| The safe disposal of radioactive waste generated by using nuclear power is the most pressing and potentially costly environmental problems.The high level liquid wastes(HLLW)contain a large number of radioactive materials,must be immobilized in a stable solid state and completely isolated from the biosphere.Regrettably,HLLW,even though partitioned,is generally a multi-component complex system,therefore it is impossible for different HLLWs to be immobilized by the same approach.Alpha high-level waste(α-HLLW)which is partitioned by Tsinghua TRPO Flowsheet is a sort of HLLWs in China,this paper attempted in view of its component characteristic to find an effective method to make its immobilization possible.Glass-ceramics is the third high-level liquid waste wasteform joining the other two,glasses and ceramics,which is rarely researched around the world.Glass-ceramics,a host for HLLW,is composed of both crystalline phase(s)and a glass matrix with high chemical durability.Hence what matters here is to select a highly durable glass matrix for a monazite glass-ceramic wasteform.In this study,La2O3 or Fe2O3 were properly mixed with H3PO4 to be the raw materials. In virtue of the oxide powder of the components in simulated HLLWs,the monazite glass-ceramic wasteforms were successfully fabricated by using the sintering process.Two sintering methods and three formulas were effectively proved.The monazite glass-ceramic samples were examined by a X-ray powder diffractometer (XRD)to evaluate crystalline phase;and by an electron probe microscopic analysis (EPMA)technique to probe into phase species,phase abundances,phase composition,and grain shape,size and distribution of each phase;and by Fourier transform infrared absorption spectra(FTIR)to detect phosphate glass species;and by differential scanning calorimetry(DSC) technique to scan glass transformation(Tg);and by Mosbauer spectra to determine both the valances and the coordination numbers of iron ions;and by scan electron microscopy(SEM)to characterize the microarea morphology of glass-ceramic samples;and by Inductively coupled plasma-atomic emission spectrometry(ICP-OES)to mensurate the the various element concentration in the leaching solution.The leaching behaviors ofα-HLLW glass-ceramic wasteforms were systemically studied according MCC-1 method.The results indicate as follows:(1)La2O3(or Fe2O3)as raw material with H3PO4 separately at 1230℃(or 1270℃)can prepare the phosphatic parent glass.In virtue of the oxide powder of the components in simulated HLLWs,the monazite glass-ceramic wasteforms were successfully fabricated in conditions of 1200℃/4h(or 1100℃/4h).There were[PO4]3-and a spot of[P2O7]4-,not any[PO3]-in the IR spectrum.The former one,showed the different glass phases:molybdenum iron phosphate glass and aluminum iron phosphate glass.It was not possible for components Al and Mo to co-exist in an iron phosphate glass.It was proved that this wasteform had low loss rate and high chemical stability.The another one,had at least three glass phases:silicon iron phosphate glass,silicon aluminum iron phosphate glass and aluminum iron phosphate glass.Iron in the glass phases of the wasteforms was in a state of high spin Fe3+,which was with a coordination number of four.As a result of low activity,this sample was good stability.The volatilization of MoO3 occured in the process of preparation;Aluminum phosphate phase existed,owing to cauterization of corundum crucible.(2)For the first time,the glass-ceramic wasteforms,made of monazite crystalling phase and the phosphate glasses phase,were successfully prepared only at 940℃without the sintering process for the parent glass.This method,which reduced sintering temperature nearly 200℃contrast to the previous two ways,was provided with superiority of reducing the energy consumption and predigesting flowsheet.Monazite growed so unevenly that the size was different up to 20 times;The glass phase whose O/P ratio was from 3.02 to 4.20 was made of Fe-Mo-P-O glass.In addition the part of SiO2 particles exsited on account of mixed grinding course,which came from incomplete response.There were[PO4]3-(mostly exist in monazite)and a spot of [P2O7]4-(exist in glass phase)in the IR spectrum.Experiments showed that weight loss was almost zero,apparent porosity was 0.89%,water absorption was 0.24%.This result was the lowest one of the current experiment,it was obvious that the method was highly feasible.(3)MCC-1 mothod showed:at 90℃the sample began to appear dilapidation from the 14th day,appear Al-Mo-Fe-P white leaching matter which looked like grape at 28th day;from 0 days to 14 days,pH was tardily changed from 6.23 to 5.49,after 14 days pH was hastily reduced to2.14.this is an attestation about the leaching of alkalescence ion.Mg,K,Ca from the deionized water were found in the wasteform,at one time,K,Na,Ca,Mg from the deionized water reduced appulsively from 3days to 7days.So there was an intercommunion between this wasteforms and the deionized water.Before 14 days the normalized leaching rates of the various elements showed a decreasing trend,and at the 14th day the leaching rates was the lowest one,hence it was proved that the wasteform had corrosion but slowly.The order of the initial normalized leaching rates:Al＞Mo＞Zr＞P＞Fe＞La＞Ce;after 28 days:Mo＞Al＞Fe＞P＞Zr＞La＞Ce.Contrast fore-and-aft leaching rates,Mo,Al,Fe,P were the largest in increased range.After the analysis of chemical stability,this paper demonstrated that the main reason about the destroyable wasteform as follows:less and uneven glass phase made the appearance of analogous[PO3]- easy; split phase was formed by the action of SiO2.
|
PDF Full Text Request