Algodoo

Every single aspect of this model matches up to the real thing.
The size, dimensions, mass, fireball size, thermal pulse size, everything.
It's not the prettiest as it doesn't have any cool effects or even a mushroom cloud, but it still does its purpose.
The fireball had a diameter of 6700m. The thermal pulse has a diameter of 51400m.
Activation key is set to 1. You can edit this by selecting the black tip of the warhead, going to "Controller" and editing the "Destroy Key" input.
MAKE SURE the scene is paused before deleting or rescaling it, it's tied to "onDie" and will auto-activate upon deletion if the scene is playing.
Credit me if you want to use this in your scenes.
Also I underestimated the sheer size of the Tsar Bomba. This basically isn't viable in any scene since it vaporizes everything.