First operating experience with the steam generators of the THTR nuclear power plant

The steam generators of the THTR nuclear power plant are once-through forced-flow boilers. The heat transfer areas consist of helically wound tubes which are arranged in concentric multiple-start tube cylinders. The primary gas flows upwards passing first along the low pressure heat transfer area arranged in co-current and then along the high pressure heat transfer area arranged in countercurrent. Evaporation in the high pressure section is thus effected in down-hill direction, however at a very small inclination of the heat transfer tubes. On October 19, 1985 the reactor power was first raised to a level at which superheated steam is generated in the steam generators. In the operating procedures performed up to now including the passage through the wet steam phase as well as stationary load conditions, the principle of down-hill boiling has been demonstrated on a large scale for the first time. Signs of instability have not been observed. In the feedwater connection lines throttle valves are arranged which, on the one hand, are used to produce an additional pressure loss to ensure stability and which, on the other hand, can be used to control the mass flow rates on the secondary side so as to maintain the steam temperature differences within specified limits. First measurements during operation have shown that the temperatures of the steam exiting from the tube cylinders were reasonably uniform. (author)