A leakage prevention seal for sealing a space between a rotating shaft rotating about an axis and a housing surrounding the rotating shaft from a radial outer side so as to regulate a flow of high-temperature pressurized water in the space from the upstream side to the downstream side in an axial direction, the leakage prevention seal comprising: a first seal ring configured to surround the rotating shaft and to be in circumferential contact with the surface of the housing which faces the upstream side; a second seal ring, which, on the upstream side of the first seal ring, is configured to surround the rotating shaft and to be in circumferential contact with the first seal ring; and a heat-driven section which, when high-temperature pressurized water reaches the heat-driven section, is configured to reduce the diameter of both the first seal ring and the second seal ring and to cause the inner peripheral surfaces (41, 51) of both the first seal ring and the second seal ring to be in contact with the rotating shaft, wherein the first seal ring has a closed loop shape to form a first gap extending over a first range (θ1) in a circumferential direction between the rotating shaft and the first seal ring during the reduction in diameter, wherein the second seal ring forms a second gap, which is configured to allow the high-temperature pressurized water to leak to the downstream side, over a second range (θ2) in the circumferential direction during the reduction in diameter, and wherein the circumferential positions of both the first range (θ1) and the second range (θ2) are different, wherein the heat-driven section is configured to reduce the diameter of the first seal ring at a first temperature and to reduce the diameter of the second seal ring at a second temperature, and wherein the first temperature is lower than the second temperature. a first seal ring configured to surround the rotating shaft and to be in circumferential contact with the surface of the housing which faces the upstream side; a second seal ring, which, on the upstream side of the first seal ring, is configured to surround the rotating shaft and to be in circumferential contact with the first seal ring; and a heat-driven section which, when high-temperature pressurized water reaches the heat-driven section, is configured to reduce the diameter of both the first seal ring and the second seal ring and to cause the inner peripheral surfaces (41, 51) of both the first seal ring and the second seal ring to be in contact with the rotating shaft, wherein the first seal ring has a closed loop shape to form a first gap extending over a first range (θ1) in a circumferential direction between the rotating shaft and the first seal ring during the reduction in diameter, wherein the second seal ring forms a second gap, which is configured to allow the high-temperature pressurized water to leak to the downstream side, over a second range (θ2) in the circumferential direction during the reduction in diameter, and wherein the circumferential positions of both the first range (θ1) and the second range (θ2) are different, wherein the heat-driven section is configured to reduce the diameter of the first seal ring at a first temperature and to reduce the diameter of the second seal ring at a second temperature, and wherein the first temperature is lower than the second temperature.