It must be some parts of nuclear reactor, like electrical supply or something. India has not imported a nuclear reactor as a whole from China till date. Nor is there any trade of critical nuke tech between India and China in my knowledge. If you look at the list carefully, you would find that it is on both import and export list. That is India is exporting AND importing nuclear reactors from China. But none of the Indian plants is based on Chinese tech. As far as I know, India imported/was helped by US/Canada.
Water boiler units for reactors can probably be imported from China. The link doesn't describes anything.
No idea, it says clearly nuclear reactors. Look carefully at the link I provided. There are two tables there.
In post 8 among the items India exported to China
Nuclear Reactors, Boilers, Machinery Etc.; Parts at listed with values of 478 million dollars for 2011 and 469 for 2012.
Those items are also listed in the basket China exported to India during 2011 and 2012.
The values are 12341 millions for 2011 and 11123 millions for 2012.