Antinuclear activists from around the world will gather here this month to launch a highway armada for peace, a fleet of white buses that will carry them and their message, ideally accompanied by extensive media coverage, to the capitals of Europe.

Later, in the fall, Stockholm will be the site of a major nuclear nonproliferation conference.

These are the kinds of events neutralist Sweden likes to host, those it feels befit its long and carefully nurtured role as the world's self-appointed conscience in the fight to outlaw nuclear weapons.

So it was with some dismay that the government of Prime Minister Olof Palme found itself in the headlines late last month, not as an antibomb crusader, but as a potential bomb builder that as recently as 1972 had conducted experimental explosions involving plutonium.

According to reports that first surfaced in a local technical journal, the explosions were the culmination of a secret military program throughout the 1950s and 1960s that, among other things, gave Sweden the capability of building its own bomb by 1961.

To much of the world, Swedish officials predicted, the reports would be perversely welcome news.

"Lots of people hate the Swedes," said Defense Ministry spokesman Bo Eriksson. "They think we're a pain in the ass, always bragging at nonproliferation conferences and arms control negotiations."

Equally distressing to Palme's Social Democratic Labor government, however, is the likelihood expressed by many officials that revelations of Sweden's former dabbling in nuclear weapons research would trouble young Swedes who know only of their country's "do-gooder" image.

"To the public today," acknowledged Pierre Schoeli, undersecretary of foreign affairs and a top Palme aide, "it might come as a surprise that we had research at all."

No one here has denied that the research, as described by the journal Ny Teknik, took place. No one, including Ny Teknik, has charged that Sweden actually built a bomb before the program ended in 1972.

But government explanations, largely limited to saying that there was no bomb and that Sweden did nothing untoward, appear to have raised more questions than they have answered. In response, Palme has ordered a government investigation of what Ny Teknik journalist Christer Larsson says is one of the principal points of his report -- that the program appears to have violated at least the spirit of a series of parliamentary bans, beginning in 1957, against research aimed at construction of a nuclear weapon.

A final ban, adopted in 1968, formally eschewed a nuclear component for Swedish defense and for all weapons-related research. Two years later Sweden ratified the nuclear nonproliferation treaty.

Until that point, however, under a 1960 parliament-approved policy, the nuclear option theoretically was still open. Parliament allowed "protective" research into anything that would contribute to Sweden's efforts to defend itself against someone else's bombs.

It was under this umbrella, with parliamentary knowledge and approval, that the research program was carried on, Palme said in an interview. Sophisticated knowledge of how nuclear weapons worked, he said, among other things, helped Sweden make its case in disarmament talks in Geneva during the late 1960s.

But Palme acknowledged that a "difficult borderline" exists between studying the bomb for defensive purposes and learning how to make one. "Maybe, sometime," he said, "the military has done research that went out of bounds. So we're going to go through" with the investigation.

Palme and other officials attributed the continued operation of the program for four years beyond the final 1968 ban to a wind-down phase prolonged by overzealous physicists and an inattentive government.

Although closely familiar with the overall program, and himself the author of the 1960 government policy, Palme said he did not know about the 1972 explosions. But he said he was "not surprised. Because the military wanted the nuclear research program very much."

And the research scientists themselves, he said, "are very difficult people. They don't want to be told. They always want one step further."

The "difficult borderline" questions arise in part from the explosions. Officials said they were part of a series of experiments to test the way conventional explosives -- in a configuration designed to trigger a "critical mass" leading to a nuclear chain reaction -- affect plutonium.

The amount of plutonium used, 5 to 10 grams for each of the 10 explosions, was only a small fraction of the several pounds needed to begin a chain reaction. Officials involved in the program described the experiments as "small bangs in a barrel."

At the same time, formerly classified documents obtained by Larsson indicate that at least two pieces of hardware were built in Sweden for use in a nuclear device.

The first was a neutron pulse generator, in effect the final trigger for a nuclear device. The second was a nuclear implosion unit, the cone of special explosive surrounding the plutonium core that when activated compresses it to a critical mass.

Questions about these and other matters remain. Although Defense Minister Anders Thunborg said in a statement that the Swedish plutonium inventory "has never exceeded 110 grams," the figure appears to refer only to plutonium imported in the early 1960s from France and Britain.

According to Ny Teknik, and other previous news reports here, the final 1968 ban found Sweden with as much as 20 kilograms of plutonium that had been produced in a local experimental reactor. The plutonium reportedly was exported through Belgium to an unnamed destination.

The idea of a thin "borderline" between protective research and bomb-building is not a new one here. In an article published in Norway in 1972, Swedish scientist Jan Prawitz, a participant in the research program, wrote:

"The research effort was limited to a study of possible measures to protect the country against nuclear attack, including a general review of atomic bomb design necessary to make an assessment of possible deployment of nuclear weapons against Sweden.

"This latter program," Prawitz wrote, "also provided spinoff information of importance for the possible manufacture of a Swedish bomb."

In overall terms, Palme said, although the investigation will be undertaken, the whole question of how far the research went now is academic because no bomb was constructed.

Despite Palme's personal involvement in the program and the fact that this is an election year in Sweden, the new reports have had little political resonance here. The once-strong antinuclear Center Party has fallen to 10 percent in the polls.

Parliament member Carl Bildt, the principal security spokesman for the largest opposition group, the Moderate Coalition Party, says he wishes the research would have continued.

The 1972 explosions, he said, were "primarily to see how small bombs could be made. It was important to study the effects of nuclear weapons, and how plutonium reacts . . . . it was part of an ongoing research program. It's not going on any longer, which is unfortunate, I'd say. We need to know."

Far from being conducted behind parliament's back, Bildt said, the "main outlines" of the research reported by Ny Teknik "were fairly well known among the informed, while perhaps not among the normal, ordinary Swedish public" at the time.

What was known here as the "nuclear option" was a subject of intense political debate during the decades in question. "All the bourgeois parties," the bulk of opposition to what were decades of uninterrupted Social Democratic Labor rule until 1976, "and half of our own party were for nuclear weapons," Palme said. Among the Social Democrats, "the women were against it. They weren't pacifists, they were for a strong defense. Just not nuclear."

While historically nonaligned, Sweden long has based its neutrality on nonpartisan support for a strong defense and domestic arms industry. With the little that was known about nuclear weapons during the 1950s and 1960s, Palme said, "the climate was that they were not something terrible. They were terrible, but not any worse than other weapons. So, we should develop them. The military was solidly behind it."

Although he now is known worldwide as a proponent of nuclear nonproliferation and disarmament, Palme said he long considered nuclear weapons a viable option for Swedish defense. He changed his mind, he said, "not for moral reasons, but . . . because I thought that it would not enhance our security" and would make Sweden a target of other nuclear powers.

"I became more moral as the years went on," he said.

Those who now profess to be shocked by Sweden's nuclear past, Palme and others say, are simply too young, or too forgetful, to have known that the nuclear option was once a serious one.

One of those too young to remember much about it, they say, is 36-year-old Ny Teknik reporter Larsson. Based on an offhand comment by a scientist he was questioning on another subject, Larsson spent four years investigating the research program. His 26-page report in the tabloid-sized journal is buttressed by more than 100 interviews and approximately 50 documents declassified under Sweden's equivalent of the freedom of information act.

While the mainline Swedish press has had a field day with his report, with new front-page revelations almost daily, and it has received wide coverage abroad, Larsson notes that the subject "was not sensationalized by us."

Despite what he sees as a wish by government and opposition alike to dismiss the issue as uncontroversial, Larsson believes that Swedish history deserves a more correct and complete accounting. "In order to protect ourselves against bombs, we needed to know everything about them, maybe even blow one up?" he asks rhetorically. "That's 'protective' research?"

During the next several weeks, Larsson plans to publish more revelations gleaned from his pile of documents and interviews. Among them, he said, will be accounts of how the United States and the Soviet Union competed to gain control of Sweden's extensive natural uranium deposits.