Local Gamma-Ray Counting Efficiency of Halogen Counter with Thick Anode

A substance for solidifying waste containing ¹²⁹I is sought that effectively sorbs iodine to inhibit its release from repository into the environment. Three candidate media—commercial alumina cement mixed with calcium sulfate and/or calcium hydroxide—were investigated. The criterion applied for evaluating iodine sorption performance was the distribution coefficient Kd of iodine between solid and solution in an emulsion of sodium iodide solution and powder prepared from the alumina cement/calcium compound mixture, which was molded, cured and ground. Batch sorption experiments were performed on different combinations of the above-mentioned calcium compounds added to alumina cement. The solidified substance was also examined for mechanical strength. The highest iodine sorption performance was obtained with calcium sulfate added to alumina cement to a SC₄/Ca mole ratio of 0.16, which ensured a Kd value raised to a level of not lower than 0.2 m³/kg from the (1.8–3.2)x10^-3 m³/kg obtainable with alumina cement before calcium compound addition. The enhancement of iodine sorption with addition of calcium sulfate is attributed to formation in the substance of monosulfate (3CaO·Al₂O₃·CaSO₄12H₂O) or tetracalcium aluminate hydrate (4CaO·Al₂O3·XH₂O (X=13 to 19)).     

Dissolution behavior of (U,Zr)O2-based simulated fuel debris in nitric acid

To explore the possibility of dissolving fuel debris into nitric acid as a potential pre-treatment for waste treatment in which the U and Pu are removed from the inventory, dissolution tests of U_1?xZr_xO₂ and (U,Pu)_1?xZr_xO₂ were carried out in 6 M HNO₃ at 353 K. At the end of the dissolution test (after 4 h), the ratio of dissolved uranium decreased with an increase in the Zr contents, x. While the dissolution of U-rich samples was congruent, a preferential leaching of U was observed with Zr-rich samples. Taking into account these different dissolution phenomena, the dissolution rate analysis was carried out using surface-area model to calculate the instantaneous dissolution rate (IDR). The IDR decreased from 10^?5 down to 10^?10 mol cm^?2 min^?1 as x increased from 0 to 0.95. From these findings, dissolution with HNO₃ is expected to be only applicable in U-rich part of fuel debris (x < 0.3) if the dissolution in 6 M HNO₃ at 353 K is assumed. Application of complexing acids, such as mixture of HNO₃ and HF, should be considered to increase the dissolution rate of the Zr-rich part.     

Solidification of “Cement-Glass”

Cement-glass, which is a mixture of sodium silicate (kNa₂O·mSiO₂nH₂O), silicon phosphate (P₂O₅·2SiO₂) and cement, was developed to solidify radioactive waste pellets in containers. The optimum molar ratio of Si(OH)₄, NaOH and P₂O₅2SiO₂ was found to be 2:2:1, based on compressive strength measurement of solidified samples. The compressive strength of solidified sodium silicate with silicon phosphate was increased when the amount of solidified SiO₂ per unit volume was increased by reducing the water content. Cement-glass had a higher Cs distribution factor than ordinary Portland cement.     

Effect of LiNO3 on Corrosion Prevention of Aluminum Wastes after Their Land Disposal

After their land disposal, LiNO₃ added to cement solidified miscellaneous wastes inhibits hydrogen gas generation due to alkaline corrosion of aluminum contained in the wastes. We considered the presence of an Li-Al preservation film prevents hydrogen gas generation, and then, we assumed a scenario in which the amount of LiNO₃ included in the waste packages is lowered by underground water penetration, resulting in dissolution of the Li-Al preservation film. This dissolution allows the alkaline underground water to reach and corrode the aluminum materials. The loss of Na₂O and K₂O in cement by underground water penetration lowers the pH, so that the aluminum corrosion in the waste packages with LiNO₃, expected when the Li-Al preservation film dissolves, is less than that without LiNO₃.

To test this scenario, we measured solubility of the Li-Al preservation film, Li⁺ ion concentration, pH variation by underground water penetration, and aluminum corrosion when the Li-Al preservation film had dissolved. The measured solubility of the Li-Al preservation film was 3 × 10^?4 _M at 283 K. At that time, pH was lowered from 12.9—13.0 to 12.2—12.3. As a result, with LiNO₃ addition the aluminum corrosion amount was reduced to 10% of that without LiNO₃ addition, because of the pH decrease.     

Melting behavior and thermal conductivity of solid sodium-concrete reaction product

This study revealed melting behavior and thermal conductivity of four samples generated by sodium-concrete reaction (SCR). We prepared the samples using two methods such as firing mixtures of sodium (Na) and grinded concrete powder, and sampling depositions after the SCR experiments. In the former, the mixing ratios were determined from the past experiment. The latter simulated the more realistic conditions such as the temperature history and the distribution of Na and concrete. The thermogravimetry-differential thermal analyzer (TG-DTA) measurement showed the temperatures of the onset of the melting (solidus temperatures) were 865–942°C, but those of the samples containing metallic Na could not be clarified. In the two more realistic samples, the compression moldings in a furnace were observed. The observation revealed the softening temperature was 800–840°C and the solidus temperature was 840–850°C, which was 10–20°C lower than the TG-DTA results. The thermodynamics calculation of FactSage 7.2 revealed the solidus temperature was caused by melting of the some components such as Na₂SiO₃ and/or Na₄SiO₄ and NaAlO₂. Moreover, the thermal conductivity was λ ~ 1–3 W/m-K, which was comparable to xNa₂O - (1 - x)SiO₂ (x = 0.5, 0.33, and 0.25), and that at 700°C was explained by NBO/T of Equation (1).     

Dissolution Phenomena of Ca0-Si02-H20 Gel at Ca/Si>l Coexisting with Ettringite System

We have studied the influence of interaction between Ca0-Si02-H2 0 (C-S-H) gel and ettringite (AFt) on their dissolution regarding the initial chemical feature of degraded cement pore water. In this work, firstly dissolution experiments were performed in C-S-H gel at Ca/Si=1.0, 1.2 and 1.47 coexisting with AFt of which amount was 5% of mol corresponding to each C-S-H gel. Secondly, assuming the congruent dissolution of AFt, dissolution of the same system was calculated by combining with the dissolution model of C-S-H gel at 1< Ca/Si ≤1.5, considering without interaction between C-S-H gel and AFt. Measured and calculated results of Ca, Si, Al and S total concentrations and pH values of C-S-H gel coexisting with AFt system were investigated. Discrepancies between experimental and calculated Ca, Si, Al and S total concentrations and pH values suggested the importance of interaction phenomena in C-S-H gel coexisting with AFt system. By considering the substitutions of Al(OH)^? ₄ and SO^3? ₄ with Si tetrahedra in the structure of C-S-H gel, these discrepancies are discussed in this study.     

Feasibility studies on the selective separation of fission palladium(II) by isoHex-BTP/SiO2-P adsorbent from HLLW

Aiming at the selective recovery of fission palladium(II) from high-level liquid waste (HLLW), the silica/polymer (SiO₂-P)-based isoHex-BTP adsorbent (isoHex-BTP/SiO₂-P) was synthesized by impregnating complexing agent isoHex-BTP into the multiporous SiO₂-P inert support. The feasibility of separation of Pd(II) from HLLW by isoHex-BTP/SiO₂-P was evaluated by batch experiment method. The results showed that isoHex-BTP/SiO₂-P exhibited much higher adsorption selectivity for Pd(II) than the other fission products, even Am(III) and Pu(IV) presented in HLLW. The ideal nitric acid concentration for the adsorption of Pd(II) by the adsorbent was shown to be ≥2 mol dm^–3. The adsorption of Pd(II) fits well to the pseudo-second-order kinetic model and Langmuir isotherm model. Quantitative Pd(II) desorption was achieved by using 0.5 mol dm^?3 SC(NH₂)₂ - 0.1 mol dm^?3 HNO₃ solution.     

Adsorption mechanism of silica/polymer-based 2,6-bis(5,6-diisohexyl-1,2,4-triazin-3-yl)pyridine adsorbent towards Ln(III) from nitric acid solution

2,6-Bis(5,6-diisohexyl-1,2,4-triazin-3-yl)pyridine (isoHexyl-BTP) is a nitrogen-donor chelating ligand which shows high extraction selectivity for minor actinides (MA) over lanthanides (Ln). We synthesized a macroporous sillica/polymer-based isoHexyl-BTP adsorbent (isoHexyl-BTP/SiO₂-P) for the separation of MA(III) from Ln(III) in high-level liquid waste. The work focused on the isoHexyl-BTP/SiO₂-P adsorption mechanism towards Ln(III) in nitric acid solution and the coordination chemistry between Ln(III) and isoHexyl-BTP/SiO₂-P through batch adsorption experiments, extended X-ray absorption fine spectroscopy, acid-base titration and ion chromatography. It was found that both H⁺ and NO₃^? directly participated in the adsorption reaction. For the middle and heavy Ln(III), Ln(isoHexyl-BTP/SiO₂-P)₃(NO₃)₃·3HNO₃ was supposed as the main product of the adsorption. The Ln(III)-N (Ln(isoHexyl-BTP/SiO₂-P)₃³⁺) bond length of the first coordination layer decreased as the atomic number of Ln(III) increased, which explains the increased adsorption affinity of isoHexyl-BTP/SiO₂-P towards middle and heavy Ln(III) in relatively high concentration nitric acid as the Ln(III) atomic number increased.     

Temperature Effect on Plutonium Leach Rate of Nuclear Waste Glass

The two kinds of nuclear waste glass with similar composition, a ²³⁸Pu-doped and nonradioactive waste glass, were leached under the ISO-test conditions at temperature between 23 and 90°C. An activation energy of 22±10kJ/mole was obtained from the initial leach rates of Pu, which was much lower than the 78±9kJ/mole obtained from those of Si, Na, Sr and Cs, It is suggested that in the initial stages of leaching. Pu is not released from the waste glass with the same mechanisms as the releases of Si, Na, Sr and Cs, but the dissolution of hydrous plutonium dioxide PuO₂·_xH₂O formed on the glass surface becomes predominant. In the long duration tests (<32d), the release of Pu appears to be affected by the solubility of PuO₂·_xH₂O remaining in the leached surface layers.     

Thermochemical and Experimental Considerations of NOx Composition and Iodine Species in the Dissolution of Spent PWR-Fuel Specimens

Abstract

The NO_x composition and iodine species in the dissolution of spent fuels are discussed on the basis of thermochemical calculations and experimental results. The influence of N0_x sparging on the expulsion of iodine is also discussed. The dissolution of a spent PWR-fuel specimen (–3g) in 30 ml of 3.5_MHNO₃ at 100°C is calculated to yield a concentration of 7×10^?2atm of N0₂, which is 80% of the total NO_X in the dissolver. This N0₂ fraction is much higher than experimental values of 15% or less that were reported for dissolver off-gas cooled near to room temperature. The high N0₂ fraction suppresses the formation of iodate (IO₃ ^?) in the dissolution. The calculations predict that IO₃ ^?) is not formed in 3.5 M HNO₃ at 100°C at an NO₂ pressure ≥3×10^?2 atm (3kPa). Attempts to expel iodine from the fuel solution indicated that the main iodine species in the solution was colloidal iodine and not iodate (I0₃ ^?) which earlier workers postulated. The obtained experimental results are consistent with the thermochemical predictions. For the decomposition of the colloidal iodine in the expulsion process, NO₂ sparging has a negative effect. This is because an increase in NO₂ pressure promotes the formation of colloidal Agl.     

Electrical resistivity and lattice parameter of uranium monocarbide

The resistivity dependence of as-cast and annealed UC on temperature (77–300 K) as well as the $\text{C}\text{U}$ ratio have been investigated experimentally. Additionally, lattice constants of UC have been measured in its nonstoichiometric regions. Estimated values of the electrical resistivity of stoichiometric UC (annealed at 1500°C for 3 h) were (10 ± 2) μΩ · cm at liquid-nitrogen temperature and (34 ± 3) μΩ · cm at room temperature, and the value of the lattice constant was (4.958 ± 0.001) Å at room temperature. It was also estimated that 1 at% of carbon vacancies in UC_1?x and oversaturated carbon interstitials in UC_1+x result in resistivity increases of (12 ± 2) μΩ · cm and 6 μΩ · cm, respectively. A very narrow nonstoichiometric region was observed in UC at 1500°C. It might lie between UC_0.98 and UC_1.01     

Effect of ion-irradiation on the microstructure and microhardness of the W-2Y2O3 composite materials fabricated by sintering and hot forging

A W-2Y₂O₃ material was developed in collaboration with the Plansee Company (Austria). An ingot of the material having approximate dimension of 95 mm × 20 mm was fabricated by mixing the elemental powders followed by pressing, sintering and hot forging. The microstructure of the W-2Y₂O₃ composite was investigated using transmission electron microscopy (TEM). The microhardness was studied using nano-indentation technique. We observed that the W-grains having a mean size of about 1 μm already formed and these grains contain very low density of dislocations. The size of the yttria particles was between 300 nm and 1 μm and the Berkovich hardness was about 4.8 GPa. The specimens were irradiated/implanted with Fe and He ions at JANNuS facility located at Orsay/Saclay, France. The TEM disks kept were irradiated/implanted at 300 and 700 °C using Fe and He ions with an energy of 24 and 2 MeV, respectively. The calculated radiation dose was about 5 dpa produced by Fe ions and total He content is 75 appm at both 300 and 700 °C. From the TEM investigation of irradiated samples, few radiation loops are present on the W grains, whereas on yttria particles, the radiation induced damages appear as voids. Berkovich hardness of the irradiated sample is higher than that of the non-irradiated sample. Results on the microstructure and microhardness of the ion-irradiated W-2Y₂O₃ composites are presented in detail.     

Comparative study of silicate glasses containing Bi2O3, PbO and BaO: Radiation shielding and optical properties

The radiation shielding and optical properties of xBi₂O₃:(100-x)SiO₂, xPbO:(100-x)SiO₂ and xBaO:(100-x)SiO₂ glass systems (where 30 ? x ? 70 is the composition by weight%) have been investigated. Total mass attenuation coefficients (μ_m) of glasses at 662 keV were improved by increasing their Bi₂O₃ and PbO content, which raised the photoelectric absorption in glass matrices. Raising the BaO content to the same fraction range, however, brought no significant change to μ_m. These results indicate that photon is strongly attenuated in Bi₂O₃ and PbO containing glasses, and but not in BaO containing glass. The results from the optical absorption spectra show an edge that was not sharply defined; clearly indicating the amorphous nature of glass samples. It is observed that the cutoff wavelength for Bi₂O₃ containing glass was longer than PbO and BaO containing glasses.     

Dissolution Behavior of Uranium Oxides with Supercritical CO2 Using HNO3-TBP Complex as a Reactant

Dissolution behavior of U₃O₈ and UO₂ using supercritical CO₂ medium containing HNO₃-TBP complex as a reactant was studied. The dissolution rate of the oxides increased with increasing the HNO₃/TBP ratio of the HNO₃-TBP complex and the concentration of the HNO₃-TBP complex in the supercritical CO₂ phase. A remarkable increase of the dissolution rate was observed in the dissolution of U₃O₈ when the HNO₃/TBP ratio of the reactant was higher than ca. 1, which indicates that the 2:1 complex, (HNO₃)₂TBP, plays a role in facilitating the dissolution of the oxides. Half-dissolution time (t_½ ) as an indication of the dissolution kinetic was determined from the relationship between the amount of uranium dissolved and the dissolution time (dissolution curve). A logarithmic value of a reciprocal of the t_½ was proportional to the logarithmic concentration of HNO₃, C_HNO3, in the supercritical CO₂. The slopes of the (l/t_½ ) vs. ln C_HNO3 plots for U₃O₈ and UO₂ were different from each other, indicating that the reaction mechanisms or the rate-determining steps for the dissolution of U₃O₈ and UO₂ are different. A principle of the dissolution of uranium oxides with the supercritical CO₂ medium is applicable to a method for the removal of uranium from solid matrices.     

Surface Alteration of Pollucite under Hydrothermal Conditions

In connection with studies for evaluating the leachability of Cs from solidified radio-active waste, the surface alteration of pollucite was studied under hydrothermal conditions. Pollucite exposed to NaCl or KCl at 300°C formed on its surface a precipitate phase of analcime in the case of NaCl or leucite in the case of KCl. Cubo-octahedral analcime built up not only on the outermost surface of sample but also in the altered layer underneath. This crystal penetration was attributed to precipitation and isomorphous substitution of Na by Cs in the pollucite. The leachability of Cs proved to increase in keeping with salt concentration.

Exposure to CaCl₂ or to MgCl₂ formed a thick precipitate layer of anorthite in the case of CaCl₂ or clinochlore in the case of MgCl₂ over a thin reaction layer. This precipitate layer had the effect of inhibiting the migration of Cs. Si and Al to restrain Cs leaching into solution, which caused the amount of Cs leached to level off beyond 0.01 mol·dm^?3 salt concentration in the case of CaCl₂, and beyond 0.01 mol·dm^?3 in the case of MgCl₂.     

Formation of columnar and equiaxed grains by the reduction of U3O8 pellets to UO2+x

The reduction of U₃O₈ pellets to UO_2+x has been investigated at 1300 °C in H₂, Ar and CO₂ gas atmospheres by TGA, SEM, and X-ray diffraction. The selected U₃O₈ pellet was prepared by sintering a U₃O₈ powder compact. The TGA results show that the reduction rate is fastest in H₂ gas, and X-ray diffraction indicates that U₃O₈ reduces to UO_2+x without any intermediate phase. The reduced pellet, UO_2+x, has a special grain structure that consists of equiaxed grains at the surface, columnar grains in the middle, and equiaxed grains in the center. The equiaxed grains and columnar grains are much smaller in H₂ gas than in Ar or CO₂ gas. The reducing gases significantly influence the morphology of the grain structure. This difference can be explained in terms of a relation between oxygen potential and critical nucleus size during the reduction.     

Silicate Anion Structural Change in Calcium Silicate Hydrate Gel on Dissolution of Hydrated Cement

High pH conditions of aqueous solutions in a radioactive waste repository can be brought about by dissolution of cementitious materials. In order to clarify the mechanisms involved in maintaining this high pH for long time, we investigated the dissolution phenomena of OPC hydrate.

In the present research, leaching tests on powdered cement hydrates were conducted by changing the ratio of mass of leaching water to mass of OPC hydrate (liquid/solid ratio) from 10∽2, 000 (wt/wt). Ordinary Portland Cement hydrate was contacted with deionized water and placed in a sealed bottle. After a predetermined period, the solid was separated from the solution.

From the results of XRD analysis on the solid phase and the Ca concentration in the aqueous phase, it was confirmed that Ca(OH)₂ was preferentially dissolved when the liquid/solid ratio was 10 or 100 (wt/wt), and that C—S—H gel as well as Ca(OH)₂ were dissolved when the liquid/solid ratio was 500 (wt/wt) or larger. ²⁹Si-NMR results showed that the silicate anion chain of the C-S-H gel became longer when the liquid/solid ratio was 500 (wt/wt) or greater. This indicates that leaching of OPC hydrate results in a structural change of C—S—H gel.     

Synthesis and characterization of lithium silicate powders

Lithium-based ceramics, such as Li₂O, LiAlO₂, Li₄SiO₄, Li₂SiO₃, Li₂TiO₃and Li₂ZrO₃, have long been recognized as promising tritium breeding-materials for D-T fusion reactor blankets. Among these candidate materials, lithium orthosilicate (Li₄SiO₄) and lithium metasilicate (Li₂SiO₃) are recommended by many ITER research teams as the first selection for the solid tritium breeder. Li₄SiO₄ has even been selected as the breeder material for the helium-cooled solid breeder test blanket module (HCSB TBM) in China and EU. In present study, the processes of solid-state reaction between amorphous silica and Li₂CO₃ powders was studied by thermogravimetry analysis-differential scanning calorimetry (TGA/DSC); the lithium silicate powders were synthesized at 700, 800 and 900 ° C with Li:Si molar ratios of 0.5, 1, 2 and 4, respectively, using solid-state reaction method. The as-prepared lithium silicates were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that the phase composition and morphology of the as-prepared samples change with the different synthesis conditions. At low temperature of 700 °C, all samples contain the amorphous silica, and the major crystalline phase is Li₂SiO₃ with different microstructure for Li/Si ratio of 0.5, 1 and 2. As for Li/Si=4, 98% purity of Li₄SiO₄ can be obtained at 700 °C. At high temperature of 900 °C, the significant sinterization effect will occur in all samples and Li₄SiO₄ will even decompose. The results also show that pure Li₄SiO₄ can be synthesized by calcining at 800 ° C for 4 h, and its’ solid-state reaction synthesis may be divided into two steps:

(1)

515-565 °C: Li₂CO₃+SiO₂→Li₂SiO₃+CO₂;

(2)

565-754 °C: Li₂CO₃+SiO₂→Li₂SiO₃+CO₂ and then Li₂SiO₃+Li₂CO₃→Li₄SiO₄+CO₂.

While Li/Si=2, 99% purity of and pure Li₂SiO₃ can be obtained at 800 and 900 °C, respectively.     

Reactions between SiC and SrO at High Temperatures

A mixture of SiC and SrO was heated in air, in Ar and in vacuum, and the reaction products were determined by powder X-ray diffraction. The reaction began above 1,000°C and the products varied, depending on the oxygen content in the heating atmosphere. For example, in the case of a mixture of 1:1 in atomic ratio, SrO·SiO₂ formed in air, while in oxygen-free atomsphere, SrC₂ and Sr were produced besides SrO·SiO₂. It was also found that the powder X-ray diffraction pattern of SrO·SiO₂ obtained in the present experiment differed in part from O'Daniel's data. It is concluded that study of the microstructural changes in the SiC coating layer at high temperatures should be most important for assuring the retainment of the fission products Sr and Ba in coated particle fuels.     

Selectivity of Alkali Metal Ion and Lithium Isotopes on Ion Exchangers in H Form Prepared from LiTi x Zr2-x (PO4)3 (x=0, 1)

Inorganic metal (IV) phosphate-based ion exchanges in the hydrogen form, HTi _x Zr_2-x (PO₄)₃ (x =0, 1), have been prepared by leaching lithium ions from the precursors, LTi _x Zr_2-x (PO₄)₃ (x=0, 1), through ion exchange with protons. The degrees of leaching of lithium ion were more than 99%. Both ion exchangers showed high selectivity toward lithium and sodium ions and little affinity to rubidium and cesium ions among alkali metal ions. The lithium ion uptakes from aqueous solutions were monotonously increasing functions of pH. Isotopically, both ion exchangers were ⁶Li-specific like other inorganic ion exchangers so far examined. The ⁷Li-to-⁶Li isotopic single-stage separation factor, S, of HTiZr(PO₄)₃ was larger than that of HZr₂(PO₄)₃ at a given pH-value. The relatively large S-values of 1.022 and 1.042 were found for HZr₂ (PO₄)₃ for HTiZr(PO₄)₃, respectively, at 25°C when the pH of the solution phase was around 5.