The separation system containing thin-film polyamide reverse osmosis membrane (tf-PA RO) of high temperature was studied. performed laboratory tests, RO silica rejection percentage was over 97%, and boron passage was about 60-65%, (molecular weight cut off of 200 or 250 Da) of the solution which contained silica in the range of 1-90 ppm and boron in the range of 7500-15,000 ppm. The separation factor (SF) between boron and silica related to boron concentration could be expressed as the relation. SF = k[boron](0.7). The separation process could be described by a mass balance model. The modeling calculation fitted the experimental results very well, within the acceptable parameters' errors. It was proposed that the boric acid in a boric acid storage tank (BAST) of one studied pressurized water reactor (PWR) plant could be treated with such tf-PA RO. In the prediction, the silica concentration in it would be removed to about I ppm. and that the boron could be reused. The waste amount of boron after such treatment was predicted to be less than 1.5%. (C) 2004 Elsevier B.V. All rights reserved.