If the containmaint fails, it means widespread contamination. This is basically the reason, Chernobyl was so bad: the graphite in the nuclear core caught fire a

If the containmaint fails, it means widespread contamination. This is basically the reason, Chernobyl was so bad: the graphite in the nuclear core caught fire and the fire launched a lot of radioactive material high into the air. So beyond the impact on the immediate region around the reactor, there were nuclear clouds moving across Europe. In consequence, a lot of forests in Bavaria are still contaminated. Even today, one needs to be a bit carefull eating mushrooms and especially wild boar (who ate a lot of mushrooms...) from the local forests. Also, that was the reason everyone was so thrilled by the news that Russian troups camped and lit fires in the contaminated forests around the reactor, both endangering themselves and spreading more radioactivity. Of course there is much less material involved than with the original accident, but still one wants to avoid any of this.