"Ukraine doesn't have nuclear weapons, so the risk of nuclear war in this scenario is if, somehow, the conflict escalated to pull in NATO countries or the US," she added.
"That raises the risk of nuclear confrontation because some of the NATO countries have nuclear weapons."
The US, for instance, has about 5,500 nuclear weapons, while Russia has about 6,000, according to the Federation of American Scientists.
When a nuclear bomb strikes, it sets off a flash of light, a giant orange fireball, and building-toppling shockwaves.
The fallout of a nuclear bomb also depends on how a country chooses to detonate it.