Energy Secretary Bill Richardson yesterday ordered an immediate investigation into reports that thousands of unsuspecting employees at a government uranium plant in Kentucky were exposed on the job to cancer-causing plutonium and other radioactive materials. His announcement followed the publication yesterday of a Washington Post article on the issue. The Natural Resources Defense Council and workers from the plant have filed suit against the government, alleging that government contractors concealed evidence of the exposure for decades and deliberately dumped plutonium and other radioactive materials into the environment.