DETERMINATION OF LONG-LIVED RADIONUCLIDE (~(10)Be, ~(41)Ca, ~(129)I) CONCENTRATIONS IN NUCLEAR WASTE BY ACCELERATOR MASS SPECTROMETRY

摘要

Radiological characterization of nuclear waste is essential for storage sites management. However, most of Long-Lived RadioNuclides (LLRN), important for long-term management, are difficult to measure since concentration levels are very low and waste matrices generally complex. In an industrial approach, LLRN concentrations are not directly measured in waste samples but assessed from scaling factors with respect to easily measured gamma emitters. Ideally, the key nuclide chosen (~(60)Co, ~(137)Cs) should be produced by a similar mechanism (fission or activation) as the LLRN of interest and should have similar physicochemical properties. However, the uncertainty on the scaling factors, determined from experimental and/or calculation data, can be quite important. Consequently, studies are performed to develop analytical procedures which would lead to determine precisely the concentration of LLRN in nuclear waste. In this context, the aim of this study was to determine the concentrations of three LLRN: ~(129)I (T_(1/2) = 15.7×10~6 a), ~(41)Ca (T_(1/2) = 9.94×10~4 a) and ~(10)Be (T_(1/2) = 1.387×10~6 a) in spent resins used for primary fluid purification in Pressurized Water Reactors using Accelerator Mass Spectrometry (AMS) for measurement. The AMS technique combined mass spectrometry and nuclear physics to achieve highly efficient molecular and elemental isobars separation. Energies of several Million Electron-Volt transferred to the ions in the first accelerating part of specifically developed tandem accelerators lead to molecular isobars destruction through interaction with the argon gas used to strip the injected negative ions to positive ones. At the exit of the tandem accelerator, the energy acquired in both accelerating parts allows an elemental isobars separation based on their significantly different energy loss (dE) while passing through a thickness of matter dx that is proportional to their atomic number (Z) and inversely proportional to ions velocity (v) according to the Bethe-Block law (1). dE/dx = (k~*Z~2)/v~2 (1) The use of a particle accelerator in conjunction with a selective ion source, mass and energy filters and a high-performance detector thus allow unambiguously identifying and measuring analyte concentration against much more abundant interfering isobars. The development of AMS and of related applications have recently been extensively reviewed. Up to now, the potentialities of the accelerator mass spectrometry technique were explored for the measurement of cosmogenic radionuclides produced in the Earth's environment either in the atmosphere or in the Earth's crust (in situ-production). Many applications aiming to date and/or quantify Earth surface processes have been developed in the fields of geology, geomorphology and planetary sciences as well as archeology paleoanthropology and biomedicine. The present study extends the scope of AMS to nuclear industry. Because AMS facilities are not widely accessible and difficult to handle, LLRN concentrations in nuclear waste are usually determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and radiometric techniques. However for the measurement of very low LLRN concentrations, AMS becomes the most effective measurement method with detection limits of 10~5 - 10~6 atoms per sample. In this study, AMS measurements were performed using the French AMS national facility ASTER located at the Centre Europeen de Recherche et d'Enseignement des Geosciences de l'Environnement (CEREGE). The challenge was to define a chemical treatment procedure allowing the measurement of the three nuclides, ~(10)Be, ~(41)Ca and ~(129)I, by AMS. Each method selection was based on three main requirements: 1) a quantitative recovery in solution of Be, Ca, I and key radionuclides after resin mineralization, 2) a selective extraction from the sample matrix and the separation from β-γ emitters (~3H, ~(14)C, ~(55)Fe, ~(59)Ni, ~(60)Co, ~(63)Ni, ~(90)Sr, ~(125)Sb, ~(134)Cs, ~(137)Cs) and isobars, 3) the precipitation of each element under the best suited forms (i.e. AgI, CaF_2, BeO) for AMS measurements. The chosen methods were optimized on synthetic solutions and finally applied for the determination of the three LLRN concentrations in spent resins from a 900 MWe Nuclear Power Reactor.