KYIV, Ukraine — Six and a half feet down a ladder inside a small shed at the back of Oleksandr Kadet’s home is an underground room with a cement hatch that he hopes he never has to use.
For the past two weeks, Mr. Kadet, 32, said that he and his wife, who live outside the Ukrainian capital, Kyiv, had been preparing for the possibility of a nuclear attack by stocking the room — an old well that they converted into a bunker — with bottled water, canned food, radios and power banks.
“We are more anxious now, especially after yesterday’s attacks,” Mr. Kadet said on Tuesday, a day after a series of Russian missile attacks across Ukraine. “But we do think that in case of a nuclear explosion, we will be able to survive if we stay in the shelter for some time.”
The fears of escalation rose on Saturday after an attack on the 12-mile Kerch Strait Bridge connecting Russia to the Crimean Peninsula, which Moscow annexed in 2014. Initially, Ukrainians celebrated, but that quickly gave way to worry that such a brazen assault on a symbol of President Vladimir V. Putin’s rule could prompt a severe retaliation.
Even before these recent events, though, concerns about the potential for a nuclear disaster had increasingly been making their way into Ukraine’s national psyche. The fear is that Russia could either use tactical nuclear arms or launch a conventional attack on one of Ukraine’s nuclear power plants.
U.S. officials have said they think the chances of Russia’s using nuclear weapons are low, and senior American officials say they have seen no evidence that Mr. Putin is moving any of his nuclear assets.
On Sunday, Mr. Putin called the assault on the bridge a “terrorist attack aimed at destroying the critically important civilian infrastructure of the Russian Federation.”
But his spokesman, Dmitri S. Peskov, appeared to tamp down fears of a nuclear reprisal, saying that the attack on the bridge did not fall within the category under Russia’s defense doctrine that allowed for such a response.
Last month, Mr. Putin raised fears that he could resort to nuclear weapons when he warned that he would “use all the means at our disposal to protect Russia and our people,” if Russian-controlled territory was threatened.
“This is not a bluff,” he said.
Days later, Russia illegally annexed four Ukrainian territories.
Mr. Kadet, who noted that he had begun preparing two weeks ago, said it felt better to have an action plan.
“It’s psychologically easier because you know you are at least somehow prepared for it,” he said. “It’s not a guarantee it will save you, but at least you’re ready.”
Residents of Kyiv said that they had felt wary even before the most recent missile strikes there on Monday.
Immediately after the bridge attack, many Ukrainians had shared their glee on social media. They toasted triumphantly in the capital’s bars over the weekend, and posed for selfies in front of posters of the burning bridge.
But the worry soon set in.
“I feel this real fear about how the Russians will answer this,” said Krystina Gevorkova, 30, who was shopping with her friend in Kyiv on Sunday. “Earlier it had felt safer here,” she added. “Now, I have this feeling like something is going to happen.”
Kyiv has for months been spared the worst of the Russian onslaught while Moscow focused its attention instead on southeastern Ukraine. But on Monday, a Russian missile struck just blocks away from where Ms. Gevorkova had spoken.
She said that she had been reading up on how to stay safe during a nuclear war, but that she was skeptical that it would help.
“We can’t really do anything,” she said.
The war has felt far from Kyiv in recent months, as life’s rhythms return to a semblance of normalcy after Russian forces were ousted from parts of northeastern Ukraine. Nevertheless, the city has also been slowly preparing for a potential nuclear attack.
The Kyiv City Council said on Friday that potassium iodide pills would be distributed to residents in case of a nuclear incident “based on medical recommendations,” adding that the pills were also available in city pharmacies.
Potassium iodide is used to saturate a person’s thyroid with iodine so that radioactive iodine inhaled or ingested after exposure will not be retained by the gland.
Alina Bozhedomova, 23, a pharmacist in Kyiv, said that customers were coming in daily looking for the pills, but added, “I haven’t seen people panicking about it.”
Some elementary schools have advised parents to prepare emergency packs for their children to keep with them at school.
Nadiia Stelmakh, 50, who works in a market selling home goods, said that one mother had come to her with a list from the school that included latex gloves, a poncho, boot covers, tissues, wet wipes and a flashlight.
“People are really concerned right now,” she said. Her husband, Volodymyr Stelmakh, who has another stall nearby, agreed.
“I have an emergency bag packed,” he said, “but I think if the nuclear threat is imminent, you will have no time to run away.
After worries grew about the security of the Zaporizhzhia Nuclear Power Plant in the country’s southeast in recent weeks, Ukraine’s Ministry of Health issued guidance about how to respond to a nuclear incident.
The risk of nuclear fallout can feel very real in Ukraine, a country that still bears the scars of the Chernobyl accident in 1986, one of the worst nuclear disasters in history. Chernobyl is only about 60 miles north of Kyiv.
And some who experienced the life-threatening fallout firsthand say that they, possibly more than anyone, understand the full risk of nuclear exposure. Oleksandr, 55, who asked that his last name not be used, said that he and his family had fled Chernobyl for Kyiv immediately after the meltdown, when he was just 18.
His family closely followed guidance to move south, as winds were pushing radioactive materials north, and he said that was the only reason they escaped unscathed.
“Now, people here are really not ready. People don’t know what to do,” he said. “There is not enough information.”
He owns a market stand that sells household necessities and said that more people had come in during the past two weeks preparing for a nuclear disaster, buying flashlights, batteries, knives, radios and small camp stoves.
While some were preparing for the worst, others remained optimistic that Russia would never carry out such an extreme attack, which would draw international outrage.
Dmytro Yastrub, 31, said he felt more concerned about Mr. Putin using conventional weapons to target Kyiv.
“I presume something will happen” after the bridge attack, he said, standing outside a bar in the Kyiv city center on Sunday evening with a group of friends. But, he added, the risk of a nuclear attack was not weighing heavy on his mind.
Svetlana Zozulia, 47, and her husband, Vladyslav Zozulia, 37, were walking in central Kyiv with their daughter, Anastasiia, 11, on Sunday night. Ms. Zozulia said she tried to remain optimistic and did not believe that Mr. Putin would launch a nuclear attack on Ukraine.
But she did buy potassium iodide tablets just in case, she said.
“I think our success disturbs him,” Ms. Zozulia said. “But there is also a threat for him if he chooses a nuclear attack.”
Oleksandra Mykolyshyn contributed reporting