Separation and determination of actinides from nuclear spent fuel solution by alpha spectrometry

Spent nuclear fuel represents an important category of radioactive waste produced in nuclear domain. The characterization of radioactive waste consists in identification and quantification of radioactive content, to evaluate and reduce radioactive risk on short and long term for the environment and population. One of the used performance techniques for the characterization of radioactive waste is alpha spectrometry, with the help of which the alpha emitter content is measured, based on destructive analysis methods and going through several phases: 1. Preliminary treatment (sample dissolution, adding of spike solutions); 2. Chemical separation (ionic exchange, electro deposition); 3. Obtaining the source and acquisition of spectra. The preliminary treatment is applied for bringing the samples in a homogeneous form and the adjustment of these for the subsequent chemical processing, such as separation of every alpha emitting element. For the measurement of the radiochemical separation yields, tracers are added in the sample in the phase of preliminary treatment. The general techniques used for separation and purification include coprecipitation, liquid-liquid extraction, ionic exchange etc. In some cases, two ore more of these techniques are combined. Co precipitation is often used for the pre-concentration and the obliteration of the radio nuclides in the matrix which will be analyzed. For example, the precipitation of calcium oxalate will eliminate the majority of the actinides from complex mixtures, leaving the majority of the organics and inorganic separated. The mechanism of coprecipitation for the majority of the components is inclusion. Several compounds, such as iron hydroxide are amorphous and usually contain water molecules. When radionuclide tracks are in a complex mixture, a procedure of co precipitation for the preliminary separation is used. For example, iron or aluminium hydroxide can be used for the separation of U from aqueous solutions by adding carbonate free of ammonium hydroxide. U can be co precipitated, also with Al, Ti, Zr or La, as fluorine or phosphate. The liquid-liquid extraction (named extraction with organic solvents) is used for the extraction of U or of other actinides. For this the following are used: organic acids, ketones, ethers, esters, alcohols and organo-phosphoric derivatives. Methyl-isobutyl ketone has been intensively used in the nuclear industry for the extraction of U and Pu from spent nuclear fuel. In some cases, solvent extraction can concentrate the nuclide 10 times or even more and can be selective for a specific nuclide. Extraction means that the nuclide is found in an ionic form in aqueous solutions. Nuclides cannot be extracted from this kind of colloidal solutions, suspensions or if the aqueous solution contains complex organic materials. Depending on the distribution coefficient, KD, for a liquid extraction system, it is necessary that a few extractions from the sample to be made, to obtain high retrievals for the radionuclide. The ionic exchange is one of the most used techniques for chemical separation. With the sample in an aqueous acid environment, the ions (actinide compounds) replace the active groups from the resin, while other ions pass through and continue the process. The followed element is eluted from the resin, using different elution agents. By carefully selecting the parameters, the ionic exchange procedures can achieve very good performances. To summarize, by applying the separation methodology of alpha emitters and with the help of the measurement technique by alpha spectrometry, the alpha emitting components of the spent nuclear fuel solution (like U, Pu, etc.) can be identified. The method can also be used for the determination of the alpha emitters concentration, if tracers (232U, for the determination of U isotopes and 242Pu, for the determination of Pu isotopes) are added at the samples that have to be analyzed. These are added in known amounts (activities), before the phase in which the elements are separated and they are measured in the final phase, to measure the chemical yields of separation of the elements which have to be analyzed, yields that be will applied to each alpha emitter