If the deal goes through, think how many reactors and other associated equipment will be imported. Just guess the money involved and the commission of the ministers, bureaucrats and scientists for awarding the contracts plus other fringe benefits like foreign visits, securing employment of their relatives in such companies which are benefitted by the purchase orders. So how can any minister, bureaucrat and scientist say that the deal is not good.
America and European countries will sell the reactors, fuel, etc. at exorbitant rates. Hence they will also want the deal to go through.
Now BJP does not want congress to pocket all money. Hence they are opposing. Once they come to power they will also bring similar deal with some minor changes to fool the general population.
Communists get lots of funds from China. China does not want India to sign the deal with Ameriaca. Hence they are telling communists to oppose the deal.