摘要:

AbstractDiffusion theory overestimates neutron transport in voided or low-density regions of a reactor when the diffusion coefficient is defined as 1/3Σtr.Alternative definitions of the diffusion coefficient for such regions have been proposed. The present paper summarizes some definitions of axial diffusion coefficient for cylindrical channels and proposes a modification to an earlier formula. The results of calculations for a channel in a fast reactor supercell model using different formulas are compared and the limitations of this method, which involves changing only the channel diffusion coefficient, are discussed.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21317

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractA method has been developed to calculate the fundamental mode decay constants in two- and three-dimensional pulsed neutron moderator assemblies using the separable form of the scattering kernel in the transport equation. The method uses the Fourier transform of the integral transport equation and is an extension of the method developed by Sahni to treat monoenergetic criticality problems for two- and three-dimensional geometries. The new kernel of the integral transform equation is factored into components depending on only one of the dimensions of the assembly. This property is further exploited by use of a single Fourier mode approximation in one or more dimensions while the kernels in the remaining dimensions are retained in their respective forms. In our numerical work, three simple forms of the scattering cross section are used for calculating the matrix elements of the relevant equations accurately. Numerical results are presented for the asymptotic decay constant in a one-dimensional slab, a one-dimensional cylinder, two-dimensional infinite rectangular prisms, and three finite cylinders of different height-to-diameter ratios. The relation between the asymptotic decay constant and the geometrical buckling in the transport and diffusion approximations are also calculated for interpreting the results in terms of extrapolation lengths.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21318

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractThe maximum likelihood method for the multivariate normal distribution is applied to the case of several individual eigenvalues. Correlated Monte Carlo estimates of the eigenvalue are assumed to follow this prescription and aspects of the assumption are examined. Monte Carlo cell calculations using the SAM-CE and VIM codes for the TRX-1 and TRX-2 benchmark reactors, and SAM-CE full core results are analyzed with this method. Variance reductions of a few percent to a factor of 2 are obtained from maximum likelihood estimation as compared with the simple average and the minimum variance individual eigenvalue. The numerical results verify that the use of sample variances and correlation coefficients in place of the corresponding population statistics still leads to nearly minimum variance estimation for a sufficient number of histories and aggregates.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21319

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractIn using a Monte Carlo transport code, particle fluxes are underestimated with a calculational model using specular reflection boundaries when a point detector estimator that scores only the direct contribution to the detector from a collision point within the model is used. This underestimation occurs because the contributions from collision points outside the calculational model are neglected. An additional scoring scheme is developed to compensate for the discrepancy; the new scheme is implemented to three-dimensional Monte Carlo transport code MORSE-GG. Validity of the method is shown by test calculations with ANISN and revised MORSE-GG.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21320

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractA recursive Monte Carlo (RMC) method for estimating the importance function distribution in three-dimensional systems, intended for importance sampling applications, is developed. The method consists of dividing the system into relatively thin geometrical regions and solving the inhomogeneous forward transport equation for each of the regions. The RMC method is found to possess a number of unique features, including the ability to infer the importance function distributions pertaining to many different detectors from essentially a single Monte Carlo run.Various technical questions concerned with the practical application of the RMC method, including the questions of the accumulation of statistical and systematic errors and their dependence on the details of the system division and source batch size, are investigated. A promising algorithm for the application of the method is formulated. The practicality and efficiency of the RMC method is investigated for a number of monoenergetic problems.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21321

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractThe simple statistical model statement relating the yield YPof fission fragments to the effective neutron binding energy B'n, namely YPαexp(−B'n/T), can be used as a basis for parallel developments, one leading to the well-known empirical delayed-neutron statementwhere Y is the number of delayed neutrons per fission.Repeating the development for prompt neutron emission leads to the analogous resultwhere v is the prompt neutrons per fission.This semi-empirical result implies that a semi-logarithmic experimental plot of v against (3Z - A) should be a family of straight lines. Currently available experimental results justify this prediction. The theoretical precision of this semi-empirical formula is estimated to be±10% or better, depending mainly on the part and shape of the fission yield-mass curve taken into account.The existence of the above empirical and semi-empirical formulas strongly suggests that the yield of fission neutrons, as well as their spectrum, can be calculated by using the standard statistical model with non-adjustable parameters.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21322

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractThe excitation function for the reaction103Rh(n,n')103mRh was measured by the activation technique from 0.2 to 6.1 MeV in 0.1-MeV steps and from 13.0 to 16.7 MeV in 1-MeV steps. This excitation function is normalized through an absolute measurement at 1.8 MeV. This measurement is based on n-p scattering for neutron flux determination and on liquid scintillation counting of103mRh separated from103Pd solutions for the activity determination. The total uncertainty of the cross-section results is typically±5% above 0.5 MeV (about±10% above 13 MeV). Concurrence with existing data is good except below 0.35 MeV, where the present results are considerably higher.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21323

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractThe fuel spheres in the core of a pebble-bed reactor are mainly distributed statistically. Regular configurations appear only near the walls and on the bottom of the core.A statistical distribution has been studied by means of a proper mock up. For this purpose the coordinates of 3024 sphere centers were defined by measurements so that Monte Carlo games for neutron diffusion could be established. From these results as well as from Monte Carlo calculations for the cubic and tetrahedral sphere configurations, the diffusion constants and the first two mean moments h and h2of the passage lengths in the holes were determined. Under the assumption that no correlation exists between the passage lengths in the holes and in the balls, an exact formula for the diffusion lengths was derived using h2/(h)2.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21324

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractA generalized integral transform technique is used to obtain various order analytical solutions to the problems of linear diffusion of radioactive atoms in an eroding slab, cylinder, or sphere subjected to a general boundary condition of the third kind.The zeroth- and first-order solutions obtained in this manner are compared with the exact solution; the results are highly accurate for most practical cases. The proposed technique is sufficiently versatile to cover a variety of heat and mass transfer problems in media with moving boundaries.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21325

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

AbstractWithin the framework of the Abajan, Bazazjantz, Bondarenko, and Nikolaev (ABBN) shielding factor approach, several methods have been proposed in the past for the treatment of resonance shielding of multigroup cross sections in heterogeneous cells of fast reactor lattices. First, the approximations made in the different methods and their interrelationship is briefly reviewed. Then, three recent efficient methods proposed by Bitelli et al., by Tone, and by Kujawski and Protsik are numerically compared by checking against exact fine-group collision probability calculations. It is found that the method of Bitelli et al. may give erroneous results, while the methods of Tone and of Kujawski and Protsik show satisfactory agreement with the exact calculations.

ISSN:0029-5639    

DOI:10.13182/NSE80-A21326

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor