Chernobyl and Hiroshima are not identical.
In Japan there were bombs, in Chernobyl - the release of radioactive substance as a result of an accident and fire. Physics of phenomena is different. After the explosion, the area radiates for a second time due to the given radiation, plus a certain number of rapidly decomposing elements. In Chernobyl, the main infection is strontium-90 (half-loss 27 years).
After the bomb, you can begin to return in a year or two, bury the most hidden objects and build houses of imported material. And the backdrop in Pripyat has fallen at least 15 times, it takes more than a hundred years. Approximately so much area and closed.