Western officials quickly rejected Russia’s claim over the weekend that Ukraine was planning to use a so-called dirty bomb in its own territory. The United States and its allies have issued a series of statements accusing leaders in Moscow of making "transparently false allegations” to create a pretext for escalating the war.

But the intense exchanges in recent days have renewed attention on the concept of the dirty bomb itself.

It is a type of weapon that was thought of and tested more than three-quarters of a century ago, in the early years of the Atomic Age, but never fielded by a military force.