Chernobyl is still incredibly threatening, actually. The concrete coffin that was built to cover it is nearing collapse from time and disrepair. So until they finish that steel dome that they should have started building 10 years ago, it's still a real threat. A collapse would send an absurd amount of radioactive dust into the air and reunite the still highly radioactive core with the air. Also, even if we were legitimately trying to ruin the Earth with radioactivity it would be damn near impossible. 71% of the Earth is covered by oceans. I'm sure if you did the math to find out how much radioactive drainage would be needed to hit the water to be a meaningful, harming percentage, it would be decades worth.