Matthew G. Watrous's research while affiliated with Idaho National Laboratory and other places

This work summarizes the radiation transport–based design for a new D2O-moderated ex-core irradiation facility in the Washington State University (WSU) TRIGA reactor for optimization of ¹³⁵Xe sources used for calibration and quality control testing of Xe gas detection equipment in support of the Comprehensive Test Ban Treaty (CTBT). Three-dimensional (3-D) particle transport analysis characterizing the WSU reactor core using MCNP6.2 (3-D Monte Carlo) and PENTRAN (3-D deterministic parallel SN) form the basis for the computational optimization. Excellent agreement between MCNP6.2 and PENTRAN predictions is observed. A fundamental fuel bundle depletion analysis is applied to enable a more accurate prediction of neutron flux and neutron spectrum distribution, which drives production rates of ¹³⁵Xe and ¹³³Xe. The results of various model simulations were used to inform recommendations for the final irradiation chamber design, which has been optimized for safe placement in the reactor tank prior to startup and will allow for insertion and rotation of xenon “bean” samples using existing WSU irradiation equipment, while remaining within operational parameters. The irradiation chamber is expected to produce samples that will remain viable for use in CTBT standards applications for durations 70% to 80% longer than samples produced using current procedures. Thus, this design is expected to improve CTBT-related calibrations and performance testing and to support the continued stability of the CTBT monitoring network.

Atmospheric radioxenon analyses conducted under the Comprehensive Test Ban Treaty Organization (CTBTO) necessitates the collection of air samples to quantify 135Xe, an indicator for nuclear explosions and reactor operations. Analytical instrument calibration for these analyses requires high quality, monoisotopic reference standards that are typically short lived, and therefore must be produced and shipped promptly to monitoring stations worldwide. The work described here outlines two methods to produce highly enriched 135Xe by irradiating stable monoisotopic targets by (1) neutron capture (n, ɣ) on monoisotopic 134Xe and (2) high energy photon irradiation (ɣ, n) of monoisotopic 136Xe. The targets were prepared by electromagnetic isotope separation of natural xenon. Results from each process were compared for 135Xe yield and purity.

Here we present a new method of irradiating ¹³⁴Xe capsules to produce ¹³⁵Xe gas standards which maximize the ratio of ¹³⁵Xe to ¹³³Xe production due to (n,g) and (n,2n) reactions, respectively. We performed “Spectral tuning” of the University of Utah TRIGA Reactor (UUTR) neutron spectrum to increase the length of time that ¹³⁵Xe dominates undesirable ¹³⁵Xe in the sample, so that the capsules – used for calibration and quality control testing of Xe gas detection equipment in support of the Comprehensive Test Ban Treaty (CTBT) – will remain viable for longer periods post-irradiation. Moreover, optimized methods of computation and analysis were developed yielding improved computational efficiency over standard Monte Carlo approaches. These methods provided valuable insight into the final design and manufacture of new, ex-core Teflon irradiation chambers tested in the UUTR. The methods of computation and analysis, as well as the physical irradiation chamber designs, were derived such that they could be readily applied to any reactor for spectral tuning of a specific reactor's flux profile. Results of the physical experiments employing the optimized irradiation chamber designs demonstrated sample viability time improvements of over 60% when compared to conventional, un-optimized methods of gas sample generation. Thus, use of these methods enhance both the CTBT-related calibrations and performance testing, and the continued stability of the CTBT monitoring network overall.

Sample preparation for Accelerator Mass Spectrometry (AMS) analysis of ¹²⁹I from environmental samples typically involves isolation and purification of the iodine followed by precipitation as silver iodide. The silver iodide is mixed with silver or niobium powder as a binder and medium for electrical conductivity and approximately 3 mg of the mixture pressed into a cathode for AMS analysis. Electrodeposited silver iodide on a silver clad niobium 50 µm diameter wire provides an attractive alternative to precipitation. Six inches of wire and electrodeposited silver iodide are easier to handle when pressing cathodes and minimizes possible cross contamination between samples. Although electrodeposition onto pure silver wires has been recently reported, the usage of silver clad niobium wires could offer an additional advantage. The use of niobium in the silver iodide matrix has been reported to result in higher intensity and long term stability of the iodide ion current. This paper presents a comparison of electrodeposition methods using silver and silver clad niobium wires to the traditional precipitated silver iodide mixed with silver and niobium powders. The comparison was done using materials with 129/127I ratios having nominal values of 5 × 10⁻¹⁰ and 5 × 10⁻¹¹. For each 129/127I ratio material, eight samples were analyzed for each preparation method on three days, with seven replicate AMS measurements per sample. For each ratio, the measurements of the separate days were combined for each method, yielding 168 measurements per method. An analysis of variance indicated minor statistical differences between the precipitated and electro-deposited materials, with the pooled standard error of the means being 0.2% for both ∼5 × 10⁻¹⁰ and ∼5 × 10⁻¹¹ ratios.

Global and regional releases of 14C have resulted from nuclear weapons testing activities; assessment of the chemical behavior and mechanisms of environmental transport and deposition of this radionuclide can assist remediation strategy development efforts and provide insights into global carbon cycling processes. This work reports a systematic evaluation of 14C in surface soils taken from the Nevada National Security Site. Surface soil samples are derived from above- and underground test locations, with underground test sites representing a range from near complete containment to uncontrolled radioactive releases. Only one surface soil taken from a underground test location (i.e. the Baneberry shot) shows elevated 14C concentrations (319 ± 9 pMC) in addition to elevated concentrations of 137Cs, 60Co and 152Eu above regional backgrounds. Surface soils from above-ground test locations show extremely high 14C content (~1000 to 10,000 pMC); elevated concentrations of 152Eu and 60Co for these soils are also observed, with 137Cs at or below background levels. Taken together, these data suggest that 14C in surface soils from above-ground tests is primarily derived from in-situ neutron activation of the native soil material, whereas 14C in surface soils from underground tests may be from either recondensed particulate material or soil activation.

Iodine environmental measurement programs are in need of new materials with certified ¹²⁹I activity. Frequently ¹²⁹I measurements are validated in the literature using the standard material IAEA-375, Chernobyl soil, which is the only soil/sediment material with a recommended ¹²⁹I activity. IAEA-375 has not been available for purchase since 2010. This study is an extension of previous work at INL to include four additional standard materials that are commercially available (NIST materials: RM 8704, Buffalo River sediment, SRM 2710a, Montana I soil, and IAEA materials: SL-1, lake sediment, IAEA-385, Irish Sea sediment). These materials have certified or recommended activities for a variety of radionuclides but not for ¹²⁹I. This paper reports a comparison of accelerator mass spectrometry (AMS) and thermal ionization mass spectrometry (TIMS) data for ¹²⁹I activity, as well as the 129/127I ratios for these standards to assist in identifying a suitable alternative for IAEA-375. Two independent chemical separation and mass spectrometric analysis techniques have been applied in an effort to corroborate the data. Both methods were validated via analyses of IAEA-375 for ¹²⁹I and show good agreement with the recommended activity of 1.7 × 10⁻³ Bq kg⁻¹ for ¹²⁹I; (1.6 × 10⁻³ Bq kg⁻¹ by AMS and 1.8 × 10⁻³ Bq kg⁻¹ by TIMS) with both sets of results within the 95% confidence interval of the recommended value.

Environmental ²³⁷Np analyses are challenged by low ²³⁷Np concentrations and lack of an available yield tracer; we report a rapid, inexpensive ²³⁷Np analytical approach employing the short lived ²³⁹Np (t1/2 = 2.3 days) as a chemical yield tracer followed by ²³⁷Np quantification using inductively coupled plasma-mass spectrometry. ²³⁹Np tracer is obtained via separation from a ²⁴³Am stock solution and standardized using gamma spectrometry immediately prior to sample processing. Rapid digestions using a commercial, 900 W “Walmart” microwave and Parr microwave vessels result in 99.8 ± 0.1% digestion yields, while chromatographic separations enable Np/U separation factors on the order of 10⁶ and total Np yields of 95 ± 4% (2σ). Application of this method to legacy soil samples surrounding a radioactive disposal facility (the Subsurface Disposal Area at Idaho National Laboratory) reveal the presence of low level ²³⁷Np contamination within 600 m of this site, with maximum ²³⁷Np concentrations on the order of 10³ times greater than nuclear weapons testing fallout levels.

Iodine environmental measurements have consistently been validated in the literature using the standard material IAEA-375, soil collected approximately 160 miles northeast of Chernobyl, which is currently the only soil/sediment material with a certified ¹²⁹I activity. IAEA-375 has not been available for purchase since approximately 2010. Two other standard materials that are available (NIST SRM 4354, freshwater lake sediment and NIST SRM 4357, ocean sediment) have certified activities for a variety of radionuclides but not for ¹²⁹I. This paper reports a comparison of TIMS and AMS data for all three standards.

Investigations of enhanced sample utilization in thermal ionization mass spectrometry (TIMS) using porous ion emitter (PIE) techniques for the analyses of trace quantities of americium and plutonium were performed. Repeat ionization efficiency (i.e., the ratio of ions detected to atoms loaded on thefilament) measurements were conducted on sample sizes ranging from 10–100 pg for americium and1–100 pg for plutonium using PIE and traditional (i.e., a single, zone-refined rhenium, flat filament ribbon with a carbon ionization enhancer) TIMS filament sources. When compared to traditional filaments, PIEs exhibited an average boost in ionization efficiency of ∼550% for plutonium and ∼1100% for americium. A maximum average efficiency of 1.09% was observed at a 1 pg plutonium sample loading using PIEs. Supplementary trials were conducted using newly developed platinum PIEs to analyze 10 pg mass loadings of plutonium. Platinum PIEs exhibited an additional ∼134% boost in ion yield over standard PIEs and ∼736% over traditional filaments at the same sample loading level.

INL has shown that a Marinelli beaker geometry can be used for the measurement of radioactive xenon in air using an aluminum Marinelli. A carbon fiber Marinelli was designed and constructed to improve overall performance. This composite Marinelli can withstand sample pressures of 276 bar and achieve approximately a 4x performance improvement in the minimum detectable concentrations (MDCs) and concentration uncertainties. The MDCs obtained during a 24 hour assay for ¹³³Xe, 131mXe, and ¹³⁵Xe are: 1.4, 13, and 0.35 Bq/m³.