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Title: Effects of radiation damage and composition on phase separation in borosilicate nuclear waste glasses
Author: Patel, Karishma Bhavini
ISNI:       0000 0004 7231 7920
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2018
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In order to increase the waste loading efficiency of nuclear waste glasses, alternative composite structures are sought that trap molybdenum in a water-durable CaMoO4 phase. In this thesis, the formation and stability of CaMoO4 in a borosilicate glass against the attack of internal radiation was investigated. It is a fundamental study that simplified the composition to known contributors of molybdate speciation, and further splits the com- ponents of α and β-decay into integral parts that replicated both nuclear and electronic interactions. Irradiation experiments using 2.5 MeV β, 7 MeV Au, and 92 MeV Xe ions were enlisted to test the hypotheses of whether 100−1000 years of radiation damage given current waste loading standards would: (i) induce phase separation in homogeneous re- gions, (ii) increase the extent of existing phase separation, (iii) induce local annealing that could cause amorphisation of crystalline phases or increase mixing between amorphous phases, or (iv) cause some combination of the above. Results from XRD, SEM, EPR, and Raman spectroscopy suggest that powellite is stable against replicated radiation damage with only minor modifications observed. The main mechanisms of alteration involved: (i) thermal and defect-assisted diffusion, (ii) relaxation from the added ion’s energy, (iii) localised damage recovery from ion tracks, and (iv) the accumulation of point defects or the formation of voids that created significant strain, and led to longer-range modifications. It can be further concluded that no precip- itation or increased phase separation was observed in single-phased glasses. In isolated cases, radiation-induced precipitation of CaMoO4 occurred, but these crystallites were reamorphised at higher doses. At high SHI fluences, minor amorphisation of powellite was also observed, but this occurred alongside bulk-to-surface reprecipitation of CaMo- species. Overall, the components of internal radiation were often found to have opposing effects on the alteration of Si−O−B mixing in the glass, ion migration, and crystallite size. This led to the prediction that a steady-state damage structure could form from cumulative decay processes. These results suggest that CaMoO4 containing borosilicate GCs are resistant to radiation, and that excess molybdenum from increased waste loading can be successfully incorporated into these structures without altering the overall dura- bility of the wasteform. Furthermore, the identified saturation in modifications occurring around 8 x 10¹⁴ Xe ions/cm² can be used as a benchmark in future investigations on more complex systems where the maximum damage state is required.
Supervisor: Farnan, Ian Sponsor: University of Cambridge ; EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Nuclear materials ; glass ceramics ; molybdenum incorporation ; nuclear glass ; borosilicate glass ; radiation damage ; radiation effects ; phase separation ; compositional study ; crystallisation ; nuclear waste ; heterogeneous composites ; immiscibility