Raphaël Hanon
email: raphael.hanon@edu.mnhn.fr

Francesco D'errico

Lucinda Backwell

Sandrine Prat

Marylène Patou-Mathis

Stéphane Péan

Christine Steininger

New evidence of bone tool use by Early Pleistocene hominins from Cooper's D, Bloubank Valley, South Africa

Bone tool-use by Early Pleistocene hominins is at the center of debates in human evolution. It is especially the case in South Africa, where 102 bone tools have been described from four Early Stone Age (ESA) archaeological sites, which have yielded Oldowan and possibly Acheulean artefacts, as well as Paranthropus robustus and early Homo remains. Here we describe a bone tool from Cooper's D. The deposit, dated between 1.4 and 1.0 Ma, has yielded seven P. robustus remains and 50 stone artefacts. Our results highlight similarities in morphology and use-wear patterns between the Cooper's D bone tool and those previously identified at nearby Sterkfontein, Swartkrans, Kromdraai and Drimolen. Our findings increase the number of Early Stone Age bone tools and provide further evidence of their association with P. robustus. They suggest P. robustus had the cognitive capacities to develop this cultural adaptation and the manipulative abilities to implement it.

Introduction

Although formal bone tools, i.e. tools entirely or almost entirely shaped with techniques specifically conceived for bone material such as scraping, grinding and grooving, are well known in the Eurasian Upper Palaeolithic and the African Later Stone Age, it is only in the last twenty years that we have begun to have information on the origin of this cultural innovation (Backwell and d'Errico, 2001;[START_REF] Backwell | Criteria for identifying bone modification by termites in the fossil record[END_REF]d'Errico et al., , 2020)). Early instances are found at Middle Stone Age sites from southern, northern, and central Africa [START_REF] Brooks | Dating and context of three middle stone age sites with bone points in the Upper Semliki Valley[END_REF][START_REF] Brooks | Projectile technologies of the MSA: implications for modern human origins[END_REF][START_REF] Yellen | A middle stone age worked bone industry from Katanda, Upper Semliki Valley, Zaire[END_REF][START_REF] Henshilwood | An early bone tool industry from the Middle Stone Age at Blombos Cave, South Africa: implications for the origins of modern human behaviour, symbolism and language[END_REF]Backwell et al., 2008[START_REF] Backwell | The antiquity of bow-and-arrow technology: evidence from Middle Stone Age layers at Sibudu Cave[END_REF][START_REF] Backwell | Criteria for identifying bone modification by termites in the fossil record[END_REF]d'Errico et al., , 2020;;[START_REF] El Hajraoui | Préhistoire de La Région de Rabat-Témara. Villes et Sites d'archéologie Marocaine[END_REF][START_REF] Backwell | Osseous Projectile Weaponry from Early to Late Middle Stone Age Africa[END_REF].

More debated issues concern the first use of bone modified with techniques generally applied to stone such as knapping [START_REF] Backwell | The first use of bone tools: a reappraisal of the evidence from Olduvai Gorge[END_REF][START_REF] Zutovski | The use of elephant bones for making Acheulian handaxes: A fresh look at old bones[END_REF][START_REF] Doyon | A 115,000-year-old expedient bone technology at Lingjing[END_REF][START_REF] Pante | Bone tools from Beds II-IV, Olduvai Gorge, Tanzania, and implications for the origins and evolution of bone technology[END_REF][START_REF] Porraz | Experimentation preceding innovation in a MIS5 Pre-Still Bay layer from Diepkloof Rock Shelter (South Africa): emerging technologies and symbols[END_REF], and the use of minimally modified bone fragments. Ever since the discovery of the species Australopithecus africanus, referred to then as A. prometheus, Dart (1949[START_REF] Dart | The osteodontokeratic culture of Australopithecus prometheus[END_REF]Dart ( , 1959aDart ( , b, 1960Dart ( , 1961aDart ( , b, 1962) ) proposed that this hominin was able to use bone as a tool. He referred to the bone collection from Makapansgat as the "osteodontokeratic culture". Dart's 'bone tool' hypothesis was challenged by the work of Brain (1967aBrain ( , 1967bBrain ( , 1976[START_REF] Brain | The Hunters or the Hunted? An Introduction to African Cave Taphonomy[END_REF], Mills (1973), Mills & Mills (1977), Skinner (1976) and Klein (1975), who emphasized the role of natural agents, particularly hyaena, in the production of pseudo-tools. While maintaining his opinion on the highly questionable nature of the Makapansgat "bone tools", it was the same Brain who published the discovery of 68 bone tools from the hominin site of Swartkrans (Brain & Shipman 1993). Since then, evidence for the existence of an early bone tool technology at Early Stone Age (ESA) sites in South Africa has grown [START_REF] Brain | Bone weathering and the problem of bone pseudo-tools[END_REF][START_REF] Brain | New evidence of early hominids, their culture and environment from the Swartkrans cave, South Africa[END_REF]Brain and Shipman, 1993;[START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]Backwell and d'Errico, 2001, 2001, 2003[START_REF] Backwell | The origin of bone tool technology and the identification of early hominid cultural traditions[END_REF], 2008;d'Errico et al., 2001;d'Errico andBackwell, 2003, 2009;[START_REF] Stammers | The first bone tools from Kromdraai and stone tools from Drimolen, and the place of bone tools in the South African Earlier Stone Age[END_REF]. A growing body of data indicate that these tools mostly consist of weathered limb bone shaft fragments of medium to large size animals with a worn and polished area at the tip (Brain and Shipman, 1993;[START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]d'Errico, 2001, 2008). According to d [START_REF] Errico | Bone tool use in termite foraging by early hominids and its impact on our understanding of early hominid behaviour[END_REF] and Backwell and d'Errico, (2001), Swartkrans bone tools are characterized by a single rounded end with a smoothed/polished area ranging from 5 to 50 mm from the tip. At microscopic scale, 5-40 μm wide sub-parallel, overlapping striations oriented along the main axis of the tool cover the smoothed area.

Broader striations are visible, transverse to the main axis of the bone, with a width ranging from 100 to 400 μm. These bones mainly derived from medial portions of long bone shafts from medium (size-class II-III) to large-size (size-class III-IV) class mammals (Brain and Shipman, 1993;[START_REF] Backwell | Additional evidence on the early hominid bone tools from Swartkrans with reference to spatial distribution of lithic and organic artefacts[END_REF]. They have been interpreted as digging sticks used to forage for termites and plant roots, and possibly to process fruit (d'Errico and Backwell, 2009). To date, four South African sites have yielded definitive evidence of early hominin bone tool technology, namely Sterkfontein, Swartkrans, Drimolen and Kromdraai, all situated in the Cradle of Humankind UNESCO World Heritage Site, located northwest of Johannesburg (Figure 1).

Here, we provide a description of the first bone tool from the P. robustus-bearing deposit of Cooper's D. This site is dated between 1.4 and 1.0 Ma (see below), a timespan that fills the chronological gap between Swartkrans Members 2 and 3. This paper also provides descriptions of associated pseudo-tools from Cooper's D, some of which feature a similar rounded tip morphology, but lacking diagnostic traces of utilisation, and a discussion of South African ESA bone tools. [START_REF] Robinson | A Bone Implement from Sterkfontein[END_REF] was a pioneer as he was the first to identify a bone tool from the Cradle of Humankind, from Sterkfontein Member 5 West, Acheulean Infill. In his original paper in 1959, this author proposed that Telanthropus (attributed today to Homo) was the most probable stone and bone toolmaker. The deposit has been dated by paleomagnetism to between 1.3 and 1.1 Ma, and has yielded Homo remains associated with Acheulean stone tool technology [START_REF] Kuman | Stratigraphy, artefact industries and hominid associations for Sterkfontein, Member 5[END_REF][START_REF] Herries | Palaeomagnetic analysis of the Sterkfontein palaeocave deposits: Implications for the age of the hominin fossils and stone tool industries[END_REF]. Subsequent studies provided descriptions of additional bone tools from the site of Swartkrans Members 1 to 3 [START_REF] Brain | New evidence of early hominids, their culture and environment from the Swartkrans cave, South Africa[END_REF][START_REF] Brain | Swartkrans. A Cave's Chronicle of Early Man[END_REF]Brain & Shipman 1993;[START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]d'Errico, 2001, 2003;[START_REF] Backwell | Additional evidence on the early hominid bone tools from Swartkrans with reference to spatial distribution of lithic and organic artefacts[END_REF], and then from Drimolen (Backwell and d'Errico 2008), Krombraai B [START_REF] Stammers | The first bone tools from Kromdraai and stone tools from Drimolen, and the place of bone tools in the South African Earlier Stone Age[END_REF] and possibly the Sterkfontein Name Chamber [START_REF] Val | The macrovertebrate fossil assemblage from the Name Chamber, Sterkfontein: taxonomy, taphonomy and implications for site formation processes[END_REF]. Based on these studies, 102 bone tools have been identified at four South African early hominin sites.

Bone tools from Early Stone Age deposits in South Africa

The bone tool-bearing deposits cover a time span of almost one and a half million years (2.4-0.96 Ma). Drimolen Main Quarry (MNQ) has been recently dated c. 2.04 -1.95 Ma [START_REF] Herries | Contemporaneity of Australopithecus , Paranthropus , and early Homo erectus in South Africa[END_REF]. In the MNQ, the bone tools are associated with both Homo and Paranthropus robustus remains, as well as Oldowan stone artefacts [START_REF] Keyser | Drimolen: a new hominid-bearing site in Gauteng, South Africa[END_REF][START_REF] Moggi-Cecchi | Early hominin dental remains from the Plio-Pleistocene site of Drimolen, South Africa[END_REF][START_REF] Stammers | The first bone tools from Kromdraai and stone tools from Drimolen, and the place of bone tools in the South African Earlier Stone Age[END_REF]. A similar situation occurs for Swartkrans Member 1 [START_REF] Brain | Swartkrans. A Cave's Chronicle of Early Man[END_REF][START_REF] Backwell | Additional evidence on the early hominid bone tools from Swartkrans with reference to spatial distribution of lithic and organic artefacts[END_REF][START_REF] Caruana | Lithic Production Strategies in the Oldowan Assemblages from Sterkfontein Member 5 and Swartkrans Member 1, Gauteng Province, South Africa[END_REF], which is dated between 2.4 and 1.8 Ma [START_REF] Pickering | U-Pb-dated flowstones restrict South African early hominin record to dry climate phases[END_REF]. In Member 2 of Swartkrans, dated c. 1.4 Ma, an Acheulean stone tool industry is found associated with bone tools and both Homo and P. robustus remains [START_REF] Brain | Swartkrans. A Cave's Chronicle of Early Man[END_REF][START_REF] Backwell | Additional evidence on the early hominid bone tools from Swartkrans with reference to spatial distribution of lithic and organic artefacts[END_REF][START_REF] Kuman | The earlier Stone Age in South Africa: site context and the influence of cave studies[END_REF][START_REF] Balter | U-Pb dating of fossil enamel from the Swartkrans Pleistocene hominid site, South Africa[END_REF]. The largest collection of bone tools was found in Swartkrans Member 3, which is dated c. 0.96 Ma, associated with an Acheulean Industry and only P. robustus remains [START_REF] Brain | Swartkrans. A Cave's Chronicle of Early Man[END_REF][START_REF] Backwell | Additional evidence on the early hominid bone tools from Swartkrans with reference to spatial distribution of lithic and organic artefacts[END_REF][START_REF] Kuman | The earlier Stone Age in South Africa: site context and the influence of cave studies[END_REF][START_REF] Gibbon | Cosmogenic nuclide burial dating of hominin-bearing Pleistocene cave deposits at Swartkrans, South Africa[END_REF]. Two bone tools have been described from the 2007; [START_REF] Braga | Stretching the time span of hominin evolution at Kromdraai (Gauteng, South Africa): Recent discoveries[END_REF][START_REF] Stammers | The first bone tools from Kromdraai and stone tools from Drimolen, and the place of bone tools in the South African Earlier Stone Age[END_REF]. The age of this deposit is unresolved, but it is thought to be >2.2 Ma [START_REF] Bruxelles | A revised stratigraphy of Kromdraai[END_REF]. Finally, one bone tool is reported from the newly excavated area of the Sterkfontein Name Chamber [START_REF] Val | The macrovertebrate fossil assemblage from the Name Chamber, Sterkfontein: taxonomy, taphonomy and implications for site formation processes[END_REF], but we still lack photographic evidence supporting its identification, and the age of the deposit is unclear; a mixture of Members 4 (2.95-1.95 Ma; Pickering and Kramers, 2010) and 5 East Oldowan (2.18±0.21 Ma; [START_REF] Granger | New cosmogenic burial ages for Sterkfontein Member 2 Australopithecus and Member 5 Oldowan[END_REF].

Cooper's D site

Cooper's Cave is located in the UNESCO Sterkfontein, Swartkrans, Kromdraai and Environs World Heritage Site in South Africa, at 1.5 km northeast of Sterkfontein, 1 km southwest of Kromdraai, and 45 km northwest of Johannesburg (Figure 1) [START_REF] Berger | Preliminary results of excavations at the newly investigated Coopers D deposit, Gauteng, South Africa[END_REF][START_REF] De Ruiter | New Australopithecus robustus fossils and associated U-Pb dates from Cooper's Cave (Gauteng, South Africa)[END_REF]. These cave deposits occur on dolomite of the Monte Cristo Formation (Malmani Subgroup, Transvaal Supergroup) and yield abundant fossil assemblages in both calcified and decalcified breccias [START_REF] Berger | Preliminary results of excavations at the newly investigated Coopers D deposit, Gauteng, South Africa[END_REF][START_REF] Steininger | A partial skull of Paranthropus robustus from Cooper's Cave, South Africa[END_REF][START_REF] De Ruiter | New Australopithecus robustus fossils and associated U-Pb dates from Cooper's Cave (Gauteng, South Africa)[END_REF].

Excavations at Cooper's between 2001 and 2009, were conducted in decalcified sediments, which has preserved an abundant fossil assemblage of large and small vertebrates (n >50,000), stone tools (n = 49), and seven hominin remains, six of them attributed to Paranthropus robustus [START_REF] Berger | Preliminary results of excavations at the newly investigated Coopers D deposit, Gauteng, South Africa[END_REF][START_REF] Steininger | A partial skull of Paranthropus robustus from Cooper's Cave, South Africa[END_REF][START_REF] De Ruiter | New Australopithecus robustus fossils and associated U-Pb dates from Cooper's Cave (Gauteng, South Africa)[END_REF][START_REF] Sutton | Early Pleistocene stone artefacts from Cooper's cave, South Africa[END_REF]. The first radiometric uranium-lead (U-Pb) dates estimated the age of the basal speleothem to be 1.526±0.088 Ma (de Ruiter et al., 2009). A flowstone layer situated in the middle of the deposit was dated to c. 1.4 Ma. A more recent study based on the resampling of the basal speleothem for U-Pb dating gives an age of the basal speleothem to be 1.375±0.113 Ma [START_REF] Pickering | U-Pb-dated flowstones restrict South African early hominin record to dry climate phases[END_REF]. A minimum age of 1.0 Ma for the deposit is proposed by [START_REF] Hanon | Comportements de subsistance des hominines du Pléistocène inférieur d'Afrique du Sud : analyse archéozoologique du site de Cooper's D (Bloubank Valley)[END_REF], based on biochronological data from the large mammal assemblage. Thus, we assume that the Cooper's D material from decalcified sediments accumulated between c. 1.4 and 1.0 Ma. Two previous taphonomic studies were conducted on large mammal sub-assemblages at Cooper's D [START_REF] De Ruiter | New Australopithecus robustus fossils and associated U-Pb dates from Cooper's Cave (Gauteng, South Africa)[END_REF][START_REF] Val | New taphonomic analysis of large-bodied primate assemblage from Cooper's D, Bloubank Valley, South Africa[END_REF]. The first one focused on a sub-sample of the large mammal assemblage, and suggested that a hyaenidparticularly the brown hyaena (Parahyaena brunnea)was the main accumulating agent of the faunal deposit [START_REF] De Ruiter | New Australopithecus robustus fossils and associated U-Pb dates from Cooper's Cave (Gauteng, South Africa)[END_REF]. The second study relied on the large-bodied primate assemblage to suggest that both leopards and hyaenas were the most probable accumulating agents [START_REF] Val | New taphonomic analysis of large-bodied primate assemblage from Cooper's D, Bloubank Valley, South Africa[END_REF].

These two studies did not report any bone surface modifications consistent with damage produced by hominins through carcass exploitation or the use of bone tools.

The first extensive taphonomic study of the Cooper's D large mammal assemblage, conducted by one of us [START_REF] Hanon | Comportements de subsistance des hominines du Pléistocène inférieur d'Afrique du Sud : analyse archéozoologique du site de Cooper's D (Bloubank Valley)[END_REF], led to the identification of butchery marks and potential bone tools that are the subject of the present study.

Materials and methods

Between 2017 and 2018, we undertook a taphonomic study of the entire large mammal faunal assemblage, composed of 21,193 specimens housed at the Evolutionary Studies Institute, University of Witwatersrand, Johannesburg. For our anatomical and taxonomic identifications, we used the modern osteological collection housed in the same institution. We also compared Cooper's material to the modern collections and fossil assemblages from All specimens from the large Cooper's D faunal collection were observed using an Olympus SZ51 binocular microscope (10-40x magnification). We recorded the following bone modifications: fracture pattern, weathering stage, cortical preservation state, abrasion and polish, manganese coating, decalcification, concretion, compaction, carnivore and rodent tooth marks, trampling and butchery marks, microbial damage and insect modification. These identifications were made based on criteria developed by several authors (e.g. [START_REF] Behrensmeyer | Taphonomic and Ecologic Information from Bone Weathering[END_REF][START_REF] Binford | Nunamiut Ethnoarchaeology[END_REF][START_REF] Brain | Some criteria for the recognition of bone-collecting agencies in African caves[END_REF][START_REF] Brain | The Hunters or the Hunted? An Introduction to African Cave Taphonomy[END_REF][START_REF] Maguire | The Makapansgat limeworks gray breccia: hominids, hyaenas, hystricids or hill wash[END_REF][START_REF] Potts | Cutmarks made by stone tools on bones from Olduvai Gorge, Tanzania[END_REF]Shipman and[START_REF] Shipman | Early hominid hunting, butchering, and carcass-processing behaviors: Approaches to the fossil record[END_REF][START_REF] Behrensmeyer | Trampling as a cause of bone surface dama and pseudo-cutmarks[END_REF][START_REF] Hill | Damage to some fossil bones from Laetoli[END_REF][START_REF] Blumenschine | An experimental model of the timing of hominid and carnivore influence on archaeological bone assemblages[END_REF][START_REF] Blumenschine | Percussion marks on bone surfaces as a new diagnostic of hominid behaviour[END_REF][START_REF] Olsen | Surface modification on bone: Trampling versus butchery[END_REF][START_REF] Fiorillo | An experimental study of trampling[END_REF][START_REF] Cruz-Uribe | Distinguishing Hyena from Hominid Bone Accumulations[END_REF][START_REF] Villa | Breakage patterns of human long bones[END_REF][START_REF] Lyman | Vertebrate Taphonomy[END_REF][START_REF] Blumenschine | Blind Tests of Inter-analyst Correspondence and Accuracy in the Identification of Cut Marks, Percussion Marks, and Carnivore Tooth Marks on Bone Surfaces[END_REF][START_REF] Patou-Mathis | Les marques de boucherie au Paléolithique[END_REF][START_REF] Kaiser | Proposed Fossil Insect Modification to Fossil Mammalian Bone from Plio-Pleistocene Hominid-Bearing Deposits of Laetoli (Northern Tanzania)[END_REF][START_REF] Pickering | Reconsideration of criteria for differentiating faunal assemblages accumulated by hyenas and hominids[END_REF][START_REF] Domínguez-Rodrigo | A new protocol to differentiate trampling marks from butchery cut marks[END_REF], 2010;[START_REF] Kuhn | Examining criteria for identifying and differentiating fossil faunal assemblages accumulated by hyenas and hominins using extant hyenid accumulations[END_REF][START_REF] Backwell | Criteria for identifying bone modification by termites in the fossil record[END_REF][START_REF] Fourvel | Hyénidés modernes et fossiles d'Europe et d'Afrique : taphonomie comparée de leurs assemblages osseux[END_REF][START_REF] Bountalis | Cave Usage by Multiple Taphonomic Agents: Issues towards Interpreting the Fossil Bearing Cave Deposits in South Africa[END_REF][START_REF] Huchet | Approche ichnologique et taphonomique des altérations ostéolytiques dues aux insectes en contexte archéologique[END_REF][START_REF] Parkinson | Traces of Insect Activity at Cooper's D Fossil Site (Cradle of Humankind, South Africa)[END_REF]. The results of this comprehensive taphonomic analysis will be published elsewhere (Hanon et al., in prep.). This led us to identify 12 possible bone tools featuring morphological characters matching, to some extent, those published by [START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]d'Errico (2001, 2004) and [START_REF] Pante | Bone tools from Beds II-IV, Olduvai Gorge, Tanzania, and implications for the origins and evolution of bone technology[END_REF]. These pieces were photographed with a Dino-Lite AD7013MTL digital microscope (20-100x magnification) and an Olympus SZX 16 multifocal microscope coupled to a digital camera (7-115x magnification).

Selected areas of these specimens were moulded with a silicone dental elastomer (Coltène President light body) and analysed with the Tescan Vega 2 LSU scanning electron microscope (SEM) housed at the Muséum national d'Histoire naturelle electron microscopy and microanalysis technical platform, Paris. The resin replicas were not metal coated, and all the images were taken using 15.00 kV, 25 Pa and a view field ranging from 15 to 1 mm with a LVSTD detector. Transparent replicas were made with the MA2+ resin (PRESI, France). These casts were examined in reflected and transmitted light using a motorized Leica Z6 APOA microscope equipped with a DFC420 digital camera and the Leica Application Suite (LAS) software, including the multifocal module (4-40x magnification). This microscope used digital images acquired at variable heights and combined them to obtain a single composite image with an extensive depth of field. As a reference we used descriptions and images of early hominin bone tools from South Africa provided by the literature, and experimental counterparts used in tasks such as digging in different sedimentary environments, digging for termites, plant roots, and fruit processing (Backwell and d'Errico, 2001[START_REF] Backwell | The first use of bone tools: a reappraisal of the evidence from Olduvai Gorge[END_REF], 2008;d'Errico andBackwell, 2003, 2009).

Results

Examination of the 21,193 large mammal bone specimens resulted in the selection of 12 potential bone tools (Table 1). Two bones were identified as fragments of bovid metapodials.

The rest of the specimens are unidentifiable to taxonomic level and skeletal element. Most of the bones belong to size class I and II mammals (n = 9/12, Table 2). This small assemblage is composed of well-preserved specimens with no sign of abrasion (n=8/12), as well as very abraded bone fragments (n=4/12, Table 3). No modification by biological agents was identified. Morphometric data on each specimen are provided in Table 1.

The two limb shaft fragments CD.9977 and CD.3046C (Figure 2) have bevelled edges that appear to be the result of fresh bone fracture. These specimens are characterized by the presence of contiguous micro flake scars along their sides. The sharpness of the edges, however, indicates the flake scars may result from post-depositional processes such as trampling or sediment compression. Six other specimens (CD.343, CD.3528, CD.7900, CD.3529, CD.1649, CD.15631) mimic the general morphology of bone tools found at early Pleistocene hominin sites (Figure 3). However, at microscopic scale, we were not able to identify the diagnostic use-wear pattern associated with the fossil tools, and the smoothing present on them generally extends to the entire bone surface, which is consistent with the action of a natural agent such as water abrasion.

CD.3538 (Figure 3d) is a limb bone shaft fragment with a morphology similar to that of early Pleistocene hominin bone tools. However, no typical use-wear pattern is visible at a microscopic scale, and the preservation of the periosteal surface is more consistent with that resulting from a digestion process. For these reasons we identified this specimen as modified by a carnivore. CD.6978A (Figure 4a) is a fragment of a small bovid metapodial (size class II). One end is bevelled, while the other shows a transverse fracture on dry bone. A crack parallel to the main axis of the bone structure is visible on its periosteal surface. We attribute this crack to a potential post-depositional process rather than to weathering, since no other evidence of exposure is observable on the bone. The specimen has a manganese coating. The general morphology is very similar to that of bone tools identified at Swartkrans, Drimolen and Kromdraai, but we did not observe the use-wear pattern found on the bone tools from these sites (Figure 4b). CD.1293 (Figure 4c) is an indeterminate long bone shaft fragment attributed to a size class II mammal. The piece is bevelled with a slightly smoothed end. Dry breakage is observed at the opposite end as well as stage 1 weathering. However, the absence of microstriations and polish restricted to the area of the tip does not permit us to identify this specimen as a bone tool (Figure 4d).

Only one specimen can be confidently identified as a bone tool. CD.7895 (Figures 567) is a fragment of an indeterminate long bone characterized by a rounded end. The other end has a dry bone fracture. The general morphology of the fragment may correspond to a bovid metapodial. The piece belongs to an animal of size class 2. The use-wear pattern is visible from the tip for 28.9 mm along the edge, medullary and periosteal surfaces of the bone (Figures 567). The microstriae are clearly visible on both multifocal and SEM photographs (Figures 567). They are mostly longitudinal or oblique to the main axis of the long bone and appear, as on well-preserved specimens from Swartkrans and Drimolen, to result from abrasion by individual particles, each following a slightly different trajectory and marking successively the bone surface. Microstriation widths are highly variable, ranging from 25 to 300 μm. However, very few striations with a width >45 μm can be observed. Most of them are curved and subparallel to the main axis of the bone. Longitudinal thin cracks indicate the bone was at a weathering stage 1 when it was used as a tool.

Discussion

During our taphonomic investigation of the large mammal collection from Cooper's D, we were able to identify 12 specimens as bone tools or pseudo-tools. However, after close microscopic examination, only one specimen can be securely identified as a bone tool. The remaining 11 pieces are interpreted as pseudo-tools produced by non-human post-depositional processes, or tools so heavily affected by natural processes that their identification as implements is impossible.

The CD.7895 bone tool from Cooper's D bears features (i.e. rounded and strongly polished end associated with longitudinal or oblique microstriae between 25 to 300 μm wide) identical to those identified on bone tools from the Sterkfontein, Swartkrans, Drimolen and Kromdraai sites (Backwell and d'Errico, 2008;d'Errico and Backwell, 2009;[START_REF] Stammers | The first bone tools from Kromdraai and stone tools from Drimolen, and the place of bone tools in the South African Earlier Stone Age[END_REF]. At Cooper's D, the faunal material is dominated by small to medium size class mammals [START_REF] Hanon | Comportements de subsistance des hominines du Pléistocène inférieur d'Afrique du Sud : analyse archéozoologique du site de Cooper's D (Bloubank Valley)[END_REF] and the only bone tool specimen is attributed to a size-class II mammal. At Sterkfontein and Swartkrans, mammal size class II-III and III-IV dominate the bone tool assemblage, while at Drimolen, the size class II-III is dominant and the mammals over or under this size class are underrepresented. The bone tool from Cooper's D has been made on a bovid metapodial, which is consistent with the trend observed at Swartkrans and Drimolen, where the majority of the bone tools were obtained from long bone shaft fragments [START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]d'Errico, 2003, 2008). Unlike these assemblages, we found no bone tools made from horncores, mandibles or ribs. This is not surprising, however, if one considers that they occur at the two sites in very low proportions.

The wear-pattern observed on the tip of the CD.7895 specimen from Cooper's D is very similar to that recorded on bone tools from Sterkfontein, Swartkrans and Drimolen (Backwell and d'Errico, 2008). Originally, these bone tools were interpreted as digging implements to dig up tubers (Brain and Shipman, 1993). Subsequent quantitative study and comparison between archaeological and experimental specimens lead some authors to suggest that these bone tools were used for termite foraging [START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]d'Errico, 2001, 2008;d'Errico et al., 2001). Subsequently, more detailed analysis of the bone surface texture indicated that even though termite foraging is the most likely task for the Drimolen bone tools, other foraging activities such as fruit processing and extraction of tubers could also be possible (d'Errico and Backwell, 2009). It is difficult, at this stage, to assess the function of the bone tool from Cooper's D, but given the orientation and fine width of the striations we propose termite foraging as most likely.

Bone tools from Swartkrans Member 1 (n = 32), which is dated between 2.249±0.077 -1.706±0.069 Ma, is associated with the Oldowan stone tool industry, P. robustus and H. cf.

erectus [START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]d'Errico, 2001, 2003;[START_REF] Backwell | Additional evidence on the early hominid bone tools from Swartkrans with reference to spatial distribution of lithic and organic artefacts[END_REF][START_REF] Caruana | Lithic Production Strategies in the Oldowan Assemblages from Sterkfontein Member 5 and Swartkrans Member 1, Gauteng Province, South Africa[END_REF][START_REF] Pickering | U-Pb-dated flowstones restrict South African early hominin record to dry climate phases[END_REF]. According to [START_REF] Herries | Contemporaneity of Australopithecus , Paranthropus , and early Homo erectus in South Africa[END_REF] the depositional age of Swartkrans Member 1 remains uncertain, and based on ESR dates and faunal evidence could have occurred somewhere between 2.4 and 1.8 Ma, most likely 1.8 Ma. Based on this assertion, Drimolen MNQ, recently dated between 2.04 -1.95 Ma, could represent the oldest occurrence in southern Africa of bone tools (n = 14), stone tools, Homo and Paranthropus [START_REF] Herries | Contemporaneity of Australopithecus , Paranthropus , and early Homo erectus in South Africa[END_REF]. The bone tools apparently disappear around 0.96 Ma, with the last occurrence in Swartkrans Member 3 (n = 41; [START_REF] Brain | Swartkrans. A Cave's Chronicle of Early Man[END_REF][START_REF] Backwell | Additional evidence on the early hominid bone tools from Swartkrans with reference to spatial distribution of lithic and organic artefacts[END_REF]. This implies that bone tools are known in South Africa from at least 2.4 to 0.96 Ma. During this time, five sites have yielded definitive evidence of bone tool technology (Table 4). The Cooper's D assemblage fills the chronological gap between Swartkrans Members 2 and 3. At Cooper's D, P. robustus is the only hominin identified [START_REF] Steininger | A partial skull of Paranthropus robustus from Cooper's Cave, South Africa[END_REF][START_REF] De Ruiter | New Australopithecus robustus fossils and associated U-Pb dates from Cooper's Cave (Gauteng, South Africa)[END_REF]. According to [START_REF] Sutton | Early Pleistocene stone artefacts from Cooper's cave, South Africa[END_REF], there are 49 stone artefacts from Cooper's D, but this small assemblage does not permit allocating them to a specific industry. Although two bone tools are reported from the Kromdraai B deposit [START_REF] Stammers | The first bone tools from Kromdraai and stone tools from Drimolen, and the place of bone tools in the South African Earlier Stone Age[END_REF], there is an absence of clear radiometric dates. A second bone tool has been reported from Sterkfontein, from the Name Chamber [START_REF] Val | The macrovertebrate fossil assemblage from the Name Chamber, Sterkfontein: taxonomy, taphonomy and implications for site formation processes[END_REF], and while a detailed study of this specimen is lacking, it confirms a bone tool technology at this site.

We agree with [START_REF] Stammers | The first bone tools from Kromdraai and stone tools from Drimolen, and the place of bone tools in the South African Earlier Stone Age[END_REF] that an overall study of these sites shows no clear pattern of associations with bone tools, not with hominins or stone tool industry. Bone tools are associated with Oldowan as well as Acheulian stone tool industries in deposits containing P. robustus and early Homo remains (Table 4). Some authors have suggested a link between the presence of P. robustus and the early bone tools [START_REF] Brain | Swartkrans. A Cave's Chronicle of Early Man[END_REF][START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]d'Errico, 2003, 2008). Indeed, the largest collection of ESA bone tools has been discovered in Swartkrans Member 3, which is rich in Paranthropus remains [START_REF] Brain | Swartkrans. A Cave's Chronicle of Early Man[END_REF]. Drimolen MNQ has also yielded a large number of P. robustus remains associated with a collection of bone tools (Backwell and d'Errico, 2008). Finally, South African bone tools disappear after 0.96 Ma in Swartkrans Member 3, as is the case for both P. robustus and early Homo. We can assume that the most parsimonious hypothesis is that Paranthropus may have been the user of the bone tools, but the presence of Homo complicates the picture (Backwell and d'Errico, 2008;[START_REF] Stammers | The first bone tools from Kromdraai and stone tools from Drimolen, and the place of bone tools in the South African Earlier Stone Age[END_REF][START_REF] Herries | Contemporaneity of Australopithecus , Paranthropus , and early Homo erectus in South Africa[END_REF].

Conclusion

In this study we identify and describe the first bone tool from the Cooper's D faunal assemblage. The general morphology and use-wear pattern observed at the tip of the Cooper's D bone tool are very similar to that observed on bone tools from Sterkfontein, Swartkrans and Drimolen. It has been shown that these tools were probably used to forage for termites and plant roots and to process fruits. Based on the longitudinal orientation of the fine striations at the tip of the specimen we tentatively propose that it was used in termite foraging, but wish to investigate further this issue in the future. The fact that P. robustus is the only hominin identified at Cooper's D supports the hypothesis that P. robustus probably used the bone tool [START_REF] Brain | Swartkrans. A Cave's Chronicle of Early Man[END_REF][START_REF] Backwell | A critical assessment of southern African 'early hominid bone tools[END_REF]d'Errico, 2001, 2003).

  Sterkfontein, Swartkrans and Kromdraai, housed at the Ditsong National Museum of Natural History, Tswane (formerly Pretoria). Potential bone tools identified during the taphonomic study of the Cooper's D assemblage were macro-and microscopically compared with previously identified bone tools from Swartkrans housed at the Ditsong National Museum of Natural History, and those published from Sterkfontein, Drimolen and Kromdraai.

Figure 1 :

 1 Figure 1: Locality of Cooper's Cave and other Early Stone Age-bearing bone tool sites in South Africa with a geological plan of the Cooper's D site (modified after de Ruiter et al., 2009).

Figure 2 :

 2 Figure 2: Photographs of specimens CD.9977 (a) and CD.3046C (b) displaying denticulated micro flake scars along their edges (arrows). Scales = 2 cm.

Figure 3 :

 3 Figure 3: Rounded bone fragments from Cooper's D interpreted as pseudo-tools; CD.1649 (a), CD.7900 (b), CD.15631 (c), CD.3538 (d), CD.3529 (e), CD.3528 (f), CD.343 (g). Scale = 2 cm.

Figure 4 :

 4 Figure 4: Photographs of bone fragments from Cooper's D interpreted as pseudo-tools, CD.6978A (a) and CD.1293 (c) and photographs of their rounded tips taken in transmitted light on resin replicas CD.6978A (b) and CD.1293 (d). Scales = 2 cm (a, c) and 1 mm (b, d).

Figure 5 :

 5 Figure 5: Photographs of the bone tool from Cooper's D (CD.7895) showing the cortical (left) and medullary (right) surfaces. Scale = 2cm.

Figure 6 :

 6 Figure 6: Periosteal surface of the bone tool tip from Cooper's D (CD.7895) showing characteristic longitudinal subparallel, intersecting striations. Photographs taken in transmitted light on resin replicas. Scales = 1 mm.

Figure 7 :

 7 Figure 7: Medullary surface (a) of the bone tool from Cooper's D (CD.7895) and close-up views of the tip (b-c) and right side of the object. Notice the myriad number of individual intersecting lines flattening the bone surface and only sparing concave areas of trabecular bone. Scales = 1 mm.

Acknowledgments

We would like to thank Sylvain Pont who helped us visualizing specimens with the SEM at the MNHN, and Eric Pubert for producing resin casts. We thank Bernhard Zipfel from the University of the Witwatersrand, and Stephany Potze, Mirriam Tawane, Heidi Fourie and Lazarus Kgasi from the Ditsong Museum, Tswane (Pretoria), for access to collections. CMS acknowledges the funding contribution of the Centre of Excellence in Palaeosciences (CoE-Palaeo) and the Palaeontological Scientific Trust (PAST). RH acknowledges the ATM MNHN project "Paranthropus Diet. Régime alimentaire et comportement de subsistance des Paranthropes sud-africains", directed by S. Prat, the UMR 7194/MNHN, the Agence Nationale de la Recherche (ANR, France): project HOMTECH, ANR-17-CE27-0005, directed by S. Prat, for funding and logistical support. FdE acknowledges support from the Research Council of Norway through its Centre's of Excellence funding scheme (SFF Centre for Early Sapiens Behaviour -SapienCE-project number 262618), the Talents programme of the University of Bordeaux Initiative d'Excellence, and the LaScArBx (ANR-10-LABX-52).

LB is grateful for a DSI-NRF Centre of Excellence in Palaeosciences grant (CEOOP2020-1).


Rébecca Dowek

Céline Cagna

Jean Noirot
email: jean.noirot@cea.fr

Isabelle Zacharie-Aubrun

Thierry Blay

Karine Hanifi

Philippe Bienvenu

Ingrid Roure

Martiane Cabié

Myriam Dumont

Determination of the pressure in micrometric bubbles in irradiated nuclear fuels

Keywords:

In oxide nuclear fuels, at high burn-up or during high temperature periods such as ramp tests, out-ofpile heating tests, or any irradiations at high linear heat rates, fission gases can form micrometric or quasi-micrometric bubbles. During nominal operations, these bubbles participate to the pellet swelling, to the decrease of the fuel thermal conductivity and are involved in the mechanisms leading to fission gas release. During events involving a temperature increase, the resulting increase in the internal pressure of the bubbles might play a role in fuel fragmentation and in the opening of grain boundaries. The gas densities inside these bubbles are therefore one of the useful experimental information for the understanding of the fuel behaviour, and for the fuel behaviour code progress and validation. Two methods were developed to evaluate the gas density in the quasi-micrometric bubbles, using electron probe micro analyser, secondary ion mass spectrometry and focused ion beam scanning electron microscope together. The first method provides a mean gas density for all quasi-micrometric bubbles in a given area. The second method provides a gas density in a single selected bubble. In addition to the gas density, the 3D size and shape of the selected bubble is measured and can be related to the gas density result. In this work, these methods were applied to the bubbles formed in the centre of a PWR Cr doped UO 2 at 38.8 GWd/t U after a ramp test in the Osiris reactor, with a 12 h plateau at 470 W/cm, and to the bubbles formed in a PWR Cr doped UO 2 at 62.8 GWd/t U in the centre of the pellet and on the bubbles of the high burn-up structure on the rim. Both show the high pressures reached in these bubbles.

Introduction

During nuclear fuel irradiation, fission reactions lead to a gradual build-up of fission products in the material. About 15% of these products are gaseous species (mainly xenon and krypton). A fraction of these gases are released in the free volumes of the rods [ 1 , 2 ], but most of them remain in the fuel matrix, as isolated atoms or in bubbles. Bubbles can form inside the grains and on the grain boundaries of the fuel, and their sizes may range from nanometric to micrometric scales. Their distribution (size, shape and density) is dependent on the burn-up, the irradiation conditions and the radial position in the pellet [ 1 , 3-5 ]. In Pressurized Water Reactor (PWR) fuels, micrometric bubbles appear in the pellets at high burn-up mainly in the central part [ 6 , 7 ] ( Fig. 1 a), and on the rim [START_REF] Stehle | Performance of oxide nuclear fuel in water-cooled power reactors[END_REF] ( Fig. 1 b), with the formation of the so-called High Burn-up Structure (HBS) [ 4 , 8 , 9 ]. Such micrometric bubbles also form in case of high temperature excursions [START_REF] White | The development of grain-face porosity in irradiated oxide fuel[END_REF] , i.e. irradiations at high linear power in Material Testing Reactors (MTR) ( Fig. 1 c). Such high temperature conditions can also be imposed out-of-pile, during high temperature tests in hot cell facilities.

Bubbles contribute significantly to the fuel's behaviour during nominal operations: they participate to the pellet swelling, to the decrease of the fuel thermal conductivity, and they are involved in the mechanisms leading to fission gas release. During events involving a temperature increase, such as LOCA (Loss Of Coolant Accident) or RIA (Reactivity Insertion Accident), the internal pressure of the bubbles increases accordingly. This can lead to fuel fragmentation or to the opening of grain boundaries [START_REF] Papin | Summary and Interpretation of the CABRI REP-Na Program[END_REF][START_REF] Noirot | Size and radial origin of fragments formed while heating a 83 GWd/tU PWR fuel up to 1200[END_REF][START_REF] Guénot-Delahaie | Simulation of reactivity-initiated accident transients on UO 2 -M5® fuel rods with ALCYONE V1.4 fuel performance code[END_REF] . Consequently, numeric codes used in the modelling of the fuel behaviour include fission gas models [ 2 , 14-17 ]. To provide accurate data for modelling, a thorough characterization of fission gases, particularly of bubbles at different scales, is therefore required. Among all needed parameters, pressure inside bubbles is one important feature but particularly complex to determine, as direct measurement is not possible. Bubble pressure estimation requires coupling of different experimental techniques to determine the molar volume of gases, as well as the use of a proper equation of state for fission gases. Moreover, pressure is supposed to evolve during irradiation and to be bubble size-dependent.

Several studies proposed experimental methodologies in order to measure the mean molar volume of the fission gases in the micrometric bubbles. In all cases, the procedure consisted in the measurement of the amount of fission gases contained in bubbles, and in the measurement of the volume occupied by these bubbles. Only techniques and analysis used to determine these values differed.

In the method used by Noirot et al. [START_REF] Noirot | Detailed characterisations of high burnup structures in oxide fuels[END_REF] , EPMA (Electron Probe Micro-Analyser) measurements were performed to determine the local amount of xenon contained in the bubbles, and the local porosity was determined by image analysis of SEM (Scanning Electron Microscopy) micrographs. This was applied to determine the molar volume of micrometric bubbles formed in the HBS on the rim of irradiated UO 2 pellets, and in plutonium rich agglomerates of mixed oxide fuels (MOX). In this paper, only values for the rim HBS of UO 2 samples are reported. In [START_REF] Noirot | Detailed characterisations of high burnup structures in oxide fuels[END_REF] , SIMS (Secondary Ion Mass Spectrometry) xenon measurements showed that the release was extremely low on the periphery of the irradiated UO 2 samples. The local quantity of fission gases contained in the bubbles was then derived from EPMA measurements, more precisely from the difference between the xenon concentration produced by fission, and the xenon concentration remaining in the fuel matrix as isolated atoms or nanometric bubbles [START_REF] Ronchi | Determination of xenon concentrations in nuclear fuels by electron microprobe analysis[END_REF] . To obtain this gas production, EPMA was used to measure the neodymium concentration, approximately proportional to the local burn-up. Finally, the porosity of the fuel was obtained by the analysis of SEM 2D images. Resulting molar volumes obtained in UO 2 rim are summarized in Table 1 . In this paper, pressure evaluation was performed by using the Van der Waals EOS (Equation Of State) at 650K. However, based on the work of (28), and considering the range of molar volumes, the EOS proposed by Soave [START_REF] Vidal | CESAR: A Code for Nuclear Fuel and Waste Characterisation[END_REF] seems to be more accurate and will be used further in this paper. As a consequence, for the sake of comparison, pressures recalculated according to Soave's EOS are also displayed in Table 1 , for results from the literature, when all data necessary for the calculation are available. The results in [START_REF] Noirot | Detailed characterisations of high burnup structures in oxide fuels[END_REF] demonstrated that rim HBS bubbles were highly pressurized, even with a systematic overestimation of the parameters resulting in a minimized pressure determination.

Walker et al. [START_REF] Walker | SIMS analysis of an UO 2 fuel irradiated at low temperature to 65 MWd/kgHM[END_REF] , carrying out a study on irradiated PWR fuels, used a similar approach with SIMS to confirm the very low release, and EPMA to quantify fission gases in the rim bubbles. They also found a high pressure of 45 MPa at room temperature in HBS UO 2 rim bubbles in a fuel irradiated at low temperatures. This time, the Brearly and MacInnes (B&MI) EOS was used. In Table 1 , the pressure was recalculated at 293 K using the Soave EOS. In addition, the size distribution of HBS bubbles was also studied. The mean pore equivalent circle diameter (ECD), and the diameter class with the highest bubble density are displayed in Table 1 .

Horvath et al. [START_REF] Horvath | Analysis of xenon gas inclusions in nuclear fuel using laser ablation ICP-MS[END_REF] conducted experimental quantification of fission gases, using Laser Ablation Inductively Coupled with Plasma Mass Spectrometry (LA-ICP-MS). This technique was applied on the HBS rim of a very high burn-up PWR fuel. The quantity of gas contained in bubbles was measured by several large laser shots (diameter ~50 μm). The porosity was determined by SEM. The ablated volume was obtained using an optical microscope. Again, a high pressure, similar to the previous values, was found. In addition to this mean value, local analyses were conducted on this sample with smaller laser spots (15 μm) aiming to open only one bubble for each laser pulse. However, quantitative measurement of individual bubbles was not achieved, as the bubble size and bubble density, estimated from the SEM, provided a basis of 2 to 4 bubbles opened for each laser pulse, with a diameter ranging from 2 to 7 μm. Consequently, the range of pressure deduced remained wide, from 0.4 MPa to 35 MPa.

Noirot et al. [ 21 , 22 ] improved the gas quantification process using the SIMS for gas measurements, and not only EPMA. Thereby, the method was extended to cases where fission gases are partially released. Indeed, in such cases, it is not possible to directly deduce the total amount of gas in the fuel from the amount of gas produced during irradiation. It therefore becomes necessary to have a method that measures both the total quantity of gas in the fuel and the quantity of gas as isolated atoms (or nano-bubbles) in the material, which is possible with SIMS calibrated by EPMA. The gas contained in micrometric bubbles is then the difference between these two values. The method is detailed in section 2.2. This improved methodology was used to estimate the mean pressure of gas in micrometric bubbles of a UO2 fuel discs sandwiched between Mo discs, irradiated in the Halden Boiling Water Reactor within the NFIR program [START_REF] Noirot | Fission gas release behaviour of a 103 GWd/tHM fuel disc during a 1200 °C annealing test[END_REF] . Considering the irradiation conditions (very high final burn-up around 103 GWd/t U and low irradiation temperature between 560 °C and 700 °C), these UO 2 discs were comparable to the rim of a standard pellet. This method was also applied to characterize micrometric bubbles localized in the centre and on the rim of a standard UO 2 pellet in [START_REF] Noirot | High burn-up change in UO2 fuels irradiated up to 83 GWd/tUin M5 claddings[END_REF] , and micrometric bubbles of the centre in [START_REF] Cagna | Influence des gaz de fission sur l'état mécanique des combustibles oxydes irradiés[END_REF] .

Results from these articles are summarized in Table 1 .

Considering these results, it appears that the different procedures used to measure the molar volume agree to state that mi- Horvath et al.

[

Noirot et al. [START_REF] Noirot | Fission gas release behaviour of a 103 GWd/tHM fuel disc during a 1200 °C annealing test[END_REF] Noirot et al. [START_REF] Noirot | High burn-up change in UO2 fuels irradiated up to 83 GWd/tUin M5 claddings[END_REF] Cagna [START_REF] Cagna | Influence des gaz de fission sur l'état mécanique des combustibles oxydes irradiés[END_REF] Pellet burn-up (GWd/t crometric bubbles are highly pressurized in the high burn-up irradiated UO 2 pellets. However, data are mainly focused on the rim HBS bubbles, with pressures at room temperature found from 7 to 20 MPa. Few data are available concerning the pellet's centre, with a higher scattering, from 6 MPa to 68 MPa. Moreover, the size distribution of bubbles is known to be wide. As the pressure is expected to be size-dependent, a deeper knowledge on the size effect on pressure as a function of the irradiation conditions is still needed. This article aims to bring new measurements for micrometric bubbles, both in the rim HBS and in the centre of fuels irradiated in PWR, by using a mean molar volume quantification by coupling SIMS, EPMA and SEM. Moreover, a new experimental approach was carried out to determine the gas molar volume of single bubbles. This allows to determine the gas pressure in selected bubbles of known size and then to explore the size effect on pressure inside micrometric bubbles.

Experimental

Samples

The first sample used for this study was a cross section of a large-grain UO 2 fuel rod, with an initial 235 U enrichment of 4.89%. These large grains, with an average size of 64 μm in diameter, were obtained by the addition of chromium (Cr 2 O 3 ) in the UO2 powder before pressing and sintering of the pellets. It was irradiated for two annual cycles in a PWR. The fuel was then ramp tested at W.cm -1 with a 12-hour plateau in the Osiris MTR. The rod did not fail during this ramp test. The burn-up at the sampling position was 38.8 GWd/t U . As the ramp was short, it did not significantly increase this burn-up. This UO 2 sample is referred to as sample A in the following.

The second sample is a cross section of a large-grain UO2 fuel rod with an average grain size around 60 μm, also obtained by the addition of chromium in the UO2. The fuel was irradiated in a PWR. The average burn-up of the rod was 63.8 GWd/t U , but the burn-up at the sampling position was a bit lower, 62.8 GWd/t U . This high burn-up sample irradiated in standard conditions is referred to as sample B . The samples were embedded in a low melting point Sn-Bi alloy under vacuum. Then, their surface was mechanically polished with SiC and then with diamond suspensions down to 0.25 μm grade.

The final step of the polishing was realized with a colloidal suspension of silica with a particle size of 0.02 μm. Fabrication characteristics, burn-up and main features for both samples are summarized in Table 1 .

Techniques

The samples were characterized by FIB-SEM (FIB = Focused Ion Beam), EPMA and SIMS. All devices are operated in shielded hot cells in the CEA LECA-STAR facility at Cadarache (France), and have been modified to allow examination of irradiated fuel samples.

The FIB-SEM was a shielded dual beam microscope Auriga (Carl Zeiss, Oberkochen, Germany). It was used in this work both for surface 2D imaging (section 3.1) and for 3D tomography by serial cuts (section 3.2). Surface imaging is done concomitantly with back scattered electron detector and a secondary electron detector. Images performed with back scattered electron detector were used for image analysis quantitative measurements, the other images had only qualitative information. Beam acceleration voltages are detailed in the course of the text. The tomography is a complex acquisition process. First, a platinum (Pt) layer is grown by FIB induced deposition, to cover the area of interest on the sample surface. An excavation is then made by abrasion of the material using the FIB perpendicular to the sample, with a high voltage. The wall of the excavation is then gradually smoothed. It forms the starting plane for the acquisition. Finally, the actual tomography begins: an alternation between abrasion of a thin layer of material with the FIB and imaging of the newly revealed wall with the SEM. It results in a series of consecutive 2D images, or serial cuts, forming a stack of images. After an alignment step and additional post-processing steps (background correction, noise reduction…), the 3D volume is reconstructed assuming that the pixel observed on a 2D image is representative of the voxel between this 2D image and the following 2D image. The FIB is used with a current 600 pA. The conditions of imaging are detailed gradually in the text. It must be noted that during the tomography, FIB and SEM columns form a 54 °angle. Images of each cut must therefore be numerically corrected after acquisition.

The EPMA was a SX100-R micro-beam from CAMECA. Xenon radial distribution (section 3.1 and 3.2) and maps (section 3.2) were measured for this work. Radial distributions were done by quantitative punctual analyses every 10 μm, and then every 2 μm on the first 100 μm from the edge on a chosen pellet radius. The radial distribution measurements were done at an electron acceleration voltage of 20 kV and an incident beam current of 200 nA. Quantitative results were obtained, as a mass percentage of xenon in the irradiated UO 2 , thanks to prior calibration on a reference standard sample, with the same conditions. For xenon, this reference was a multi-characterized 25 GWd/t U UO 2 , the same as for [START_REF] Boidron | Microprobe measurement calibration of absorbed xenon in a PWR fuel[END_REF] . Regarding the xenon maps, they were composed of 512 × 512 pixels, and covered an area of 20 0 × 20 0 μm 2 for sample A , and 150 × 150 μm 2 for sample B , at 30 kV-200 nA conditions. Each pixel required 30 ms for acquisition.

SIMS measurements were performed with an IMS 6f-R from CAMECA. Xenon can be measured, thanks to its ionization under the primary beam [START_REF] Desgranges | Measurement of xenon in uranium dioxide (UO 2 ) with SIMS[END_REF] . The isotope measured was 132 Xe, as the best compromise between the ratio 132 Xe/Xe, the ability to be collected by the spectrometer, and the relatively low interferences from other species. SIMS was used first to obtain a relative radial xenon distribution. The primary beam was generated from an oxygen ion source (O 2 + ), at an acceleration voltage of 15 kV, and with a current about 60 nA. Further details on the SIMS settings are exposed in [START_REF] Desgranges | Measurement of xenon in uranium dioxide (UO 2 ) with SIMS[END_REF] . The radial distribution was done through punctual fixed measurements, resulting in craters in the material, approximately every 150 μm along the same radius as for EPMA. For each point, 5,0 0 0 acquisitions were done, at a rate of one acquisition every 0.2 s, providing a result in counts detected by the spectrometer. The volume of each crater V c was measured afterwards by profilometry (profilometer STIL, with a step 1 μm). The measured val-ues were about 7,0 0 0 μm 3 in the previously stated conditions. For this work, the volume of each crater was controlled by profilometry (profilometer STIL, with a 1 μm step). The first 150 seconds of a typical SIMS measurement is displayed in Fig. 2 a. For the single bubble measurement, SIMS is used with exactly the same conditions (primary beam, acquisitions and isotope), apart from the acquisition time since the acquisition is stopped as soon as the selected bubble is opened (cf. 3.2).

In the method used in [ 1 , 22 , 23 ] to estimate mean molar volume of gas in micrometric bubbles, EPMA and SIMS are coupled to perform gas quantification. A typical SIMS measurement is displayed on Fig. 2 a. It was evidenced [START_REF] Lamontagne | Detection of gas bubble by SIMS in irradiated nuclear fuel[END_REF] that a SIMS xenon measurement, representative of all the gas remaining in the UO 2 fuel, consists of two components: a baseline, which is associated to 132 Xe present as isolated atoms or in nanometric bubbles, and peaks, corresponding to the opening of one or a few bubbles. The dissociation between these two parts, described in [START_REF] Desgranges | A method for the quantification of total xenon concentration in irradiated nuclear fuel with SIMS and EPMA[END_REF] , gives the local relative quantity of fission gas contained in the micrometric bubbles. EPMA measurements are then used to calibrate the SIMS measurements ( Fig. 2 b). This is done by the superimposition of the different radial profiles, in a zone where precipitation and release are minimal, and considering that the ratio 132 Xe/Xe is almost constant over the pellet radius. EPMA gives a value for the mass concentration of xenon in the fuel. If the EPMA average value in the zone of calibration is noted w Xe , and the average counts intensity measured by the SIMS is noted I, the calibration constant Cin mol per counts, is expressed by:

C = w Xe I • V c,mean . ρ U O 2 M Xe ( 1
)
with V c,mean the mean volume of SIMS craters in this location (μm 3 ), ρ U O 2 the density of the UO 2 sample (g.μm -3 ), and M Xe the xenon molar mass (g.mol -1 ). After such a calibration, SIMS measurements give the quantitative amount of xenon n Xe contained in micrometric bubbles in the fuel, as a function of the radial position (0R refers to the centre of the rod and 1R to the rim). Knowing the production molar ratio r = Xe/Kr thanks to neutron calculation [START_REF] Vidal | CESAR: A Code for Nuclear Fuel and Waste Characterisation[END_REF] , the quantitative amount of fission gases is:

n = n Xe 1 + 1 / r (2)
It should be noted that Xe/Kr ratio and gas isotopies used here are calculated for each sample (cf. Table 2 ) but that their radial changes are neglected. Using SEM porosity p, gas molar volume in micrometric bubbles v m can be estimated for bubbles at

v m = p × V c
/n , with V c the volume of the crater resulting of the SIMS measurement [START_REF] Desgranges | A method for the quantification of total xenon concentration in irradiated nuclear fuel with SIMS and EPMA[END_REF] .

Results

In this section, the mean densities of the two samples were calculated with the quantification of gas combining SIMS and EPMA, and the porosity determined by SEM image analysis. The procedure was applied to a large population of micrometric bubbles, leading to a mean molar volume determination. The approach was then refined to measure the gas density in one single bubble. From the molar volume obtained, pressures at selected temperatures were deduced.

Considering that all the molar volumes obtained were higher than 4.0 × 10 -5 m 3 .mol -1 , the Soave equation of state was used [START_REF] Cagna | Influence des gaz de fission sur l'état mécanique des combustibles oxydes irradiés[END_REF] . The equation is a modification of the Redlich-Kwong equation, both being a two parameters equations, with a more general second thermal member. The equation for the pressure P was:

P = R.T v m -b - a ( T ) v m ( v m + b ) (3)
with R the gas constant = 8,315 J.K -1 .mol -1 , T the temperature (in K), and a (T ) and b the parameters depending on gas nature.

They are formulated as follows:

a ( T ) = 0 . 42747 1 + f 1 - T T crit 2 R 2 . T crit 2 P crit ( 4
)
with f fugacity coefficient (dimensionless), and T crit and P crit respectively the critical temperature (in K) and the critical pressure (in Pa) for the gas being studied, and:

b = 0 . 08664 R. T crit P crit (5)
The a (T ) and b have to be calculated for the gas mixture, considering the ratio α of the sample.

Mean molar volume determination

Fuel after a power ramp -Sample A

The SEM images in Fig. 3 a and b illustrate respectively the centre 0R and the radial position 0.75R of sample A . These images were acquired with a secondary electron detector, with a beam acceleration voltage of 25 kV. In the centre, the large intergranular bubbles were highly interconnected, and formed a network through which part of the fission gases were released. The majority of these bubbles were therefore gas-empty cavities, and were not taken into account when measuring the porosity to establish the mean molar volume in bubbles full of gas. Outside this central area, between 0.75R and 0.9R, very few bubbles were observed, as displayed in Fig. 3 b (where intra-granular porosity is close to 0,2%). This area was used as a calibration reference for the SIMS xenon measurements.

The 2D porosity was determined for sample A thanks to a binarization of SEM images, acquired with a backscattered electron detector at a beam acceleration voltage of 10 kV. Binarization is a process of assigning to each pixel of the image a label 0 or 1, in order to split this image in two areas. Here, the two labels correspond to bubbles for 1, and fuel (or non-bubble) for 0. The discrimination was based only on a simple grey level threshold using the Otsu criterion [START_REF] Otsu | A threshold selection method from gray-level histograms[END_REF] : bubbles were represented by pixels whose grey level is lower than a selected value. Among the bubble label, as explained above, for this sample, only the intra-granular porosity was considered: the grain boundaries were manually drawn and all bubbles on a grain boundary were removed from the measurement. This intra-granular porosity was 2.5% in the fuel centre. Then, a distribution of the surface fraction represented as a function of the bubble ECD was calculated: the class with the highest surface percentage was that of bubbles of ECD 2-2.2 μm, and the biggest class was for ECD 3.4-3.6 μm, which is a small size compared to the observed area of 250 × 190 μm 2 .

SIMS measurements were conducted along a radius of the pellet. As said previously, the calibration was done for the referent area at a relative radial location around 0.8R. In the centre, the total quantity of gas in the bubbles was found to be 3.17 × 10 -12 mol, for 8177 μm 3 of fuel sputtered. Knowing the porosity, the corresponding bubble volume was 204.3 μm 3 .

The molar volume was therefore 6.46 × 10 -5 m 3 .mol -1 , i.e. an atomic volume corresponding to 2.6 times the volume of one Schottky defect/atom ( = 40.9 Å 3 ). In the centre of sample A , the mean pressure of all bubbles larger than about 0.1 μm in diameter was evaluated to 20.5 MPa, for a temperature of 293 K.

High burn-up fuel -Sample B

Fig. 4 illustrates the centre and the rim of sample B , at the radial positions studied in this work. A secondary electron detector and a backscattered electron detector with a beam acceleration voltage of 15 kV were used. Several images of the same area were acquired for each location, with a pixel size from 10 nm to 100 nm. There was a high density of intra-granular bubbles in the central area ( Fig. 4 a andb), and of inter-granular bubbles on the rim of the pellet, with a typical rim HBS ( Fig. 4 c). In addition to these bubbles formed during the irradiation, fabrication pores exist in the fuel. Such pores were already visible on Fig. 3 b, where they constitute the scarce small intra-granular cavities and bigger inter-granular cavities. On Fig. 4 a andc, the two large single objects pointed out by yellow arrows were also fabrication pores. Indeed, almost all the small pores completely disappear during the early stage of the irradiation [START_REF] Freshley | Irradiation-induced densification of UO 2 pellet fuel[END_REF] , but large pores remain. They are unrepresentative of the bubbles formed during irradiation, and are therefore not taken into account for the molar volume measurement. For this, a size threshold was fixed in order to dissociate the two types of objects, knowing the bubbles rarely exceed one micrometre in size: only objects with ECD smaller than 3 μm were considered. Some pores validating this criterion could exist, but they are rare and not distinguishable from bubbles.

For sample B , SEM images at five radial positions were analysed, one at 1R, with HBS, the others in the central area, respectively at 0R, 0.20R, 0.25R and 0.30R. The surface fraction of the bubbles on the rim was 11.2%, and it was between 1.5% at 0.30R and 4.1% at 0R, in the centre. These values are reported in Table 3 . The SIMS was then calibrated for sample B measurements ( Fig. 2 b). The %weight of xenon contained in bubbles (relative to the total mass of the material) for each of these positions are also reported in Table 3 with the gas densities and pressures deduced from these measurements.

Single-bubble -Molar volume

The mean density measurements were applied on microstructures with a wide distribution of bubble sizes, between 0.1 μm and 3 μm. This section is dedicated to the development of a complementary methodology, characterising the density in a single fission gas bubble, first in sample A centre, then in sample B centre and rim. The same set of equipment was used as in the mean density approach described in the previous section. The goal was to detect and to open a single selected bubble to measure its content, and then to measure its volume.

Fuel after power ramp -Sample A

Such a methodology, able to isolate a single bubble and evaluate the gas density in it, was first developed for sample A [ 23 , 32 ]. This methodology is referred to as the "single-bubble" process in the rest of this text. Sample A was chosen to be investigated first, because after such a power ramp, the bubbles in the fuel centre were large and scarcely distributed ( Fig. 3 ). This made the measurement of a single bubble easier.

The characterization required several steps, described in Fig. 5 . Two bubbles were investigated for this sample, both located in the same area in the centre (0R) of the sample, where the temperature was the highest during the ramp test.

Bubble selection. The first step a was the detection and the selection of a gas-filled bubble very close to the polished surface in this area. Its detection was allowed by SEM observations ( Fig. 6 a) with two different electron beam acceleration voltages, and so different material penetration. Here, voltages of respectively 10 kV and 25 kV were used. Step b is a comparison with the EPMA xenon map of the area ( Fig. 6 b), to confirm that the selected bubble appeared in this map as a bright spot, showing that it was full of fission gases.

Gas quantification. After the SIMS calibration (step c), SIMS xenon measurement was done on the selected bubble location for step d, corresponding to the quantification part. For each bubble the sputtering was stopped immediately once a peak was measured ( Fig. 7 ). After background subtraction, the peak area gave the xenon content n Xe of the selected bubble. From this quantity, the total fission gas content n of the bubble was calculated, like for the mean pressure, knowing the xenon/krypton ratio α by equation ( 2) . The quantity of fission gas (xenon + krypton) due to this peak was 1.03 × 10 -13 mol for the first bubble, and 1.62 × 10 -13 mol for the second one.

Bubble control. After the SIMS measurement, a second EPMA map (step e ), confirmed that only the expected bubble was opened and emptied by showing a dark contrast in the place of the original white spot ( Fig. 8 ). It is also possible to verify that no other bubble was affected by the SIMS beam.

Bubble volume. The selected bubble volume evaluations (step f) were done by the use of the FIB-SEM. In fact, in case of complex shapes [START_REF] Noirot | Focused ion beam-scanning electron microscope examination of high burn-up UO 2 in the center of a pellet[END_REF] , SEM image of the polished surface is unable to provide a good estimation of the size and volume of the object. With FIB-SEM, a 3D tomography by FIB serial cuts was realized in the SEM for the selected bubble. This technique was possible, because the SIMS measurement was immediately stopped after the opening of the bubble. Consequently, the remaining cavity volume was almost unaffected by the SIMS beam. The mechanical properties of UO 2 are such that the volume of the cavity did not change significantly when the gas escapes from the bubble.

For this tomography, serial cuts were made with a regular step of 40 nm step. Each image was acquired with a pixel of the same size, in order to obtain an image made of cubic voxels with 40 nm sides. To acquire the images of each cut, the secondary electron detector was used, with a beam acceleration voltage of 10 kV. Some of these aligned images are presented Fig. 9 a, with a yellow arrow pointing out the measured bubble on each image. A Pt layer, deposited on the sample surface after the SIMS measurement and necessary in the SEM tomography process, is visible on each slice and is hatched in blue. On slice 10, where the bubble started, a sharp conical cavity is visible, it is simply a hole made by the FIB-SEM before the measurement. A red arrow indicates the point where the bubble reached the surface of the sample (slide 73). From this 3D tomography, a reconstruction of the bubble was performed, giving access to its volume V bubble and mean size. The Fig. 9 b is a volume rendering of this 3D reconstruction. The bubble shape was close to spherical, the depression visible at the top left is due to the presence of a metallic precipitate, visible on the slices 50-73-90. Such association bubble-precipitate is common.

Results. The molar volume v m was obtained directly by v m = V bubble / n . Once the molar volume was determined, the pressure was estimated using Soave's EOS at 293K. Two bubbles in the centre of sample A were investigated using this single-bubble procedure. All results are reported in Table 4 . The atomic volumes, equivalent to the molar volume, were also calculated, expressed as a number of Schottky defect volumes per gas atoms in the bubble.

High burn-up fuel -Sample B

This technique was then tested on a high burn-up fuel. As seen from the SEM images ( Fig. 4 ), bubbles are much smaller and more densely distributed in this sample than in sample A . Because of that, it was not possible to apply directly the previous procedure.

Table 4

Sample A -Results obtained using the single-bubble process. As demonstrated in [ 23 , 27 ], the problematic step is the gas quantification d: it was not possible to restrict the SIMS measurement to one single bubble. Improvements were therefore added in the process to take into account these additional constraints.

To overcome this difficulty, a new step was added, between the selection of the bubble and the problematic measurement with SIMS: in the FIB-SEM, a platinum layer was deposited around the bubble location, thanks to the ion beam of the FIB. The objective was the protection of the surface surrounding the selected bubble, to prevent the SIMS sputtering around it, and the opening of unwanted other bubbles. The platinum acted as a mask for the SIMS beam, limiting the sputtering area to the central hole, where the bubble was located ( Fig. 10 ). This mask has a thickness of 600 nm.

This modified process was applied to sample B , in the centre and on the rim. In total, four bubbles were characterized: two in the central area, one at 0R and the other at 0.3R, and two in the rim area, at 1R.

Bubble selection

Steps a and b, for bubble selections were the same as previously. Then, the platinum masks were deposited using the FIB equipment of the SEM ( Fig. 10 ). The SIMS sputtering area was set at a diameter of about 50 μm for a 60 nA primary beam current, therefore the outer diameter of the mask was set to 60 μm. The inner hole allowing the SIMS beam to sputter the sample, open the bubble and measure its content, had a diameter of 5 μm.

Gas quantification

After the SIMS calibration (step c, Fig. 2 b), the bubble locations, protected by the platinum mask, were sputtered by SIMS. Only the sputtering time was different, since it was stopped as soon as the opening of the bubble was detected (step d). For each SIMS profile, a unique peak was clearly visible ( Fig. 11 ). The SEM observation of the masks confirmed their integrity: the platinum layer was only superficially sputtered. This validated the platinum deposit technique. The content of fission gas measured for the two bubbles of the centre and the two bubbles of the rim are reported in Table 5 .

Bubble volume

As in the case of sample A , 3D tomographies by FIB-SEM were used to measure the precise volume of each opened bubble (step f). This was particularly necessary because bubbles in the centre in the high burn-up fuels had complex shapes [START_REF] Noirot | Focused ion beam-scanning electron microscope examination of high burn-up UO 2 in the center of a pellet[END_REF] . Therefore, only a 3D volume measurement was relevant.

As previously, a serial cuts of FIB in the materials, a series of 2D images were acquired on the bubble locations. These series of images, after alignment, formed 3D images of the fuel. Each voxel was a cube of 15 nm side. A secondary electron detector is used to acquire the images of each cut, with a beam acceleration voltage of 2 kV. A selection of these 2D images of cuts are presented in Fig. 12 a for the bubble of the centre (0R). On each image, the thin platinum layer deposited as mask is visible (double-hatched in green), as well as a second, thicker, platinum layer, deposited after the SIMS measurement and necessary in the SEM tomogra-phy process (hatched in blue). Furthermore, in Fig. 12 a, a yellow arrow points out the measured bubble on each image. The bubble appears for the first time in slice 220 and ends in slice 332. As expected, the bubble was just below the surface and had a very small opening towards the surface (red vertical arrow on slice 264). 3D reconstruction gave the volume of the bubble ( Fig. 12 b), completing step e. The bubble selected in the centre was quite flat, far from a spherical shape. For indication, its sphericity was 0.11, and its geodesic diameter and diameter of the biggest inscribed sphere were respectively 1.91 μm and 0.36 μm. Its volume was 0.31 μm 3 , corresponding to an equivalent spherical diameter (ESD) of 0.84 μm.

The same process was applied for the bubble on the rim. Fig. 13 shows the 3D reconstruction of the bubble volume. It was closer to a spherical-shape, but the surface was influenced by surrounding small grains. This surrounding resulted in a low sphericity value of 0.25, but its compact shape was evidenced by the bubble geodesic diameter of 1.64 μm and the values of the biggest inscribed sphere of 0.93 μm, two values close to the ESD 1.17 μm. Results. The same method as in 0 was used to calculate the molar volume and the pressure of the fission gases in the four investigated bubbles. The results for the four bubbles at different radial positions are summarized in Table 5 .

Discussion

This work has shown the possibility of implementing a "singlebubble" process, even in the case of high burn-up fuels presenting a high density of small bubbles. In addition to the gas molar volume in selected bubbles, it also gives the volume, size and shape of these bubbles. Also, it allows to focus on bubbles really full of gas. It is therefore a way to have access to parameters possibly linked with the pressure, such as the bubble volume, even with complex shapes. The results obtained by using this method are more precise than the mean pressure evaluations, and complementary to it. The main disadvantage of this single-bubble methodology is that it is a long and difficult process, limiting the number of measurements.

In the case of sample A , the three measurements (mean value and values for the two single bubbles) were consistent, with the same order of magnitude. The mean pressure estimated to 20.5 MPa lied between the two single-bubble pressures, 41.2 and 9.4 MPa respectively. Regarding the single-bubbles themselves, the most pressurized bubble had the smallest size, as expected. However, the evolution of the pressures of the two bubbles did not fol- Concerning the size of the bubbles, it can be compared to the size distribution obtained by 2D SEM images of the sample A surface. At 0R, the most represented class (in terms of percent surface area) was that of the bubbles with an equivalent circular diameter of 2.0-2.2 μm. Note that only the bubble cross-section with the surface was considered, so the actual representative diameter was probably slightly larger. The two single-bubbles were therefore on either side of this majority size class. Consequently, it was expected that the pressure results would reproduce this order, with the mean pressure between the two single-bubble pressures.

Fig. 14 provides a comparison between the mean pressure and the single-bubble pressure results for high burn-up fuels, i.e. sample B, as compared to some data from the literature. Again, for this sample, for the two single-bubbles of the centre (0R and 0.30R), and for the two single-bubbles of the rim (1R), the smallest bubble had a higher pressure than the biggest one. Likewise, for each radial position, the values obtained by the two methods (mean and single-bubble) were similar. The pressure values coming from the literature were also in the same order of magnitude (except for one higher value in the centre for a sample at 72.7 GWd/t U ), although showing some dispersion.

However, it must be noticed that in these cases, the singlebubble pressures were systematically higher than the corresponding mean pressure, whereas the bubbles characterized were among the large bubbles in the local size distributions. Indeed, 2D analyses of the SEM images for sample B , at 0R, 0.30R and 1R, showed that the most represented equivalent circular diameter class, in terms of percent surface, were respectively 0.5-0.6 μm, 0.2-0.3 μm, and 0.8-0.9 μm. Considering this, we would expect that the gas pressures in the measured single-bubbles, with higher diameters, were lower or equal, and not systematically higher, than the mean pressure. This could indicate an overestimation of single-bubble pressure, or, more likely, an underestimation of the mean pressure for high burn-up fuels. This underestimation could be due to the existence of fabrication pores smaller than 3 μm which would bias the resulting pressure, or to an overestimation of to-tal bubble porosity due to polishing effects or to image analysis bias.

The pressure results obtained by the single-bubble methodology for the two samples are reported in Fig. 15 , as a function of the bubble equivalent diameter. For each bubble the pressure was calculated at ambient temperature, but also at the local temperature at the end of the irradiation. Consistently with previous mean results, all these pressures are higher than 2 γ /R, therefore bubbles are over-pressurized, even at room temperature. In addition, the bubbles from sample A , (12h plateau at 470 W.cm -1 ) showed the highest pressures when calculated for the central temperature at the end of the power ramp plateau. The gases in the bubbles were therefore still over-pressurised at the end of the ramp. For sample B , although HBS bubbles were larger than the bubbles in the centre, at 293 K, their pressure can be higher. However, when taking into account the local temperature at the end of the irradiation, the trend is reverse. Even taking into account the local temperatures at the end of the irradiation and considering the case of sample B bubbles only, this limited set of "single-bubble" measurements tends to show that there is no direct link between bubble size and bubble content for all radial positions. This is not necessarily surprising, as the mechanisms inducing the formation of these bubbles are different.

Conclusion

Two methodologies were used to measure fission gas densities in micrometric bubbles in irradiated nuclear fuel samples. These two methods combine three devices in a high activity laboratory: FIB-SEM, EPMA and SIMS.

The first method was used to measure the mean gas density in bubbles as a function of the local radial position in two samples. The results were compared to data from the literature.

The second method, called here the "single-bubble" process was developed in this work. It can be applied to any irradiated nuclear fuel. In this work, it was applied to a PWR UO 2 fuel after a power ramp and a PWR UO 2 high burn-up fuel. It provides values for the fission gas densities inside selected micrometric bubbles. From these gas densities, gas pressures can be obtained using the relevant equation of state. The feasibility of the process on single bubbles was demonstrated, and the pressure was measured for six bubbles distributed in two samples. The results obtained for these two fuels were similar for the two methods, but question a possible underestimation of pressure for the mean gas density method, more likely than an overestimation for the "single-bubble" method. New bubbles in other irradiated samples, exhibiting various bubble concentrations and bubble size and shapes must now be investigated to enrich this, still limited, set of data.

Fig. 1 .

 1 Fig. 1. Images of different microstructures in irradiated nuclear fuels (a) high burn-up centre, (b) high burn-up rim, (c) fuel after power ramp. An example of micrometric bubble is pointed out by an arrow in each image.

Fig. 2 .

 2 Fig. 2. Sample B -132 Xe SIMS measurement of an irradiated nuclear fuel (a) SIMS profile (b) SIMS-EPMA calibration of the sample.

  critical temperature and pressure values are f Xe = 0 . 4926 , T crit, Xe = 289 . 7 K and P crit, Xe = 5 . 84 × 10 6 Pa for xenon, f Kr = 0 . 4879 , T crit,Kr = 209 , 4 K and P crit,Kr = 5 . 50 × 10 6 Pa for krypton.

Fig. 3 .

 3 Fig. 3. Sample A -SEM images (a) at 0R and (b) at 0,75R.

Fig. 4 .

 4 Fig. 4. Sample B -SEM images, in the centre of the pellet, (a) at 0R and (b) at 0.3R, and on the rim HBS of the pellet (c) at 1R. The two fabrication pores, visible respectively in (a) and (c), are pointed out by a yellow arrow.

Fig. 5 .

 5 Fig. 5. Schematic view of the single-bubble process.

Fig. 6 .

 6 Fig. 6. Sample A -(a) SEM image of the centre 0R, and (b) xenon EPMA map of the same area, for the single-bubble process. A yellow circle indicates on each image the location of the selected bubble.

Fig. 7 .

 7 Fig. 7. Sample A -SIMS single-bubble measurement.

Fig. 8 .

 8 Fig. 8. Sample A -Xenon EPMA maps (a) before, (b) after the SIMS measurement.

Fig. 9 .

 9 Fig. 9. Sample A -(a) Selection of 2D serial FIB cuts of the tomography, (b) 3D reconstruction of the opened bubble (bubble 1). Yellow arrows point out the measured bubble, red arrow points out the bubble opening towards sample surface. Blue hatching covers the platinum layer deposited after the SIMS measurement.

  volume (m 3 .mol -1 ) 5.5 × 10 -5 8.7 × 10 -5 Atomic volume of fission gases (Schottky defect volume/atom)

Fig. 10 .

 10 Fig. 10. Sample B -Preparatory steps (a) platinum layer deposit as a mask preventing SIMS sputtering, (b) SEM image of such a platinum deposit on the rim.

Fig. 11 .

 11 Fig. 11. Sample B -SIMS measurements for a bubble of the centre (0R) and for a bubble of the rim (1R).

Fig. 12 .

 12 Fig. 12. Sample B , 0R -(a) Selection of 2D serial FIB cuts, (b) 3D reconstruction of the bubble. Yellow arrows point out the measured bubble, red arrow points out the bubble opening with the sample surface. Green double-hatching indicates the platinum layer deposited as a mask before the SIMS measurement, blue hatching indicates the layer deposited after the SIMS measurement.

Fig. 13 .

 13 Fig. 13. Sample B -3D reconstruction of the bubble on the rim.

Fig. 14 .

 14 Fig. 14. Comparison of the mean pressures and single-bubble results for high burn-up fuels.

Fig. 15 .

 15 Fig. 15. Sample A and sample B -Results of pressure for the single-bubble process, at 293 K and at the approximate temperature at the end of the irradiation (for γ taken as 1 N/m).

Table 1

 1

		Walker et al. (15)
	Bibliographic data -micrometric bubbles in UO 2 samples irradiated in PWR.	References Noirot et al. [4]

Table 2

 2 UO 2 samples A and B -fabrication characteristics ( 235 U enrichment, initial grain size), irradiation characteristics (burn-up, gas release of the rods) and calculations (molar ratio Xe/Kr and temperature at the centre of sample, at the end of the irradiation).

	Sample	235 U enrichment	Initial grain size	Burn-up (GWd/t U )	Gas release (%)	Ratio α= Xe/Kr	Temperature (end of irradiation)
	A ramped	4.89 %	≈64 μm (centre)	38.8 (2 cycles)	7,52%	9.0	1878 °C (centre, ramp)
	B not ramped	4.89 %	≈60 μm (centre)	62.7 (4 cycles)	3,04 %	9.7	840 °C (centre) 380 °C (rim)
	a selected radial position, even if local gas release had occurred:

Table 3

 3 Sample B -Mean pressure in micrometric bubbles for several radial positions.

		0R	0.20R	0.25R	0.30R	1R
	Porosity (% area)	4.1	3.4	2.6	1.5	11.2
	Xenon in micrometric bubbles (%weight)	3.9	5.1	4.3	2.5	11.1
	Molar volume of fission gases (m 3 .mol -1 )	12.1 × 10 -5	8.3 × 10 -5	7.3 × 10 -5	8.4 × 10 -5	12.1 × 10 -5
	Atomic volume (Schottky defect/atom)	4.92	3.36	2.98	3.42	4.92
	Mean pressure at 293 K -Soave (MPa)	7.2	10.1	13.3	9.8	7.2

Table 5

 5 Sample B -Results of density measurements by single-bubble process.

	Bubble	1	2	3	4
	Area	0R	0.30R	1R	1R
	Gas quantity ( × 10 -15 mol)	3.38	3.03	13.81	14.69
	Volume (μm 3 )	0.306	0.215	0.845	1.183
	Equivalent spherical diameter ESD (μm)	0.84	0.74	1.17	1.31
	Molar volume (m 3 .mol -1 )	9.04 × 10 -5	7.08 × 10 -5	6.12 × 10 -5	8.05 × 10 -5
	Atomic volume of fission gases (Schottky defect volume/atom)	3.7	2.9	2.5	3.4
	Pressure at 293 K -Soave (MPa)	8.8	14.7	25.1	9.9

Acknowledgments

The authors are grateful to EDF and Framatome for their support in the studies by providing studied irradiated fuels that they made and used in nuclear power plants.

FIJI [ 33 , 34 ], ilastik [35] and 3Dslicer [36] ( http://www.slicer.org ) have been used in the image processing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.