It's the secondary effects that would likely cause this. Having read a good deal about the Chernobyl (Rhodes, Arsenals of Folly) and Fukushima disasters, basically it's the loss of coolant in the reactor cores and the evaporation of water from the cooling ponds for the 'spent' (i.e. too hot to handle) reactor fuel that lead to a runaway meltdown event and associated fire.

So, a 500kt blast over a reactor site will destroy all the surrounding infrastructure, evaporate the water from the cooling ponds, and while perhaps the steel reactor containment shell itself might not be completely obliterated/vaporized at once (or it might), the whole zone is going to be unapproachable, and the kind of heroic efforts employed at Chernobyl (notably a graphite core reactor that burned, Fukushima having no flammable graphite) and Fukushima are not going to happen. So, total meltdown of reactor core, hydrogen explosion, venting to atmosphere, some fraction of the core liquifies the rest vaporizes. The spent fuel in the ponds, all the water flashes to steam, they burn / melt. Nobody comes rushing in to manage the situation, it's full-on runaway.

So Chernobyl IIRC lost about 1% of one of its reactor mass to the atmosphere, and that was one of four reactors at the site. This resulted in significant fallout across Europe and a 30-km radius long-term exclusion zone. Now let's say instead oh, at least 50% of each reactor mass + spent fuel goes into the atmosphere, the rest melts into the ground... I don't know how this would scale. A 300-km radius exclusion zone perhaps at minimum? To get accurate estimates you then have to do a bunch of modeling, atmospheric fluid dynamics, etc.

I can't imagine there aren't top-secret studies in some military branch or other on how this would all pan out, but they've never been publicly released to my knowledge.