摘要:

AbstractA hybrid multigroup/continuous-energy Monte Carlo algorithm is developed for solving the Boltzmann-Fokker-Planck equation. This algorithm differs significantly from previous charged-particle Monte Carlo algorithms. Most importantly, it can be used to perform both forward and adjoint transport calculations, using the same basic multigroup cross-section data. The new algorithm is fully described, computationally tested, and compared with a standard condensed history algorithm for coupled electron-photon transport calculations.

ISSN:0029-5639    

DOI:10.13182/NSE124-369

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractIf the scattering interaction in linear particle transport problems is highly peaked about zero momentum transfer, a common and often useful approximation is the replacement of the integral scattering operator with the differential Fokker-Planck operator. This operator involves a first derivative in energy and second derivatives in angle. In this paper, higher order Fokker-Planck scattering operators are derived, involving higher derivatives in both energy and angle. The applicability of these higher order differential operators to representative scattering kernels is discussed. It is shown that, depending upon the details of the scattering kernel in the integral operator, higher order Fokker-Planck approximations may or may not be valid. Even the classic low-order Fokker-Planck operator fails as an approximation for certain highly peaked scattering kernels. In particular, no Fokker-Planck operator is a valid approximation for scattering involving the widely used Henyey-Greenstein scattering kernel.

ISSN:0029-5639    

DOI:10.13182/NSE96-A17918

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractMethods for coupling multiple forward and adjoint radiation transport Monte Carlo calculations with no statistical error propagation are presented. Correlated forward and adjoint particle histories are uniformly initialized on arbitrarily placed intermediate source boundaries throughout the calculational system. In applying the method to multilegged duct streaming problems, these source boundaries are placed at the duct leg intersections. The necessary forward and adjoint fluxes for the coupling procedure are each computed from an opposite-mode calculation. The no-error-propagation feature is the result of an exact correlation of all phase-space variables for coupled forward-adjoint particle histories at each boundary. For ducts of more than two legs, next-event estimation between forward and adjoint collision sites across arbitrarily placed intermediate scoring boundaries is necessary to achieve the variable correlation. Comparison of calculational results between the coupled and standard methods for two- and three-legged ducts are presented.

ISSN:0029-5639    

DOI:10.13182/NSE96-A17919

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractWe solve the neutron diffusion equation by a wavelet Galerkin scheme in this paper. Wavelet functions are generated by dilation and translation operation on a scaling function. The wavelet functions are localized in space and have a recursive property, so these properties may be utilized to solve a differential equation that has severe“stiffness.”The wavelet Galerkin method (WGM) represents the solution as a summation of Daubechies’scaling functions, which are also used as the weighting function. The Daubechies’scaling functions have the properties of orthogonality and high smoothness. Unlike the finite element method, the weighting function is the Daubechies’scaling function, and the unknowns determined are not the fluxes of the nodes but the coefficients of the scaling functions. The scaling functions are overlapping in the nodes and require special treatment at interfaces between nodes and at the boundaries.We tested the WGM with several diffusion theory problems in reactor physics. The solutions are very accurate with increasing Daubechies’order and dilation order. The boundary conditions are also satisfied very well. In particular, the WGM provides very accurate solutions for heterogeneous problems in which the flux distribution exhibits very steep gradients.We conclude that it is worthwhile investigating further the WGM for reactor physics problems and that numerical integration and acceleration of the matrix equation must be improved so as to reduce computing time.

ISSN:0029-5639    

DOI:10.13182/NSE96-A17920

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThe derivation and proper use of a formula originally published in 1959 for thermal neutron self-shielding computations in scattering bodies are presented. It is shown that the f0entity in this formula must be computed using the total macroscopic cross section∑tinstead of the macroscopic absorption cross section∑a, as was stated in previous publications. The validity of the formula is verified by Monte Carlo computations.

ISSN:0029-5639    

DOI:10.13182/NSE96-A17921

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractIrradiated light water reactor fuel from the BR3 reactor was thermally annealed up to 2500 K in a Knudsen cell, and the effusing vapors were measured by mass spectrometry. The experiments provide data on the stoichiometry evolution of the fuel during release as well as a reliable method to evaluate the diffusion coefficients of volatile and less-volatile fission products.The analysis of the data starts from diffusion of xenon, which clearly shows three typical release stages respectively controlled by radiation damage annealing, self-diffusion, and matrix vaporization. The experimental measurements are also in agreement with the predictions of intragranular trapping models.Barium and cesium showed faster release than xenon, the former being likely to diffuse atomically to the grain boundaries where no evidence of formation of stable zirconates was found. These results were compared with those obtained by a burnup-simulated fuel, where barium was initially present in a perovskite phase, producing essentially different release patterns.

ISSN:0029-5639    

DOI:10.13182/NSE96-A17922

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractThree types of experiments with a252Cf neutron source are proposed to evaluate systematically the neutron shielding effects of a material. The type 1 experiment deals with each shielding material alone, the type 2 experiment combines a shielding material and a structural material, and the type 3 experiment constructs the optimization with the materials used in the type 2 experiment. In the stainless steel (SS) + polyethylene shielding system, because of the location of the SS slabs at the source side, the tenth layer of the polyethylene becomes approximately one-half the value as when the polyethylene is employed alone. This is the enhancement effect of the SS. In the type 3 experiment, the total thickness of the SS + polyethylene shielding system is 40 cm, and the total thicknesses of the SS and the polyethylene slabs are fixed at 25 and 15 cm thick, respectively. The minimum total dose-equivalent rate (neutron + secondary gamma rays) is observed when the polyethylene slabs are located at a 20-cm depth from the source side, with an arrangement of 20-cm-thick SS + 15-cm-thick polyethylene + 5-cm-thick and SS, and with a ratio of the maximum to the minimum dose-equivalent rate of 2.5. The shielding optimization can be constructed by combining the materials having different shielding characteristics. The experimental results of the three types of experiments are reproduced fairly well by using the continuous-energy Monte Carlo code MCNP 4A with a next-event surface crossing estimator.

ISSN:0029-5639    

DOI:10.13182/NSE124-455

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractCalculations related to the temperature coefficient of reactivity of enriched gas-cooled reactors show the high sensitivity of this parameter to the proper description of thermalization effects in the moderator. Additionally, the calculation of the temperature dependence of the inelastic-scattering cross section with current ENDF/B formalisms correlates the errors of the cross sections as functions of the temperature. Neglecting this temperature correlation introduces unnecessary conservatism in the estimation of the error of the reactivity coefficient.These two facts drove our efforts to characterize the present status of the inelastic cross section of graphite and to calculate its covariance file. The ENDF/B evaluation of the scattering matrixS(α,β, T) is still based on the approximations (incoherent component only) andphonon spectra of the early 1960s. Subsequent measurements showed that the structure observed inS(α,β, T) cannot be described using the incoherent approximation, and soon after the availability of highly intense neutron beams and large specimens of pyrolitic graphite have allowed the direct measurement of elastic constants of relevance for a better calculation of the phonon spectra. Calculations of the probability distributions of the moment and energy transfer, a andβ, in a Maxwellian spectrum allow us to define a range ofαandβfor which comparison of experimental and theoretical data are of most interest for reactor analysis, and to point out regions of deficient resolution or excessive details in the presentα,βmesh used in the ENDF/B files. Because the phonon spectrum definesS(α,β, T), mathematical formulas have been found that allow the calculation of the covariance matrix ofSby propagating the errors of the phonon spectra.

ISSN:0029-5639    

DOI:10.13182/NSE96-A17924

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractTo develop a new process for the production of99Mo using low-enriched uranium targets, uranium dissolution in alkaline hydrogen peroxide was studied. Molybdenum-99 is a parent of the widely used medical isotope99mTc.The rates of uranium dissolution in alkaline hydrogen peroxide solution were measured in an open, batch-type reactor and were found to be a 0.25th order reaction with respect to equilibrium hydrogen peroxide concentration. In general, uranium dissolution can be classified as a low-base (0.2 M hydroxide) process. In the low-base process, both the equilibrium hydrogen peroxide and the hydroxide concentrations affect the rate of uranium dissolution. In the high-base process, uranium dissolution is independent of alkali concentration! the presence of base affects only the equilibrium concentration of hydrogen peroxide. An empirical kinetics model is proposed and discussed.

ISSN:0029-5639    

DOI:10.13182/NSE96-A17925

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor

摘要:

AbstractTransmutation of actinide waste into fission products could be enhanced by using resonance fission of odd-odd target materials; those of interest are232Pa,238Np, and242Am. Fission cross-section measurements of two of these short-lived materials were performed at Los Alamos National Laboratory. Samples were produced by the (d,2n) reaction in the Los Alamos Ion Beam Facility followed by fast radiochemistry to separate the odd-odd target of interest. The fission cross section of the nanogram samples was measured in a high intensity pulsed neutron beam produced by 800-MeVproton spallation. Using this procedure, the fission cross sections of the 1.3-day232Pa and 2.1-day238Np were successfully measured in the energy range from 0.01 eV to 50 keV. The fission cross section of the relatively long-life isotope2S6Np was also measured in the same system while the short half-life isotopes were being prepared. The results and resonance analysis are presented.

ISSN:0029-5639    

DOI:10.13182/NSE96-A17926

出版商:Taylor&Francis    

年代:1996

数据来源: Taylor