Overall Conclusions and Future Projections OVERALL CONCLUSIONS

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1 OVERALL CONCLUSIONS This article brings the thesis to a close by presenting the conclusions drawn from the outcome of the radiation effects on the structural and optical properties of heavy metal oxide borosilicate glasses. The main objective of the thesis was to develop a durable nuclear waste glass. It should melt easily with reduced cost, higher damage resistance and the possible higher sensitivity to irradiation so that it could be used in nuclear industry for the radiation dosimetric and nuclear waste immobilization purposes. The experimental investigations on the possibility of use of these glasses in the nuclear field are extensively explained. For this, XRD, FTIR and UV-visible spectroscopic techniques were employed to investigate the changes brought about in the structure of the glasses by the addition of heavy metal oxides and by impact of gamma rays and heavy ions. The X-ray diffraction studies confirm the amorphous nature of the prepared glass samples. The general conclusions drawn from the FTIR spectra of the prepared borosilicate glasses are as follows: i. The peaks corresponding to B O Si linkages indicate that borate groups mix with SiO 4 tetrahedral structural units to form the glass structure. ii. There is no effect of composition on the types of the structural groupings. This may be due to the presence of symmetric (BO 3 ) triangles, asymmetric (BO 3 ) units with non-bridging oxygen (NBO) and tetrahedral BO 4 and SiO 4 units which remain intact in each of the samples of HMO borosilicate glasses even after the addition of modifier oxides. iii. With the addition of heavy metal oxide CdO, BaO, PbO and Bi 2 O 3, the number of the [BO 3 ] groups with non-bridging oxygen decreases because some trigonal [BO 3 ] structural units got transformed into tetrahedral [BO 4 ] units. iv. The heavy metal oxide ions enter the glass network and alternate with BO 4 pyramidal units and form linkages of the type B O M where M= Cd, Ba, Pb, Bi. 143

2 v. The increase in the BO 4 units increases the interconnectivity of glass atoms making it more stable and rigid. Such harder glass systems are highly suitable for shielding the radiation effects. vi. Gamma irradiation of HMO borosilicate glasses result in the transformation of tetrahedral BO 4 group to the triangular BO 3 one along with non bridging oxygens. vii. Gamma ray interaction with glass may cause the displacement of lattice atoms or electronic defects which involve changes in the valence state of the lattice or impurity atoms. This leads to disruption of already random network in glasses and as a result the arrangement of groups becomes unsymmetrical leading to the possible weakness of network grouping vibrations which is reflected in terms of changes of some bond angles or bond lengths. The fundamental building units (BO 3, BO 4, SiO 4 ) however, remain intact even after irradiation. viii. In the cadmium borosilicate glasses, none of the composition seems to be highly radiation resistant and only Cd5 glass composition containing a maximum of 25 mol% CdO seems to sustain radiation dose upto 15 kgy. This fact can be related to the relatively low molecular weight of CdO as compared to other heavy metal oxides such as barium and bismuth oxide. ix. The addition BaO alters the structure of the glass to large extent by the conversion of BO 3 units present in glass into BO 4 units. This results in the production of a stable glass configuration that can sustain the high energy gamma radiation up to large doses. In the present work it is observed, that Ba5 glass system with 25 mol% of BaO is most resistant to the effect of gamma radiation. x. The glass compositions containing higher amount of PbO show the obvious resistance or retardation effect towards gamma irradiation. This radiation xi. shielding effect is related to the presence of heavy metal PbO forming PbO 4 structural groups in the glass structure. Studies of the composition dependence of IR absorption spectra show that bismuth borosilicate glasses are built up of the [SiO 4 ], [BO 3 ], [BO 4 ], [BiO 3 ] and [BiO 6 ] structural units. It is also observed that the present glass system is a good shielding glass and with increasing the Bi 2 O 3 content in the glass its radiation 144

3 hardness increases. In fact all the compositions containing bismuth show radiation retardation effect and is highly dominant in samples containing 15, 20 and 25 mol percent of Bi 2 O 3. xii. The overall conclusions of FTIR spectral studies indicate that gamma radiations are capable of provoking structural damage in the glasses in terms of the change in bond length, bond angles etc. But with the addition of heavy metal oxide the effect of gamma radiations can be minimized to a large extent and as a result the glass compositions can be practically used in nuclear industry as well as in the applications implying the use of high energy radiations such as in medicine, aircrafts etc. UV-visible spectroscopic investigations of the prepared glasses led to the following conclusions: i. There are strong absorption bands in all the glass samples in the region nm which can be attributed to the presence of trace unavoidable iron impurities in the raw materials. ii. The bands observed in this region in lead and bismuth glasses were assigned to divalent Pb 2+ ions and Bi 3+ ions respectively. iii. The decrease in the optical band gap before irradiation is related to the creation of BO 4 units on introduction of the modifier oxide. The presence of heavy metal oxide such as CdO, BaO, PbO and Bi 2 O 3, modifies the structural arrangements in the glass resulting in a more compactness by the transformation of three coordinated BO 3 units into four coordinated BO 4 units. With increase in the mole fractions of HMO, the number of tetrahedral borate and silicate groups increases. The higher number of dense tetrahedral BO 4 and SiO 4 units along with the heavy metal oxide ions form a highly compact structure. This close packing of the resultant local structures provide an increased interaction between modifier ions and glass atoms. Therefore, the formation of a compact glass network and a corresponding decrease in the optical mobility gap is observed. iv. The percentage decrease in optical band gap is maximum in bismuth borosilicate glasses followed by lead, barium and cadmium borosilicate glasses before irradiation. This shows that highly polarisable and large sized Bi 3+ ions increases 145

4 rigidity and results in a highly compact glass structure as compared to other metal oxides. v. After gamma irradiation the decrease in band gap is related to the creation of NBOs in the glass. The decrease in the band gap energy is minimum in bismuth and lead borosilicate glasses which show that these glass compositions can sustain impact of high energy radiations. The percentage decrease in band gap is minimum in bismuth borosilicate glasses particularly in compositions containing 15, 20 and 25 mol% bismuth oxide. vi. Owing to high toxicity of lead, bismuth based glasses are good alternatives for shielding purposes due to their stable structure and this can be attributed to the high atomic number of bismuth ion. vii. The increase in values of the Urbach energies of the glass samples shows that the disorder in the glass structure increases resulting even more amorphicity of the glass samples after irradiation The high energy heavy 120 MeV Ag 7+ ions by undergoing inelastic collisions with the glass matrix were capable of inducing displacements in the glass structure. The decrease in the optical band gap from optical absorption analysis indicates the change in the glass network. The increase in Urbach energy signifies that degree of disorder increases with increase in the fluence rate of SHI. This fact has been supported by IR studies in which the number of non-bridging oxygen s (NBOs) increases after irradiation. Out of the three glass compositions subjected to SHI irradiation bismuth borosilicate glass seems to be least affected by the radiation damage. This is reflected in the UV-visible and FTIR measurements. It can be stated that glasses containing cadmium and barium can be used for neutron and X-ray shielding while lead and bismuth containing glasses can be used for shielding the effect of high energy radiations such as gamma rays. The nuclear shielding strength of the prepared glasses follow the trend Bi 2 O 3 > PbO>BaO>CdO. PROPOSED USES OF THE PREPARED GLASS SYSTEMS The prepared HMO glasses can be used as: 146

5 i. Windows and lenses in nuclear safety monitoring and inspection equipment. Lenses for cameras will enable the monitoring of high energy environments. ii. Radiation hard fiber which will allow for the creation of a fiber optic device that will continually operate and monitor inside the core of a nuclear reactor and detect potential cracks, leaks, or other problems that may exist before they become major threats. iii. Radiation resistant laser hosts for nuclear fusion. iv. Safer, more stable glass for the vitrification of nuclear waste. v. Radiation shielding purposes and applications involving the use of ionizing radiations such as nuclear reactors, nuclear power stations, spacecrafts, satellites and military aircraft. FUTURE SCOPE OF THE WORK Radiation hard glasses will be subjected to harsh environments and various kinds of physical and chemical stresses. Long-term exposure of such materials in a radiative environment could result in significant alterations in materials during the service life. The presence of HLW can also lead to elevated temperatures and furnish high levels of radiation. Due to these stresses, various forms of degradation can be expected. Also, radiation effects, such as radiation hardening and embrittlement, enhanced diffusion, and enhanced creep rate that must be taken into account since all materials are susceptible to these phenomena. The high density and homogeneity of borosilicate glasses facilitate their use as hard nuclear waste materials which can ensure optimum radionuclide containment. The decay of radio nuclides, immobilized in the glass matrix may lead to increase in the temperature of the glass product and may create the thermal gradient. This temperature gradient can lead to the diffusion of radio nuclides in the glass matrix, which is one of the major aspects of the waste immobilization process affecting the leaching behavior of the radio nuclides from the glass matrix. As the glass system is proposed for high level nuclear waste management therefore thermal studies on such a glass system are also important which were beyond the scope of this thesis. Thus, present work can be extended to study the thermal behavior of the prepared glass system. The electrical conduction in almost all oxide 147

6 glasses containing alkali oxide is due to the motion of the alkali ions present in interstitial positions within the glass network. Since the present prepared glasses contain Na 2 O therefore electrical measurements would bring interesting results. Also, the knowledge of conductivity of the glass melt as a function of composition is required to optimize the composition of the glass, which will be required to facilitate higher waste loading at higher temperature. These glasses can also be used as solid-state nuclear track detectors (SSNTDs). The chemical etching studies on these glasses are therefore, important to evaluate their chemical durability. Thermo luminescence (TL) properties of all the glass compositions can be investigated to find their use as thermo luminescent dosimeters. 148

CHAPTER-2 HEAVY METAL OXIDE GLASSES CONTAINING BISMUTH AND LEAD

CHAPTER-2 HEAVY METAL OXIDE GLASSES CONTAINING BISMUTH AND LEAD There is an increasing interest in the scientific community regarding heavy metal oxide (HMO) glasses containing bismuth and lead oxide.