The Environmental Stability Study Of Actinide Borates

Owing to the increasing demand of energy worldwide,the nuclear energy occupies a very important portion in the current international energy structure.However,the nuclear power is a double-edged sword.While it could provide clean energy for our society,it also produces a tremendous amount of radioactive waste at the same time.What's more,the nuclear power plant accidents such as Chernobyl and Fukushima have destructive impacts for the surrounding ecological environment and threatens to human beings.Actinide elements,especially Uranium,Plutonium,and Thorium,are the crucial component of nuclear fuels.Therefore,the research regarding the chemistry of actinides will expand our knowledge on the migration and transformation in the natural and extreme environments of these elements,and provide supporting information on the design of the suitable solid form of spent nuclear fuels.In this work,by adjusting the reaction conditions,a series of the uranium and neodymium borate compounds with various structures have been synthesized using the molten boric acid method.The novel KUBO-4 was characterized with single crystal XRD,SHG and fluorescence spectra,and TG/DSC measurements.Further understanding regarding the dissolution behaviours of uranyl borates and neodymium borates were achieved via solubility studies of the KUBO-1,KUBO-4,Nd BO-1 and Nd BO-2 using a combination of powder XRD,ICP-MS,and spectroscopy techniques.The results are as follows:1)By increasing the reaction temperature to 250 ?(in comparison to the usual 190 ?),a new potassium uranyl borate K[(UO2)B6O10(OH)](KUBO-4)compound was synthesized.The KUBO-4 adopted the same layer topology as the previously reported 2D layered compound KUBO-1.A profound understanding concerning the dissolution behaviours of uranyl borates was obtained via solubility studies of the KUBO-1 and KUBO-4.The results verified that the stability of uranium borates in aqueous solutions was greatly affected by the complexing ligands rather than their structure topologies.For example,in the presence of phosphate,nitrate,and humic acid,the resulting solids were characterized as uranium phosphate and hydroxide,respectively,whereas both uranium borates would completely dissolve in carbonate solutions.2)By adjusting the reaction stoichoimetry,two three-dimensional neodymium borate compounds with distinct structure topology were synthesized from the molten boric acid reactions.Their dissolution behaviors were studied in simulated aqueous condition similar as the groundwater at WIPP(the Waste Isolation Pilot Plant in United States).In general,limited amounts of aqueous Nd(III)were detected in sulfate,carbonate,and tetraborate systems.What's more,it was found that the neodymium borate can be converted into carbonate precipitation in carbonate solutions.Lanthanides are generally considered as good surrogates for trivalent actinides,therefore,the study of Nd(III)would provide useful information of the environmental behavior of the trivalent actinide borates under similar conditions.