David Loaiza

Measurements of absolute delayed neutron yield and group constants in the fast fission of 235U and 237Np

The delayed neutron activity resulting from the fast induced fission of {sup 235}U and {sup 237}Np has been studied. The six-group decay constants, relative abundances, and absolute yield of delayed neutrons from fast fission of {sup 235}U and {sup 237}Np were measured using the Godiva IV fast assembly at the Los Alamos Critical Experiments Facility. The absolute yield measured for {sup 235}U was 0.0163 {+-} 0.0008 neutron/fission. This value compares very well with the well-established Keepin absolute yield of 0.0165 {+-} 0.0005. The absolute yield value measured for {sup 237}Np was 0.0126 {+-} 0.0007. The measured delayed neutron parameters for {sup 235}U are corroborated with period (e-folding time) versus reactivity calculations.

Results and analysis of the spherical 237Np critical experiment surrounded by highly enriched uranium hemispherical shells

Abstract An experiment to investigate the critical mass of 237Np was performed at the Los Alamos Critical Experiments Facility. The critical configuration consisted of a 6.07-kg neptunium sphere surrounded by 62.555 kg of highly enriched uranium hemispherical shells. The experiment was performed in order to decrease the large uncertainty in the critical mass of 237Np for criticality safety and nonproliferation issues. The critical configuration had an experimental keff of 1.003. Comparison of the experimental results with computational methods used to predict the keff of the system led to identification of a large discrepancy in the 237Np cross-section data from ENDF/B-VI used by the analysis performed with the MCNP code. In an effort to bound the uncertainty on the experimental keff, a sensitivity analysis was performed. This analysis systematically examines uncertainties associated with the critical experiment as they affect the calculated multiplication factor. The systematic analysis is separated into uncertainties due to mass measurements, uncertainties due to geometry of materials, and uncertainties due to impurities. Each type of uncertainty is analyzed individually, and a total combined uncertainty is derived. The sensitivity analysis on this experiment yielded a total combined uncertainty on the measured keff of ±0.0032.

Sensitivity Analyses for Polyethylene-Moderated and Polyethylene-Reflected Highly Enriched Uranium Experiments Mixed with Waste Matrix Materials

Abstract Critical experiments are carried out in order to validate, improve, and benchmark the extensive data calculations available. A series of such experiments was performed at the Los Alamos Criticality Experiments Facility. These experiments were performed to provide criticality safety data for waste matrix materials. These critical experiments were fueled with highly enriched uranium, moderated and reflected with polyethylene, and mixed with silicon dioxide (SiO2), aluminum (Al), magnesium oxide (MgO), and gadolinium (Gd). The uncertainties affecting the experiment were divided into three broad categories: mass measurement, geometry, and material composition. Each category is considered in turn, and then the total experimental uncertainty is derived. All four experiments had a measured keff of 1.001. The sensitivity analyses of these critical experiments yielded uncertainties in the measured keff of ±0.0026 for SiO2, ±0.0026 for Al, ±0.0021 for MgO, and ±0.0029 for Gd. These experiments were judged to be of benchmark quality.

Critical experiments with highly enriched uranium and matrix elements (Si, Mg, Al, Gd, and Fe)

Abstract Scientists at the Los Alamos National Laboratory measured the critical masses of square prisms of highly enriched uranium diluted in various X/235U with matrix material and polyethylene. The configuration cores were 22.86 and 45.72 cm square and were reflected with 8.13-cm-thick and 10.16-cm-thick side polyethylene reflectors, respectively. The configurations had 10.16-cm-thick top and bottom polyethylene reflectors. For some configurations, the Rossi-α, which is an eigenvalue characteristic for a particular configuration, was measured to establish a reactivity scale based on the degree of subcriticality. These experiments provided critical mass data in the thermal energy range for systems containing Si, Mg, Al, Gd, and Fe. The measured keff from these experiments was compared with the calculated keff from MCNP using ENDF/B-V and ENDF/B-VI cross-section data. The observed biases were +0.005 Δk and +0.008 Δk for Si, +0.0006 Δk and +0.008 Δk for Al, +0.0023 Δk for Mg, +0.004 Δk and +0.01304 Δk for Gd, and +0.0123 Δk and -0.00106 Δk for Fe.

Examination of the 2x2 SiO{sub 2}, Al, and Fe Experiments Mixed with Highly Enriched Uranium on the Thermal Energy Region

Abstract The basic characteristics of waste materials such as silicon dioxide, aluminum, and iron fueled with highly enriched uranium (HEU) and moderated and reflected by polyethylene were investigated. These critical experiments were performed at the Los Alamos Criticality Experiments Facility. The primary intention of these experiments is to provide supplementary data that can be used to validate and improve criticality data for the Yucca Mountain and the Hanford Storage Waste Tanks Projects. The secondary intention of the 2×2 experiments is to reduce the H/U ratio and increase the waste material/U ratio from previously published experiments. These experiments were designed to supply data for interlaced waste material/fuel/moderator systems on the thermal region. The experiments contained silicon dioxide (SiO2), aluminum (Al), and iron (Fe) mixed with 93.23% enriched uranium and moderated and reflected by polyethylene. This analysis systematically examines uncertainties associated with the critical experiments as they affect the calculated multiplication factor. The systematic analysis is separated into uncertainties due to mass measurements, uncertainties due to fabrication, and uncertainties due to composition. Each type of uncertainty is analyzed individually, and a total combined uncertainty is derived. The SiO2-HEU experiment had a measured keff of 0.993, the Al-HEU experiment had a measured keff of 0.990, and the Fe-HEU experiment had a measured keff of 1.000. The calculated keff values tend to agree well with the experimental values. The sensitivity analysis of these critical experiments yielded a total combined uncertainty on the measured keff of ±0.0044 for SiO2, of ±0.0048 for Al, and of ±0.0046 for Fe.