Passive shutdown sealing device for a system of shaft seals of a reactor coolant pump set

Abstract

The present invention relates to a passive shutdown sealing device ( 20 ) for a reactor coolant pump unit comprising: a split sealing ring ( 23 ) having an inactivated position in which a leakage flow is permitted and an activated position in which said ring stops said leakage flow; at least one piston ( 22 ) designed to position said split sealing ring ( 23 ) in its activated position; locking/unlocking means ( 25 ) designed to lock said at least one piston ( 22 ) in its inactivated position when the temperature of said locking/unlocking means is below a temperature threshold and to release said at least one piston ( 22 ) when the temperature of said locking/unlocking means is above said temperature threshold; elastic means ( 24 ) designed to move said at least one piston ( 22 ) when said piston is released, so as to position said sealing ring ( 23 ) in its activated position.

What is claimed is: 
     
       1. A passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit comprising:
 a split sealing ring having an inactivated position wherein a leakage flow is permitted and an activated position wherein said ring stops said leakage flow; 
 at least one piston separate from said split sealing ring in the inactivated position designed to position said split sealing ring in its activated position; 
 a piece that mechanically degrades at a temperature above a temperature threshold, said piece configured to lock the movements of the at least one piston and to maintain said at least one piston in its inactivated position when the temperature of said piece is below the temperature threshold, and to release said at least one piston when the temperature of said piece is above said temperature threshold; 
 a source of pressure positioned to move said at least one piston when said piston is released at the temperature above said temperature threshold, so as to position said sealing ring in its activated position; 
 the piece and the split sealing ring forming two separate pieces at the inactivated position. 
 
     
     
       2. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said device is designed to be integrated in a system of shaft seals of a reactor coolant pump unit in use. 
     
     
       3. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said ring includes a chamfered side wall designed to cooperate with a chamfered wall of said at least one piston. 
     
     
       4. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said piece is designed to be mechanically degraded from a temperature threshold between 80° C. and 200° C. 
     
     
       5. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said sealing ring is made of a polymeric material resisting to temperatures above 300° C. 
     
     
       6. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said sealing ring is made of PEEK. 
     
     
       7. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said sealing ring is made of a composite material with PEEK matrix and glass or carbon reinforcement fibres. 
     
     
       8. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said sealing ring is made of a metal material. 
     
     
       9. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said sealing ring is a composite material formed by a metal core and coated with a material more malleable than said metal core. 
     
     
       10. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 9 , wherein said material more malleable than said metal core is a polymer or nickel or silver. 
     
     
       11. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein said source of pressure is a compression coil spring, a wave spring or a spring washer. 
     
     
       12. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein the device includes a plurality of pistons and a plurality of locking/unlocking means distributed about the perimeter of said ring, said pistons and said locking/unlocking means being angularly spaced from each other by a constant angle. 
     
     
       13. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein the device includes three pistons and three pieces that mechanically degrade at a temperature above a temperature threshold. 
     
     
       14. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 1 , wherein the device includes a split collar integrated in said sealing ring and designed to ensure sealing of said device in case of degradation of said ring. 
     
     
       15. A reactor coolant pump unit comprising:
 a system of seals designed to perform a controlled leakage occurring along a leakage path arranged along the pump shaft of the reactor coolant pump unit; 
 a passive shutdown sealing device according to  claim 1  designed to seal at least partly said leakage path of said system of seals when said system of seals is failing and when said sealing ring is activated, so as to perform a controlled leakage. 
 
     
     
       16. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 4 , wherein said piece is designed to be mechanically degraded from 150° C. 
     
     
       17. A method for controlling a primary coolant leakage resulting from a failing system of seals of a reactor coolant pump unit and after accidental shutdown of the pump unit, wherein the pump unit comprises a passive shutdown sealing device according to  claim 1 , said method comprising:
 positioning the split sealing ring to an inactivated position at temperature below the temperature threshold, 
 positioning the piece configured to lock the movements of the at least one piston and to maintain at least one piston in the inactivated position a t the temperature below the temperature threshold, 
 allowing the piece to mechanically degrade so as to release the at least one piston when the temperature of the piece is above the temperature threshold, and 
 allowing the source of pressure to move the at least one piston so as to position said sealing ring in the activated position. 
 
     
     
       18. The method for controlling a primary coolant leakage resulting from a failing system of seals of a reactor coolant pump unit by activating a passive shutdown sealing device according to  claim 17 , wherein the piece is designed to be mechanically degraded from a temperature threshold between 80° C. and 200° C. 
     
     
       19. The method for controlling a primary coolant leakage resulting from a failing system of seals of a reactor coolant pump unit by activating a passive shutdown sealing device according to  claim 17 , wherein said sealing ring is made of a polymeric material resisting to temperatures above 300° C. 
     
     
       20. The method for controlling a primary coolant leakage resulting from a failing system of seals of a reactor coolant pump unit by activating a passive shutdown sealing device according to  claim 18 , wherein the piece is designed to be mechanically degraded from 150° C. 
     
     
       21. The passive shutdown sealing device for a system of shaft seals of a reactor coolant pump unit according to  claim 11 , wherein the piston comprises a recess for the compression coil spring, the wave spring or the spring washer.