This leakage prevention seal is provided with: a first seal ring which surrounds a rotating shaft and which is in circumferential contact with the surface of a housing which faces the upstream side; a second seal ring, which, on the upstream side of the first seal ring, surrounds the rotating shaft and which is in circumferential contact with the first seal ring; and a heat-driven section which, when high-temperature pressurized water reaches the heat-driven section, reduces the diameter of both the first seal ring and the second seal ring and causes the inner peripheral surfaces of both the first seal ring and the second seal ring to be in contact with the rotating shaft. The circumferential positions of both a first range between the first seal ring and the rotating shaft and a second range formed by the second seal ring are different.