Patent Application
20090052604
The present invention relates to a temperature detector for a natural circulation boiling water reactor in which coolant is circulated by natural circulation.
In the natural circulation boiling water reactor (simply referred to as “natural circulation reactor” hereinafter) a chimney is provided at an upper end of a core shroud which encloses a core. The mixture flow (two-phase flow) of cooling water and steam (bubbles, also called voids) ascends in the core and the chimney. The cooling water being supplied to the reactor pressure vessel from the feed water pipe and the cooling water that flows out from the chimney are mixed in a circular flow path called downcomer, which is formed between the outer surface of the core shroud and the reactor pressure vessel. The mixed flow descends in the downcomer. Thus, the cooling water circulates inside and outside the core shroud. The natural circulation reactor does not have any forced circulation devices such as a recirculation pump or the like, and the density difference between the density of the two-phase flow inside the core shroud and the density of the cooling water outside the shroud causes the circulation flow.
When the natural circulation reactor is started up from the low pressure sub-critical state, control rods are withdrawn from the core, and the reactor becomes critical state. This process is called as a critical control process. Then reactor power is controlled to a few percent of a rated thermal power by the control rod operation and temperature and pressure of the reactor are at last reached the rated ones. This process is called as a heat-up control process.
Subsequently, in the state that the reactor pressure is kept to be constant, the control rods are withdrawn from the core. As a result, the reactor condition changes from a high pressure, low power state to a high pressure, high power state. It is known that an instability phenomenon called natural circulation instability may occur at the beginning of the heat-up control process during a low pressure, low power state to the high pressure, low power state.
First, the principle of the instability phenomenon in this state will be described. If the boiling starting position of the coolant in the chimney moves to the upstream for some reasons, the amount of the steam in the chimney increases (void fraction increases), and the density of the mixed flow in the chimney is lightened. Thus, the density difference between the core shroud inside and the downcomer increases, and the cooling water flow rate being supplied into the core increases too. When this occurs, the core is more cooled than before and the cooling water temperature decreases at the core outlet. The boiling starting position in the chimney moves to downstream and the amount of generated steam decreases. Thus, the void fraction decreases. As a result, the density difference between the density of the core shroud inside and the downcomer becomes smaller and the cooling water flow rate being supplied into the core decreases.
When this core cooling water flow decreases, the cooling water temperature at the core exit becomes higher and the boiling starting position in the chimney moves to the upstream and the void fraction increases. The density difference between the core shroud inside and the downcomer increases and the cooling water flow rate supplied to the core increases. At low pressure, the density difference between the steam and the cooling water is larger compared to that of high pressure. For example, at 1 atmospheric pressure, the density ratio of water and steam is approximately 1000:1, while at 70 atmospheric pressures, the density ratio is about 20:1. As a result, at low pressure, the change of the natural circulation force due to the void fraction change inside the chimney becomes large. This phenomenon is called natural circulation instability. In this manner, in the heat-up control process of the natural circulation reactor, the boiling starting position in the chimney undulates up and down and the natural circulation instability may occur, in which the core flow rate undulates.
In addition, at the beginning of the start-up control process, because the reactor power is low and the absolute value of the natural circulation flow rate is smaller than that of high pressure, the amplitude of the flow variation becomes relatively larger. Even though the flow instability occurs, fuel rods are not damaged because the reactor power is very low at the heat-up control process. However, the temperature of the cooling water in the core may vary due to the flow variation and that causes nuclear reactivity changes. It may cause the short signal of reactor period which indicates sudden increase in neutron flux. If this signal is detected, the operation of withdrawing control rods cannot be permitted.
A known method for preventing this flow instability is, for example, the technique in Japanese Patent Laid-open No. Sho 59(1984)-143997 of utilizing the heat of boiler used for periodic inspection. At first, the reactor water temperature and pressure are increased by the boiler heat and the high pressure state in which the instability is unlikely to occur is obtained, and next the reactor power is increased. In Japanese Patent Laid-open No. Hei 5(1993)-72387, a method is disclosed in which a pressurizing device is introduced and the natural circulation reactor is started up from the high pressure state in which the instability is unlikely to occur. In both cases the technique is used of increasing power after obtaining high pressure state in which the instability is unlikely to occur. However, in the former case, a large capacity boiler must be provided in order to perform start-up in a short time, while in the latter case a pressurizing device for start-up must be separately provided and this increases construction costs.
Furthermore, Japanese Patent Laid-open No. Hei 8(1996)-94793 describes the technique that the upper portion of the core and lower portion of the chimney are equipped with pressure gauges and thermometers and the saturation temperatures of the upper portion of the core and lower portion of the chimney are calculated from measured pressures and when the core outlet is in saturated condition and the lower portion of the chimney is in the sub-cooling condition, the reactor pressure is forced to reduce or the reactor power is done to increase, and the entire region of the chimney reaches a saturated state and stability is improved. This is based on the knowledge that flow stability is improved if the entire regions in the core and the chimney are in the two-phase flow condition even at low pressure, but when actual start-up is considered, starting up at a high power that makes entire regions of the core and chimney a saturated state is difficult at the beginning of the heat-up control process. In addition, reducing the reactor pressure while reactor pressure is increasing causes start-up time elongation.
The methods for measuring the temperatures inside the reactor pressure vessel include the method in the invention described in Japanese Patent Laid-open No. Hei 8(1996)-94793 in which pressure gauges and thermometers respectively are disposed at the upper portion of the core and lower portion of the chimney, but it is extremely difficult to replace the thermometer when it malfunctions (or is damaged). That is to say, in these structures, if the chimney is not removed from the reactor pressure vessel, replacement of the instrumentation pipe which has a built-in thermometers or signal cables and is disposed at the lower portion of the chimney or upper portion of the core, is not possible. The replacement operation of measuring devices is performed when the fuel assemblies are replaced, or at the time of periodic inspection. But if the replacement operation of the instrumentation pipe can be performed without detaching the chimney from the core shroud, the operation rate as well as economic efficiency of the reactor will be improved.
An object of the present invention is to provide a temperature detection apparatus for boiling water reactor in which replacement of the thermometer inside the reactor pressure vessel is easy when it was damaged or at the time of periodic replacement.
The present invention for attaining the above object is characterized by comprising the chimney having lattices and arranged in the reactor pressure vessel, a temperature detection apparatus for a natural circulation boiling water reactor, in which the replacement of the fuel assemblies is possible between the chimney lattices, and a thermocouple extension wire pulling conduit mounted to the upper end of the chimney lattices. The temperature detection apparatus includes a temperature detection thermocouple and a cable connected to the thermocouple. The thermocouple and the cable are inserted into the thermocouple extension wire pulling conduit.
In another preferable embodiment of the present invention, the thermocouple extension wire pulling conduit is mounted to the upper end portion of the chimney lattice. In another preferable embodiment, the thermocouple and a thermocouple extension wire pulling conduit which have the cable connected to the thermocouple are disposed on the vertical line which is the intersection line of the chimney lattices such that the temperature detection thermocouple is disposed at a suitably selected position on the intersection line of the chimney partition. In this case, the lower end of the chimney portion of the thermocouple extension wire pulling conduit is connected to the neutron instrumentation pipe assembly for neutron flux measurement that is disposed inside the core provided under the chimney lattices.
According to the natural circulation reactor of the present invention, the temperature detection apparatus (for example thermocouple) and the extension wire pulling conduit do not become obstacles at the time of replacement of the fuel assemblies, and replacement of a damaged temperature detection apparatus can be performed relatively easily without detaching the chimney.
The following is a description of an embodiment of the temperature detection apparatus of the natural circulation reactor of the present invention based on the drawings, but prior to that, a general outline of the reactor power control system of the natural circulation reactor using the temperature detection apparatus of this embodiment will be described.
As shown in
Also, the lower portion of the reactor pressure vessel 6 is equipped with a control rod drive mechanism 8 which drives the control rods 3 so as to be inserted in the core 4 in the vertical direction. A main steam pipe 12 and a feed water pipe 13 are connected to the reactor pressure vessel 6. Though it doesn't show in the figure, there is some plants with two or more main steam pipes and feed water pipes according to the power scale. A cylindrical core shroud 5 is disposed so as to enclose the core 4 in the reactor pressure vessel 6. Ascending paths in which the coolant (cooling water) ascends in the direction of the arrow in the drawing is formed inside of the core shroud 5. Downcomer 7 that is a descending path in which the coolant descends is formed in the space between the core shroud 5 and the reactor pressure vessel 6. The cylindrical chimney 9 is disposed at the upper portion of the core shroud 5 and a steam separator 10 and a steam dryer 11 are provided at the upper side of the chimney 9.
The coolant of two-phase flow including gas and liquid that is boiled in the core 4 passes through the inside of the chimney 9. The coolant descends the downcomer 7 due to the density difference between the two-phase flow in the shroud 5 and liquid flow passing through the downcomer 7. The coolant in the downcomer 7 is introduced to the core 4 and ascends in the chimney 9. A circulation path having the ascending path formed in the core 4 and the chimney 7 and the descending path formed in the downcomer 7 is formed in the reactor pressure vessel 6. When the mixture of cooling water and steam that ascends in the chimney 9 passes through the steam separator 10, the steam is separated from the mixture by the steam separator 10. The cooling water separated at the steam separator 10 descends down the downcomer 7 and passes the lower portion of the reactor pressure vessel 6 and is supplied into the core 4 inside the core shroud 5.
In the steam dryer 11, the tiny water droplets are removed from the steam that came from the steam separator 10. Then the steam is supplied to the turbine 18 via the main steam pipe 12. The turbine 18 and the generator 21 connected thereto are rotated by this steam flow and power is generated.
The steam that rotated the turbine 18 is led to the condenser 23 and becomes condensed water. The condensed water is returned to the reactor pressure vessel 6 through a feed water pipe 13 by the feed water pump 24. The feed water pipe 13 has a flow rate adjusting valve 25. By adjusting the feed water flow rate by the flow rate adjusting valve 25, the reactor water level in the reactor pressure vessel 6 can be controlled. The feed water pipe 13 also has feed water heaters 26. The steam extracted at a middle stage in the turbine 18 is introduced to the feed water heater 26 via the extraction pipe 22. At the feed water heater 26, the cooling water introduced from the condenser 23 is heated to a suitable temperature and injected into the reactor pressure vessel 6.
The main steam pipe 12 has a main steam isolation valve 27 and a steam flow adjusting valve 28 which adjusts the amount of steam that is introduced into the turbine 18. A relief pipe 29 having a safety valve and a bypass pipe 30 having a turbine bypass valve 31 are connected to the main steam pipe 12. When the turbine steam flow adjusting valve 28 is closed, the turbine bypass valve 31 is opened. Thus, some steam is directly introduced into the condenser 23 via the bypass pipe 30 without any of the steam being introduced into the turbine 18. When the main steam isolation valve 27 is closed, the safety valve 32 is opened. As a result, the steam generated in the reactor pressure vessel 6 is led into a pressure suppression pool (not shown) in the containment vessel and the steam is condensed in the pressure suppression pool.
In this embodiment of the present invention, the upper portion of the chimney 9 in the reactor pressure vessel 6 has a temperature detection section (temperature detection apparatus) 37 of the gas-liquid mixed flow and a pressure detection section 38 to measure the pressure of the liquid. The temperature and pressure signals that are detected here are transferred to the temperature measuring apparatus 39 and the pressure measuring apparatus 40 respectively. The temperature measuring apparatus 39 and the pressure measuring apparatus 40 convert the electrical signals to actual temperature and pressure units and output the converted electrical signals to the reactor power control apparatus 35. The reactor power control apparatus 35 incorporates an controller to control the reactor power not to occur the natural circulation instability by using the temperature detection section 37. The reactor power control apparatus 35 generates control rod operation signals to realize stable reactor operation and outputs the signals to the control rod drive control apparatus 36. The control rod drive control apparatus 36 controls the control rod drive mechanism 8 having an electric motor or a hydraulic piston which drives the control rods 3.
A display apparatus 43 is also connected to the reactor power control apparatus 35. This display apparatus 43 displays information relating to coolant temperature of the chimney 9 and the stable boundary temperature at which the flow instability will occur, on the same screen. Thus, the reactor operator can look at this display screen and confirm the stability of the operation state of the reactor. The temperature of the coolant in the chimney 9 is an important information to keep the flow state in the reactor pressure vessel 6 stable at the start-up operation.
Prior to describing an example of an embodiment of this temperature detection apparatus of the present invention, the coolant flow of the reactor pressure vessel 6 and the temperature in this embodiment will be described.
The saturation temperature of 2 atmospheric pressures is approximately 120° C., and sub-cooling temperature of cooling water (difference between the saturation temperature and the cooling water temperature) at 95° C. is 25° C. The cooling water supplied to the core 4 from the lower plenum 6a is warmed at the core 4 (c-d at the right of
At the beginning of the heat-up control process at the low pressure and low rector power state, the natural circulation instability may occur due to the boiling starting position movement in the chimney 9. The saturation temperature at which boiling begins can be easily obtained using a steam-water property chart based on the pressure, therefore, if the cooling water temperature and pressure at the upper end of the chimney 9 are measured, it is possible to determine that the boiling is occurring at the chimney upper end. If periodic temperature change is observed in the chimney upper end, it can be confirmed that the natural circulation instability occurs. If there is little or no boiling at the chimney upper end at start-up, a stable flow state is obtained. Thus, if the temperature at the outlet of the chimney 9 is measured, generation of natural circulation instability can be confirmed and the reactor power can be controlled so as to prevent instability generation. It is to be noted that there is a temperature distribution with respect to the radial direction position due to the power difference between the fuel assemblies 2 in the core outlet in the core shroud 5. One portion of the chimney 9 divided by partitions usually corresponds to a plurality of the fuel assemblies. At the lower end of the chimney 9, the coolants with different temperatures exhausted from the each fuel assembly 2 are in a state of beginning of mixing, thus there are large temperature variations according to the temperature measurement position in the horizontal plane. Meanwhile, at the upper end of the chimney 9, mixing of the coolants with different temperatures has developed and variations in the coolant temperature due to temperature measurement position and time become less. Thus, in the case where the temperature in the chimney is measured, measurement at the chimney upper portion which has comparatively little cooling water temperature variation is suitable.
Because the fuel assembly 2 must be taken out from the core 4 by using a fuel exchange apparatus (not shown), the position for installation of the temperature detection section 37 and the pressure detection section 38 must be such that they do not become obstacles when the operation of the fuel exchange is performed. That is to say, in the natural circulation reactor, when the operation of periodic exchange of the fuel assemblies 2 is performed, the fuel assembly 2 is usually taken out through the chimney 9 and it is suitable for the temperature detection section 37 and the pressure detection section 38 to be at position where they do not become obstacles at the time of this operation such as at the upper portion of the chimney partition (not shown), or at a position adjacent thereto.
The fuel assemblies 2 are disposed in the core 4 inside the core shroud 5 and under the chimney 9. When viewed from the upper side of the chimney 9, as shown in
The thermocouple extension wire pulling conduit 52 for inserting the thermocouples 51a-51c (together called thermocouple 51 hereinafter) which function as the temperature detection section 37 are disposed at the upper portion of the chimney partition wall 50. The plurality of thermocouple extension wire pulling conduits 52 for the thermocouple 51 are usually provided so as to correspond to the number of thermocouples 51. In
In the example shown in
By forming the chimney partition wall 50 and the thermocouple extension wire pulling conduit 52 described above, an upper end 50a of the other chimney partition wall 50 and the upper end of the thermocouple extension wire pulling conduit assembly 55 are almost flush and thus there is no need to subject the support bracket 52 to special bending processing, and as shown in the drawing, and the support bracket 52 can be fixed to the upper end 50a of the chimney partition wall 50 using a fixing piece such as a bolt 54 or the like.
In the example shown in
The placement of the thermocouple extension wire pulling conduit 60 and the method for retrieving the signal will be described based on
Generally, in the natural circulation reactor, as shown in
The neutron instrumentation pipe assembly 70 shown in
According to the second embodiment shown in
The method to form the insertion hole of the thermocouple extension wire pulling conduit 60 shown in
According to this example, the support bracket 64 can be detached without detaching the chimney and the thermocouple and the thermocouple extension wire pulling conduit 60 which form the temperature detection section 63 can be replaced.
It is to be noted that unlike the coolant temperature for which distribution in the radial direction occurs due to difference in thermal power of the fuel assembly, the pressure distribution in the radial direction at the same height in the chimney 9 is small, therefore the pressure inside of the chimney 9 can be measured by installing the detection section for the pressure conduit at an outer peripheral wall of the chimney 9 which contacts the downcomer 7. An example of measurement methods is to introduce the coolant from the upper portion of the chimney 9 to the pressure gauge through the hole formed in the reactor pressure vessel 6. Another example of measurement methods is connecting the chimney upper portion and the steam dome with a differential conduit and connecting a differential pressure gauge to the differential conduit and then measuring the pressure difference between the steam dome pressure which measures the absolute pressure. As is the case with the thermocouple extension wire pulling conduit which measures temperature, a differential conduit can be disposed on the upper portion of the chimney 9 to measure the pressure of the upper portion. In this case, the thermocouple extension wire pulling conduit 60 and the differential conduit may be stored in a common thermocouple extension wire pulling conduit assembly 55.
Embodiments of the present invention were described above, the present invention is not to be limited by the examples of the embodiments above and various other embodiments may be included in the present invention without departing from the spirit of the present invention described in the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-050917 | Feb 2006 | JP | national |
This application is a continuation of U.S. application Ser. No. 11/678,740, filed Feb. 26, 2007, the contents of which are incorporated herein by reference. The present application claims priority from Japanese application serial no. 2006-050917, filed on Feb. 27, 2006, the content of which is hereby incorporated by reference into this application.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11678740 | Feb 2007 | US |
| Child | 12250762 | US |
