The nuclear forces responsible for radioactive decay are short ranged and so isolated from other forces (such as the much weaker E$M forces) that this results in the exponential decay law. I believe there have been experiments that demonstrate that these decays can be slightly affected by exposure to very strong E&M fields, but this is a special circumstance that normally does not occur. I would have to Google to find references to those experiments. I believe the experiments were conducted on nuclear isomers.
Edit: The experiments that I was remembering took place between 1998 and 2007, but a Google search reveals that those experiments have now been discredited. You may read about this episode here. The search terms that I used were: nuclear, isomer, decay, stimulated. If you follow the links resulting from this search, you may find the original sources.