The article concerns the modelling of stresses and strains of the enclosure of the active zone and chamber housing inside a WWER-1000 reactor, taking into consideration welding, heat treatment and the operation of the structure in the conditions of intense radiation. The work presents the input of welding internal stresses and contact stresses into the total stress of the chamber housing inside the reactor vessel. The article-related research involved testing the relaxation of welding internal stresses in the active zone enclosure caused by radiation creep.