I'd say much worse than that. The total fuel release from Chernobyl was only 1.5 - 3.5%, but radiation was detected hundreds of miles away - in the Scottish Highlands for example. The west learned about the accident from Geiger counters in Sweden.

The explosive force of the steam that would have erupted underneath the already exposed reactor is, without a doubt highly debatable. But suppose it only ejected 30% of total fuel - about 20x what happened in Chernobyl. Chernobyl's exclusion zone is - today - about 1000 square miles. Would it have to be 20x that? I don't know. Would very much depend on the weather probably.

The wikipedia article on the disaster is a pretty good introduction to the topic.

https://en.wikipedia.org/wiki/Chernobyl_disaster#Spread_of_r...