A safety system automatically shut down a nuclear reactor in eastern Ukraine on Monday, and there was no increase in radiation levels, officials said. The cause of the shutdown of the reactor No. 6 at the Zaporizhsky nuclear power plant was not immediately clear, the state-run nuclear operator Energoatom said in a statement. Four other reactors at the plant are in operation now, and another is undergoing planned repairs, it said. Ukraine was the site of the world's worst nuclear accident, when a reactor at the Chernobyl nuclear plant exploded and caught fire in April 1986, spewing radiation over the western part of the Soviet Union and northern Europe. Chernobyl was closed down in 2000. Ukraine has 15 working nuclear reactors; officials said 11 are in operation now.