摘要:

AbstractThis paper describes a method for predicting the deposition of particles entrained in turbulent flow. The method predicts the deposition of particles ranging from molecular size to∼100µin diameter. Brownian diffusion, turbulent diffusion, and inertial effects are accounted for, and the existence of an equilibrium concentration of particles close to the wall is postulated. The method shows reasonable agreement with published data.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18874

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe thermal-neutron-absorption cross sections of6Li and10B were measured by the pulsed-neutron method. The 2200 m/sec absorption cross sections are 938±6 b for6Li and 3855±26 b for10B.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18875

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe total one-phonon coherent neutron scattering cross section for graphite is evaluated using an approximate model of the dispersion relations. A relatively simple phonon frequency distribution is derived and multiphonon terms are calculated in the incoherent approximation. The results are compared with experimental and other theoretical calculations based upon the incoherent approximation. The latter have been consistently lower than the experimental results by a factor of two or more. It is found that the out-of-plane orz-mode contributes∼75% of the one-phonon cross section. The value of [υΣ01(υ)]minis found to be∼70% greater than the same quantity obtained with the same model in the incoherent approximation. Moreover, theλ-law or constant collision frequency domain only begins with neutron wavelengths>∼20Å(the Bragg cutoff is 6.7Å).

ISSN:0029-5639    

DOI:10.13182/NSE70-A18876

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractA new method has been proposed for the selection of a single resonance structure or ladder used for the calculation of the cross sections in the unresolved resonance region. This method is especially useful for fissile nuclides, where level spacings are quite narrow, and is capable of taking the rather complicated energy-variation of alpha values into consideration. By use of the present method, studies have been made of the cross sections of235U,238U,239Pu, and240Pu. For the construction of the cross sections of these nuclides, new evaluations have also been made of the average resonance parameters, which were used in generation of the ladders. The calculated results are shown to be a very good representation of the low-resolution experimental data.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18877

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe general problem of unfolding is considered from the point of view of linear vector space theory as applied to the more specific problem of spectral unfolding. It is shown that the basis for many of the methods currently in use is an expansion of the unknown spectrumφ(E) in terms of some set of functionswn(E). The coefficients in the expansion are determined by the measured outputs of the detectors. The relationships between the various solutions obtained by using different sets ofwn(E) functions are explored. It is shown that the particular solution obtained by using the response functions of the detectors as thewn(E) is effectively an orthogonal decomposition ofφ(E) whereas all other expansions are nonorthogonal decompositions. As a result of these properties, the response function expansion, for example, has a bounded square deviation fromφ(E) and is less sensitive to errors in the measured detector outputs, whereas other expansions can lead to solutions that may differ violently fromφ(E). Conditions under which the latter situation can occur are of a fundamental nature and do not owe their origin to calculational inaccuracies.The square-wave solution is given particular attention and the theoretical basis is investigated of the standard practice of requiring an all positive solution with theoretical outputs that differ least from those measured. It is shown that the correct square-wave representation forφ(E) results in theoretical detector outputs that necessarily differ from those produced byφ(E) itself—possibly by a large amount. Thus, except for cases where this difference is known,a priori, to be small, there is no theoretical basis for this standard practice.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18878

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractSpectral indexes have been measured in homogeneous systems (solutions of europium and boron). The measured values have been compared with the results of spectra calculations and, independently, have been utilized for a correlation method devised to deduce the reaction rates which are undetectable directly.The reliability of the THERMOS code for spectra calculations in such systems has been shown. A satisfactory test of the correlation method has been performed. The adequacy of the cross-section sets available in the literature has been shown for the detectors197Au,63Cu,55Mn,176Lu,239Pu, and235U. The sets available for151Eu and175Lu are not satisfactory: new evaluations have been carried out by properly utilizing measurements reported in the literature and/or new measurements performed for this purpose in a known spectrum. When using the new sets, the agreement between calculated and experimental spectral indexes in the absorber solutions is significantly improved.When reliable cross sections are available for the detectors, the correlation method can be considered a powerful tool for the evaluation of an unknown reaction rate: for instance the239Pu absorption rate in the solutions is evaluated by means of the measured activation rates without introducing uncertainties due to the correlation procedure itself.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18879

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe multigroup neutron kinetics equations are derived and investigated for the case when the within-group weighting spectra, which are used in defining group constants, are space- and time-dependent. New terms are introduced by the space- and time-dependence of these weighting spectra. The derivation is carried through the hierarchy of operations by which the continuous space, time, and lethargy dependence is replaced by a discrete representation. The new terms do not alter the usual positivity properties associated with the discrete multigroup kinetics equations, provided that certain conditions are satisfied. Conditions are also established which are sufficient to ensure that the discrete representation is adjoint consistent; i.e., the discrete representation of the adjoint equation is mathematically adjoint to the discrete representation of the direct equation.A similar development is presented for the Spectral Synthesis approximation. Conditions are established for the adjoint consistency of the discrete representation. The type of positivity argument made for the multigroup equations is shown to be invalid for the Spectral Synthesis equations.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18880

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractA numerical method for the solution of the time- and space-dependent multigroup diffusion equations is presented. The method permits a significant reduction in the computer time required to solve these equations by substantially increasing the allowable time step size. In the point reactor case, a form of the method considerably simplifies the calculation by removing the explicit dependence on the generation time and the delayed-neutron terms.The space-time equations are transformed into the Laplace domain and after multiplication by a weighting function they are transformed back into the time domain. By appropriate choice of the weighting function the equations appear either as coupled convolution integrals, where numerically difficult (e.g., generation time and delayed neutron) terms have been canceled, or as coupled integral equations in the weighted residual form, which permits very large time steps to be taken.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18881

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThe full-range Monte Carlo program RECAP was used in a consistent analysis of parameters measured in seven H2O-moderated uranium lattices: four were lattices of slightly enriched rods and three were natural-uranium-slab lattices. The most important parameters were: epithermal-to-thermal ratios for238U capture (ρ28) and for235U fission (δ25), ratio of238U fission-to-235U fission (δ28), eigenvalue, integral fast/epithermal spectrum comparisons with a variety of detectors, and resonance integrals for isolated uranium and UO2rods (calculated with the Monte Carlo program RESQ, supplemented by ZUT and TUZ at higher energies). The analysis of these lattices was fairly unambiguous and served to test the input nuclear data for238U,235U, and H2O. With one exception, leakage was small (<20%) and parameters were measured in nearly asymptotic flux spectra. These covered a wide range of hardness as reflected in the change of parameters:ρ28ranged from 0.5 to 12;δ25from 0.04 to 1.1; andδ28from 0.05 to 0.44. The Monte Carlo cell calculations were corrected for leakage by means of the homogenized lattice programs P3MG and MUFT-MAGMA. With a straightforward choice of current“best”nuclear data, the calculations reproduce virtually all the parameters over the whole range of lattices. There are two qualifications: some reduction of smooth238U capture integral is required to match the lattice measurements (by an amount barely compatible with the isolated rod measurements), and there is too much calculated epithermal235U fission in the very tight lattices.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18882

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor

摘要:

AbstractThermal disadvantage factors and spectral indexes have been measured in a variety of light-water-moderated lattices. One series contained slightly enriched uranium rods in hexagonal geometry and another series used natural-uranium fuel in slab geometry. The detectors used were164Dy,176Lu, and239Pu. Full energy range (0 to 10 MeV) Monte Carlo calculations with explicit cell-geometry representations were performed using the RECAP program. In addition, thermal energy range (0 to 0.625 eV) calculations were obtained with the Monte Carlo program MARC as well as with the integral transport-theory-code THERMOS. The purpose of these investigations was to test the adequacy of the various water scattering kernels—Nelkin, Koppel, and Haywood—for a broad range of thermal-flux characteristics: from a soft moderator spectrum with a steep spatial gradient to a very hard spectrum which was relatively flat as a function of position.The conclusions obtained were as follows. Calculated spectral indexes using the Haywood kernel were 2 to 3% higher than experiment, on the average, in the fuel region of these cells. Use of the Koppel kernel removed most of this disagreement in the case of176Lu but the comparison for239Pu remained unchanged. On the basis of these results, the thermal-flux spectrum obtained with the Haywood model appears to be slightly too hard. With regard to the disadvantage factors, good agreement was generally obtained between theory and measurement except for the tightest lattices. The calculated disadvantage factors were found to be insensitive to the kernel model selected. The effects from including thermal-scattering-pattern treatments aboveP1as well as a spatially dependent and anisotropic source-to-thermal description were found to be small in these cells,<2%.

ISSN:0029-5639    

DOI:10.13182/NSE70-A18883

出版商:Taylor&Francis    

年代:1970

数据来源: Taylor