Chapter 8.7. Nuclear materials

The term nuclear materials refers to a very wide range of materials that are encountered in the nuclear fuel cycle (the specific case of nuclear weapons is not addressed here). Nuclear materials usually contain radioactive elements such as actinides, fission and activation products, and they are generally heterogeneous, chemically diverse and in some cases amorphous (glasses) or have a crystalline structure affected by radioactive decay. In such samples, the availability of synchrotron radiation allows the identification and quantification of the different oxidation states and the characterization of the molecular, chemical and physical forms of the various elements present in a matrix/material without the need to have previously separated them. As demonstrated by Antonio & Soderholm (2006), XAS techniques in particular have revolutionized actinide chemistry, considering the complexity inherent to the 5f elements, with their multiple oxidation states and varied coordination polyhedra. In the case of nuclear materials, the obtained results provide crucial fundamental data to increase the reliability of performance codes or to drive further theoretical developments (Calvin & Nowak, 2010). Another essential asset of XAS techniques is the penetrating nature of X-rays in the 3–50 keV energy range (from the M edges of actinides to the K edges of heavier fission products), which allows the sample to remain encapsulated during analysis/handling. Moreover, the increasing brilliance provided by third-generation synchrotrons, the availability of focused beams (<1 µm) and the improved sensitivity/resolution of detectors have proved to be extremely beneficial in studies dealing with radioactive materials. Another essential parameter is that a growing number of synchrotron facilities accept radioactive samples if their activities remain limited,¹ which is sufficient for most environmental samples or samples containing radionuclides with very long half-lives (uranium and thorium). After proper encapsulation and validation by local radiation-safety and health groups, such samples can be analyzed on any `non-active' beamline. For radioactive samples above the exemption limits (International Atomic Energy Agency, 2012), experiments have to be performed at dedicated endstations that are especially equipped to handle such materials (in particular equipped with a ventilation system, shielding etc.). Several synchrotron sources have such beamlines equipped with a `nuclearized' XAS endstation: the INE and ACT beamlines at ANKA in Germany (Rothe et al., 2012), the Rossendorf Beam Line (ROBL) at the European Synchrotron Radiation Facility (ESRF) in France (Matz et al., 1999), the micro-XAS beamline X05LA at the Swiss Light Source (SLS) in Switzerland,² the MARS beamline at SOLEIL in France (Llorens et al., 2014), the 11-2 beamline at Stanford Synchrotron Radiation Laboratory, California, USA³ and the BL-27B beamline at the Photon Factory in Japan (Konishi et al., 1996). The Actinide Facility for Synchrotron Research in Molecular and Environmental Sciences operated by the Chemistry Division at Argonne National Laboratory (ANL), Argonne, Illinois, USA provides researchers with access to hot-laboratory⁴ facilities for sample manipulation before and after experiments at the Advanced Photon Source at the ANL site. These factors, combined with the possibility of reducing the sample size and activities in hot laboratories using, for example, a focused ion beam, has contributed to the growth in the application of XAS and related scattering techniques to research on nuclear materials.

The nuclear fuel cycle refers to the industrial processes associated with the production of electricity from fission of ²³⁵U and ²³⁹Pu in nuclear power reactors. Mining and milling, conversion, enrichment and fuel fabrication (Barré, 2010) are the first steps that are necessary to prepare uranium for use in a nuclear reactor. The great majority of commercial reactors in current operation, referred to as Generation II or Generation III reactors, are of several types: Advanced Gas-cooled Reactors (AGRs), Canadian Deuterium Uranium (CANDU) reactors, pressurized water reactors (PWRs) and boiling water reactors (BWRs) (Abe & Asakura, 2012). These reactors are mostly loaded with uranium dioxide (UO₂), with some differences in the production processes to fit each nuclear fuel design (Abe & Asakura, 2012). In the so-called `once through' cycle, after three years in a reactor producing electricity the spent fuel is temporarily stored, to be ultimately disposed of as high-level nuclear waste in dedicated deep multi-barrier geological repositories. However, in some countries the spent fuel may undergo a further series of steps including temporary storage, reprocessing and recycling before the waste materials are disposed of, which corresponds to the so-called `closed fuel cycle'. During reprocessing of spent fuel, valuable elements such as uranium and plutonium are recovered (Glatz, 2012) and introduced into the fresh fuel-manufacturing process. The plutonium dioxide is mixed with uranium dioxide to obtain a mixed-oxide fuel (MOX) (U,Pu)O₂ with an average plutonium content equal to 7–8 mol% (Abe & Asakura, 2012). The different steps involved in the different versions of the nuclear fuel cycle are shown in Fig. 1.

Figure 1 The nuclear fuel cycle. Reprinted from Glatz (2012) with permission from Elsevier.

Studies including XAS and related scattering techniques have been used in every step of the nuclear fuel cycle, and in each case on a large range of elements/materials; therefore, it will not be possible to give an exhaustive presentation in the current context. As readers can find reviews dealing with the application of XAS to actinides and their mobility through the environment (Antonio & Soderholm, 2006; Denecke, 2006; Plaschke et al., 2011) and XAS related to spent nuclear fuel (SNF) and waste-storage topics, we will illustrate some of the recent key studies using XAS performed on almost exclusively `industrial' samples: cladding, fuels and nuclear glasses.

Zirconium alloys have been used for over 50 years as a cladding material in almost all water-cooled reactors (PWRs, BWRs and CANDU). These alloys (Zircaloys, ZIRLO, M5) contain small additions (less than 3 wt% in total) of tin, iron, chromium, nickel and/or niobium (Lemaignan, 2012), which do not negatively affect their low neutron absorption cross section. As extensively detailed by Allen et al. (2012), the water-side corrosion performance is a factor that can limit the in-reactor lifetime of fuel assemblies. Furthermore, in parallel to the corrosion process, a fraction of the hydrogen produced by the oxidation process (Zr + 2H₂O → ZrO₂ + 4H) and the radiolysis of water diffuses into the cladding and ultimately leads to hydride precipitation ZrH_2−x, which can cause cladding embrittlement.

Most of the studies performed on cladding materials are based on diffraction experiments. However, XAS is very valuable to determine the chemical state of the alloying additions, as the latter strongly influence corrosion and hydrogen pick-up properties. In particular, tin is added (≤2.5 wt%) to enhance strengthening properties but has a detrimental effect on corrosion. Tin exhibits a high solubility in zirconium and is heterogeneously distributed either throughout the zirconium metal matrix or within the corroded layer (Lemaignan, 2012). A recent study (Hulme et al., 2016) aimed to identify the oxidation state of tin in oxidized cladding samples using Sn L₃ XANES. The samples were coupons of fully recrystallized Zircaloy-4 and ZIRLO sheet (≤1.8 µm thickness ZrO₂ layer) corroded in a 360°C autoclave in simulated primary water chemistry (2 p.p.m. lithium as LiOH and 100 p.p.m. boron as HBO₃). Measurements were performed at varying incidence angles (12–45°) of the incoming X-ray photon beam to alter the penetration depth and hence probe only the corrosion layer. For the first time, clear evidence of the presence of Sn²⁺ and Sn⁴⁺ in the corroded layer was found and the Sn²⁺/Sn⁴⁺ ratio was obtained as a function of depth. An increase in this ratio was observed, suggesting a gradual oxidation of tin as the oxide layer thickens. These results support the hypothesis that the tetragonal zirconia phase formed in the early stage of the corrosion cycle is stabilized by tin(II) (Wei et al., 2013).

Froideval et al. (2009) compared the composition and thickness of the corroded layer on a Zr–2.5% Nb alloy sample retrieved from a fuel rod irradiated in a PWR (burnup of 41.4 GW d t⁻¹) and on the same alloy oxidized in an autoclave. The micro-XAS data collected at the Zr and Nb K edges combined with focused ion beam scanning electron microscopy and micro X-ray fluorescence (µ-XRF) experiments allowed the authors to conclude that both zirconium and niobium exhibited the same local environments in both samples. Consequently, neutron irradiation does not induce a modification of the oxide corrosion layer observed for such a zirconium alloy.

The typical fresh UO₂ fuel for commercial light-water reactors (PWRs and BWRs) is a dense ceramic (≥95% of theoretical density) with a grain size of 8–10 µm. The fuel is inserted into the reactor core as axial stack of cylindrical fuel pellets encased in a cladding tube called a fuel rod (Abe et al., 2014). As already mentioned, the MOX fuel is also industrially used in LWRs in countries with a closed fuel cycle. Both uranium and plutonium dioxides crystallize in the same fluorite or CaF₂ structure (space group Fm3m) and UO₂ and PuO₂ form a continuous solid solution (Guéneau et al., 2011, 2012). The fluorite-type structure of the mixed oxide (U,Pu)O₂ has the ability to tolerate both the addition of oxygen and its removal, leading to the formation of a wide homogeneity range, i.e. (U,Pu)O_2±x. The oxygen stoichiometry or oxygen/metal (O/M) ratio of the (U,Pu)O_2±x fuel is a crucial parameter as it affects most of the fuel properties (thermal conductivity, melting temperature, diffusion phenomena, fuel/cladding interactions etc.; Beauvy, 1992; Philipponneau, 1992; Duriez et al., 2000).

3.1. Nuclear fuels

Due to the high radiotoxicity of SNF, the number of XAS studies performed on SNF remains extremely limited. In 2011, Degueldre and coworkers performed the first direct examination of an irradiated fuel sample using XAS (Degueldre et al., 2011). The investigated material was an MOX fuel synthesized by an internal gelation process with the initial composition (U_0.943Pu_0.047)O₂ and was irradiated up to a 60 GW d t⁻¹ burnup in the Beznau-1 PWR. To comply with the radioactivity limit authorized at the micro-XAS beamline (SLS, Switzerland), a replicate method described in Fig. 2 was used and allowed SNF particles to be retrieved from both the rim area and the central part of the fuel.

Figure 2 Replicate methodology to produce sample particles on adhesive Kapton (top). Sample (4) is shown in the inner cell and mounted in the outer cell. Reprinted from Degueldre, Mieszczynski et al. (2014) with permission from Elsevier.

The encapsulated SNF particles were scanned by µ-XRF mapping to identify the locations of elemental `hotspots', after which L₃-edge micro-XANES (µ-XANES) spectra of the matrix elements uranium and plutonium (Degueldre et al., 2011; Degueldre, Pin et al., 2014), curium (Degueldre, Borca et al., 2013), americium (Degueldre, Cozzo et al., 2013) and thorium (Cozzo et al., 2014) were collected. Americium and curium were found to be in the trivalent state. The plutonium spectrum collected from the SNF sample was compared with a spectrum collected from the fresh fuel material, showing that plutonium remained as Pu⁴⁺ in particles located in the rim area of the fuel, whereas a minor fraction of Pu³⁺ was detected in grains extracted from the central part of the fuel (Degueldre, Pin et al., 2014).

Mixed oxides (U,Pu)O₂ with a plutonium content higher than the 7–8 mol% currently used in LWRs are expected to be a favourable fuel for future fast neutron reactors and have been studied within the framework of the development of fourth-generation (GEN-IV) nuclear reactors.⁵ In this context, Vigier et al. (2015) have recently studied stoichiometric U_0.7Pu_0.3O_2.00 and hypo-stoichiometric U_0.7Pu_0.3O_2−x synthesized by a sol–gel route by X-ray diffraction (XRD), ¹⁷O-NMR and XAS. The XANES data showed a mixed valence of plutonium cations (Pu³⁺/Pu⁴⁺) in the U_0.7Pu_0.3O_2−x sample, whereas only Pu⁴⁺ was observed in the stoichiometric sample. Both ¹⁷O-NMR and EXAFS data collected from the latter sample clearly demonstrated a random distribution of uranium and plutonium, corroborating the assumption of the formation of an ideal solid solution. Furthermore, no significant differences between the first U–O and Pu–O distances were observed in stoichiometric MOX. These results contrast with those reported for ThO_2.00 solid solutions with MO_2.00 [M = U, Pu (Hubert et al., 2006) or Am (Carvajal-Nunez et al., 2012)], where the M–O distances remain close to those observed in the different pure binary oxides.

3.3. Fission-product behaviour

During irradiation, the fission of ²³⁵U and ²³⁹Pu leads to the formation of more than 40 main fission products. Their creation yields as a function of their masses follow a curve in the shape of a `camel's humps' (Baron & Hallstadius, 2012) basically covering every element from selenium to gadolinium. The highest production is of zirconium, molybdenum, xenon, cerium and neodymium. The fission products may be distributed among several phases, depending on the temperature, pressure and composition (burnup) of the fuel as well as their particular chemical and physical properties (Lewis et al., 2012). A special case is the rare gases (xenon and krypton) which are considered to be totally insoluble in the fuel. During irradiation, these elements can be released from the fuel and induce an increase of pressure within the pin and eventual stresses on the cladding (Rest, 2012). By combining EXAFS, high-energy-resolution fluorescence detection (HERFD) data and theoretical calculations using the FDMNES software (Bunău & Joly, 2009) on fresh UO₂ samples implanted with krypton or xenon (0.5 and 0.37 at%, respectively), Martin et al. (2015) and Bès et al. (2015) have shown that these two rare gases are soluble in UO₂ at such concentrations. Their incorporation site into the UO₂ fluorite lattice is a neutral Schottky defect, and therefore the solubility of rare gases in UO₂ should be re-evaluated.

Curti et al. (2014) studied the redox speciation of selenium in fragments extracted from a UO₂ SNF sample irradiated in a BWR (Leibstadt, Switzerland) up to a burnup of 78.7 GW d t⁻¹. Despite the very low concentration (100–200 p.p.m.) of selenium, Se K-edge µ-XANES spectra could nevertheless be collected using a 2–3 × 5 µm beam. Based on a linear combination fit using Se²⁻, Se(0) and Se⁴⁺ references and theoretical XANES, the occurrence of selenide species was clearly demonstrated, supporting the hypothesis that selenium would be sparingly soluble as Se²⁻ in SNF.

The aforementioned illustrations concern normal reactor operation, but accidental conditions must also be considered, as exemplified by the severe accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in 2011, which was the largest nuclear incident since the 1986 Chernobyl disaster and has been rated at the maximum level of 7 on the International Nuclear Event Scale (Abe et al., 2014). Adachi et al. (2013) found spherical 2 µm particles containing radioactive caesium in aerosol samples collected on 14 and 15 March 2011 in Tsukuba, 172 km southwest of the FDNPP. Three particles were analyzed on the BL37XU (Terada et al., 2004, 2010) beamline at Spring-8 in Japan using µ-XRF, µ-XANES and µ-XRD. As illustrated in Fig. 3, in addition to caesium, ten other heavy elements were detected: iron, zinc, rubidium, zirconium, molybdenum, tin, antimony, tellurium and barium, together with uranium in two of the three analyzed particles. The XANES data collected showed that each element exhibited a high oxidation state (i.e. Fe³⁺, Zn²⁺, Mo⁶⁺ and Sn⁴⁺). The authors concluded that the sources of the elements identified within the three radioactive microparticles would most probably be the uranium fuel, fission products and components of the reactors.

Figure 3 Results of synchrotron-radiation (SR) µ-XANES analyses. (a) Comparison of the U L3-edge SR µ-XANES spectra of the three radioactive particles, demonstrating the presence of uranium in particles A and B. (b, c, d) Comparisons of the (b) Fe K-edge, (c) Mo K-edge and (d) Sn K-edge SR µ-XANES spectra of the three particles and the reference materials. Reprinted with permission from Abe et al. (2014). Copyright 2014 American Chemical Society.

In the closed fuel cycle, the fuel is reprocessed and the uranium and plutonium are removed from the spent fuel, leaving fission products (FPs) such as technetium, molybdenum, iodine, caesium, lanthanides and minor actinides (MAs) such as neptunium, americium and curium as waste. The prevalent separation method used at the reprocessing plants at la Hague (France), Rokkasho Mura (Japan) and Sellafield (UK) is the plutonium and uranium extraction (PUREX) process (Vernaz et al., 2012). After dissolution of the spent fuel in nitric acid, the key step involves a hydrometallurgical method of chemical separation of uranium and plutonium from the solution. The remaining solution containing FPs and MAs (called high-level waste; HLW) must be immobilized and converted to a durable form allowing storage in deep geological repositories. The vitrification of HLW is currently the only method capable of the safe confinement of high-activity nuclear waste that has been validated on an industrial scale. Over 30 elements (FPs, activation products and MAs) can be immobilized in industrial nuclear glasses such as the French technological R7T7 glass produced at la Hague (Vernaz et al., 2012). Moreover, to ensure the long-term behaviour of such glass, various additives (Li₂O, CaO, ZnO, ZrO₂ etc.) are introduced, leading to a quite complex final glass formulation [see, for example, see Table 2 in Vernaz et al. (2012) and Table 1 in Dardenne et al. (2015)].

The lack of long-range order in glass makes XAS a very useful technique to reveal the binding properties either of the network-forming glass constituents or of the various impurities/dopants which modify the properties or stability of the glass matrix. XAS studies on nuclear waste glasses have mainly been performed on surrogate glasses (glasses with the same composition) doped with fission products (mostly non­radioactive isotopes) or actinide elements. For example, McKeown et al. (2015) studied the behaviour of iodine using the ¹²⁹I isotope in low-activity borosilicate glasses simulating the glasses produced at the Handford site in Washington state (USA). The XAS spectra collected at the I K edge showed that iodine is present only as iodide (I⁻) in the samples. The EXAFS results showed that the increased iodine incorporation in glasses containing Na₂O and Li₂O could be explained by the presence of I—Na and I—Li bonds. Other FP behaviour in simulated nuclear waste glasses, such as technetium (Antonini et al., 1983; Lukens et al., 2007; Muller et al., 2014; Soderquist et al., 2014), selenium (Bingham et al., 2011), molybdenum (Calas et al., 2003; Hyatt et al., 2006), zirconium (McKeown et al., 1999; Galoisy et al., 1999; Connelly et al., 2011; Calas et al., 2014), rhenium (Lukens et al., 2007; Okamoto et al., 2016), ruthenium (Okamoto et al., 2016), caesium (Stefanovsky & Purans, 2012) and lanthanides (Larson et al., 1990; Jollivet et al., 2005, 2007), as well as uranium (Greaves et al., 1989; Connelly et al., 2013) and plutonium (Richmann et al., 2001; Cachia et al., 2006), have also been studied.

Dardenne et al. (2015) performed the first XAS study on a glass fragment extracted from a real nuclear waste glass. This glass was produced by the Karlsruhe vitrification plant (VEK) in Germany to immobilize the HLW generated at the Karlsruhe Reprocessing Plant (WAK) from 1971 to 1991. This glass contains 25 FPs, uranium, neptunium, plutonium, americium and curium with contents varying between 0.01 and 1.19 wt% (oxide mass). The XAS sample consisted of a millimetre-sized fragment extracted from the nuclear waste glass rod (shown in Fig. 4). XRF data were collected at different excitation energies (shown in Fig. 4) to identify the chemical elements that were present in the sample. XANES spectra were collected at the Tc and Se K edges and the U, Np, Pu and Am L₃ edges. Comparison with reference compounds showed that selenium is present in the form of selenite () in the glass. Technetium was found to be in the form. Americium was found as Am³⁺ and plutonium as Pu⁴⁺ whereas, according to the XANES spectra, uranium and neptunium were found as U⁶⁺ and Np⁵⁺ with the uranyl and the neptunyl geometries, respectively.

Figure 4 Picture of a rod extracted from real nuclear waste glass and the particle used (blue arrow) for investigation on the INE beamline (left) and XRF spectra taken from this fragment at different excitation energies (right). Reprinted from Dardenne et al. (2015) with permission from Elsevier.

The examples presented give a selective overview of research and development activities in the nuclear energy scientific XAS community. The use of XAS in this domain is still in its infancy, as it is only recently that the combination of sample preparation in hot laboratories and beamlines dedicated to radioactive materials has allowed researchers to work on actual highly radioactive samples such as nuclear glasses and spent nuclear fuels. Nevertheless, as shown, XAS can hopefully provide crucial data for the fundamental comprehension of these materials which was not possible without such experiments. Standard XANES/EXAFS will remain extremely valuable, but the next big step is the development of HERFD, X-ray emission spectroscopy, resonant inelastic X-ray scattering (RIXS) and nonresonant inelastic X-ray scattering experimental setups on `active' beamlines. These techniques have already shown their power for samples containing actinides (Kvashnina et al., 2013; Bès et al., 2016; Butorin et al., 2016; Butorin, 2011; Vitova et al., 2010, 2013; Popa et al., 2016; Conradson et al., 2013). A good example is the expected sensitivity of a RIXS map collected at the U L₃ edge to the substitution of uranium by heavier actinides, allowing direct probing of the homogeneity at the molecular level of an (U,An)O₂ solid solution (Kvashnina et al., 2015). Another great advantage of using a crystal analyzer spectrometer (CAS) is the possibility of isolating a specific fluorescence line from a specific element in a sample containing more than 20 chemical elements with very similar Z. Thanks to the use of a CAS, XAS studies of minor elements that were otherwise unattainable are now possible in nuclear glasses and SNF. Furthermore, the development of XAS endstations dedicated to radioactive samples allows the development of in situ experiments (high temperature and atmosphere) on real samples such as molten salt fuels containing heavier elements than uranium and fission products (Beneš & Konings, 2012) or the investigation of thermal calcination of advanced precursors for the manufacture of nuclear fuel or transmutation targets (Caisso et al., 2015). To summarize, XAS and associated scattering techniques will become even more relevant in the future for research and development on nuclear materials.