The Research of Steam Generator Thermal Performance Degradation in Nuclear Power Plant

As with any heat exchanger, the function of a nuclear steam generator is compromised by operational changes that increase the resistance to heat transfer between the primary and secondary fluids, lowering the efficiency and leading to thermal performance degradation. This type of performance degradation, the underlying causes, industry experience, and plant operational consequences had been discussed by ERRI and DEI reference documentations. After 6-10 years operation the steam pressure degradation of most plants occurred. The steam pressure of steam generator (SG) in Ningde nuclear power plant (NPP) decreased after the unit was in commercial operation. It decreased by 1 bar in first fuel cycle, and it was down by 0.5 bar at the second and third cycle. And the same type of nuclear power plant in France had no similar status. It was also different with the situations of American units. In this paper it concludes that the main cause of steam pressure loss is the sludge or fouling which accumulated at tubes and supporting plates at the second loop. The eddy current imaging and video inspection results showed that the sludge or deposit were found at the cold side tubes nearby the tube supporting plate one, the sludge amount at hot side tubes was smaller than the cold tubes. In general, there were no deposit signals at the upper tube bundle. And the total amount of sludge on SG tubes was fewer, which was possibly contradiction with the fouling factor. The bundle flushes equipment have been applied at Ningde NPP unit 1, the sludge amount was five times larger than the normal tube sheet flush. This special bundle flush applications reduce the sludge in the SGs, avoid the corrosion of heat transfer tubes and ensure the SGs safety. It is a pity that the SG thermal performance was not recovered quickly after this special flush. The sludge nearby the TSP1 was not the main cause of steam pressure loss. At present, the project team is studying polyacrylic acid (PAA) and chemical cleaning to expand the scope of washing and cleaning effect, and further recover the thermal performance of steam generator.