Abstract: Disclosed are a nuclear reactor adapted for mitigating a loss-of-coolant accident and a mitigation method thereof. The nuclear reactor includes a nuclear reactor vessel, a first pipe part connected to the nuclear reactor vessel to supply or drain fluid, and a first opening/closing part connected to the first pipe part. When the first pipe part is broken, the first opening/closing part closes the first pipe part to stop discharge of coolant. A supplementary coolant injection part is connected to the nuclear reactor vessel through a second pipe part. When the second pipe part is broken, a second opening/closing part closes the second pipe part to stop discharge of coolant.