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: 

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.

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.

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."

Many analysts have dismissed the possibility.