摘要:

AbstractWork has been done to develop improved methods for treating resonance cross sections in both the resolved and the unresolved energy regions. The methods are natural extensions of previous methods using theJ* treatment. Efficient numerical algorithms have been developed to make the methods highly economical for the routine fast reactor calculations. For the resolved energy region, a new technique is proposed for evaluating the generalized form of the resonance integral which allows the use of the multilevel formalism. By taking advantage of the asymptotic behavior of the Doppler-broadened line-shape functions, the technique of rational transformation is introduced. Gauss-Jacobi quadrature is used to evaluate the integral with the transformed integrand. The integration can be carried out efficiently. For the unresolved energy region, an accurate and efficient method has been developed for estimating the effect due to in-sequence overlap. In contrast to the previously developed methods, the new method is capable of treating practically all energy regions of practical interest in fast reactor applications. The Dyson correlation function is used to account for the proper correlation of levels. The method has been made economical for routine calculations.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28186

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractThe neutron-induced fission cross section of238Pu was measured from 18 eV to 3 MeV. The nuclear explosion Persimmon provided an intense, single-pulse neutron source spanning an energy range of more than five decades. Neutrons from this underground source were sorted by time of flight over a 304-m vertical path, and interacted with a238Pu sample and with flux monitors at ground level. The neutron flux was measured relative to the reactions3He(n,p),6Li(n,αt), and235U(n,f). Fission cross-section areas are reported for 49 resonances below 500 eV. A number of previously unobserved resonances are reported, yielding an average level spacing of 9.5±0.7 eV. There is strong evidence for intermediate structure in the fission cross section with an average spacing of∼1 keV.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28187

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractThe radiative capture cross section of238Pu has been measured from 18-eV to 200-keV neutron energy. A time-of-flight experiment with a 306-m flight path was carried out in conjunction with the underground nuclear explosion Persimmon. Fission-fragment detectors viewed a thin238Pu target to measure the fission cross section, while modified Moxon-Rae detectors viewed a second, thicker238Pu target to measure the gamma-ray emission. Subtraction of the fission gamma-ray contribution from the Moxon-Rae signal yielded the contribution due to radiative capture. Single-level area analysis of the measured fission and capture cross sections gave values for the neutron and fission widths of 49 resonances below 500 eV, under the assumption of a known, constant radiative capture width. Thes-wave neutron strength functionwas determined to be (1.27±0.25)×10-4. The derived fission widths exhibit a distinct maximum near 300-eV neutron energy. At higher energies, the fission-to-capture ratio shows pronounced intermediate-structure peaks attributed to second-well effects in the fission barrier.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28188

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractSystematic errors responsible for the large discrepancy between the measured and calculated central reactivity coefficients were examined. These errors were narrowed to two sources: the normalization integral (or perturbation denominator) and the conversion factor of inhour, or dollars, toΔk/kunits. The magnitude of both sources of error is uniquely determined by the ratio of the measured-to-calculated normalization integral when the measurement is carried out using the252Cf source-reactivity method. The measured-to-calculated normalization integral ratios for ZPR-6 Assemblies 6A and 7, two typical demo-plant-size Liquid Metal Fast Breeder Reactor criticals, were 1.19 and 1.21, respectively. The magnitude of this discrepancy is essentially the same as that found for the central reactivity coefficient.Analysis of the available fission rate distribution in both assemblies indicates that the calculated normalization integral may be underestimated by 6 to 8% and that the remainder of 10 to 14% must come from the conversion factor. The delayed-neutron data of Krick and Evans, when used with the appropriate average number of neutrons per fission in each assembly, yield conversion factors 9 to 13% higher than the delayed-neutron data of Keepin. This would provide the explanation of the central reactivity discrepancy. Unfortunately, the method of calculatingβeffcould also produce errors of this magnitude even if one has an absolutely correct set of delayed-neutron fractions.More definitive measurements of the delayed-neutron fractions of pertinent isotopes, as a function of the incident neutron energy, are needed. In addition, measurements ofβeff in various assemblies by different methods are required.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28189

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractIn a class of experiments using extended planar sources, the age of fission neutrons is calculated by“correcting”the measured second moment of the flux through the use of a series in the higher flux moments. In this paper, the“correction”is generalized to include terms in addition to the leading term of an eigenfunction expansion of the neutron source distribution. In the generalized correction series, expansion coefficients are shown to be series themselves, which cannot be shown to converge in general. Examination of physically reasonable examples, one of which included only the effect of the energy-dependent extrapolation length of a published experiment, reveals divergences in the series for all expansion coefficients but that of the leading term in the correction series. Since the assumption of an energy-independent extrapolation length was central to the derivation of the correction series in question, this indictment is quite general.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28190

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractNeutron waves in a reflected reactor are analyzed using the two-group diffusion equation. The solution is composed of a transient which disappears within a few milliseconds, a steady-state part, and an oscillatory part. The last is comprised of four terms corresponding to four waves: two propagating waves and two reflected waves.When a sinusoidal source of thermal neutrons is applied to a water-moderated multiplying medium, the source wave amplitude decays with a relaxation length equal to the diffusion length of the medium. A wave of fast neutrons is then produced by fission which is induced by the source wave. This fast-neutron wave carries the wave phenomenon across the core. The two propagating thermal-neutron waves are (a) the source wave, and (b) a wave of thermalized fission neutrons. The two reflected waves are the reflections of the two propagating waves. The amplitudes and phases of the four waves are calculated, together with those of the total wave which is the sum of the four partial waves. The agreement of the measured data of the total wave with the calculations is reasonable.Modulated poison and source oscillators were used for the experiments. The reactor response to both was essentially the same except in the region near the oscillators; while the first decreased the number of neutrons, the second increased their number.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28191

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractA description is presented of an experiment which provides verification of the accuracy of the available neutron cross sections for use in transport calculations of deep penetration of neutrons through up to 3 ft of iron and 18 in. of stainless steel. Calculations of the experiment were performed with a special version of the MORSE multigroup Monte Carlo code which uses point total cross sections. Comparison of the calculations using the new MAT 4180-Mod 1 iron cross-section set with experiment indicates that the absolute neutron leakage spectra above thermal energies arising from scattering that penetrate up to 1 ft of iron or 18 in. of stainless steel can be calculated to within∼20%, and that the total neutron leakage above thermal energies penetrating up to 3 ft of iron can also be calculated to about the same accuracy.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28192

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractBy employing the invariant four-dimensional representation of the photon-electron interaction, obtained from lowest order quantum electrodynamics, the Compton scattering kernel is easily found in any coordinate frame. This procedure provides a simple alternative to the usual Lorentz transformation of the scattering kernel (from electron rest frame to frame of interest) used in radiation-hydrodynamics computations and associated moving-media problems in transport theory. Furthermore, arbitrary distributions of electrons can be conveniently handled in this representation, and standard predictions for electrons initially at rest can be recovered easily.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28193

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractExperimental techniques previously used for measuring neutron spectra in fast critical assemblies have been applied to reactor shielding problems. Fast-neutron spectra in the energy range from 25 keV to 1 MeV have been measured in the water shield surrounding a small thermal research reactor. Three different water thicknesses were investigated, as well as a combination of water with a 2-in.-thick slab of iron. The spectrometer system developed for these measurements employs small proton-recoil proportional counters, together with electronic pulse-shape discrimination for the rejection of gamma-ray signals. Since this discrimination is extremely important for shielding measurements, the detectors and electronic system are described in some detail. The basic theory underlying proton-recoil measurements of neutron spectra is reviewed, as well as the techniques used to correct for errors arising due to finite detector size and irregularities in the electric field within the counters. Results of the water measurements indicate that the spectrometer is capable of measuring the absolute neutron flux as well as relative variations within the spectrum at a given point. In addition, predictions of spectral hardening with increasing water thickness are verified. The water-iron measurements indicate that the system is capable of measuring spectral features caused by resonances in the cross section of the shield material.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28194

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor

摘要:

AbstractThe variances of pulse fluctuations of a periodically pulsed fast reactor have been calculated. The characteristic parameters of SORA, the 1-MW pulsed fast reactor, under development by Euratom at Ispra, have been used as input data. As a result, the relative standard deviation was found to be independent of the duration of the power pulse and the pulse shape and inverse proportional to the square root of the operation power.

ISSN:0029-5639    

DOI:10.13182/NSE73-A28195

出版商:Taylor&Francis    

年代:1973

数据来源: Taylor