Aren't Canada and the United States, the nations that share the world's longest undefended border, supposed to be good friends?

If so, what possible explanation and justification can there be for Canada's recent decision to let the United States become weaker than it really needs to be? Could Canadians seriously doubt that when their neighbors to the south become more vulnerable, Canada is flirting with danger, too?In a disappointing move, Canada has refused to sell the United States some of its surplus tritium, a vital component of nuclear warheads.

A radioactive gas, tritium is a by-product of nuclear reactors cooled by heavy water. It has the ability to vastly increase the explosive power of nuclear weapons, allowing them to be made smaller, lighter and more easily deliverable.

The United States' only sources of tritium, the Department of Energy's three reactors at Savannah River, S.C., are closed because of safety problems, and it is uncertain when they can resume production.

Tritium also decays at about 6 percent a year, forcing the DOE to replace the gas in warheads approximately every five years. Without fresh supplies, DOE will have to scrap or cannibalize warheads in another year or so, diminishing this country's nuclear deterrent.

Meanwhile, Scripps Howard News Service reports that Ontario Hydro, which supplies electricity to Canada's most populous province, is building a tritium extracting facility for its 16 heavy water reactors at Darlington. Canada has almost no domestic need for the isotope and, starting in 1990, could earn around $112 million a year by exporting tritium to the DOE.

Sensibly, Ontario Hydro drew up export plans. Unfortunately, someone leaked the memo to anti-nuclear groups, which leaked it to the press. Fearful that he would be regarded as insufficiently anti-Pentagon, Ontario Premier David Peterson quickly proclaimed that there would be no exports of tritium if they would help the American military.

From that, one would think the U.S. armed forces were enemies of Canada and not, as they are, its allies in NATO and Norad. The fact is that Canada is protected by an umbrella of American conventional and nuclear forces. This umbrella permits Canada to spend less than it otherwise would have to on defense and to invest more in its economy and social programs.

Under the circumstances, Canada's embargo on tritium is clearly unjustified and shortsighted.