss improvements relative to previous single-size models for intra-granular bubble evolution. We validate the model against experimental data, both in terms of bubble number density and average bubble radius. Lastly, we perform an uncertainty and sensitivity analysis by propagating the uncertainties in the parameters to model results.

Uranium silicide ( U 3 Si 2 ) is a concept explored as a potential alternative to UO 2 fuel used in light water reactors (LWRs) since it may improve accident tolerance and economics due to its higher thermal conductivity and increased uranium density. U 3 Si 2 has been previously used in research reactors in the form of dispersion fuel which operates at lower temperatures than commercial LWRs. The research reactor data illustrated that significant gaseous swelling occurs as the fuel burnup increases. Therefore, it is imperative to understand the fission gas behavior of U 3 Si 2 under higher temperature LWR operating conditions. In this work, molecular dynamics and phase-field modeling techniques are used to reduce the uncertainty in select modeling assumptions made in developing the fission gas behavior model for U 3 Si 2 in the BISON fuel performance code. To support the implementation of a U 3 Si 2 fission gas model in BISON, cluster dynamics simulations of irradiation enhanced Xe diffusion have been carried out. Similarly, MD simulations were used to predict the athermal contribution due to atomic mixing during ballistic damage cascades. By combining our results with literature DFT data for thermal equilibrium diffusion, Xe diffusivity has been described over a wide range of temperatures for in-reactor conditions. These lower length scale informed models are then utilized in the assessment of BISON U 3 Si 2 modeling capabilities by simulating the ATF-1 experiments irradiated in the Advanced Test Reactor (ATR). Sensitivity analysis (SA) and uncertainty quantification (UQ) are included as part of the assessment process to identify where further experiments and lower length scale modeling would be beneficial. The multiscale modeling approach utilized in this work can be applied to new fuel concepts being explored for both LWRs and advanced reactors (e.g., uranium nitride, uranium carbide).

The formation of hydrides, a result from excess hydrogen uptake by the cladding during nuclear reactor operation, can significantly impact cladding integrity. We present a sensitivity and uncertainty analysis of a hydrogen predictive model in the BISON fuel performance code to characterize the key input parameters involved in the model. This includes identifying the key parameters necessary to simulate hydrogen behavior in the fuel cladding, revealing the impact of environmental conditions on hydrogen distribution, and informing the envelope of conditions for ongoing experimental work conducted by the University of Michigan. The Sobol sensitivity analysis reveals the quantitative impacts of environmental conditions on the predicted total hydrogen concentration, as well as the respective impact on their sensitivity with respect to the physical parameters. Overall, the precipitation of hydrogen that occurred at the cold end of the sample is the most important phenomenon in the prediction of hydrogen concentration. The optimization study using the results from sensitivity analysis indicates that the BISON simulations produce accurate hydrogen predictions when the sets of parametric ranges are shifted to enable more precipitation to occur at the cold end. Lastly, the sensitivity and uncertainty (S/U) analysis for the ongoing benchmarking experiments supports the focus of experiments that lies on the measurement of Soret effect of hydrogen driven by linear temperature gradients. The outputs are expected to better characterize the various parameters involved in the hydrogen transport model in the BISON code, and improve the understanding of the hydrogen transport behaviors in zirconium-based fuel cladding in a range of expected environmental conditions.

In the rim zone of UO2 nuclear fuel pellets, the combination of high burnup and low temperature drives a microstructural change, leading to the formation of the high burnup structure (HBS). In this work, we propose a semi-empirical model to describe the formation of the HBS, which embraces the polygonisation/recrystallization process and the depletion of intra-granular fission gas, describing them as inherently related. For this purpose, we performed grain-size measurements on samples at radial positions in which the restructuring was incomplete. Based on these new experimental data, we infer an exponential reduction of the average grain size with local effective burnup, paired with a simultaneous depletion of intra-granular fission gas driven by diffusion. The comparison with currently used models indicates the applicability of the herein developed model within integral fuel performance codes.

In this work, we present a model of fission gas behavior in U3Si2 under light water reactor (LWR) conditions for application in engineering fuel performance codes. The model includes components for intra-granular and inter-granular behavior of fission gases. The intra-granular component is based on cluster dynamics and computes the evolution of intra-granular fission gas bubbles and swelling coupled to gas diffusion to grain boundaries. The inter-granular component describes the evolution of grain-boundary fission gas bubbles coupled to fission gas release. Given the lack of experimental data for U3Si2 under LWR conditions, the model is informed with parameters calculated via atomistic simulations. In particular, we derive fission gas diffusivities through density functional theory calculations, and the re-solution rate of fission gas atoms from intra-granular bubbles through binary collision approximation calculations. The developed model is applied to the simulation of an experiment for U3Si2 irradiated under LWR conditions available from the literature. Results point out a credible representation of fission gas swelling and release in U3Si2. Finally, we perform a sensitivity analysis for the various model parameters. Based on the sensitivity analysis, indications are derived that can help in addressing future research on the characterization of the physical parameters relative to fission gas behavior in U3Si2. The developed model is intended to provide a suitable infrastructure for the engineering scale calculation of fission gas behavior in U3Si2 that exploits a multiscale approach to fill the experimental data gap and can be progressively improved as new lower-length scale calculations and validation data become available.

In 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.

The formation and binding energetics that control the nucleation behavior of Xe-vacancy defect clusters in uranium monocarbide (UC) is investigated via density functional theory (DFT) calculations. Based on previous work, assumptions on the effective formation energy of U and C vacancies are made, and the substitutional defect Xe U is considered as the most stable Xe defect under carbon-rich conditions. The DFT calculations for the binding energy indicate that, depending on the non-equilibrium vacancy concentration under the specific irradiation conditions, clusters can grow by absorbing uranium vacancies, while a saturation effect exists for the absorption of carbon vacancies. It is also found that, depending on the supersaturation and correspondingly, the effective formation energy of Xe U under irradiation, the stable nucleus for a Xe bubble consists of a small cluster of 2 or 3 Xe atoms.

One of the advanced nuclear fuel concepts for current commercial water-cooled reactors focuses on microstructural modification of UO2 fuel via dopants. Dopants can effectively promote grain growth and suppress fission gas release (FGR), a key parameter that dictates the overall nuclear fuel performance. This work improves the BISON FGR model for chromia/alumina-doped UO2 fuel through statistical calibration with in-reactor experimental data. The high computing cost and nonintrusive nature of BISON limit the application of conventional techniques under the Bayesian framework. Dimensionality reduction is performed using principal component analysis (PCA) to deal with the FGR time series data. Kriging is used as metamodel of BISON to reduce the computing cost. A novel optimization framework, Variational Bayesian Monte Carlo (VBMC) is demonstrated as a low-cost nonintrusive approach for Bayesian calibration. The performance of VBMC is compared to the conventional statistical Markov Chain Monte Carlo (MCMC) sampling showing similar accuracy but superior efficiency.

In the first part of this work, we discussed BISON fuel performance code developments for Zircaloy cladding behavior under loss-of-coolant accident (LOCA) conditions and validation to separate-effects experiments. In this paper, we present modeling developments for UO2 fuel behavior during LOCAs and validation to integral experiments. Code developments are related in particular to modeling axial relocation of fuel fragments during cladding ballooning. Code validation is performed against three integral fuel rod LOCA tests from the Halden IFA-650 series, covering fresh and pre-irradiated fuel rods with and without significant axial fuel relocation observed. Calculated results are systematically compared to experimental data of fuel rod inner pressure evolution during the LOCA transients, time to cladding burst and post-test cladding diameter profile. Comparisons in terms of rod pressure evolution and time to burst appear adequate. Also, the ability to reproduce axial relocation of fuel fragments is demonstrated. Calculated cladding diameter profiles are shown to be considerably sensitive to the selection of the burst failure criterion, highlighting the difficulty to accurately evaluate cladding strains at burst during postulated LOCA scenarios with fuel performance codes.

During the operation of a light water reactor, a fraction of the hydrogen produced by waterside corrosion is absorbed into the nuclear fuel cladding. When the hydrogen concentration reaches its solubility limit, a brittle zirconium hydride phase precipitates, leading to a loss of ductility of the cladding. To assess cladding integrity, an accurate simulation tool is needed to predict hydrogen distribution within the cladding and hydride precipitation. Recent studies have developed an improved understanding of the physical processes involved in hydrogen redistribution, and hydride precipitation and dissolution. This research led to the development of a new model, called Hydride Nucleation-Growth-Dissolution (HNGD). The present work describes the implementation of HNGD into the fuel performance code BISON, developed at Idaho National Laboratory. The main innovative feature of the HNGD model is that it accounts for hydride nucleation and growth as two distinct precipitation components, using the Johnson-Mehl-Avrami-Kolmogorov model to describe hydride growth kinetics. Each step of the model implementation into BISON was systematically verified, and simulations of experiments performed for validation, showing that the HNGD model provides improved predictions, and captures some experimentally observed physical phenomena related to hydride growth that the previous model could not.

One of the important roles of cladding in light water reactor fuel rods is to prevent the release of fission products. To that end, it is essential that the cladding maintain its integrity under a variety of thermal and mechanical loading conditions. Local geometric irregularities in fuel pellets caused by manufacturing defects known as missing pellet surfaces (MPS) can in some circumstances lead to elevated cladding stresses that are sufficiently high to cause cladding failure. Accurate modeling of these defects can help prevent these types of failures. The BISON nuclear fuel performance code developed at Idaho National Laboratory can be used to simulate the global thermo-mechanical fuel rod behavior, as well as the local response of regions of interest, in either 2D or 3D. In either case, a full set of models to represent the thermal and mechanical properties of the fuel, cladding and plenum gas is employed. A procedure for coupling 2D full-length fuel rod models to detailed 3D models of the region of the rod containing a MPS defect is detailed here. The global and local model each contain appropriate physics and behavior models for nuclear fuel. This procedure is demonstrated on a simulation of a boiling water reactor (BWR) fuel rod containing a pellet with an MPS defect, subjected to a variety of transient events, including a control blade withdrawal and a ramp to high power. The importance of modeling the local defect using a 3D model is highlighted by comparing 3D and 2D representations of the defective pellet region. Parametric studies demonstrate the effects of the choice of gaseous swelling model and of the depth and geometry of the MPS defect on the response of the cladding adjacent to the defect.

The 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.

BISON is a modern finite element-based nuclear fuel performance code that has been under development at Idaho National Laboratory (INL) since 2009. The code is applicable to both steady and transient fuel behavior and has been used to analyze a variety of fuel forms in 1D spherical, 2D axisymmetric, or 3D geometries. Code validation is underway and is the subject of this study. A brief overview of BISON's computational framework, governing equations, and general material and behavioral models is provided. BISON code and solution verification procedures are described, followed by a summary of the experimental data used to date for validation of Light Water Reactor (LWR) fuel. Validation comparisons focus on fuel centerline temperature, fission gas release, and rod diameter both before and following fuel-clad mechanical contact. Comparisons for 35 LWR rods are consolidated to provide an overall view of how the code is predicting physical behavior, with a few select validation cases discussed in greater detail. Results demonstrate that (1) fuel centerline temperature comparisons through all phases of fuel life are very reasonable with deviations between predictions and experimental data within ±10% for early life through high burnup fuel and only slightly out of these bounds for power ramp experiments, (2) accuracy in predicting fission gas release appears to be consistent with state-of-the-art modeling and with the involved uncertainties and (3) comparison of rod diameter results indicates a tendency to overpredict clad diameter reduction early in life, when clad creepdown dominates, and more significantly overpredict the diameter increase late in life, when fuel expansion controls the mechanical response. Initial rod diameter comparisons are not satisfactory and have led to consideration of additional separate effects experiments to better understand and predict clad and fuel mechanical behavior. Results from this study are being used to define priorities for ongoing code development and validation activities.

Accurately predicting changes in the thermal conductivity of light water reactor UO2 fuel throughout its lifetime in reactor is an essential part of fuel performance modeling. However, typical thermal conductivity models from the literature are empirical. In this work, we begin to develop a mechanistic thermal conductivity model by focusing on the impact of gaseous fission products, which is coupled to swelling and fission gas release. The impact of additional defects and fission products will be added in future work. The model is developed using a combination of atomistic and mesoscale simulation, as well as analytical models. The impact of dispersed fission gas atoms is quantified using molecular dynamics simulations corrected to account for phonon-spin scattering. The impact of intragranular bubbles is accounted for using an analytical model that considers phonon scattering. The impact of grain boundary bubbles is determined using a simple model with five thermal resistors that are parameterized by comparing to 3D mesoscale heat conduction results. When used in the BISON fuel performance code to model four reactor experiments, it produces reasonable predictions without having been fit to fuel thermocouple data.

U3Si2 is a potential accident-tolerant fuel that shows promise due to its high thermal conductivity and higher uranium density relative to UO2. However, its swelling and fission gas release behavior in light water reactor (LWR) conditions is relatively unknown. To provide mechanistic insight and determine parameters for engineering-scale fuel performance modeling of pellet-form U3Si2, phase-field simulations of the growth, interconnection, and venting of intergranular fission gas bubbles were performed. The fractional coverage of the grain boundary and the fraction of bubble area that is vented were calculated as a function of time. From the simulation data, the fractional grain boundary coverage at saturation, an important parameter needed in engineering-scale modeling of swelling and fission gas release, was determined. Multiple simulations were run to determine the uncertainty in the calculated value. The effect of model assumptions and input parameters that are not well known was evaluated. Simulation results are compared to related theoretical and computational work. Based on the simulation results, a value of 0.60 for the fractional grain boundary coverage at saturation is recommended for U3Si2 fuel.

Biogeochemical weathering of bedrock is the most important input of silicon (Si) and phosphorus (P) to forest ecosystems. While soil microbes, and in particular P-solubilizing bacteria (PSB), are known to accelerate the solubilization of Si and P from silicate rocks, our understanding of the mechanisms driving biogenic weathering are still limited. To fill this gap, incubation experiments with weathered parent materials (i.e. basalt, andesite and paragneiss) of four soils and water extracts of the four soils, differing in P stocks, were conducted. We found that the net Si solubilization rate ranged from 5.0 (±1.8) to 91.0 (±2.4) nmol m−2 d−1 across all examined soils. The silicate dissolution rates were negatively related to the decrease in pH (ΔpH) and positively related to the amounts of organic acids released by microbes. We found that the gross P solubilization rates from the parent materials were ~11 times higher at the P-rich site (BBR) compared to the P-poor site (CON). In addition, we determined the abundance and the taxonomic composition of PSB communities in the four soils. The abundance of PSB ranged from 2% at the P-rich site to 22.1% at the P-poor site, indicating that a selective pressure exists in P-poor soils towards a higher abundance of P-solubilizers. Yet, despite the relative high abundance of PSB, the gross P solubilization rates were low in the soils derived from P-poor parent material. The genus Pseudomonas was found only in the PSB community at the P-poor site. Burkholderiales and Bacillales together were by far the two most abundant orders among the PSB communities in all soils and depths. In conclusion, this study shows that PSB are more abundant in P-poor soil than in P-rich soil, while the weathering rate seems to be mostly dependent on the bedrock.