TORONTO — FOR the first time since the Three Mile Island nuclear power scare in April 1979, a North American utility has committed to building new nuclear reactors. Ontario, Canada's most populous and industrialized province, already generates half of its electricity through nuclear power. Now in the midst of a cold winter that has seen threatened blackouts because of an electricity shortage, the government-owned utility wants to build more nuclear generating plants.
Ontario Hydro wants to install $20 billion (Canadian: US$17.2 billion) worth of CANDU nuclear reactors so it can meet electricity demands into the next century. Use of electricity in Ontario has grown by 5 percent a year over the past six years. To meet that demand it proposes installing 10 reactors at three sites.
``While the demand for electricity continues to grow, the number of facilities we now have to meet that demand will decrease. By 2014, more than one-quarter of the existing generating facilities in the province - over 8,500 megawatts worth - will have been retired because of age,'' said Robert Franklin, president of Ontario Hydro, Canada's largest public utility.
The utility's nuclear proposals now go before an environmental assessment committee of the provincial legislature.
The record cold snap in December pressed Ontario Hydro to the limit. It lowered voltage levels by 3 percent to save energy. It told customers, long used to cheap and plentiful electricity, that if they did not conserve power at peak periods, it would have to cut the electricity off in certain places for 15 minutes or more at a time. That didn't happen. Critics of the utility say it was a scare tactic to prepare the way for the request for more nuclear power.
The spike in electricity demand has meant that Ontario, usually a seller of electricity, has been buying it from the neighboring province of Manitoba and a utility in Cleveland, Ohio.
The purchases from Manitoba were 250 megawatts a day but had to be scaled back to 105 megawatts because Manitoba had to deal with the cold snap as well.
The purchases from American Electric Power of Cleveland amounted to 1,400 megawatts a day in December. That is from coal-fired generators which Ontario Hydro itself is not allowed to operate because of government-imposed acid-rain restrictions.
The utility would not have had to buy all that power if its new nuclear station at Darlington, 50 miles east of Toronto, had opened on schedule in the fall. There were delays because of computer software problems in the emergency shutdown procedure.
But on Dec. 28, Darlington started testing its first reactor. It should be producing 900 megawatts by the spring. By 1993 Darlington will produce 3,600 megawatts of electricity when all four of its reactors are operating.
Although it plans to rely heavily on nuclear power, Ontario Hydro has other plans to meet its electricity needs. They include a conservation program, the building of generating stations to run on natural gas - nonpolluting and Canada has plenty of it - as well as extracting more electricity from Niagara Falls.
Another Canadian province - New Brunswick - is thinking of buying a nuclear reactor from Atomic Energy of Canada Ltd. This reactor would not be to meet rising domestic demand but to supply electricity for export to the northeastern United States.
New Brunswick already has one CANDU reactor at Point Lepreau on the Bay of Fundy, about 100 miles from the border with Maine. It produces about 635 megawatts of electricity, much of it exported to the United States. New Brunswick may buy another similar reactor, according to Mac Keeler, a spokesman for Atomic Energy of Canada Ltd. ``Our hottest domestic prospect is New Brunswick,'' says Mr. Keeler.
The hottest foreign prospect is South Korea, which already has one CANDU 5 reactor.