The Influence of Gamma Irradiation on the Leaching Behavior of a Modified Synroc-B Ceramic Waste Form
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THE IFILUME OF GAt4A IRRADIATION ON THE LEACMING BEHAVIOR OF A MODIFIED SYNROC-B CEPRAMC WASTE FORM A. G. SOLOtMAH School of Engineering, North Carolina State University, Raleigh, North Carolina, USA ABSTRACT The leaching behavior of a sintered 1cbdified SYNRIC-B ceramic waste form containing l0wt% simulated high level radioactive waste, under the influence of y-irradiation, has been investigated. Similar leaching experiments have been carried out in the absence of y-irradiation, and the leach rates of Ba and Cs are presented for ccoparison studies. Acid/Base formation has been monitored through pH measurements of the leachants. A decrease in pH is found with yirradiation due to radiolytic nitric acid generation. Higher initial leach rates with a more pronounced subsequent drop in the case of Ba and Cs were observed with yirradiation than without irradiation. The results of the study are presented and discussed. INTRODTION The radiochemical behavior of a proposed or selected high level radioactive solid waste form is an important factor in understanding and, subsequently, evaluating the performance of this solid waste form especially under long-term deep geologic disposal conditions (105 - 106 years). Some synthetic materials, whose naturally-occurring analog minerals contain naturally radioactive species, have shown exceptional leaching resistance or retention capacity for radioactivity when they incorporated simulated high level radioactive waste calcine [1, 2, 3]. As originally proposed by A. E. Ringwood of Australia [4], SYNRDf-B is a hot isostatic pressed assemblage of three mineral phases: Ba-Hollandite (BaAl 2 Ti 6O16 ), Perovskite (CaTi03) and Zirconolite (CaZrTi207). A solid-state sintering technique has been developed as an alternate process for making SYNRX2-B ceramic waste forms [5]. Results are reported here on a study of a sintered version of this solid waste form identified as sintered Modified SYNROD-B [6]. Table I presents the composition of the proposed solid waste form which is investigated in this study, as determined by electron microprobe analysis [7]*. The leaching performance of a sintered Nodified SYNROC-B waste form containing lOwt% simulated high level radioactive waste (PW-4b) [8], under the influence of ganma irradiation from a "5 Co source, has been investigated. The acid/base behavior of the leachant (triple distilled H20)was monitored through pH measuremnts performed at the end of the sequential leaching periods.
Electron Microprobe Analyzer, Philips model MAR/3, Nominal operating voltage 20 kV at 60 pA, and IBM system 370 model 165 Computer for data analysis.
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TABLE I Electron microprobe analysis for Modified SYNROC-B containing lOwt% simulated high level radioactive waste (PW-4b) [7].
Element
Oxide
Al Ba Ca Ce Cs Fe Mo Nd Ni Sr Ti U Zr
A120 3 Ba0 Ca0 CeO2 Cs 2 0 Fe 2 0 3 Mo0 3 Nd 2 0 3
NiO SrO Ti0 2 U02 ZrO2
Calculated Ncainal composition (Oxide wt%)
Electron microprobe analysis on fracture surfacea) At% wt% Oxide wt%
Difference Oxide wt%
8.18 10.31 8.25
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