Radiochim. Acta 94, 343–346 (2006) / DOI 10.1524/ract.2006.94.6.343  by Oldenbourg Wissenschaftsverlag, München Effect of glass composition on diffusion of cesium in borosilicate glass By Sumit Kumar 1 , Neetika Rawat 1 , A. Bhattacharya 1 , B. S. Tomar 1 , ∗ , V. K. Shrikhande 2 and G. P. Kothiyal 2 1 Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India 2 Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India (Received September 5, 2005; accepted in revised form January 18, 2006) Heavy ion Rutherford backscattering spectrometry / 19 F beam / Cesium diffusion / Borosilicate glass Summary. Heavy ion Rutherford backscattering spectrom- etry, with 30 MeV 19 F beam, has been used to study the diffusion of cesium in borosilicate glasses of different com- positions. The coefficient of cesium diffusion in borosilicate glass was obtained by analyzing the depth profile of cesium in samples annealed at different temperatures in the range of 100–350 ◦ C. The activation energy of diffusion of cesium was found to be in the range of 9–22 kJ/mol, indicating the inter-diffusion between the cesium and sodium as the mechanism of the diffusion process. The activation energy was found to decrease with increasing sodium content of the glass, indicating that the alkali metal ion diffusion is governed by the interaction between the jumping ion and the dipole field around the oxygen ions. 1. Introduction Borosilicate glass is considered to be one of the most suitable matrices for immobilization of the long-lived ra- dionuclides present in the high level radioactive waste (HLW), which is generated by reprocessing the spent nu- clear fuel [1–4]. Owing to the presence of certain non active chemicals in the waste, the basic matrix is usually modified to meet the requirement of product quality. Addition of fis- sion product oxides to the base glass matrix may alter the properties of the product. The base glass consisting of SiO 2 , B 2 O 3 and Na 2 O is modified by adding other oxides, e.g., BaO, TiO 2 , MnO 2 , etc. depending upon the non-radioactive chemical constituents of the HLW, in order to maintain the glass network of the final products. The most important as- pect while optimizing the composition of the glass matrix is the rate of leaching of the long lived radionuclides from the glass matrix into the surrounding, which in turn depends upon the diffusion of the radionuclides in the bulk matrix due to concentration and temperature gradients. Addition of boron in the silica results in the network formation while sodium is added for charge compensation. As long as the Na/B ratio is less than or equal to one, Na + ions occupy *Author for correspondence (E-mail: bstomar@apsara.barc.ernet.in). the interstitial space near the BO 4 − tetrahedra. However, for Na/B ratio > 1, the excess sodium ions present in the glass lead to the modification in the network by formation of Si−O−Na groups in place of Si−O−Si and Si−O−B groups. Thus the ratio of sodium to boron plays an important role in the property of the glass matrix with regard to diffu- sion of long lived fission products and actinides present in the glass. Studies of the rate of diffusion of the long-lived fission products, such as, 137 Cs, 90 Sr, 99g Tc, 129 I, etc., and actinides such as, 237 Np, 239,240,242 Pu, 241,243 Am, etc., are of impor- tance from the point of view of long term behaviour of these radionuclides in the glass matrix. Conventionally, the diffusion of these elements in the glass matrix has been studied by radiotracer method [5], wherein the radiotracer is deposited on the surface of the sample which is then annealed at fixed temperatures for a fixed length of time and the amount of radioactivity at various depths is meas- ured by ion beam sputter sectioning followed by gamma counting. Other techniques include, low energy ion scatter- ing [6], in which, first a depleted cesium layer is created near the surface by bombardment with a low energy ion beam, such as 3 keV 4 He. After annealing, the migration of cesium to the surface is monitored using scanning elec- tron microscope in combination with an energy dispersive X-ray spectrometer (SEM/EDX). However, this method can be used to study diffusion only in the range of 30–40 nm. In the concentration couple method [7], a concentration gra- dient is introduced by melting together the glasses with dif- ferent amounts of the element of interest. After prolonged annealing the concentration profile as a function of depth, is determined using SEM/EDX. Hench et al. [8] used Ruther- ford Backscattering Spectrometry (RBS) using 2 MeV alpha particles beam to analyze the surface of the nuclear waste glasses after one year burial under different temperature conditions. This method involves long experimental time to observe the leaching. Recently our group used a method based on Heavy ion Rutherford backscattering spectrome- try (HIRBS) to study the diffusion of cesium in borosilicate glass [9]. HIRBS offers better depth resolution (∼ 10 nm, as- suming 20keV energy resolution of the energy spectra) over conventional RBS, due to higher linear energy transfer and better mass resolution owing to large variation in kinematic factor with mass number. Brought to you by | Brigham Young University Authenticated Download Date | 5/19/15 5:27 PM