Patent Number: 
Section: claims

1. A leakage prevention seal 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 which surrounds the rotating shaft and which contacts with an upstream side surface of the housing along a circumferential direction in the rotating shaft;a second seal ring, which, on the upstream side of the first seal ring, surrounds the rotating shaft and which contacts with an upstream surface of the first seal ring along the circumferential direction in the rotating shaft; anda 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,wherein the first seal ring has a closed loop shape to form a first gap extending over a first range 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 allows the high-temperature pressurized water to leak to the downstream side, over a second range in the circumferential direction during the reduction in diameter, andwherein the circumferential positions of both the first range and the second range are different. 2. The leakage prevention seal according to claim 1, further comprising a first gap positioning unit determining the circumferential position of the first range where the first gap is formed by applying a force in a radial direction to the first seal ring. 3. The leakage prevention seal according to claim 2, further comprising a pressing unit pressing the second seal ring toward the first seal ring. 4. The leakage prevention seal according to claim 3,wherein the first seal ring includes:a ring main body extending in the circumferential direction of the rotating shaft; anda pair of soft members disposed at both respective ends of the ring main body, formed of a material softer than the material of the ring main body, and brought into contact with each other during the reduction in diameter. 5. The leakage prevention seal according to claim 4,wherein the heat-driven section reduces the diameter of the first seal ring at a first temperature and reduces the diameter of the second seal ring at a second temperature, andwherein the first temperature is lower than the second temperature. 6. The leakage prevention seal according to claim 5,wherein the second seal ring has a closed loop shape when reduced in diameter, andwherein the second gap extends over the second range between the second seal ring and the rotating shaft. 7. The leakage prevention seal according to claim 6, further comprising a second gap positioning unit determining the circumferential position of the second range where the second gap is formed by applying a force in the radial direction to the second seal ring. 8. The leakage prevention seal according to claim 5,wherein the second seal ring has a C shape for both circumferential ends to face each other in the circumferential direction during the reduction in diameter, andwherein the second gap is formed between both of the ends of the second seal ring. 9. A pump for a nuclear reactor cooling material comprising:a rotating shaft rotating about an axis;a housing surrounding the rotating shaft from a radial outer side; anda leakage prevention seal according to claim 8 sealing a space between the rotating shaft and the housing so as to regulate a flow of high-temperature pressurized water in the space from the upstream side to the downstream side in the axial direction. 10. The pump for a nuclear reactor cooling material according to claim 9,wherein an annular groove recessed to the radial outer side and extending in a circumferential direction is formed in an inner peripheral surface of the housing,wherein the first seal ring and the second seal ring are arranged to be accommodated in the annular groove, andwherein the surface of the annular groove facing the upstream side is an abutting surface.