If I remeber it right then one can simulate upto second order decay reactions in Mt3DMS.
Honestly it depends upon what do you want to simulate using radioactive transport modeling.
The range of radionuclide transport mechanisms and whether you treat porosity as a variable or a constant is just a part of the decision.
There can be a number of reactions, number of sorption, desorption mechanisms of just a few of a long list to consider.
Ask the right question and get the right and the simplest code to model is all I can suggest.
regards
hpc