I rode through in the reverse direction in June. You'd have to be a pretty brave man to venture into the Polygon.
To the best of my knowledge there are no fences, nor markings to indicate the hot spots. As a result it's pretty much up to you to carry your own geiger counter and monitor it constantly.
I stuck to the main road west to Pavlodar, then south to Karaganda. Roads are pretty poor, espec. between Pavlodar and Karaganda.
Enjoy, Garry from Oz.
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