Replacing Australia’s coal-fired power stations with small modular nuclear power reactors would cost A$387 billion ($249 billion), according to Climate Change and Energy Minister Chris Bowen.

The high cost makes such a transition, advocated by the country’s opposition, a “unicorn and a fantasy,” Bowen said on ABC Radio National Monday.

Australia would need 71 small modular reactors to replace the 21 gigawatts of coal-fired power in the system, he said, discussing analysis from the Climate Change Department. “I can’t think of a worse fit for Australia than nuclear power,” Bowen said.

Read more: Can Small Nuclear Reactors Really Help The Climate?: QuickTake

Backers of small modular reactors include Bill Gates and Warren Buffett, though there’s currently only a handful of operating plants and key developers have recently flagged cost challenges.