Daniel Gehman’s research while affiliated with Los Alamos National Laboratory and other places

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Publications (3)


Fig. 1. Nb-1Zr fueled with HEU experiment mounted on the planet assembly.  
Fig. 2. Subcritical configuration with BF 3 detectors.  
Fig. 3. Nested HEU fuel plates.  
Fig. 4. Refractory metal plates.  
Fig. 5. Representative fission distribution for the Ta-2.5W experiments in the center of the core.  

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Critical Mass and Subcritical Experiments Interlaced with Nb-1Zr, Re, Mo, Ta-2.5W Fueled with Highly Enriched Uranium in Support of the Prometheus Project
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  • Full-text available

October 2008

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355 Reads

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2 Citations

David J. Loaiza

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Daniel Gehman

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The National Aeronautics and Space Administration is considering nuclear power sources for space exploration. A series of critical mass experiments was designed to address the development, performance, and design of a space nuclear reactor being considered to support the Prometheus project. These experiments consisted of interlacing the refractory metals rhenium (Re), molybdenum (Mo), tantalum2.5 wt% tungsten (Ta-2.5W), and niobium-1 wt% zirconium (Nb-1Zr) with moderating materials (graphite or polyethylene) and were fueled by highly enriched uranium plates. These experiments are designed to assess the adequacy of and uncertainty in refractory metal neutron cross-section evaluations for use in Prometheus nuclear reactor design work. The critical experiments were designed in the energy spectrum closely resembling or bracketing that in the proposed space reactor. For each material (Re, Mo, Ta-2.5W and Nb-1Zr), four critical configurations were designed and performed to measure the sensitivity of keff to the material under four different and progressively softer neutron spectra (core center spectrum, harder than core average spectrum, softer than core average spectrum, and accident flooded spectrum). The thicknesses of the graphite or polyethylene moderator and reflector plates were adjusted to achieve the desired neutron spectrum. One critical and 18 subcritical experiments provided for measurements of material neutronic behavior in a simple cylindrical geometry configuration that was modeled in MCNP with ENDF/B-VI.6 cross-section data and compared to the extrapolated or predicted critical mass for all the experiments. These experiments were performed at the Los Alamos National Laboratory using the Planet vertical lift critical assembly at the Los Alamos Critical Experiment Facility.

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Temperature dependency analysis of light output from an NE-213 liquid sintillator

January 2007

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17 Reads

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4 Citations

NE-213 liquid scintillation detectors are currently used in radiation fields consisting of both gamma rays and fast neutrons and are an excellent tool for differentiating between each type of radiation via their respective interactions. In this experiment, an analysis was performed on an NE-213 liquid scintillation detector to investigate the effects of temperature changes on the light output. The two effects measured were the amount of the individual light decay components have to the total pulse height and the total gain of the system as a function of temperature.


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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159 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.

Citations (2)


... 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]. ...

Reference:

Radiation characterization summary for the WSMR fast burst reactor environment at the 6-inch location
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

... The only crystal that can resist higher temperatures is the p-terphenyl[37] at 380 K (107 °C). In contrast, liquid[108] scintillators have to be used in environments with temperatures below 303 K (30 °C) because of their low flash point (≈26 C°). Only recently, new liquid scintillators have been developed that can resist higher temperatures, such as the EJ-309[109] [110] whose flash point is at 417 K (144 °C). ...

Temperature dependency analysis of light output from an NE-213 liquid sintillator
  • Citing Conference Paper
  • January 2007