David Loaiza’s research while affiliated with Los Alamos National Laboratory and other places

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Publications (9)


Hernia incisional gigante en un adulto mayor
  • Article
  • Full-text available

June 2019

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20 Reads

Revista Ecuatoriana de Ciencia Tecnología e Innovación en Salud Pública

Ines Zoraya Vasquez-Santander

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David Fabricio Loaiza-Ramírez

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Diego Fernando Velasco-García

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Milton Daniel Tite-Naranjo

La hernia incisional (HI) figura dentro de las principales complicaciones posteriores a una laparotomía en los servicios de cirugía a nivel mundial, esta incidencia ha ido disminuyendo a través de los años principalmente debido al desarrollo de nuevas técnicas quirúrgicas, nuevos materiales de sutura y mallas protésicas, sin embargo, las hernias de gran tamaño aun representan un desafío para los especialistas cirujanos. Respecto a la clasificación de HI la evidencia recomienda utilizar la clasificación de la European hernia society (EHS). Al no existir un Gold standard diagnóstico, la TC proporciona el diagnóstico más sensible para HI seguido del ultrasonido y el examen físico. El abordaje quirúrgico en estos casos dependerá de un amplio abanico de factores, ya sean propios del paciente o de sus enfermedades concomitantes, así mismo las características del defecto aponeurótico, su tamaño, calidad de los tejidos, como también de factores relacionados con la destreza quirúrgica, y los recursos disponibles. Se presenta el caso de un paciente de 66 años, con diagnóstico de hernia incisional gigante de 8 meses de evolución posterior a laparotomía exploratoria, con obesidad grado I y diabetes mellitus tipo II, ingresó al servicio de cirugía del hospital José María Velasco Ibarra de la ciudad de Tena y bajo evaluación multidisciplinaria se decidió tratamiento quirúrgico mediante abordaje abierto, realizándose abdominoplastia y cirugía de reparación de hernia.

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Criticality of a237Np sphere

January 2008

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19 Reads

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16 Citations

R. Sanchez

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D. Loaiza

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R. Kimpland

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[...]

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M. Chadwick

A series of critical-mass experiments using a 6-kg neptunium sphere was performed on the Planet vertical-assembly machine at Los Alamos National Laboratory (LANL). The purpose of the experiments was to obtain a better estimate of the critical mass of 237Np. The configurations that were studied included surrounding the neptunium sphere with highly enriched uranium (HEU) shells as well as reflecting it with iron and polyethylene. An additional experiment using a 4.5-kg α-phase plutonium sphere surrounded with HEU was performed to demonstrate how well the computer transport code and the existing cross-section data for uranium and plutonium could reproduce the experiment. For some of the configurations, the prompt-neutron decay constants at delayed critical were measured. These experiments provided an integral measurement of the cross sections for 237Np in the fast-energy and possibly in the intermediate-energy regions. The measured keffthese experiments was compared with the calculated keffthe Monte Carlo N-Particle (MCNP) transport code using ENDF/B-V and ENDF/B-VI and cross-section data evaluated by the Nuclear Theory and Applications group (T-16) at LANL. In all the neptunium experiments, the calculated keffvalues based on ENDF/B-VI data were ∼1% lower than the experimental keff. After adjusting the cross sections for neptunium and 235Np to match the bare neptunium/ HEU experiment as well as Godiva keff criticality and spectra indexes, the MCNP code yielded a value of 57 ±4 kg for the bare critical mass Of237Np.


Criticality of a Np-237 sphere

January 2008

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103 Reads

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13 Citations

A series of critical-mass experiments using a 6-kg neptunium sphere was performed on the Planet vertical-assembly machine at Los Alamos National Laboratory (LANL). The purpose of the experiments was to obtain a better estimate of the critical mass of Np-237. The configurations that were studied included surrounding the neptunium sphere with highly enriched uranium (HEU) shells as well as reflecting it with iron and polyethylene. An additional experiment using a 4.5-kg alpha-phase plutonium sphere surrounded with HEU was performed to demonstrate how well the computer transport code and the existing cross-section data for uranium and plutonium could reproduce the experiment. For some of the configurations, the prompt-neutron decay constants at delayed critical were measured These experiments provided an integral measurement of the cross sections for Np-237 in the fast-energy and possibly in the intermediate-energy regions. The measured k(eff) from these experiments was compared with the calculated k(eff) from the Monte Carlo N-Particle (MCNP) transport code using ENDF/B-V and ENDF/B-VI and cross-section data evaluated by the Nuclear Theory and Applications group (T-16) at LANL. In all the neptunium experiments, the calculated keff values based on ENDF/B-VI data were similar to 1 % lower than the experimental k(eff). After adjusting the cross sections for neptunium and U-235 to match the bare neptunium/ HEU experiment as well as Godiva k(eff) criticality and spectra indexes, the MCNP code yielded a value of 57 +/- 4 kg for the bare critical mass of Np-237.


End of an Era for the Los Alamos Critical Experiments Facility: History of critical assemblies and experiments (1946–2004)

November 2006

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161 Reads

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23 Citations

Annals of Nuclear Energy

The Los Alamos Critical Experiments Facility (LACEF) was the last operational, general-purpose, critical-mass laboratory in the United States. The long history of remote operations and large-scale critical-mass experiments at LACEF began in 1948, and it effectively ended in July 8th, 2004, when the last critical experiment was performed on the Planet critical assembly. The experimental activities at the Pajarito Site began in April 1946 as a way to obtain subcritical measurements for weapons safety guidance. A year later, the first Kiva (a concrete-reinforced building) was constructed, and 18 months afterward the first remote critical operation was reported with the Topsy critical assembly. In the early years, the Pajarito Site primarily supported the weapons program; later, for almost 17 years, the neutronics of the Rover nuclear-propulsion program dominated activities at Pajarito Site. More recently, Pajarito Site added some new dimensions to its operations in order to support emergency response, the Nuclear Criticality Safety Program, and radiation-detection development. The long history of critical-assembly measurements and operations is documented in hundreds of peer-reviewed technical papers, laboratory reports, personal files, and video sessions with some of the pioneers. It is the intent of this paper to capture, in one single document, a summary and the highlights of the glorious days of this facility. In essence, this paper is a summary of the programs conducted in the last 58 years and of the numerous critical assemblies and reactors that operated at LACEF. It also provides a list of references to the reader who might want to learn more about this facility’s rich history.


Results and Analysis of the Spherical 237 Np Critical Experiment Surrounded by Highly Enriched Uranium Hemispherical Shells

January 2006

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33 Reads

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13 Citations

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 k eff of 1.003. Comparison of the experimental results with computational methods used to predict the k eff 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 k eff, 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 k eff of ± 0.0032.


Critical Experiments with Highly Enriched Uranium and Matrix Elements (Si, Mg, Al, Gd, and Fe)

July 2004

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7 Reads

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2 Citations

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 k{sub eff} from these experiments was compared with the calculated k{sub eff} 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.


Critical Masses of Uranium Diluted with Matrix Material [Slides]

October 2002

Critical masses of square-prisms of highly enriched uranium diluted in various X/235U ratios with matrix material and polyethylene were measured. The Configuration cores were 22.86-cm and 45.72-cm square and were reflected with 8.1 3-cm and 10.1 6-cm thick side polyethylene reflectors, respectively. The configurations had 10.1 6-cm thick top and bottom polyethylene reflectors. For some configurations, the Rossi-a, which is an eigenvalue value characteristic for a particular configuration, was measured to establish a reactivity scale based on the degree of subcriticality . Finally, the critical mass experiments are compared with values calculated with MCNP and ENDF/B-VI cross-sections.


BENCHMARK CHARACTERIZATION FOR THE DILUTED HIGHLY ENRICHED URANIUM EXPERIMENTS WITH WASTE MATRIX MATERIALS

5 Reads

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 (LACEF). These experiments were performed to provide criticality safety data for waste matrix materials. These critical experiments were fueled with highly enriched uranium (HEU), moderated with polyethylene, and mixed with silicon dioxide (SiO 2), 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. These uncertainties were calculated using neutronic codes (MCNP, KENO, DANTSYS). All four experiments had a measured k eff of 1.000. The benchmark of these critical experiments yielded uncertainties in the measured k eff of ± 0.0023 for SiO 2 , ± 0.0024 for Al, ± 0.0026 for MgO, and ± 0.0024 for Gd. These experiments were judged to be of benchmark quality.


Citations (5)


... The critical mass experiment with the 237 Np sphere was performed on the Planet critical assembly machine in which it was surrounded by RF hemishells. 34,46,47 To support this experiment, subcritical multiplication measurements were performed in advance at LACEF to provide approach-to-critical information. 48 Experiments with the 237 Np sphere at NCERC are described in Sec. ...

Reference:

A New Era of Nuclear Criticality Experiments: The First 10 Years of Radiation Test Object Operations at NCERC
Criticality of a Np-237 sphere
  • Citing Article
  • January 2008

... . Los Alamos National Laboratory estimates the critical mass of 237 Np at 57 ± 4 kg[108].Due to the relatively small size of a critical 237 Np sphere and the relative stability of 237 Np (2.144 × 10 6 years), a 237 Np-based weapon is potentially feasible. Typical attributes that make a nuclear material unattractive for use in a nuclear weapon, such as heat generation, spontaneous fission, and self-protecting dose rate are nearly nonexistent for 237 Np[109]. ...

Criticality of a237Np sphere
  • Citing Article
  • January 2008

... In the range of thermal to fission neutrons for the fission and capture reaction rates of 237 Np, 241 Am, and 243 Am, numerical analyses by Monte Carlo methods [9][10][11] have been conducted together with numerous nuclear data libraries. [12][13][14][15][16][17][18][19] Other experimental analyses have been conducted for the uncertainty of 237 Np inelastic scattering cross sections for criticality at the Los Alamos Criticality Experiments Facility, [20] the validity of cross sections for MAs in the JEFF-3.1 library [21] at the fast reactor Phénix, [22] and the evaluation of MA sample reactivity worth at the Fast Critical Assembly [23] by JENDL-5. [24] Minor actinide irradiation experiments have been carried out at the neutron spectra of the thermal and intermediate regions obtained using highly enriched uranium (HEU) 93 wt% fuel in the solid-moderated and solid-reflected cores (A cores) at the Kyoto University Critical Assembly (KUCA). ...

Results and Analysis of the Spherical 237 Np Critical Experiment Surrounded by Highly Enriched Uranium Hemispherical Shells
  • Citing Article
  • January 2006

... The matrix materials in these experiments included SiO 2 , MgO, Al, Gd, CH 2 , Fe, and concrete embedded in polyethylene. 21 Figure 10 shows examples of configurations with a variety of matrix materials. In most cases, MCNP simulations overpredicted the reactivity of the experimental configurations. ...

Critical Experiments with Highly Enriched Uranium and Matrix Elements (Si, Mg, Al, Gd, and Fe)
  • Citing Article
  • July 2004

... WSMR FBR can operate in a steadystate mode with the operating power level limited to 8kW. In pulse mode, a maximum pulse size of 2.0 MJ (∆T of 250°C) with a full-width half-maximum (FWHM) of 50 s can be attained [16]. ...

End of an Era for the Los Alamos Critical Experiments Facility: History of critical assemblies and experiments (1946–2004)
  • Citing Article
  • November 2006

Annals of Nuclear Energy