Abstract. In this paper, viewed thermophysical characteristics of the moderator water graphite reactor RBMK. Indicated possibilities of controlling thermal state of graphite stack by regulation composition of the purge gas. Presents experimental results, but static thermal state characteristics of graphite moderator RBMK-1000. Developed a software code for integral characteristic engineering calculations, that determine value of margin reactivity reactor RBMK-1000, in the slow transients.