25 years later? The thing you have to remember about radioactive substances is that the gas tank is finite. The stuff that stays active a long time isn’t very radioactive to begin with. The stuff that’s fiercely radioactive doesn’t stay that way very long.
The half life of cesium-137 is 30 years. That means after 30 years half of it will have decayed and half of it will still be there. After 60 years one quarter of it will still be there, etc.
Unfortunately nature and the physical world is much more complex then humans will ever grasp. Internal contamination in humans from Chernobyl actually increased over time 5-10 years after the accident. And we are seeing the same thing happen in Japan. After the initial large radiation spike settled back down, we are now in the period where nuclear particulate contamination increases over time from current levels. You can see that in Fukushima City the past few months.
Also contamination of soil near Chernobyl is much higher then it should be given normal half life rates.