Evaluation of Impulse Turbine Performance Under Wet Steam Conditions

After the Great East Tohoku Earthquake of 2011, operations at three critical reactors at the Fukushima Daiichi site were disrupted. All three reactor plants had automatic shutdowns. After a reactor is shutdown from power, decay heat must continue to be removed for several hours. The earthquake caused electrical equipment disruption which interrupted power supply from off-site. In Unit 2, emergency diesel generators and battery power were also affected. This interruption of on-site and off-site power affected operability of decay heat removal systems. In Unit 2, the Reactor Core Isolation Cooling System (RCIC) was an installed means of removing decay heat. The RCIC System uses a tangential flow impulse turbine called a Terry turbine to power a pump, which directs cooling water into the Reactor Pressure Vessel (RPV). Though DC and AC power were unavailable, the RCIC System was able to operate for 70 hours. One reason for this may be lowered efficiency due to moisture carryover in the steam line. The events of the plants which were operating at Fukushima Daiichi have been modelled using several computational codes. Instrument data is limited due to the loss of electrical power, so assumptions were necessary. To more fully understand the events of Fukushima Daiichi Unit 2, it was desired to investigate the performance of a tangential flow impulse turbine under two-phase flow conditions. Estimation techniques have been made of the effect of two-phase injection on turbine performance, but these were also tested for this unique turbine design. A Terry turbine similar to those used in the RCIC Systems was installed in an experimental facility. The facility was equipped to inject compressed air or steam, along with a water component, into the inlet of the turbine. The turbine shaft work was measured by a water brake dynamometer. Dry and wet mixes from 60 g/s to 0 g/s were injected into the turbine down to the lower limit of operability. Torque, shaft work, and isentropic efficiency were obtained for 1500, 2000, 2500, and 3000 shaft revolutions per minute (RPM). The steam tests produced higher shaft work and torque than the air tests due to the higher enthalpy content of steam. Air, however, achieved similar isentropic efficiency to the steam tests. The 1912 approximations of Baumann for low-quality steam had good agreement with the results from tests using the wet mixes