Uncertainties in Actinide Solubility Calculations Illustrated using the Th-OH-PO4 System

Doktorsavhandling, 1999

Computer simulations have become increasingly popular in many different areas over the years, owing mainly to more effective and cheaper machines. In many cases, the trend seems to be that computer simulations are replacing experiments, at least in areas in which experiments are very difficult (expensive) or impossible. One such area is that of attempting to foresee what will happen in the future. Such analyses are very important for a durable construction such as a repository for spent nuclear fuel, for example. In the modelling effort, several computer codes are used and input data are often used without scrutiny. However, this work shows that even the rather simple task of calculating the solubility of a solid phase in a given water is encumbered with the effects of different uncertainties. These uncertainties may make the calculated solubility vary by several orders of magnitude. Thus the input to the more complex codes, simulating processes in connection with the repository, will also be affected. This thesis presents some computer programs for uncertainty and sensitivity analyses of solubility calculations. They are then used to make predictions of uncertainty intervals for a case at the Äspö site in Sweden. Some of the input data treated as uncertain parameters are the stability constants for the reactions between the metal ion concerned and the elements present in the selected water or the rock. Stability constants and the enthalpies and entropies of reaction for the thorium-water-acetylacetone-phosphate system have been determined experimentally. In addition to the values determined for these entities, uncertainty intervals are also estimated. A complexing mechanism for the thorium-phosphates at pH 8 is also suggested.