The successful use of an acid reducing chemistry for the resolution of flow variations in the RCS

In 2009, one 900 MW PWR operated by EDF measured a significant increase in core pressure drop, along with a decrease in the primary coolant flow rate. These variations appeared after the reactor had been maintained for 1.5 months in hot standby, near 290 deg. C. Due to this variation in flow rate, the startup of the plant had to be interrupted. Several solutions were examined to solve the issue and it was finally proposed to implement an acid reducing chemistry with the objective of dissolving the deposits which were thought to have caused the flow rate variations. The plant was then cooled down to 170 deg. C, with 2000 mg/kg boron, dissolved H₂ around 15 Ncm³/kg and Li removed from the coolant. As a result, the core pressure drop started to decrease as soon as the Li concentration went below 0.5 mg/kg, and it was within 1% of the initial value after about 48 hours of the applied treatment. At this point, the flow rate issue was considered as solved. The analyses performed on the coolant confirmed that there had been an effective dissolution of the corrosion products in the primary coolant during the acid reducing phase. A subsequent operational experience feedback analysis was also performed, in order to understand the origin of the flow rate variation. It showed the importance of the chemistry during extended times spent in hot standby. As a consequence, it was proposed to change the standard chemistry during these phases, in order to prevent the occurrence of flow rate variations in the primary loops. This paper describes the operational experience feedback of the event, how it appeared and how it was finally solved. The results of the subsequent analysis carried out to understand the phenomenon are also detailed, together with the preventive actions implemented for the whole fleet. (authors)