Short answer…
A civil union is not, in itself, an impediment to receiving the sacraments.
Long answer…
I am no authority on this, as I only make a point of knowing the Church discipline (wrt. marriage) which applies in my own situation, but, from what I have seen here in CAF, my answer is that a civil union is a valid marriage, so long as there were no impediments, and hence either or both partners are free to receive the sacraments, under normal conditions (eg. RCIA and being received into the church). The two possible impediments which immediately come to mind are 1. Either party were a baptised Catholic at the time of marriage and entered a civil union without dispensation, or 2. either party were not free to marry, because of a previous marriage. In either of these cases one would need to do further work (eg. have the civil union validated in the Catholic Church) before receiving the sacraments.
A proper answer can only be given with more information, and by someone better qualified than myself.
The best way to get a quick and correct answer is to talk to a priest (and not the first person who answers the phone!) at your local Catholic parish. This is very much part of their job.