Loads Applied to Clogged Steam Generators Internal Structures

Abstract

During their commissioning, steam generators are clean, which means there is no fouling of the heat transfer surface of tubes and no clogging of the flow area on the secondary side. Then sludge appears steadily at a slow pace during operation. Sludge initiates a partial loss of cooling capacity which is modeled by a fouling factor and which mainly results in vapor pressure decrease. Sludge also initiates a reduction of the secondary side flow area, known as clogging. Four safety-related issues are dependant on clogging [1]: the secondary water mass balance, the thermohydraulics oscillations, the tube vibration risk and the resistance of internal structures. This paper focuses on the last of these issues. A numerical application, based on the modeling of a fictitious steam generator, is detailed in this presentation. The order of magnitude is an 8-times increase of the loads in normal operating conditions in case of a typical 60% clogging ratio of the upper tube support plate, and a 12-times increase in case of incidental depressurization transient. These theoretical results emphasize the need to take these loads properly into account in the checking of the mechanical behavior of the internal structure of the steam generators in operation in case of significant sludge deposits.

Simulation of eddy current testing of steam generator tubes with wear and secondary side magnetite deposits

“… Local increase of the secondary fluid velocity with a risk of tube rupture for small row U-bends unsupported by anti-vibration bars (AVB) due to fluid-elastic instability;  A reduction of the circulation ratio with a risk of water level oscillations detrimental to primary temperature and reactivity stability;  A reduction of the water mass inventory, detrimental to accident transients associated with a loss of feedwater;  An increase on the loads on SG internals (TSP, tie-rods) in the event of fast depressurisation of the secondary such as the rupture of a main steam line (6) .…”