The universal critical assembly UKS-1M was built at the IPPE for experimental studies of the basic neutron transport regularities in coupled pulsed fast and thermal reactors. The assembly simulates a nuclear pumped laser system. The mock-up consists of a small-sized fast neutron core and an annular subcritical booster with simulators of laser-active elements. The assembly is equipped by a pulse neutron generator to simulate pulse operation of the reactor and a stationary Cf fission source. Central fast critical core is surrounded by subcritical block with thermal neutron spectrum. Effective multiplication factor of the booster is less than 1.     Hexagonal thermal block consist of 2-mm-thick aluminium tubes with diameter 49 mm and height 150 cm, placed in triangular lattice with step 51 mm. Fissile material is comprised inside these tubes, 26 gramms of 235U per one tube (the number of tubes with fissile fuel is 858). In order to moderate the fast neutrons, the 20-mm polythelene rod is inserted into each tube. Thermal subcritical multiplicative zone is surrounded by neutron reflector: two rows of 50-mm-diameter polythelene rods (222 rods).     A small-size uranium-molibdenum fast reactor is placed in the center of a thermal block (internal zone). Its core is constructed from 37 fuel pins surrounded by 6 rows of molibdenum reflector rods.     Experimental results, obtained on the UKS-1M assembly, were used for testing of the mathematical models and computer codes for simulation of neutron kinetics of coupled reactors. It was shown that experimental and computed results are in a good agreement.
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