Using reactive transport modeling to evaluate the source term at Yucca Mountain

Abstract

The conventional approach of source-term evaluation for performance assessment of nuclear waste repositories uses the dissolution rate of waste form and the solubility of pure phases of radioactive elements to constrain radionuclide concentrations. This standard practice has obvious shortcomings. First, it is unrealistic to use the dissolution rate of spent fuel to constrain source terms because of its short life under the repository conditions. Second, it is well recognized that most radionuclides may incorporate into secondary uranium minerals as solid solutions. A source-term model without considering the role of secondary minerals would deviate from the reality. As a result, source term predicted by the conventional approach may be several orders of magnitude higher than the concentrations of radioelements measured in spent fuel dissolution experiments. This paper presents the author's attempt of applying reactive-transport modeling approach to realistically evaluate source term by including the precipitation and dissolution of secondary minerals for Yucca Mountain. Based on the general reactive-transport code AREST-CT, a model for spent fuel dissolution and secondary phase precipitation has been constructed. Its predictions have been compared against laboratory experiments and natural analogues. It is found that without calibration, the simulated results match laboratory and field observations very well in many aspects. More important is the fact that no contradictions between them have been found. By considering Np incorporation into uranyl minerals, the model not only predicts a lower Np source term than that given by the conventional model but also produces results that are consistent with laboratory measurements and observations. Moreover, two hypotheses, whether Np enters tertiary uranyl minerals or not, have been tested by comparing model predictions against laboratory observations; the results suggests that the former hypothesis is a better assumption for Np source-term prediction. It is concluded that this non–conventional approach of source-term evaluation not only eliminates over-conservatism in the conventional approach to some extent but also gives a realistic representation of the system of interest. Therefore, it is a promising alternative approach for source-term evaluation.