Spent Nuclear Fuel Experiments During the Last 20 Years Performed at JRC-Karlsruhe

Twenty years ago, Enresa initiated an ambitious research program aimed at exploring the behavior of high burn-up spent nuclear fuel within the expected conditions of a deep geological repository. This research has been conducted within the hot cells at the Joint Research Center, Karlsruhe (JRC), in the frame of a collaboration agreement between Enresa, JRC, Eurecat and UPC-Barcelona Tech.<br/>The primary aim of this investigation was to gain insights from leaching experiments regarding various factors, including the characteristics of the spent fuel such as Burn-up, Linear Power Density, and sample type (powder, segments), alongside the diverse experimental conditions such as redox conditions and leaching solutions (bicarbonate water, cement water and bentonitic groundwater). The radionuclide segregation during irradiation was also a factor studied. From a radial point of view comparing internal and external powder samples, and from an axial point of view, determining the effect of the interpellet space, and using cladded segments from different positions of the pin.<br/>Seven specimens of spent fuel have been studied in total, with four originating from PWR reactors and the remaining three from BWR reactors. Burn-ups varies from 45 to 63 MWd/kgU.<br/>In this work, we present results on spent fuel characterization as well as on leaching experiments in terms of Instant Release Fraction for significant fission products such as Rb, Sr, Cs, Mo and Tc and matrix dissolution rates based on uranium release.<br/>Leaching data have been modelled by using Segregated Radionuclide Identification and Quantification Model (SERNIM). The main advantages of SERNIM is its capacity to separate the release of a radionuclide from the different locations in the fuel at any time, including the gap, dishing and fractures; pre-oxidized fuel surface; internal grain boundaries and finally UO₂ matrix dissolution.