The principle of defense-in-depth in nuclear safety consists of five steps, each of which must be independently successful in preventing or mitigating radiation damage in its own right. Even if the concept of defense-in-depth is qualitatively ideal, it is always challenging to evaluate its effectiveness quantitatively. The deterministic approach with margin has been a main tool to prove it, while the probabilistic approach with risk assessment has begun to support the conventional realm. In particular, in Korea, the scope of safety analysis is expanding and getting complicated due to the pending issues of multi-unit accident, the legalization of severe accident management, and emerging new reactor types.