摘要:

AbstractThe CNmethod of solving the transport equation has been developed at Saclay during the past few years. This method is based on a lemma proved by Placzek; an integral equation is provided for the angular flux at the boundary of the various media, and its kernel is the infinite medium Green's function. Four plane geometry problems are solved in one-velocity theory, with a linearly anisotropic scattering kernel: the albedo for the Milne problem, the extrapolation length for the same problem, albedo and transmission factor for slabs, and the critical thickness for slab reactors. Numerical results are obtained and compared with data computed by reference methods.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27152

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractUsing slab geometry, generalized rebalance is presented as a class of iteration acceleration schemes applicable to the neutron transport equation. We demonstrate that the diffusion-synthetic, variable-Eddington-factor, and conventional-rebalance schemes can be shown to be special cases of generalized rebalance. Expressing these schemes within the generalized-rebalance framework leads one to consider a new scheme labeled third-moment rebalance. Numerical results are presented that indicate that Alcouffe's diffusion-synthetic schemes are presently the best available methods.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27153

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractThe fictitious scattering radiation transport model, suitable for Monte Carlo applications in geometrically complex systems, has been extended for use in deep-penetration calculations by the development of an appropriate next-flight estimator. Mathematical derivations are given, and it is shown that the estimation scheme is actually a one-dimensional version of the general model. Sample problems are solved to illustrate the use of the next-flight estimator, its variance characteristics, and the time-saving features of the model. Other items discussed are coupling techniques with standard methods, systems with large cross sections, and inclusion of the fictitious scattering model in multigroup cross-section structure.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27154

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractUnsymmetric perturbations were introduced into the core of a large, critical, heavy-water-moderated, multiregional reactor. The resulting three-dimensional changes in flux level and shape with time were measured. Perturbations included:1.Free-fall insertion of rods near the reactor center. Each rod contained235U slugs in the bottom half and lithium slugs in the top half2.Free-fall insertion of rods into an off-center radial position. Each rod contained235U slugs in the bottom half and aluminum in the top half.3.Withdrawal of cadmium control rods from the central 20% of the reactor core.Flux tilts calculated with the TRIMHX code were within 5% of measured flux tilts. TRIMHX provides a three-dimensional (hex-z geometry) solution of the few-group neutron diffusion and delayed precursor equations without feedback. Inputs to the calculations are available in sufficient detail to allow other methods of solution to be tested.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27155

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractPresent theories for predicting expected Monte Carlo errors in neutron transport calculations apply to estimates of flux-weighted integrals sampled directly by scoring individual collisions. To treat track-length estimators, the recent theory of Amster and Djomehri is generalized to allow the score distribution functions to depend on the coordinates of two successive collisions.It has long been known that the expected track length in a region of phase space equals the expected flux integrated over that region, but that the expected statistical error of the Monte Carlo estimate of the track length is different from that of the flux integral obtained by sampling the sum of the reciprocals of the total cross sections for all collisions in the region. These conclusions are shown to be implied by the generalized theory, which provides explicit equations for the expected values and errors of both types of estimators. Sampling expected contributions to the track-length estimator is also treated.Other general properties of the errors for both estimators are derived from the equations and physically interpreted. The actual values of these errors are then obtained and interpreted for a simple specific example.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27156

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractUsing the most recent evaluations of power loss by impurity radiation, we have calculated the maximum permitted impurity concentration for various species as a function of Q, the ratio of deuterium-tritium (D-T) fusion power to injected beam power. These criteria for maximum impurity concentration must be satisfied before applying the usual neτEversus Ticonditions for obtaining a given Q value. For, the critical impurity concentration fczvaries as Z−2.2 to−2.5. The tolerable concentration of medium- and high-Z impurities for operation at lowcan be at least one order of magnitude larger than the concentration allowed for ignition, provided that the plasma temperature is maintained by reacting ion beams.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27157

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractWe show that in a medium consisting of asymmetric cells, neutrons can“drift,”or diffuse, in a special preferred direction. The drift is caused by selective asymmetric changes in the cross sections in each cell. We describe several physical mechanisms that produce a drift, and we briefly discuss a possible application in a reflector design. (A reflector constructed of asymmetric cells, oriented so that the drift is always directed toward the reactor core, would be more efficient than a homogeneous driftless reflector.)Our theoretical treatment consists of an asymptotic analysis of the one-dimensional neutron transport equation. We show that a simple modification of the diffusion equation describes the neutron drift, and we provide numerical results for several problems. We also numerically compare the solution of an initial value problem for the transport equation in an asymmetric cellular medium to the corresponding diffusion theory problem. The results are in reasonably good agreement for both short and long times.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27158

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractCalculations of neutron and gamma-ray fluences from 14-MeV neutron and252Cf sources in liquid air and liquid nitrogen have been performed. These calculations were made specifically for comparison with experimental data measured at Stohl, Federal Republic of Germany. The discrete-ordinates method was utilized with neutron and gamma-ray cross sections from ENDF/B-IV. One-dimensional calculational models were developed for the sources and tank. Limited comparisons are made with experimental data.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27159

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractThe accuracy of unfolding neutron spectra with threshold activation foils was investigated by comparing the unfolded spectra with“known”spectra obtained from calculations verified with more accurate spectrometry techniques. Because of the lack of spectral data below 1 MeV (only one useful activation threshold exists between 0.01 and 1 MeV), reliable results are generally obtained only with the aprioriconstraint of a physically complete trial spectrum. A family of trial spectra useful for reactor spectra [and any nuclear“evaporation”spectra, such as from high-energy (γ, n) reactions] has been formulated. The sensitivity of the solutionϕ(E) to the specific unfolding code used, to the threshold-foil cross sections and related nuclear data, and to foil-activation measurement accuracy was also investigated.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27160

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor

摘要:

AbstractMeasurements of differential cross sections for fast neutrons scattered by carbon are presented for 14 incident neutron energies between 8.0 and 14.5 MeV. The measurements were performed with the four-detector neutron time-of-flight facility of the Centre d'Etudes de Bruyères-le-Châtel tandem Van de Graaff accelerator. The angular distributions for elastic and inelastic scattering to the first excited level of12C were obtained over the angular range from 10 to 160 deg. The experimental uncertainties vary from 7 to 15% for the differential elastic scattering cross sections and from 8 to 22% for the differential inelastic scattering cross sections. These measurements partially fill in an energy range that was previously characterized by a lack of data. The angle-integrated cross sections are compared to the evaluated values of ENDF/B, and significant discrepancies are shown. The present data were included in an evaluation work on carbon carried out in this laboratory. The consistency of total cross-section measurements with our data and other partial cross sections is discussed.

ISSN:0029-5639    

DOI:10.13182/NSE78-A27161

出版商:Taylor&Francis    

年代:1978

数据来源: Taylor