Khondab, Iran ? As the U.S. and Europe struggle to stop Iran’s uranium development, the Iranians are pushing ahead on another track that could also lead to a nuclear bomb – construction of a heavy-water reactor that can produce weapons-grade plutonium.

It will take at least another four years for Iran to complete the reactor, making it a less immediate worry for the West than the uranium program, parts of which are either in operation or ready to go at a moment’s notice.

But ultimately, the heavy-water reactor could prove more dangerous, since bombs made with plutonium are smaller and easier to fit onto a ballistic missile.

In a comprehensive package aimed at reining in Iran’s nuclear program, Europe proposed that it give up the heavy-water project in return for a light-water reactor, seen by arms control experts as easier to monitor to ensure it’s not being used for weapons.

Iran – which says its nuclear program is peaceful – rejected the entire package this week. The head of Iran’s Atomic Energy Organization called the heavy-water reactor offer a “joke.”

“We have developed this capability. The heavy-water project today is a reality,” Gholamreza Aghazadeh, who is also vice president, said on state-run television. “This knowledge belongs to Iran. Nobody can take it from us. As they (Europeans) see Iran’s determination, they will be forced to show flexibility and accept it.”

While Iran has agreed to suspend parts of its uranium program as a gesture in negotiations with Europe, it has repeatedly rejected European calls for it to freeze the heavy-water project, which is moving full steam ahead.