Patent Application
20130156143
The following relates to the nuclear reactor arts, nuclear power generation arts, nuclear safety arts, and related arts.
Nuclear reactor safety centers upon maintaining the radioactive core in an immersed condition with adequate heat removal. During normal operation, the reactor core is disposed in a sealed reactor pressure vessel that is filled (or mostly filled) with primary coolant (e.g., light water, in the case of a light water reactor). Heat removal is provided by circulation of the primary coolant through a “heat sink”. In the case of a nuclear power plant, the “heat sink” usually takes the form of a steam generator or turbine. In a conventional boiling water reactor (BWR) the primary coolant is converted to steam inside the pressure vessel and piped out to directly drive a turbine where the act of performing useful work on the turbine cools the steam. In a conventional pressurized water reactor (PWR) primary coolant in a subcooled liquid phase is piped into an external steam generator where heat is transferred to a secondary coolant that in turn drives the turbine. In an integral PWR design an “integral” steam generator is located inside the pressure vessel. In this variant design secondary coolant feedwater is piped into the steam generator and secondary coolant steam is piped out of the steam generator.
Safety systems are designed to remediate various possible events that could compromise the objective of keeping the reactor core immersed in primary coolant and adequately cooled. Two possible events that are addressed by the safety systems are: a loss of coolant accident (LOCA); and a loss of heat sinking accident. Conventionally, safety systems include: (1) a steel containment structure surrounding the pressure vessel and of sufficient structural strength to contain released primary coolant steam; (2) an ultimate heat sink (UHS) comprising a pool of water located outside of the containment; and (3) an emergency core cooling system (EGGS) comprising a combination of condensers to condense steam into water and reject the heat to the UHS. Additionally, a refueling water storage tank (RWST) located inside the containment structure to provide water during refueling operations can also serve as a source of water in emergencies. The steam recaptured by the condensers is optionally fed back into the RWST or the reactor coolant inventory and purification system (RCIPS). The UHS is designed to contain a sufficient supply of water to dissipate heat from the reactor for a designated time period (e.g. 72 hours or for two weeks in some regulatory schemes) without replenishment of the water in the UHS.
In a LOCA, a rupture in the pressure vessel or in connecting piping (e.g., pipes conducting primary coolant to/from an external turbine or steam generator) may cause the pressure vessel to depressurize and possibly leak primary coolant. Remediation of a LOCA includes (1) containing and condensing primary coolant steam in order to depressurize the system; and (2) replenishing water to the pressure vessel in order to keep the reactor core immersed. The RWST provides replenishment water, while the ECCS condensers condense the steam to control pressure within containment.
In a loss of heat sinking event the “heat sink” is lost either through loss of primary coolant flow to the turbine (in a BWR) or to the external steam generator (in a PWR), or through loss of feedwater to the steam generator (in either a PWR or an integral PWR). Response to loss of heat sinking includes venting steam from the pressure vessel to the ECCS condensers in order to remove heat and controllably depressurize the pressure vessel. Ideally this will be performed using a closed system in which steam from the pressure vessel is vented into the condensers. However, if the pressure rise due to loss of heat sinking is too rapid it may be necessary to vent into the containment structure (in effect, converting the loss of heat sinking event into a controlled LOCA).
The high pressure (i.e., steam) loop of the ECCS condensers are connected with the pressure vessel in a loss of heat sinking accident, and/or are connected inside the containment structure in a LOCA or a heat sinking accident in which the pressure vessel is vented to containment. The low pressure (i.e., coolant fluid) loop of the ECCS condensers are connected with the UHS via suitable piping, which must pass through the containment structure in order to provide fluid communication between the EGGS condensers inside containment and the UHS which is outside of containment. This piping presents a potential safety hazard since a break in the pipe may result in a primary leak from containment. In a LOCA this can result in venting of radioactive primary coolant steam into the outside atmosphere.
In one aspect of the disclosure, a nuclear reactor includes a pressure vessel and a nuclear reactor core contained in the pressure vessel. A containment structure contains the nuclear reactor. An ultimate heat sink pool is disposed outside of the containment structure. A condenser includes a plurality of closed-path heat pipes or closed-path thermosiphons having first ends and opposite second ends. The closed path heat pipes or thermosiphons are embedded in the containment structure with the first ends contained inside the containment structure and the second ends disposed outside of the containment structure. The second ends are in thermal communication with the ultimate heat sink pool.
In another aspect of the disclosure, a condenser includes a plurality of closed path heat pipes or closed path thermosiphons having first ends and opposite second ends, and a plenum chamber containing the first ends. The plenum chamber has a fluid inlet and a fluid outlet.
In another aspect of the disclosure, a containment structure contains an interior volume, and a nuclear reactor is disposed in the interior volume. An ultimate heat sink pool is disposed outside of the containment structure. A condenser includes a plurality of closed-path heat pipes or closed-path thermosiphons having first ends and opposite second ends. The closed-path heat pipes or closed-path thermosiphons are embedded in the containment structure with the first ends protruding into the interior volume and the second ends protruding outside of the containment structure.
The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.
With reference to
The PWR 10 includes other components known in the art that are not shown, such as a “basket” or other structure supporting the reactor core 14 in the pressure vessel 12, neutron-absorbing control rods selectively inserted into the reactor core 14 by a control rod drive mechanism (CRDM) to control the nuclear chain reaction, a central riser that defines a primary coolant pressure boundary, primary coolant pumps, or so forth. These various components may be variously disposed inside or outside the pressure vessel. For example, the CRDM may be external, as is conventionally the case, or may be located internally inside the pressure vessel as described in Stambaugh et al., “Control Rod Drive Mechanism for Nuclear Reactor”, U.S. Pub. No. 2010/0316177 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety; and Stambaugh et al., “Control Rod Drive Mechanism for Nuclear Reactor”, Intl Pub. WO 2010/144563 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety. The reactor coolant pumps may be internal or external, and in some embodiments may be omitted entirely in which case heat generated by the reactor core 14 drives primary coolant flow via natural circulation.
The illustrative PWR 10 is an integral PWR design, by which it is meant that an internal steam generator is disposed in the pressure vessel 12. The installed steam generator is not shown due to the opacity of the pressure vessel 12; however,
The PWR 10 is contained in a containment structure 22. The containment structure 22 is typically a steel structure in order to provide structural strength. The use of a steel structure also provides high thermal conductivity to facilitate removal of heat from inside the containment structure 22. Instead of steel, other materials are also contemplated for the containment structure 22. For example, portions or all of the containment structure 22 may be made of steel-reinforced concrete, a composite material such as a steel host with embedded nanoparticles to enhance thermal conductivity, or so forth.
The illustrative containment structure 22 is generally cylindrical, and further includes a lower flood well 24 and an upper dome 26. The lower flood well 24 contains the lower portion of the pressure vessel 12 including the reactor core 14. The flood well 24 enables the lower portion of the pressure vessel 12 to be flooded with water in certain emergency situations in order to assist in cooling the reactor core 14. The upper dome 26 provides enhanced structural strength and serves as a steam condensation surface in certain emergency situations. The containment structure 22 is large enough to accommodate the PWR 10 and to additionally provide space for operations such as removing the steam generator 20 during installation and/or maintenance.
The illustrative containment structure 22 is subterranean, by which it is meant that at least a portion of the containment structure 22 lies below grade, that is, at least partially below the ground level 30. A secondary containment structure 32 contains the (primary) containment structure 22. The secondary containment structure 32 is typically made of concrete, steel-reinforced concrete, or another suitably robust building material. The illustrative secondary containment structure 32 is also subterranean in order to “contain” the subterranean primary containment structure 22. An upper “roof” 34 of the secondary containment structure 32 is above-ground. In some embodiment the upper roof 34 includes vents 80 arranged to allow water evaporated or boiled off of the UHS pool 40 to escape from the secondary containment structure 32. Refilling inlets 82 may also be utilized to refill and/or maintain ultimate heat sink water level.
In the illustrative system of
The upper portion (i.e., roof 34) of the secondary containment structure 32 is optionally omitted. Including the roof 34 enables better control over the composition (e.g., chemistry) of the UHS pool 40, and prevents debris from falling into the UHS pool 40. In some embodiments the UHS pool is provided with a cover that is separate from the secondary containment structure. On the other hand, in some embodiments the sidewalls 46 and optional bottom portion 48 of the UHS pool may form part of the secondary containment structure. More generally, various levels and degrees of integration and/or separation between the walls and bottom of the UHS pool 40, on the one hand, and the walls and roof of the secondary containment 32 on the other hand, are contemplated. It is also contemplated to omit the secondary containment structure 32 entirely, if such an omission does not compromise safety and does not violate applicable nuclear regulatory standards.
The UHS pool 40 provides passive heat removal as follows. Primary coolant released from the pressure vessel 12 (whether in an uncontrolled LOCA or in a controlled fashion such as may be performed in a loss of heat sinking event) naturally rises and contacts the inside surface 44 of the dome 26. The UHS pool 40 in contact with the dome 26 (or, more generally, with a wall and/or roof of the containment structure 22) keeps the dome 26 at outside ambient temperature (or, more precisely, at about the temperature of the water in the UHS pool 40, which is at or close to outside ambient temperature). The primary coolant steam thus condenses onto the inside surface 44 of the dome 26 to form condensate, and its latent heat and any additional kinetic energy is transferred through the dome 26 to the UHS pool 40.
The condensate is in the form of water (or water droplets) adhering to the inside surface 44 of the dome 26. This water falls or runs downward along the surface under the influence of gravity. Advantageously, this may result in a substantial portion of the condensed water flowing into the flood well 24 to contribute to flooding the flood well 24. Alternatively, baffles 50 may be provided to guide the flow of the condensed water. In the illustrative embodiment the baffles 50 are arranged to guide the condensed water into a refueling water storage tank (RWST) 52 which is used in some emergency conditions (such as some LOCA events) to replenish water in the pressure vessel 12.
Arrangements such as that shown in
With continuing reference to
Closed-path thermosiphons are closed tubes containing a working fluid such as liquid water. When one end of the thermosiphon is heated relative to the opposite end, this creates natural convection of the working fluid in the thermosiphon which transfers heat via convection from the hotter end to the cooler end. The convection currents within the thermosiphon are defined in part by the gravitational acceleration, and accordingly thermosiphons generally operate most efficiently when oriented vertically (as defined respective to the gravitational field) with the hotter end facing down and the cooler end facing up. In normal operation, the working fluid inside the thermosiphon is generally single-phase (e.g., liquid water). However, phase change (e.g., boiling of liquid water inside the thermosiphon) may occur when the temperature of the working fluid at the hot end of the thermosiphon exceeds its boiling point. This is sometimes referred to as the “reboiling” operational mode of the thermosiphon. In this case the higher buoyancy of the gas phase versus the liquid phase helps drive the convection that provides the heat transfer.
A heat pipe is also a closed tube containing a working fluid. However, the heat pipe does not transfer heat by convection, but rather by performing an evaporation/condensation cycle with evaporation of the working fluid occurring at the hotter end and condensation of the working fluid occurring at the opposite lower end. Mass transfer of the condensate back to the hotter end can be driven by gravity, in which case the heat pipe exhibits a high degree of orientational dependence with most efficient operation occurring in the vertical orientation with the hotter end facing down and the cooler end facing up. Optionally, a wicking structure can be disposed inside the heat pipe in order to provide mass transfer of condensate to the hotter end by capillary action. The wicking structure substantially reduces the dependency of heat transfer on heat pipe orientation, and indeed a heat pipe with a suitable wicking structure can be operated efficiently even in a horizontal position.
The term “closed-path” (i.e., “closed path heat pipes or closed-path thermosiphons”) clarifies that the heat pipe or thermosiphon operates as a closed system, with the tube being sealed and having no inlet and no outlet. In other words, the working fluid is permanently trapped inside the closed-path thermosiphon or heat pipe.
With continuing reference to
The closed-path heat pipes or closed-path thermosiphons 60 are closed heat transfer pathways 60, in contrast with conventional condenser arrangements in which water from the UHS pool is piped through the wall (or roof) of the containment structure into its interior and back out. A single break in the closed heat transfer pathways disclosed herein cannot breach containment. Rather, to breach containment a minimum of two breaks in a single thermosiphon or heat pipe 60 is needed—a break of the first end protruding into the containment structure 22, and also a break of the second end protruding out of the containment structure 22. This is a consequence of the thermosiphon or heat pipe 60 being a closed path, i.e. sealed at both ends. In contrast, a pipe conducting water to or from the UHS pool can breach containment with a single pipe break.
The approach shown in
Toward this end, with reference to
In the embodiment of
The condensers of
With reference to
The embedding of the heat pipes into a wall or roof of the containment structure can be done in various ways. If the containment is made of steel-reinforced concrete, it may be possible to embed the heat pipes during formation of the concrete. If the containment structure is steel, then one suitable approach is to employ a tubesheet, which can form a portion of the containment wall or roof, and the closed-path heat pipes or closed-path thermosiphons are suitably embedded in the openings of the tubesheet. If the chamber plenum is used to define a controlled volume around the first ends (e.g. plenum chamber 162 surrounding first ends 1601), then the tubesheet can also define part of the plenum chamber. The working fluid for either thermosiphons or heat pipes is suitably water, although other working fluids or fluid mixtures are also contemplated. In general, the working fluid can be tailored to cover the temperature and pressure range expected to be reached in any credible accident scenario. It is also worth noting that the isolation provided by the closed heat transfer pathways disclosed herein is well-suited to accommodating a hydrogen burn inside containment.
The embodiments of
With reference to
On the cold side, piping 280 conveys water from the UHS pool 40R into an inlet of the plenum 263 where it receives heat from the second ends of the closed-path heat pipes or closed-path thermosiphons 260. Heated water or steam flows back to the UHS pool 40R via piping 282 to complete the loop.
The preferred embodiments have been illustrated and described. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
