Is HeySel SuqSIv the standard term for nucleus?
Unknown. But fortunately, the English sentence here has core of an atom, not nucleus.
What, then, does the core of an atom refer to?
I suppose it refers to the nucleus. But you asked whether HeySel SuqSIv was the standard way to say nucleus. We don't know what Klingon-speaking scientists call the atomic nucleus. It's just what the author of this particular sentence chose to say.
Thanks. I am certainly not physicist, so I thought there might be some other core thing that I had not remembered.