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On the Oxidation Mechanism of U3Si2 Accident TolerantNuclear FuelDOI:10.1016/j.corsci.2020.108822

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Corrosion Science

journal homepage: www.elsevier.com/locate/corsci

Short Communication

On the oxidation mechanism of U3Si2 accident tolerant nuclear fuel

R.W. Harrisona,*, C. Gasparrinib,c, R.N. Wortha, J. Buckleya, M.R. Wenmanb, T. Abrama

aNuclear Fuel Centre of Excellence, Department of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Sackville Street, Manchester, M13 9PL,United Kingdomb Centre for Nuclear Engineering, Department of Materials, Imperial College London, London, SW7 2AZ, United Kingdomc Consorzio RFX, Corso Stati Uniti 4, Padova, 35127, Italy

A R T I C L E I N F O

Keywords:Uranium silicideIntermetallicOxidationNuclear fissionNuclear fuel

A B S T R A C T

The oxidation mechanism and products of U3Si2 accident tolerant nuclear fuel in flowing air up to 750 °C isreported. Differences between observed and theoretical mass gains for complete oxidation is due to un-oxidisedSi that forms nano-crystalline regions of Si. Some Si rich regions are protected by the formation of UO2, which isthermodynamically preferential to oxidise before Si. The UO2 is further kinetically oxidised to form nano-crystalline U3O8. The nanostructure formed, accompanied by large volumetric expansions during oxidationproduces pulverisation of fragments into powder, which may have serious consequences for fuel integrity ifexposed to oxidative atmospheres.

1. Introduction

The accident at the Fukushima Daiichi nuclear power plant in 2011highlighted weaknesses in current uranium oxide and zirconium alloyfuel-cladding assemblies. Since then, the development and deploymentof so called ‘accident tolerant fuels’ (ATF) and cladding materials toreplace current fuel assemblies in water cooled reactors has been un-derway internationally [1]. The intermetallic compounds of uraniumand silicon, primarily U3Si2, are being promoted as ATF candidates toreplace currently used UO2 fuel material in light water reactors (LWRs)[2]. U3Si2 has a superior thermal conductivity (∼15W/m/K at 500 °C)[3] than UO2 (∼4W/m/K at 500 °C), which lowers the centrelinetemperature of the fuel pellet and reduces thermally active mechanismsthat degrade fuel during reactor operation (such as fission gas bubblegrowth). In addition, the higher U metal density of U3Si2 compared toUO2 has economic advantages by requiring lower 235U enrichments.Although U3Si2 has been found to have much lower radiation inducedswelling compared with other U-Si compounds, such as U3Si [4,5],there is much that is still unknown about its properties including itsoxidation mechanism(s). The fuel material may be exposed to hightemperature oxidising atmospheres during its manufacture/processing,in reactor operation such as a loss of coolant accident (LOCA) withwater and/or air ingress into the fuel pin, or during spent fuel storageafter discharge from the reactor core.

Sooby Wood et al. [6] have shown that the onset of oxidation ofU3Si2 in air occurs at ∼384 °C and the final observed mass gain was

∼21wt.%, lower than the expected 24.9 wt.% mass gain for completeoxidation to U3O8 and SiO2, indicative of incomplete oxidation.Johnson et al. [7] oxidised U3Si2 fragments which were produced by arcmelting. Fragments were heated to 800 °C under flowing air and sam-ples began to oxidise at a much higher onset temperature of 470 °Ccompared to the onset temperature found in Ref. [6]. However, theauthors [7] similarly observed a lower than expected mass gain of∼20wt.% (again, around 5wt.% lower than that expected for completeoxidation to U3O8 and SiO2). This mass discrepancy was attributed tothe oxidation products formed being U3O8 and SiO. However, no evi-dence of SiO in the final products is given and is simply inferred fromthe mass gains observed. SiO is a metastable compound that decom-poses into Si and SiO2 after high temperature treatment so confirmingits presence experimentally would be challenging [14]. Furthermore,SiO readily oxidises to SiO2 under oxidative atmospheres and non-ambient temperatures [8] so it would not be anticipated to be the finaloxidation product of Si after the conditions studied in Ref. [7]. In othernon-oxide uranic materials, such as UC [9,10], as well as other carbidesand nitrides [11,12], oxidation leads to the nucleation of nano-crys-tallite oxide grains, with gaseous release of carbon dioxide or nitrogen.However, in U3Si2 fuel materials it is important to fully understand theoxidation processes and the product(s) of Si during oxidation so as toevaluate any effects this may have on the resulting material composi-tion and properties as well as being able to engineer more resistant ATFcandidates.

The reaction products, especially the fate of Si and mass discrepancy

https://doi.org/10.1016/j.corsci.2020.108822Received 21 April 2020; Received in revised form 1 June 2020; Accepted 16 June 2020

⁎ Corresponding author.E-mail address: R.W.Harrison@Manchester.ac.uk (R.W. Harrison).

Corrosion Science 174 (2020) 108822

Available online 18 June 20200010-938X/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

T

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during oxidation of U3Si2 have so far eluded previous works [6,7]. Thismay be due to the use of X-ray diffraction (XRD) techniques as the maincharacterisation tool, which only probes crystalline material while theSi products could be amorphous and/or in quantities below XRD de-tection limits, and scanning electron microscopy (SEM) with energydispersive spectroscopy (EDS) that can detect chemical segregation tothe μm scale. In a recent work [13], scanning transmission electronmicroscopy (STEM) and EDS was used to study the oxidation of Ce3Si2as an isostructural non-radioactive surrogate and investigate the massdiscrepancy observed during oxidation. This revealed the formation ofnano-grained CeO2, amorphous regions of SiO2 and nano-crystallineregions of free Si accounting for the mass discrepancy observed. Theamount of unreacted Si was equal to around 1wt.%, which would bedifficult to detect by XRD (particularly in actinide materials) and todistinguish via SEM-EDS due to the lower spatial resolution comparedto S/TEM. However, the surrogate work on the Ce3Si2 system is limiteddue to the highest oxidation state of Ce being +4 which only allows theformation of CeO2, whereas U has the potential to transition throughmany oxidation states up to +6 which will give rise to much morecomplicated oxidation kinetics and thermodynamics and thus elucida-tion of the mass discrepancy observed during oxidation.

In the current work, we have followed on from our previous studyand used S/TEM-EDS and high resolution (HR)TEM to characterise as-oxidised U3Si2 to explore the presence of unreacted Si on the nanoscaleand if this could account for the mass discrepancy observed duringoxidation.

2. Experimental

U3Si2 samples were fabricated by arc melting (Centorr 5SA arcfurnace) uranium metal and silicon pieces (Si, Alfa Aesar, 99.9999 %)in an inert atmosphere glovebox (Kiyon). As-melted fragments and as-oxidised powders were characterised using XRD (further details givenin [13]). Mass gains of the as-melted U3Si2 fragments during oxidationwere measured using thermogravimetric analysis (TGA, NetzschSTA449 F1 Jupiter) to 750 °C (+5 °C/min, 50mL/min synthetic airflow). Samples were prepared for S/TEM by dropping crushed powdersdispersed in ethanol onto carbon film grids. S/TEM and energy dis-persive X-ray spectrometry (EDS) were performed on a JEOL 2100 F(200 kV) equipped with an Ultim® Max EDS detector (Oxford instru-ments). XRD, selected area diffraction patterns (SADPs) and fast Fouriertransforms (FFTs) of high resolution (HR)TEM images were indexedwith reference patterns from the International Centre for DiffractionData (ICDD) and the Inorganic Crystal Structure Database (ICSD).

3. Results and discussion

Fig. 1 shows the mass gain of U3Si2 during oxidation in this work,alongside previous works by Sooby Wood et al. [6] and Johnson et al.[7] and shows that there is a range of onset temperatures between350−450 °C, though the work here more closely matches that of SoobyWood et al. [6]. However, all oxidation profiles show similar reactionrates, initially with a rapid increase in weight gain, which may be at-tributed to the formation of UO2 and then a second, slower oxidationstep with the conversion of UO2 to U3O8. Fig. 1 also shows the theo-retical weight gains anticipated from the oxidation products of Si, eitherremaining as free Si, formation of SiO or SiO2. In this work, the finalplateaued mass gain was 21.1 wt.% (similar to that of Sooby Wood et al.[6] and Johnson et al. [7]) and below the anticipated mass gain to24.9 wt.% for the formation of U3O8 and SiO2, indicating an incompleteoxidation reaction.

XRD patterns of the material before and after oxidation are shown inFig. 2. The as-melted fragments (before oxidation) indexed as tetra-gonal U3Si2 with space group P mbm4/ (PDF 04−003-0520 [15]). Postoxidation, these fragments pulverised into powder that indexed as or-thorhombic U3O8 with space group of Amm2 (PDF 01−072-1257

[16]). This showed that following oxidation all peaks in the XRD pat-tern indexed with orthorhombic U3O8 and no other crystalline phaseswere observed, similar to previous work [6,13], which raises thequestion of the fate of the Si which may be forming amorphous pro-ducts or present as a crystalline material in quantities below the de-tectability of XRD.

Fig. 3 shows bright field (BF) and dark field (DF)-STEM images of aparticle in the as-oxidised powder after exposure to flowing air at750 °C showing two regions, one with high Z number (darker regions inBF image) and a region with a low Z number. Subsequent EDS mappingrevealed the presence of U and O rich regions along with some Si richregions that were deficient in both U and O. This is similar to ourprevious work [13] where Si rich regions were observed in oxidisedCe3Si2 using STEM-EDS and energy filtered (EF)TEM. This indicates thepresence of unreacted Si after the oxidation of U3Si2 fragments ap-peared complete by the plateau in the TGA.

Fig. 4a shows a TEM image of the region analysed by STEM-EDS inFig. 3. The HRTEM shown in Fig. 4b revealed that the lighter region(lower Z number, Si rich from EDS mapping) was found to consist ofnano-crystallites (∼10 nm in size). The FFT (inset in Fig. 4b) indexed as

Fig. 1. Mass gains as a function of temperature during oxidation of U3Si2 to750 °C (+5 °C/min) along with data from [6] and [7]. Theoretical weight gainsare shown by horizontal lines.

Fig. 2. XRD patterns of U3Si2 before oxidation and after oxidation to 750 °Cshowing the formation of U3O8.

R.W. Harrison, et al. Corrosion Science 174 (2020) 108822

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face centred cubic (FCC) Si close to the [110] zone axis along withforbidden {002} reflections observed. The FFT filtered image in Fig. 4cshows an enlarged image of the area highlighted in the white box. Al-though the {002} reflections are forbidden in the diamond like FCClattice of Si these reflections are typically observed in TEM due to dy-namical scattering giving rise to the appearance of these reflections[17]. As the oxidation temperature of 750 °C in this work was above thecrystallisation temperature of Si (in the region of 550−700 °C [18])then it is anticipated that the Si would be crystalline agreeing with theHRTEM observations.

The HRTEM image in Fig. 4d shows a region that was found to be

rich in U and O in the EDS mapping in Fig. 3. The FFT of the areahighlighted in Fig. 4d is shown in Fig. 4e which revealed polycrystallinerings and indexed as orthorhombic U3O8, agreeing with the XRD pat-tern after oxidation. These nano-crystallites can also be observed in thebottom left hand side of Fig. 4a, which was found to be U and O richand also likely to be U3O8. This shows that the material undergoes atransition from a solid fragment of as-melted material to a powderconsisting of nano-crystallites of U3O8 during oxidation. Volumetricexpansion during the formation of UO2 from U3Si2 is ∼17 vol.% andsubsequent oxidation of UO2 to U3O8 leads to a further 36 vol.% ex-pansion [10]. Coupled with the volumetric expansion of the formation

Fig. 3. BF and DF-STEM images of as-oxidised U3Si2 to 750 °C along with EDS maps of Si, U and O.

Fig. 4. a) TEM image of region in EDS maps, area highlighted in dashed blue box is enlarged in b) and shows a HRTEM image revealing crystalline material indicatedby the FFT inset, which indexed as Si and the FFT filtered image is shown in c). The region indicated in solid red box is enlarged in d) showing the HRTEM image andthe FFT is shown in e) showing polycrystalline materials which indexed as U3O8 (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article).

R.W. Harrison, et al. Corrosion Science 174 (2020) 108822

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of SiO2 (assuming a density of 2.2 g/cm3 for amorphous silica [19]) andfree Si leads to a total volume change of 133 vol.% during oxidation.This strain arising from this large volumetric change and associatednano-grain formation leads to the pulverisation of the material from asolid fragment into powder.

The oxidation of U to UO2 is much more favourable (ΔG =−1000 kJ/mol O2 [20]) at the onset temperature of ∼350 °C comparedwith ΔG = −800 kJ/mol O2 for the oxidation of Si to SiO2 [21]. Thissuggests that the U will begin to oxidise preferentially over the Si in-itially. Once the U is converted to UO2, by around 400 °C (as observedby change in kinetics in Fig. 1) the Gibbs free energy of further oxi-dation to U3O8 is −200 kJ/mol O2 [22] and so at this point, the oxi-dation of Si to SiO2 becomes more thermodynamically favourable. Thisdiffers to the Ce3Si2 system studied previously where unreacted Si wasalso observed along with a mass discrepancy during oxidation [13]. TheGibbs free energy of formation of CeO2 is lower (ΔG = −950 kJ/molO2 at the onset temperature of oxidation) than Si to SiO2 across theentire temperature range, and so unreacted Si would be anticipatedthermodynamically. Thus, in the U3Si2 system studied here, there mustbe a shift from the thermodynamically favoured products of UO2 andSiO2 to kinetically favourable products i.e. the full conversion of UO2 toU3O8 before all the Si is oxidised to SiO2.

Oxidation of Si to SiO2 must occur as the total mass gain observed inFig. 1 is 21.1 wt.% and so this goes beyond the mass gain if the sampleoxidised to U3O8 and free Si (∼9wt.%). Although TGA data from thecurrent and previous oxidation of U3Si2 [6,23] indicates the final pro-duct is SiO by mass we believe this will not be the case as SiO readilyforms SiO2 under high temperature oxidative atmospheres [8]. Fur-thermore, previous works [24–26] examining the disproportionation ofSiO into SiO2 and Si under high temperature (> 850 °C) reductive at-mospheres produced nanocrystalline Si particles surrounded by anamorphous SiO2 matrix. For example, Mamiya et al. [26] annealed SiOat temperatures between 900−1400 °C under reductive argon (99.99%) atmosphere finding the formation of crystalline Si particles (be-tween 1−20 nm in size dependent on annealing temperature) withinamorphous SiO2 at temperatures below 1300 °C. At temperatures above1300 °C, crystalline SiO2 (cristobalite) was observed to form. A furtherregion from this work (Fig. 5), identified as Si rich and U and O defi-cient from EDS in Fig. 4 shows the presence of ∼10 nm size crystalliteswhich indexed with Si. No amorphous regions between grains wereobserved as would be expected from the structures observed in Refs.[24–26] resulting from disproportionation reaction of SiO into Si andSiO2. This indicates that the Si observed in this work is remnant fromunreacted Si rather than formed via the production of SiO, showing the

incomplete mass gain observed in the TGA can be attributed to amixture of oxidation of Si to SiO2 and unreacted Si.

Following oxidation to UO2 the most thermodynamically favourablereactions following that would be full conversion to SiO2 before theUO2 began to oxidise further to U3O8. Sooby Wood et al. [20] per-formed isothermal anneals of oxidation of U3Si2 at 350 °C (50 °C belowtheir observed oxidation onset) for 10 h. The reactions products ob-served by XRD in [20] were a mix of UO2, U3O8 and USi3 showing thatas the U is oxidised there is a formation of Si rich uranium silicides.Following the formation of these Si rich uranium silicides (USi3), thismay then lead to the segregation of nanoscale islands of unreacted Sisurrounded by layers of UO2. The UO2 layers then react and consumesfurther oxygen, leading to the formation of U3O8. This, in turn, reducesthe oxygen diffusion to the unreacted Si core. This reveals the kineti-cally favoured products of the reaction, accounting for the full oxida-tion of all UO2 to U3O8 (as observed by XRD) before the Si oxidation toSiO2 has been completed which would be anticipated from the ther-modynamic products. The regions of crystalline Si observed in Figs. 3and 4 are indeed regions of Si that are surrounded by nano-crystallineU3O8 that may have protected the Si from oxidation to SiO2 while theUO2 layer surrounding it formed U3O8. During the powder preparationfor TEM, this un-reacted Si will have been exposed. We have reported[13] similar nanoscale regions of unreacted Si in Ce3Si2 oxidised undersimilar conditions.

Assuming that all U is oxidised to U3O8 (accounting for 9.35 wt.%gain) and the remaining 11.75 wt.% mass gain is due to the oxidation ofSi to SiO2 then the unreacted mass of Si can be determined by sub-tracting the theoretical mass gain of full Si oxidation (15.58 wt.%). Theunreacted Si in the material is equal to ∼1.4 wt.% in the total sample,which is approaching the limits for typical laboratory XRD detection,particularly considering the high scattering cross section of U for X-rays. This may explain the reason that observation of crystalline Si haseluded previous works [6,20,23] and thus the reason for the dis-crepancies between the oxidation plateau mass observed experimen-tally and theoretical expected mass gain remaining unknown.

4. Conclusion

In conclusion, TGA was used to study the oxidation process of as-melted fragments of U3Si2 from room temperature to 750 °C, finding adiscrepancy between the experimentally observed and theoreticallyexpected mass gain of full oxidation to U3O8 and SiO2. Oxidation pro-ducts were characterised by XRD and S/TEM-EDS revealing the for-mation of nano-crystalline U3O8 and regions of free crystalline Si,

Fig. 5. HR-TEM image of Si rich and U and O deficient region identified by EDS, showing the 10 nm size crystallites, FFT from boxed area of grain 1 (G1) and grain 2(G2) indexed as crystalline Si.

R.W. Harrison, et al. Corrosion Science 174 (2020) 108822

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which accounts for the mass discrepancy observed in the TGA and in-dicates incomplete oxidation of Si. From thermodynamic assessment ofthe reaction pathway the U undergoes oxidation preferentially over theSi which then leads to the formation of Si rich USi3 phases. Althoughthe majority of Si oxidises to SiO2, some nanoscale (< 100 nm) regionsof free Si may be protected by UO2, which is then oxidised to U3O8 toreveal the kinetically favoured product as opposed to the thermo-dynamically anticipated products. The formation of the nanostructurealongside the large volumetric expansion of U3Si2 during oxidationleads to pulverisation of the fragments into powder which may haveserious consequences on fuel manufacturability, performance and be-haviour if exposed to high temperature oxidative atmospheres duringin-reactor operation or on fuel durability during spent fuel storage.

Data availability

The raw and processed data required to reproduce these findings areavailable to download from https://doi.org/10.17632/rfd7zwf3m7.

CRediT authorship contribution statement

R.W. Harrison: Conceptualization, Data curation, Formal analysis,Methodology, Visualization, Writing - original draft. C. Gasparrini:Investigation, Methodology, Writing - review & editing. R.N. Worth:Investigation, Methodology, Resources, Writing - review & editing. J.Buckley: Investigation, Methodology, Resources, Writing - review &editing. M.R. Wenman: Supervision, Writing - review & editing. T.Abram: Supervision, Writing - review & editing, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgements

RWH, RNW, JB and TA are thankful to the EPSRC (EP/S011935/1ATLANTIC) and the UK Government Nuclear Innovation Programme onAdvanced Fuels for financial support of this project. CG and MRW arethankful to the EPSRC (EP/P005101/1) for funding of this work.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in theonline version, at doi:https://doi.org/10.1016/j.corsci.2020.108822.

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On the oxidation mechanism of U3Si2 accident tolerant nuclear fuelIntroductionExperimentalResults and discussionConclusionData availabilityCRediT authorship contribution statementDeclaration of Competing InterestAcknowledgementsSupplementary dataReferences