摘要:

AbstractA study is made of the dynamic interaction between the bolts and fuel segments in a pulse reactor assembly which undergoes rapid heating. The analysis of the propagating thermoelastic stress waves in the bolted assembly indicates that the maximum dynamic stresses in the bolts can be many times greater than the static thermal stress. A comparison of the results obtained in the continuum analysis with those from an approximate stereomechanical (spring-mass) model indicates that the latter model gives good results for moderately large ratios of segment mass-to-bolt mass. Analytical expressions are derived which give the peak dynamic bolt stresses.

ISSN:0029-5639    

DOI:10.13182/NSE67-A18395

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

摘要:

AbstractA reasonable physical model for the slowing down of gamma rays in infinite media is presented, and a method of numerical solution is described. Equilibrium energy spectra due to a fission source of gamma rays are shown for water, aluminum, iron, zirconium, and lead. In addition, energy spectra in aluminum, iron, and lead, due to the corresponding (n,γ) source in each metal, are presented. The use of infinite medium calculations to obtain a lower energy cutoff for a gamma heating problem is suggested. It is shown that for the case of a fission source, essentially all of the source energy is absorbed above 0.05 MeV in the materials studied, except in the case of water where approximately three percent of the energy is absorbed below 0.05 MeV. The infinite medium spectra are used to average absorption and slowing down cross sections for fuel materials and metals, and the resulting group constants are compared with similar calculations using a fission-source spectrum as a weighting function. Large differences are noted in many instances. Calculations of spatial energy deposition in simple model problems indicate that such differences in group constants can lead to local errors of significant magnitude.

ISSN:0029-5639    

DOI:10.13182/NSE67-A18396

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

摘要:

AbstractMonte Carlo calculations, using the albedo concept, have been carried out to determine subcadmium and epicadmium neutron flux distributions along the centerline of a straight, a two-legged, and a three-legged square concrete duct arising from the slowing down of incident epicadmium neutrons for a particularly demanding source geometry and spectrum. The calculations used albedo data differential both in the reflected angles and reflected energy which have been reported previously for concrete. A comparison of the results of these calculations with those from a geometrically similar experiment shows good agreement and places on a firm foundation the concept of treating neutron slowing down in a concrete duct as a reflection phenomenon at a point which is describable by the differential albedo properties of the walls. The conclusion is also reached that the dose rates arising from the subcadmium neutrons (whether due to an epicadmium source or a subcadmium source) and associated secondary wall-capture gamma rays can comprise a very important part of the total absorbed dose rate in tissue deep inside a multilegged duct.

ISSN:0029-5639    

DOI:10.13182/NSE67-A18397

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

摘要:

AbstractThe neutron total cross section of238Pu has been measured from 0.008 to 6500 eV. These data give single-level Breit-Wigner parameters for resonances below 200 eV. The observed total cross section at 2200 m/sec isb. A value ofb has been calculated for the effective (equivalent 1/v) thermal absorption cross section. Parameters of individual resonances below 200 eV and average parameters at higher energies give a resonance absorption integral of 164±15 b, and a value of (1.10±0.20)×10-4for the s-wave neutron strength function (/D).

ISSN:0029-5639    

DOI:10.13182/NSE67-A18398

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

摘要:

AbstractAsymptotic decay constants for pulse-induced“thermalized”neutron fields have been measured for graphite cubical assemblies having geometric bucklings varying from 9.30×10–4cm–2to 13.44×10–3cm–2. A value of 700μsec was observed to be a sufficient time after the neutron pulse to identify and evaluate fundamental-mode decay in the smallest system included in the above interval of buckling. Values of the infinite-medium neutron lifetime,“Fick’slaw”diffusion coefficientD0, as well as the so-called“diffusion-cooling”coefficientC, were obtained from least-squares fits to the experimentalα/ρvsB2/ρ2data and are mutually consistent and stable over a large interval ofB2and in good agreement with theory. The existence of a well-defined negativeFB6term has been verified. An“effective”higher-mode decay of (3570±80)sec–1, independent of system buckling, was obtained and is consistent with the concept of a continuum lying above a critical limit for fundamental-mode decay. An apparent critical limit (v∑t)minhas been identified in the interval 2392 sec–1<(v∑t)min<2648 sec–1which corresponds to the interval of buckling 13.44×10–3cm–2to 16.53×10–3cm–2.

ISSN:0029-5639    

DOI:10.13182/NSE67-A18399

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

摘要:

AbstractThe Doppler effect in238U capture and235U fission has been measured by means of a foil activation technique in the fast-neutron spectrum core of the Mixed Spectrum Critical Assembly. Experimental results were obtained for two238U foil thicknesses and one235U foil thickness. The amount of scattering material between the foil and surrounding core fuel was varied to determine the effect on the Doppler measurement of change in the incident flux fine-energy structure in the resonances. In this experiment, only the foil is heated, while the core fuel remains at room temperature.The experiment is analyzed by means of the collision-probability method which is used to develop an expression for the resonance integral of a thin absorber which is separated from a homogeneous reactor fuel region by a purely scattering medium. The general expression for the foil resonance integral is simplified and numerical results are presented for the case in which the dominant resonances are weak; that is, for a fast reactor in which the 0.5 to 3.0-keV energy region dominates the238U Doppler effect.The measured238U Doppler effect expressed as the ratiotypically was of the order of 0.015±0.002. This was a factor of 2 higher than that calculated using a neutron energy spectrum derived from“nominal”material cross sections. Presently available cross sections in the energy range of interest are sufficiently uncertain so that it is possible to infer from them“hard”or“soft”neutron energy spectra such that the value ofR-l varies by a factor of 2. The measured values for238U agreed quantitatively with those found from the“soft”neutron energy spectrum. Within the precision of the measurement no235U Doppler effect was observed. The calculated235U Doppler effect was smaller than the sensitivity of the experiment, thus, within its precision (±0.002), the measurement confirms the theory.

ISSN:0029-5639    

DOI:10.13182/NSE67-A18400

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

摘要:

AbstractA detailed Monte Carlo analysis in one, two, and three dimensions and with different multigroup scattering kernels is presented for a number of actual reactor systems. Several variance reducing sampling techniques, which we believe to be unusual, are employed and, in addition to the prediction of reactivity, much emphasis is placed on generation time calculations with reference to the“life cycle”point of view. One of the main points of interest in the numerical results obtained is the comparison of the reactivity and time eigenvalues with those obtained from the equivalentSNandjNcalculations. The excellent agreement with these two methods establishes the necessary confidence in the Monte Carlo procedure described here. As a further illustration of the method, it was thought to be of interest to compare the numerical results obtained from different scattering kernels (transport approximation, linear anisotropy, and exact anisotropy) with a view to assessing the influence of these different approximations on the reactivity, absorption, leakage, generation time, etc. Simultaneously, an examination of two different Monte Carlo sampling techniques is presented. To apply a physical test to the method, some highly enriched uranium spheres, some cylinders of extreme geometry reflected by a variety of materials, and some cylindrical annuli were analyzed and the results compared with experiments. In addition, some systems requiring the full use of the three-dimensional scope of the method are studied. The efficiency of the Monte Carlo procedure is finally illustrated by listing, for several calculations, the probable errors in the reactor eigenvalues and other parameters after 10 min of IBM-7090 computer time. This analysis proves that statistical methods can be used to carry out three-dimensional assessments of reactor assemblies with sufficient accuracy without the expenditure of a prohibitive amount of computer time. Such a goal has not yet been achieved by the numerical or analytical methods which solve the neutron transport equation.

ISSN:0029-5639    

DOI:10.13182/NSE67-A18401

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

摘要:

AbstractA space-dependent reactor kinetics approximation, called the Natural Mode Approximation (NMA), has been applied to the calculation and interpretation of reactor dynamic experiments. The NMA is based on a modal expansion technique where the space- and time-dependent reactor variables are approximated by a series of products of time-dependent coefficients and space-dependent expansion modes. The modes are the eigenvectors of a linear operator derived from the complete set of equations describing the reactor system at an initial reference condition. A pair of computer codes, MUDMO-II and SYNSIG, are used to synthesize approximate modes in two-dimensional systems without feedback. An oscillation test is proposed which may be used to verify key parameters of the NMA. The experimental technique is described and applied to both numerical and actual measurements. In addition, it is shown how a natural mode expansion may be used to interpret standard dynamic experiments when the observations are functions of space and time. The results of calculations of kinetic problems are compared with those of independent calculations which are considered to be exact. Good agreement is established. It is shown that the flux tilting following a localized perturbation is a sensitive function of the relative magnitudes of the measurable parameters of the NMA. The novel idea of“correction modes”is introduced which increases the accuracy of a low-order NMA without appreciable increase in computation time.

ISSN:0029-5639    

DOI:10.13182/NSE67-A18402

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

摘要:

AbstractA solution of the zero-power kinetic equations for sinusoidal excess reactivity insertions, previously obtained by the author by Wentzel-Kramers-Brillouin approach (WKB), is further discussed. Explicit equations for the reactor period, reactivity bias, and stabilized reactor response, within the range of applicability of the method, are derived. Harmonic contents of the logarithm of flux for both pure and properly biased sinusoidal reactivity variations are analyzed. Fourier components of flux yielding the new steady-state mean power, the fundamental and the second harmonic are given. Results of the treatment are extended to the describing function of a low-power nuclear reactor and the major error involved in the earlier literature is indicated. The procedure, although developed under the assumption of one average group of delayed neutrons, is expected to yield very satisfactory results even if generalized to multigroup treatment.

ISSN:0029-5639    

DOI:10.13182/NSE67-A18403

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor

ISSN:0029-5639    

DOI:10.13182/NSE67-A18404

出版商:Taylor&Francis    

年代:1967

数据来源: Taylor