[en] Purpose: To effectively remove organic substances incorporated in the water of a condensate storage tank. Constitution: A clean-up device having a radioactivity irradiation source and an ion exchanging performance is disposed to the outside of a condensate storage tank in a nuclear power plant. Then, water in the condensate storage tank is introduced to the irradiation source to radiolyze and ionize the organic substances incorporated therein, filtered and then subjected to ion exchange in a desalter to remove the organic substances. Spent fuels are used as the radioactivity irradiation source. Further, the clean-up device uses a filter device for the condensate system, a filter device for the fuel pool clean-up system and a desalter. (Ikeda, J.)