Fabrication and characterization of silicon nitride-based inert matrix fuels sintered with magnesium silicates

The effects of sintering additives of magnesium silicates, i.e. enstatite (MgSiO₃), steatite (MgSiO₃) and forsterite (Mg₂SiO₄), on the sintering behaviors and characteristics of the silicon nitride ceramics-based inert matrix fuels (IMFs) were experimentally investigated. Fabrication tests and characterizations of Si₃N₄-based IMFs with the sintering additives were carried out using cerium oxide (CeO₂) to represent minor actinide oxides. Sintered bodies were characterized in terms of their densities and thermal conductivities. In addition, a solubility test with hot nitric acid was carried out for evaluation of applicability to the existing reprocessing technology. The densification of sintered bodies was enhanced by using additives of magnesium silicates at relatively low sintering temperature. In particular, the relative density of Si₃N₄-based IMFs with Mg₂SiO₄ was above 90% at 1723 K. The thermal conductivities of Si₃N₄-based IMFs varied according to sintering temperature, and for the IMFs sintered at 1923 K were above 34 W/m K. The solubility test results revealed that only grain boundary phases in Si₃N₄-based IMFs can be dissolved into hot nitric acid.     

Densification of nano-CeO2 ceramics as nuclear oxide surrogate by spark plasma sintering

The sintering and resulting microstructure of nano-grained CeO₂ ceramics were investigated as functions of the spark plasma sintering (SPS) parameters. Ceria powders could be sintered to a relative density over 97% with a grain size of about 30 nm. The applied uniaxial pressure during sintering had a significant effect on densification. The combination of high pressure and fast heating rate produces a marked reduction in the sintering temperature to densify CeO₂ with very limited grain growth. Heating rate and holding time, however, had insignificant effect on density but a measurable effect on grain size.     

Reformation of hazardous wastes into useful supporting materials for fast reactor fuels

Novel concepts for effective utilization of molybdenum (Mo) from nuclear waste and magnesium silicates from hazardous asbestos wastes are proposed. A fast reactor cycle scheme that incorporates each material is described in the present paper. Results of basic studies on several fundamental technologies for the present cycle are given. Basic separation features of Mo by using the LIX63 microcapsule and tertiary pyridine resin were investigated. A simple chemical synthesis route for Mo precursor powder from Mo-containing HNO₃ solution was demonstrated. Effects of impurities in recovered Mo on sintering behavior of Mo-pellets were experimentally investigated.     

Effect of oxygen potential on the sintering behavior of MgO-based heterogeneous fuels containing (Pu, Am)O2−x

The effect of oxygen potential on the sintering behavior of MgO-based heterogeneous fuels containing (Pu, Am)O_2−x was experimentally investigated. Sintering tests in various atmospheres, i.e. air, moisturized 4%H₂-Ar, and 4%H₂-Ar atmosphere, were carried out. The sintering behavior was found to be significantly affected by the oxygen potential in the sintering atmosphere. The sintered density decreased with decreasing oxygen potential. The (Pu, Am)O_2−x phase sintered in a reductive atmosphere had hypostoichiometry. The aggregates of the (Pu, Am)O_2−x phase sintered in the reductive atmosphere grew, in comparison with those in the oxidizing one. The sintering mechanism was discussed in terms of the difference in sintering behavior of (Pu, Am)O_2−x and MgO.     

A study on the burn-up characteristics of inert matrix fuels

Proper disposal of minor actinides (MA), long-lived fission products (LLFPs), and transuranium element (TRU) plays a key role in the sustainable development of fission nuclear power. Adoption of inert matrix fuels (IMFs) can effectively reduce the amount of ²³⁷Np and Np element in the spent fuel of present-day commercial power reactors. In order to study the burn-up characteristics of IMFs caused by the unique composition, burn-up calculations and MA accumulation of two typical IMFs, PuO₂ + ZrO₂ + MgO and PuO₂ + ThO₂, are performed in this paper. Results indicate that k_inf at beginning of life (BOL) and reactivity drop with burn-up for PuO₂ + ZrO₂ + MgO are much larger than those of PuO₂ + ThO₂ IMF. The yields of ²³⁷Np and Np element in IMFs are two orders smaller than those of UO₂ and mixed oxide (MOX) fuels. For the same PuO₂ volume fraction and a certain burn-up, the masses of ²³⁷Np, Np element, and ²⁴¹Am for PuO₂ + ZrO₂ + MgO are smaller than those of PuO₂ + ThO₂; however, the mass of total MA is larger. IMF has high destruction efficiencies of TRU and plutonium (Pu). The results and conclusion provide basic data for the ongoing IMF design and application study.     

Fabrication of particle dispersed inert matrix fuel based on liquid phase sintered SiC

In the present work, liquid phase sintered SiC (LPS-SiC) was proposed as an inert matrix for the particle dispersed inert matrix fuel (IMF). The fuel particles containing plutonium and minor actinides were substituted with pure yttria stabilized zirconia beads. The LPS-SiC matrix was produced from the initial mixtures prepared using submicron sized α-SiC powder and oxide additives Al₂O₃, Y₂O₃ in the amount of 10 wt.% with the molar ratio 1Y₂O₃/1Al₂O₃. Powder mixtures were sintered using two sintering methods; namely conventional high temperature sintering and novel spark plasma sintering at different temperatures depending on the method applied in order to obtain dense samples. The phase reaction products were identified using X-ray diffraction (XRD) and microstructures were investigated using light microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) techniques. The influence of powder mixing methods, sintering temperatures, pressures applied and holding time on the density of the obtained pellets was investigated. The samples sintered by slow conventional sintering show lower relative density and more pronounced interaction between the fuel particles and matrix in comparison with those obtained with the fast spark plasma sintering method.     

A practical fabrication method for the advanced heterogeneous fuel with magnesia containing minor actinides

Fabrication tests on advanced heterogeneous fuel with MgO were carried out for the purpose of establishing a practical fabrication method. Advanced heterogeneous fuel consists of spheres (diameter greater than 100 μm) of a minor actinide oxide and MgO inert matrix (macro-dispersed type fuel). Macro-dispersed type fuel pellets with a high density above 90% T.D. were successfully fabricated. In addition, the fabricated pellets showed a homogeneous dispersion of near spherical host phase granules. These were attained by optimization of the fabrication process and conditions; i.e. a preliminary heat treatment of raw powders of host phase, an adjustment of pressure at the granulation process, deployment of a spray-dry process for MgO sphere preparation and sintering in a He atmosphere. From these results, a practical fabrication method for MgO-based macro-dispersed type fuel based on a simple powder metallurgical technique was established.     

Influences of the ZrC coating process and heat treatment on ZrC-coated kernels used as fuel in Pu-burner high temperature gas-cooled reactor in Japan

ABSTRACT

The concept of Pu-burner high temperature gas-cooled reactor (HTGR) has been proposed to more safely reduce the amount of recovered Pu. In the Pu-burner HTGR concept, coated fuel particles with ZrC-coated yttria-stabilized zirconia containing PuO₂ (PuO₂-YSZ) kernels are employed for very high burn-up and high nuclear proliferation resistance. The role of ZrC layer is that of oxygen getter. CeO₂-YSZ kernels were fabricated to simulate the PuO₂-YSZ kernel and coated with a ZrC layer. In this study, we clarified that both Ce-rich grains and Zr-rich grains were densely distributed in surface regions of the as-fabricated CeO₂-YSZ kernel. However, we have already clarified that the surface region of the CeO₂-YSZ kernel coated with a ZrC layer was porous and mainly consisted of Zr-rich grains. These experimental results confirmed that Ce-rich grains were selectively corroded during the ZrC coating process. Then, the chemical stability of Zr-rich grains would be higher than that of Pu-rich grains. Thus, it would be more difficult to extract Pu from PuO₂-YSZ kernels (in which almost all grains are Zr-rich) than from PuO₂-YSZ kernels (in which many Pu-rich grains are included). Influences of the sintering of fuel compact on the microstructure of the ZrC-coated CeO₂-YSZ kernel is also reported.     

Recovery of neutron-irradiation-induced defects of Al2O3, Y2O3, and yttrium-aluminum garnet

SiC fiber-reinforced SiC matrix composites (SiC_f/SiC) are considered as one of the candidates for blanket materials in future fusion reactors and as an advanced fuel cladding material for next-generation fission reactors. Generally, the densification of SiC needs sintering additives and oxides such as Al₂O₃, Y₂O₃, and yttrium-aluminum garnet (YAG, Y₃Al₅O₁₂), which are frequently added to SiC. However, the effects of neutron irradiation on sintering additives are still unclear. In this study, we performed the neutron irradiation of Al₂O₃, Y₂O₃, and YAG at fluences up to 2.0–2.5 × 10²⁴ n/m² (E > 0.1 MeV) at 60–90 °C. The isochronal recovery of the macroscopic volume of Al₂O₃ against annealing temperature showed smooth and continuous shrinkage at a temperature of up to 1200 °C, and the volume slightly increased above that temperature. In contrast, the volume of Y₂O₃ showed quick shrinkage at the low temperature range, and slower and smooth recovery was observed up to ～1100 °C. In the case of YAG, the recovery of volume occurred in a step-wise manner at 600–750 °C, and continuous shrinkage occurred at temperatures lower and higher than that temperature range. The activation energies for the macroscopic volume recoveries of three oxides were obtained from the Arrhenius plots of the rate coefficients. Two-stage recovery was observed for Al₂O₃, whereas more complicated recovery processes were suggested for Y₂O₃ and YAG.     

Synthesis and characterization of zirconia-magnesia inert matrix fuel: Ce homolog studies

X-ray powder diffraction, microscopy, thermal analysis and electron probe microanalysis were used to characterize a ZrO₂-MgO inert matrix containing CeO₂ as a homolog for PuO₂ and Er₂O₃ as a burnable poison. The synthesis was carried out using a precipitation method. A large composition range of MgO to ZrO₂ was evaluated to determine phases present, phase mixing, phase composition, microstructure and thermal properties. It was found that most compositions of the material consist of two phases: MgO (periclase) and ZrO₂ (cubic zirconia). The zirconia phase incorporates 5% (wt/wt) MgO and up to 14% and 12% (wt/wt) CeO₂ and Er₂O₃, respectively. The MgO phase remains pure, which will enable it to retain its heat transfer and solubility properties and will improve the overall thermal conductivity and reprocessing component of the inert matrix fuel. The results with Ce will be used as the basis of future studies with actinides.     

Reactivity enhancement of chemical materials used in packed bed reactor of chemical heat pump

In this study, a mixture of expanded graphite (EG) and magnesium hydroxide (Mg(OH)₂) was used to enhance the thermal conductivity and reactivity of a magnesium oxide/water (MgO/H₂O) chemical heat pump, because EG is chemically stable and has high thermal conductivity and high moldability to form the heat exchange structure. Calcium chloride (CaCl₂) was also introduced into the mixture of EG and Mg(OH)₂ to ensure smooth diffusion of vapor in materials and enhance the fittability between EG and Mg(OH)₂. The reaction kinetics of pure Mg(OH)₂, a mixed material containing Mg(OH)₂ and CaCl₂ (termed MC), and a mixed material containing EG, Mg(OH)₂, and CaCl₂ (termed EMC) were examined under the same reaction conditions by performing thermobalance measurements. EMC exhibited a higher dehydration rate than the other materials. It also exhibited hydration reactivity at temperatures of up to 200 °C; at this temperature, pure Mg(OH)₂ exhibited low reactivity. The addition of CaCl₂ also enhanced the hydration reactivity of MgO because of the high water adsorption ability of CaCl₂ in EMC. A reaction rate equation for the hydration of EMC was proposed on the basis of an assumed reaction model. The thermal performance of a MgO/H₂O chemical heat pump manufactured using EMC was evaluated from this equation. EMC was concluded to have good potential for use as a packed bed material in the MgO/water chemical heat pump owing to its low cost, high hydration reactivity, high thermal conductivity, and high moldability to form the heat exchange structure.     

Preparation,characterisation and dissolution of a CeO2 analogue for UO2 nuclear fuel

The behaviour of spent nuclear fuel under geological conditions is a major issue underpinning the safety case for final disposal. This work describes the preparation and characterisation of a non-radioactive UO₂ fuel analogue, CeO₂, to be used to investigate nuclear fuel dissolution under realistic repository conditions as part of a developing EU research programme. The densification behaviour of several cerium dioxide powders, derived from cerium oxalate, were investigated to aid the selection of a suitable powder for fabrication of fuel analogues for powder dissolution tests. CeO₂ powders prepared by calcination of cerium oxalate at 800 °C and sintering at 1700 °C gave samples with similar microstructure to UO₂ fuel and SIMFUEL. The suitability of the optimised synthesis route for dissolution was tested in a dissolution experiment conducted at 90 °C in 0.01 M HNO₃.     

Production and investigation of thorium-containing fuel compositions

A technology has been developed for obtaining fuel tablets with the compositions (U, Th)O₂, (U, Th, Ca)O₂, and (U, Th)O₂+MgO by combined precipitation of uranium, thorium, magnesium, or calcium components from inert solutions, followed by heat treatment of the powders, compression into pellets, and sintering of the pellets. Work on optimizing the technological processes for obtaining fuel pellets so as to obtain good pellet quality was performed. The effect of the properties of the precipitates and powders, fabricated using different technological regimes on the properties of the finished objects was studied. The work includes detailed investigations of powders (x-ray phase analysis, electron-microscopic investigation) and sintered fuel tablets (change in the geometric dimensions as a result of sintering, determination of the density, and study of the microstructure). The behavior of fuel compositions (U, Th)O₂ and (U, Th)O₂+MgO in contact, with coolants under conditions where the fuel elements become unsealed was studied: with water at 300°C and sodium at 700°C. 3 figures, 3 tables, 6 references. State Science Center of the Russian Federation-A. I. Leipunskii Physics and Power-Engineering Institute. Translated from Atomnaya énergiya, Vol. 88, No. 5, pp. 346–353, May, 2000.     

Characterization and densification studies on ThO2-UO2 pellets derived from ThO2 and U3O8 powders

ThO₂ containing around 2-3% ²³³UO₂ is the proposed fuel for the forthcoming Indian Advanced Heavy Water Reactor (AHWR). This fuel is prepared by powder metallurgy technique using ThO₂ and U₃O₈ powders as the starting material. The densification behaviour of the fuel was evaluated using a high temperature dilatometer in four different atmospheres Ar, Ar-8%H₂, CO₂ and air. Air was found to be the best medium for sintering among them. For Ar and Ar-8%H₂ atmospheres, the former gave a slightly higher densification. Thermogravimetric studies carried out on ThO₂-2%U₃O₈ granules in air showed a continuous decrease in weight up to 1500 °C. The effectiveness of U₃O₈ in enhancing the sintering of ThO₂ has been established.     

Effect of CuO on CaTiO3 perovskite ceramics prepared using a direct sintering process

Effect of CuO on CaTiO₃ (CT) ceramics prepared using a direct sintering process (reaction-sintering process) was investigated. The mixture of raw materials was pressed and sintered into ceramics without any calcination stage involved. Pure CT could be obtained. The degree of densification in CT via reaction-sintering process is lower than traditional oxide route but the grains grew easier in CT via reaction-sintering process. A density 3.63 g/cm³ (90.3% of ρ_th) is obtained in CT pellets after 1500 °C/16 h sintering. With 3 wt.% CuO addition, density 3.92 g/cm³ (97.5% of ρ_th) is obtained after 8 h sintering at 1500 °C due to the liquid phase sintering. The liquid phase at grain boundaries appeared significantly at a lower sintering temperature for longer soak time.     

Spark plasma sintering of iodine-bearing apatite

The high chemical durability of iodine-bearing apatite makes it strongly prospective for conditioning of radioactive iodine. The synthesis and consolidation of iodine-bearing compounds require low temperatures to avoid iodine volatilization. Spark plasma sintering therefore appears to be a suitable process because of its shorter treatment time and lower sintering temperature compared with other processes such as HUP or HIP. Two alternatives were examined: SPS sintering of iodine-bearing apatite powder and SPS reacting of a stoichiometric lead iodide and lead phosphovanadate powder mixture. The degree of densification and the microstructure of bulk materials in both cases are described and compared. Reactive sintering appears to involve a three-stage mechanism: (i) PbI₂ coalescence, (ii) solid-state iodoapatite synthesis and consolidation and, (iii) iodoapatite consolidation in the presence of a liquid phase. The SPS reacted products reveal the finest and most homogeneous microstructure, and a density exceeding 96%.     

Research and Development of Minor Actinide-containing Fuel and Target in a Future Integrated Closed Cycle System

Research and development of minor actinide-containing fuels and targets, i.e., (Pu,Am)O₂–MgO, (Pu,Np)O₂–MgO, (U,Pu,Np)O₂, (U,Pu,Np)N and (Pu,Np,Zr)N, for use in a future integrated closed cycle system that includes fast reactor and accelerator driven sub-critical system is underway. The present statuses of fabrication test and property measurements are given. Design concept of the oxide target is described in detail together with a screening of the support material. A new apparatus for the measurement of mechanical properties at the elevated temperature is installed for use in evaluating the fuel-cladding mechanical interaction. Development histories with future prospects of two types of Np-containing fuels for the fast reactor are mentioned. Preliminary test results for a new nitride target for the accelerator driven sub-critical system are given. Finally, an irradiation test plan in the experimental fast reactor JOYO is briefly described.     

Melting behavior of MgO-based inert matrix fuels containing (Pu,Am)O2−x

The melting behavior of MgO-based inert matrix fuels containing (Pu,Am)O_2−x ((Pu,Am)O_2−x-MgO fuels) was experimentally investigated. Heat-treatment tests were carried out at 2173 K, 2373 K and 2573 K each. The fuel melted at about 2573 K in the eutectic reaction of the Pu-Am-Mg-O system. The (Pu,Am)O_2−x grains, MgO grains and pores grew with increasing temperature. In addition, Am-rich oxide phases were formed in the (Pu,Am)O_2−x phase by heat-treatment at high temperatures. The melting behavior was compared with behaviors of PuO_2−x-MgO and AmO_2−x-MgO fuels.     

Fission-induced densification of UO2

A new technique has been developed to study fission-induced densification and hot-pressing of UO₂ at very low temperatures without complications from fracturing or other concurrent thermal effects. Thin disks of UO₂ of 0.22 to 20% enrichment and differing microstructural stability, were irradiated in the core of the CP — reactor at temperatures below 200°C and at pressures from 1 atm to 20.7 MPa. Results indicate that the pressurizing medium, NaK, had penetrated the open porosity at high pressure and impeded densification. To rationalize this effect, the previously proposed models for radiationinduced densification are critically reviewed. Modifications to models involving thermal sintering and hot-pressing, and pore resolution appear the most tenable. The former mechanism leads to predictions that fission-induced hot-pressing can occur, and ex-reactor sintering and hot-pressing should correlate with in-reactor densification. The proximity of pores and grain boundaries is also stressed. The effect of NaK logging is rationalized by changes in pore surface energy and by stabilization of small pores aginst complete resolution.     

Ion-induced amorphization in ceramic materials

Amorphization induced by swift heavy ions is discussed, the main features are reviewed and explained by the author’s model. In Al₂O₃ and MgAl₂O₄ the recrystallization reduces considerably the track diameters. The threshold electronic stopping power for amorphization S_et can be estimated reliably for these solids from the position of the amorphous-crystalline boundary formed at high ion fluences. The results are in good agreement with the predictions of the model. Experiments on CeO₂ and UO₂ are discussed. Estimates of S_et are made for β-Si₃N₄, CeO₂, pure ZrO₂, ZrSiO₄, and UO₂, and 10.1 > S_et > 6.2 keV/nm is obtained at room temperature for fission fragment energies. At about 1000 °C operation temperature, S_et is reduced by about 35-50%. No amorphization is expected by ion bombardment in AlN, SiC (semiconductors) and MgO (ionic crystal).