Neutron Irradiated Uranium Research Articles

The application of synchrotron micro-computed tomography to characterize the three-dimensional microstructure in irradiated nuclear fuel

This research demonstrated the first time that synchrotron X-ray micro-computed tomography (μ-CT) was utilized to analyze the three-dimensional (3-D) morphology of the microstructure of irradiated nuclear fuel. Both absorption contrast and propagation-based phase contrast enhanced μ-CT techniques were evaluated. Porosity and phase region volumes were determined in neutron irradiated uranium 10 wt% zirconium (U–10Zr) using propagation-based phase contrast enhanced μ-CT. Volumes were calculated using image processing software and algorithms, with the porosity volume subsequently compared to the ASTM E562 method. The 3-D porosity of a small region (∼8 × 105 μm3) located in the intermediate fuel redistribution zone that was removed via a focused ion beam was determined to be 7.2% via image processing. A detailed 3-D analysis of the porosity with respect to volume and morphology was provided. Five growth stages of pore evolution were observed including: nucleation, growth, coalescence, interconnected porosity, and extended/interconnected porosity. Four distinct phase regions were identified, including the pores, via both absorption contrast and propagation-based phase contrast enhanced μ-CT. A quantitative comparison of fuel swelling between this localized microscopic U–10Zr fuel region that was previously irradiated in the Fast Flux Test Facility, and historic U–10Zr fuels irradiated in the Experimental Breeder Reactor-II (EBR-II), was conducted. It was determined that for similar burnups, the localized swelling in this microscopic fuel region was 7.7%, which was approximately one-half of the value of the macroscopic swelling reported in EBR-II irradiated fuel. With the advent of new specimen preparation tools for irradiated materials, as well as advances in characterization techniques, researchers can now obtain previously inaccessible microstructural insight into irradiated fuels. This possibility naturally lends itself to improved fuel performance models for the nuclear community.

Performance testing of a Compton suppressed coincidence measurements using the Advanced Radionuclide Gamma-spectrOmeter

A Compton-suppressed multidimensional gamma-spectrometer is being developed within the Shallow Underground Laboratory at Pacific Northwest National Laboratory (PNNL, USA). This Advanced Radionuclide Gamma-spectrOmeter system consists of two Broad Energy Germanium detectors, surrounded by a 12-detector sodium iodide Compton suppression shield, situated inside a low-background lead and copper shield, fitted with a cosmic veto background reduction system. The detector has advanced functionality, including operation in single or combined detector mode, coincidence mode and Compton suppressed coincidence mode. Performance testing experiments measuring a neutron irradiated natural uranium sample, have demonstrated order of magnitude improvements in gamma detection sensitivity compared to conventional gamma-spectrometry techniques. Importantly this research demonstrates the feasibility of processing and analysing high-multiplicity datasets and the potential for high-detection sensitivity radionuclide measurements.

Lise Meitner and the discovery of fission

Pearson replies: I am grateful to Ruth Sime for raising the issue of the incorrect positioning of element 93 in the periodic table. I intended to do so in the original article but space limitations prevented it. (A longer version of the article can be found at http://dirac.lps.umontreal.ca/~pearson/belated.pdf.) But one might ask whether the outcome would have been any different even if the transuranics had been correctly positioned in the periodic table: Would Enrico Fermi have then taken Ida Noddack more seriously? Conceivably not, since he failed to address another problem—namely, that the observed multiplicity of half-lives was serving as a warning that something more complex than a simple radiative capture of neutrons was taking place. Actually, in his Nature paper,11. E. Fermi, Nature 133, 898 (1934). https://doi.org/10.1038/133898a0 Fermi was very cautious in claiming that he had formed transuranics: It was his successors who accepted that interpretation uncritically, even as the anomalies accumulated.Concerning Lise Meitner, the object of my article was not to attribute credit for the eventual discovery of fission but rather to understand why it took so long. In that respect I must remind the reader of Meitner’s 1936 rebuff of Fritz Strassmann when he reported finding barium in neutron-irradiated uranium: “Leave that to us physicists, and throw your results in the garbage can.” Meitner’s earlier opposition to the very suggestion of fission makes all the more remarkable the assurance she gave to Otto Hahn in late 1938, as Sime mentions. That assurance is discussed in more detail in Sime’s biography of Meitner (Lise Meitner: A Life in Physics, page 235), but one wonders whether Meitner was recalling Noddack’s proposal from long before, probably without even identifying the source of her memory.As for Sime’s last point, I did not intend to suggest that Hahn had explicitly promised to include Meitner’s name on the paper with Strassmann, had she come up with a physical explanation. I certainly believed, though, that such a promise was implicit in his request to the exiled Meitner seeking her advice on his puzzling results. However, a closer reading of Hahn’s letter of 19 December 1938 to Meitner (see, for example, pages 233–34 of Sime’s biography) shows that I was wrong: Hahn expresses the hope that Meitner will have something to publish on her own, so that “it would still in a way be work by the three of us!” Presumably Hahn wrote that for precisely the reasons Sime states in her letter; he acted in the only way that was open to him at the time. Any suggestion of deceit on his part at this stage would be inappropriate.REFERENCESSection:ChooseTop of pageREFERENCES <<1. E. Fermi, Nature 133, 898 (1934). https://doi.org/10.1038/133898a0, Google ScholarCrossref© 2015 American Institute of Physics.

Preliminary investigation into suitable techniques to measure the chemical yield of radionuclides in thermal neutron irradiated samples at AWE (Atomic Weapons Establishment)

Radiochemical separation methods are required for the accurate measurement of radionuclides in complex matrices. To be able to quantify the results after the separation of radionuclides, known amounts of either radioactive tracers or inactive carrier solutions are added at the start of the separation. The added isotopes/elements are then measured at the end of the analysis to calculate the chemical yield for the element or isotope. The chemical yield data is applied to the counting data of the analytes of interest to produce quantified results. Recently, changes have been made to the techniques used to measure chemical yields at the Atomic Weapons Establishment for samples containing fission products. Preliminary development work using wavelength dispersive X-ray fluorescence and inductively coupled plasma–atomic emission spectrometry is discussed, including a comparison of results and evaluation of measurement uncertainty. The use of the techniques to measure fission products in a thermal neutron irradiated uranium sample is described.

Irradiation-induced heterogeneous nucleation in uranium dioxide

Using classical molecular dynamics simulations, we have studied the first stages of defect cluster formation resulting from 10 keV displacement cascades in uranium dioxide. Nanometre size cavities and dislocation loops are shown to appear as a result of the irradiation process. A specifically designed TEM experiment involving He implanted thin foils have also been carried out to support this modelling work. These results, in conjunction with several other observations taken from the literature of ion implanted or neutron irradiated uranium dioxide, suggest a radiation damage controlled heterogeneous mechanism for insoluble fission product segregation in UO 2.

Primary result of 236U measurement with accelerator mass spectrometry at CIAE

The rare isotope 236U has a half-life of 2.342(3) × 107 years, and is produced principally by thermal neutron capture on 235U. The isotopic atom ratio of 236U/238U depends on the integral thermal neutron flux received by the material of interest. 236U is potentially useful as a “fingerprint" for indicating the presence of neutron-irradiated uranium usually originating from nuclear activity. By extracting negative molecular ion UO− from the uranium oxide target, simulating the 236U16O− beam transport with 238U16O− and 208Pb216O− pilot molecular ion beam, transporting the 236U-containing ion beam with a high resolution injection magnet analyzer and electrostatic analyzer system, and finally identifying and detecting 236U with a time-of-flight detector (TOF), a method for AMS (Accelerator Mass Spectrometry) measurement of 236U was established on the HI-13 Accelerator AMS system at China Institute of Atomic Energy.

Unique challenges with recent gamma spectroscopy measurements at Los Alamos National Laboratory

A variety of unique radioactive samples have been measured recently at Los Alamos National Laboratory (LANL) using an electrically-cooled high-purity germanium detector. In each case the purpose of the measurements included one or more of the following objectives: (1) an accurate determination of the isotopic weight fractions of different plutonium or uranium materials; (2) an accurate determination of the isotopic quantity in the absence of relevant calibration standards; and (3) a qualitative determination of various sample impurities for additional forensic information. This paper discusses how simple modifications to the PC-FRAM parameter sets enabled a better determination of the isotopic content of the following samples: (1) high-purity plutonium metal, (2) plutonium-beryllium (PuBe) neutron sources, (3) neutron-irradiated natural uranium, and (4) re-processed HEU fuel with elevated 236U content. The isotopic quantity in a variety of samples was determined using a combination of the Spectral Nondestructive Assay Platform (SNAP™) routine from Eberline Services and the Monte Carlo Neutral Particle (MCNP) code developed at LANL. The non-traditional sources that were quantified with these gamma ray modeling codes included dozens of neutron-irradiated targets of natural uranium, several plutonium-beryllium neutron sources, and three high-purity samples of plutonium metal.

Establishing reactor operations from uranium targets used for the production of plutonium

This paper presents results from the examination of a number of archived neutron-irradiated uranium targets used for past plutonium production testing. Three of these targets were destructively characterized using Los Alamos National Laboratory actinide analytical chemistry capabilities. A validated conduct-of-operations protocol was followed for this characterization effort. Chemical analyses included measurements for radionuclides, uranium assay, uranium isotopic abundances, trace actinides, trace metals, and non-metals. Material scientists also examined materials for morphological and microstructural properties and individual particles were examined for trace impurities. After characterization of the targets was completed, a reactor modeling effort was undertaken to corroborate target details in historical records. Time since irradiation calculations utilized both activation and fission products. The described examination of uranium targets has a tremendous impact from a safeguards verification and nuclear forensics perspective.

Chemical and nuclear studies of hassium and element 112

After the discovery of the neutron by Chadwick in 1932, the group led by Fermi in Rome began bombarding different elements with neutrons to study the radioactivity induced through (n,γ) reactions. The decay characteristics of the radioactivity in neutron-irradiated uranium was interpreted to indicate that some of the products were probably transuranium elements. Similar conclusions were reached by other research groups; for example in 1937 Meitner, Hahn and Strassman in Berlin proposed the following reaction/decay series (omitting mass numbers)

Speciation of239Np in neutron irradiated uranium compounds

The chemical state of239Np formed in the β-decay of239U produced by thermal neutron capture in239U, has been determined in simple uranium compounds as well as in macrocyclic complexes of this element. It is shown that the behavior of neptunium depends on such factors as the nature of the target, the counter-ion of the complex, the dissolution medium and the pH. The change of the oxidation state of239Np with time and the effect of the presence of macroscopic amounts of238UO22+ and of238U(IV) in the solution have also been investigated. The results are discussed in terms of the hot atom behavior of239U and of the genuine effects of β-decay.

A new method for separation of niobium from zirconium with silica gel.

The adsorption behaviors of niobium and zirconium on silica gel and some other adsorbents were investigated in various concentrations of nitric acid or hydrochloric acid. Niobium is strongly adsorbed on silica gel in a concentrated nitric acid solution, while zirconium is not. In a concentrated hydrochloric acid solution, niobium is weakly adsorbed. Based on the results, a new simple separation method of niobium from other radionuclides especially zirconium is developed. In this method, niobium can be recovered by 81%, and separation from zirconium is complete.The method was applied to separation of radioniobium from the neutron-irradiated zirconium wire and from the neutron-irradiated uranium with the results of complete elimination of other radionuclides present.

Neutron Irradiated Uranium Silicides Studied by Neutron Diffraction and Rietveld Analysis

The irradiation behavior of high-density uranium silicides has been a matter of interest to the nuclear industry for use in high power or low enrichment applications. Transmission electron microscopy studies have found that heavy ion bombardment renders U3Si and U3Si2 amorphous at temperatures below about 250 C [1], and that U3Si becomes mechanically unstable suffering rapid growth by plastic flow [2,3]. In this present work, crystallographic changes preceding amorphization by fission fragment damage have been studied by high-resolution neutron diffraction as a function of damage produced by uranium fission at room temperature. Initially, both silicides had tetragonal crystal structures. Crystallographic and amorphous phases were studied simultaneously by combining conventional Rietveld refinement of the crystallographic phases with Fourier filtering analysis of the non-crystalline scattering component [4].

Neutron Irradiated Uranium Silicides Studied by Neutron Diffraction and Rietveld Analysis

Uranium silicides have been considered for use as reactor fuels in both high power and low enrichment applications. However, U3 Si was found to become amorphous under irradiation [1] and to become mechanically unstable to rapid growth by plastic flow [2,3]. U3Si2 appears to be stable against amorphization at low displacement rates, but the extent of this stability is uncertain. Although the mechanisms responsible for plastic flow in U3Si and other amorphous systems are unknown, as is the importance of crystal structure for amorphization, it may not be surprising that these materials amorphize, in light of the fact that many radioactive nuclide - containing minerals are known to metaminctize (lose crystallinity) under irradiation [4]. The present experiment follows the detailed changes in the crystal structures of U3Si and U3Si2 introduced by neutron bombardment and subsequent uranium fission at room temperature. U-Si seems the ideal system for a neutron diffraction investigation- since the crystallographic and amorphous forms can be studied simultaneously by combining conventional Rietveld refinement of the crystallographic phases with Fourier-filtering analysis [5] of the non-crystalline scattering component.

Magnetic properties of uranium mononitride with fission fragment damage

Abstract Magnetic disorder in neutron-irradiated (fission fragment damage) uranium mononitride (UN) has been investigated by magnetic susceptibility measurements. The Neel point (TN ) shifted to lower temperatures with increasing fission dose and attained the lowest value at a dose of around 1018 f/cm3. After further irradiations, however, it recovered to the non-irradiated value. The shift of TN was in accordance with the change of the concentration of fission-induced vacancies. Other magnetic parameters showed a similar trend. These fission-fragment damage results are discussed in terms of the electronic state of 5 f electrons.

Micro-Hardness of Neutron Irradiated Uranium Dioxide at Room Temperature

Micro-Hardness of Neutron Irradiated Uranium Dioxide at Room Temperature

Decay Heat and Decay Rate of Actinides in Highly Neutron-Irradiated Uranium Initially of High 235U Content

The contributions from actinides to the decay heat and the decay rate relative to those from fission products in highly irradiated 235U-enriched uranium has been assessed. This assessment, which is based on measured and associated calculated actinide concentrations in a sample of uranium in which the 235U had been burned to 17% of its original >97% content (i.e., to ∼17% 235U), indicates that for most practical times (<108 s) after reactor shutdown, the actinide contribution to the decay heat and to the decay rate is a reasonably small fraction (<7%) of the total and comes mainly from 237U, 238Np, and 238Pu. These results differ from those for uranium only slightly enriched in 235U.

Micro-hardness of neutron irradiated uranium dioxide at room temperature.

Knoop hardness tests were performed on irradiated UO2 in the fission range of 1014–1018 fiss./cm3. Hardness values were obtained as a function of applied load (25–1,000 g). Three different regions of load dependence on hardness [work hardening or plastic (25–70 g), plateau (70–100 g) and crack formation (over 100 g) regions] were observed in a similar way to those of non-irradiated ones. The elastic effect was not observed on the hardness values of specimens irradiated to a dose more than 7.8 × 1014 fiss./cm3 in the load around 25~30 g in contrast to those of non-irradiated ones. Hardness change after irradiation was discussed on the basis of the variation of lattice strain, lattice volume and whole volume for the similar specimens at the same irradiation condition. The hardness change in work hardening or plastic region and that in plateau region increased with dose up to 4.8 × 1017 fiss./cm3 and decreased above that dose, and correlated to the lattice volume change and lattice strain.

Radiochemical separation of 99Mo, 99mTc and 131I from irradiated uranium

A rapid and simple method for the determination of molybdenum ( 99Mo), technetium ( 99 m Tc) and iodine ( 131I) from neutron irradiated uranium and its fission products has been developed. The procedure is based on the simultaneous extraction of these nuclides with 0.1 M solution of 4(5-nonylpyridine) in benzene from 2 M HNO 3. These nuclides after back extraction into 8 M HNO 3 were separated from each other by solvent extraction techniques.

Chemical separation of molybdenum from uranium and fission product nuclides

A very rapid method for the separation of molybdenum(VI) from neutron irradiated uranium and its fission products is described. The procedure is based on the selective extraction of molybdenum(VI) by a 0.1M solution of 2-hexylpyridine in benzene from 4M HCl+0.04M KSCN. Decontamination factors were estimated to be >104 for the radionuclides of niobium, zirconium, ruthenium, lanthanum, cerium, promethium, yttrium, strontium and barium.

Separation of heavier rare earths from neutron irradiated uranium targets

A radiochemical method is described for the separation of heavier rare earths from the fission of uranium. The method is particularly suitable for the separation of low yield (10−5%–10−7%), highly asymmetric rare earth fission products viz.179,177Lu,175Yb,173Tm,172,171Er,167Ho and161,160Tb in the neutron induced fission of natural and depleted uranium targets. Additional separation steps have been incorporated for decontamination from239Np (an activation product) and93-90Y (a high fission-yield product) which show similar chemical behaviour to rare earths. Separation of individual rare earths is achieved by a cation exchange method performed at 80°C by elution with α-hydroxyisobutyric acid (α-HIBA).