According to nuclear bomb simulator Outrider.org, if the Tsar Bomba was used on some of the most populated places in the world, it would lead to millions of immediate deaths.
If a Tsar Bomba was used on Buckingham Palace, it would result in 4.6 million deaths, and an additional three million injuries.
The radius of the fireball, which would burn 10,000 times hotter than the sun, would reach 43.75 miles around the explosion site, destroying almost all of the capital.
If the Tsar Bomba was dropped on the US capital city of Washington, directly on the White House, it would result in 1.8 million immediate fatalities.
source: express.co.uk