[en] The first Czechoslovak experimental modular steam generator, the 30 MW BOR-1, was manufactured in Czechoslovakia in 1973 and was put into trial operation at the BOR-60 nuclear power plant in Dimitrovgrad in the USSR. The steam generator was designed as a tri-metallic system. The casing tubes of the superheater were made of austenitic steel of the DIN 1.4306 type designated as 3R12. The heat exchange tubes in the superheater were made of high alloy material of the DIN 1.4876 type designated as SANICRO 31. The modules of the evaporator and the economizer were made of low-alloy ferritic-perlitic Nb-stabilized steel of the DIN 1.6770 type designated as HT8X6. In 1979, small and large leakages of the steam generator were simulated by injecting steam into sodium. After termination of the experiment the materials were subjected to metallographic investigation. In the 3R12 material long-term operation had not caused any substantial changes with the exception of precipitation of carbides at the grain boundaries under the surface. The SANICRO 31 material showed a negligible increase in strength and reduction in plastic properties. At the inlet of sodium into the superheater where the highest temperature is recorded the material showed a susceptibility to intergranular corrosion. The HT8X6 did not show any substantial changes in mechanical properties. On the inner surface of the tubes on the water side a layer of corrosion products and deposits was found up to 0.3 mm in thickness. In some places corrosion in form of pits and small cracks to a depth of 0.2 mm was found under the deposits. (M.D.)