Expect a full day of interviews, including a presentation. As an academic focused institution, they'll be very interested in your school, GPA, and professors. They expect you to become "world class" at the national laboratory (PNNL), so tell them about your scientific interests and how you'll make them famous. The presentation should include not just something you did, but how it relates to the DOE Office of Science mission specific to the directorate you're interviewing into.