Looking at today's posts you seem to have spent the whole day looking for answers.
Tense relates to tension meaning tightness - so you could say that the atmosphere was tight or taut - but those are not common usages and sound clumsy - tense is definitely the best word.
Nervous comes from nerves, and since the situation and the atmosphere do not have any nerves, the atmosphere cannot be nervous. The PEOPLE IN the situation can be nervous though, and that is usually what makes the atmosphere tense.
Nervous should only be used to describe people or animals