[en] Safety analysis of fast reactors requires knowledge of the vapor pressure of the fuel material under the condition of rapid heating in the temperature region from 3000 K upwards, where up to now no measured data have been available. An experimental technique to determine the vapor pressure of refractory fuel materials in the temperature range between 3000 and 5000 K is based on laser beam heating of the specimen surface for approximately 1 msec and measurement of both the recoil momentum of the specimen and the mass and momentum carried away by the vapor jet flowing into a vacuum. The determination of the vapor pressure requires application of gas dynamics and development of a suitable mode. The application limits of such a measuring technique using laser heating and the reliability of the vapor pressure data thereby determined have been assessed. (U.S.)