Flow Instability Inside Heat Transfer Tubes of MSR

Moisture separator reheater for nuclear steam turbine system of light water reactor, a combined equipment of mist separator and in-tube side condensing multi-tube type heat exchanger, has an inherent issue of flow instability inside the tubes. The issue causes the temperature oscillation and possible thermal fatigue at tube to tube-sheet welding of the tube outlet end. Construction of its heat transfer is that heating main steam supplied by NSSS inside the tubes transfers its latent heat by condensing to the colder high pressure turbine exhaust steam cooling outside of the tubes. The condensed water is exhausted by friction of venting excess steam through the tubes which makes the flow in tubes gas-liquid two phase regime and results the steady drainage without sub-cooling and associated temperature oscillation at tube end. A measuring was conducted on MSR of an existing 600 MW class unit to clarify the unstable behaviors without excess steam, which showed the temperature oscillation at tube end with synchronizing to oscillation of both pressure difference between tube inlet and outlet chambers and tube surface temperature along the tube. The cycle and the amplitude of the temperature oscillation were 55 second and 40K. Only the effects of venting steam for the actual unit and presumption of the mechanism of subcool were reported in the original paper, Journal of Engineering for Gas Turbines and Power 2010 Vol. 132/ 102905, here is added qualitative and quantitative study for both dynamics of the condensed water and behavior at tube outlet end to assess adequacy of the mechanism.