Canada has not sold one of its much-touted ''Candu'' nuclear reactors abroad since 1978. But sales hopes are rising.
A senior official in the Department of Energy expresses confidence that the government-owned Atomic Energy of Canada Ltd. will win a bumper $7 billion nuclear power plant contract from Mexico next autumn. The Latin nation is planning to have the third world's largest nuclear industry.
The initial Mexican requirement is for a nuclear power complex with a capacity of 1,600 to 2,400 megawatts. It will be the start of a massive project to supplement the country's large production of oil before that resource begins to run out.
The original Canadian bid is in two parts. It offers four 600-megawatt Candu nuclear units of the same kind now in Canadian commercial use. It also offers a technology package. This would permit Mexican industries to manufacture some of the equipment needed for the new nuclear plants.
By the year 2000, Mexico hopes to have 20,000 megawatts of nuclear power installed, or about one-fifth of present world capacity.
There are significant reasons behind Canadian government enthusiasm about securing the Mexican order. Mexican President Jose Lopez Portillo and Canadian Prime Minister Pierre Trudeau have developed a strong friendship. In addition, Mexico has chosen some other types of Canadian high technology recently.
Most noticeable was the 1981 sale (against fierce French competition) of $100 million in subway cars for Mexico City's expanding metro. These will be built by Bombardier Inc. of Montreal.
Canada is also one of five ''middle powers'' selected by Mexico as external sources of needed technology. The others include Sweden, France, Brazil, Japan, and West Germany.
The toughest competition facing the Canadians for this first nuclear plant order (which could mean further sales for future reactors) is coming from the Swedes and French.
In mid-January, hot on the heels of the departing Mr. Trudeau and his four-day official visit with Mr. Lopez Portillo, came King Karl Gustaf XVI of Sweden. The King rules a social democracy which is popular in Mexico and also has nuclear reactors for export.
The French are reportedly offering the most competitive concessionary terms, with interest as low as 73/4 percent, for the loan Mexico will need to pay for the new reactor.
The Canadians may not be prepared to go below 8 percent. United States nuclear power companies seeking the Mexican business could be out of the running because of the Reagan administration's reported directive that concessionary terms offered to developing nations from the Export-Import Bank must not go below 10 percent. The Mexican decision will not be made until mid-November.
The US bid could be killed by an older, ideological issue, namely the strong anti-Americanism which has existed in Mexican national governments since the nationalization of the country's infant oil industry in 1938.
Reliance now on US nuclear technology would reverse the political process of the late 1930s especially at the time when a strong, conservative Washington administration is opposed ideologically to the Mexican policy of government control over energy exploration and production.
Many Canadian exporters to the Mexican market have learned how to play on the country's deeply entrenched and anti-US economic nationalism, picking up contracts that might otherwise have gone to US companies.
This strategy is also a major, if unspoken, element in the official Canadian campaign to sell its initial nuclear package to Mexico. The Candu reactor and its subsystems were designed and built with no US technical know-how.
But the French and the Swedes are also using the same nationalistic pitch. Thus nongovernment observers judge that a Canada-Mexico nuclear marriage remains far from certain.