The aim of this article is to observe anomalous behaviour of strong acid cation (SAC) exchanger in VVER and PWR power plants. This ion exchanger is a part of primary water coolant purification system and shows a non-standard operation in the end of the fuel cycle during high-performance effect. This behaviour consists in process' changes of potassium cation capture from boric acid solution, which is a major chemical of primary cooling water. The experiments were conducted under laboratory conditions on ion exchanger type DOWEX HCR-S NG and the samples with different concentrations of analytes (K+, NH4+, Li+, H3BO3) were measured. The results confirmed the anomalous behaviour of ion exchanger during high-performance effect when the potassium ions are captured on SAC due to its size and the lack of boric acid in water coolant. Therefore, the potassium hydroxide must be dosed into the primary water to control the content of alkali metals.