WHAT? SWEDEN a secret testing member, or almost-member, of the nuclear club? The thought is astonishing, cutting across 1) Sweden's reputation as a state whose nuclear self-denial is a matter of domestic law and international treaty and 2) Prime Minister Olof Palme's personal reputation as a preachy Mr. Nuclear Clean. But it may be only a thought. Notwithstanding the furor created by a report in a Swedish technical journal, there is no evident basis for thinking that the Swedes have been living a nuclear lie.

In the 1950s, in an atmosphere colored by NATO's deployment of battlefield nuclear weapons, Sweden considered building such weapons too. It has not been in a war since 1814 but, counting as it does on a balance of power kept by others, it has its moments of strategic loneliness. Eventually, however, the Swedes decided that going nuclear would be a misuse of limited defense resources and, far from firming up deterrence, might instead make the country a target for preemption.

According to what is known, Sweden then turned its formidable scientific capability in the nuclear field strictly to matters of defense. In 1972, it seems, it conducted the tests -- it insists they were conventional explosions to measure effects on different materials, including plutonium -- whose disclosure by the Swedish journal produced the recent stories about a nuclear test.

In these sensitive matters, it is best not to take any government's denials too categorically. The Swedish government's further detailed accounting of its past work in nuclear defense research has been promised and should be helpful. It's only fair, though, to keep in mind that Sweden is not the sort of desperate or defiant country commonly suspected of harboring nuclear aspirations.

It is an exposed but well armed and reasonably secure country with a non-antagonistic foreign policy. It is an open society in which nuclear self-denial has long enjoyed informed consensus support. And it is a leading, even evangelical supporter of international efforts to check the spread of nuclear arms and to control the arms already possessed by the nuclear powers. It's also a country facing elections; the Swedish press is asking whether the journal article was an attempt to embarrass Prime Minister Palme and his party.

People are right to be nervous about the spread of nuclear weapons to more countries. Sweden, however, strikes us as just about the last country that would be tempted to cheat.