Simulated High-level Radioactive Waste Research Articles

Neutron resonance absorption imaging of simulated high-level radioactive waste in borosilicate glass

We performed a preliminary study of neutron resonance absorption imaging to investigate the spatial distribution of constituent elements in borosilicate glasses containing simulated high-level radioactive waste, in which elemental inhomogeneities affect the physical and chemical stabilities of the glass. Dips generated by the resonance absorptions of Rh, Pd, Na, Gd, Cs, and Sm were observed in the neutron transmission spectra of the glass samples. The spatial distributions of these elements were obtained from the neutron transmission images at the resonance energies. The distributions of Rh and Pd visualized the sedimentation of these platinum group elements. In contrast, the lanthanides (Gd and Sm) and Cs were uniformly dispersed. These results show that neutron resonance absorption imaging is a promising tool for characterizing borosilicate glasses and investigating the vitrification mechanism of high-level radioactive waste.

Influence of Rock Structure on Migration of Radioactive Colloids from an Underground Repository of High-Level Radioactive Waste

Studies of leaching of vitrified simulated high-level radioactive waste (HLW) evidence that most of actinides or their simulators enter leaching water in a colloidal form. In this paper, we consider a mechanism of colloid-facilitated migration of radionuclides from an underground repository of HLW located at a depth of a few hundreds of meters in fractured crystalline rocks. The comparison between data of field and laboratory measurements showed that the bulk permeability of the rock massif in field tests is much greater than the permeability of rock samples in laboratory experiments due to an influence of a network of fractures in the rock massif. Our theoretical analysis presents evidence that this difference can take place even in a case when the network is not continuous, and the fractures are isolated with each other through a porous low-permeable matrix of the rock. Results of modelling revealed a possibility of mechanical retention of radionuclide-bearing colloid particles in the frame of rock during their underground migration.

Recovery of Se, Zr, Pd, and Cs from simulated high‐level radioactive waste glass through phase separation

AbstractIn this study, elemental recovery was performed using phase separation from simulated high‐level radioactive waste (HLW) glass. To cause phase separation, SiO2 and B2O3 were added to the simulated HLW glass and adjusted the ratio of SiO2: B2O3: other oxides to 40:50:10. The phase separated glass was immersed in aqueous solutions of 0–3 mol/L of HNO3, H2SO4, and a 1:1 mixture of HCl–HNO3 at 363 K for 20 h, and the dissolution behavior of 17 elements was examined. The relationship between the dissolved mass fraction of each element and the acid concentration in the immersion liquid could be approximated by the modified sigmoid function. The recovery of stable nuclei Se, Zr, Pd, and Cs instead of long‐lived radioactive nuclei was tested using a four‐stage leaching process in which the sample was immersed sequentially in four aqueous solutions at 363 K of distilled water, HNO3, H2SO4, and a 1:1 mixture of HCl–HNO3 for 20 h. It was confirmed that Se, Zr, Pd, and Cs could be recovered selectively. Furthermore, the recovery result could be predicted based on the individual dissolution results described above.

Selective Pd Separation from Simulated Radioactive Liquid Waste by Precipitation Using Xenon Lamp Irradiation for a Simplified Procedure.

In this study, we developed a simple and one-step Pd separation technique based on photoreduction with Xe lamp irradiation for the determination of 107Pd in highly radioactive samples. A simulated high-level radioactive liquid wastes (HLLW) solution, consisting of 14 major elements in a 3 mol L-1 HNO3 solution, was used to evaluate the separation performance. The Pd precipitate was formed by Xe lamp irradiation, and recovered by centrifugation. The Pd recovery from the simulated HLLW solution reached up to 50%, while 99.5% of the other 13 elements was separated. These results indicate that the applicability of the proposed separation technique to HLLW samples.

Studies on Ca substituted CePO4 as waste form matrix for the immobilization of simulated high level radioactive waste

Studies on Ca substituted CePO4 as waste form matrix for the immobilization of simulated high level radioactive waste

Phase separation of molybdenum, cesium and selenium from the borosilicate glass containing simulated nuclear wastes under a CO2-rich heating atmosphere

Molybdenum, cesium and selenium along with potassium-rich materials were phase-separated onto the surface from the borosilicate glass containing simulated high level radioactive wastes by heat treatment under a CO2-containing atmosphere. The phase separation behaviors of potassium, molybdenum, cesium and selenium were characterized by FE-SEM, EDS, XPS and Raman analysis. These elements could be further extracted by leaching for recovery. The leaching results show that the extraction efficiencies of these elements under CO2-containing atmosphere were much higher than that under air atmosphere. Attempts were further made to investigate the effects of the melting temperature, melting time, the concentration of CO2 gas and the amount of potassium carbonate flux on the extraction efficiencies of molybdenum, cesium and selenium from the glass phase under the CO2-containing atmosphere. As a result, more than ca. 80% molybdenum, almost 100% selenium and ca. 50% cesium were extracted in the leachate from the highly simulated borosilicate glass under the CO2-containing heat treatment. It is expected that this technique can be extended to study the separation of other fission products by optimizing the process parameters in the future research.

Real‐time monitoring of crystal accumulation in the high‐level waste glass melters using an electrical conductivity method

AbstractDuring the vitrification of high‐level radioactive waste (HLW) in HLW melters in the Waste Treatment and Immobilization Plant located in Washington State, spinel crystals may precipitate from glass and accumulate in the melter riser, preventing the discharge of molten glass into canisters. Therefore, an effort is being made to develop an electrical conductivity method to monitor crystal buildup in the melter riser. A vertically configured electrical conductivity (EC) probe with an alumina shaft and Pt‐10%Rh‐electrodes was designed and tested in standard conductivity solutions and glass melts both with and without spinel crystals. The EC probe measured conductivity in conductivity solutions within 10% of their certified values and showed a linear relationship with increased spinel layer thicknesses. Testing in silicate glass containing spinel crystals allowed for the determination of spinel conductivity as a function of temperature. The conductivities of spinel crystals of 20‐24 S/m at 800°C were in excellent agreement with the conductivity of trevorite (NiFe2O4) crystals at 800°C reported in the literature. The conductivities of spinel crystals and measured changes in the conductivity across the accumulated layer allowed for a successful measurement of spinel crystal accumulation in simulated HLW glass.

Modelling the sulfate capacity of simulated radioactive waste borosilicate glasses

The capacity of simulated high-level radioactive waste borosilicate glasses to incorporate sulfate has been studied as a function of glass composition. Combined Raman, 57Fe Mössbauer and literature evidence supports the attribution of coordination numbers and oxidation states of constituent cations for the purposes of modelling, and results confirm the validity of correlating sulfate incorporation in multicomponent borosilicate radioactive waste glasses with different models. A strong compositional dependency is observed and this can be described by an inverse linear relationship between incorporated sulfate (mol% SO42−) and total cation field strength index of the glass, Σ(z/a2), with a high goodness-of-fit (R2 ≈ 0.950). Similar relationships are also obtained if theoretical optical basicity, Λth (R2 ≈ 0.930) or non-bridging oxygen per tetrahedron ratio, NBO/T (R2 ≈ 0.919), are used. Results support the application of these models, and in particular Σ(z/a2), as predictive tools to aid the development of new glass compositions with enhanced sulfate capacities.

Platinum group metal particles aggregation in nuclear glass melts under the effect of temperature

The viscosity of simulated high level radioactive waste glasses containing platinum group metal particles is studied over a wide range of shear stress, as a function of the particles content and the temperature, thanks to a stress imposed rheometer, coupled to a high-temperature furnace. The system shows a very shear thinning behavior. At high shear rate, the system behaves as a suspension of small clusters and individual particles and is entirely controlled by the viscosity of the glass matrix as classical suspensions. At low shear rate, above a certain fraction in platinum group metal particles, the apparition of macroscopic aggregates made up of chains of RuO2 particles separated by thin layers of glass matrix strongly influences the viscosity of the nuclear glass and leads, in particular, to the apparition of yield stress and thixotropic effects. The maximum size of these clusters as well as their effective volume fraction have been estimated by a balance between Van der Waals attractive forces and hydrodynamic forces due to shear flow. We showed experimentally and theoretically that this aggregation phenomenon is favored by an increase of the temperature, owing to the viscosity decrease of the glass matrix, leading to an unusual increase of the suspension viscosity.

REDOX state analysis of platinoid elements in simulated high-level radioactive waste glass by synchrotron radiation based EXAFS

Extended X-ray Absorption Fine Structure (EXAFS) analyses were performed to evaluate REDOX (REDuction and OXidation) state of platinoid elements in simulated high-level nuclear waste glass samples prepared under different conditions of temperature and atmosphere. At first, EXAFS functions were compared with those of standard materials such as RuO2. Then structural parameters were obtained from a curve fitting analysis. In addition, a fitting analysis used a linear combination of the two standard EXAFS functions of a given elements metal and oxide was applied to determine ratio of metal/oxide in the simulated glass. The redox state of Ru was successfully evaluated from the linear combination fitting results of EXAFS functions. The ratio of metal increased at more reducing atmosphere and at higher temperatures. Chemical form of rhodium oxide in the simulated glass samples was RhO2 unlike expected Rh2O3. It can be estimated rhodium behaves according with ruthenium when the chemical form is oxide.

Viscoplasticity of simulated high-level radioactive waste glass containing platinum group metal particles

The shear rate dependency of the viscosity of three simulated high-level radioactive waste glasses containing 0, 1.2 and 4.5wt% platinum group metals (PGMs) was examined at a temperature range of 1173–1473K by a rotating viscometer. Shear stress when the shear rate equals zero, i.e. yield stress, was also measured by capillary method. The viscosity of the glass containing no PGM was shear rate-independent Newtonian fluid. On the other hand, the apparent viscosity of the glasses containing PGMs increased with decreasing shear rate, and nonzero amount of yield stresses were detected from both glasses. The viscosity and yield stress of the glass containing 4.5wt% PGMs was roughly one to two orders of magnitude greater than the glass containing 1.2wt% PGMs. These viscoplastic properties were numerically expressed by Casson equation.

Solid-phase Extraction of Palladium(II) Using Resins Impregnated with 8-Quinolinol Derivatives with a Long-chain Alkyloxymethyl Group

The solid-phase extraction (SPE) of palladium(II) using resins impregnated with 8-quinolinol derivatives (HQs) with a long-chain alkyloxymethyl group from acidic solutions was investigated. The HQs used were 5-octyloxymethyl-8-quinolinol (HO8Q) and 7-bromo-5-octyloxymethyl-8-quinolinol (HBrO8Q). Palladium(II) was quantitatively extracted from 5 M HNO3 with HQs-impregnated resins and from 2 M HCl with HBrO8Q-impregnated resins. The selective extraction and recovery of palladium(II) from a simulated spent autocatalysts solution and a simulated high-level radioactive waste solution could be accomplished using HQs-impregnated resins.

Effect of temperature on the extraction of Pd by liquid tin from molten borosilicate glass containing simulated radwaste

Separation of Pd from borosilicate glass-melt containing simulated high level radioactive waste (Pd, Mo, Cs, Sr, Zr, Nd, Ni, Fe) using Sn as the solvent metal was studied on lab scale. The separation of Pd was studied using varying amounts of Sn, and the percentage of extraction of Pd and other elements was investigated by varying the process temperature and the starting material for Pd. The process temperature was observed to have a profound influence on the separation of Pd from the glass melt to the metallic tin phase. The % extraction of Pd was found to be less for the batches heat-treated at temperatures below 900°C. The decrease in the % extraction was due to the entrainment of Pd in the glass phase for the samples heat-treated at low temperatures. The entrainment of Pd leads to the formation of Pd-rose like structures in the glass phase as observed by SEM and EDAX. Similarly, Mo, Fe and Na also formed separate phases inside glass matrix when the samples were heated below 900°C. However, entrainment was not seen when the samples were heat-treated above 1050°C in Ar. At 1200°C, a recovery of about 99% of Pd was obtained. The formation of a PdSn4 type intermetallic phase was noticed in tin metallic phase. The extraction behavior of Pd in borosilicate glass melt was also studied by using Pd(NO3)2 instead of Pd powder as staring material, and the results showed that the process temperature is the key determinant of the extraction behavior of Pd.

Effective separation of palladium from simulated high level radioactive waste

Aiming the selective recovery of palladium from high level radioactive liquid waste (HLW), a chelating thiamide type sorbent, CWP–TU, was prepared by the modification of Japanese cedar wood powder (CWP). Convection oven and microwave heating were separately used for modification purpose and found that microwave heating is more effective over oven heating. CWP–TU was extensively studied for the adsorption of Pd(II) from nitric acid medium. The batch test showed that nitric acid concentration of 3 M or higher is favorable for Pd(II) loading. Consistent adsorption of Pd(II) under gamma irradiation condition demonstrated the feasibility of using CWP–TU in real HLW. Also, Pd(II) only adsorption from simulated HLW solution verified the palladium only selectivity of the sorbent as well as the lack of influence of coexisting metal ions on its affinity toward Pd(II). CWP–TU holds maximum Pd(II) loading capacities of 0.98 mol/kg at 30 °C and 1.04 mol/kg under gamma irradiation. A comparative study using some ion exchange resins revealed that the resins are either ineffective in nitrate medium or lack stability under irradiation.

Pitting and Cracking of Steel in Simulated High-Level Radioactive Waste

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Recovery of Metals from Simulated High-Level Radioactive Waste Glass through Phase Separation

The recovery of metals from simulated high-level radioactive waste (HLW) in a glass form using the phase separation of borosilicate glass was studied in order to satisfy possible future demands such as the adoption of some new treatments for the waste in the glass or use of the vitrified elements as resources. The simulated HLW glass was separated into SiO2-rich and B2O3-rich phases at 973 and 873K when the ternary mass ratio of SiO2:B2O3:Na2O was adjusted to 68:27:5 by the addition of SiO2 and B2O3 to the simulated HLW glass. Annealing at the lower temperature of 878K promoted the distribution of the elements in the B2O3-rich phase. Approximately 90% of the Ni, Zn, Fe, Nd, Te, Zr, and Mo were distributed in the B2O3-rich phase, and these elements, except Zr, were almost completely leached into 1 mol dm−3 nitric acid at 363 K. The leaching of Zr was also achieved using 1.5 mol dm−3 sulfuric acid after the nitric acid leaching. The leached fractions of the glass-network components, such as B and Al, were lower than the other elements at approximately 70–80% due to the existence in the SiO2-rich phase. An increase in the concentration of CaO in the glass in the range of 2–5 wt% inhibited the distribution of elements into the B2O3-rich phase. The increase in the concentration of CaO also changed the structure of the B2O3-rich phase from the continuous unit shape to the discontinuous spherical shape. Consequently, the leaching fraction of every element dramatically decreased. The structure of the B2O3-rich phase returned to continuous when the concentration of CaO was more than 10 wt%. From this glass, the leaching of 69% Zr was possible using only nitric acid without any sulfuric acid.

Recovery of Metals from Simulated High-Level Radioactive Waste Glass through Phase Separation

The recovery of metals from simulated high-level radioactive waste (HLW) in a glass form using the phase separation of borosilicate glass was studied in order to satisfy possible future demands such as the adoption of some new treatments for the waste in the glass or use of the vitrified elements as resources. The simulated HLW glass was separated into SiO2-rich and B2O3-rich phases at 973 and 873K when the ternary mass ratio of SiO2:B2O3:Na2O was adjusted to 68:27:5 by the addition of SiO2 and B2O3 to the simulated HLW glass. Annealing at the lower temperature of 878K promoted the distribution of the elements in the B2O3-rich phase. Approximately 90% of the Ni, Zn, Fe, Nd, Te, Zr, and Mo were distributed in the B2O3-rich phase, and these elements, except Zr, were almost completely leached into 1 mol dm−3 nitric acid at 363 K. The leaching of Zr was also achieved using 1.5 mol dm−3 sulfuric acid after the nitric acid leaching. The leached fractions of the glass-network components, such as B and Al, were lower than the other elements at approximately 70–80% due to the existence in the SiO2-rich phase. An increase in the concentration of CaO in the glass in the range of 2–5 wt% inhibited the distribution of elements into the B2O3-rich phase. The increase in the concentration of CaO also changed the structure of the B2O3-rich phase from the continuous unit shape to the discontinuous spherical shape. Consequently, the leaching fraction of every element dramatically decreased. The structure of the B2O3-rich phase returned to continuous when the concentration of CaO was more than 10 wt%. From this glass, the leaching of 69% Zr was possible using only nitric acid without any sulfuric acid.

Durability of CMPO Impregnated Silica Adsorbent under Irradiation Conditions

Separation of Cs and Sr from simulated high-level radioactive liquid waste (HLLW) was performed using an octylphenyl-N,N-diisobutylcarbomoyl phosphine oxide impregnated silica adsorbent (CMPO-adsorbent). As part of HLLW treatment, the adsorbent is subjected to strong irradiation. Therefore, the durability of CMPO-adsorbent under gamma-ray irradiation conditions was studied. CMPO decomposed to phosphoric acid after a transition to octylphenyl phosphinic acid (OPP). The adsorbed amount of actinoids and lanthanoids were found to decrease linearly with increasing irradiation dose. However, at a dose of 2 MGy, the CMPO-adsorbent was able to maintain adsorption at a rate of 80% of its initial capacity. Based on this finding, it is estimated that the adsorbent can be used in the treatment of HLLW for 400-500 cycles. Radiolytic degradation products increase with increasing irradiation dose caused by HLLW. If OPP accumulates in a CMPO-adsorbent, actinoids and lanthanoids are not recovered. OPP dissolves easily by washing with an eluent, suggesting that OPP would not accumulate in the adsorbent. After washing, separation performance was maintained with a dose of 2 MGy.

Durability of CMPO Impregnated Silica Adsorbent under Irradiation Conditions

Separation of Cs and Sr from simulated high-level radioactive liquid waste (HLLW) was performed using an octylphenyl-N,N-diisobutylcarbomoyl phosphine oxide impregnated silica adsorbent (CMPO-adsorbent). As part of HLLW treatment, the adsorbent is subjected to strong irradiation. Therefore, the durability of CMPO-adsorbent under gamma-ray irradiation conditions was studied. CMPO decomposed to phosphoric acid after a transition to octylphenyl phosphinic acid (OPP). The adsorbed amount of actinoids and lanthanoids were found to decrease linearly with increasing irradiation dose. However, at a dose of 2 MGy, the CMPO-adsorbent was able to maintain adsorption at a rate of 80% of its initial capacity. Based on this finding, it is estimated that the adsorbent can be used in the treatment of HLLW for 400-500 cycles. Radiolytic degradation products increase with increasing irradiation dose caused by HLLW. If OPP accumulates in a CMPO-adsorbent, actinoids and lanthanoids are not recovered. OPP dissolves easily by washing with an eluent, suggesting that OPP would not accumulate in the adsorbent. After washing, separation performance was maintained with a dose of 2 MGy.

Clay mineral weathering and contaminant dynamics in a caustic aqueous system: II. Mineral transformation and microscale partitioning

Microscopic and spectroscopic studies were conducted to assess mineral transformation processes in aqueous suspensions of illite (Il), vermiculite (Vm) and montmorillonite (Mt) that were subjected to weathering in a simulated high-level radioactive tank waste leachate (0.05 m Al T, 2 m Na +, 1 m NO 3 −, pH ∼14, Cs + and Sr 2+ present as co-contaminants). Time series (0 to 369 d) experiments were conducted at 298 K, with initial [Cs] 0 and [Sr] 0 concentrations from 10 −5 to 10 − mol kg −. Incongruent clay dissolution resulted in an accumulation of secondary aluminosilicate precipitates identified as nitrate-sodalite, nitrate-cancrinite and zeolite X, by molecular spectroscopy and electron microscopy (XRD, IR, NMR, SEM-EDS and TEM-EDS). Contaminant fate was dependent on competing uptake to parent clays and weathering products. TEM-EDS results indicated that high Il affinity for Cs was due to adsorption at frayed edge sites. The Il system also comprised Sr-rich aluminous precipitates after 369 d reaction time. In Mt systems, Cs and Sr were co-precipitated into increasingly recalcitrant spheroidal precipitates over the course of the experiment, whereas contaminant association with montmorillonite platelets was less prevalent. In contrast, Cs and Sr were found in association with weathered Vm particles despite the formation of spheroidal aluminosilicate precipitates that were comparable to those formed from Mt dissolution.