Sludge Removal at Krško NPP Steam Generators during Outage

Abstract

Sludge removal is performed on two steam generators (SG’s) at the Krško Nuclear Power Plant (NEK) during every outage. SG’s are a meeting point of four major plant systems: Reactor Coolant System (RC) on the primary side and three systems on the secondary side – Auxiliary Feedwater System (AF), Main Feedwater System (FW) and Main Steam System (MS). Sludge removal activities take place on the secondary side of the SG’s on the top of the tube sheet. It consists of classical Sludge Lancing (SL) which is done by spraying water at different angles (30°, 90°, 150°) between the tube gaps in the steam generator tube bundle with a pressure around 220 bars. Another method is Inner Bundle Lancing (IBL) which means spraying water at a much higher pressure (NEK’s contractor reached a pressure of approximately 590 bars). Such water is sprayed directly on the top of the tube sheet with a robot guided lance which is placed inside a steam generator. The robot is controlled by an operator and at times fully autonomous to provide the highest protection measures possible. After these activities, a televisual inspection (TVI) of the top of the tube sheet is performed to access the hard sludge area and to search for potential foreign objects in the SG’s. If an object is found, an attempt to retrieve it would usually take place. Other methods of sludge removal as upper bundle flushing or chemical cleaning have not been implemented in NEK thus far. Since the power plant uprate in May 2000, NEK conducted SL on both SG’s every outage with IBL in 2013 and 2015 and the same method was used in the 2018 outage. The purpose of these activities is mainly to extend the full load operation of the plant, prevent denting processes in the SG’s from occurring, stop the buildup of hard sludge area to increase/sustain efficiency and remove foreign objects found in the SG’s. SG’s U-tubes are a barrier between the primary side coolant and the secondary side of NEK and the environment. Therefore, it is crucial to keep the highest level of integrity of the U-tubes because any leak could potentially mean a release of radioactive material to the atmosphere. This paper describes the purpose and workflow of sludge removal in NEK.