When it comes to nuclear construction sites worldwide, Iran has been under the most scrutiny over the past decade.
However, for the sheer multitude of nuclear reactors under construction, nowhere can compete with China. 24 of them were under construction there as of July 01 2015 while a further 27 were operational. Russia is in second place with 8 reactors being built while India rounds off the top three with 6. The US has 5 nuclear plants under construction with 99 operational. According to the World Nuclear Industry Status Report 2015, there are 62 active building sites globally, the highest number since 1990.
This chart shows nuclear reactors under construction and operational worldwide as of 1 July 2015
You will find more statistics at Statista