Effect of Different Modifiers on Melting Points and pH under Leaching in Nuclear Waste Glasses in Borosilicate and Phosphate Systems

Glasses were melted in three systems viz., borosilicate, phosphate and pure silicate system. In the present work we could melt the glass at much lower temperature (800-950oC) with a soaking period of 30 min-1 hr which were earlier reported to be melted not below 1000oC. The effect of different modifier ions like Pb2+, Ba2+, Na+, Fe3+, Al3+, Y3+, Ca2+, Ce4+ and Sr2+ on melting temperature and time of melting in each system has been found to be evident. The pH determination in the range 6.43 to 8.18 of the leachate solution at ambient temperature under varying time intervals shows interesting and regular variations. The findings have been corroborated in terms of ionic size, ionic radii and hence ionic potential of the modifier ions.


INTRODUCTION
The future development of nuclear waste energy depends largely on the success of programmes for handling and management of radioactive wastes generated at various stages of the nuclear fuel cycle.Besides fission products, in a reprocessing of spent fuel operation, the waste contains unrecovered U, Pu, corrosive products like Fe, Co, Ni, Al, Na and anions NO 3 -and SO 4 2-.This constitutes a high level waste (HLW) and that HLW, which contains more than 99.1% of fission products, is called nuclear waste.
Waste disposal experts all over the world believe that the best way of ultimate disposal of HLW is to immobilize them by fixation in solid matrix, preferably glass 1 .The reason why fixation in glass appealed so many people can be found in various advantages that glass offers as a medium for waste contaminant.Glass has the ability to dissolve most of the element of the periodic table.The fission product constitutes become a part and parcel of the glass structure as compared to any mechanical entrapment.These are finally stored in burial (Georepositories).Usually glass in both phosphate and borosilicate systems are used as nuclear waste glasses.

EXPERIMENTAL
The composition of such waste generated in Indian reactors is as shown in table I (as supplied by Dr. S. D. Mishra, Director, Nuclear Recycle group, BARC, Trombay, Mumbai).Glasses were melted in three systems viz., a) Borosilicate system b) Phosphate system and c) Pure silicate system.Glass batches in the above three systems with predetermined compositions were synthesized in acetone medium.They are next dried and taken in a high quality alumina crucible and fired in muffle furnace.In melting operation, the temperature and time of melting are the principle factors to be monitored.The melting operation was done in a programmable muffle furnace with window and temperature controller.The temperature and time of melting was varied.
The effect of different modifier ions like Pb 2+ , Ba 2+ , Na + , Fe 3+ , Al 3+ , Y 3+, Ca 3+ , Ce 4+ and Sr 2+ in each of the basic glass systems viz.borosilicate and phosphate networks on melting points and time of melting has been studied.X-ray diffraction study of the final products was made to confirm its amorphous nature.The pH of the leachate solution at ambient temperature was studied under varying time intervals.
Ten glass forming systems (Table II), seven in Borosilicate system (BS1 to BS7), two in the Phosphate and one in pure Silicate system were studied.Quartz powder (AR grade, Oxford laboratory reagent, Mumbai) was used as the source of silicate and Borax (AR grade, RANKEM, New Delhi) in the borosilicate system, while both For the pH determination of the leachate solution, the bulk glass was powdered.This was done by grinding in a mortar.It was then allowed to pass through a sieve (size 330 and 420 mesh, B. S.).Accurately 1 g of the glass powder of definite size (0.30 -0.42mm) was taken in a beaker with a fixed volume of distilled water of 40 c.c.It was stirred with a magnetic stirrer for a definite period of time of 2 mins.The pH of the liquid was determined by a pH meter (Systronics, Model no 335).Such measurement of pH was done at regular intervals of 1 hr, 2 hr, 3 hr, 4 hr and 5 hr respectively.The mixture was stirred after each 15 min with the help of the magnetic stirrer.Results of such pH study on selected glasses are shown in Table IV.

RESULTS AND DISCUSSIONS
It was observed in the present work that for some initial melting operations (e.g. in BS1), there were heavy loss due to frothing.This resulted in loss of glass from the crucible.Increasing the former content (SiO 2 ) in the composition (e.g.BS4) resulted in successful melts.In general it is found that the m.p. of glasses in the borosilicate system (BS1-BS7) is lowered in comparison to that in the phosphate system P1 (Table III after dissociation.It is followed by recombination leading to liberation of higher amount of energy which is lowered in case of P-O bond energy. The variation in melting points is due to mixed effect of different modifier ions, the exact trend being difficult to speculate.In the present work we could melt the glass composition at much lower temperature (800-950°C) with a soaking period of 30 min.to 1 hr.which were earlier reported to be melted not below 1000° C 2 .Considering the fact that the glass will be utilized to incorporate nuclear waste oxides with some of the fission fragments like oxides of Cs and Ru, which are volatile, lowering of glass processing temperature leads to the betterment of the utility of the glass melt.
In general it is found that the melting point of glasses in borosilicate system (BS1-BS7) is lowered in comparison to that in the phosphate system (P-1) (Table III).Thus more energy and time is required to form the phosphate network.This is possibly due to the presence of simultaneous formers in borosilicate system, namely boron and silicon (Fig. 1) whereas in case of phosphate network 3 , there is only one former atom (phosphorous, Fig. 2).The difference in bond strengths also contributes to this.In addition to these effects, the different modifier ions (added as oxides) can also lower the melting temperature of a particular composition of the phosphate system or borosilicate systems 4 .Here the mechanism is that the modifier ion (M n+ ) dissociates a Si-O (or B-O or P-O) bond of network and generates anionic O sites (O -) and gets attached to such sites ionically.Additionally the variation in m. p. among different compositions in the particular borosilicate system (BS1 to BS7) can be explained due to different types of modifier oxides being added.IV there is a definite change in pH of the leachate solution between the phosphate system and the borosilicate system.The slightly increasing trend in the pH values in borosilicate system may be due to the mixed oxide network viz.[BO 4 ] and [SiO 4 ] in comparison to that in single oxide phosphate system [4][5] .The observed pH values are in good agreement with the equilibrium pH values of glass system having composition close to present work.The range of pH obtained in a borosilicate glass with different modifier cation 2 covers that of ours.It is seen from the table that there is a slight decreasing tendency as we go from 1 hr to 5 hr in steps of 1 hr.If we consider that the following dissociation is being operated in our aqueous system: ... (1)   The extent of which (forward or reverse direction) is influenced by the different modifier ions released during the leaching of the glass sample.

As shown in Table
The ionic radii (Å) of the different modifier ions in our case are as follows:

(
) n M OH .The dissociation of H 2 O as above will be shifted to the forward direction releasing more number of H + in the medium.Thus the resulting solution will increasingly become acidic with lower value of pH.The decreasing trend in the pH value from 1 hr to 5 hr data can be explained in the light that with higher extent of leaching, more modifier cations are released into the solution increasing the ionic potential and therefore leading to lowering of pH values.

CONCLUSIONS
In the present work we could melt glass composition at much lower temperature (800-950ºC) with a soaking period of 30 min.to 1 hr.This is on the lower side than the melting temperatures reported earlier (~1000º C).The lowering of temperature is possibly due to the presence of modifier ions.The difference in bond energies as well as number of glass formers (two in borosilicate compared to one in phosphate) might have contributed to difference in melting point in the two systems.The findings on pH of leachate solution have been explained in terms of ionic size, ionic radii and ionic potential of the modifier ions incorporated into the glass network.In order to reach a definite conclusion, more pH data of varying glass composition are necessary, the explanation of which will be obviously analytical.

Table 1 . Compositional details of simulated low and intermediate level radioactive liquid waste S. No.
).The bond dissociation energy of Si-O is 115 Kcal/mole, B-O is 125 Kcal /mole and P-O is 96 K Cal/mole.In borosilicate glass, the glass forms Si-O and B-O

Table 4 : pH of leachate after different time intervals
This is reflected in our observed data clearly.Glass BS5 with Al 2 O 3 (highest I. P.) shows lower value of pH than BS 3 with no Al 2 O 3 in the composition.