Spark plasma sintering of UO2 fuel composite with Gd2O3 integral fuel burnable absorber

Journal Pre-proof Spark plasma sintering of UO2 fuel composite with Gd2O3 integral fuel burnable absorber E.K. Papynov, O.O. Shichalin, A.A. Belov, A.S. Portnyagin, I.Yu Buravlev, V.Yu Mayorov, A.E. Sukhorada, E.A. Gridasova, A.D. Nomerovskiy, V.O. Glavinskaya, I.G. Tananaev, V.I. Sergienko PII:

S1738-5733(19)30885-X

DOI:

https://doi.org/10.1016/j.net.2020.01.032

Reference:

NET 1057

To appear in:

Nuclear Engineering and Technology

Received Date: 18 October 2019 Revised Date:

31 December 2019

Accepted Date: 30 January 2020

Please cite this article as: E.K. Papynov, O.O. Shichalin, A.A. Belov, A.S. Portnyagin, I.Y. Buravlev, V.Y. Mayorov, A.E. Sukhorada, E.A. Gridasova, A.D. Nomerovskiy, V.O. Glavinskaya, I.G. Tananaev, V.I. Sergienko, Spark plasma sintering of UO2 fuel composite with Gd2O3 integral fuel burnable absorber, Nuclear Engineering and Technology (2020), doi: https://doi.org/10.1016/j.net.2020.01.032. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Korean Nuclear Society, Published by Elsevier Korea LLC. All rights reserved.

Spark plasma sintering of UO2 fuel composite with Gd2O3 integral fuel burnable absorber 1,2

1

Papynov E.K. , Shichalin O.O. , Belov A.A.1,2, Portnyagin A.S.1, Buravlev I.Yu.1,2, Mayorov V.Yu.1, Sukhorada A.E.2, Gridasova E.A.2, Nomerovskiy A.D.1,2, Glavinskaya V.O.1,2, Tananaev I.G.2, Sergienko V.I.1 1

Institute of Chemistry, Far Eastern Branch of Russian Academy of Sciences, 159, Prosp. 100-letiya Vladivostoka, Vladivostok 690022, Russia 2

Far Eastern Federal University, 8, Sukhanova St., Vladivostok 690091, Russia

E-mail: [email protected]

Abstract The paper studies spark plasma sintering (SPS) of industrially used UO2-based fuel containing integral fuel burnable absorber (IFBA) of neutrons Gd2O3. Densification dynamics of pristine UO2 powder and the one added with 2 and 8 wt.% of Gd2O3 under ultrasonication in liquid has been studied under SPS conditions at 1050, 1250, and 1450 °C. Effect of sintering temperature on phase composition as well as on O/U stoichiometry has been investigated for UO2 SPS ceramics. Sintering of uranium dioxide added with Gd2O3 yields solid solution (U,Gd)O2, which is isostructural to UO2. SEM with EDX and metallography were implemented to analyze the microstructure of the obtained UO2 ceramics and composite UO2-Gd2O3 one, particularly, open porosity, defects, and Gd2O3 distribution were studied. Microhardness, compressive strength and density were shown to reduce after addition of Gd2O3. Obtained results prove the hypothesis on formation of stable pores in the system of UO2-Gd2O3 due to Kirkendall effect that reduces sintering efficiency. The paper expands fundamental knowledge on pros and cons of fuel fabrication with IFBA using SPS technology. Keywords: uranium dioxide; ceramic nuclear fuel; burnable neutron absorbers; gadolinium oxide; spark plasma sintering. 1 Introduction To prolong lifecycle of the fuel in modern nuclear power generators (above 12 months), it is important to increase enrichment level of the fuel material (U235) by more than 4.5% [1]. However, economically and technically efficient route is to introduce integral fuel burnable absorbers (BA) of neutrons into the fuel composition. BA suppress high starting reactivity and enables to increase fuel burnup owing to greater neutron absorption cross-section [1,2]. Additionally, energy production can be substantially improved by using the fuel pellet with certain distribution of integral fuel burnable absorbers (IFBA) and by arranging such pellets in 1

an optimal way. For example, “Weistinghouse” company proved the fuel containing ZrB2 for PWR reduces the fuel cycle cost by ~3% [3]. Apart from ZrB2, boron carbide, gadolinium, samarium, erbium, dysprosium, europium, and hafnium oxides have been extensively studied as burnable absorbers [1,2]. Such BAs are well compatible with uranium dioxide, their burn-up rate (at optimal concentration) is close to the one of U235, and they form no decay byproducts that poison chain fission reaction of the fuel (parasite neutron absorption). Particularly, large neutron absorption cross-section of 157

155

Gd and

Gd isotopes in the range of heat energy and low cross-sections of secondary 156Gd and

158

Gd

nuclides make Gd2O3 a promising BA for the nuclear fuel [4]. Exhaustion cycle for this type of fuel increases up to 18 months [5]. However, active industrial manufacturing of UO2-Gd2O3 fuel is limited due to its sintering is rather complicated. Namely, Gd2O3 content (optimal value is assumed to range in 0.1-14.0 wt.%) in the sintered fuel determine the quality of the final product. At high Gd2O3 concentrations (above 10 wt.%), density, mechanical strength, thermal conductivity, and melting point of the fuel drop drastically. [6–9]. There were numerous attempts to explain the “sintering blockage” effect in the works by M. Durazzo et al. [10,11]. First hypothesis was based on formation of the diffusion barrier in Gd2O3-rich areas with low interdiffusion coefficients. That theory was proved several times, but it was finally declared illground [12]. Second hypothesis assumed formation stable pores in the local regions of the sintered material, which are different in terms of interdiffusion coefficients of gadolinium into UO2 and, alternatively, uranium into Gd2O3 during solid solution formation (Kirkendall effect). The latter hypothesis is still not proved, because mentioned effect was revealed for other IFBA, e.g., for Er2O3 [13]. Additionally, researchers indicate “sintering blockage” increase depending on how fuel precursors are mixed, their fractional compositions, gaseous medium, and heating regimes [9,14,15]. That effect can be minimized via using various homogenization, fractionation, and sintering routines to prepare starting materials. There are many individual and complex technological approaches based on wet synthesis and solid-phase mixing, heterogeneous graining and high-temperature reductive consolidation of starting powders for fuel compositions etc. In addition to mentioned conventional technologies, we suggest to pay attention to modern technology of spark plasma sintering (SPS) to obtain fuel added with IFBA. Principle of electropulse consolidation via SPS has been shown for a plethora of ceramics [16–20] including promising UO2-based fuel compositions [21–25]. Moreover, our earlier works demonstrated main technological advantages of SPS for fabrication of high-quality UO2 ceramics [26]: rapid heating, low temperatures, and short sintering cycles adopted for commercial powders and for recovered ones of various fractional compositions and binding agents. However, information on SPS consolidation of UO2-Gd2O3 nuclear fuel is practically absent. 2

Thus, this work aims to investigate SPS consolidation of pristine UO2 powders and the ones containing 2 and 8 wt.% of Gd2O3 neutron burnable absorber. The starting powders were prepared via ultrasonication in the liquid phase. Densification dynamics of the powders was studied as well as physico-chemical and mechanical characterizations of the final pellets were done as well. Earlier unknown results of the work would provide fundamental knowledge on “sintering blockage” effects in (U,Gd)O2 systems revealed in [10,11] and would also reveal technological features of nuclear fuel fabrication in the presence of IFBA using SPS technique. 2 Experimental 2.1 Materials and equipment To obtain ceramics we used the UO2 powder of non-ceramic type obtained via thermoreductive annealing of U3O8 powder (99%, LLC “Reakhim”, Russia) at 400 °C for 4 hours in a tubular furnace RSR-B 120/500/11 (“Nabertherm”, Germany), according to [27]. Gd2O3 powder (99%, LLC “Reakhim”, Russia) was used as a neutron burnable absorber additive introduced into the sintering mixture via ultrasonication in acetone on a device Bandelin Sonopuls HD 3200 (“Bandelin Electronic GMBH”, Germany). SPS consolidation was conducted on a SPS-515S installment (“Dr. Sinter*LABTM”, Japan) using graphite die (graphite «Graphite grade Ellor+30» manufactured by Mersen France Gennevilliers S.A.S, France). Size dimensions of the dies were: height 30 mm, external diameter 30 mm, internal diameter 10.5 mm. Size dimensions of the obtained cylindrical samples: diameter 10.3 mm, height 4-5 mm. The graphite paper of 0.2 mm thick was used to prevent the contact between the die and the sintered material.

2.2 Synthesis methods 5.5 g of UO2 powder containing 0, 2, and 8 wt.% of Gd2O3 were placed into graphite die, prepressed (20.7 MPa), then the green body was installed in the vacuum chamber (6 Pa) and sintered followed by cooling and extracting from the chamber. Owing to high oxidizability of UO2 in air, sintering was conducted in vacuum to preserve O/U stoichiometry equal to 2.00, as in the starting uranium dioxide powder. The following sintering conditions were employed: sintering temperature 1050-1450 °C, pressure – 60 MPa, heating rate – 100 °C min-1, holding time – 5 min, cooling time – 30 min. SPS temperature was controlled using the optical pyrometer focused on the gap (5.5 mm deep) in the middle of outer die wall. The die was covered with thermal insulating fabric to reduce heat loss during heating. Because of SPS specifics, powder sintering was done using direct electric current in the pulse 3

On/Off regime, 12 (pulse) / 2 (pause), pulse duration – 39.6 ms, pause – 6.6 ms. Maximal current was 1000 A with the voltage being 4 V.

2.3. Characterization methods Granulometric analysis of the oxide powders was performed by the means of laser diffraction on a laser particles analyzer Analysette-22 NanoTec/MicroTec/XT (“Fritsch”, Germany). Crystal phases in the powders and sintered UO2 pellets were identified using XRD on a multi-purpose diffractometer D8 Advance (“Bruker AXS”, Germany). The following parameters were used to record XRD patterns: CuKα-source, Ni-filter, mean wavelength (λ) – 1.5418 Å, angle range – 10-80°, scanning step – 0.02°. scanning rate – 5 ° min-1. Microstructure (surface morphology) was investigated based on SEM images on a Carl Zeiss Ultra 55 microscope (Germany) with the field emission cathode (FE-SEM) at an accelerating voltage of 1-5 kV and beam current ≈ 100 pA. Element composition of the sample was identified using energy-dispersive spectroscopy (EDX) on an addon to the mentioned microscope. Vickers microhardness (HV) was measured at a load of 0.5HV on a hardness tester HMV-G-FA-D (“Shimadzu”, Japan). Compressive strength was assessed on a tensile machine Autograph AG-X plus 50 (“Shimadzu”, Japan). Specific weight of the materials was measured via hydrostatic weighing in water on a balance AdventurerTM (“OHAUS Corporation”, USA). To calculate theoretical density of the samples containing Gd2O3 we used the formula suggested in the paper [28]: = 10.96 − 0.031 (

),

where η(Gd2O3) – weight fraction of Gd2O3. 2.4 Stoichiometry (O/U) determination Two different methods were implemented to identify O/U stoichiometry for initial UO2 powder: Method #1: Dissolution of the preliminary weighed pristine UO2 powder in the concentrated nitric acid followed by precipitation with NH4OH UO2 + 4HNO3 → UO2(NO3)2 + 2NO2 + 2H2O 2UO2(NO3)2+6NH4OH→ (NH4)2U2O7 ↓+4NH4NO3+3H2O Precipitate was washed till neutral reaction and dried in air. Then the precipitate was annealed in air at 900 °C till constant mass: 3(NH4)2U2O7 + 3½O2 → 2U3O8 + 3N2 + 12H2O Obtained U3O8 was weighed and mass of uranium content was identified (Table 1). 4

Mass fraction of uranium in U3O8 is 0.848. Thus, mass of the uranium in the 1 and 2 sample of U3O8 is 0.8827 and 0.8780 g, respectively. Therefore, mass of oxygen in starting UO2 samples was 0.1204 and 0.1202, correspondingly. The resulting stoichiometric formula in each is UO2.03 and UO2.035, which closely corresponds to the pristine UO2 compound. The small deviation is caused by slight oxidation of the sample.

Method #2: Preliminary weighed pristine UO2 powder was put into crucible and annealed in air at 900 °C till constant weight: 3UO2 + O2 → U3O8 Obtained U3O8 was weighed and uranium content was identified (Table 2). The stoichiometric formula obtained from samples 1 and 2 was UO2.032 and UO2.036, respectively, also evidencing the partial oxidation of the UO2 oxide.

3 Results and discussion Granulometric analysis and SEM images provided fractional composition of the starting oxide powders. Particle size of the UO2 powder ranges in 5-60 µm, where the size of the main fraction is 30 µm (Fig. 1a). Particle size range of the IFBA additive is narrower with the mean value lying within 0.1–18 µm (Fig. 1b). Densification dynamics during SPS treatment enables to investigate consolidation efficiency of UO2 powder. Dilatometry curves (Fig. 2) reveal pristine UO2 powder densification proceeds identically at 1050, 1250, and 1450 °C. UO2 undergoes compaction in two steps: during first 2-3 minutes during heating to 650-700 °C and second step on 3-7 minutes at 800 °C and above. Main compaction is achieved in 4 minutes of heating. Displacement does not depend on sintering temperature. SPS consolidation of UO2 powders containing Gd2O3 was performed at 1250 °C. This temperature was chosen primarily to avoid carbon diffusion from graphite die and press punches into ceramics occurring at high temperatures. Earlier we have shown the negative presence of carbon in the final product for SPS consolidation of ceramic type UO2 powder [26]. Additionally, other researchers demonstrated earlier [29,30] that fuel sintering with the addition of 10 wt.% of Gd2O3 in oxygen-free and oxidative medium is intense at the beginning of (U,Gd)O2 solid solution formation below 1300 °C. Then, sintering rate drops and it is necessary to increase temperature up to1700 °C and to maintain it for 4 hours. In this respect, temperature 1250 °C may be optimal to obtain UO2-Gd2O3 fuel of required density owing to SPS specifics based on pulse current treatment in vacuum leading to rapid (~minutes) powder consolidation. 5

This is proved by densification dynamics of the composite powders presented on Fig. 3. Adding 2 and 8 wt.% of Gd2O3 into UO2 powder increases densification by 14% as compared to the pristine sample. UO2 compaction with the IFBA additive proceeds via two steps similarly to the example discussed above. Both concentrations of Gd2O3 have the same effect on the compaction (Fig. 3). Non-stoichiometry character of the UO2 (O/U ratio may vary within 1.6-2.5) poses a problem of avoiding phase oxidation for the fuel production technology. That is why conventional UO2 fuel sintering is conducted in the reducing atmosphere. However, obtained XRD data (Fig. 4) revealed no changes in phase composition and in stoichiometric ratio (O/U=2.00) caused by SPS consolidation of the UO2 powder within the studied temperature interval. Moreover, the paper [31] showed that UO2.00 stoichiometry can be achieved under vacuum conditions of SPS treatment of the non-stoichiometric powders. This can be provided by partial carbothermic reduction with carbon coming from the dies and press punches under electric pulse heating. XRD results for UO2-Gd2O3 samples are in agreement with the reported data on the similar systems obtained using conventional sintering routines [10,12,13,32–34]. XRD patterns (Fig. 5) show diffraction peaks corresponding to Gd2O3 are absent. This proves that (U,Gd)O2 solid solution phase of fluorite type is formed from UO2 powder with the addition of Gd2O3 as demonstrated by UO2-Gd2O3 phase diagram [32]. Separate Gd2O3 phase is observed in UO2 ceramics at concentrations above 14 wt.% as was shown in the references cited above. Diffraction patterns of the obtained samples are characterized by line broadening and shifts to higher 2θ values. This effect in our study is observed for sample containing 8 wt.% of Gd2O3 (Fig. 5). There are a number of factors driving the change of UO2 lattice parameters when the (U,Gd)O2 solid solution forms [32–34]. UO2 and Gd2O3 oxides are isomorphic to fluorite, therefore, they may generate a series of substitution solutions with variable stoichiometry. The paper [33] studied all 8 possible models of UO2-Gd2O3 solid solution formation. Analysis of the described substitution mechanisms indicates the ability of Gd+3 to substitute U+4 in the UO2 structure leading to the formation of U+5 and U+6 cations. Such processes do not proceed in the reductive medium, where U4+ concentration in the UO2 structure is maximal favoring Gd3+ substitution into it with the formation of pre-stoichiometric solid solution, which is highly prone to oxidation followed by increase of lattice parameters. Under normal conditions including the one with limited oxygen concentration (vacuum annealing), one introduced Gd3+ cation results in formation of one U5+ cation yielding stoichiometric solid solution. Additionally, one oxygen vacancy at constant concentration of the rare earth metal gives rise to two cation vacancies of 6

U5+. Size of U5+ (0.88 Å) is smaller than U4+ (1.00 Å), that is why accumulation of U5+ leads to contraction of the cubic lattice, even though the Gd3+ cations are larger (1.053 Å). Superstoichiometric solid solution (U,Gd)O2+x forms at high temperature oxidative annealing. However, excess amount of O2- anions with larger diameter (1.37 Å) has little effect on lattice parameters as compared to the cations mentioned above. Thus, the contraction of the UO2 lattice has been found to be a result of (U,Gd)O2 formation in the present study. The works [10,13] showed that efficiency of conventional sintering of UO2-Gd2O3 followed by formation of (U,Gd)O2 solid solution as a fluorite-type phase is the highest, because the densest particle packing is achieved. Gd3+ cations actively diffuse according to the mechanism indicated above and substitute U4+ cations in the fluorite structure that increases sintering efficiency. If non-fluorite type UO2 phases are formed, e.g., at Gd2O3 concentrations above 50 wt.%, sintering rate and compaction reduce. It is noteworthy that rapid SPS consolidation of UO2-Gd2O3 at 1250 °C in our study yields monophase (U,Gd)O2 system (Fig. 5) with high compaction (14%) being achieved regardless of the amount (2 or 8 wt.%) of Gd2O3 additive (Fig. 3). Microstructure of UO2 SPS ceramics obtained at various temperatures is characterized by dense packing of consolidated particles (Fig. 6). There are open pores observed on the surface with their amount being inversely proportional to sintering temperature due to grain growth. UO2 ceramics microstructure exhibit significant changes after adding Gd2O3 (Fig. 7). Samples observe structural defects with the size depending on the amount of additive. Adding 2 wt.% of Gd2O3 leads to formation of small defect regions as intergranular inclusions in the places of particle contacts. On the contrary, 8 wt.% of IFBA yield large defects that are similar to non-consolidated parts of the material. Additionally, interparticle porosity in ceramics increases with the amount of additive. Presence of such pores in the samples is in agreement with the reported data [11]. These pores form as a result of active Gd diffusion into UO2 matrix at the moment of (U,Gd)O2 solid solution formation indicated by XRD above (Fig. 5). As was shown in [10,11], gadolinium phase dominates in these pores over uranium one. That fact proves the occurrence of Kirkendall effect that is responsible for such pore formation due to differences diffusion of indicated metal atoms. Gd3+ cations diffuse into UO2 faster than U+4 into Gd2O3. High flow of Gd3+ is established towards UO2 phase, which expands to obtain additionaly cations, while Gd2O3 additive leaves cavities after it [11]. This effect is often observed in various mixed powder systems with nonuniform diffusion and solubility between the powders [35]. EDX-microanalysis (Fig. 8) of the UO2-Gd2O3 ceramics surface reveals Gd2O3 distribution depending on its content. Surface area of the regions corresponding to Gd increases 7

with its concentration in the pellet (Fig. 8). Regions with high concentration of pores and defects revealed by SEM images (Fig. 7) correspond to areas with maximal Gd2O3 content (Fig. 8). Gd distribution in the ceramics was also found using metallography analysis (Fig. 9). Optical images reveal the correlation between the area of the local region containing Gd and its concentration in the pellet. It is noteworthy that the majority of large pores is formed in the places, where the additive appears. It is reported that indicated pores are formed as closed pores at the moment of (U,Gd)O2 solid solution formation at high temperatures and, apparently, they cannot be fully eleiminated from the compact. That leads to residual porosity of the sintered fuel pellets explaining low density and mechanical characteristics of the obtained products [11]. For example, the paper [9] showed that UO2-(2 and 5 wt.%)Gd2O3 samples obtained via conventional sintering in hydrogen atmosphere at 1700 °C and 3 hours of holding exhibit 91% from theoretical density, while the minimal required value is 93.5-96.0 % [36]. Here we report on density, microhardness, and compressive strength of UO2- Gd2O3 samples obtained via SPS are also reduced depending on IFBA content (93.6-95.2% from theoretical) as shown in Table 3. It is noteworthy that technological regime for fabrication of UO2-Gd2O3 ceramics with such characteristics is rather mild (1250 °C, 14 minutes holding time) as compared to conventional many-hours high-temperature consolidation as was shown in references.

Conclusions The paper has studied SPS consolidation of UO2 powders including the ones containing 2 and 8 wt.% of Gd2O3 with the powders being prepared via ultrasonication in the liquid phase. Powders densification under SPS conditions has been demonstrated to proceed in two steps: during first 2-3 minutes of heating to 650-700 °C and on 3-7 minutes at 800 °C and above. Main compaction occurs in the first 4 minutes of heating. Densification value does not depend on sintering temperature within the studied range 1050-1450 °C. Adding Gd2O3 to UO2 powder increases compaction by 14% regardless of the IFBA amount (2 or 8 wt.%). UO2-Gd2O3 ceramics was found to correspond to single phase system of (U,Gd)O2 solid solution. Ceramics microstructure is characterized by presence of open interparticle porosity, which is partially eliminated at increased SPS temperature. Structural defects similar to large pore agglomeration as non-consolidated fragments are observed for UO2-Gd2O3 samples. EDX and metallography proved that these defects are formed in the places of Gd2O3 distribution and their amount is proportional to IFBA concentration. Mechanical characteristics and density decrease with the increasing amount of IFBA. 8

New results on SPS consolidation of UO2-Gd2O3 fuel prove the hypothesis suggested earlier [11] related to pore formation in the places of Gd2O3 presence within the UO2 powder due to Kirkendall effect causing bad ceramics sintering. However, technological parameters for fabrication of UO2-Gd2O3 fuel characterized by TD 95.2% using SPS are rather mild (1250 °C, 60 MPa, 100 °C min-1) and, therefore, more attractive than conventional sintering methods.

Acknowledgements The work was financially supported by Russian Science Foundation (project No. 17-7320097). Equipment of common use centers “Far Eastern Center for Structural Research” (Institute of Chemistry, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok) and “Laboratory of mechanical tests and structural studies of materials” (Far Eastern Federal University, Vladivostok) were used in the work.

9

References [1]

D. Gosset, Absorber materials for Generation IV reactors, in: P. Yvon (Ed.), Struct. Mater. Gener. IV Nucl. React., Elsevier Ltd, NY, 2017: pp. 533–567. doi:10.1016/B978-0-08100906-2.00015-X.

[2]

M. Lovecký, J. Závorka, J. Jiřičková, R. Škoda, Increasing efficiency of nuclear fuel using burnable absorbers, Prog. Nucl. Energy. 118 (2020). doi:10.1016/j.pnucene.2019.103077.

[3]

R.L. Simmons, N.D. Jones, F.D. Popa, D.E. Mueller, J.E. Pritchett, Integral Fuel Burnable Absorbers with ZrB2 in Pressurized Water Reactors, Nucl. Technol. 80 (2017) 343–348. doi:10.13182/nt88-a34058.

[4]

K. Hesketh, Burnable Poison-Doped Fuel, in: R.J.M. Konings (Ed.), Compr. Nucl. Mater., Elsevier, Oxford, 2012: pp. 423–438. doi:10.1016/B978-0-08-056033-5.00037-9.

[5]

M. Asou, J. Porta, Prospects for poisoning reactor cores of the future, Nucl. Eng. Des. 168 (2002) 261–270. doi:10.1016/s0029-5493(96)01322-2.

[6]

M. Hirai, Thermal diffusivity of UO2-Gd2O3 pellets, J. Nucl. Mater. 173 (1990) 247–254.

[7]

K. Iwasaki, T. Matsui, K. Yanai, R. Yuda, Y. Arita, T. Nagasaki, N. Yokoyama, I. Tokura, K. Une, K. Harada, Effect of Gd2O3 Dispersion on the Thermal Conductivity of UO2, J. Nucl. Sci. Technol. 46 (2009) 673–676. doi:10.1080/18811248.2007.9711574.

[8]

K.W. Kang, J.H. Yang, J.H. Kim, Y.W. Rhee, D.J. Kim, K.S. Kim, K.W. Song, The solidus and liquidus temperatures of UO2-Gd2O3 and UO2-Er2O3 fuels, Thermochim. Acta. 455 (2007) 134–137. doi:10.1016/j.tca.2006.11.027.

[9]

M.M.F. Lima, W.B. Ferraz, A.M.M. dos Santos, L.C.M. Pinto, A. Santos, Study of UO210wt%Gd2O3 fuel pellets obtained by seeding method using AUC co-precipitation and mechanical mixing processes, in: 18 CBECiMat - Congr. Bras. Eng. e Ciência Dos Mater., 2008: pp. 1224–1242.

[10] M. Durazzo, F.B.V. Oliveira, E.F. Urano De Carvalho, H.G. Riella, Phase studies in the UO2-Gd2O3

system,

J.

Nucl.

Mater.

400

(2010)

183–188.

doi:10.1016/j.jnucmat.2010.03.001. [11] M. Durazzo, A.M. Saliba-Silva, E.F. Urano De Carvalho, H.G. Riella, Sintering behavior of UO 2 -Gd 2 O 3 fuel: Pore formation mechanism, J. Nucl. Mater. 433 (2013) 334–340. doi:10.1016/j.jnucmat.2012.09.033. [12] M. Durazzo, A.M. Saliba-Silva, E.F.U. De Carvalho, H.G. Riella, Remarks on the sintering behavior of UO2-Gd2O 3 fuel, J. Nucl. Mater. 405 (2010) 203–205. doi:10.1016/j.jnucmat.2010.08.002. [13] M. Durazzo, A.C. Freitas, A.E.S. Sansone, N.A.M. Ferreira, E.F.U. de Carvalho, H.G. Riella, R.M. Leal Neto, Sintering behavior of UO2Er2O3 mixed fuel, J. Nucl. Mater. 510 10

(2018) 603–612. doi:10.1016/j.jnucmat.2018.08.051. [14] R. Yuda, K. Une, Effect of sintering atmosphere on the densification of UO2-Gd2O3 compacts, J. Nucl. Mater. 178 (1991) 195–203. doi:10.1016/0022-3115(91)90386-L. [15] M. Durazzo, H.G. Riella, Effect of Mixed Powder Homogeneity on the UO2-Gd2O3 Nuclear

Fuel

Sintering

Behavior,

in:

Key Eng.

Mater.,

2001:

pp.

60–66.

doi:10.4028/www.scientific.net/kem.189-191.60. [16] R. Orru`, R. Licheri, A.M. Locci, A. Cincotti, G. Cao, Consolidation/synthesis of materials by electric current activated/assisted sintering, Mater. Sci. Eng. R Reports. 63 (2009) 127–287. doi:10.1016/j.mser.2008.09.003. [17] M. Tokita, Spark Plasma Sintering (SPS) Method, Systems, and Applications, in: S. Somiya (Ed.), Handb. Adv. Ceram. Mater. Appl. Process. Prop., 2nd ed., Elsevier Inc., 2013: pp. 1149–1178. doi:10.1016/B978-012654640-8/50007-9. [18] D. Dudina, B. Bokhonov, E. Olevsky, Fabrication of Porous Materials by Spark Plasma Sintering: A Review, Materials (Basel). 12 (2019) 541. doi:10.3390/ma12030541. [19] E.P. Simonenko, N.P. Simonenko, E.K. Papynov, O.O. Shichalin, A. V. Golub, V.Y. Mayorov, V.A. Avramenko, V.G. Sevastyanov, N.T. Kuznetsov, Preparation of porous SiC-ceramics by sol–gel and spark plasma sintering, J. Sol-Gel Sci. Technol. 82 (2017) 748–759. doi:10.1007/s10971-017-4367-2. [20] E.K. Papynov, O.O. Shichalin, V.Y. Mayorov, E.B. Modin, A.S. Portnyagin, I.A. Tkachenko, A.A. Belov, E.A. Gridasova, I.G. Tananaev, V.A. Avramenko, Spark Plasma Sintering as a high-tech approach in a new generation of synthesis of nanostructured functional

ceramics,

Nanotechnologies

Russ.

12

(2017)

49–61.

doi:10.1134/S1995078017010086. [21] L. Ge, G. Subhash, R.H. Baney, J.S. Tulenko, E. McKenna, Densification of uranium dioxide fuel pellets prepared by spark plasma sintering (SPS), J. Nucl. Mater. 435 (2013) 1–9. doi:10.1016/j.jnucmat.2012.12.010. [22] M. Cologna, V. Tyrpekl, M. Ernstberger, S. Stohr, J. Somers, Sub-micrometre grained UO2 pellets consolidated from sol gel beads using spark plasma sintering (SPS), Ceram. Int. 42 (2016) 6619–6623. doi:10.1016/j.ceramint.2015.12.172. [23] T. Yao, S.M. Scott, G. Xin, B. Gong, J. Lian, Dense nanocrystalline UO2+ x fuel pellets synthesized by high pressure spark plasma sintering, J. Am. Ceram. Soc. (2017) 1–11. doi:10.1111/jace.15289. [24] Z. Chen, G. Subhash, J.S. Tulenko, Master sintering curves for UO2 and UO2-SiC composite processed by spark plasma sintering, J. Nucl. Mater. 454 (2014) 427–433. doi:10.1016/j.jnucmat.2014.08.023. 11

[25] B. Gong, T. Yao, C. Lu, P. Xu, E. Lahoda, J. Lian, Consolidation of commercial-size UO2 fuel pellets using spark plasma sintering and microstructure/microchemical analysis, MRS Commun. (2018) 1–9. doi:10.1557/mrc.2018.121. [26] E.K. Papynov, O.O. Shichalin, A.Y. Mironenko, A. V Ryakov, I. V Manakov, P. V Makhrov, I.Y. Buravlev, I.G. Tananaev, V.A. Avramenko, V.I. Sergienko, Synthesis of High-Density Pellets of Uranium Dioxide by Spark Plasma Sintering in Dies of Different Types, Radiochemistry. 60 (2018) 362–370. doi:10.1134/S1066362218040045. [27] M. Pijolat, C. Brun, F. Valdivieso, M. Soustelle, Reduction of uranium oxide U3O8 to UO2 by hydrogen, Solid State Ionics. 101–103 (1997) 931–935. doi:10.1016/S01672738(97)00385-8. [28] A. V. Fedotov, E.N. Mikheev, A. V. Lysikov, V. V. Novikov, Theoretical and experimental density of (U, Gd)O2and (U, Er)O2, At. Energy. 113 (2013) 429–434. doi:10.1007/s10512-013-9657-3. [29] R. Manzel, W.O. Doerr, Manufacturing and irradiation experience with UO2/Gd2O3 fuel, Am.

Ceram.

Soc.

Bull.

59

(1980)

1980.

https://jglobal.jst.go.jp/en/detail?JGLOBAL_ID=201002067108164492&rel=0. [30] K.W. Song, K. Sik Kim, J. Ho Yang, K. Won Kang, Y. Ho Jung, Mechanism for the sintered density decrease of UO2-Gd2O3 pellets under an oxidizing atmosphere, J. Nucl. Mater. 288 (2001) 92–99. doi:10.1016/S0022-3115(00)00721-2. [31] V. Tyrpekl, M. Naji, M. Holzhäuser, D. Freis, D. Prieur, P. Martin, B. Cremer, M. Murray-Farthing, M. Cologna, On the Role of the Electrical Field in Spark Plasma Sintering of UO 2+x, Sci. Rep. 7 (2017) 1–9. doi:10.1038/srep46625. [32] A. Baena, T. Cardinaels, K. Govers, J. Pakarinen, K. Binnemans, M. Verwerft, Lattice contraction and lattice deformation of UO2 and ThO2 doped with Gd2O3, J. Nucl. Mater. 467 (2015) 135–143. doi:10.1016/j.jnucmat.2015.09.018. [33] S.M. Ho, K.C. Radford, Structural Chemistry of Solid Solutions in the UO2-Gd2O3 System, Nucl. Technol. 73 (1986) 350–360. doi:10.13182/nt86-a16077. [34] T. Cardinaels, J. Hertog, B. Vos, L. De Tollenaere, C. Delafoy, M. Verwerft, Dopant solubility and lattice contraction in gadolinia and gadolinia-chromia doped UO 2 fuels, J. Nucl. Mater. 424 (2012) 289–300. doi:10.1016/j.jnucmat.2012.02.014. [35] R.M. German, Sintering Theory and Practice, John Willey & Sons, New York, 1996. [36] International Atomic Energy Agency, Experiences and trends of manufacturing technology of advanced nuclear fuel, IAEA-TECDO, IAEA, Vienna, 2012.

12

Figure captions Figure 1. Granulometric composition and SEM images of the (a) UO2 and (b) Gd2O3 starting powders. Figure 2. Pristine UO2 powder densification dynamics at various SPS temperatures: (1) 1050 °C; (2) 1250 °C; (3) 1450 °C; applied pressure (60 MPa) and heating rate (100 °C min-1) are constant. Figure 3. Powder densification dynamics: (1) pristine UO2, (2) 2 wt.% Gd2O3, (3) 8 wt.% Gd2O3, SPS sintering temperature – 1250 °C, pressure (60 MPa) and heating rate (100 °C min-1) remain constant. Figure 4. XRD patterns of the pristine UO2 powder and SPS ceramic pellets based on it obtained at various temperatures in the graphite die. Figure 5. XRD patterns of the pristine UO2 powder (non-ceramic type) and its SPS ceramic derivatives sintered at 1250 °C including the ones containing 2 and 8 wt.% of Gd2O3. Figure 6. SEM images of the surface of UO2 SPS ceramics obtained at various temperatures. Figure 7. SEM images of the surface of UO2-Gd2O3 ceramics obtained via SPS (1250 °C, 60 MPa, 100 °C min-1) with different amount of IFBA. Figure 8. EDX surface microanalysis for UO2-Gd2O3 samples obtained using SPS (1250 °C, 60 MPa, 100 °C min-1) with different IFBA concentration. Figure 9. Metallography images from the cross-section surface of UO2- Gd2O3 samples obtained using SPS (1250 °C, 60 MPa, 100 °C min-1) with different amount of IFBA additive.

13

Table 1. O/U stoichiometry for initial UO2 according Method #1 Sample #

Weight (g) UO2

U3O8

1

1.0031

1.0409

2

0.9982

1.0355

Table 2. O/U stoichiometry for initial UO2 according Method #2. Sample #

Weight (g) UO2

U3O8

1

1.0022

1.0398

2

1.0005

1.0378

Table 3. Characteristics of UO2 ceramics including the ones with Gd2O3.

Sample

Tsint., °C

ρ, g·cm-3

TD, %

HV*

UO2-1050 UO2-1250 UO2-1450 UO2-(2%)Gd2O3 UO2-(8%)Gd2O3

1050 1250 1450

10.1566 10.4177 10.5339 10.3757 10.0273

92.6 95.0 96.1 95.2 (for mixture) 93.6 (for mixture)

488 570 675 693 582

1250

Compressi ve strength, MPa 584.3 849.3 1084.4 857.8 399.7

Declaration of interests ☒ 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.