31 research outputs found
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Review of the neutron capture process in fission reactors
No full textThe importance of the neutron capture process and the status of the more important cross section data are reviewed. The capture in fertile and fissile nuclei is considered. For thermal reactors the thermal to epithermal capture ratio for /sup 238/U and /sup 232/Th remains a problem though some improvements were made with more recent measurements. The capture cross section of /sup 238/U in the fast energy range remains quite uncertain and a long standing discrepancy for the calculated versus experimental central reaction rate ratio C28/F49 persists. Capture in structural materials, fission product nuclei and the higher actinides is also considered
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Fast-neutron capture cross sections of importance in technological applications. [Review]
No full textThe importance of the capture cross section of the major fertile nuclei, /sup 238/U and /sup 232/Th, leads to the consideration of these data. The /sup 238/U (n,..gamma..) cross section is considered of priority as it is part of the /sup 238/U-/sup 239/Pu cycle. Experimental techniques used in the measurements of these data are considered. Data measured more recently are compared with provisions made for the possible explanations of differing results. It is concluded that the /sup 238/U (n,..gamma..) cross section is known with approx. 5% above 10 keV and fulfills the uncertainty limit for this cross section set to achieve design accuracy for k/sub eff/ and the breeding ratio above 500 keV. Below 500 keV, the present uncertainty falls short of the required 1.5 to 3.0% uncertainty. Specific recommendations are made to resolve existing discrepancies and data uncertainties. 84 references
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Review of measurement techniques for the neutron radiative-capture process
No full textThe experimental techniques applied in measurements of the neutron capture process are reviewed. The emphasis is on measurement techniques used in neutron capture cross section measurements. The activation technique applied mainly in earlier work has still its use in some cases, specifically for measurements of technologically important cross sections (/sup 238/U and /sup 232/Th) with high accuracy. Three major prompt neutron radioactive capture detection techniques have evolved: the total gamma radiation energy detection technique (mainly with large liquid scintillation detectors), the gamma-energy proportional detectors (with proportional counters or Moxon-Rae detectors), and the pulse-height weighting technique. These measurement techniques are generally applicable, however, shortcomings limit the achievable accuracy to a approx. = 5 to 15% uncertainty level
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Simultaneous evaluation of interrelated cross sections by generalized least-squares and related data file requirements
No full textThough several cross sections have been designated as standards, they are not basic units and are interrelated by ratio measurements. Moreover, as such interactions as /sup 6/Li + n and /sup 10/B + n involve only two and three cross sections respectively, total cross section data become useful for the evaluation process. The problem can be resolved by a simultaneous evaluation of the available absolute and shape data for cross sections, ratios, sums, and average cross sections by generalized least-squares. A data file is required for such evaluation which contains the originally measured quantities and their uncertainty components. Establishing such a file is a substantial task because data were frequently reported as absolute cross sections where ratios were measured without sufficient information on which reference cross section and which normalization were utilized. Reporting of uncertainties is often missing or incomplete. The requirements for data reporting will be discussed
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The relationship between integral experimental data and nuclear fission parameters
No full textHigh sensitivities of critical assembly and reactor design parameters to the fission cross sections, prompt and delayed neutron yields, and fission spectra parameters have resulted in an important role of experimental integral data for the testing and verification of differential data and computational methods. The higher accuracy of the experimental integral data compared with the uncertainties of reactor parameters which result from the uncertainties of the differential data has led to their utilization in data adjustment procedures. Improvements of up to a factor of ten are obtained for reactor parameters, however, the uncertainties of the basic data are reduced by smaller amounts. Other integral data like the fission spectra averaged cross sections are used for the evaluation of cross sections and fission spectra. 33 refs., 4 figs., 4 tabs
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Status of nuclear data of importance for LMFBR applications prior to the evaluation of ENDF/B-VI
No full textThe evaluation of nuclear data of importance to the LMFBR program has shifted to a Nuclear Data Evaluation Task Force. It is anticipated that the results of these evaluations will be incorporated in ENDF/B-VI. However, several cross sections for reactor applications are included in a simultaneous evaluation of the standard cross sections for ENDF/B-VI organized by the Standard Subcommittee of CSEWG. Cross sections included in this simultaneous evaluation are those of /sup 6/Li(n,..cap alpha..), /sup 6/Li(n,n), /sup 10/B(n,..cap alpha../sub 0/), /sup 10/B(n,..cap alpha../sub 1/), /sup 10/B(n,n), /sup 197/Au(n,..gamma..), /sup 235/U(n,f), /sup 238/U(n,..gamma..), /sup 238/U(n,f), and /sup 239/Pu(n,f). The change of the evaluation methodology for ENDF/B-VI will result in a much improved definition of the data, their uncertainties and cross correlations. Trends which can be seen in new data and which are caused by the change of the evaluation procedure are toward, lower /sup 239/Pu(n,f), /sup 235/U(n,..gamma..), modestly lower /sup 235/U(n,f), and higher /sup 10/B(n,..cap alpha..) data. The data base for /sup 238/U(n,..gamma..) below 30 keV remains poorly defined and a resolution of the C/E discrepancy of C/sup 28//F/sup 49/ cannot be expected from the infinite dilute capture cross section of /sup 238/U. Anti nu of /sup 252/Cf remains unchanged and therefore also the nu(E) of the fissile isotopes, except at thermal energy
