Proportioning of 79Se and 126Sn long life radionuclides in the fission products solutions coming from spent fuels processing

The determination of radionuclides present in waste resulting from the nuclear fuel reprocessing is a request from the regulatory authorities to ensure an optimal management of the storage sites. Long-lived radionuclides (T1/2 > 30 years) are particularly concerned owing to the fact that their impact must be considered for the long term. Safety studies have established a list of long-lived radionuclides (LLRN) whose quantification is essential for the management of the disposal site. Among these, several are pure β emitters, present at low concentration levels in complex matrices. Their determination, by radiochemical method or mass spectrometry, involves selective chemical separations from the others β/γ emitters and from the measurement interfering elements. The work undertaken in this thesis relates to the development of analytical methods for the determination of two long-lived radionuclides: selenium 79 and tin 126, in acid solutions of fission products present in nuclear fuel reprocessing plant. For selenium 79, a β emitter with a half live estimated to be 106 years, the bibliography describes different chemical separation methods including precipitation, liquid-liquid extraction and chromatography on ionic resins. After optimisation on a synthetic solution, two of these techniques, precipitation by potassium iodine and separation with ion exchange resins were applied to a genuine solution of fission products at Cogema La Hague. The results showed that only the ion exchange method allows us to obtain a solution sufficiently decontaminated (FDβγ = 250) with a significant selenium recovery yield (85%). This separation allows the measurement of the 79Se by electrothermal vaporization coupled with inductively coupled plasma mass spectrometry (ETV-ICP/MS), after transfer of the samples to CEA/Cadarache. The concentration of 79Se measured is 0,42 mg/L in the solution of fission products with an isotopic ratio 79Se/82Se equal to that recommended by the modelling codes for this fuel. Further analyses were carried out by liquid scintillation counting after two additional chemical treatment steps for purification. For tin 126, a β emitter with a half-live estimated to 105 years, the studies of separation methods described in the literature made it possible to define a liquid-liquid protocol of extraction with N-benzyl-phenylhydroxylamine (BPHA). Its application on genuine solution of fission products was performed with high chemical recovery yield (90%) and decontamination factors (FDβγ = 105). The measurements carried out by ICP/MS made it possible to quantify 126Sn (2,3 mg/L) and to determine the isotopic composition of tin in the fission product solution. The isotopic composition of tin obtained was in good agreement with the data recommended by modelling codes. Complementary measurement on 126Sn, and its decay products, were carried out by gamma spectrometry. Therefore, it was possible to deduce a value of the half-life of 126Sn found equal to 2,63.105 years. (author)