In the sentence, "Well, that concludes another very fine experiment", the interjection word would be Well because it shows their expression/feelings as they say it. It could be like, "Oh well" type of way of saying it, or "well this has been fun" type of way.
I know this because interjection is a word that expresses emotion or feelings and is mostly/usually used in the beginning of a sentence.
You're welcome in advanced! I truly hope this helped!
P.S. I put this in my own words.