Material Parameters and Experiments - Corrosion Behavior of Spent Fuel in Non-saline Solutions Experimental Studies at FZK/INE

摘要

With respect to the assessment of the long-term behavior of the waste form spent fuel it is of high importance to understand in particular the UO_(2) matrix dissolution behavior and the associated release or retention of mobilized radionuclides during an assumed contact with groundwater. Due to the radionuclide inventory of the fuel radiolysis of the groundwater will start, followed by oxidation and subsequent dissolution of the fuel. Additionally, the overall corrosion behavior of the fuel is governed by the geochemical constraints in the near field. Related to this objective we studied experimentally the impact of geochemical conditions possibly encountered in close vicinity to the spent fuel on fuel matrix dissolution under anoxic conditions. Special attention was focused on (1) the nature of solution contacting the matrix (5 M NaCl brine, granite water [low ionic strength, SiO_(2)-rich], granite bentonite water [high ionic strength, low in SiO_(2)]), the (2) presence/absence of CO_(2), and (3) the presence or absence of corroding container material (Fe and Fe corrosion products). Pre washed powdered fuel samples (grain size > 3 (mu)m) were used for these studies to achieve a faster reaction progress and to mask contributions from the grain boundary inventories. The fuel matrix dissolution rates were found to be within the same range in 5 M NaCl-brine, granite and granite bentonite water (about 10-4/d). In the presence of iron and its corrosion products considerable amounts of H_(2) were generated due to the corrosion of the added iron. In these cases, the matrix dissolution rates have been slowed down by a factor of 10 - 20, associated with strong retention effects for various radioelements. Studies upon spent fuel corrosion behavior in a clay environment are presently in the planning stage.