 Complete the following phrase:  The area has also been severely damaged by the plutonium-producing facility Mayak, opened in Chelyabinsk-40 later called Chelyabinsk-65, Ozyorsk, in the Southern Ural, after World War II. Its plants went into operation in 1948 and, for the first ten years, dumped unfiltered radioactive waste into the river Techa and Lake Karachay. In 1990, efforts were underway to contain the radiation in one of the lakes, which was estimated at the time to expose visitors to 500 millirem per day. As of 2006, 500 mrem in the natural environment was the upper limit of exposure considered safe for a member of the general public in an entire year though workplace exposure over a year could exceed that by a factor of 10. Over of land were contaminated in 1957 from a storage tank explosion, only one of several serious accidents that further polluted the region. The 