Localized problem? Maybe now, long after the fact, but at the time...
"Four hundred times more radioactive material was released than had been by the atomic bombing of Hiroshima. The fallout was detected over all of Europe except for the Iberian Peninsula"
-- http://en.wikipedia.org/wiki/Chernobyl_disaster
And as for the worst disaster ever...
"The suck-and-salvage technique was developed in desperation across the Arabian Gulf following a spill of mammoth proportions -- 700 million gallons -- that has until now gone unreported, as Saudi Arabia is a closed society, and its oil company, Saudi Aramco, remains owned by the House of Saud. But in 1993 and into '94, with four leaking tankers and two gushing wells, the royal family had an environmental disaster nearly sixty-five times the size of Exxon Valdez on its hands..."
-- http://www.esquire.com/blogs/politics/gulf-oil-spill-supertankers-051310
Which doesn't mean this isn't the worst disaster the Western Hemisphere has seen, which is very likely will be by the time it's done.