摘要:

AbstractWe present evaluations of the interaction of neutrons with energies between 20 and 100 MeV with oxygen and nitrogen nuclei, which follows from our previous work on carbon. Our aim is to accurately represent integrated cross sections, inclusive emission spectra, and kerma factors, in a data library that can be used in radiation transport calculations. We apply the Feshbach-Kerman-Koonin-GNASH nuclear model code, which includes Hauser-Feshbach, pre-equilibrium, and direct reaction mechanisms, and use experimental measurements to optimize the calculations. We determine total, elastic, and nonelastic cross sections; angle-energy-correlated emission spectra for light ejectiles with A≤4 and gamma rays; and average energy depositions. Our results for charged-particle emission spectra agree well with measurements of Subramanian et al. We compare kerma factors derived from our evaluated cross sections with experimental data, providing an integral benchmarking of our work.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24209

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractWe present an evaluation of the interaction of neutrons with energies between 20 and 100 MeV with carbon nuclei. Our aim is to accurately represent integrated cross sections, inclusive emission spectra, and kerma factors, in a data library for use in radiation transport simulations offast neutron radiotherapy. We apply the Feshbach-Kerman-Koonin-GNASH nuclear model code, which includes Hauser-Feshbach, pre-equilibrium, and direct reaction mechanisms, and use experimental measurements to optimize the calculations. We determine total, elastic, and nonelastic cross sections; angle-energy-correlated emission spectra for light ejectiles with A≤4 and gamma rays; and average energy depositions. Coupled-channel optical model calculations describe the total, elastic, and nonelastic cross sections well. Our results for charged-particle emission spectra agree fairly well with University of California-Davis as well as new Los Alamos National Laboratory and Louvain-la-Neuve measurements. We compare our results with the recent ENDF/B-VI evaluation and argue that some of the exclusive channels between 20 and 32 MeV should be modified. We also compare kerma factors derived from our evaluated cross sections with the measurements, providing an integral benchmark for our work. The evaluated data libraries are available as electronic files.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24210

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe finite element and lumped finite element methods for the spatial differencing of the even-parity discrete ordinates neutron transport equations (EPSN) in two-dimensional x-y geometry are applied. In addition, the simplified even-parity discrete ordinates equations (SEPSN) as an approximation to the EPSNtransport equations are developed. The SEPSNequations are more efficient to solve than the EPSNequations due to a reduction in angular domain of one-half, the applicability of a simple five-point diffusion operator, and directionally uncoupled reflective boundary conditions. Furthermore, the SEPSNequations satisfy the same diffusion limits as EPSNin an optically thick regime, appear to have no ray effect, and converge faster than EPSNwhen using a diffusion synthetic acceleration (DSA). Also, unlike the case of EPSN, the SEPSNsolutions are strictly positive, thus requiring no negative flux fixups. It is also demonstrated that SEPSNis a generalization of the simplified PNmethod. Most importantly, in these second-order approaches, an unconditionally effective DSA scheme can be achieved by simply integrating the differenced EPSNand SEPSNequations over the angles. It is difficult to obtain a consistent DSA scheme with the first-order SNequations. This is because a second-order DSA equation must generally be derived directly from the differenced first-order SNequations.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24211

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractA locally exact numerical scheme (LENS) based on the concept of locally exact numerical differencing is presented. The essence of the LENS scheme consists in determining the coefficients of the difference scheme so that the resulting equation interpolating numerical fluxes at the control volume surface satisfies the analytical solution of transport equations with absorption and source terms. The spatial distribution of the coefficients of transport equations is taken into consideration based on a four-region model among three adjacent control volumes, in which continuous conditions for solutions are imposed on the boundary between two adjacent regions. An analysis of nonoscillation properties of the present LENS scheme was performed using the characteristic polynomial analysis method. It was found that the LENS scheme possesses the potential for nonoscillation properties for stationary convection-diffusion equations with absorption. The LENS scheme is examined through numerical experiments and shows stable and accurate solutions for transport equations with absorption and source terms.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24212

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractA singular perturbation technique is applied to the time-independent one-dimensional neutron transport equation with isotropic neutron scattering. The technique reduces the transport problem to a series of diffusion theory problems in the interior medium and a series of simplified transport problems solved analytically in the boundary layer. The analysis provides a consistent method for deriving and comparing various diffusion theory approximations to the transport equation. In addition, the resulting scheme provides a systematic method for enhancing the accuracy of diffusion theory calculations of global flux distributions. A general asymptotic expansion of c, the number of secondary neutrons per collision, is obtained and an O(ε2) correction to the diffusion theory boundary condition at a material interface is derived. The perturbation technique has been applied analytically to both fixed source and criticality problems. The technique is also incorporated in a multigroup diffusion theory computer code. In test calculations, the error in flux distributions is reduced to about one-half that achieved with standard diffusion theory techniques.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24213

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe FLEXBURN neutron transport code is developed by the discrete ordinates (Sn) method to analyze heterogeneous fuel assemblies in light water reactors. The transport equations are formulated with transmission and leakage probabilities in arbitrary convex square meshes. Arbitrary convex square meshes precisely describe fuel assemblies as lattices of cells. The code deals with fuel assemblies including gadolinia doped fuel rods, water rods, or plutonium mixed fuel rods with control blades. The code can make burnup calculation sequentially to high burnup. The results computed by the FLEXBURN code are validated by comparing them with those of the ANISN typical transport code and the KENO-IV Monte Carlo code. The FLEXBURN code provides control blade worth and detailed distributions of flux, power, burnup, and atomic densities in complicated boiling water reactor and pressurized water reactor fuel assemblies.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24214

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe method to treat randomly distributed spherical fuels in continuous energy Monte Carlo calculations has been established. In this method, the location of a spherical fuel is sampled probabilistically along the particle flight path from the spatial probability distribution of spherical fuels, called the nearest neighbor distribution. The necessary probability distribution was evaluated by a newly developed Monte Carlo hard sphere packing simulation code, which employs a random vector synthesis method to reduce overlaps of spherical fuels. The obtained probability distribution was validated by comparing a cross-section photograph of a real fuel compact and an X-ray diffraction experimental result. This method was installed in a Monte Carlo particle transport code and validated by an inventory check of spherical fuels and criticality calculations of ordered packing models. Also, an analysis of a critical assembly experiment was performed with the new code. As a result, it was confirmed that the method was applicable to practical reactor analysis. The method established is quite unique in the respect of probabilistically modeling the geometry of a great number of spherical fuels distributed randomly without any loss of the advantage of the continuous energy method.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24215

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractWe consider the two-region Milne problem, defined as the steady-state monoenergetic linear transport problem for two adjoining homogeneous source-free half-spaces, with a particle source coming from infinity in one of the half-spaces. We demonstrate that the asymptotic (Case discrete mode) component of the solution for the scalar flux is easily and explicitly written in terms of Chandrasekhar’s H-function for each medium. This asymptotic solution is shown to exhibit a discontinuity in both the scalar flux and current at the interface between the two half-spaces. Numerical benchmark results for the linear extrapolation distance and the discontinuities are given for various combinations of the mean number of secondaries (c) characterizing the two media. Contact is also made with a variational treatment. In particular, the variational formalism is shown to predict the linear extrapolation distance and these asymptotic discontinuities correct to first order in the difference between the values of c characterizing the two half-spaces.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24216

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractCalculations of resonance absorption for simple temperature profiles both in a slab and in a pin of238UO2are presented to show the influence of a nonuniform fuel temperature profile on the choice of the equivalent uniform temperature, or the effective fuel temperature. The effective fuel temperature is given as a weighted average of the temperatures in the fuel zones. Two simple theoretical expressions for this weighted average, derived from the literature, are discussed. First, for high absorption, the effective fuel temperature is given by the so-called chord-averaged fuel temperature. Second, for low absorption, the effective fuel temperature is given by the volume-averaged fuel temperature. The results for a slab of238UO2show that a bruteforce method is necessary to calculate an accurate effective fuel temperature. A set of weights for one specific238UO2pin is calculated. This set agrees well with the chord-averaged fuel temperature. However, this appears to be a coincidence because the results for specific neutron energy ranges do not agree with this set of weights.

ISSN:0029-5639    

DOI:10.13182/NSE94-108

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractCountercurrent flow limitation in channels with evaporation taking place inside them is examined. Countercurrent flow limitation in short, small-diameter channels subject to purely axial, purely radial, and combined axial and radial gas injection is studied. Experiments were performed using air and water, with channel diameters 0.475 to 1.91 cm and channel lengths 1.27 to 5.72 cm. Purely axial gas injection data are shown to agree with Wallis’s correlation but with coefficients that strongly depend on channel dimensions. Purely radial gas injection data and data obtained with combined axial and radial gas injection result in flooding curves significantly different from those representing the purely axial gas injection data and indicate that near complete flooding (zero liquid penetration) can occur in small-diameter and short channels due to relatively small radial gas injection rates. Flooding curves for long or large-diameter channels are insensitive to the gas injection configuration, however.

ISSN:0029-5639    

DOI:10.13182/NSE96-A24218

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor