There are a few important issues here
1. Venezuela is treated as rouge state by US
2. The ties between Russia and Venezuela are very warm.
While building a nuclear reactor has nothing to do directly with the Russia - US ties, it can be seen as an indication of Russia's superpower status,
although all may not agree
Just read an article that some Russian nuclear submarines were headed to Venezuela.
Thus Russia is certainly working within US's geo - political sphere of influence.
en.rian.ru
(visit the link for the full news article)

