A problem with my simulation with EOS9nT

I have some problems in my modelling project with EOS9nT. In my example I want to simulate the nuclide release and transport process in a disposal site with a 3-D model. But the nuclide concentration gets always lost during the simulation. (The considered nuclide is Ur235 with a long half life time). Sometimes the output of the advective transport rate is NaN, where the nuclide concentration also disapeares. However, the water flow simulation part runs correctly.

Has some one experience about this problem?

Thank you very much for your help!

Xiaoshuo Li