How bad could Chernobyl get?
Although the original explosion itself was may be already worst-possible, the worst possible scenario of entire disaster would be:
1) Slower firefighting response resulting in fire spread to 3rd reactor too (it was a close call IOTL), diluting respondents resources and slowing later explosion mitigation efforts
2) Gusty winds immediately after disaster, spreading radioactive material to larger area, and preventing effective use of helicopters to quell reactor fire
3) Some man not as competent as OTL B.E. Ssherbina assigned as head of disaster mitigation team
4) Secondary explosion after reactor meltdown (inevitable if (2) and (3) factors are sufficiently grave)
Overall, we can see in worst case the permanent evacuation of Kiev (over 2 million people).