The increase in the boron concentration is obtained by adding boric acid to the coolant: the maximal rate of boron increase in the coolant is about 25 ppm/ min. On the other hand, reducing the concentration of boron in the coolant is done by diluting the coolant with fresh, non-borated water, the time needed for halving the boron concentration is about five hours. The rate of decrease in boron concentration hence depends on the initial boron level and is particularly slow at low concentrations, which makes this method less efficient at the end of the fuel cycle. In addition, coolant dilution with fresh water generates a large amount of effluents that must be treated and disposed of by the chemical volume control system.