Emergency coolant accumulator for nuclear reactor

Abstract: An emergency reactor coolant comprising means for preventing the flow of accumulated gas into a reactor system. If an accident occurs in the reactor system and the pressure of the reactor system decreases, accumulated water (19) in the accumulator body (11), which is pressurized by accumulated gas (18), is injected to the reactor system through an accumulated water outlet line (12) via a check valve (13) to cool the reactor core, and at the same time, the accumulated water (19) in the accumulated body (11) is introduced into a depressurizing vortex damper body (21) through a lower inlet (21c) of the damper (21) in the tangential direction to form a vortex. The increase in resistance due to the vortex reduces the amount of water discharged from the accumulator body (11) to a gas release line (22) via the depressurizing vortex damper (21). When the water level of accumulated water (19) drops below the lower inlet (21c) of the depressurizing vortex damper (21), the accumulated gas (18) enters into the depressurizing vortex damper body (21) through a top inlet (21b) and the lower inlet (21c) and is released from the reactor system through a gas outlet (21d) and the gas release line (22).

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT This invention generally relates to an emergency reactor coolant accumulator for an emergency reactor core cooling system in a nuclear power plant and more particularly to an emergency reactor coolant accumulator typically used in a pressurized light water reactor (PWR). A typical emergency reactor core cooling system conventionally used includes various kinds of hydraulic lines and components such as valves, borated water storage tanks, pipes, pumps, spray nozzles and so on. An emergency reactor coolant accumulator in which an accumulator tank contains a borated water under pressurization by a compressed gas such as nitrogen gas is one of the most familiar components included in the emergency core cooling system before-mentioned. A typical accumulator is generally shown in Fig.10. In the drawing, a tank 1 communicates with a primary reactor cooling loop (not shown) through a water injection line 2 provided with a check valve 3. The accumulator tank 1 also communicates, at an upper region, with a compressed gas supply source (not shown) through a gas inlet line 4 provided with an on-off valve 5. A compressed gas such as nitrogen is introduced into a gas plenum 8 at an upper region when the valve 5 is open, while the valve 5 is kept closed in the normal operation of a nuclear power plant thereby maintaining a pressurized condition of a predetermined pressure in the tank 1. On the other hand, an emergency reactor coolant such as a borated water is supplied into the tank 1 through a water inlet line 6 when a supply valve 7 is opened and an accumulated borated water 9 is held under such pressurized condition as before-described in the normal operation of the nuclear plant. In the normal operating condition, a pressure in the accumulator tank 1 is maintained at a predetermined degree lower than that in the primary reactor cooling loop thereby keeping the check valve 3 in a closed condition. If a leakage or breakdown occurs in the primary reactor cooling loop, for example, as a credible accident, a pressure in the primary cooling loop decreases thereby causing the check valve 3 to open. Under such an accidental condition, the accumulated water 9 in the tank 1 is driven, under the pressure of the compressed nitrogen gas, into the primary cooling loop through the water injection line 12 so as to cool a nuclear core in a reactor vessel communicating with the primary cooling loop. Accordingly, as the accumulated water 9 continues to flow out of the tank in such a manner, the level of the water 9 comes down with the expansion of the gas and a decrease in the pressure thereof. It is desirable to keep the pressure of the gas as high as possible so as to inject a remaining water into the loop without delay when the level of the water 9 comes down near the bottom of the tank 1 and the inlet of the injection line 2. There is a risk, however, in which some of the gas flows out together with the water thereby giving undesirable effects to the cooling of the nuclear core. OBJECT AND SUMMARY OF THE INVENTION An object of this invention is to provide an emergency reactor coolant accumulator in which all of the accumulated reactor coolant is surely supplied into a primary reactor cooling loop without accompanying gas. Another object of this invention is to provide an emergency reactor coolant accumulator which can accommodate an emergency reactor coolant under a relatively higher pressure and release an accumulated gas at a good timing in a last stage of an injection of the reactor coolant. Another object of this invention is to provide an emergency reactor coolant accumulator in which a compressed gas accumulated can be released at the best timing predetermined without any delay and/or premature release. With said object in view, according to this invention, there is provided an emergency reactor coolant accumulator as set out in Claim 1. Thus an embodiment of the invention may include an accumulator tank capable of accommodating a reactor coolant such as a borated water as well as a compressed gas such as nitrogen gas and have a gas damper which can be opened at a predetermined timing to as to release an accumulated gas out of the tank. The gas damper may be opened in a last stage of the injection of the borated water into a primary reactor cooling loop so as to release the accumulated gas out of the tank thereby preventing the gas from flowing into the cooling loop together with the water. Owing to the gas damper, the initial pressure of the accumulated gas can be set at a relatively higher value so as to surely purge the borated water into the primary reactor cooling loop and supply it to the nuclear core. According to a preferred embodiment of this invention, a vortex fluid diode is employed as the gas damper. As appreciated by those skilled in the art, the vortex fluid diode has a casing defining a vortex chamber therein, a central opening communicating with a gas release line arranged out of the tank, a gas inlet open above a level of the water through vertical pipe and a water inlet usually open below the water level. When the water inlet is below the water level, water flows into the casing through the water inlet and a vortex flow of the water closes the gas inlet. On the other hand, when the level comes down below the water inlet as the injection of the water progresses, no water flows into the casing thereby communicating the gas inlet with the central opening. Thus, the accumulated gas is released at the predetermined height of the level without any control unit or other means. Further preferred features are set out in the dependent claims. BRIEF DESCRIPTION OF THE DRAWING Fig.1 is a schematic view of a nuclear reactor system to which this invention is applied, Fig.2 is a longitudinal sectional view of an embodiment of accumulator of this invention, Fig.3 is a side view of a depressurizing vortex damper, Fig.4 is a front view of a depressurizing vortex damper, Fig.5 is a front sectional view of a depressurizing vortex damper showing its operation in case where accumulated water flows into it, Fig.6 is a front sectional view of a depressurizing vortex damper showing its operation in case where accumulated gas flows into it, Fig.7 is a graph for illustrating the operation of the accumulator shown in Fig.2, Fig.8 is a longitudinal sectional view of another embodiment of accumulator of this invention, Fig.9 is a graph for illustrating the operation of the accumulator shown in Fig.8, and Fig.10 is a longitudinal sectional view of an example of conventional accumulator. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The embodiments of this invention will be described with reference to the drawings. Fig.1 schematically shows a reactor core cooling system (RCS) 30 for a pressurized light water reactor including an emergency reactor coolant accumulator (ACC) 10 according to this invention. A nuclear core 32 in a reactor vessel 31 is cooled by a reactor coolant flowing through a cold leg pipe 34 via a reactor coolant pump 33. The reactor coolant flowing from a coolant nozzle goes down through a down comer 35, turns upward at a lower plenum 36, and flows into the nuclear core 32, where the reactor coolant cools the nuclear core 32. Then, the reactor coolant flows to a hot leg pipe 37. The reactor coolant, which flows from the hot leg pipe 37 to a steam generator 38, passes many heat transfer tube 39 to heat the feed water 40 supplied as a shell side fluid. Then, the reactor coolant flows to a pipe 41 and returns to the reactor coolant pump 33. During the operation of the reactor, the reactor coolant flows as described above. The pressure for this flow is appropriately maintained usually by a pressurizer 43 having a heater and a spray nozzle. The coolant injection pipe 12 attached to the bottom of a tank 11 of an accumulator 10 is connected to the cold leg pipe 34 of the primary reactor cooling loop via a check valve 13. A high-pressure injection system 45 and a low-pressure injection system 46, which compose a multiple emergency cooling system, are also connected to the coolant injection pipe 12. The high-pressure injection system 45 and the low-pressure injection system 46 have a high-pressure injection pump 48 and a low-pressure injection pump 49, respectively, and are connected to a not illustrated emergency cooling water storage tank. The emergency coolant in the tank 11 and the emergency cooling water in the storage tank are usually light water, being injected in the event of loss of coolant accident as described above. The reactor coolant is also light water. Fig.2 is an expanded view showing the construction of the accumulator 10. The accumulator of this embodiment will be described with reference to Figs.2 through 6. Reference numeral 11 denotes an accumulator body, 12 denotes an accumulated water outlet line or coolant injection pipe extending to a reactor system (not shown), 13 denotes a check valve installed halfway in the accumulated water outlet line 12, 14 denotes a gas inlet line, 15 denotes a gas inlet valve installed halfway in the gas inlet line 14, 16 denotes an accumulated water inlet line, 17 denotes an accumulated water inlet valve installed halfway in the accumulated water inlet line, 18 denotes an accumulated gas (pressurized gas, such as nitrogen gas) in the accumulator body 11, and 19 denotes an accumulated water in the accumulator body 11. Reference numeral 21 denotes a depressurizing vortex damper, which is most characteristic of this invention. The damper 21 is installed in the accumulator body 11. The depressurizing vortex damper 21 comprises a depressurizing vortex damper body 21a, a top inlet 21b, a lower inlet 21c, and a gas outlet 21d. Reference numeral 22 denotes a gas release line connected to the gas outlet 21d of the depressurizing vortex damper. Gas release valves 23 are disposed in parallel branch lines installed halfway in the gas release line 22. Next, the operation of the accumulator shown in Figs.2 through 6 will be described. The accumulator body 11 contains the accumulated gas 18 and the accumulated water 19. Comparing the pressure in this accumulator body 11 with that of the reactor system, the latter is higher, and the accumulator is isolated from the reactor system by the check valve 13. If the pressure of the reactor system decreases in the event of accident of reactor system, the accumulated water in the accumulator body 11, which is pressurized by the accumulated gas 18, is injected to the reactor system through the accumulated water outlet line 12 via the check valve 13, thereby the reactor core being cooled. In this process, the accumulated water 19 in the accumulator body 11 enters the depressurizing vortex damper body 21a through the lower inlet 21c in the tangential direction as indicated by the arrows of solid line in Fig.5, by which a vortex is formed. The increase in resistance due to the vortex reduces the quantity of water discharged from the accumulator body 11 to the gas release line 22 through the depressurizing vortex damper 21, whereas there is substantially no release of accumulated gas. The discharged water is ineffective; therefore, design is made so as to minimize the quantity of this discharged water. The accumulated gas 18 in the accumulator body 11 expands and its function ends as the injection of the accumulated water 19 proceeds. The level of accumulated water 19 lowers below the lower inlet 21c of the depressurizing vortex damper 21 as shown in Fig.6. Under this condition, the accumulated gas 18 enters into the depressurizing vortex damper body 21a through the top inlet 21b and the lower inlet 21c. The gases entering through the two inlets join and are released out of the reactor system through the gas outlet 21d and the gas release line 22. In the embodiment shown in Fig.2, the core injection characteristic of the accumulated water 19 changes at the time when the water level of accumulated water 19 comes down to the lower inlet 21c of the depressurizing vortex damper 21 (refer to Fig.7). Fig.8 shows another embodiment of this invention. As seen from the figure, the constitution of the reactor core cooling system (RCS) is the same as that described in the above embodiment; therefore, the description thereof is omitted. The accumulator (ACC) 50 of this embodiment has the same construction as that of the accumulator 10 shown in Figs.1 and 2, except that the accumulator 50 has a vortex diode 51 at the inlet portion of the coolant injection pipe 12. Like reference numerals designate like parts, and the description thereof is omitted. In Fig.8, which is an expanded view showing the construction of the accumulator 50, the central outlet pipe 51d of a casing 51a of the vortex diode 51 is connected to the inlet of the injection pipe 12, and a rise pipe 51b extending from the casing 51a is open at a position above the gas damper 21. A vortex fluid inlet 51c is open at a relatively low portion. This vortex diode 51 provides a multi-stage injection characteristic of emergency coolant i.e. borated or accumulated water 19 to meet the requirement described below. For example, if a loss of coolant accident occurs, a large amount of borated water 19 must be injected at the start of injection to cover the exposed nuclear core 32 with water again; however, after the nuclear core has been covered with water, it is sometimes required to supply borated water at a low flow rate for a long period of time to replenish the coolant to provide against the leakage of water at a broken place. The operation of the accumulator 50 will be described with reference to Figs.8 and 9. In the normal operation of reactor, the borated water 19 supplied from the water inlet line 16 maintains a predetermined water level, whereas the nitrogen gas supplied from the gas inlet line 14 to a gas plenum 18 keeps the pressure at a predetermined value. At this time, the release valves 23 on the gas release line 22 are closed. Under this condition, for example, if a break occurs on the hot leg pipe 37, the coolant flows out from that portion, resulting in a decrease in pressure of the reactor core cooling system 30; the pressure becomes lower than that of the tank 11. The change of pressure difference opens the check valve 13, so that the borated water 19 is injected to the cold leg pipe 34 through the injection pipe 12 to cool the nuclear core 32. At this time, the gas release valves 23 are open. However, the water flowing through the lower inlet 21c of the gas damper 21, which is a vortex fluid diode, produces a vortex in the casing 21a, cutting off the communication between the gas release line 22 and the vertical pipe 21b. In the vortex diode 51, the borated water flows into the vortex diode through both the rise pipe 51b and the vortex fluid inlet 51c; therefore, no vortex occurs in the casing 51a, resulting in a water flow of a relatively high flow rate. Fig.9 shows an example of the change in pressure P₁ in the accumulator, pressure P₂ in the reactor core cooling system, and the injection flow rate Q of borated water 19 to the reactor core cooling system. The before-described condition is designated by Stage I in Fig.9. When the water level of borated water 19 lowers below the top opening of the rise pipe 51b, there is no water flowing into the vortex diode 51 through the rise pipe 51b; water enters into the vortex diode 51 through the vortex fluid inlet 51c only. Therefore, a vortex occurs in the casing 51a, so that the resistance due to the vortex increases, resulting in low injection flow rate Q. The condition of the gas damper 21 does not change; only a small amount of water flows through the gas release line 22, and nitrogen gas is not released. The vortex fluid diode used as the gas damper 21 is designed so that the quantity of discharged water is minimized. This condition is designated by Stage II. When the water level of the borated water 19 lowers below the lower inlet 21c of vortex fluid diode 21, no vortex of water occurs as described above, so that the vertical pipe 21b communicates with the gas release line 22. Therefore, the compressed nitrogen gas 18 in the tank 11 begins to be released through the gas release line 22, promoting the decrease in internal pressure P₁. As a result, the difference between internal pressure P₁ and pressure P₂ in the reactor core cooling system decreases; as a result, the force for pushing out the borated heater 19 decreases and in turn the injection flow rate Q decreases further. This condition is indicated by Stage III in Fig.9. Thus, when the remainder of borated water 19 flows from the vortex diode 51, the gas pressure decreases, thereby the gas not flowing into the injection pipe 12 together with the water. In the embodiment shown in Fig.8, an outlet vortex damper 51 is disposed at the lower part in the accumulator body 11. When the water level of accumulated water 19 comes down to the lower inlet 51c of the outlet vortex damper 51 or when it comes down to the lower inlet 21c of the depressurizing vortex damper 21, the core injection characteristic of the accumulated water 19 changes. Even if the gas release valves 23 are opened due to misoperation or failure of equipment, a vortex occurs, so that the increase in resistance reduces the quantity of water discharging from the depressurizing vortex damper 21 to the gas release line 22. Therefore, the check of failure and the closing of valve can be performed while replenishing the water of discharged amount without stopping the plant. Thus, the operation rate of plant is enhanced. In the accumulator according to this invention, if an accident occurs in the reactor system and the pressure of the reactor system decreases, the accumulated water in the accumulated body, which is pressurized by the accumulated gas, is injected to the reactor system through the accumulated water outlet line via the check valve to cool the reactor core, and at the same time, the accumulated water in the accumulated body is introduced into the depressurizing vortex damper body through the lower inlet of the damper in the tangential direction to form a vortex. The increase in resistance due to the vortex reduces the quantity of water discharged from the accumulator body to the gas release line via the depressurizing vortex damper. When the water level of accumulated water lowers below the lower inlet of the depressurizing vortex damper, the accumulated gas enters into the depressurizing vortex damper body through the top inlet and the lower inlet and is released out of the reactor system through the gas outlet and the gas release line. Therefore, the accumulator can be depressurized at an appropriate timing, thereby preventing the flow of accumulated gas into the reactor system.

Claims: An emergency reactor coolant accumulator (10) including an accumulator tank (11) comprising a coolant injection pipe (12) located at a bottom of the tank (11), a gas inlet (14) for introducing compressed gas into the tank (11), and a reactor coolant inlet (16) for introducing a reactor coolant into the tank (11), said injection pipe (12) being provided with a check valve (13) and connected to a primary reactor cooling loop (33-41), characterized in that the accumulator tank (11) has a gas damper (21) therein communicating with a gas release line (22) arranged outside the tank (11). An emergency reactor coolant accumulator as defined in Claim 1 wherein the gas damper (21) is a vortex fluid diode. An emergency reactor coolant accumulator as defined in Claim 1, characterized in that the accumulator tank (11) further includes a vortex fluid diode (51) having a vertical pipe (51b) open to an upper interior of the tank and a lower inlet (51c) and communicating with the coolant injection pipe (12).

Inventors: MATSUOKA TSUYOSHI, MATSUOKA, TSUYOSHI
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1992
Main classification: G21C15
IPC codes: G21D1, G21C15, G21C1