Self-irradiation Damage Research Articles

Observable consequences of self-irradiation damage in a MIMAS-type MOX nuclear fuel as analyzed by x-ray diffraction, electron microprobe analysis, and Raman imaging. A possible methodological approach

One of the challenges of multi-recycled Pu, to be used to produce MOx fuel, lies in its isotopic composition. Further recycling enriches the isotopy toward 238Pu, 240Pu, and 241Pu, which have much higher specific activities than the 239Pu isotope, meaning that those fuels are subjected to strong self-irradiation, provoking defect accumulation in the (U,Pu)O2 crystal lattice. A combination of three different techniques, XRD, EPMA, and RS (x-ray diffraction, electron probe micro-analysis, and Raman spectroscopy, respectively) was implemented to characterize a particular self-irradiated, 238Pu, 240Pu, and 241Pu-enriched MIMAS (MIcronized-MASter blend)-type MOx fuel sample, which had been stored for 15 years at room temperature under an inert atmosphere, to maximize irradiation effects. For comparison purposes, a specimen from the same batch was submitted to a thermal treatment and was completely analyzed in the two months following this treatment. Two of these methods (EPMA and RS) were used in their imaging mode. In particular, four spectral characteristics could be extracted from the Raman spectra. However, because of the inherent heterogeneity of this particular MOx material, the results had to be analyzed in part in a rather statistical way. This combination of techniques first allowed for determining the local Pu content. Then, the effects of self-irradiation were analyzed in terms of lattice parameter swelling, defect injection, and resonant scattering. The merits and uncertainties associated with these methods are discussed in terms of macro- and/or micro-strains. Finally, the Raman spectroscopy of (U,Pu)O2, in the 0%–40% range, was revisited in part, in an indirect way, however.

Using external ion irradiations for simulating self-irradiation damage in nuclear waste glasses: State of the art, recommendations and, prospects

Due to the complexities associated with handling and characterizing the radiation damage in radioactive materials such as nuclear waste forms, ion irradiation has been widely used to simulate the effects of the self-irradiation damage. This often involves ion irradiation of nuclear waste surrogate glasses using ion accelerators. Such facilities allow using a wide range of ion types ranging from H to U and energies from tens of keV to hundreds of MeV. For various reasons, the researchers often have to use ions and energies that do not necessarily match with what is expected in actual nuclear waste forms. Due to a lack of detailed and dedicated studies, there are currently no guidelines or consensus regarding the use of the ion irradiation as a means of simulating the self-irradiation damage. The purpose of this article is to delve into this subject by presenting systematic studies of a number of glasses carried out in the last decade or so. By doing so, guidelines and easy to follow ion-mass Vs ion energy contour maps are provided to best simulate the long-term radiation damage in nuclear waste forms. We also discuss how the ions of different masses and energies can be exploited for defect healing and provide a roadmap to undertake systematic studies on this subject. For the sake of brevity, a quick summary of the important results and the guidelines is presented towards the end of this article and not here. Besides the nuclear waste community, the study is also of interest to a wide research community dealing with defect formation and defect healing in glasses for use in radiation environments experienced in nuclear reactors, decommissioning and space applications to name a few.

The In-Situ Quantification of Structural Radiation Damage in Zircon Using Laser-Induced Confocal Photoluminescence Spectroscopy

We present a new methodology for laser-induced steady-state photoluminescence (PL) spectroscopy of Dy3+ that aims at a direct quantification of the amorphous fraction fa present in zircon (ZrSiO4), which undergoes a transition from a crystalline to a metamict state due to cumulative self-irradiation damage caused by the radioactive decay of substituted U and Th. Using state-of-the-art confocal spectrometers attached to optical microscopes, measurements may be performed non-destructively on the micrometre length-scale with the option to visualize radiation-damage patterns as revealed by hyperspectral PL maps. Zircon from the Ratnapura district (Sri Lanka, ~520 Ma), was used as reference material to substantiate the applicability of the proposed method. The accumulation of radiation damage in this material was investigated in detail and obtained fa values correlate with calculated α-doses in accordance to the direct impact model reported variously in the literature. The impact of chemically-induced, heterogeneous broadening of Raman and Dy3+ emission spectral bands is discussed on two examples from Mt. Malosa district, Malawi. A mean weighted U-Pb isotope age of 111 ± 1 Ma (pegmatitic-type) and a discordia age of 112 ± 1.6 Ma (hydrothermal-type) as obtained by LA-ICP-MS confirm their close genetic and temporal relationship. Studied zircon examples demonstrate that the amount of radiation damage present may have a substantial effect on the precision of LA-ICP-MS ages, but cannot be considered an exclusive cause for bias of obtained isotope ages.

4He irradiation of zircon, ZrSiO4, using a micro-patterned, Si-based energy filter

The quantitative evaluation of alpha-particle damage in the mineral zircon, ZrSiO4, using 4He irradiation experiments is difficult because the vast majority of atomic knock-ons in the target are concentrated in a narrow depth range near the ends of the He-ion trajectories. Here we present a new concept to overcome this problem, namely, tailoring the depth profile of damage by means of a micromechanically fabricated “energy filter”. Lamellae of 1.5 μm thickness, prepared from ZrSiO4 using the focused-ion-beam technique, were subjected to irradiation with 8.8 MeV 4He ions. Five irradiations with ion fluences in the range 2.5 × 1015–1 × 1017 cm−2 have resulted in mild to severe damage, as monitored by the broadening and downshift of SiO4-stretching Raman bands. Our results may provide a means for quantifying the contribution of alpha particles to the total self-irradiation damage in zircon.

Radiations effects in ISG glass: from structural changes to long-term aqueous behavior

The aim of the studies about the stability of nuclear glasses is to predict as accurately as possible their behavior over geological timescale. It requires the development of experimental methods to accelerate their irradiation and leaching ageing. This study focuses on the International Simple Glass behavior under irradiation by evaluating its structural and macroscopic property evolutions, and also its long-term chemical durability. Two irradiation ageing scenarios are considered: an alpha self-irradiation of a 244Cm-doped-ISG glass and an external irradiation with few MeV gold ions of non-radioactive ISG glass coupons. The results obtained from the Raman spectroscopy, density, and hardness measurements led to the conclusion that ballistic effects from the recoil nuclei are responsible of the changes with dose observed under alpha self-irradiation and that the ISG glass is a good surrogate of the French nuclear SON68 complex glass under such irradiation conditions. The assessment of the structural evolution with dose of this glass reinforces the mechanistic concepts about the glass response under alpha self-irradiation damage, where the glass response to a recoil nucleus event is the result of a very fast quenching of the damaged zone in the displacement cascade. Furthermore, the irradiation damage induced by gold ions does not fully mimic the 244Cm-doped glass evolution with nuclear dose, since the results of this simulation method overestimates the amplitude of the irradiation damage in the glass because the effects of the alpha particle are not included. Finally, this study demonstrates that the ISG glass water alteration at high-reaction progress is impacted by the glass damage, which indicates that the long-term glass chemical durability will depend on the glass structure when water arrives in contact.

Neoproterozoic amorphous “ekanite” (Ca2Th0.9U0.1Si8O20) from Okkampitiya, Sri Lanka: A metamict gemstone with excellent lead-retention performance

A rare gemstone form of ekanite from Okkampitiya, eastern Sri Lanka, has a chemical composition of Ca2Th0.9U0.1Si8O20, corresponding to that of the tetragonal mineral ekanite. The Okkampitiya material, however, has undergone amorphization and is found in a fully metamict state, which is ascribed to the long-term accumulation of high levels of self-irradiation damage (1.38 x 10(20) alpha decay events per gram). The ekanite nevertheless does not show any post-growth chemical alteration, and the radiogenic Pb has been retained. The Okkampitiya ekanite yielded a Neoproterozoic Pb-207/Pb-206 age of 562.1 +/- 0.8 Ma (uncertainty quoted at the 95% confidence level) that is concordant within the uncertainties of decay constants. The remarkable Pb-retention performance of ekanite contradicts the common hypothesis that metamictization in minerals results in U-Pb discordance. The exceptional chemical durability of the heavily radiation-damaged geological material described here has also implications for materials-science research. High radiation resistance, a key criterion in the search for advanced nuclear waste forms, may not be a prerequisite for high aqueous durability of a solid.

Investigation on the use of Americium Oxide for Space Power Sources: Radiation Damage Studies

Within Europe 241 Am is a feasible alternative to 238 Pu that can provide a heat source for radioisotope thermoelectric generators (RTGs) and radioisotope heating units (RHUs) to be used in Electrical power sources used in outer planet missions. In the EPSO-SPACE* project developed as an exploratory research at JRC, a new type of nuclear power source, to provide heat and electricity for deep space missions is considered. It will consist of a 241 Am based compound with aerogel thermal insulation and thermocouples in close contact with the heat source. The assembly is inside an iridium encapsulation to withstand all relevant accident scenarios. The design will be modular and combinable for required electrical output.Several candidate Americium compounds will be investigated for chemical stability at high temperature and for self-irradiation damage. New thermo-electric converter materials containing actinides will be assessed, and a robust encapsulation designed. Safety analyses will be performed including launch explosion and re-entry accidents. The research will conclude in a conceptual design of a prototype power source. In the first part of this study, americium dioxide will be considered from the point of view of its chemical durability and of its behavior against radiation damage and helium formation, two aspects to be carefully investigated due to the high alpha-activity of the americium. Transmission electron Microscopy (TEM) and helium thermal desorption spectrometry (TDS) experiments will be described and results on aged (more than 30 years) AmO2 reported. Some comparison with 238 PuO2 based RTG’s will be discussed.

Electron-beam-induced annealing of natural zircon: a Raman spectroscopic study

The annealing of radiation damage in zircon by low-energy electron irradiation was explored systematically. Natural zircon samples spanning a wide range of self-irradiation damage were irradiated with the focused electron beam of an electron probe microanalyser. The effects of beam current and irradiation time were tested systematically, and the changes in zircon were measured using Raman spectroscopy. Our results confirm the damage-annealing effect of an accelerated electron beam. We demonstrate that non-thermal annealing occurs through electron-enhanced defect reactions and that the extent of the annealing is a function of both the irradiation time and the beam current. The complete annealing of radiation damage in zircon by an accelerated electron beam was not possible under the conditions of our experiments. Our results indicate that Raman band broadening in ion-irradiated zircon can possibly be explained through phonon confinement, as the estimated domain sizes of the crystalline volume amid recoil clusters decrease with increasing α dose. The results underlay the importance of doing Raman spectroscopy before electron-beam and ion-beam analysis. To avoid unwanted beam-induced annealing of damage in zircon during EPMA analysis, the electron energy transferred per volume unit of sample should be minimised, for instance by keeping the integrated charge low and/or by defocusing the electron beam.

Spectroscopic study of inclusions in gem corundum from Mercaderes, Cauca, Colombia

Mineral inclusions in gem corundum from Mercaderes, Cauca, Colombia, were investigated non-destructively using Raman and photoluminescence spectroscopy, with special focus lying on phases containing radioactive elements. Besides abundant rutile, the minerals zircon, apatite, feldspar, and an epidote-group species, most probably allanite–(Ce), were found. The latter is detected easily from its characteristic Nd3+ emission pattern, which may prove useful in future provenance studies. Zircon inclusions range from well crystalline to moderately radiation damaged [FWHM (full width at half band maximum) of the ν3(SiO4) Raman band 1.8–10.9 cm−1]. Both the zircon inclusions and their neighbouring host corundum are affected by compressive stress, which is assigned to (1) heterogeneous volume expansion of the host-inclusion couple upon pressure release during the uplift following primary growth and (2) volume expansion of the zircon inclusion due to the accumulation of self-irradiation damage. Internal stress of zircon inclusions averages ~1.1 GPa. Heat treatment of corundum leads to structural reconstitution of zircon (narrowed zircon Raman bands with FWHMs in the range 1.8–2.7 cm−1) and accompanying stress release in the adjacent corundum (indicated by downshifts of the R1 emission of Cr3+). The observation of broadened Raman band of zircon inclusions therefore allows one to exclude high-temperature enhancement of the host corundum.

Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized δ-Pu

The effects on the local structure due to self-irradiation damage of Ga stabilized δ-Pu stored at cryogenic temperatures have been examined using extended x-ray absorption fine structure (EXAFS) experiments. Extensive damage, seen as a loss of local order, was evident after 72 days of storage below 15 K. The effect was observed from both the Pu and the Ga sites, although less pronounced around Ga. Isochronal annealing was performed on this sample to study the annealing processes that occur between cryogenic and room temperature storage conditions, where damage is mostly reversed. Damage fractions at various points along the annealing curve have been determined using an amplitude-ratio method, a standard EXAFS fitting, and a spherical crystallite model, and provide information complementary to the previous electrical resistivity- and susceptibility-based isochronal annealing studies. The use of a spherical crystallite model accounts for the changes in EXAFS spectra using just two parameters, namely, the crystalline fraction and the particle radius. Together, these results are discussed in terms of changes to the local structure around Ga and Pu throughout the annealing process and highlight the unusual role of Ga in the behavior of the lowest temperature anneals.

Charged defects during alpha-irradiation of actinide oxides as revealed by Raman and luminescence spectroscopy

We have recently evidenced an original Raman signature of alpha irradiation-induced defects in UO2. In this study, we aim to determine whether the same signature also exists in different actinide oxides, namely ThO2 and PuO2. Sintered UO2 and ThO2 were initially irradiated with 21MeV He2+ ions using a cyclotron device and were subjected to an in situ luminescence experiment followed by Raman analysis. In addition, a PuO2 sample which had accumulated self-irradiation damage due to alpha particles was investigated only by Raman measurement. Results obtained for the initially white ThO2 showed that a blue color appeared in the irradiated areas as well as luminescence signals during irradiation. However, Raman spectroscopic analysis showed the absence of Raman signature in ThO2. In contrast, the irradiated UO2 and PuO2 confirmed the presence of the Raman signature but no luminescence peaks were observed. The proposed mechanism involves electronic defects in ThO2, while a coupling between electronic defects and phonons is required to explain the Raman spectra for UO2 and PuO2.

High temperature X-ray diffraction study of the oxidation products and kinetics of uranium-plutonium mixed oxides.

The oxidation products and kinetics of two sets of mixed uranium-plutonium dioxides containing 14%, 24%, 35%, 46%, 54%, and 62% plutonium treated in air were studied by means of in situ X-ray diffraction (XRD) from 300 to 1773 K every 100 K. The first set consisted of samples annealed 2 weeks before performing the experiments. The second one consisted of powdered samples that sustained self-irradiation damage. Results were compared with chosen literature data and kinetic models established for UO2. The obtained diffraction patterns were used to determine the temperature of the hexagonal M3O8 (M for metal) phase formation, which was found to increase with Pu content. The maximum observed amount of the hexagonal phase in wt % was found to decrease with Pu addition. We conclude that plutonium stabilizes the cubic phases during oxidation, but the hexagonal phase was observed even for the compositions with 62 mol % Pu. The results indicate that self-irradiation defects have a slight impact on the kinetics of oxidation and the lattice parameter even after the phase transformation. It was concluded that the lattice constant of the high oxygen phase was unaffected by the changes in the overall O/M when it was in equilibrium with small quantities of M3O8. We propose that the observed changes in the high oxygen cubic phase lattice parameter are a result of either cation migration or an increase in the miscibility of oxygen in this phase. The solubility of Pu in the hexagonal phase was estimated to be below 14 mol % even at elevated temperatures.

Thermal expansion and self-irradiation damage in curium nitride lattice

A powder sample of curium nitride (CmN) containing 0.35%-Pu and 3.59%-Am as actinide impurities was prepared by carbothermic nitridation of the oxide. The lattice expansion induced by self-irradiation damage at room temperature was measured as a function of time. The saturated Δa/ao value was 0.43%, which is greater than those for transuranium dioxides available in literature. The undamaged lattice parameter at 297±1K was determined to be 0.50261±0.00006nm. Temperature dependence of the lattice parameter was measured by a high temperature X-ray diffractometer in the temperature range up to 1375K. The linear thermal expansion from 293 to 1273K was 0.964% and the corresponding thermal expansion coefficient was 9.84×10−6K−1. Comparing with the other actinide nitrides, it was found that CmN lies between the higher expansion nitrides (PuN and AmN) and the lower expansion nitrides (UN and NpN).

Self-irradiation damage to the local structure of plutonium and plutonium intermetallics

The effect of self-irradiation damage on the local structure of δ-Pu, PuAl2, PuGa3, and other Pu intermetallics has been determined for samples stored at room temperature using the extended x-ray absorption fine-structure (EXAFS) technique. These measurements indicate that the intermetallic samples damage at a similar rate as indicated in previous studies of PuCoGa5. In contrast, δ-Pu data indicate a much slower damage accumulation rate. To explore the effect of storage temperature and possible room temperature annealing effects, we also collected EXAFS data on a δ-Pu sample that was held at less than 32 K for a two month period. This sample damaged much more quickly. In addition, the measurable damage was annealed out at above only 135 K. Data from samples of δ-Pu with different Ga concentrations and results on all samples collected from different absorption edges are also reported. These results are discussed in terms of the vibrational properties of the materials and the role of Ga in δ-Pu as a network former.

Topological analysis of the structure of self-irradiated sodium borosilicate glass

High-level nuclear waste glasses must maintain their mechanical and chemical stability over very long time scales. Self-irradiation damage in these glasses induces bond-breakage and atomic displacements by two distinct mechanisms: radiolysis (principally from energetic beta-decay electrons) and ballistic mechanisms involving collision cascades initiated by energetic fission nuclei and recoil of alpha-emitting actinide nuclei. This study investigates collision-cascade-induced alteration of the glass network in a simplified sodium borosilicate model nuclear waste glass, using molecular dynamics (MD) simulation and efficient topological assessment algorithms. Network topologies of the initial and resulting altered glass structures were determined by enumerating the primitive-ring-based local cluster atom complements at each atom site. The topological description is seen to provide a revealing assessment of network structural changes in the simulated radiation environment.

Investigation of defects in actinide-doped UO 2 by positron annihilation spectroscopy

The nuclearization and validation of a new positron annihilation lifetime spectroscopy (PALS) system was ideally used to investigate vacancy defects generated by α self-irradiation in the UO 2 matrix of several plutonium-doped samples. The damage levels studied ranged from 0 to 0.3 dpa. This study validated the operational protocols for actinide-doped materials. A lattice lifetime of about 170–180 ps was determined for the undoped UO 2 matrix, which is consistent with the values reported in the literature. Alpha self-irradiation damage systematically increases the mean positron lifetime, resulting in a difference of 133 ps for a damage level of 0.3 dpa. Even at low damage values, a positron trapping site appears that corresponds to point defects involving an uranium vacancy, with a specific lifetime of about 310 ps. When annealed at 1373 K, some of these defects coalesce to form larger extended defects. The initial results for actinide-doped UO 2 also confirm the high sensitivity of PALS to the presence of vacancy defects even at low integrated α dose.

Annealing behavior of (Pu,Cm)O 2 lattice and bulk expansion from self-irradiation damage

In order to investigate the effect of self-irradiation damage and accumulation of He on oxide fuel pellets containing minor actinides, the expansion and annealing behavior of (Pu 0.95Cm 0.05)O 2 lattice and bulk were examined comparatively. Since the lattice and bulk expansion at room temperature showed a similar dependence on the storage duration, the main factor of bulk expansion was found to be the lattice expansion due to the generation of point defects. The lattice parameter recovered to the undamaged value by annealing at 1429 K for 2 h, whereas the bulk expanded again by annealing at 1433 K and did not recover to the undamaged value. In the micrographs of the fracture surface of the annealed pellet, the formation of gas bubbles along grain boundaries was confirmed. The He gas bubble formation resulted in the pellet swelling, and it may affect the pellet thermal conduction.

Alteration of crystalline zircon solid solutions: a case study on zircon from an alkaline pegmatite from Zomba–Malosa, Malawi

A natural, altered zircon crystal from an alkaline pegmatite from the Zomba–Malosa Complex of the Chilwa Alkaline Province in Malawi has been studied by a wide range of analytical techniques to understand the alteration process. The investigated zircon shows two texturally and chemically different domains. Whereas the central parts of the grain (zircon I) appear homogeneous in backscattered electron images and are characterised by high concentrations of trace elements, particularly Th, U, and Y, the outer regions (zircon II) contain significantly less trace elements, numerous pores, and inclusions of thorite, ytttrialite, and fergusonite. Zircon II contains very low or undetectable concentrations of non-formula elements such as Ca, Al, and Fe, which are commonly observed in high concentrations in altered radiation-damaged zircon. U–Pb dating of both zircon domains by LA-ICPMS and SHRIMP yielded statistically indistinguishable U–Pb weighted average ages of 119.3 ± 2.1 (2σ) and 118 ± 1.2 (2σ) Ma, respectively, demonstrating that the zircon had not accumulated a significant amount of self-irradiation damage at the time of the alteration event. Electron microprobe dating of thorite inclusions in zircon II yielded a Th–U-total Pb model age of 122 ± 5 (2σ) Ma, supporting the age relationship between both zircon domains. The hydrothermal solution responsible for the alteration of the investigated zircon was alkaline and rich in CO3 2−, as suggested by the occurrence of REE carbonates and CO2-bearing fluid inclusions. The alteration of the crystalline, trace element-rich zircon is explained by an interface-coupled dissolution-reprecipitation mechanism. During such a process, the congruent dissolution of the trace element-rich parent zircon I was spatially and temporally coupled to the precipitation of the trace element-poor zircon II at an inward moving dissolution-precipitation front. The driving force for such a process was merely the difference between the solubility of the trace element-rich and -poor zircon in the hydrothermal solution. The replacement process and the occurrence of mineral inclusions and porosity in the product zircon II is explained by the thermodynamics of solid solution-aqueous solution systems.

Evolution of Static Physical Properties in Plutonium by Self-irradiation Damage

AbstractThe alpha-decay of plutonium leads to the age-related change in physical properties. This paper presents updated results of age-related effects on enriched and reference alloys measured from immersion density, dilatometry, and mechanical tests. After nearly 100 equivalent years of aging, both the immersion density and dilatometry show that the enriched alloys are decreasing in density by less than 0.002% per year and now exhibit a near linear density decrease, without void swelling. The tensile tests show that the aging process increases the strength of plutonium alloys, followed by possible saturation past 70 equivalent years of age. The ultimate goal of this work is to develop capabilities to predict physical properties changed by aging effects.

Raman study of radiation-damaged zircon under hydrostatic compression

Pressure-induced changes of Raman band parameters of four natural, gem-quality zircon samples with different degrees of self-irradiation damage, and synthetic ZrSiO4 without radiation damage, have been studied under hydrostatic compression in a diamond anvil cell up to ~10 GPa. Radiation-damaged zircon shows similar up-shifts of internal SiO4 stretching modes at elevated pressures as non-damaged ZrSiO4. Only minor changes of band-widths were observed in all cases. This makes it possible to estimate the degree of radiation damage from the width of the ν3(SiO4) band of zircon inclusions in situ, almost independent from potential “fossilized pressures” or compressive strain acting on the inclusions. An application is the non-destructive analysis of gemstones such as corundum or spinel: broadened Raman bands are a reliable indicator of self-irradiation damage in zircon inclusions, whose presence allows one to exclude artificial color enhancement by high-temperature treatment of the specimen.