Nuclear Science question im curious to know.?

I just watched a doentery on Chernoyl, when the explosion happened, all this radioactive stuff filled the air and messed up a good portion of the country. (Ukraine plus Belarus and Russia). So when you see videos of giant nuclear bombs tested around the world, why dosen't it effect the world then? Chernobyl nearly effected almost all of Europe I thought. So even if the bomb is tested somewhere in the ocean, wouldn't it still effect the world somehow?