Surprise. Today is when it is most prevalent, so possibly askhistorians might not be the right venue for you. Nuclear power has grown in capacity though almost no new plants have been built in the US in decades. Design changes and increased efficiency in the existing plants have increased nuclear's percentage share of total US electricity.

Environmental protesters in the late 70s and forward were successful in effectively shutting down new reactors, but the reactors are designed to function for quite a few decades. In the last few years, some of those early environmentalists have done an about face about nuclear power. One is Stuart Brand of the whole Earth Catalog. He advocates extensive use of nuclear power as the only viable option to reduce carbon emissions. One of the protesters specializing in nuclear issues was/is Helen Caldicott, who has, like Brand, written several books.

Other than the list of reactors from the Nuclear Regulatory Commision, there's a very interesting maps project here showing the power plants in different locations. Just type in your zip code.

In terms of total number of operating power reactors, it peaked in 1990, at 111 reactors. (Decommissioning has dropped it down to 99 today.) Though almost no new reactor constructions were begun between 1977 and 2007 (the renewed cheapness of oil made utilities lose interest, even before Three Mile Island), many of those that were under construction took increasingly long amounts of time to complete, in part because of litigation and oversight issues raised in the wake of the Three Mile Island accident. So no reactor active today had its construction begun any later than 1977. The late 1960s was the peak of interest in constructing new reactors.

In terms of raw megawattage, I'm not sure — I don't have the historical data for that at hand. They get more energy out of the reactors today than they used to, because of design changes and increased experience and efficiency.