It is a popular misconception that nuclear fusion power is free of radioactivity; in fact, the deuterium-tritium reaction that nuclear scientists are currently exploring with such zeal produce both alpha particles and neutrons.
Studying GRE, I just faced this phrase.
Just based on the above sentence, do you think the writer intends to make those scientists seem to be pretty much preoccupied with their own research, without taking account of other things like its practicality or negative impacts, whatever, by using “with zeal”? or the opposite?
Zeal can be positive, negative, or neutral. You can only determine that from context.
In the example sentence you give, I would say it’s neutral. The ‘popular misconception’ is not connected to the scientists, and there’s nothing that indicates that the scientists are being dishonest or immoral.