Regis Braucher
email: braucher@cerege.fr

Alan J Hidy

Matmon Ari

D L Bourles

Georges Aumaitre

Keddadouche Karim

Braucher Régis

Bourlès Didier

; Aumaître Georges

Towards successful cleaning of chert samples for improved 10Be and 26Al measurements

Keywords: Beryllium, Aluminum, Cosmogenic, Silicate, ASTER. 10

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Measurements of in situ-produced 10 Be and 26 Al are routinely used for determining exposure or burial durations and denudation rates. In quartz, these cosmogenic nuclides are mainly produced through spallation reactions on oxygen and silicon. Quartz is presently the mineral of choice because it is ubiquitous, resistant to alteration, and a means of purification is established. Thus, huge efforts have been made to calibrate 10 Be and 26 Al production rates within this mineral (e.g. [START_REF] Nishiizumi | Cosmic ray production rates of 10 Be and 26 Al in quartz from glacially polished rocks[END_REF][START_REF] Evans | Cosmogenic chlorine-36 production in K-feldspar[END_REF][START_REF] Balco | Regional Beryllium-10 production rate calibration for late-glacial northeastern North America[END_REF][START_REF] Putnam | In situ cosmogenic 10 Be production-rate calibration from the Southern Alps, New Zealand[END_REF][START_REF] Kaplan | In-situ cosmogenic 10 Be production rate at Lago Argentino, Patagonia: Implications for late-glacial climate chronology[END_REF][START_REF] Borchers | Geological calibration of spallation production rates in the CRONUS-Earth project[END_REF]). However, for other silicate minerals (feldspars, sanidine, chert), obtaining pure mineral fractions is not as straightforward and requires more attention [START_REF] Zerathe | Dating chert (diagenetic silica) using in-situ produced 10Be: Possible complications revealed through a comparison with 36 Cl applied to coexisting limestone[END_REF][START_REF] Zerathe | Toward the feldspar alternative for cosmogenic 10 Be applications[END_REF]; [START_REF] Kober | In situ cosmogenic 10 Be and 21 Ne in sanidine and in situ cosmogenic 3He in Fe-Ti-oxide minerals[END_REF]). 27 Al), must be accurately and precisely determined before sample dissolution. Beryllium is generally negligible in most silicate minerals and thus the addition of a 9 Be carrier is required. Conversely, aluminum is relatively abundant in most silicate minerals, with 27 Al concentrations often sufficient for AMS measurement without addition of a carrier. Therefore, measurements of 26 Al require the natural 27 Al concentration to be accurately determined. In many laboratories [START_REF] Mifsud | Extraction and purification of quartz in rock using hot phosphoric acid for in situ cosmogenic exposure dating[END_REF][START_REF] Corbett | An approach for optimizing in situ cosmogenic 10 Be sample preparation[END_REF], the cleaning procedure ends when this natural content, measured on small aliquot (1-2 g) is below 100 ppm Al. These previous approaches have been shown to be suitable for well crystallized silica with homogeneous aluminum content. However, for amorphous silica with a potential heterogeneous chemical composition, this procedure may be inappropriate and could result in 26 Al and/or 10 Be concentrations that either do not solely reflect in-situ production or reflect in-situ production from uncharacterized mineral impurities [START_REF] Boaretto | Dating of prehistoric cave sediments and flints using 10 Be and 26 Al in quartz from Tabun Cave, Israel[END_REF][START_REF] Matmon | Complex exposure histories of chert clasts in the late Pleistocene shorelines of Lake Lisan, southern Israel[END_REF][START_REF] Guralnik | 10 Be exposure ages of ancient desert pavements reveal Quaternary evolution of the Dead Sea drainage basin and rift margin tilting[END_REF][START_REF] Zerathe | Dating chert (diagenetic silica) using in-situ produced 10Be: Possible complications revealed through a comparison with 36 Cl applied to coexisting limestone[END_REF]) as evidenced by the assumed "clean" material having anomalously high 27 Al concentrations.

In this paper, ten samples of various siliceous rocks (chert, granite, vein quartz and quartzolite (silicified carbonate rock)) were processed and measured for 10 Be and 26 Al analyses with an adapted chemical protocol based on [START_REF] Zerathe | Dating chert (diagenetic silica) using in-situ produced 10Be: Possible complications revealed through a comparison with 36 Cl applied to coexisting limestone[END_REF]. The purpose of this study is to 1) determine if cosmogenic 10 Be and 26 Al concentrations measured from samples of different lithologies, but collected from the same location with presumably identical exposure histories, yield identical values; and if not, 2) determine the effectiveness of additional cleaning steps at resolving any lithologic concentration differences. To do this, we monitor the 10 Be, 27 Al and 26 Al concentration evolutions as a function of cleaning steps. Additionally, we measure in-situ produced 36 Cl in carbonate-rich samples at one site (ZEB) for an independent exposure age comparison.

Samples (Braucher.kmz here)

Samples were collected in the Negev Desert, southern Israel, are from four locations named ZEB, YEH, HEM, and ARAD (refer to kmz and table 1). These locations were chosen for this experiment because each site featured geomorphic surfaces with long surface exposure histories and provided multiple silica-bearing lithologies (ZEB, HEM, and ARAD were collected from desert pavements, and YEH from eroding bedrock). From each location, samples of different lithology were collected. Considering their short identifications (Short ID, Table 1), samples 1 and 2 are from the ZEB location (chert and quartzolite respectively), samples 3, 4, and 5 are from the YEH location (chert, granite and quartz vein respectively), samples 6 and 7 are from the ARAD location (chert and quartzolite respectively) and samples 8, 9 and 10 are from the HEM location (quartzolites and two distinct types of chert, respectively). For more information on the regional geological context of these samples, one can refer to Matmon et al (2017).

With the exception of the carbonate-rich ZEB samples, all samples were crushed, decarbonated, and processed at CEREGE. These steps were not performed on the ZEB samples because it was possible to measure 36 Cl from the carbonate fraction as well as 10 Be and 26 Al from the chert fraction (Figure 1).

Chemical preparation and cleaning strategy for silica samples

Original samples were crushed and sieved (0.25-.85 mm), then divided into four sub-samples (Bulk; HF; KOH and KOH-bis) (Table 2) which were subjected to different cleaning steps. Following each cleaning step, sample concentrates were rinsed several times in ultra-pure water and dried for 24 hours at 90°C.

The Bulk sub-sample underwent no cleaning steps.

The three other sub-samples were subjected to Aqua Regia to remove the carbonate fraction and purified by selective dissolutions with HCl and H2SiF6. To eliminate the potential atmospheric 10 Be adsorbed to grain surfaces, the remaining dried and weighted samples were cleaned using sequential 48% HF dissolutions [START_REF] Brown | Examination of surface exposure ages of antarctic moraines using insitu produced Be-10 and Al-26[END_REF][START_REF] Kohl | Chemical isolation of quartz for measurement of in-situ -produced cosmogenic nuclides[END_REF][START_REF] Cerling | Geomorphology and in-situ cosmogenic isotopes[END_REF], with each step dissolving ~10% of the material. Sub-sample HF was dissolved and processed without further cleaning.

Sub-samples KOH and KOH-bis were subjected to one and two rounds, respectively, of a 48 hours leaching in 3N KOH at 80°C prior to spiking and complete dissolution. As shown in [START_REF] Verstraete | Approche multi-technique et multi-échelle d'étude des propriétés structurales des matériaux hétérogènes : application à un granulat siliceux naturel[END_REF] and tested in [START_REF] Zerathe | Dating chert (diagenetic silica) using in-situ produced 10Be: Possible complications revealed through a comparison with 36 Cl applied to coexisting limestone[END_REF], the 3N KOH solution gently dissolves amorphous silica, enabling a smooth cleaning of the material and, allowing, if they are present the accumulation of micro-crystallized quartz spherules (see [START_REF] Zerathe | Dating chert (diagenetic silica) using in-situ produced 10Be: Possible complications revealed through a comparison with 36 Cl applied to coexisting limestone[END_REF]).

Samples were dried to quantify sample loss between cleaning steps and enable the calculation of efficiency cleaning by: Reagent efficiency = (Mass dissolved by reagent) / (Total reagent volume). This efficiency unit is in (g sample) / (ml reagent).

After cleaning, all samples were spiked with ~0.1 g of an in-house 9 Be carrier (3025±9 ppm; [START_REF] Merchel | Towards more precise 10 Be and 36 Cl data from measurements at the 10-14 level: influence of sample preparation[END_REF], totally dissolved in an acid cocktail (concentrated HF, 10 mL HNO3, and 3 mL HClO4), and then re-dissolved in 7N HCl. Final solutions were weighed and a 100µl aliquot was collected and diluted in HNO3 2% in order to determine the 27 Al concentration by ICP-OES on a 6500 ICAP from Thermo Scientific. Finally, 10 Be and 26 Al were extracted by ion exchange resins (200 mesh DOWEX® 1X8 and 50WX8), precipitated as hydroxides and converted to oxides by ignition at 700°C. Beryllium oxide was mixed with 325 mesh niobium powder and aluminum oxide with silver powder prior to AMS measurements, which were carried out at ASTER, the 5 MV French AMS national facility at CEREGE [START_REF] Arnold | The French accelerator mass spectrometry facility ASTER: Improved performance and developments[END_REF].

The 10 Be data were calibrated directly against STD11 in house standard ( 10 Be/ 9 Be value of (1.191±0.013) x10 -11 , [START_REF] Braucher | Preparation of ASTER in-house 10 Be/ 9 Be standard solutions[END_REF]. Analytical uncertainties (reported as 1σ) include uncertainties associated with AMS counting statistics, chemical blank measurements and AMS internal error (0.5%).

The measured 26 Al/ 27 Al ratios were calibrated against the ASTER in-house standard SM-Al-11 whose nominal value is 26 Al/ 27 Al = (7.401 ± 0.064) x10 -12 [START_REF] Merchel | First international 26[END_REF]). 10 Be/ 9 Be blank ratio was 3.38x10 -16 (± 37%), and 26 Al/ 27 Al blank ratio was 5.26x10 -15 (±26 %).

Chemical preparation for 36 Cl measurements

The crushed samples (0.250-1 mm fraction) experienced a 4h water-leaching, followed by a 10%-dissolution using HNO3 (2 mol.l -1 ). The resulting material was totally dissolved in HNO3 after addition of ~1.5 mg of a 35 Cl-enriched carrier ( 35 Cl/ 37 Cl ~ 800) allowing natural chlorine determination by isotope AMS dilution. Potential residues were recovered from the solution for further 10 Be and 26 Al preparations. After taking an aliquot for Ca-determination by ICP-OES, 1 ml of an AgNO3 solution (10%) was added to the dissolved sample to precipitate AgCl. To reduce 36 S isobaric interference during AMS measurement of 36 Cl, the AgCl precipitate was re-dissolved using NH4OH, and sulfur was co-precipitated with BaCO3 as BaSO4 by addition of a slightly ammoniac saturated Ba(NO3)2 solution. The final AgCl-target was produced by reprecipitation using HNO3, repeated washing cycles with HNO3 and H2O and drying at 80°C. Chlorine AMS measurements were performed at the ASTER AMS national facility (CEREGE, Aix en Provence, France) [START_REF] Klein | Performance of the HVE 5 MV AMS system at CEREGE using an absorber foil for isobar suppression[END_REF][START_REF] Finkel | Improved 36 Cl performance at the ASTER HVE 5 MV accelerator mass spectrometer national facility[END_REF], and normalized using KNSTD1600 calibration material ( 36 Cl/ 35 Cl=2.112x10 -12 ) [START_REF] Sharma | Development of 36 C1 standards for AMS[END_REF].

Results and discussion

Cl measurements

The 36 Cl, natural 35 Cl and Ca concentrations of carbonate fractions of samples 1 and 2 are presented in sub table 3. As the 35 Cl concentrations are low, no extra isotopic determination were performed on these samples. Long term denudation rates or minimum exposure durations have been determined using a mean production rate of (42.2 ±2) at/g/yr [START_REF] Schimmelpfennig | Sources of in-situ 36 Cl in basaltic rocks. Implications for calibration of production rates[END_REF][START_REF] Braucher | Production of cosmogenic radionuclides at great depths: A multi element approach[END_REF]) and the muon scheme of [START_REF] Braucher | Production of cosmogenic radionuclides at great depths: A multi element approach[END_REF]. These denudation rates and exposure ages are 2.19 ± 0.07 m/Ma and 275 ± 9. ka for sample1 and 0.18 ± 0.01 m/Ma and 1065 ± 37ka for sample 2. These denudation rates and exposure durations are in agreement with previous measurements from the same location [START_REF] Matmon | Controls on aggradation and incision in the NE Negev, Israel, since the middle Pleistocene[END_REF].

10 Be concentrations:

The 10 Be concentrations behave as expected. They indeed decrease as the number of cleaning steps increases until a plateau corresponding to the sole in situ-produced 10 Be concentration is reached, [START_REF] Brown | Examination of surface exposure ages of antarctic moraines using insitu produced Be-10 and Al-26[END_REF]). Thus at this stage, one can say that the proposed cleaning procedure (3HF cleaning steps plus 2 cleanings in 3N KOH) allows obtaining accurate in situ-produced 10 Be concentrations for chert (Figure 2 and Table 3).

As for 10 Be, 27 Al concentration evolutions were determined all along the cleaning process. It is thus possible to monitor the cleaning efficiency of each individual step (Table 4).

Samples 5 (quartz) and 7, 8, (quartzolites) yield the lower efficiencies for both HF and KOH, between 0.28 and 0.32 g/ml for HF and below 0.06 g/ml for KOH. The HF efficiency corresponds to that expected from a stoichiometric reaction of HF (48%) on well crystallized SiO2. However, there is a reduced effect of KOH on the quartzolite.

For other samples both cleaning efficiencies are higher. HF efficiency ranges from 0.38 up to 0.75g/ml and the KOH efficiency ranges from 0.075 up to 0.26 g/ml. A value higher than 0.32g/ml for HF and higher than 0.075 g/ml for subsequent KOH leachings appears to be related to poorly-crystallized or amorphous mineral phases.

Four samples (2, 3, 4 and 10) with high HF efficiencies exhibit a high release of 27 Al after the HF cleaning. This is unexpected since selective dissolution is typically employed to reduce the relative abundance 27 Al with respect to the silica concentrate, and this is in spite of the careful washing that was performed before drying the remaining sample. In fact, such high 27 Al content (several hundred to thousands of ppm!) would be expected in the leachate rather than the remaining silica concentrate These unexpected 27 Al peaks may therefore be linked to a highly inhomogeneous distribution of the elements within the studied samples. To better understand this, it would be interesting in future to perform in situ measurements (such as X-ray fluorescence spectrometry) to localize if they exist, potential 27 Al enriched phases or potential sites that may have the ability do complex cations. Sample 4, described as granite presents a dual behavior with a high efficiency for the 3 HF leachings as observed for chert samples but a low efficiency with KOH solution as observed for pure quartz or quartzite. This sample also exhibit unexpected high 27 Al content.

To conclude, it appears important to monitor the 27 Al content after each cleaning step (Figure 3). If a plateau is reached, as for samples 3, 7, 8 and probably 9, the material is ready for total dissolution and for 26 Al measurement. However, the magnitude of the plateau in 27 Al concentration should also be considered. If it is of the order of several hundred of ppm Al, then it might result in a 26 Al/ 27 Al too low to be measured by AMS.

Al concentration.

Among the 38 prepared samples, no 26 Al was detected in two of them (4-HF and 4-KOH), only one event was counted (100% uncertainty) for two others (4-Bulk and 4-KOH-bis) and 10 of them did not reach the minimum 27 Al -current (20 nA at the high energy faraday cup) for an accurate AMS measurement. For the remaining samples, the mean 27 Al -currents were between 300 and 20 nA. Based on these data, it seems that the chemical composition of the mineral may be the reason of such poor currents. This was also the case in a previous attempt to measure samples 13-IL-ZEB-02C and 13-IL-HEM-17 at LLNL where low currents were also obtained.

Denudation rates and minimum exposure durations.

The main purpose of this paper was to develop a chemical procedure enabling to measure in situproduced cosmogenic nuclides ( 10 Be and 26 Al) within silica samples of various compositions. Although the results concerning aluminum must imperatively be improved, it is important to present the output of these measurements (mainly 10 Be concentrations) in term of geomorphological information such as denudation rates or minimum exposure durations (Table 5).

Basically, denudation rates were determined assuming steady state and minimum exposure ages were calculated assuming no denudation. Production parameters have been scaled using Stone polynomial [START_REF] Stone | Air pressure and cosmogenic isotope production[END_REF]after Lal 1991) and a standard atmosphere. The spallation sea level production rate is the one presented by [START_REF] Borchers | Geological calibration of spallation production rates in the CRONUS-Earth project[END_REF] that is (4.02 ± 0.18) at/g/a and the muon contributions from [START_REF] Braucher | Production of cosmogenic radionuclides at great depths: A multi element approach[END_REF].

As the bulk fractions were not clean, their results will not be discussed.

Denudation rate and minimum exposure duration have been determined for the ZEB location from 10 Be and 36 Cl concentrations. Both nuclides yield to similar results. Sample 1 yields exposure ages of 256 ± 25 ka and 275 ± 9 ka for 10 Be and 36 Cl respectively in total agreement with the ages of 241±26 ka and 248±27ka determined by [START_REF] Matmon | Controls on aggradation and incision in the NE Negev, Israel, since the middle Pleistocene[END_REF] using 10 Be only.

Considering sample 2, despite a constant 10 Be concentration on the order of 7.5 ± 0.2 Mat/g it seems that this sample is saturated. For this sample, Matmon et al. deduced exposure ages of 1671 ± 267 ka and 1950 ± 338 ka for the coarse and fine fractions from 10 Be measurements. For 36 Cl the carbonate fraction allows determining an age of 1065 ± 37 ka.

For the YEH samples, the chert sample number 3 appears to be more resistant to denudation than the neighboring granite (Sample 4) or quartz vein (sample 5). This surface experienced denudation rates ranging from 7 to 17 m/Ma and is exposed at least since 40 ka.

For the ARAD samples composed of a chert (sample 6) and a quartzite (sample 7), the outputs of both samples are comparable and yield to a denudation rate lower than 1.5 m/Ma and a minimum exposure duration of 350 ka. Again, chert samples seem to be more resistant than the co-existing quartzite.

The last group, HEM, presents similar results for the quartzite sample 8 but a slightly lower denudation rate for the 2 others chert samples (9 and 10). Here again, chert fraction appears to be more resistant than quartzite. Based on the two chert sub samples (9 and 10), the minimum exposure duration of the surface is of the order of 1.1 Ma.

Conclusions

In this paper, a cleaning procedure derived from that used for quartz mineral, but adapted for other silica minerals, is presented. The main outputs are:

-It is important to monitor the leaching efficiency throughout the cleaning procedure. A value closed to 0.3 g of material dissolved per milliliter of HF indicates "well" crystallized quartz. A higher value may indicate the presence of an amorphous phase. -In case of amorphous phase, the use of a 3N KOH solution at 80°C for 2 days may help to gently dissolve the amorphous phase. This step has to be done at least twice. -It is important to measure 27 Al content after each cleaning step in KOH to ensure that a concentration plateau is reached. In this study, 2 KOH leachings were not sufficient for all samples. -Based on 10 Be only, chert samples seem to be more resistant than quartz to denudation processes in desert environment.

As described in [START_REF] Zerathe | Dating chert (diagenetic silica) using in-situ produced 10Be: Possible complications revealed through a comparison with 36 Cl applied to coexisting limestone[END_REF], thin sections of the chert samples may help to better characterize the chert structure. In some chert types, micro crystalline quartz can be present. If it is the case, the cleaning procedure described in this paper will then allow the accumulation of this micro crystalline quartz throughout all cleaning steps.

Table 1: Sample identification (as named when sampled and simplified short identification).

Table 2: Presentation of the different cleaning steps. The names (Bulk, HF, KOH and KOH-bis) refer to cleaning sequences (see text); "Yes" in a cell means that the cleaning step indicated in the first column has been performed.

Table 3: Tables 3: Sample analytical data and cosmogenic nuclide concentrations. (1-KOH and 1 KOH-bis not prepared because of lack of sample)

Table 4 : Leaching efficiency; This "efficiency" represents, for a given cleaning step ( HF, KOH) the mass dissolved divided by the volume of solution used; (In case of HF, the volume is the total volume used for the 3 sequential leachings)

Table 5: Denudation rates and minimum exposure ages for the studied samples.

Figure 1 :

 1 Figure 1: Photography of Sample 1 (13-IL-ZEB-01). Limestone (light brown) has been used to determine the 36 Cl concentrations and the chert fraction (dark brown) for the determination of 10 Be and 26 Al concentrations.

Figure 2 :

 2 Figure 2: Evolution of 10 Be concentrations as a function of the cleaning process steps.

Figure 3 :

 3 Figure 3: Evolution of 27 Al concentration as a function of the cleaning process steps.

Table 1 :

 1 Sample identification (as named when sampled and simplified short identification). Eocene Chert transported from east of the Dead Sea rift to lts current location by Miocene drainage systems. * Senonian chert : chert clasts derived from local bedrock outcrops

	Sample ID	Short ID Latitude Longitude Alt.(m)	Type
	13-IL-ZEB-01	1	31.020	35.285	83	"Zebra surface" Limestone clast with chert inside; chert band from ~2 cm depth; coarse fraction
	13-IL-ZEB-02	2	31.020	35.285	83	"Zebra surface" Quartzolite sample; at surface and homogeneous; coarse fraction
	13-IL-YEH-09	3	29.561	34.892	357	"Wadi Yehoshafhat" fractured chert amalgam (50-60 clasts)
	13-IL-YEH-10	4	29.561	34.892	359	"Wadi Yehoshafhat" fractured granite amalgam (~80 clasts)
	13-IL-YEH-11	5	29.561	34.892	359	"Wadi Yehoshafhat" quartz vein in Precambrian granite bench
	13-IL-ARAD-14	6	31.227	35.210	637	"Mount Kina" near Arad; ~40 clast chert amalgam from gently sloping pads near hill crest
	13-IL-ARAD-15	7	31.227	35.210	637	"Mount Kina" near Arad; ~40 clast quartzalite amalgam from gently sloping pads near hill crest
	13-IL-HEM-17	8	31.132	35.170	434	"Hemar Formation" desert pavement atop mesa; quartzalite pebble amalgam (40-50)
	13-IL-HEM-18	9	31.132	35.170	434	"Hemar Formation" desert pavement atop mesa; Imported Chert * pebble amalgam (40-50)
	13-IL-HEM-19	10	31.132	35.170	434	"Hemar Formation" desert pavement atop mesa; senonian chert ** pebble amalgam (40-50)
	* Imported chert:

*

Table 2 :

 2 Presentation of the different cleaning steps. The names (Bulk, HF, KOH and KOH-bis) refer to cleaning sequences (see text); "Yes" in a cell means that the cleaning step indicated in the first column has been performed.

	Cleaning steps

Table 4 :

 4 Leaching efficiency; This "efficiency" represents, for a given cleaning step ( HF, KOH) the total mass dissolved divided by the volume of solution used; (In case of HF, the volume is the sum of the three HF volumes used to perform the three sequential leachings)

	Short ID	Sample type	Leaching efficiency (g/ml)
			HF	KOH KOH-bis
	1-HF	Chert within carbonate matrix	0.51	-	-
	2-HF		0.65	-	-
	2-KOH	Quartzolite/dispersed chert and carbonate	0.59	0.05	-
	2-KOH-bis		0.75	0.02	0.001
	3-HF		0.52	-	-
	3-KOH	Amalgamated pebbles (chert)	0.55	0.08	-
	3-KOH-bis		0.55	0.07	0.075
	4-HF		0.52	-	-
	4-KOH	Amalgamated pebbles (granite)	0.54	0.01	-
	4-KOH-bis		0.51	0.04	0.004
	5-HF		0.32	-	-
	5-KOH	Amalgamated pebbles (quartz vein)	0.32	0.05	-
	5-KOH-bis		0.29	0.06	0.003
	6-HF		0.38	-	-
	6-KOH	Amalgamated pebbles (chert)	0.42	0.22	-
	6-KOH-bis		0.42	0.17	0.087
	7-HF		0.29	-	-
	7-KOH	Amalgamated pebbles (quartzite)	0.28	0.01	-
	7-KOH-bis		0.28	0.01	0.005
	8-HF		0.29	-	-
	8-KOH	Amalgamated pebbles (quartzite)	0.28	0.03	-
	8-KOH-bis		0.30	0.01	0.004
	9-HF		0.46	-	-
	9-KOH	Amalgamated pebbles (chert)	0.38	0.17	-
	9-KOH-bis		0.39	0.16	0.103
	10-HF		0.34	-	-
	10-KOH	Amalgamated pebbles (chert)	0.51	0.16	-
	10-KOH-bis		0.36	0.26	0.112

Table 5 :

 5 Denudation rates and minimum exposure ages for the studied samples.

		Location name	10 Be Denudation rates Exposure ages	36 Cl Denudation rates Exposure ages
	Short ID		m/Ma	ka	m/Ma	ka
	1-Bulk 1-HF		0.72 ± 0.02 2.15 ± 0.07	638 ± 69 256 ± 25	2.19 ± 0.07	275 ± 9
	2-Bulk 2-HF 2-KOH	ZEB	n.a. 0 0	Saturated Saturated Saturated	0.18 ± 0.01	1065 ± 37
	2-KOH-bis		0	Saturated
	3-Bulk		2.08 ± 0.06	267 ± 26
	3-HF		8.41 ± 0.3	75 ± 7
	3-KOH		7.3 ± 0.35	86 ± 9
	3-KOH-bis		7.11 ± 0.35	88 ± 9
	4-Bulk		0.79 ± 0.03	606 ± 67
	4-HF 4-KOH	YEH	9.66 ± 0.82 14.59 ± 0.8	66 ± 8 45 ± 5
	4-KOH bis		12.75 ± 1.08	51 ± 6
	5-Bulk		1.28 ± 0.04	406 ± 42
	5-HF		16.01 ± 1.28	41 ± 5
	5-KOH		13.55 ± 1.53	48 ± 7
	5-KOH-bis		9.31 ± 0.5	69 ± 7
	6-Bulk		n.a.	Saturated
	6-HF		0.92 ± 0.03	536 ± 56
	6-KOH		0.79 ± 0.02	610 ± 65
	6-KOH-bis	ARAD	0.46 ± 0.03	927 ± 134
	7-Bulk		1.28 ± 0.03	407 ± 40
	7-HF		1.42 ± 0.05	371 ± 37
	7-KOH		1.38 ± 0.04	382 ± 38
	7-KOH-bis		1.45 ± 0.05	365 ± 37
	8-Bulk		0.04 ± 0.00	3090 ± 646
	8-HF		1.15 ± 0.04	444 ± 45
	8-KOH		1.17 ± 0.04	439 ± 45
	8-KOH-bis		1.11 ± 0.04	457 ± 47
	9-Bulk		n.a.	Saturated
	9-HF 9-KOH	HEM	0.63 ± 0.02 0.4 ± 0.01	722 ± 79 1018 ± 122
	9-KOH-bis		0.45 ± 0.01	935 ± 108
	10-Bulk		n.a.	Saturated
	10-HF		0.35 ± 0.01	1119 ± 134
	10-KOH		0.35 ± 0.01	1111 ± 135
	10-KOH-bis		0.29 ± 0.01	1259 ± 159

Acknowledgements

ASTER AMS national facility (CEREGE, Aix en Provence) is supported by the INSU/CNRS, the ANR through the "Projets thématiques d'excellence" program for the "Equipements d'excellence" ASTER-CEREGE action and IRD. RB thanks Laetitia Leanni and Valery Guillou for fruitful discussions. A component of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This is LLNL-JRNL-745527. The authors thank S. Zerathe for thoughtful suggestions that improved the manuscript.


Mohan S Chandra
email: cyrielle.rey@umonpellier.frxavier.deschanels@cea.fr

Conversion of actinide nitrate surrogates into oxide using combustion synthesis process: A facile approach

Keywords:

In recent years, the focus of research in the conversion routes for the actinide nitrates into oxides for the generation IV nuclear fuel cycle have attracted rapidly growing interest. One process is the self-sustained exothermic reaction of Solution Combustion Synthesis (SCS) methodology. It was applied here to the reaction of an actinide surrogate Gd(NO3)3,6H2O with glycine in air to produce ~10 -3 mol amounts of Gd2O3. The primary process parameters were the Gd/glycine ratio and heating rate, which affected the ignition temperature of the precursor mixture. An optimum ratio of one and a heating rate of 10 K.min -1 resulted in nearly complete SCS conversion over the 483-573 K temperature range and ultrafine gadolinia powders. The 100-to 500-fold swelling of the reactant mixture plays a dynamic role in the reactivity of the system. Final products were characterized by X-Rays Diffraction (XRD), surface area analysis, carbon content, and Scanning Electron Microscopy (SEM). The structural characteristics of the final gadolinia phases have been interpreted by the adiabatic flame temperature calculation.

Introduction

The recycling of actinides, in particular uranium and plutonium, resulting from the treatment of spent nuclear fuel, is of great interest for the deployment of Generation IV reactors (GEN IV) [START_REF]A technology roadmap for generation IV nuclear energy systems[END_REF][START_REF]GEN IV International Forum. Technology roadmap update for generation IV nuclear energy systems[END_REF]. This recycling process involves a step of conversion of the actinides obtained in the form of nitrate in solution into an oxide form or more precisely a solid solution of mixed oxides (MOX). The conversion path should facilitate the formation of homogeneously mixed oxide compounds, containing different amounts of Pu (GEN III / GEN IV transition), in a continuous process. The resulting powders of mixed oxides must be adapted to a subsequent shaping of the fuel without any grinding step or sieving of the powders, in order to reduce the generation of dust.

In this direction, new routes have been experimented for the effective conversion of nitrates of actinides into oxides. Among the various routes, Solution Combustion Synthesis (SCS) is a simple and convenient path.

Moreover, this is a self-sustaining exothermic redox reaction process between an oxidant (metal nitrate) and a reducing agent (organic fuel) mixed in an aqueous solution at the molecular level [START_REF] Merzhanov | Self-propagating high-temperature synthesis of refractory compounds[END_REF][START_REF] Merzhanov | SHS-process: combustion theory and practice[END_REF][START_REF] Patil | Advanced ceramics: Combustion synthesis and properties[END_REF][START_REF] Patil | A combustion process for the instant synthesis of γ-iron oxide[END_REF][START_REF] Patil | Combustion synthesis: an update[END_REF]. Under the thermal treatment of aqueous precursor, a self-sustaining reaction will occur when it reaches the ignition temperature.

During this thermal treatment, the water is evaporated which leads to the conversion of the aqueous mixture into a gel (~ 373 K), and then, the gel is allowed to pyrolysis at a temperature of about 573 K. These two-step processes generate a large amount of gas, which makes the final product in porous and finely dispersed form [START_REF] Manukyan | Solution Combustion Synthesis of Nano-Crystalline Metallic Materials: Mechanistic Studies[END_REF]. These peculiarities of the SCS reactions seem interesting as regards to the synthesis of the solid solution of nanoscale actinide oxides [START_REF] Mukasyan | Solution combustion synthesis of nanomaterials[END_REF][START_REF] Kumar | Bulk synthesis of nanocrystalline urania powders by citrate gel-combustion method[END_REF][START_REF] Kumar | Studies on the synthesis of nanocrystalline Y2O3 and ThO2 through volume combustion and their sintering[END_REF][START_REF] Kumar | A novel gel combustion procedure for the preparation of foam and porous pellets of UO2[END_REF][START_REF] Kumar | reaction mechanism and kinetic analysis of citrate gel-combustion synthesis of nanocrystalline Urania[END_REF][START_REF] Anthonysamy | Combustion synthesis of urania-thoria solid solutions[END_REF][START_REF] Naik | Synthesis and characterization of nano-structured Th1-xCexO2 mixed oxide[END_REF][START_REF] Peter-Soldani | Approche structurale et phénoménologique de la conversion directe ou modifiée de nitrates d'actinide(s) en oxyde[END_REF][START_REF] Peter-Soldani | Process for preparing a powder comprising a solid solution of uranium dioxide and of a dioxide of at least one other actinide and/or lanthanide element[END_REF]. However, the various controlling processing parameters plays a vital role to obtain the good characteristics for the finished products, but it is not well documented. In addition, the mechanism involved in the SCS reaction is not fully understood. The mechanism of SCS reaction mainly depends on the techniques used for the conversion reaction. Two modes can be distinguished: volume combustion, also known as a thermal explosion, and self-propagating combustion mode [START_REF] Varma | Combustion synthesis of advanced materials: Fundamentals and applications[END_REF][START_REF] Varma | Solution Combustion Synthesis of Nanoscale Materials[END_REF]. In the first mode, the entire volume of the precursor is uniformly heated to the ignition temperature and the temperature increases to a maximum value. In the second case, the precursor is heated locally to trigger the exothermic reaction, which propagates automatically over the entire volume of the precursor in the form of a combustion front. In this, the maximum temperature reached is lower than that observed in "volume combustion" mode [START_REF] Varma | Combustion synthesis of advanced materials: Fundamentals and applications[END_REF].

Several equilibria such as thermal, chemical and structural are involved in a SCS reaction, and it depends on the kinetics of the reactions. The kinetics of the reaction could be strongly influenced by the various processing parameters. It is well known that the heating/cooling rate affects the SCS reaction leading to metastable phases [START_REF] Peter-Soldani | Process for preparing a powder comprising a solid solution of uranium dioxide and of a dioxide of at least one other actinide and/or lanthanide element[END_REF]. The heating rate can also act on the generation of the gaseous compounds produced during the process, which can induce swelling of the viscous gel obtained by the dehydration of the precursors and it indirectly results in a modification of the reactivity. The fuel to oxidizer molar ratio, also called richness (see SI-1 for definition of ), is another parameter which can affect the kinetic and the thermodynamic of the SCS reaction. For example, the adiabatic temperature reached during the SCS reaction is dependent on this parameter, and consequently it changes the characteristic of the powders synthesized.

In this present study, we studied the specific parameters such as swelling of the gel, temperature rate that provide a new and unique information for the SCS process. The experiments were carried out with gadolinium nitrate and glycine. The oxidation-reduction rate might be important in the conversion of actinides. So, gadolinium has been considered as a surrogate for trivalent actinides (Cm, Am). Syntheses were carried out in a tubular furnace to perform a homogeneous volume heating of the sample and to control the thermal treatment. To gain better insights, modifications in the controlling parameters (heating rate, richness) were applied in the combustion process. The precursor was characterized by infrared spectroscopy (IR), thermal analysis coupled with mass spectrometry (TG-MS), the final products were analysed by X-ray diffraction (XRD), elemental analyses, scanning electron microscopy (SEM) and nitrogen adsorption-desorption isotherms (BET).

Experimental procedure

Gadolinium (III) nitrate hexahydrate (purity: 99.9%), Glycine anhydrous (purity: 99.5%) was purchased from Sigma Aldrich. Gd/Glycine was prepared by dissolving under stirring 2.10 -3 mole of gadolinium nitrate hexahydrate in 3 mL of purified water, and an appropriate quantity of glycine to obtain the targeted richness  (0.3

< < 2.2).
Syntheses were performed in a tubular furnace (Nabertherm, Controller P320) under static air atmosphere. The heat treatment consists of rapid heating, up to 393 K, followed by a temperature plateau (dwell time 15 minutes) to dehydrate it and to obtain a gel called "dried gel" containing a mass of about 65-490 mg of Glycine and 550 mg of gadolinium nitrate. The resulting gel is then heated to 573 K with a variable heating rate (1 -10 K.min -1 ). The ignition of the SCS reaction was observed visually for a temperature in the range of 458 K and 513 K depending on the mixture chemical composition. After this treatment, a white powder was obtained.

IR spectra were recorded by using a spectrometer (Perkin-Elmer IR-ATR, spectrum 100) with a typical resolution of 8 cm -1 in the range of 300-1800 cm -1 . The Gd/Glycine gel was analysed after dehydration of the solution on a hot plate at 398 K and powders obtained after syntheses on tubular furnace were pressed on the lens thanks to a poppet.

The volume increase of the gel was measured in operando by direct observation of a graduated test tube (volume equal to 25 mL or 50 mL) put in the furnace. The initial volume of the dried gel is about 3 ml. A thermometer put on the test tube checked the temperature of the mixture.

ThermoGravimetric and Differential Thermal Analyses coupled with Mass Spectrometry (TGA-DTA-MS) analyses were performed in air atmosphere on a sample of about 3 mg obtained after dehydration on a hot plate at 393 K of 7.5 µL of the Gd/Glycine mixture solution. These analyses were performed in a Setaram Setsys Evolution 16 apparatus coupled to a mass spectrometer Hiden QGA300 for gas analysis. X-Ray Diffraction patterns were obtained by a Bruker D8 diffractometer equipped with a Lynxeye detector and using Cu K radiation (λ = 1.54184 Å). XRD patterns were recorded at room temperature in the 10° ≤ 2θ ≤ 60° range with a time of analysis equal to 2'47'', and Debye-Scherrer calculation has been used to determine average crystallites size.

Nitrogen adsorption/desorption analysis (77 K) was carried out with a Micromeritics Tristar 3020 instrument.

Before each measurement, samples were outgassed at 398 K for 24 hours. The specific surface area was calculated by using the classic BET equation (Brunauer, Emmett, Teller).

Scanning electron microscope (SEM) observations were directly conducted on powder samples without prior preparation, using a FEI Quanta 200 electronic microscope, equipped either with a Back-Scattered Electron Detector, in high vacuum conditions with a low accelerating voltage equal to 2 kV. These conditions were chosen in order to induce a beam deceleration effect that led to high resolution images.

Finally, the amount of carbon in the powder was evaluated by the means of a LECO CS230 analyser using the complete combustion of the samples in a large excess of O2. Added Fe powder was used to assist the combustion.

The IR detection of the CO2 formed was used to detect the carbon content. In order to get quantitative values, a blank and a series of standards (steel containing wt% of carbon) were analysed before the samples.

Results

1) Comparison of SCS / thermal conversion of Gd(NO3)3

The comparison of TGA curves (Fig. 1a) of Gd/Glycine dried gel (= 1) and gadolinium nitrate shows a clear difference in the evolution of the mass losses according to the temperature. Concurrently with these mass losses, DTA analyses show a small endothermic peak at 673 K for the former and a broad exothermic peak at 483 K for the latter. These results confirmed that the ignition has occurred by the SCS reaction of the Gd/Glycine dried gel according to reaction R1 (Table 1). The TGA analysis of glycine presented in SI-2 is identical to that presented in [START_REF] Varma | Solution Combustion Synthesis of Nanoscale Materials[END_REF]. Table 1 Equations and mass losses associated to thermal events observed during the TG analyses of Gd/Glycine (= 1) dried gel and of gadolinium nitrate hexahydrate. mi and mf : mass before and after the reaction. The thermal treatment was done in air under a heating rate of 10 K / min. (*) Reactions described in reference [START_REF] Melnikov | Computerized modeling of intermediate compounds formed during thermal decomposition of gadolinium nitrate hydrate[END_REF] Reaction More precisely, TGA curve of the gel (Gd/Glycine) displayed a mass loss of ~4% between 298 and 483 K caused by evaporation of water, a quick mass loss of ~ 52% at 483 K was caused by the ignition of the gel, and a total mass loss of ~ 62% at 973 K, is in agreement to theoretical mass loss 62.5% which is calculated on the basis of the SCS reaction of the dried gel (Table 1). At the end of ignition step, a white powder was formed, and regular decrease of mass of the SCS product was observed beyond 573 K, which was due to an elimination of organic residues as attested by carbon analyse showing a decrease of the carbon content from 0.5 wt% to 0.01 wt% respectively for powders heated at 573 K and 973 K.

The thermal decomposition of gadolinium nitrate hexahydrate reported by Melnikov [START_REF] Melnikov | Computerized modeling of intermediate compounds formed during thermal decomposition of gadolinium nitrate hydrate[END_REF] in nitrogen atmosphere, presents 5 endothermic steps (from R2 to R6). Although the TGA analyses were carried out under different atmospheres, the results are agreed with the report (Fig. 1a). All mass losses and peaks were assigned to the formation of oxynitrate as described in table 1.

MS curves (Fig. 1b) confirm the release of CO2 and H2O, during the ignition step of the SCS reaction, in agreement with reaction R1. N2 is used as gas vector during the analysis that is why it cannot be detected as a product of the reaction R1. In addition, the release of NO2 gas corresponds to a secondary reaction. This gas is also detected during the thermal decomposition of gadolinium nitrate hexahydrate in agreement with the reactions R3, R4, R5, R6.

Final products obtained after the TGA-DTA analyses were analysed by XRD (SI-3). XRD patterns show the formation of a cubic gadolinium oxide after a thermal treatment up to 973 K of gadolinium nitrate alone, whereas the products of the SCS reaction from synthesis in tubular furnace is a monoclinic gadolinium oxide obtained at 483 K (SI-2). This difference, which is a consequence of the temperature increase during the SCS reaction, will be discussed in the next part of this article.

2) Effect of processing parameters on the characteristics of the final products

As said previously, the SCS reaction was performed in a furnace leading to a volume combustion mode for the reactants. This mode of heating allows a precise control of the temperature ramp. Syntheses were done with Gd/Glycine (= 1) mixtures with different ramp and XRD analyses of the final products were performed according to this parameter (Fig. 2). The results show that the crystalline compound Gd2O3 monoclinic was obtained for a heating rate higher than 3 K.min -1 , in these conditions, the final product is a white powder and ignition phenomenon were observed. On the contrary, tests carried out at lower heating rate did not lead to the ignition phenomenon. So the colour of the obtained powder is brown, associated to the high carbon content of these samples (> 4.7%). At heating rate of 3 K.min -1 , the powder was heterogeneous, the upper part of the sample was white, and the bottom was brown, which can mean a suffocation of the reaction at the bottom. For = 1, the ratio C/Gd in the precursor is equal to 3.3 (see R1), depending on the processing conditions this ratio change from 0.08 to a value higher than 2 in the final product. The highest values were obtained when the ignition did not take place. Based on these results, a heating rate of 10 K.min -1 was selected for the elaboration of the following samples (Table 2).

In order to have a better understanding of the effect of glycine on the SCS reaction, we have undertaken several tests with various amounts of glycine (0.3 < < 2.2). Several authors studied the effect of fuel/nitrate molar ratio on the final material, and their results provided a strong basis for the next studies [START_REF] Mukasyan | Solution combustion synthesis of nanomaterials[END_REF][START_REF] Bera | Studies on Cu/CeO2: A New NO Reduction Catalyst[END_REF][START_REF] Deshpande | Direct synthesis of iron oxide nanopowders by the combustion approach: Reaction mechanism and properties[END_REF][START_REF] Patil | Chemistry of nanocristalline oxides materials, Combustion synthesis, properties and applications[END_REF][START_REF] Nair | Role of glycine-to-nitrate ratio in influencing the powder characteristics of La(Ca)CrO3[END_REF][START_REF] Yulin | Combustion synthesis and stability of nanocrystalline La2O3 via ethanolamine-nitrate process[END_REF][START_REF] Wang | Characteristics of YSZ synthesized with a glycine-nitrate process[END_REF][START_REF] Jung | Quantitative effects of fuel on the synthesis of Ni/NiO particles using a microwave-induced solution combustion synthesis in air atmosphere[END_REF][START_REF] Valefi | New insights of the glycine-nitrate process for the synthesis of nano-crystalline 8YSZ[END_REF][START_REF] Erri | Controlling combustion wave propagation for transition metal/alloy/cermet foam synthesis[END_REF]. By comparing these studies, it clearly shows an effect of fuel richness on the morphology of the material and chemical composition, which changes with the mixture and the process (SI-1).

XRD patterns on the final products give several information's (SI4). The first one was the loss of crystallinity of the oxide when ignition does not occur, for low heating rate (< 3 K.min -1 ) and with the lowest and the highest richness. The final products have a black colour, and a relatively high amount of carbon, meaning a low rate of transformation. The second observation relates to the phase transition observed according to the value of the richness in between 0.8 and 1.2. For 0.8 < < 1.2, monoclinic Gd2O3 was formed and cubic structure for the higher value of the richness. Not only the phase obtained, but also the morphology (specific area, crystallites size…) was affected by the change in the processing parameters (Table 2). For = 1, the specific surface area increases with respect to the heating rate, and it is inversely proportional to the carbon content which can be likened to the efficiency of the SCS reaction. The high heating rate at 10 K.min -1 , is the best condition for the conversion, the specific surface area increasing slowly according to the richness in the range of 0.8 < < 2.2. As said previously, for the lowest richness (= 0.5) the obtained amorphous powder has a quite high specific surface area which could be the result of the incomplete thermal decomposition of the mixture glycine/gadolinium nitrate. The increase of the richness leads also to an increase in the amount of carbon in the final products, which ultimately has the effect of reducing the size of the crystallites. However, the calculated surface area representative of the surface of the crystallites is higher than the measured one that can be explained by the embedment of the nanoparticles in a carbon matrix. This is revealed through the SEM observations, which clearly shows a porous structure for the sample obtained in the best conditions of reactivity (= 1, heating rate = 10 K.min -1 ). During ignition, the decomposition reaction of reagents caused a large emission of gaseous species, which causes the formation of this porous structure (Figure 3). On the contrary, SEM images of the amorphous product (Fig. 3a) shows the formation of a coarse structure led to the low specific area. An amorphous compound was also obtained for = 2 (Fig. 3d), but in this case, the high value for a specific area could be caused by the release of gas arising from the incomplete conversion, i.e.

suffocation of the SCS reaction.

Discussion 1) Complexation

Glycine is used as a fuel for the SCS reaction, but can also be considered as a complexing agent for gadolinium nitrate in particular because of amino and carboxyl functional groups in its structure. To understand these interactions, comparative IR spectroscopy analyses were performed on glycine and Gd/Glycine gel at = 1 (Fig. 4). Fig. 4. IR spectra of the gel Gd/Glycine (= 1) compared to the glycine alone and the gadolinium nitrate hexahydrate in the wavenumber range a) 1800-300 cm -1 and b) 750-300 cm -1 [START_REF] Wang | Characteristics of YSZ synthesized with a glycine-nitrate process[END_REF][START_REF] Gomez-Zavagliaab | Low-temperature solid-state FTIR study of glycine, sarcosine and N,N-dimethylglycine: observation of neutral forms of simple α-amino acids in the solid state[END_REF][START_REF] Fischer | The FT-IR spectra of glycine and glycylglycine zwitterions isolated in alkali halide matrices[END_REF] It is difficult to obtain information from the domain of organic vibration bond between 1800 and 1000 cm -1 because of simultaneous detection of nitrates and fuel bonds. The missing of organic vibrating bonds modification suggests there was no chemical transformation of the reagents during gel formation step.

Information about interactions between gadolinium and glycine has been obtained by observing the shift of the carboxylate function peaks between 750 and 300 cm -1 . This shift in the characteristic band associated with the stretching of carboxylate is due to the strong interactions between the carboxylates and gadolinium. This interaction was caused by a complexation of gadolinium ions with glycine during the formation of the gel. The interaction between Gadolinium and Glycine is possible in with the mesomeric form of Glycine, which is a dipolar molecule with a complexing carboxylate function. This complexation is very important for the combustion redox reaction between the fuel and the nitrates. Indeed, results obtained during TG and MS analyses (Fig. 1, and SI-2)

show a transition from two endothermic thermal decompositions of reagents alone to one exothermic simultaneous decomposition of nitrates and glycine during the ignition.

2) Swelling effect and ignition

Only a few studies [START_REF] Deshpande | Direct synthesis of iron oxide nanopowders by the combustion approach: Reaction mechanism and properties[END_REF][START_REF] Patil | Chemistry of nanocristalline oxides materials, Combustion synthesis, properties and applications[END_REF] report a swelling during the heat treatment of the gel obtained after mixing the nitrate with fuels (glycine, urea…). This phenomenon was not well studied, but it seems to have an impact on the ignition step as shown in Figure 5. It can be observed that the ignition was inhibited for the precursor, that has the highest swelling (Gd/Glycine =2). For a temperature of 483K, the volume of glycine alone increases more than twice, which is the same order of magnitude as the swelling of the precursors Gd/Glycine. This means that glycine has an important role in this process. This swelling is a consequence of the high viscosity of the gel and the generation of gases due to thermal decomposition. It is feasible that the viscosity of the gel with respect to glycine alone, is increased due to the interaction between the chemical groups of glycine and nitrates (see Fig. 4). However, two regimes can be highlighted: one corresponding to the thermal decomposition of a gel having a richness close to 1, i.e. in these conditions the final product obtained is a crystalline oxide, and the other corresponding to a higher richness (> 2)

leading to an amorphous oxide (table 2). In the first case (richness close to 1), the heating rate has an effect on the ignition phenomenom. For a heating rate lower than 3 K.min -1 the ignition was not observed (Fig. 6). This effect can be a consequence of the too low density of matter in the system induced by the higher volume increase. In these conditions, even if ignition can occur at a local scale, the reaction can not be self-propagated. At the transition-heating rate of 3 K.min -1 , two products were obtained, an amorphous brown powder with high carbon content (0.74%) on the bottom of the sample and a crystalline white powder with lower carbon content (0.08%) on the top (see table 2).The heterogeneity observed in this sample is probably due to the release of gaseous products by reaction R1. These gases rise from the bottom to the top of the mixture, and influenced the swelling and the reactivity of the system. For higher heating rates (> 3 K.min -1 ), the SCS reaction occurs easily, because the density of matter is high enough to generate selfpropagation of the reaction.

In the last case (> 2), not only the swelling of glycine but also the penetration of oxygen is limiting factor for the reactivity. Indeed, under these conditions according to R1, oxygen is necessary for this reaction. The large volume of gas released by the reaction when the ignition occurs (500 liters / mol Gd for  = 1 to 500K) limits the ingress of oxygen and the progress of the reaction as indicated by the high carbon content measured in the final products (~2%wt.).

To conclude, the density of matter or more precisely the density of the chemical complex is a key parameter for the reactivity of the SCS reaction and for the ignition phenomena. In the case of an excess of glycine (> 2), the access of oxygen could be another parameter that explains the lack of reaction (Fig. 2, Table 2).

3) Phase transformation

Powders obtained from the SCS reaction of different Gd/Glycine mixtures shows a transition phenomenon of gadolinium oxide crystalline structure formation when the fuel richness increases. The effect of fuel richness on SCS reaction can be explained through the estimation of the temperature attained during ignition. This temperature can be measured experimentally or calculated by thermodynamic considerations. In the former case, the temperature (Tc) is generally called "flame temperature" or "combustion temperature". Several reports [START_REF] Chick | Glycine-nitrate combustion synthesis of oxide ceramic powders[END_REF][START_REF] Hwang | Synthesis and characterization of nanocrystalline ZnO powders by a novel combustion synthesis method[END_REF] described that an increase of Tc with richness up to a value in the range of 1 < < 1.5 and after, a decrease of this parameter for higher values of . In the latter case, the adiabatic temperature (Tad) can be calculated using a simple thermodynamic approach. Tad increases substantially with the richness [START_REF] Purohit | Ultrafine ceria powders via glycine-nitrate combustion[END_REF][START_REF] Tahmasebi | The effect of starch addition on solution combustion synthesis of Al2O3-ZrO2 nanocomposite powder using urea as fuel[END_REF]. However, in a general point of view, Tc is lower than Tad. Several factors have been invoked to explain this difference: the experimental conditions are not purely adiabatic [START_REF] Varma | Solution Combustion Synthesis of Nanoscale Materials[END_REF], the fuel does not completely reacts with oxygen especially for fuel-rich conditions [START_REF] Varma | Solution Combustion Synthesis of Nanoscale Materials[END_REF][START_REF] Hwang | Synthesis and characterization of nanocrystalline ZnO powders by a novel combustion synthesis method[END_REF] and a part of heat produced by the ignition is dissipated by the considerable amount of gas generated. As said previously, Tc decreases for high values of , which is the opposite trend compared to Tad. This difference can be caused by the utilization of an incorrect reaction route, which can change the energy balance of the process and finely the calculation of Tad. Only one report elucidated [START_REF] Mukasyan | Solution combustion synthesis of nanomaterials[END_REF] the decrease of Tad for > 1, in the system of ironnitrate-glycine, certainly because of the set of the thermodynamic data used is representative of the SCS reaction.

To conclude, it is not so easy to obtain the value of the temperature reached during the ignition, especially when ≠ 1. In this study, for stoichiometric conditions (= 1), a low amount of residual carbon was measured in the final products obtained by the SCS reaction R1 (Table 2), indicating that the reaction route proposed in Table 1 is consistent. This result allows us to use a basic calculation to estimate Tad as follow:

𝑻 𝒂𝒅 = 𝑻 𝟎 -𝑸/𝒄 𝒑 (1)
𝑸 = ∑ 𝒏 𝒋 𝒋 ∆ 𝒇 𝑯 𝒋 𝟎 -∑ 𝒏 𝒊 𝒊 ∆ 𝒇 𝑯 𝒊 𝟎 (2)
𝒄 𝒑 = ∑ 𝒏 𝒋 𝒄 𝒋 𝒋 (3)
Where 𝑄 is the heat release during the reaction and, 𝑐 the average specific heat capacity of the products at room temperature. To is the ambient temperature, i and j respectively specify reactants and products and ni and nj are the amounts of each compound. The ∆𝐻 and 𝑐 values of the compounds are presented Table 3.

Table 3.

Standard enthalpies of formation and heat capacities for Gadolinium nitrate hexahydrate -Glycine system from [START_REF] Varma | Solution Combustion Synthesis of Nanoscale Materials[END_REF][START_REF] Xiao | Thermochemical analyses on rare earth complex of gadolinium with salicylic acid and 8-hydroxyquinoline[END_REF] Compounds The adiabatic temperature calculated (Tad) for = 1 is close to 1600 K, which is higher than the temperature of structure of the gadolinium oxide powders synthesise in this study is in agreement with this result (Table 2). As discussed previously for ≠ 1, a decrease of the temperature reached by the SCS reaction was observed, that can explain the formation of the cubic gadolinium oxide structure obtained under the same combustion temperatures for fuel-lean or fuel-rich (= 0.8 and 1.5 <  < 1.7). However, the thermodynamic calculation is not able to predict the lack of ignition and the formation of amorphous final products, for large excess of one of the reagents ( < 0.5 or  > 2). In these conditions, the reagent acts as a diluting compound for the SCS reaction and was associated with a diluting effect and its endothermic decomposition induces an inhibition for ignition.

∆𝑯 𝒇 𝟎 (kJ.mol -1 ) 𝒄 𝒑 (J.mol -1 .K -1 ) Gd(NO3)3, 6H2O Glycine, solid Gd2O3
The lack of ignition with = 0.5 and = 2 Gd / Glycine mixtures and the optimal ignition of Gd / Glycine stoichiometric richness mixtures were confirmed by IR analyses on the final powders (Fig. 7). IR spectra of the compound obtained without ignition (= 0.5, = 2, Fig. 7) show a lack of gadolinium-oxygen bonds between 700 and 400 cm -1 , and the main presence of organic compounds between 1800 and 1000 cm -1 .

These organic bonds are also present in the IR spectra of the final products, but gadolinium-oxygen bonds were detected too. Furthermore, organic residues are fewer presents in IR spectra of final powders obtained with stoichiometric richness mixtures, which was confirmed by carbon content measures (Table 2). IR spectra of powders are similar than their XRD patterns (SI-4) and confirm the formation of crystalline nanoparticles after ignition and of organic residues at = 0.5 and = 2. In the case of = 0.5 and = 2 Gd/Glycine mixtures, a thermal treatment at 973 K is necessary to obtain the final cubic gadolinium oxide.

Conclusion

The conversion of gadolinium nitrate into oxide was obtained by SCS reaction with glycine as fuel at temperature about 723 K lower than the classical thermal denitration temperature. For a stoichiometric ratio of glycine / gadolinium nitrate (= 1), monoclinic Gd2O3 powder are obtained with a specific surface area of about 10 m 2 .g -1 , a small amount of residual carbon and good crystallinity. The ignition of the precursors leading to the conversion nitrate / oxide, was observed at a temperature of about 483 K.

The implementation of this reaction with glycine causes significant swelling of the precursors during the temperature increase up to the ignition. This swelling is related to the decomposition of glycine. Several experimental parameters: the heating rate, the richness influence the amplitude of this phenomena. When the swelling is too high (rate of heating rate < 3 K.min -1 , richness ϕ > 2), the ignition is not observed and an amorphous compound is obtained. These results indicate that the density of matter before the ignition is a key parameter for the reaction that modifies the reactivity of the system and induces structural changes. For intermediate richness (= 0.8, 1.5, 1.7), the end-products have a cubic structure, which is the stable structure of this oxide at low temperature, or a mixture of monoclinic and cubic Gd2O3. Adiabatic temperature calculations of the reaction are consistent with the crystalline phases observed.

These findings of actinide surrogates suggest that, the new route will be a promising path for the recycling of actinides of the Generation IV reactors systems. However, the high swelling of the glycine would be a drawback for the application of such fuel in an industrial process, notably for the conversion of actinides. Works are in progress to evaluate other fuels such as citric acid, -alanine.

Fig. 1 .

 1 Fig. 1. a) TGA, DTA, and b) MS curves of gadolinium nitrate, = 1 Gd/Glycine gel during a thermal treatment (air atmosphere, heating rate 10 K.min -1 )

Fig. 2 .

 2 Fig. 2. XRD patterns of SCS reactions applied to Gd/Glycine = 1 mixture with different heating rates (Gd2O3 monoclinic JCPDS 043-1015 (C))

Fig. 3 .

 3 Fig. 3. SEM images of final products obtained after SCS reaction of Gd/Glycine mixtures a) 2 K.min -1 -= 1, b) 10 K.min -1 -= 1, c) 10 K.min -1 -= 1.5, d) 10 K.min -1 -= 2

Fig. 5 .

 5 Fig. 5. Effect of the richness on the thermal swelling of the precursors (heating rate 10 K.min -1 )

Fig. 6 .

 6 Fig.6. Swelling of the precursors during the heat treatment: Effect of the heating rates Gd/glycine (= 1)

Fig. 7 .

 7 Fig. 7. IR spectra of SCS reaction final products obtained with Gd/Glycine mixtures with different  in the wavenumber range a) 1800 -400 cm -1 and b) zoom in the area 600 -400 cm -1 (air atmosphere, heating rate 10 K.min -1 )

  𝐆𝐝 𝟒 (𝐍𝐎 𝟑 ) 𝟏𝟐 , 𝟒𝑯 𝟐 𝐎 → 𝐆𝐝 𝟒 𝐎(𝐍𝐎 𝟑 ) 𝟏𝟎 , 𝟒𝑯 𝟐 𝐎 + 𝐍 𝟐 𝐎 𝟓 𝐆𝐝 𝟒 𝐎(𝐍𝐎 𝟑 ) 𝟏𝟎 , 𝟒𝑯 𝟐 𝐎 → 𝐆𝐝 𝟒 𝐎 𝟒 (𝐍𝐎 𝟑 ) 𝟒 , 𝑯 𝟐 𝐎 + 𝟑𝐍 𝟐 𝐎 𝟓 + 𝟑𝑯 𝟐 𝐎 𝐆𝐝 𝟒 𝐎 𝟒 (𝐍𝐎 𝟑 ) 𝟒 , 𝑯 𝟐 𝐎 → 𝐆𝐝 𝟒 𝐎 𝟓 (𝐍𝐎 𝟑 ) 𝟐 + 𝐍 𝟐 𝐎 𝟓 + 𝑯 𝟐 𝐎 𝐆𝐝 𝟒 𝐎 𝟓 (𝐍𝐎 𝟑 ) 𝟐 → 𝟐𝐆𝐝 𝟐 𝐎 𝟑 + 𝐍 𝟐 𝐎 𝟓

					R3(*)	92.5	87	513
					R4(*)	71.7	74	626
					R5(*)	86.8	87	738
					R6(*)	87	93	850
						Theoretical	Experimental	Temperature
						mf/mi (%)	mf/mi (%)	onset (K)
	𝟓 𝟔	𝝓 +	𝟐 𝟑	𝑵 𝟐	R1	37.5

𝐆𝐝(𝐍𝐎 𝟑 ) 𝟑 , 𝟔𝑯 𝟐 𝐎 + 𝟓 𝟑 𝝓 𝐍𝐇 𝟐 𝐂𝐇 𝟐 𝐂𝐎 𝟐 𝐇 + 𝟏𝟓 𝟒 (𝝓 -𝟏) 𝐎 𝟐 → 𝟏 𝟐 𝑮𝒅 𝟐 𝑶 𝟑 + 𝟏𝟎 𝟑 𝝓 𝑪𝑶 𝟐 + ( 𝟐𝟓 𝟔 + 𝟔)𝝓 𝑯 𝟐 𝑶 + (for  = 1) 38 (for  = 1) 483 𝟒(𝐆𝐝(𝐍𝐎 𝟑 ) 𝟑 , 𝟔𝑯 𝟐 𝐎) → 𝐆𝐝 𝟒 (𝐍𝐎 𝟑 ) 𝟏𝟐 , 𝟒𝑯 𝟐 𝐎 + 𝟐𝟎𝑯 𝟐 𝐎 R2(*) 80 83 323

Table 2 .

 2 Characteristics of final products obtained after SCS reaction of Gd/Glycine. The calculated surface area S (m².g -1 )

	is obtained by the classical formulae S = 3/d* (d(nm) calculated by Debye-Scherrer formulae,  density of Gd2O3
	= 7.41 g.cm -3 ). The error bars are about 1% for carbon content, 10% for crystallites size and 1% for the surface
	area. Phases : amorphous (am.), Monoclinic (mon.), Cubic (cub.)
	Fuel	Heating	Structure	Carbon	C/Gd	Average	Calculated	Measured
	richness	rate		(wt%)	(at./at.)	crystallites	specific area	specific
		(K.min -1 )				size (nm)	(m².g -1 )	area (m².g -1 )
	0.3		am.	2.6	0,4	-	-	27.2
	0.5	10	am.	1.8	0,27	-	-	25.3
	0.8		mon., cub.	0.7	0,1	28.6	14.2	10.3
		1		8.8	1,4	-	-	0.3
		2	am.	7.8	1,27	-	-	0.5
	1.0	3 3		4.7 0.5	0,74 0,08	-29.3	-13.7	0.5 2.0
		5	mon.	0.5	0,08	29.5	13.7	8.8
		10		0.5	0,08	29.5	13.7	9.8
	1.2		mon., cub.	1.4	0,21	22.3	18.1	12.1
	1.5		cub.	1.8	0,27	12.4	32.7	14.6
	1.7	10	cub.	2.0	0,31	8.0	50.4	15.0
	2.0		am.	11.2	1,9	-	-	16.1
	2.2		am.	12.1	2,1	-	-	18.1

Acknowledgements

This work was supported by CEA.