Two government agencies that oversee the Rocky Flats nuclear weapons plant near Denver signed a draft agreement assigning responsibility to begin cleaning up the contaminated complex "almost immediately." The Department of Energy put off signing the document until next week because of a financial snag. The EPA and Colorado Department of Health signed the document, which would be a blueprint for carrying out cleanup priorities agreed on in principle by Gov. Roy Romer and Energy Secretary James D. Watkins. Cleanup is estimated to cost $500 million to $1 billion and to take more than a decade, officials said.