Experiments simulating flooding scenario in various aqueous media for a long period were carried out to determine the adequacy of cement-clay composite for solidification/stabilization of spent organic radioactive liquid scintillator wastes. The final cement waste form blocks were immersed in three aqueous media, namely, seawater, groundwater, and tapwater. The immersion process lasted for increasing periods up to 540 days. Following each predetermined interval period, physical and mechanical evaluations of the immersed blocks were determined. In addition, the change in the hydration products was followed by X-ray diffraction and infrared spectroscopy as nondestructive analyses to recognize the deterioration in the microstructure that may occur due to the flooding event. Thermal analysis and scanning electron microscopy were performed to confirm the data obtained.
To determine the radioactivity contents in any specimen based on liquid scintillation technique, the sample under quantification is dissolved or suspended in a cocktail containing an aromatic solvent (e.g., benzene, toluene, and dioxan) and predetermined amounts of other additives known as flours, that is, scintillator. The particles emitted from the radioisotope in the sample transfer their energies to the solvent which in turn transfers that energy to the flours’ molecules that dissipating the energy by emitting light. After this quantification, the spent liquid scintillators are counted as a hazardous organic radioactive waste and should be managed safely [
The practice of immobilizing radioactive waste with ordinary Portland cement began during the early years of the nuclear industry. This was primarily due to its low cost, availability, and compatibility with aqueous waste. It was soon realized, however, that specific wastes, like scintillator liquid, interact with the cement components causing inhibition or retarding the hydration reaction. To overcome these drawbacks, one or more selected additives were added to the Portland cement mixtures. Several of the more successful mixtures such as sodium silicate, reactive silica, lime, clay, and slag have been identified and commercialized [
Solidification/stabilization (s/s) using cement is a chemical treatment process aiming at either binding or incorporating the compounds of a hazardous waste stream into a stable insoluble form (stabilization) or to entrap the waste within a solid cementitious matrix (solidification) [
A very little researches dealt with the behavior of that category of the waste form under prolonged flooding in various aqueous media, so that the objective of the present work is to evaluate of the suitability of cement-clay composite to solidify and stabilize this waste and to confirm the conformance of cemented liquid scintillator waste to the regulatory requirements during long-term storage.
Experimental studies were undertaken on a laboratory scale, aiming at the determination of the influence of prolonged flooding in various aqueous media on the physicomechanical properties of the final waste form (FWF). Physicomechanical properties were evaluated by measuring the most important parameters the strength under compression, mass loss, bulk density, and porosity. Likewise, water absorption, volume of open pores, volume of impervious portion, and apparent specific gravity were, also, determined at the end of predetermined immersion time intervals.
The chemical composition of spent liquid scintillation waste (LSW) varies largely and it is based on its origin and applications. The chosen LSW for this study is a widely used one in very large volumes, as waste, resulting from liquid scintillation counting applications, was collected and segregated at Nuclear Research Center, Egyptian Atomic Energy Authority. LSW simulate was prepared and used for formulation of the FWFs subjected to all studies. The simulated waste was composed of Bray’s cocktail and 1,4-dioxan. Bray’s cocktail contains 5 g PPO (2,5-diphenyloxazole), 0.5 g POPOP 1,4-bis(5-phenyloxazol-2-yl) benzene, 30 g naphthalene, and 20 mL ethylene glycol. For preparation of one liter of LSW simulate 5.5 g from Bray’s cocktail was completed to the determined volume with 1,4-dioxan as a solvent in a dark glass bottle.
The cement used for immobilization in the present study was supplied by Al Masria Cement Company, Suez, Egypt. It is local ordinary Portland cement (OPC), 42.5N Cement (I), manufactured according to the Egyptian Standard Specifications, EN 197-1/2000.
Natural clay was used as additive material to cement. It was obtained from Belbeis desert, El Sharkia, Egypt. Chemical and phase compositions of both cement and clay materials used are given in Table
Chemical composition of ordinary portland cement and natural clay used.
Composition | OPC, wt% | Natural clay, wt% |
---|---|---|
SiO2 | 20 | 59.54 |
Al2O3 | 6 | 22.24 |
Fe2O3 | 3 | 9.47 |
CaO | 63 | 0.56 |
MgO | 1.5 | 2.21 |
SO3 | 2 | 0.19 |
Na2O | 0.5 | — |
K2O | 0.5 | 1.29 |
Chloride | — | 0.63 |
Ignition loss | 2.0 | 10.50 |
Three types of water, namely, seawater, tapwater and groundwater were applied as immersion aqueous media for evaluation of the physicomechanical characterization of solid waste forms that subjected to flooding test. The groundwater was collected from Inshas, East of Cairo, Egypt, well number 202, which is the nearest groundwater container to Egyptian Nuclear Facility where a disposal site is constructed. The source of tapwater was from Giza district while the seawater was obtained from Alexanderia on the Mediterranean Sea. The concentrations of some salts of interest in tapwater, groundwater, and seawater are represented in Table
Concentrations of some salts of interest in tapwater, groundwater, and seawater.
Composition g/L | Seawater | Tapwater | Groundwater |
---|---|---|---|
Sodium chloride | 27.32 | 0.0615 | 0.0923 |
Magnesium chloride | 3.807 | 0.021 | 0.041 |
Magnesium sulphate | 1.658 | 0.0010 | 0.0011 |
Calcium sulphate | 1.260 | 0.00028 | 0.0006 |
Potassium sulphate | 0.863 | 0.00015 | 0.0002 |
Calcium carbonate | 0.123 | 0.0510 | 0.102 |
Magnesium bromide | 0.076 | 0.00 | 0.048 |
Conductivity | — | 370 | 850 |
pH | 7.9 | 6.8 | 8.0 |
Cement-clay composite with LSW simulate as a (FWF) was prepared by adding the natural clay to cement paste then LSW simulate was dispersed into the cement-clay composite. The components were mixed entirely until homogeneous slurry was reached and then poured into polyethylene cylinder molds. The paste was compacted manually for few minutes to remove any air bubbles. The resulting slurry was allowed to set and hard at room temperature (
After 28 days of curing time, cylindrical blocks (
The data obtained from the immersion of the hard blocks were compared to that of non-immersed samples, which which were prepared by the same way and exposed to the same conditions. The results from various experiments for the blank samples as well as those immersed in various water media for increasing periods are given in Figures
Compressive strength and mass-loss values of the FWFs under different immersion environments.
Bulk density and specific gravity values of the FWFs under different immersion environments.
Volume of open pores and impervious portion of the FWFs under different immersion environments.
Apparent porosity and water absorption values of the FWFs under different immersion environments.
For gathering, the obtained data revealed the following. In general, an increase in compressive strength, bulk density, specific gravity, and volume of impervious portion values was recorded; on the other hand, decrease in mass loss, volume of open pores, water absorption, and apparent porosity values with the increase of immersion time up to 18 months were detected. The Nuclear Regulatory Commission (NRC), USA, recommended a minimum compressive strength value of 6 MPa to ensure that a waste form remains stable under the compression load inherent in a disposal environment [ It was seen from these data that the durability of a hardened cement matrix depends on the degree of hydration. As the hydration progressed due to the prolonged curing time, the amount of hydration products was increased, which fill the pores present in the cement specimens leading to enhancement in the microstructural properties, especially, the porosity. Durability of the hardened cement matrix immersed in the three types of water was higher than that of the blank sample (non-immersed one) especially in the case of seawater and had the sequence: seawater > groundwater > tapwater > nonimmersed.
Elevation in the mechanical durability for the immersed samples in seawater may be due to the formation of CaCO3 (calcite) as a result of the reaction of lime liberating during hydration of cement paste with CO2 present in seawater. Calcite can act as filler for pore cement matrix and also accelerate the hydration of tricalcium aluminate (C3A) to form calcium carboaluminate hydrates, that is, filling up the empty pores inside the FWF blocks; therefore, the durability of the FWF blocks was enhanced [
Migration of moisture through pore structure of the cement matrix can induce buildup of internal hydraulic pressure, which causes cracking in the composite and consequently can affect negatively on the physicomechanical properties. The refinement of pore structure in the FWF by adding clay leads to reduce the permeability of the hardened cement composite and can dispose retardation in moisture migration through a cement matrix so that physicomechanical properties would be enhanced [
FT-IR analysis data for the FWF that incorporating liquid scintillator waste simulates after subjecting to flooding treatment are represented in Figures sharp absorption bands at 3641 cm−1 which refer to bending vibration of the free O–H group coordinated to the Ca+2 in the Ca(OH)2; the broad absorption stretching band near 3445 cm−1 that attributed to the –OH group bonded to hydrogen and may also to the molecular water of calcium silicate hydrates gels (C–S–H); the shoulder assigned in the region 1641–1647 cm−1 characterizing water H–OH deformation vibration; the presence of calcite CaCO3, formed due to the carbonation of Ca(OH)2, was identified at two absorption bands in the region of 1452–1461 cm−1 and characterizing the stretching vibration in the broad medium band appeared close to 980 cm−1 in all analyzed specimens is due to Si–O–Si asymmetric stretching vibration while the deformation of
IR spectra of FWF blocks, cured for 28 days, aged up to 18 months, and not subjected to immersion test.
IR spectra of FWF blocks immersed in tapwater up to 18 months.
IR spectra of FWF blocks immersed in groundwater up to 18 months.
IR spectra of FWF blocks immersed in seawater up to 18 months.
Collectively, it should be notified that no essential differences were detected between the IR data for the various FWFs subjected to variable leachants for increasing immersion periods existed up to 18 months. Moreover, the addition of simulated scintillator imposed no effect on the chemical configurations of the FWF subjected to the immersion treatment; this indicated that the candidate cement-clay composite can immobilize the spent liquid scintillator fairly even under accidental flooding of the disposal site.
XRD analysis is useful for detecting the hydrates and unhydrates components of the OPC composites. The families of hydrates of calcium aluminate, such as ettringite and monosulphate hydrate, showed the highest peaks on the low angle side of XRD profile. On the other hand, the peaks of aggregate minerals (e.g., alite, belite, etc.) appeared at the high angle side [
X-ray diffraction of the nonimmersed FWF and those immersed in various types of water.
Clearly, portlandite constitutes the main hydration products in all non-immersed and immersed samples in various leachants media even after 18 months; calcite and C–S–H come as minor constituents then gypsum, quartz, and ettringite present as traces. Any how, the following observations can be also deduced from the XRD spectra for all samples under consideration. Portlandite is one of the two cement hydration main products responsible to the mechanical integrity of the FWF and recognized at 2 C–S–H is the second principal hydration product of cement and gives the product about 50% of its strength [ An appreciable amount of unavoidable calcite at 2 The environment in both seawater and groundwater favors the deposition of the formed calcite inside the bulk of the immersed samples and consequently enhances the total porosity and the mechanical durability of these samples as previously reported, Figure Traces contents of quartz, ettringite, and gypsum, at 2
These data are in a good agreement with the previously published work and confirmed the data obtained from FT-IR spectroscopy.
Scanning electron microscopy (SEM) has been used to describe the structure of the hardened FWF component and to evaluate the influence after the immersion in various water solutions.
FWF is characterized by microstructures, which is mainly composed of [ Portlandite crystals that are locally embedded in amorphous C–S–H appear as well crystalline plates like structures and exhibit hexagonal habit. They were detected in the samples which subjected to immersion conditions or not, Figures The clusters of portlandite crystals fill up the pores inside the bulk of the hydrated cement paste for non-immersed samples, Figure C–S–H and ettringite phases were found collapsed with portlandite for nondipped samples and those exposed to groundwater and seawater as in Figures It was clear that the pores (that look black in SEM graphs) were little for samples immersed in seawater and groundwater as in Figures
Hence, the microstructure appeared to be more compacted and contentious for samples immersed in seawater and groundwater followed by those immersed in tapwater and non-immersed ones. This can explain the data reached from the porosity measurement and reported the low porosity values for blocks immersed in seawater relative to that dipped in the two other media. SEM configuration could verity the adequate performance of the proposed cement-clay mixture as an immobilizing composite for organic liquid scintillator waste under exposer to aqueous media during the long-term disposal process.
Scanning electron microscopy of the non-immersed FWF after 18 months at ambient conditions.
Scanning electron microscopy of the FWF immersed in tapwater for 18 months.
Scanning electron microscope of the FWF immersed in groundwater for 18 months.
Scanning electron microscope of the FWF immersed in seawater for 18 months.
In order to asses thermal stability of the FWF blocks and the effect of immersion for long periods on the hydration products, thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTGA), and differential thermal analysis (DTA) were performed for composite samples before and after immersion in different leachants. On the basis of DTA and TGA analyses, changing in the properties of the cement mixtures occurred due to mechanical activation that cannot be perceived from X-ray diffraction analysis. Reactivity of the activated mixtures increases by the decrease in melting temperature of certain minerals and the temperatures of the characteristic reactions. The energy received during mechanical activation leads to changes in the crystal structure, which causes reduction in the reaction enthalpy and, therefore, to a reduction of the decomposition temperature [
TGA and DTGA data obtained for the immersed FWF blocks in various aqueous media and for non-immersed ones were recorded in Table The mass losses at temperatures less than 120°C, were due to the dehydration of pore’s water and release of hygroscopic water. The loss in the range In temperature range from 460°C to 560°C, the mass losses could describe the dehydroxylation (decomposition of portlandite). The fourth characterizing zone in TGA diagrams that started approximately at 700°C up to 850°C was attributed to the decarbonization of calcite according to the following reaction:
Mass loss in the FWF* due to thermogravimetric analysis (TGA) following the immersion in different aqueous media.
Immersion media | The mass loss due to heating up to 1000°C | ||||
---|---|---|---|---|---|
<120°C |
|
|
|
Total mass loss | |
Non-immersed | 1.9 | 5.5 | 1.4 | 6.0 | 22.21 |
Tapwater | 5.8 | 12.1 | 2.3 | 2.8 | 27.8 |
Groundwater | 5.9 | 13 | 2.2 | 4.6 | 27.24 |
Seawater | 5.8 | 13.2 | 2.1 | 6.0 | 27.12 |
TGA and DTGA thermograms of the FWF immersed in different types of water.
The mass due to the decarbonization reaction is higher in case of sample dipped in seawater compared to that immersed in both tapwater, and groundwater. To fortify the results, they are depicted through XRD analysis.
Figure
DTA thermograms of the FWF immersed in different types of water.
Portlandite represented the predominant cementitious products formed due to hydration reaction of the neat cement with water as the following reaction:
Calcite was formed through a subsequent carbonization process due to the reaction between the formed portlandite and the atmospheric CO2 or that present in groundwater or seawater as following reaction:
Correlating the TGA data to the DTA thermograves, it can deduce numbers of characteristic endothermic peaks, Figure The first endothermic peak was below 100°C, which indicated the presence of hygroscopic and pore’s water. The second endothermic peaks up to The third endothermic effect, at peak The fourth effect at peak centered near 770°C showed the loss of carbonate ions in the form of CO2 from calcite.
The data in Table
Based on the experimental data obtained, it could be deduced that the candidate cement-clay composite can solidify and stabilize up to 15% organic liquid scintillator waste safely and has acceptable resistibility toward the immersion in different aqueous media even under an aggressive prolonged flooding scenario. This waste form composite can optimally comply with the requirements for the disposal process.