Solubility of Np(IV), Zr(IV), and Reactive Transport Modeling with Uncertainty Calculations

Doctoral thesis, 2004

Sweden, as some other countries, is planning to construct a repository for long-lived low and intermediate level radioactive waste. The planned repository is a concrete construction in the bedrock a few hundreds of meters below the surface. With time ground water will leach alkaline substances from the concrete which will raise pH of the intruding water from its natural near neutral to 12-13.5. Under these conditions the water will start to react with the rock. To understand the overall behavior of the repository it is necessary to understand several processes. One very important one is the dynamics between the water's reaction with the rock and its transport in the rock. Another is the solubility behavior of certain elements under these conditions. Geochemical modeling in the form of reactive transport modeling can be an important instrument to gain greater knowledge of the system mentioned above. As in using any kind of modeling, it is important to understand its possibilities and limitations. This is chiefly what has been investigated in this work by means of program development and testing, and various calculation exercises. As a complement, solubility experiments were carried out for some relevant elements. This work is divided into two parts, where the governing idea in both is their application to radioactive waste disposal in geological formations: The first is the modeling of Coupled Reaction Transport (CRT). The focus has been on the investigation of uncertainty of the calculations or how accurately predictions can be made given the accuracy of the input data. To do this, a CRT modeling program was developed, TACK. This program was then further developed into TACK-UAP, which adds the ability to make probabilistic variations in input data in a controlled and fairly easy-to-use manner. TACK-UAP was then employed to investigate how the uncertainty of the input data can affect the outcome of modeling. The second part is a solubility investigation of Zr(IV) and Np(IV), which will both be abundant in radioactive waste from nuclear power plants. Both elements were investigated under conditions similar to repository conditions, that is, a low oxygen concentration or reducing atmosphere and alkaline pH. Some interesting behavior was identified at the most alkaline conditions, an example being the indication of the formation of the zirconate ion. Another is the solubility of NpO2(cr), which seems to be somewhat higher than expected from calculations using Np(OH)4(aq) as the most hydrolyzed complex.