Joetta M. Goda

Development of a model of an X-ray tube transmission source

In support of the development of an x-ray tube based source for transmission measurements of UF6 gas, we have developed a one-dimensional, spreadsheet-based model of the source. Starting with the spectrum produced by an x-ray tube we apply the linear attenuation coefficients for various notch filters, the aluminum pipe, and UF6 gas. This model allows calculation of the transmitted spectrum based on the type of filter, the thickness of the filter, the x-ray tube high voltage, the Al pipe thickness, and the UF6 gas pressure. The sensitivity of the magnitude of the transmission peak produced by the notch filter to any of these variables can be explored quickly and easily to narrow the choices for experimental measurements. To validate the spreadsheet based model, comparisons have been made to various experimental data.

Effect of Temperature on Counting Measurements in a Uranium Enrichment Monitor Based on a NaI(Tl) Spectrometer and Transmission Source

We have studied the stability of a NaI(T1) scintillation detector and a commercial, general-purpose, digital, multichannel analyzer (MCA) used in uranium enrichment monitoring and have evaluated their combined sensitivity to temperature. The method used to calculate enrichment is very sensitive to any error that may be introduced by the instrument when measuring the intensity of the transmission peak corrected by the dead time (DT). An error as small as 0.1% in the attenuation of the transmission-peak count rate results in about a 3% error in the enrichment calculation. The demand for such an unusually high level of accuracy challenges the capabilities of an off-the-shelf instrument. The experimental data show that a variation of +/-10degC in the room temperature can cause a 0.4% change in the count rate of the transmission peak, even though the gain stabilization system holds the transmission peak at a specified channel. We have observed that the temperature changes the MCAs pileup rejector efficiency and, thus, the DT correction. This is because the shapers in the fast and slow channels have different temperature characteristics. We have utilized a custom-designed detector preamplifier that stabilizes the preamplifier pulse shape over the working temperature range.

Lead Void Reactivity Worth in Two Critical Assembly Cores with Differing Uranium Enrichments

Abstract To validate lead (Pb) nuclear cross sections, a series of integral experiments to measure lead void reactivity worths was conducted in a high-enriched uranium (HEU)/Pb system and a low-enriched uranium (LEU)/Pb system using the Comet Critical Assembly at the National Criticality Experiments Research Center. There is a follow-on experiment to measure the lead void reactivity worths in a plutonium/Pb system that is currently under investigation. The critical experiments in the two uranium systems were designed to provide complementary data sets having different sensitivities to scattering cross sections of lead. The larger amount of the 238U present in the LEU/Pb core increases the neutron importance above 1 MeV compared with the HEU/Pb core. Since removal of lead from the core shifts the neutron spectrum to the higher energy region, positive lead void reactivity worths were observed in the LEU/Pb core while negative values were observed in the HEU/Pb core. This technical note is a preliminarily report of the experimental analysis results for the lead void reactivity worths with the Monte Carlo calculation code MCNP® version 6.1 together with nuclear data libraries JENDL-4.0 and ENDF/B-VII.1. The calculation values were found to overestimate the negative reactivity worths for the HEU/Pb core while being consistent for the LEU/Pb core.

New technology for transmission measurements in process pipes

Transmission measurements of radiation through process pipes provide a non-intrusive method of determining the amount of product present in the pipes. The product could be a liquid, a slurry, or a gas, which is the most challenging because of the low density. Traditionally, these techniques have used a radioactive source that has to be replaced periodically. We have developed a transmission technique based on an X-ray tube instead of a decaying source. A notch filter is used to provide a narrow transmission line, and a thin silicon transmission detector is used to monitor the X-ray tube output. The transmitted X-rays are measured with a high-throughput gamma spectrometer that consists of a NaI(Tl) detector and an MCA with precise dead time correction. This spectrometer provides stable transmission measurements with an accuracy of a fraction of a percent. The shielding and collimator are made of machinable tungsten for thermal mechanical stability, as well low-cost, low-weight tungsten powder in polymer castings. We describe two methods of measuring the pipe wall thickness without evacuating the pipe. Our particular application was for enrichment monitors for UF(6) in process pipes. Enrichment monitors that are independent of the plant data require two measurements: a transmission measurement to determine the total amount of uranium in the pipe and a measurement of the 186-keV gamma-ray line to determine the amount of (235)U present. The ratio of these values gives the enrichment. Previous designs used a decaying radioactive source such as (57)Co (122 keV, T(½)=272 days) or (109)Cd (22 keV, T(½)=1.2 years). A major effort was required to access and periodically replace these sources in operating plants. In this report, we describe the use of an X-ray tube, which eliminated the source problem, and other innovations. Then we present data from an enrichment monitor that incorporates these innovations.

Calculations in Support of JAEA Experiments. Update Oct 2015

An update on calculations provided in support of the Japanese Atomic Energy Agency (JAEA) experiments is summarized in PowerPoint form.

Reaction rate, fission product yield, and Rossi-α measurements using a HEU metal, copper reflected critical assembly

A critical experiment was performed on the Comet assembly to provide nuclear data in a non-thermal neutron spectrum and to re-establish experimental measurement capabilities relevant to the United States Department of Energys general purpose nuclear criticality experiments capability and to the Technical Nuclear Forensic program. Activation and fission foils were placed at specific locations in the Zeus all-oralloy core, copper reflected critical experiment to infer spectral indices data and obtain reaction rates data. After the irradiation, passive gamma-ray measurements were performed on all the foils and several of them were packaged and shipped to Los Alamos National Laboratory for further radiochemical analysis. The results from the non-destructive and radiochemical analyses are presented in this paper. Finally, Rossi-α measurements were performed on a slightly modified configuration from the configuration used for the activation measurements. The Rossi-α results are presented and compared to past measurements performed using other critical assemblies.

Use of a COTS X-ray scanner for 2-D neutron activation analysis

Results from the use of a commercial, off-the-shelf X-ray scanner using storage phosphors to measure neutron activation in 1- and 2-D are presented. The technique consists of irradiating thin foils or wires of various elements, then placing the activated material on the storage phosphors to expose them. The amount of exposure is proportional to the activation obtained. Examples of wires, small foils, and large area foils with asymmetric irradiation using critical assemblies are presented. Combined with isotope-specific gamma counting of the entire foil or wire, the technique offers a simple way to obtain both qualitative and quantitative 2-D activation information.