Can Yuan be used throughout China and Tibet?

We will be traveling from Beiging to Xian to Chendu to Lhasa to Yangtze River Cruise to Shanghai. Can we use the same currency throughout our whole trip? Is Yuan the proper currency? Also we will have several hours layover in Seoul. South Korea in both directions going to China. Will they take American Visa cards at the airport or do we need to get Korean money too?