[en] Development of effective procedures for recovery from the 1979 accident at the Three Mile Island 2 nuclear station requires a detailed and quantitative description of the postaccident configuration of the core. This report describes the techniques, equipment, and procedures used for making precise ultrasonic, sonar-like measurements of the cavity left in the upper core region as a result of the accident and details the primary results of the measurements. The system developed for the measurements uses computer techniques for the command and control of remote mechanical and electronic equipment, and for data acquisition and reduction. The system was designed, fabricated, and tested; procedures developed; and personnel trained in 4-1/2 months. The primary results are detailed topographic maps of the cavity. A variety of visual aids was developed to supplement the maps and aid in interpreting companion videotape surveys. The measurements reveal a cavity of 9.3 m³, approximately 26% of the total core volume. The cavity occupies most of the full diameter of the core to an average depth of about 1.5 m and approaches 2 m in places