The pebble-bed prototype thorium high-temperature reactor represents the second step of high-temperature gas-cooled reactor development in the Federal Republic of Germany. Nuclear commissioning of the plant began in August 1983 with the loading of the spherical elements, and first criti-cality was achieved in September 1983 with the loading of 198 180 spherical elements. A very good agreement of 0.004Δk was achieved between measured and calculated values. After full loading of the core with 674200 elements in October 1983, core physics tests were performed in air and nitrogen in August 1984 to verify the design calculations. In these tests the temperature coefficient, the control rod worths, and the reactivity of the reactor core were measured. The measured values of the temperature coefficient were within 10% of the expected values. The agreement between measured and expected control rod worths (5%) is excellent. The reactivity of the cold core with all rods withdrawn was determined to be 0.112 ± 0.005Δp. Taking into account values of the packing density of the spherical elements, which were higher than expected, the calculated value of 0.11Δp was in very good agreement.