Self-propagation High-temperature Synthesis For Radiocative Waste Immobilization

With the growing exploitation of nuclear energy, more and more radioactive wasteshave been produced. In order to promote the exploitation of nuclear energy as well as toobtain sustainable growth in the exploitation of nuclear energy, it is very important todispose the radioactive wastes that can lead to the nuclear pollution. In this paper, the high level radioactive wastes (HLW) were disposed using self–propagating high–temperature synthesis (SHS), CrO₃ was originally used as an oxygenagent to enhance the absolute temperature Tad ,which lead to the reaction occurd in aliquid stage. As a resuct, the density and uniformity of product was improved with adecreased porosity. In addition, the loadage of the product was enhanced also. For some high level radioactive wastes, for example Sr²⁺, SrO was chosen as a directagent and SrTiO₃ was produced during SHS. So the hold might basically resolve. In this study, double-SHS immobilization methods was developed, in which the hot-pressing or hot isotropic pressing was replaced by the second SHS immobilization. It wasfound that the SHS immobilization was energy-saving and cost-effective processes. SHS processes were developed to form a synrock including Sr²⁺ and CaTiO₃. Theabsolute combustion temperatures ( Tad ) for the immobilizing could be calculatedaccording to the chemical reactive systems. It was found that the Tad for CaTiO₃ andSrTiO₃ was much higher than the critical absolute temperature (1800K), which implied thatthe SHS could be proceeded. The structural stability of CaTiO₃ and SrTiO₃ immobilized was studied, and theirphysical and chemical properties were also analyzed. It was known that CaTiO₃ and SrTiO₃are naturaly stability with big loadage, high density, low leaching rate and strong anti-radioactive properties. Compared with glass immobilized, CaTiO₃ and SrTiO₃ are moreideal immobilization for the high level radioactivity wastes because of better heat-stabilityand higher heat conductivity.The leaching rates, ingredients, microstructures and loadage of CaTiO₃ and SrTiO₃immobilized were studied by PCT, MCC-1, XRD, SEM, TEM.It was found that the leaching rate in the compounds ratios was less than 2.1×10-3g/m2?d (CaTiO₃) and 8.58×10-3g/m2?d (SrTiO₃) to the result of PCT and MCC-1. Andboth of their leaching rates were less than 0.1×10-1g/m2? d (in boiling water), whichwere 2 or 3 orders less than that of the immobilized glass (12±0.4). Addition, the leachingrates and normalized leaching rates of CaTiO₃and SrTiO₃ gradually decreased and finallyreach a stable level, indicating that they could hold the high level radioactivity wastes in astable way.The elements of the immobilized CaTiO₃ were analyzed by X-diffraction ray. Theresults showed that the main element was CaTiO₃, which indicated that SrO haddecomposed during the chemical reaction and element Sr2+ had been immobilized intoCaTiO₃ lattices. The maximum holding ability of SrO in CaTiO₃ could reach 36%. Thesimilar analysis was also conducted to SrTiO₃ immobilized, it was observed that the mainproduct was SrTiO₃, which was due to the reaction between SrO and the other reactionagent during the process.The microstructure, element distribution and crevice shape of CaTiO₃ immobilizedwas analyzed using be scanning electron microscope STM/EDX. The results showedthat CaTiO₃ base was polygon and Sr2+ was evenly distributed in the CaTiO₃ base. Sowhen the weight of SrO was below 36%, Sr2+and CaTiO₃ had formed an evenlyimmobilized product, Sr2+ was immobilized in the CaTiO₃ base and no free SrO couldbe detected. The main product was CaTiO₃. However, when the weight of SrO is over36%, free SrO could be found, CaTiO₃ and SrO were found in the product.Based on the results above-mentioned, it was concluded that CaTiO₃ and SrTiO₃immobilized with self–propagating high–temperature synthesis (SHS) are of high density,little porosity, low leaching rate and large holding ability, and SHS is suitable for thefinally dispose of the nuclear wastes.