Influence of the Near-Field Conditions on the Mobile Concentrations of Np and Tc Leached from Vitrified HLW

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 near-field 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 1.5×10⁻¹⁰M for Np and 5.6×10⁻¹⁰M for Tc. This is due at least partially to sorption on the clay, but there are indications for precipitation as well.