Post irradiation examination of RBMK CPS channel, removed due to detected leakage after 32 years of operation, has been carried out. It was found out that depressurization occurred in the nipple of the lower channel adapter. The main reason of the damage is hydrogen absorption. Hydrogen concentration in the area of technological damage is higher than 1000 ppm. Hydrogen generates hydrides, which are distributed through the nipple wall thickness; hydride blisters are observed on the inner surface of the channel. This leads to embrittlement of nipple material.