Passive reactor cooling system

Abstract

A nuclear reactor cooling system with passive cooling capabilities operable during a reactor shutdown event without available electric power. In one embodiment, the system includes a reactor vessel with nuclear fuel core and a steam generator fluidly coupled thereto. Primary coolant circulates in a flow loop between the reactor vessel and steam generator to heat secondary coolant in the steam generator producing steam. The steam flows to a heat exchanger containing an inventory of cooling water in which a submerged tube bundle is immersed. The steam is condensed in the heat exchanger and returned to the steam generator forming a closed flow loop in which the secondary coolant flow is driven by natural gravity via changes in density from the heating and cooling cycles. In other embodiments, the cooling system is configured to extract and cool the primary coolant directly using the submerged tube bundle heat exchanger.

What is claimed is: 
     
         1 . A method for passively cooling a nuclear reactor after shutdown, the method comprising:
 heating a primary coolant in a reactor vessel with a nuclear fuel core;   extracting the heated primary coolant from the reactor vessel;   flowing the heated primary coolant through a tube bundle submerged in an inventory of cooling water in a heat exchanger pressure vessel;   cooling the heated primary coolant to lower its temperature; and   returning the cooled primary coolant to the reactor vessel;   wherein the primary coolant circulates through a first closed flow loop between the tube bundle and reactor vessel.   
     
     
         2 . The method according to  claim 1 , further comprising:
 heating the cooling water in the pressure vessel by the primary coolant;   converting a portion of the cooling water into cooling water steam;   extracting the cooling water steam from the pressure vessel;   flowing the extracted cooling water steam through heat dissipater ducts integrally attached to a shell of a reactor containment vessel in a thermally conductive relationship;   condensing the cooling water steam in the heat dissipater ducts; and   returning the condensed cooling water steam in liquid phase to the pressure vessel to replenish the inventory of cooling water.   
     
     
         3 . The method according to  claim 2 , wherein the extracted cooling water steam circulates through a second closed flow loop between the heat exchanger pressure vessel and the heat dissipater ducts. 
     
     
         4 . The method according to  claim 2 , wherein the reactor containment vessel shell is in direct thermal communication with an external heat sink operable to extract heat from and condense the cooling water steam inside the heat dissipater ducts. 
     
     
         5 . The method according to  claim 4 , wherein the shell of the reactor containment vessel is formed of a thermally conductive metal. 
     
     
         6 . The method according to  claim 5 , wherein the external heat sink is a reservoir of secondary cooling water. 
     
     
         7 . The method according to  claim 6 , wherein the secondary cooling water in the reservoir is in direct thermal contact with and wets an exterior surface of the shell of the reactor containment vessel. 
     
     
         8 . The method according to  claim 7 , wherein heat from the cooling water steam is transferred directly through the metal shell of the reactor containment vessel to the reservoir of secondary cooling water via thermal conduction. 
     
     
         9 . The method according to  claim 7 , wherein the reservoir is annular in shape and surrounds an entire circumference of the reactor containment vessel. 
     
     
         10 . The method according to  claim 9 , wherein the annular reservoir continuously extends 360 degrees around the circumference of the reactor containment vessel. 
     
     
         11 . The method according to  claim 10 , wherein the shell of the reactor containment vessel is cylindrical. 
     
     
         12 . The method according to  claim 1 , wherein the external heat sink is an annular reservoir of secondary cooling water which direct contacts and wets an exterior surface of the reactor containment vessel shell, and wherein heat from the cooling water steam is transferred directly through the reactor containment vessel shell to the secondary cooling water to condense the cooling water steam inside the heat dissipater ducts. 
     
     
         13 . The method according to  claim 12 , wherein the annular reservoir of secondary cooling water is formed between the reactor containment vessel shell and a shell of a containment enclosure structure which surrounds the reactor containment vessel shell. 
     
     
         14 . The method according to  claim 13 , wherein the secondary cooling water in the annular reservoir direct contacts and wets an interior surface of the containment enclosure structure shell. 
     
     
         15 . The method according to  claim 14 , wherein the reactor containment vessel shell and containment enclosure structure shell are each cylindrical metal shells arranged in concentric relationship. 
     
     
         16 . The method according to  claim 14 , wherein the annular reservoir is fluidly connected directly to atmosphere. 
     
     
         17 . The method according to  claim 16 , further comprising steps of heating the secondary cooling water in the annular reservoir via heat extracted from the heat dissipater ducts, boiling the secondary cooling water to produce secondary cooling water steam, and venting the secondary cooling water steam to atmosphere. 
     
     
         18 . A method for passively cooling a nuclear reactor after shutdown, the method comprising:
 heating a primary coolant in a reactor vessel with a nuclear fuel core;   extracting the heated primary coolant from the reactor vessel;   flowing the heated primary coolant in a first closed flow loop to a tube bundle submerged in an inventory of cooling water contained in a heat exchanger vessel;   heating the inventory of cooling water via the heated primary coolant;   converting a portion of the cooling water in the inventory into cooling water steam;   extracting the cooling water steam from the heat exchanger vessel;   flowing the extracted cooling water steam in a second closed flow loop through heat dissipater ducts integrally attached to a cylindrical metal shell of a reactor containment vessel in a thermally conductive relationship;   condensing the cooling water steam in the heat dissipater ducts; and   returning the condensed cooling water steam in liquid phase to the heat exchanger vessel to replenish the inventory of cooling water.   
     
     
         19 . The method according to  claim 18 , further comprising:
 cooling the heated primary coolant flowing through the tube bundle in the cooling water to lower its temperature; and   returning the cooled primary coolant from the tube bundle to the reactor vessel in the first closed flow loop.   
     
     
         20 . The method according to  claim 18 , further comprising transferring heat conductively through the metal shell of the reactor containment vessel to secondary cooling water in an external annular reservoir to condense the cooling water steam in the heat dissipater ducts, the secondary cooling water in direct thermal contact with the metal shell of the reactor containment vessel. 
     
     
         21 . The method according to  claim 20 , further comprising heating the secondary cooling water in the annular reservoir to produce secondary cooling water steam, and venting the secondary cooling water steam to atmosphere. 
     
     
         22 . The method according to  claim 20 , wherein the heat dissipater ducts comprise a plurality of C-shaped structural channels or half-pipe sections integrally welded to an interior surface of the metal shell of the reactor containment vessel, the cooling water steam being in direct wetted contact with the interior surface of the metal shell of the reactor containment vessel.