Why Russia is claiming Ukraine wants to use a 'dirty bomb' on its own soil

The Kremlin continues to insist that, as part of a campaign to fuel anti-Russia sentiment in the world, Ukraine will detonate a "dirty bomb" on its own territory, and blame Moscow for the subsequent destruction.

Russia on Tuesday took its case to a closed-door meeting of the UN Security Council, after those same allegations were made over the weekend by Russian Defence Minister Sergei Shoigu, who took them to his counterparts in the West.

Ukrainian President Volodymyr Zelenskyy has said Russia's accusation was a sign that Moscow was planning such an attack and preparing to shift the blame to Ukraine.

So what's behind Russia's allegations? Are they credible? Or part of a strategy? CBC explains:

What is a dirty bomb?

A dirty bomb is considered a relatively primitive weapon that mixes conventional explosives, such as dynamite, with radioactive powder or pellets. When the explosives are set off, the blast carries the radioactive material over the surrounding area, according to the U.S. Centers for Disease Control and Prevention.

They are not, however, nuclear weapons and do not create an atomic blast. They cause contamination.

How dangerous are they?

The most immediate danger from a dirty bomb is the explosion, though this is not its true purpose. Nor is the short-term risk from exposure to the scattered radioactive material — experts say that danger would probably be limited, since most people in an affected area would be able to escape before experiencing lethal doses of radiation.

Rather, such bombs would be most effective by — rather than flattening — forcing the evacuation of large urban areas, even entire cities.

A bomb using radioactive caesium from a medical device might require the evacuation of several city blocks, making it unsafe for decades, said physicist Henry Kelly, during testimony to the U.S. Senate during the Obama administration.

Enough radioactive cobalt could, if blasted apart in New York City, contaminate and potentially make the island of Manhattan uninhabitable, he said.

The economic damage could be massive.

What's been the reaction to Russia's claims?

So far, Western countries have dismissed Russia's allegations. The U.S., U.K. and France in a joint statement on Sunday rejected them as "transparently false."

NATO Secretary-General Jens Stoltenberg also said the alliance rejects such claims, adding that Russia must not use them "as a pretext for escalation."

Are the claims credible?

Many analysts have dismissed the possibility.

"I couldn't see Ukraine ever use a dirty bomb," said John Hardie, deputy director of the Russia Program at the Foundation for Defense of Democracies in Washington.

Ukraine would certainly not use a dirty bomb on Russian territory, he said, because the blowback from the West and the rest of the world would be enormous and risk its "lifeblood of Western military support."

Hardie says he also can't see Ukraine detonating a bomb on its own territory — risking the lives of its own citizens, as some sort of "false flag" operation to blame Russia and prompt worldwide condemnation.

The Institute for the Study of War (ISW) called Moscow's claims "false and ridiculous," in a recent report while Nigel Gould-Davies, a senior fellow for Russia and Eurasia at the International Institute for Strategic Studies, tweeted that Ukraine has neither "ability nor need" to use a dirty bomb.

No dirty bomb attack has ever been recorded, though two failed attempts to detonate one were reported in the southern Russian province of Chechnya during its separatist conflicts more than two decades ago.

Why is Russia making these claims?

There could be a series of reasons for Moscow to make these allegations. Much of this could be for domestic consumption, aimed mostly at the Russian population to garner support for the war, Hardie says.

Michael Clarke, a professor of war studies at King's College London, told NBC News that this is "classic Russian vranyo — a lie that I know you don't believe, and I don't believe it either."

It's a "clumsy double bluff trying to make the West frightened of pushing Moscow too hard," he said.

Tom Nichols, professor emeritus of national security affairs at the U.S. Naval War College, says the allegations could be a signal that Russia wants to launch its own "false flag" operation, to explode its own dirty bomb in Ukraine then demand Ukraine surrender or face nuclear retaliation.

"Let's hope that this is just the Kremlin trying to engage in scare tactics," Nichols wrote in The Atlantic.

But while there's certainly a heightened risk of nuclear war, because of the war in general, Hardie says the dirty bomb gambit isn't the excuse Russia is relying on to potentially launch a nuclear weapon.

It wouldn't be "sufficient to lay that ground work," he said.

The scenarios in which Putin would decide or consider the use of a nuclear weapon in Ukraine wouldn't depend on one particular disinformation claim or another, he said.

The ISW echoed that sentiment in its report, saying it doubts the Kremlin is preparing an imminent false-flag dirty bomb attack, or that its dirty bomb allegations signal preparations to use nuclear weapons in Ukraine.

Putin's stated red lines for nuclear weapon use have already been crossed several times without any Russian nuclear escalation, it said.

"Russia does not 'need,' under formal Russian nuclear doctrine, a further event to justify nuclear weapons use Ukraine is not apparently on the verge of tripping some new Russian redline, on the other hand, that might cause Putin to use non-strategic nuclear weapons against it at this time," the report said.

Moscow may just be hoping such threats would intimidate Western states into cutting or limiting support for Ukraine, the ISW said.