Ottawa disclosed the amount in a supplementary budget estimate.
The funding, according to the document, "will be used to address a cash shortfall caused by unexpected technical challenges on CANDU reactor refurbishment contracts.
It's the same explanation given in February, when $100 million was allocated to cover overruns. The same month, the federal budget earmarked $351 million to federally owned Atomic Energy so it could continue design work on its Advanced CANDU Reactor and conduct repairs of the troubled Chalk River research reactor.
"That's a total subsidy for AECL in 2009 of $651 million so far," said Shawn-Patrick Stensil of the anti-nuclear environmental group Greenpeace.
"It's telling, when you consider the federal government isn't likely to fetch more than $300 million for AECL as part of its privatization efforts."
Atomic Energy's two refurbishment projects in Canada the restart of two Bruce Power reactors northwest of Toronto and the overhaul of the Pointe Lepreau plant in New Brunswick are over budget and delayed.