[en] This report is a summary of the lessons learned by the US Department of Energy during its decade-long participation in the research and accident cleanup project at Three Mile Island Nuclear Power Station Unit 2 near Harrisburg, Pennsylvania. It is based on a review of a wide range of project documents and interviews with personnel from the many organizations involved. The lessons are organized into major subjects with a brief background section to orient the reader to that subject. The subjects are divided into sub-topics, each with a brief discussion and a series of lessons learned. The lessons are very brief and each is preceded with a keyword phrase to highlight its specific topic. References are given so that the details of the subject and the lesson can be further investigated. 99 refs., 24 figs