CHORNOBYL-- The black concrete sarcophagus that covers the wreckage of Chornobyl9s unit four reactor was never meant to be an eternal resting place for the estimated 190 tons of nuclear fuel still in its bowels. Although designed to last a century, the structure, thrown up in 6 months after the April 1986 explosion, is eroding faster than expected. Now, after years of false starts and political and scientific wrangling, the Ukrainian government decided earlier this month to erect an arched structure, as big as a baseball stadium, to cover the sarcophagus.