Probable Causes of Unexpected Multiple Destructions of Heat Exchanger Tubes of Some Low Pressure Reheaters on Nuclear Power Plants with VVER-1000

Abstract

Reheaters (LPR-3 and LPR-4). Heat exchange surfaces were made of welded or all-welded tubes of 08Х18Н10Т stainless steel (AISI 321) with a diameter of 16х1mm. LPR-3 and LPR-4 have identical design. The heating steam moves horizontally between the seven horizontal partitions and crossing successively vertical tubes of the second running and then the main condensate first running, condenses on the vertical heat exchange tubes and flows down along the tubes surface. The pressure in the tubes is about 1.2 MPa, and the pressure of the heating steam is 0.3 MPa. After the ban on the use of copper-containing materials in the second circuit of nuclear power plants, their gradual replacement began with steel ones. After a relatively recent installation of stainless steel tubes in LPR-3, significant damages of the tubes occurred with a violation of their tightness. Overwhelming majority of the damaged tubes were along the first running of the heated water and inside the three lower horizontal sections. The form of the damages appeared to be of a cavitation character, although the distribution of temperature and pressure did not completely satisfy this statement. It was shown that the presence of such effects can be explained using the model of internal non-stationary nature of two-phase flows proposed in the NPO CKTI JSC. Relative values of local pressure fluctuations correspond to the maximum of the density (or quality) fluctuations and can be about 0.03-0.05 MPa. Thus, the pulsations of local saturation temperature can reach 5-7°C and cause the effects of steam bubbles formation and their collapse in the condensate near the partitions. At the same time, it is possible to explain the location of the injuries and formulate some proposals for weakening or even elimination the observed effects. Although the described effects were found at NPP, but they, of course, can occur at conventional Power Plants.