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The pressures from machinery use and continuous agricultural production practices exacerbate soil compaction. Deep tillage (DT) can reduce subsoil compaction, but the spatial composition patterns and community assembly mechanisms of subsoil vs. topsoil bacteria mediated by DT are unknown. Here, we collected soil profile samples from 18 agricultural fields within a multiple cropping area of southern China to investigate the status of soil compaction. The effects of DT on soil physicochemical properties and bacterial communities at different soil depths were investigated by establishing three long-term experimental sites, and the spatial composition mechanisms of the bacterial community were preliminarily explored. Our results showed that soil compaction occurred at soil depths greater than 20 cm, as evidenced by higher soil bulk density, and sharp decreases in water and nutrient contents and bacterial community diversity. Although the ameliorative effect of DT on deep soil compaction diminished in the fourth year, the water and organic matter content and bacterial α diversity remained high. DT resulted in a more homogeneous bacterial community across the soil profile in terms of community similarity and compositional stability, along with increased alpha diversity, all of which were associated with reduced heterogeneity in the soil variables, increased soil organic matter content, and the importance of homogeneous selection in the community assembly mechanism. Additionally, the homogenization of bacterial communities under DT may promote enhancement of bacterial network complexity and stability. Collectively, our findings reveal the importance of deep tillage for deep soil improvement and spatial homogenization of bacterial communities, which has far-reaching implications for comprehensively understanding the spatial dynamics and assembly mechanisms of tillage-mediated soil microbial communities in agroecosystems. • Soil compaction occurred at soil depths greater than 20 cm in multiple cropping field. • The effect of deep tillage continued to influence deep soils after four years. • Deep tillage enhances bacterial community similarity and compositional stability. • Higher organic matter at 20–30 cm soil depth promotes bacterial spatial homogenization. [ABSTRACT FROM AUTHOR]

Abstract A key challenge for full-core transport methods is reactor agnostic multi-group cross section (MGXS) generation. Monte Carlo (MC) presents the most accurate method for MGXS generation since it does not require any approximations to the neutron flux. This paper introduces a framework that replaces the traditional multi-level approach to MGXS generation with a single MC calculation to generate the fine-spatial mesh MGXS that are needed by high-fidelity transport codes. In addition, three pin-wise spatial homogenization schemes are introduced to model inter-pin spatial self-shielding spectral effects with varying levels of spatial fidelity. Most notably, the Local Neighbor Symmetry (LNS) scheme uses a nearest neighbor-like analysis of a reactor geometry to determine which fuel pins should be assigned the same MGXS. The single-step MGXS framework with the three spatial homogenization schemes are evaluated for two PWR benchmarks. The results demonstrate the potential for single-step MC simulations of the complete heterogeneous geometry as a means to generate reactor agnostic MGXS for deterministic transport codes. [ABSTRACT FROM AUTHOR]

When collapsing multi-group cross sections, a flux separability approximation is often used. This assumes the angular variation of the flux is independent of the energy dependence, which avoids angular dependence of the total multi-group cross section. This paper investigates the impact of this approximation on fine-mesh deterministic multi-group transport methods for two PWR pin-cell benchmarks, which demonstrate errors of more than 1% in energy groups with large U-238 capture resonances and an eigenvalue bias of approximately 200 pcm between continuous energy Monte Carlo and deterministic transport methods, even when the “true” scalar flux is used to collapse cross sections. This paper also investigates two means of resolving this issue, but both are seen to have significant short-comings. First, the most direct and mathematically consistent approach is to use angularly-dependent multi-group cross sections. These cannot be easily computed for arbitrary geometries using traditional multi-group cross section generation methods, are not supported by most standard transport codes, and require significant spatial discretization. Second, SuPerHomogéneísation (SPH) factors are used to preserve reaction rates between continuous energy Monte Carlo and deterministic transport methods, but the SPH scheme requires knowledge of the reference source distribution, is dependent on the spatial discretization mesh, and is indiscriminate between various sources of approximation error. [ABSTRACT FROM AUTHOR]

Numerous researchers have been unsuccessful in obtaining accurate LWR power distributions using two-group nodal diffusion codes and neutronic data generated from single-assembly lattice calculations. Researchers using the Serpent Monte Carlo code to generate data for the ARES nodal code have determined that large buffer zones surrounding each fuel assembly are required to obtain data that permits accurate nodal calculation of core power distributions. While successful, such larger buffer zones result in extremely large computational requirements for lattice data generation. Since accurate commercial LWR power distributions have been obtained using single-assembly lattice calculations in the CASMO and SIMULATE lattice/nodal codes, it clearly should be possible to use Monte Carlo to replace deterministic lattice calculations. In this paper, we present numerous important refinements of nodal diffusion methods that are required to obtain accurate commercial LWR power distributions using single-assembly lattice data. Many of these refinements have never been published (because of the proprietary nature of their development), and consequently, many subtleties of nodal methods are not well known in the nuclear reactor analysis community. The importance and impact of numerous refinements of basic nodal methods are documented here using the Hot Zero Power (HZP) BEAVRS PWR benchmark. This paper documents only those nodal refinements needed to accurately predict assembly-averaged power distributions without thermal-hydraulic feedback (e.g. isothermal conditions) and fuel depletion. Additional refinements needed for thermal feedback, fuel depletion, and pin power reconstruction are deferred for subsequent publications. [ABSTRACT FROM AUTHOR]

This paper describes the methods used in the Serpent 2 Monte Carlo code for producing homogenized group constants for nodal diffusion and other deterministic reactor simulator calculations. The methodology covers few-group reaction cross sections, scattering matrices, diffusion coefficients and poison cross sections condensed in infinite and B 1 leakage-corrected critical spectra, as well as the calculation of discontinuity factors, pin-power form factors, delayed neutron parameters and total and partial albedos. Also included is a description of an automated burnup sequence, which was recently implemented for the handling of restart calculations with branch variations. This capability enables covering the full range of local operating conditions required for the parameterization of group constants within a single run. The purpose of this paper is to bring the methodological description provided in earlier publications up to date, and provide insight into the developed methods and capabilities, including their limitations and known flaws. [ABSTRACT FROM AUTHOR]

Abstract: The accuracy of static neutronic parameters in the nuclear reactors depends upon the determination of group constants of the diffusion equation in the desired geometry. Although several methods have been proposed for calculating these parameters, there is still the need for more reliable methods. In this paper a powerful and innovative method based on Spatial Homogenization and Temperature Variation (SHTV) of physical properties of a WWER-1000 nuclear reactor core for calculating the relative power distribution of Fuel Assemblies (FA) and the hot fuel rod, is presented. The method is based on replacing the heterogeneous lattices of materials with different properties by an equivalent homogeneous mixture of these material for determining the few group constants, while the effect of temperature variation in the fuel and coolant density along the axial core direction is considered. All calculations are performed using WIMS and CITATION codes. The obtained results are compared with the results of Final Safety Analysis Report (FSAR) prepared by the designer, and good agreement between the two results is shown. [Copyright &y& Elsevier]

The mechanisms that regulate the spatial distribution of species are an essential aid to understanding the effects of the environment on the persistence of populations and communities. The effects of spatial structure on the persistence and robustness of ecological communities can, in turn, prove useful in uncovering their functioning, e.g., in the decomposition of leaf detritus. We applied the framework of complex networks to evaluate the effects of spatial structure on the colonization process of leaf detritus in a patchy aquatic environment, with a spatial network of six pools at different salinity. We found three well-defined modules formed by groups of taxa sharing the same pools, observing an association between modularity and spatial proximity of pools. Modules maximize the number of links within modules, and minimize the number of links among modules, showing the presence of a strong site-specific association between taxa and pools. The topological characteristics of the network show robustness against random perturbations and a lower tolerance of targeted perturbations. These findings suggest that random events, such as flooding or heavy rains, slightly affect the robustness of the system, while localized perturbations on the most connected nodes could have a negative effect on the connectivity of the whole network. The consequences could lead to a structural and functional homogenization of the system, with potential effects for the entire trophic chain. Here we discuss the topological properties of the network in relation to the spatial distribution of pools, showing how network analysis can yield valuable insight for conservation and management. [ABSTRACT FROM AUTHOR]

Abstract: Overgrazing has led to severe degradation and desertification of semi-arid grasslands in Northern China over the last decades. Despite the fact that vegetation is often heterogeneously distributed in semi-arid steppes, little attention has been drawn to the effect of grazing on the spatial distribution of soil properties. We determined the spatial pattern of soil organic carbon (SOC), total nitrogen (Ntot), total sulphur (Stot), bulk density (BD), pH, Ah thickness, and carbon isotope ratios (δ13C) at two continuously grazed (CG) and two ungrazed (UG79=fenced and excluded from grazing in 1979) sites in Leymus chinensis and Stipa grandis dominated steppe ecosystems in Inner Mongolia, Northern China. Topsoils (0–4cm) were sampled at each site using a large grid (120m×150m) with 100 sampling points and a small plot (2m×2m) with 40 points. Geostatistics were applied to elucidate the spatial distribution both at field (120m×150m grid) and plant (2m×2m plot) scale. Concentrations and stocks of SOC, Ntot, Stot were significantly lower and BD significantly higher at both CG sites. At the field scale, semivariograms of these parameters showed a heterogeneous distribution at UG79 sites and a more homogeneous distribution at CG sites, whereas nugget to sill ratios indicated a high small-scale variability. At the plant scale, semivariances of all investigated parameters were one order of magnitude higher at UG79 sites than at CG sites. The heterogeneous pattern of topsoil properties at UG79 sites can be attributed to a mosaic of vegetation patches separated by bare soil. Ranges of autocorrelation were almost congruent with spatial expansions of grass tussocks and shrubs at both steppe types. At CG sites, consumption of biomass by sheep and hoof action removed vegetation patches and led to a homogenization of chemical and physical soil properties. We propose that the spatial distribution of topsoil properties at the plant scale (<2m) could be used as an indicator for degradation in semi-arid grasslands. Our results further show that the maintenance of heterogeneous vegetation and associated topsoil structures is essential for the accumulation of SOM in semi-arid grassland ecosystems. [Copyright &y& Elsevier]

Abstract: The C5G7 MOX benchmark specifying a sixteen-assembly core with asurrounding water reflector was proposed as a basis to measure current transport code abilities in the treatment of reactor core problems without spatial homogenization. Seven-group cross sections for all materials were used as initial information. Just that fact allows to test an accuracy of solving the neutron transport equation excluding additional errors connected with preparing the group cross sections. In this paper, Surface Harmonics Method (SHM) is applied to calculation of the two-dimensional configuration of this benchmark. Different approximations of SHM were applied, both with and without spatial homogenization. Additionally, this fact allowed evaluating the effect of spatial homogenization of cells. Comparisons were carried out for keff and pin powers both with the reference results and between the results calculated by different SHM approximations. [Copyright &y& Elsevier]

• Homogenized few-group XS are generated using Serpent 2.1.29 and MCU–PTR. • High-leakage problems with IRT-3M fuel and light water reflector are analyzed. • Different methods for the generation of diffusion coefficients are compared. • The consistency between MCU–PTR and Serpent few-group XS is shown. • The diffusion model is validated against a Monte Carlo 3D model. In this study, we analyze different methods for the generation of homogenized few-group cross sections (XS) using continuous-energy Monte Carlo codes Serpent 2.1.29 and MCU–PTR for full-core diffusion calculation of small-size light water research reactors. Few-group reaction cross sections, scattering matrices and diffusion coefficients generated in an infinite lattice model with and without B 1 -leakage correction of spectra, and in full-scale 3D core calculations, are compared. Few-group XS are used in the TIGRIS diffusion nodal simulator to calculate the simplified cores with IRT-3 M fuel and a light water reflector. Special attention is paid to the impact of diffusion coefficients on the results of diffusion calculations. We compare the following different methods for the generation of diffusion coefficients: the usual "out-scatter" approximation, the transport correction for hydrogen implemented in Serpent, and the migration method implemented in MCU–PTR. [ABSTRACT FROM AUTHOR]