TORONTO - Ontario has narrowed it down to four companies in the running to build the province's first new nuclear reactors in more than a decade, Energy Minister Gerry Phillips said Friday.

While Ontario is on track to close its coal-fired plants by 2014 and is counting on residents to conserve energy, Phillips said the lights can't stay on without new nuclear capacity at either the Darlington or Bruce power plants.

"Nuclear has been the backbone of our electricity system,'' Phillips said, noting more than half the province's energy last year came from nuclear plants. "With any major facility ... it requires refurbishment or replacement over time. That's essential for our plan over the next 20 years.''

Four companies have been asked to submit proposals, including AREVA, Atomic Energy of Canada, GE Hitachi Nuclear Energy and Westinghouse Electric Company, Phillips said.

Their proposals -- due by the end of June -- will be examined by a large team of senior bureaucrats from various government ministries, as well as representatives from Ontario Power Generation and Bruce Power.

The process will also be scrutinized by a review board and a "fairness monitor'' to ensure a level playing field, he said.

Those proposals will be judged based on their benefit to the Ontario economy, their ability to deliver new nuclear reactors by 2018, the cost of power and the risk assumed by the vendor, Phillips said.

The province doesn't want to see taxpayers or ratepayers on the hook for cost overruns or delays, he said.

"Our objective will be to minimize the risk to ratepayers, to the public,'' he said. "One of the key criteria we'll use is demonstrating that they're prepared to take on the maximum risk (and) that they're prepared to deliver on time.''

The aim is to negotiate a tentative agreement by the end of the year, with shovels in the ground by 2012, Phillips said. The reactors likely wouldn't come online until 2018, leaving Ontario with a four-year gap between the closures of the coal-fired plants in 2014 and the new nuclear generation.

But Phillips said the province has planned for this and doesn't expect a shortage of electricity that would cause utility rates to skyrocket.

"I think we've got a really good plan,'' Phillips said, adding the province is increasingly relying on new sources of renewable energy and conservation.

Ontario has come under pressure recently to choose the CANDU reactors built by Atomic Energy of Canada as opposed to a foreign company. Premier Dalton McGuinty has said the government is looking at the international market to get the best deal for Ontario taxpayers.