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The Gas Turbine Modular Helium Reactor (GT-MHR) is a nuclear fission power reactor design under development by a group of Russian enterprises (OKBM Afrikantov, Kurchatov Institute, VNIINM and others), an American group headed by General Atomics, French Framatome and Japanese Fuji Electric. It is a helium cooled, graphite moderated reactor and uses TRISO fuel compacts in a prismatic core design.
The core consists of a graphite cylinder with a radius of 4 m and a height of 10 m which includes 1 m axial reflectors at top and bottom. The cylinder allocates three or four concentric rings, each of 36 hexagonal blocks with an interstitial gap of 0.2 cm. Each hexagonal block contains 108 helium coolant channels and 216 fuel pins. Each fuel pin contains a random lattice of TRISO particles dispersed into a graphite matrix. The reactor exhibits a thermal spectrum with a peak located at about 0.2 eV. The QUADRISO fuel  concept conceived at Argonne National Laboratory has been used to better manage the excess of reactivity. The reactor and containment structure are located below grade and in contact with the ground, which serves as a passive safety measure to conduct heat away from the reactor in the event of a coolant failure.
The Gas Turbine Modular Helium Reactor utilizes the Brayton cycle turbine arrangement, which gives it an efficiency of up to 48% - higher than any other reactor, as of 1995. Commercial light water reactors (LWRs) generally use the Rankine cycle, which is what coal-fired power plants use. Commercial LWRs average 32% efficiency, again as of 1995.
In 2010 General Atomics presented a new version of the GT-MHR, the Energy Multiplier Module (EM2). The EM2 uses fast neutrons and is a gas-cooled fast reactor, enabling it to reduce nuclear waste considerably by transmutation.