MOSCOW — In online posts and calls to local officials, Russians on Monday expressed anger that the explosion of a small nuclear reactor at a military test site last week has gone unacknowledged for days by their government.
“Even if it is not as dangerous as it seems, we deserve to know,” said Danil Kotsyubinsky, a resident of St. Petersburg who has been pressing local officials for information.
The accident, which has been cloaked in secrecy, took place on Thursday at the Nenoska naval weapons range on the coast of the White Sea in northern Russia, and it apparently involved a test of a new type of cruise missile propelled by nuclear power, American analysts say.
The explosion killed at least seven people, released radiation that briefly elevated readings in a city 25 miles away and set off a scramble by Western experts to ascertain what happened. The military and a state nuclear energy company announced the deaths but few details of the accident.
So Russians have been mostly left guessing.
Some Moscow television broadcasts were mysteriously interrupted for as long as 53 minutes on the night of the accident. A government broadcast agency later described the disruption as a malfunction of a storm warning system. Screens went blue. A text urged people to stay at home because of a storm with strong winds, but it never arrived.
When a state nuclear energy company first conceded the accident involved nuclear materials — on Saturday, two days after the explosion — the disclosure did not make the evening news on all television channels. Channel One, the main state broadcaster, gave the story just 36 seconds.
A flurry of murky, misleading reports surfaced, which for some Russians recalled the lethal delays in acknowledging the Chernobyl accident three decades ago, although the radiation release last week was far smaller. Only on Sunday did Russian scientists explain that a small nuclear reactor had malfunctioned.
Tass, the state news agency, initially carried a report saying radiation levels were normal near the accident, which the scientists disputed on Sunday.
A post by the authorities in the city of Severodvinsk, the closest population center to the testing range, reported that radiation levels rose briefly. But the message was subsequently taken offline.
“I’m not a critic of the government, but in this situation its behavior is ugly,’’ Dmitry Zhukov posted on a message board for residents of Severodvinsk on Vkontakte, a social networking site similar to Facebook. “We got a mini-Fukushima right beside us and they pretended nothing scary happened.”
Mr. Zhukov also fumed that residents of Moscow, though far away, were apparently urged to stay indoors under the ruse of a storm warning while no such precautions were taken for those near the explosion.
On Saturday, the Russian nuclear energy company Rosatom said in a statement released at 1 a.m. that the accident involved nuclear materials, using language even experts saw as confusing. Its scientists were studying an “isotope power source for a liquid engine unit,” the company said.
Last year, President Vladimir V. Putin spoke more plainly in a speech to Parliament about Russia’s development of a nuclear-propelled cruise missile. Mr. Putin said Russian scientists had developed a “small-scale, heavy-duty nuclear energy unit that can be installed in a missile.”
When Mr. Putin spoke, “it was a rocket with a nuclear engine,” Yulia Latinina, a columnist for Novaya Gazeta, an opposition newspaper, wrote. “And when it blows up it is a liquid unit with isotopic sources.”
President Trump posted a tweet about the accident Monday evening, writing, “The United States is learning much from the failed missile explosion in Russia,” adding that the American military has “similar, though more advanced, technology.”
The explosion, Mr. Trump wrote, “has people worried about the air around the facility, and far beyond. Not good!”
Boris L. Vishnevsky, a member of the St. Petersburg City Council, said dozens of people have called, asking for clarification about radiation risks.
“People need reliable information,” Mr. Vishnevsky said. “And if the authorities think there is no danger, and nothing needs to be done, let them announce this formally so people don’t worry.”
Five scientists killed in the explosion were buried on Monday. Alexei Likhachev, the head of Rosatom, praised them as the “pride of the atomic sector” at a memorial.
For Mr. Kotsyubinsky, the St. Petersburg resident who said he was worried about radiation, the statement released in the wee hours, the contradictory accounts and the silence of senior officials have conjured dark memories of Chernobyl.
As a 21-year-old college student, he said, he was ordered to march in a May Day parade in 1986. The Soviet authorities, wary of revealing that a nuclear accident had occurred just days earlier, did not cancel parades around the country in which millions of children participated.
In Ukraine, where Chernobyl is, wind bearing radioactive dust swirled around the clueless children. In northern Russia, a cancerous rain fell. With these memories still fresh, Russia adopted a Constitution in 1993 that prohibits classifying information about public health risks.
“They made us go to these demonstrations,” Mr. Kotsyubinsky said. “We didn’t know. We wound up under the rain. It wasn’t lethal, but it was radioactive. That shouldn’t happen in a society where the government answers to the people.”