Requirements analysis phase plays a vital role in drawing the performance and characteristics of critical software systems. As the requirements were global, detailed and complementary as the system was successfully functioning, free of errors and flaws, and adapted to environment dynamicity. In critical systems, such as Nuclear Power Plants (NPPs), implementing software functional requirements (FRs) is not enough to ensure system safety. Non-functional requirements NFRs implementation beside FRs becomes crucial for ensuring such function. NFRs performs other functions that are essentials for system availability, reliability, and dependability. NFRs should be supportive, not precluding to FRs, and keep system complexity and cost as low as possible. To this end, this paper proposes a model for NFRs which have importance in nuclear field based on safety system classification, and graded approach which assign the quality attributes and constraints to a given system based on its importance to safety. This model helps in enhancing the system overall safety without increasing the system complexity and implementation cost without need.