Patent ID: 12243661

DETAILED DESCRIPTION

FIG.1is a schematic illustration of a nuclear reactor including a pressure vessel10. The pressure vessel10contains a nuclear reactor core11(shown in phantom) disposed at or near the bottom of the pressure vessel10and immersed in primary coolant water also disposed in the pressure vessel10. The pressure vessel10further contains numerous internal components that are not shown inFIG.1but which are known in the art, such as structures defining a primary coolant flow circuit, e.g. a hollow cylindrical central riser defining a hot leg inside the riser and a cold leg in a downcomer annulus (e.g., flow region) defined between the central riser and the pressure vessel10, and neutron-absorbing control rods and associated drive mechanisms for controlling reactivity of the nuclear reactor core. Some embodiments, e.g. integral pressurized water reactor (PWR) designs, also include one or more steam generators disposed inside the pressure vessel, typically in the downcomer annulus.

A reactor coolant inventory and purification system (RCIPS)12is provided to maintain the quantity and purity of primary coolant inside the pressure vessel. A letdown line14removes primary coolant water from the pressure vessel10into the RCIPS12, and a makeup line16delivers makeup primary coolant water from the RCIPS12to the pressure vessel10. The RCIPS12includes a pump17and other water processing components (not shown) for purifying and storing reserve primary coolant, injecting optional additives such as a soluble boron compound (a type of neutron poison optionally used to trim the reactivity), or so forth. Isolation valves20,21are provided at respective vessel penetration locations where the letdown line14and makeup line16, respectively, pass through an outer wall18of the pressure vessel10. During ordinary operation, makeup water flows into, and/or letdown water flows out of, the pressure vessel10through the letdown line14and makeup line16to maintain desired operating volume and composition (e.g, purity) of the primary coolant water in the pressure vessel10. However, if a break occurs in one of the fluid flow lines14,16, or elsewhere, such that a LOCA is initiated and uncontrolled primary coolant water discharge might occur, then flow of coolant out of the pressure vessel10is automatically blocked by the affected valve20,21.

With reference toFIG.2, an exemplary letdown isolation valve assembly20includes an isolation valve vessel (IVV) with a small pressure boundary containing redundant isolation valves. The pressure boundary is designed to withstand operating pressure and temperature conditions of primary coolant inside the pressure vessel10. The isolation valve vessel is mounted to the side of the lower vessel with a flanged arrangement32, which in the illustrative example is a spool piece32. As used herein, a spool piece includes two flanges connected by piping or another passageway. The spool piece is rated to withstand the operating pressure of the pressure vessel10, and in some embodiments the spool piece32is a forging. One flange of the spool piece32is connected with a mating flange of the pressure vessel10to connect the isolation valve assembly20directly to the wall18of the pressure vessel10. The other flange of the spool piece32is connected with a flanged open end of the isolation valve vessel to define a sealed volume. Any leakage at the valves is contained within this sealed volume.

With additional reference toFIG.3, the details of the exemplary isolation valve assembly20in accordance with the disclosure will be described. The illustrated valve assembly20is a letdown isolation valve that can be used to control the flow of fluid out of the reactor core. However, it will be appreciated that the valve20could also be installed on a makeup line for adding fluid to the reactor core, or in another fluid line feeding into and/or out of the pressure vessel10.

The valve20includes the spool piece32and an isolation valve vessel34secured together via a mating flange36at a (single) open end of the isolation valve vessel34and a flange38of the spool piece32. The spool piece32also includes a mounting flange42having a centrally located inlet/outlet44and a plurality of bolt holes surrounding the inlet/outlet44for securing the valve assembly20to a mating flange48of a pressure vessel, such as pressure vessel10. Thus, the spool piece32includes a first flange (namely the mounting flange42) and a second flange (namely the flange38that connects with the isolation valve vessel34). The spool piece32further includes a passageway46connecting the first and second flanges42,38. In the illustrative example, the mounting flange42is spaced apart from the flange38and connected by the passageway46which is a reduced diameter section. The isolation valve vessel34includes a hemispherical or elliptical head52(e.g., a valve cover) having flange36which connects with the flange38of the spool piece32. The connection of the isolation valve vessel34and the flange36defines a sealed volume contained by the isolation valve vessel34. A fluid flow line54includes a “U”-shaped portion disposed inside the isolation valve vessel34and then continues on coaxially inside the spool piece32to flow fluid into or out of the flange42. In the illustrative example of letdown valve assembly20, fluid flows from the pressure vessel10through the fluid flow line54and into the letdown line14(seeFIG.1) to reduce the quantity of primary coolant in the pressure vessel10.

When the letdown valve assembly20is mounted to pressure vessel10, the inlet/outlet44serves as an inlet that is in fluid communication with the interior of the pressure vessel10such that primary coolant can flow from the pressure vessel10through the letdown valve assembly20via valve fluid line54to an inlet/outlet56of the valve assembly20. In the illustrative case of letdown valve assembly20, the inlet/outlet56serves as an outlet that is connected to the letdown line14of the RCIPS12. The illustrated “U”-shaped portion of the fluid flow line54inside the isolation valve vessel34advantagely accommodates thermal expansion.

Isolation valve vessel34together with the flange38define a sealed interior volume or chamber C in which a pair of valves60and62are supported. (In view of this, the hemispherical or elliptical head52is alternatively referred to herein as valve cover52). In the illustrative example of letdown valve assembly20which is configured for a letdown application, the valves60and62are suitably actuated valves which are opened (or closed) by an actuation signal. Typically, it is preferable to have the valves60,62be “normally closed” valves such that the actuation signal causes the valves to open so that the valves are closed in the passive state, although a “normally open” configuration is also contemplated. In some embodiments the valves60,62are pneumatically actuated ball valves, although valves employing electrical, hydraulic, or manual actuation are also contemplated, as are valves other than ball valves.

In the makeup valve configuration (e.g., the makeup valve assembly21ofFIG.1), the valves60and62can be swing check valves or another type of check valve, which is configured to prevent fluid flow into the flange42(i.e., configured to prevent flow of primary coolant out of the pressure vessel10). The valves60and62are arranged in series for redundancy, and it will be appreciated that additional valves, or a single valve, could be provided in the chamber C as desired. The isolation valve vessel34optionally includes various penetrations for the plant instrument air system to pressurize the chamber C for vessel leak testing, and for air lines64for piloting/actuating the pneumatic actuators in case of pneumatically actuated valves.

An optional internal support structure68is secured to flange38to support the actuated valves60and62(or to support the check valves in the case of makeup isolation valve assembly21). The support structure68optionally also serves as a mechanical guide for installing the valve cover52so that it does not impact any internal components (e.g., valves and/or actuators, etc.) when it is removed and/or installed to allow maintenance access. Thermal insulation, although not illustrated, can be provided and its location will depend on the design of the actuator and/or position indicators. If high temperature actuators are utilized, the insulation can be placed on the outside of the support structure68and cover52. If actuator temperature limitations prevent such positioning of the insulation, multi-layer metal insulation can be provided on the piping and a component cooling water line can be added to actively cool the valve20to assure acceptable temperatures. The support structure68is optional—in some embodiments the “U” shaped portion of the fluid flow line54has sufficient rigidity to support the valves60,62.

In the illustrated embodiment, an optional third isolation valve70disposed outside of the chamber C is provided to isolate the valve fluid line54in the event of a pipe break inside of the isolation valve vessel34. The external valve70can be pneumatically operated, for example, and configured to close the valve fluid line54in the event of a leak within the valve20. The third isolation valve70can be used, for example, to block flow through the valve fluid line in the event the other valves are disabled due to flooding of the chamber C during an internal pipe break and/or leakage event. Third isolation valve70provides a level of redundancy.

Turning toFIGS.4and5, another exemplary isolation valve assembly100in accordance with the disclosure is illustrated. In this embodiment, the valve assembly100is similar to the valve assembly20ofFIGS.2and3. However, the valve assembly100has valves supported by the “U”-shaped portion of the fluid flow line (i.e., the support structure68is omitted), and valve actuators are mounted external to the pressure vessel. To this end, the valve100generally includes a spool piece104and an isolation valve vessel108comprising a valve cover112including a flange116that is removably secured to a mating flange120of the spool piece104with bolts or other fasteners (not shown). The valve assembly100is mountable to a pressure vessel of a nuclear reactor or other component via a mounting flange124of the spool piece104that is axially spaced from flange120of the spool piece104by a passageway122. A fluid flow line128fluidly connects an inlet/outlet (not shown) of the mounting flange124with an inlet/outlet132.

As with valve assembly20, the valve assembly100includes an interior chamber C formed by the valve cover112and the flange120secured to the flange120of the spool assembly104, and a pair of valves140and142are supported inside the chamber C. Valves140and142are supported by valve fluid line128and are arranged in series for redundantly blocking flow through the valve fluid line128.

In embodiment ofFIGS.4and5, externally mounted valve actuators146and148are provided for actuating valves140and142. To this end, the actuators146and148are mounted to respective actuator flanges152and154on the valve cover112with bolts or other suitable fasteners (not shown). A connecting shaft156(seeFIG.5) extends from the valves140through the valve cover112for coupling with the actuator146. In one embodiment having ball valves, rotation of the connecting shaft156by the actuator146moves a ball of the valve140between respective open and closed positions. Valves142includes a similar configuration, although its connecting shaft is not visible in the drawings.

This configuration places the actuators146,148outside of the relatively harsh environment of the chamber C, and therefore can increase component longevity and/or allow the use of conventional actuators. This generally simplifies the design and potentially eliminates the need for thermal insulation inside the pressure vessel. The connecting shafts for connecting the actuators to the valve member introduce the potential for some leakage around the connecting shafts, but leakage up to several gallons per minute or more can be accommodated while still achieving acceptable performance. As an alternative approach, a wireless actuation signal is also contemplated, which would eliminate the penetrations through the valve cover112.

The isolation valve vessel of the present disclosure provides isolation for any pipe break of the makeup or letdown lines, assuming any active component failure. The makeup lines with check valves will automatically close if flow reverses, isolating the LOCA. The letdown lines require closure of the ball valves which is effected via the pneumatic actuators and occurs on a low RCS pressure signal.

Elimination of the low break LOCA simplifies design basis accident analysis and eliminates sump recirculation after a LOCA. The valves in the vessel would isolate the broken line and long term makeup and letdown would continue using the non-effected lines. Because of the limited volume of the vessel, the amount of debris that can flow into the RCS is significantly limited, reducing concerns of debris plugging of flow passages in the fuel assemblies.

It will now be appreciated that the present disclosure provides at least one or more of the following advantages:

1. Eliminates the two separate valve rooms used in conventional reactors.

2. Eliminates the Type 1 LOCA described above. Type 1 LOCA is generally considered the most difficult type of failure in which to provide long term cooling because most of the water spills on the refueling cavity floor. The RWST level drops to approximately 8 ft above the lower vessel penetrations minimizing the driving head to inject water.

3. The higher driving head allows greater flexibility in automatic depressurization valve sizing because very low differential pressures (e.g., less than 5 psi) are not required for long term injection.

4. During long-term cooling, there is a potential for water to flow through the break back into the reactor vessel. The invention limits the water that can flow back into the vessel and, because it is a closed structure, limits the amount of fibrous debris that can be mixed with the water.

5. By eliminating the Type 1 LOCA and its low passive injection pressure, the ADV and upper vessel penetration sizes may be reduced, making any upper breaks more benign.

6. The vessel reduces the length of ASME Class I piping.

The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.