ThO$_{2}$-basierte Keramik zur Fixierung von Minoren Actiniden

Abstract

The minor actinides (Pu, Np, Am), which occur as decay products in nuclear reactors, are the most important nuclides determining the long-time risk of final disposal. Therefore after separation from the fission products the minor actinides must be finally disposed of in a safe matrix or they must be further processed as fuel. In this work, it was investigated whether these actinides can be finally disposed of as a stable component in ThO$_{2}$-based ceramic. In the experimental work Ce was used as a simulator for these actinides. The source material of (Th$_{x}$Ce$_{1-X}$)O$_{y}$ was made of Th and Ce nitrates by treatment with ammonia gas (co-precipitation). Compacts were produced from this powder after processing (drying, milling, calcination etc.) by a uniaxial hydraulic press under special conditions such as a pressure of 20-80 kN (175-900 MPa). According to a sintering program, the compacts were sintered in a furnace at 1500-1600°C. BET, DTA-TG, SEM-EDX, XRD and chemical analyses were carried out to characterise the source materials and the ceramic products. The density of the compacts was determined geometrically and that of the final products by Archimedes' method. According to the experimental results, the ceramic products (ultimate waste disposal materials) can be produced with a relative density of 80-95% of the theoretical density depending on the preparation conditions of the source material. The preparation conditions do not only influence the density but also the impurities in the product. It is assumed that these factors (calcination and milling) could play an important role in the leaching behaviour of the products. For the determination of the stability as well as the behaviour of the ceramic products in leachate media the leaching tests were performed in deionised water in a Soxhlet (dynamic) and in a sealed glass vessels (static leaching test) for a duration of 48 and 84 days. The ICPMS analyses show that the elements Ce and especially Th were found in the leachate in extremely small concentrations and that the ceramic materials produced have very good hydrolytic stability. As described in this work, the minor actinides can be finally disposed of in ThO$_{2}$-based ceramic