Influence of the Near-Field Conditions on the Mobile Concentrations of Np and Tc Leached from Vitrified HLW
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Influence of the Near-Field Conditions on the Mobile Concentrations of Np and Tc Leached from Vitrified HLW Véra Pirlet, Karel Lemmens, Pierre Van Iseghem Waste and disposal Department, SCK•CEN (Belgian Nuclear Research Center) Boeretang 200, B-2400 Mol, Belgium ABSTRACT The solubility values usually used in Belgium for Performance Assessment of the disposal of HLW in deep geological formations are based on migration experiments in the far-field and, thus, can be considered as "far-field solubilities". The particular chemistry in the near-field of the repository will also influence the solubility of the released radionuclides. Therefore, the determination of maximum concentrations of the radionuclides in the nearfield is also useful. This was approached by leach tests with doped glass in near-field conditions. The experiment consisted of static leach tests with the french R7T7 reference glass doped with 237Np (0.34 wt%) and 99Tc (0.12 wt%), two key radionuclides in the performance assessment of the disposal system. The glass was put in contact with FoCa7-clay together with pyrite, metallic iron and Boom clay water to simulate the near-field conditions under reducing conditions. The interpretation of the results is based on the measurement of the steady-state Np and Tc concentrations in the leachates after ultrafiltration through 100,000 MWCO membranes and on the activity measured on the clay after different test durations. Most of the Np and Tc activity (99.5%) was found associated to the clay and increasing as a function of time while the Np and Tc concentration in the leachates were stable and extremely low with a mean value of 1.5x10-10M for Np and 5.6x10-10M for Tc. This is due at least partially to sorption on the clay, but there are indications for precipitation as well. INTRODUCTION The Boom Clay, which is the candidate geological formation for the disposal of longlived and high-level nuclear waste in Belgium, is considered as the most important barrier for the radionuclides in the disposal concept. In the Performance Assessment (PA) of the geological disposal, the radionuclide solubility is a critical safety parameter. The solubility values used by PA in Belgium are often based on migration experiments in the Boom Clay, so they can be considered as 'far-field' solubilities. The uncertainty on the solubility is still relatively large, though. To get a coherent picture of the solubility range and influencing parameters, another approach, namely by leach tests with doped glass in near-field conditions, can be performed to determine maximum or realistic radionuclides concentrations in near-field conditions which can be then compared with the far-field concentrations. This was done for 99Tc and 237Np, two main critical radionuclides for vitrified HLW in Belgium. In previous leach tests performed in a slurry of FoCa7-clay1 with synthetic clay water, showing a high redox potential, the Np and Tc concentrations were much higher than in a mixture with Boom Clay, known to be reducing [2]. The experiments pres
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