摘要:

AbstractThe neutron diffusion length of graphite with an added 1/vabsorber has been calculated for temperatures from 200 to 2000°K. At each temperature, the entire range of absorption for which a diffusion length exists has been examined. The largest such range, which occurs for 2000°K, extends from the absorption of pure graphite to a value corresponding to 7.3 b at 0.0253 eV. For all values of temperature and absorption, the diffusion length,L, in centimeters is given within 1% bywhereTis the absolute temperature andσ′(b) is the absorption cross section at 0.0253 eV. The temperature-dependent functionsα(T) andβ(T) are given in Figs. 1 and 2. Very good agreement exists between the values implied by this formula and recent measurements.Each calculated value ofLhas been obtained as an eigenvalue of aP11approximation to the transport equation. The neutron energy spectrum is obtained accurately as the eigenfunction in the calculation. This spectrum shows the characteristic discontinuity at the Bragg limit, previously known for beryllium, which varies in magnitude with the absorption. There is also a marked hardening of the spectrum with increasing absorption, which significantly affects the value of the diffusion length.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18852

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

摘要:

AbstractA multiple foil activation iterative method has been used to experimentally determine neutron flux spectra in various types of neutron environments. The method involves irradiation of a set of different foil detectors, measurement of resultant activities, and adjustment of a spectrum selected as an initial approximation to obtain a good-fit solution for a set of simultaneous activation integral equations. A computer code, SAND-II, is used to obtain this solution. Spectra from thermal and fast reactors and from beam sources have been measured. In each experiment, a set of more than ten foil detectors, encompassing low- and high-energy neutron-induced reactions, was irradiated and used as input to SAND-II. Solutions obtained are compared with diffusion, transport, or Monte Carlo calculations or with spectrometer measurements. It is concluded that the multiple foil activation iterative method is an important adjunct to calculational and neutron spectrometer techniques used to determine neutron flux spectra.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18853

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

摘要:

AbstractA space-dependent noise formulation is developed on the basis of the modal-analysis technique. Application of the method is illustrated by numerical calculations carried out for a one-dimensional model of a coupled-core Argonaut type reactor. It has been found that the suggested method may be readily used to calculate the auto- or cross-spectral-density functions of the neutron-level fluctuations for any of the group fluxes of a multienergy group calculation when the system is subjected to arbitrary stochastic input functions. Results obtained from the numerical calculations serve to illustrate the importance of the space dependence of the spectral functions and in particular the sensitivity of the cross-spectral-density function to the two points of observation.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18854

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

摘要:

AbstractPulsed-neutron experiments have been made on two organic-moderated and -reflected coupled cores of the ROSPO (a zero-power organic-moderated reactor) nuclear system in order to investigate their time response and to measure some kinetic parameters. At least for loose coupling, the fundamental mode has been observed to have two time-decay constants in any point of the system (except in the vertical symmetry plane), which are strongly interrelated. For two identical cores, these decay constants can give separate information on the intrinsic and coupling reactivity as well as on the reduced neutron lifetime of the system.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18855

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

摘要:

AbstractTwo methods for evaluating the effects of nonuniformities in reactor fuel on resonance integrals are described. In the first method, the volume and the surface effects on the resonance integral are separated by dividing the rod into two homogenized regions and a general formalism is developed which is adequate for numerical computation. In the second method, which uses the moment technique of Cady and Clark, a generalization of the Nordheim equation, available for the study of any nonuniformity of the rod is presented. As an example, an analytical expression for a uniform temperature equivalent to a parabolic distribution is given.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18856

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

摘要:

AbstractAn approximate analytic solution to the infinite-medium slowing down equation is obtained for a weakly absorbing mixture of isotopes. It derives from a moment expansion of the integral equation and, by truncation, involves the average lethargy gain and the average square of the lethargy gain per collision in the mixture. It applies to the vicinity of a resonance if the isotope masses are not much different from each other or if the scattering power (ξΣS) of the resonant isotope at the resonance peak is much higher than the scattering power of the background. It offers a simple description of the strong fluctuations in the collision density caused by wide or strong resonances of light and structural elements in fast mixtures. An important application of the theory is the evaluation of group cross sections. The theoretical estimate of the group removal cross section was compared with numerically-exact values and a discrepancy of a few percent was found.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18857

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

摘要:

AbstractAs a part of a series of studies now under way, this paper discusses the analytical aspect of the problems encountered in the application of multilevel formalism to the fast reactor Doppler effect analysis in the unresolved region. The concept of the“statistical collision matrix”introduced by Moldauer1was used. The paper is divided into two parts. Part I describes the formulation and statistical consideration of the problem. ForS-matrix formulation, the Doppler broadened cross sections using ideal gas model can be expressed in terms of the well-known broadened line shape functions. These functions are readily amenable for reactor calculations using any existing resonance integral code with some trivial modifications. The statistical behavior of theS-matrix parameters is also discussed in some detail. In order to improve understanding of the nature of the problem, an illustrative example was carried out analytically for the case of two interfering levels. Two more realistic examples pertinent to the fissile isotopes of interest are also given by numerical calculations using 50 interfering levels. These examples provide good qualitative descriptions of the statistical behavior of the S-matrix parameters that one may expect in the reactor Doppler effect studies. Part II deals with the application of the multilevel formalism in the Doppler effect studies.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18858

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

摘要:

AbstractAs the second part of this series, this paper describes the application of the statistical theory and multilevel formulation to the reactor Doppler effect studies. Analytical studies were carried out on the basis of the narrow resonance approximation. The studies were made in both the high-energy and the intermediate-energy regions, which are of great interest in fast reactor Doppler effect calculations. Some qualitative conclusions on the influence of the multilevel interference effect on the Doppler effect of fissile isotopes are also given.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18859

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

摘要:

AbstractSolutions obtained by expansion in a series of spatial modes and by an iterative method are compared for both space and space-time problems. In the space problem, the modal expansion is used to justify the iterative results. A useful nonlinear transformation is introduced to aid in solving multi-mode approximations. The space-dependent fast adiabatic excursion model, or Fuchs-Nordheim model, is solved by a novel iterative approach. This iterative solution is valid for large disturbances, as well as small, thus improving results obtained by approximate modal expansions. The derivation of the space-independent Fuchs-Nordheim model from the space-dependent equation is shown to follow in a more straightforward manner than derivations based on modal approximations.

ISSN:0029-5639    

DOI:10.13182/NSE69-A18860

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor

ISSN:0029-5639    

DOI:10.13182/NSE69-A18861

出版商:Taylor&Francis    

年代:1969

数据来源: Taylor