Why some people said nuclear reactors are actually green power? And some said it isn’t?
Nuclear Reactors
Some argued that using nuclear as the source of generating electric, can actually using less earth resources as nuclear materials can last for very very long. It is actually using the heat produced from the atom fission to heat up waters and create steam to move the turbines.

As you’ve been studied in school, the uranium atoms are broken and then they will combine back become the uranium molecule again. This process will produce heat and radiation rays, and the heat will be used for heat up the waters in order to push the turbines to generate electric. Uranium is just used to replace traditional fossil fuels in this process.

A more detail information on how a nuclear reactor works, can be read from this link:
http://www.wired.com/science/discoveries/news/2005/07/68074?currentPage=all

As for the nuclear fission process, radiation and nuclear waste will be produced. Although using nuclear as a replacement of fossil fuel can avoid the use of too many of the earth resources, but radiation and nuclear waste is produced, which bring concerns to the people.

Although if it is managed properly, but still, there are some amount of radiation rays are released to the environment, and it will causing health issues to people who live nearby the reactor. And if it was not handled properly, when a nuclear melt down happens, it will going to kill off a lot living beings, like what happened to Chernobyl, the biggest nuclear melt down in the history.
Here is the details of the Chernobyl disaster at Wikipedia: http://en.wikipedia.org/wiki/Chernobyl_disaster

What kind of effect it will cause to the living beings?
Mutated Dog In Chernobyl
This will be what going to happen after a few year a nuclear reactor melt down.

So trying to not use fossil fuels and causing something much more dangerous? I don’t think it is green at all.
In fact, it will going to cause more pollution to the environment.