Well burning "fossil fuels" is environmentally unfriendly and using nuclear power is "cleaner"

from this article

"In Paris last week, U.S. Ambassador Constance Morella told conferees that nuclear energy was "clean" and "reliable" and necessary in order for the world to have a "secure energy supply."

Mohammad Saeidi, a vice president of the Atomic Energy Organization of Iran, delivered more or less the same message to the conferees.

Oil and natural gas "are limited and belong to all subsequent generations" and "unrestrained use of this source of energy is not prudent," he said.

Iran's goal, Saeidi added, is nothing less than "self-sufficiency in all aspects of the peaceful use of nuclear energy."

So even the US supports the use of nuclear power. Iran is simply doing what the US supports