In an article, it is stated that the probability of contamination in severe accidents like that of Chernobyl and Fukushima will be far higher than earlier estimates. Though various conditions lead to the probability, the calculation described in the articles gives us an idea of probabilities. Numbers look absurd, but nothing is wrong in being pessimistic and try to improve safety systems.
http://cleantechnica.com/2012/05/24/likelihood-contamination-severe-nuclear-reactor-accidents-higher-expected/