[en] The designer of a nuclear plant must know the plant's associated risk limitations so that he can design the plant accordingly. To design a safety system, he must understand its importance and how it relates to the overall plant risk. The computer program RIO can aid the designer to understand a system's contribution to the plant's overall risk. The methodology developed and presented was sponsored by the Nuclear Research Applications Division of the Department of Energy for use in the Gas Cooled Fast Breeder Reactor (GCFR) Program. The principal motivation behind its development was the need to translate nuclear plants safety goals into reliability goals for systems which make up that plant. The method described herein will make use of the GCFR Accident Initiation and Progression Analyses (AIPA) event trees and other models in order to determine these reliability goals