Pranesh Sengupta

A review on immobilization of phosphate containing high level nuclear wastes within glass matrix--present status and future challenges.

Immobilization of phosphate containing high level nuclear wastes within commonly used silicate glasses is difficult due to restricted solubility of P(2)O(5) within such melts and its tendency to promote crystallization. The situation becomes more adverse when sulfate, chromate, etc. are also present within the waste. To solve this problem waste developers have carried out significant laboratory scale research works in various phosphate based glass systems and successfully identified few formulations which apparently look very promising as they are chemically durable, thermally stable and can be processed at moderate temperatures. However, in the absence of required plant scale manufacturing experiences it is not possible to replace existing silicate based vitrification processes by the phosphate based ones. A review on phosphate glass based wasteforms is presented here.

Preliminary study on calcium aluminosilicate glass as a potential host matrix for radioactive 90Sr--an approach based on natural analogue study.

Given the environmental-, safety- and security risks associated with sealed radioactive sources it is important to identify suitable host matrices for (90)Sr that is used for various peaceful applications. As SrO promotes phase separation within borosilicate melt, aluminosilicate bulk compositions belonging to anorthite-wollastonite-gehlenite stability field are studied in this work. Tests for their homogeneity, microstructural characteristics and resistance to phase separation narrowed the choice down to the composition CAS11 (CaO=35 wt%, Al(2)O(3)=20 wt%, SiO(2)=45 wt%). We find that up to 30 wt% SrO can be loaded in this glass without phase separation (into Ca, Sr-rich and Sr-poor, Si-rich domains). Leaching behaviour of the glasses differs depending on the content and distribution of Sr. In general, the elemental leach rates determined from conventional PCT experimental procedure yield values better than 10(-7)gcm(-2)day(-1) for both CAS11 base glass as well as SrO doped glass. It was noted that leach rates calculated on the basis of Ca(2+) and Sr(2+) were of the same order and bit higher compared to those calculated on the basis of Si(4+) and Al(3+). During accelerated leaching tests, zeolite and zeolite+epidote were found to have developed on CAS11 base glass and SrO doped glasses respectively. The Sr bulk diffusion coefficients is found to vary from ∼ 10(-15) to 10(-13)cm(2)/s at temperature intervals as high as 725-850°C. Based on the experimental observations, it is suggested that CAS11 glass can be used as host matrix of (90)Sr for various applications of radioactive Sr-pencils.

Immobilization of High Level Nuclear Wastes: The Indian Scenario

Nuclear power stands as an immediate and sustainable soluton for satisfying the emerging energy crisis in India. Successful execution of any national ‘nuclear power program’ is keyed to its effective ‘high level nuclear waste’ management strategy. Towards this, India has recently developed sodium-barium-borosilicate glass matrix to immobilize sulfate containing high level waste. Currently, efforts are underway to explore the possibilities of using the same matrix or its modified versions to condition nuclear wastes likely to be generated from ‘closed thorium fuel cycle’. Apart from conventionally used ‘hot wall induction furnace technology’, India has recently acquired expertise in operations of indegineously developed ‘Joule heated ceramic melter’ and ‘Cold crucible induction melter’ for development of suitable inert glass matrices.

Perovskite-Ni composite: a potential route for management of radioactive metallic waste.

Management of nickel - based radioactive metallic wastes is a difficult issue. To arrest the release of hazardous material to the environment it is proposed to develop perovskite coating for the metallic wastes. Polycrystalline BaCe0.8Y0.2O3-δ perovskite with orthorhombic structure has been synthesized by sol-gel route. Crystallographic analyses show, the perovskite belong to orthorhombic Pmcn space group at room temperature, and gets converted to orthorhombic Incn space group at 623K, cubic Pm3m space group (with a=4.434Å) at 1173K and again orthorhombic Pmcn space group at room temperature after cooling. Similar observations have been made from micro-Raman study as well. Microstructural studies of BaCe0.8Y0.2O3-δ-NiO/Ni composites showed absence of any reaction product at the interface. This suggests that both the components (i.e. perovskite and NiO/Ni) of the composite are compatible to each other. Interaction of BaCe0.8Y0.2O3-δ-NiO/Ni composites with simulated barium borosilicate waste glass melt also did not reveal any reaction product at the interfaces. Importantly, uranium from the waste glass melt was found to be partitioned within BaCe0.8Y0.2O3-δ perovskite structure. It is therefore concluded that BaCe0.8Y0.2O3-δ can be considered as a good coating material for management of radioactive Ni based metallic wastes.

Adsorption of actinides within speleothems

Abstract Stalagmites and stalactites, as observed within natural caves, may develop inside geological repositories during constructional and post-operational periods. It is therefore important to understand actinide sorptionwithin such materials. Towards this, experimental studies were carried out with 233U, 238Np (VI), 238Np (IV), 239Pu and 241Am radiotracers using natural speleothem samples collected from the Dharamjali cave of the Kumaon Lesser Himalayas, India. Petrological/mineralogical studies showed that natural speleothems have three general domains: (1) columnar calcite; (2) microcrystalline calcite; and (3) botryoidal aragonite - each with ferruginous materials. Results showed that all domains of speleothems can take up >99% actinides, irrespective of valence state and pH (1-6 range) of the solution. However, distribution coefficients were found to be at a maximum in aragonite for most of the actinides. Such data are very important for long-term performance and safety assessments of the deep geological repositories planned for the disposal of high-level nuclear wastes.

Sorption of Cs and Sr radionuclides within natural carbonates

Understanding the mobility of radiocesium and radiostrontium within geological environment is important from ‘deep geological repository system’—safety assessment point of view. Cs and Sr radionuclide sorption studies have been carried out with a stalagmite sample collected from Lesser—Himalayas. Detailed microstructural studies, backed up by micro-Raman and LIBS analyses, identified three different domains within the sample; constituted of microcrystalline calcite, botryoidal aragonite and palisadic calcite respectively. Experimental studies showed that both the radionuclides exhibit moderate to low sorption coefficients within all the different domains of stalagmite under acidic environment.

Adsorption of Ru, Ce and Eu radionuclides within naturally precipitated polycrystalline calcium carbonate under acidic environment

Abstract106Ru, 144Ce and 152+154Eu radiotracers sorption experiments (pH: 1–6) have been carried out with polycrystalline columnar/microcrystalline calcite and aragonite obtained from a stalagmite of Dharamjali cave, India. The different domains of the sample were powdered and thoroughly studied using electron microscopic and X-ray spectroscopic techniques. Both 106Ru and 144Ce exhibited higher sorption within calcite varieties compared to aragonite and increased with rising pH, while it showed reverse relation in case of 152+154Eu. Further, it is noted that aragonite offers the highest Kd values for 152+154Eu, whereas 106Ru and 144Ce prefer calcite.

Surface protection coating material for controlling the decay of major construction stone

Degradation of the building stones are creating instability in the old building and monuments which is to be protected. To investigate the characteristics of such a stones used for the construction in eastern India, we have collected the khondalite stones. The microstructural and elemental composition analysis of the khondalite stones are analyzed by using SEM, EDX and PIXE trace elemental analysis. We have prepared surface protection coating material with graphene oxide and cobalt ferrite as a base material along with other residuals. The prepared coating materials is coated on the galvanized iron substrate for further characterization. The surface morphology characteristics of the coating material is analyzed by SEM and AFM. The corrosion resistance characteristics of the prepared coating material is studied by the electrochemical impedance spectroscopy. The results suggests that the prepared coating material can be used as a surface protection materials to control the self-destruction of khondalite stones.

Investigating the effect of V2O5 addition on sodium barium borosilicate glasses

V2O5 doped sodium barium borosilicate glasses were characterized by photoluminescence spectroscopy and electron probe microanalyzer (EPMA). The glass remains homogeneous for lower concentration of V2O5 but a phase separation is observed when V2O5 doping is increased beyond 5 mol%. Detailed microanalysis reveals that the phase separated glass consists of a phase containing V, Ba and Si and a separate Si rich phase within the glass matrix. The luminescence study demonstrated that at low concentration the vanadium mainly interacts with the structural units of B/Si while at higher concentrations, V-O-V/ V-O− Na+/Ba2+ linkages are formed.