A Suggestion for a Science Debate Topic
There is evidence for the existence of a natural nuclear fission reactor on Earth two billion years ago based on the nuclear waste found in rocks of that age. Candidates can be asked if they accept such evidence and what they think of the implications of the fact that the waste did not move with repect to the surrounding rock (in particular, the implications for nuclear waste disposal).
On the other hand, it might make more sense to ask them general questions about the scientific method. For example, they might be asked what a control group is or what the difference is between Type I and Type II errors.