摘要:

AbstractThere are few exact numerical solutions to which results from multigroup transfer matrix algorithms can be compared. Therefore, for isotropic elastic scattering in the center-of-mass system, exact numerical values have been calculated for, the average Legendre coefficient of the scattered particle in the laboratory system. Both projectile and target masses are allowed to vary between 1 and 238 amu. It can be concluded that significant numerical uncertainties are possible in the calculation of the higher order transfer matrices.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28694

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractUsing the authors’recently suggested thermal neutron scattering kernel for liquid hydrogen, which explicitly takes into account the presence of collective motions and two elastic field quantum exchange processes, an extensive study is made on the thermal neutron transport processes by solving the appropriate multigroup Boltzmann transport equation in liquid para and liquid para-orthohydrogen mixtures.The calculated values of the asymptotic decay constant of a neutron pulse in liquid parahydrogen and a 47% para and 53% ortho mixture at 20 K for different assembly sizes are found to agree with the corresponding measured values of Bryan and Waltner, when the chemical binding energy represented by the Debye temperature,θD, is equal to 80 K. The critical buckling for liquid parahydrogen (= 0.16 cm-2) is much smaller in comparison with that of the mixture (=0.8 cm-2). The thermalization time,τthinµs, as calculated by using the eigenvalue spectrum and corresponding eigenfunctions in both liquid parahydrogen and the mixture, increases linearly with buckling but with different slopes and can be represented, respectively, as s/bτth= (68.0 + 750 B2) andτth= (68.0 + 180 B2). Unlike the case of ice at 20 K, there is only one discrete mode in both liquid parahydrogen and the mixture.The calculated values of the diffusion length for natural absorption in pure parahydrogen and in a 47% parahydrogen and 53% orthohydrogen mixture are 5.8 and 2.4 cm, respectively. From the space-dependent spectra studies, it is found that the asymptotic spectra in both the systems for natural absorption are reached in a distance about twice the value of the respective diffusion lengths. The value of critical absorption in parahydrogen is only 1.5 times the natural absorption, while in the mixture it is very large and is greater than ten times the natural absorption.The steady-state calculations were made in liquid parahydrogen at 20 and 11 K, and liquid para-ortho mixtures at 20 K. The computed steady-state spectra at these temperatures for assembly size B2= 0.133 cm-2turn out to be in good agreement with the corresponding experimental spectra of Whittemore. As the temperature of liquid parahydrogen is decreased from 20 to 11 K, the effective temperature of the spectra decreased from 34 to∼25 K, resulting in a substantial increase of cold neutrons. In addition, several studies on steady-state spectra for different sizes of liquid para- and orthohydrogen assemblies, H2-D2mixtures at 20 and 11 K, and solid H2, D2at 4 K were made to assess the production of intense beams of cold neutrons. As D2is added to liquid parahydrogen, the effective temperature of the spectrum decreases for buckling values<0.005 cm-2at 20 K, 0.002 cm-2at 11 K, and 0.001 cm-2at 4 K. For infinite size, pure D2is the most intense cold neutron source, as the effective temperature will closely correspond to the temperature of the assembly at 20 and at 11 K, and at 4 K it will correspond to∼6 K. For a practicable size of B2= 0.0158 cm-2, however, it is found that parahydrogen, or any ortho-para mixture at 11 K is the best source of cold neutrons, better than even the optimum mixture of H2O-D2O. The increase is by more than an order, at very low energies, making it the practicable source for most intense beams of cold neutrons.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28695

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractIn this paper, we point out that data from earlier experiments carried out to measure beta-strength functions for short-lived fission products can also be used to provide average beta- and gamma-decay energy values for these nuclides. In our evaluation of decay data for the ENDF/B-V fission product file, we have used this approach as a means of deducing average decay energy values for a number of these isotopes for which experimentally based average values would otherwise not have been available. The methods employed are discussed, and the results for the average beta-decay energies per decay,, are presented. Where available,values deduced from decay scheme studies and from direct beta-spectrum measurements are given for purposes of comparison. Evidence is presented that suggests that the conventional decay scheme studies may not be a reliable source of average decay energy data for nuclides with large Qβvalues. We propose that different types of experimental measurements, possibly involving total absorption techniques (of which the beta-strength work treated here might be considered as one example), may provide a better means of producing this important information.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28696

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractNeutron capture cross sections have been measured for169Tm from 3 to 2000 keV at the Oak Ridge Electron Linear Accelerator 40-m station. The data were analyzed for individual resonance parameters up to 4.2 keV. Average strength functions have been deduced. Compound nucleus calculations, made with deformed optical model parameters, agree with experimental cross sections. Our cross sections for lower neutron energies tend to be somewhat less than those from earlier measurements.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28697

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractPerturbation theory along with a binary fuel shuffling technique is applied to predict the effects of various core configurations and, hence, the optimization of in-core fuel management. The computer code FULMNT has been developed to shuffle the fuel assemblies in search of the lowest possible power peaking factor. An iteration approach is used in the search routine. A two-group diffusion theory method is used to obtain the power distribution for the iterations.A comparison of the results of this method with other methods shows that this approach can save computer time and obtain better power peaking factors. The code also has a burnup capability that can be used to check power peaking throughout the core life.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28698

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractThe feasibility and relative merits of a quasi-time-dependent approach to burnup calculations is investigated. This method, which is shown to be practically equivalent to a true time-dependent approach, uses one iterative level less than the conventional method and is less liable to nonconvergence problems. The method has been formulated using the finite difference form of the neutron diffusion equation and is implemented in a computer code named TDB.Several one- and two-dimensional pressurized water reactor cores were analyzed using both proposed and conventional methods. The calculations show that the proposed method is about twice as fast as the conventional one with a relative accuracy of<5% in material power fractions and critical boron value.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28699

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractTemperature- and density-dependent functions were constructed for the pressure, heat capacity, and thermal conductivity of sodium. In evaluating these functions, the latest available sodium data were used. The near critical part of the properties was modeled with the method of the critical exponents. Each function is defined for all temperatures above the melting point and for all densities below the density of the liquid.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28700

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractA detailed model of the interaction of ruthenium and urania is developed and compared to experimental data. The mechanism involves physical solution of the metal in the grain boundaries of the ceramic followed by simultaneous diffusion and chemical reaction to produce URu3intergranular inclusions. The process occurs only when the oxide is substoichiometric, the reduction being effected by oxygen absorption by the refractory metal crucible containing the specimen. Reaction ceases when the URU3 product in the grain boundary reaches a thickness that prevents removal of the other reaction product, oxygen. Fitting the model predictions to the isothermal ruthenium spreading data from a source plane of the metal held between oxide pellets provides quantitative estimates of the parameters of the model The theory also correctly predicts the shape and magnitude of ruthenium migration in UO2in a temperature gradient, in which thermal diffusion does not appear to play a significant role.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28701

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractGeohydrochemical factors that affect the transport of low-level wastes in saturated-unsaturated porous media are described. Depending on the availability of those geohydrochemical parameters and the detail of information desired, three levels of analyses can be undertaken. Two examples used to illustrate these three levels of analyses are the seepage pond problem and the shallow trench burial problem. The former example indicates that the lower level of resolution gives the more conservative estimate of the breakthrough time for the contaminant. The latter example exemplifies the cases that simple levels of resolution are not adequate nor possible because the flow variables and parameters cannot be determined with rational assumptions. The level 1 model can best be used for screening purposes while level 2 analysis can be employed to rank the alternative sites. Level 3 models should be used for detailed studies of the impact of the chosen site or for predictive assessment of operational sites and decomission scenarios.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28702

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor

摘要:

AbstractA critical analysis of the Koppel-Young model is presented and compared with Nelkin’s model and their equivalence is asserted. It is shown that the only distinction between the two models is in the orientational averaging of the rotational-vibrational intermedial scattering function. Based on total cross sections, the Krieger-Nelkin orientation averaging has been confirmed to give excellent agreement with the Koppel-Young orientation averaging. However, significant quasi-periodical differences are observed when calculating differential cross sections. As a result of these insights, a new unified model is proposed for microdynamics of water molecules.

ISSN:0029-5639    

DOI:10.13182/NSE82-A28703

出版商:Taylor&Francis    

年代:1982

数据来源: Taylor