R.B. Perez

Non linear analysis of out of phase oscillations in boiling water reactors

Abstract Out of phase oscillations have been observed recently in many boiling water reactors during stability tests and also in start up conditions. Many authors have attempted to explain these regional oscillations, but the explanations given are not complete. In this paper, we develop a non linear phenomenological model that can explain, both in phase and out of phase oscillations. The neutronic loop has been described on the basis of an expansion in terms of A-modes. Furthermore, for a semiquantitative representation of the dynamics, reduced order model have been obtained reducing the number of regions, modes and energy groups considered in the problem. In this line, we propose a model that qualitatively explains the dynamic behavior of these oscillations verifying that in phase oscillations only appear when the azimuthal mode has not enough thermal-hydraulic feedback to overcome the eigenvalue separation and also, that it is possible that self-sustained out of phase oscillations arise due to the different thermal-hydraulic properties of the two reactor core lobes, if the modal reactivities have appropriate feedback gains.

Measurement of the uranium-238 capture cross section for incident neutron energies up to 100 keV

AbstractThe neutron capture cross section of 238U was measured for incident neutron energies between 5 eV and 100 keV using a pulsed electron Linac neutron source and the time-of-flight technique. Capture gamma rays were detected by a large liquid scintillator located on a 40-m flight path. The incident neutron flux was monitored by a 10BF3 ionization chamber. The cross section was normalized by the saturated resonance technique.The data have uncertainties which increase from ∼5% at 1 keV to 10% at 100 keV. These data are compared with results from other measurements and with various evaluations.

Measurement and Resonance Analysis of Neutron Transmission Through Uranium-238

Neutron transmissions through 0.076-, 0.254-, 1.080-, and 3.620-cm-thick samples of isotopically enriched /sup 238/U were measured from 0.88 to 100.0 keV by means of a time-of-flight technique over a path length of 150 m, the ORELA pulsed neutron source, and a 13-mm-thick lithium-glass detector. To obtain resonance parameters, these transmissions from 0.88 to 4.00 keV were simultaneously least-squares shape-fitted with a multilevel Breil--Wigner cross-section formalism. In general, large neutron widths were obtained, resulting in an s-wave strength function of (1.208 +- 0.045) x 10/sup -4/ over the interval from 0.0 to 4.0 keV. An absolute energy scale accurate to 2 parts in 10,000 was established. 11 figures, 16 tables.

A new resonance region evaluation of neutron cross sections for sup 235 U

A new evaluation of the resolved resonance range for the neutron cross sections of {sup 235}U is described. Up to 110 eV, the evaluation is based on an R-matrix analysis of several fission, capture, and transmission measurements. Levels above 110 eV are no longer resolved so that many resonances are missed; from 110 to 500 eV, most of the important resonances can be identified and analyzed so that the cross section and transmission data are well represented by the proposed parameters. From 500 to 2250 eV, fictitious parameters are provided that describe fairly well the results of thick sample transmission measurements and recent fission cross-section data. Such a parameterization is likely to yield a better approximation of resonance self-shielding than the current ENDF/B-V unresolved resonance treatment.

Measurement of the Uranium-238 Subthreshold Fission Cross Section for Incident Neutron Energies Between 0.6 and 100 keV

The neutron-induced /sup 238/U subthreshold fission cross section has been measured in the neutron energy range between 0.6 and 100 keV. A total of 28 fission clusters were identified. The well-known clusters at 721 and 1210 eV appeared resolved into their Class I components. Average /sup 238/U subthreshold fission cross sections were determined and compared with available results in the literature. The measurement is interpreted in terms of fission doorway (Class II levels) arising from the assumption of the existence of a double-humped fission barrier for the (/sup 238/U + n) compound nucleus at large deformations. On the basis of this model, several fission barrier parameters were determined.

Sensitivity of computed uranium-238 self-shielding factors to the choice of the unresolved average resonance parameters

The influence of different representations of the unresolved resonances of /sup 238/U on the computed self-shielding factors is examined. It is shown that the evaluated infinitely diluted average capture cross section does not provide sufficient information to determine a unique set of unresolved resonance parameters; different sets of unresolved resonance parameters equally consistent with the evaluated average capture cross section yield significantly different computed self-shielding factors. In the conclusion it is recommended that the resolved resonance description of the evaluated /sup 238/U cross sections be extended to higher energies and that thick sample transmission data and self-indication data be used to improve the evaluation of the unresolved resonance region.

Multilevel effects in the unresolved resonances region of the cross sections of fissile nuclides

For the specification of the cross sections of the fissile isotopes in the neutron energy region of unresolved resonances, the single-level formalism is often used, while an analysis of the cross sections in the resolved region indicates that a multilevel formula may be more appropriate. The statistical properties of the cross sections generated by using the single-level formalism are compared with those obtained by a multilevel formulation. The multilevel parameters were chosen to give the same average cross sections as the single- level formalism. The comparison indicates that there are small, but significant, differences between the statistical properties of the cross sections obtained with the multilevel formalism and those obtained with the single-level formula. The differences are probably too small, particularly when Doppler broadening is considered, to affect reactor calculations. (11 figures, 10 tables) (auth)

URR (Unresolved Resonance Region) computer code: A code to calculate resonance neutron cross-section probability tables, Bondarenko self-shielding factors, and self-indication ratios for fissile and fertile nuclides

The URR computer code has been developed to calculate cross-section probability tables, Bondarenko self-shielding factors, and self- indication ratios for fertile and fissile isotopes in the unresolved resonance region. Monte Carlo methods are utilized to select appropriate resonance parameters and to compute the cross sections at the desired reference energy. The neutron cross sections are calculated by the single-level Breit-Wigner formalism with s-, p-, and d-wave contributions. The cross-section probability tables are constructed by sampling the Doppler broadened cross-section. The various shelf-shielded factors are computed numerically as Lebesgue integrals over the cross-section probability tables. 6 refs.

R-matrix analysis of the 239Pu neutron cross sections

Abstract 239Pu neutron cross-section data in the resolved resonance region were analyzed with the R-Matrix Bayesian Program SAMMY. Below 30 eV the cross sections computed with the multilevel parameters are consistent with recent fission and transmission measurements as well as with older capture and alpha measurements. Above 30 eV no suitable transmission data were available and only fission cross-section measurements were analyzed. However, since the analysis conserves the complete covariance matrix, the analysis can be updated by the Bayes method as transmission measurements become available. To date, the analysis of the fission measurements has been completed up to 300 eV.

Multilevel analysis of the low-energy /sup 239/Pu cross sections

It is well known that the ENDF/B-V representation of the /sup 239/Pu resonance region is not satisfactory: below 1 eV the cross sections are given by smooth files (file 3) rather than by resonance parameters. Above 1 eV the single-level formalism used in ENDF/B-V necessitates a structured file 3 contribution. The present paper presents the results of an R-matrix multilevel analysis of several recent data sets as well as of the older data. The analysis is concerned with the region up to 30 eV, an energy region that is important for thermal reactors and for which no recent multilevel analyses had previously been available.