Federally owned AECL is up against Areva Group of France and U.S.-based Westinghouse Electric Co. for a contract to build two nuclear reactors in Ontario.
But May is urging the province to look at the fact AECL built two reactors in Chalk River, Ont., that "simply don't work.
She says AECL loves to research and design new products, but she warns the Crown company is not so good at delivering.
The Organization of Candu Industries, which represents 140 nuclear-related companies employing more than 30,000 Canadians, has taken exception to May's comments, calling them "incredibly irresponsible."
The main reactor at Chalk River was shut down May 15 after inspectors detected a heavy-water leak inside the facility its second shutdown in less than two years.
Officials say the reactor, which provides up to half the world's supply of medical isotopes used to detect cancer and heart ailments, will be down for more than a month.
May pointed to the last crisis in 2007, which ended when Parliament voted to overrule the regulator and reopen Chalk River. The Conservative government subsequently fired the head of the Canadian Nuclear Safety Commission, who ordered the initial shutdown.
"Rather than attacking the problem, we're back where we started with a reactor that doesn't work, with hundreds of millions of taxpayers' dollars squandered on a design that doesn't work and with AECL continuing to evade responsibility," May said.
But Neil Alexander, president of the Candu industries group, said the Green party leader is comparing apples and oranges Ontario wants to build power reactors while the facilities at Chalk River are research reactors.
"It's a complete lie and it's not actually substantiated by any fact," said Alexander, whose member companies supply parts and build reactors.
"If you look at the projects where AECL has been building power reactors, they have probably one of the best records of any constructor in the world.... The comments are incredibly irresponsible."
Candu reactors come in on or ahead of schedule and are "the No. 1 performer in the world," he added.
AECL ran out of medical isotopes over the weekend and doctors are scrambling to hoard a scarce supply from the world's four other isotope-producing reactors.
Areva Group wrote two federal cabinet ministers offering to help Canada find European reactors that can make the medical isotopes. The French firm also told the government its engineers can help AECL repair the 52-year-old Chalk River reactor.
Ontario is expected to award its new-reactor contract this summer, though the province's energy minister, George Smitherman, has said the decision could be delayed.