The energy firm Anadarko has agreed to pay more than $5 billion in what the Justice Department hailed as the largest payment ever for cleaning up environmental contamination. The case involves thousands of sites across the country polluted by nuclear fuel and other toxic material and more than 8,000 people who say they were sickened by chemical exposure. Anadarko had tried to avoid liability for pollution caused by Kerr-McGee, an energy company it acquired in 2006. The payments include $1 billion to the Navajo Nation for damages from Kerr-McGee’s uranium mining operations.