[en] Development of effective procedures for recovery from the 1979 accident at the Three Mile Island 2 (TMI-2) nuclear station requires a detailed knowledge of the core configuration following the accident. Remote video surveys of limited scope in the summer of 1982 revealed a cavity in the upper core region that seemed to be about 1.5 m deep, extending out to perhaps two-thirds of the core radius. A clearer, more detailed and quantitative description of the postaccident core configuration was needed to plan defueling procedures. This work describes the techniques used for precise ultrasonic, sonar-like measurements of the core void, and details the primary results