TOPAZ nuclear reactor  The TOPAZ nuclear reactor is a lightweight nuclear reactor developed for long term space use by the Soviet Union. Cooled by liquid metal, it uses a high-temperature moderator containing hydrogen and highly enriched fuel and produces electricity using a thermionic converter. In initial discussions, it was unclear that TOPAZ and the somewhat similar YENISEI reactors were different systems, and when the existence of the two Russian thermionic reactors became generally known, US personnel began referring to TOPAZ as TOPAZ-I and YENISEI as TOPAZ-II. The first thermionic converter reactors were discussed by scientists at the Los Alamos Scientific Laboratory (LASL) in 1957. Following the visit of Soviet scientists to LASL in 1958, they carried out tests on TI systems in 1961, initially developing the single cell ENISY reactor (also known as TOPAZ-II). Work was carried out by the Kurchatov Institute of Atomic Energy and the Central Bureau for Machine Building to develop the multi-cell TOPAZ (also known as TOPAZ-I), a Russian acronym for "Thermionic Experiment with Conversion in Active Zone". It was first ground tested in 1971, when its existence was acknowledged. It was under the auspices of Krasnaya Zvezda. The first TOPAZ reactor operated for 1,300 hours and then was shut down for detailed examination. It was capable of delivering 5 kW of power for 3–5 years from of fuel. Reactor mass was ~ . TOPAZ was first flown in 1987 on the experimental Plazma-A satellites Kosmos 1818 and Kosmos 1867, which were intended to test both the TOPAZ reactor and the Plasma-2 SPT electric engine. Both reactors were damaged in the 1990s, causing a leak of radioactive coolant. A proposed follow-up Plasma-2 was to have been equipped with an improved reactor. One reactor operated for 6 months, the other for a year. The program was canceled by Mikhail Gorbachev in 1988. In the TOPAZ-II or ENISY reactor each fuel pin (96% enriched UO2) is sheathed in an emitter which is in turn surrounded by a collector, and these form the 37 fuel elements which penetrate the cylindrical zirconium hydride (ZrH) moderator. This in turn is surrounded by a beryllium neutron reflector with 12 rotating control drums. Liquid metal coolant (NaK) surrounds each fuel element.