CALGARY (CP) — Canada’s largest natural gas shipper, TransCanada PipeLines Ltd., made a major move into nuclear power Monday, expanding the electricity business it has grown in recent years by buying and building gas-fired power plants across North America. In a high-profile move, the big Calgary energy company made its first foray into the nuclear field Monday by becoming a key partner in Ontario’s Bruce Power complex on the eastern shores of Lake Huron. Chief executive Hal Kvisle insisted the company’s nearly one-third ownership in Bruce, which will cost about $376 million, is keeping with TransCanada’s strategy of smart acquisitions in markets it knows well. ‘‘We always look to the underlying supply fundamentals and to our competitive position in the market,’’ Kvisle told a news conference in Calgary. ‘‘And it’s on that basis that we then go forward and kick the tires if you will on individual opportunities.’’ TransCanada completed a successful turnaround of its sagging fortunes recently by retreating from a variety of international ventures and selling billions of dollars worth of non-core assets. The company instead trained its sights on its core business of shipping natural gas across Canada and into the northern United States as well as expanding into power generation. And while the past year has been calamitous for many North American gas shippers and utility companies, TransCanada has remained in strong financial shape. Kvisle said Monday that his company was very impressed by ‘‘the low-cost, reliable nature,’’ of nuclear power in Ontario — which has Canada’s most extensive network of nuclear generating plants — and shareholders should be as well. TransCanada currently has interest in eight power plants in Alberta and three in the U.S. Through a limited partnership called TransCanada Power the company is also involved in five other plants in Ontario, one in B.C. and one in New York state. The decision to buy into Bruce is also one of a very few expansionist moves by private power companies into Ontario, which recently spooked the market by reverting from its full privatization plan and re-capping electricity prices. TransCanada also announced last week that it was studying the viability of building a big gas-fired power plant for downtown Toronto.