One complaint here in Denmark is that we can't really set aside any meaningful areas to unmanaged nature, because we don't have the space. Both Chernobyl and Fukushima indicates that we might not need as much spaces as we think, but it does need to be one large chunk of land. Sure a Chernobyl size area might be a bit much, but half the size would still go a long way.
1. having 1000 sq km of total reserves but highly fragmented over the territory is better than nothing but almost certainly worse than having only 500 at a single location, because in the fragmented version you'll basically have lots of copies of "edge" ecological niches, but will be missing the "deeper" niches entirely
2. areas being "undeveloped" is not sufficient, and possibly misleading, because it's really the human activity which is disruptive to wildlife, if you decide to have wildlife refuge you need complete removal of humans for it to truly be effective
It shows quite flagrantly how much human activity damages wildlife, that radioactivity-infested nuclear exclusion zones become biodiversity hotspots over just decades.