[en] Within the framework of cooperation with ANL under the RERTR program testing of low enrichment U-Mo pin type mini fuel elements manufactured at the Bochvar institute was started in the MIR reactor in August 2003. The cross-section of fuel elements is square, sized 2.93 x 2.93 mm, cladding material is aluminium alloy SAV-1, the length of the fuel part ∼200 mm, uranium density 4 and 6 g/cm³. The testing is being performed under the conditions as follows: distilled water with pH∼(5,5-6,0) as coolant, coolant pressure ∼1,2 MPa, coolant temperature (30-70)deg. C, coolant velocity ∼2,6 m/s, maximum temperature of fuel elements cladding ∼110 deg. C, maximum heat flux ∼0,9 MW/m². The dismountable design of the irradiation device for fuel elements testing makes it possible to perform inspections and replacement of fuel elements during the breaks between the tests. During the test coolant inlet and outlet temperature, outlet flow rate and pressure are measured in the experimental channel. The thermal power of the tested fuel elements is calculated by the measured thermodynamic coolant parameters. Fuel elements cladding leakage is continuously checked by measuring fission products' delayed neutrons penetrating into the coolant. (author)