Nuclear reactor protection device and nuclear reactor protection method for fast reactor

Abstract

A nuclear reactor protection device 100 for a fast reactor includes: a temperature measurement unit 131 that measures the temperature of a primary coolant of a nuclear reactor 1 of the fast reactor; a calculation unit 132 that calculates the change rate of the temperature of the primary coolant measured by the temperature measurement unit 131; and a signal output unit 133 that outputs a trip signal when the temperature of the primary coolant measured by the temperature measurement unit 131 exceeds a first threshold value or when the change rate of the temperature of the primary coolant calculated by the calculation unit 132 exceeds a second threshold value.

Description

Fast Reactor Reactor Protection Device and Reactor Protection Method

[0001] The present invention relates to a fast reactor reactor protection device and a reactor protection method.

[0002] Conventionally, in order to protect the fast reactor reactor, the output of the fast reactor has been monitored. For example, in Patent Document 1, a coolant temperature, which is the temperature of the primary coolant of the fast reactor reactor, is measured, and an output change of the fast reactor is calculated based on the temperature change of the measured coolant temperature, and a monitoring device for monitoring the output of the fast reactor is disclosed.

[0003] Japanese Patent Application Laid-Open No. 2001-318184

[0004] An abnormal increase in the output of a fast reactor may occur due to an incorrect extraction of control rods. In this case, positive reactivity is added to the core, and it may become impossible to maintain the thermal margin of the reactor and ensure fuel integrity. In the conventional technology, when the control rods are withdrawn at high speed, the rate of change of the coolant temperature increases, so an abnormal output of the fast reactor can be detected. On the other hand, when the control rods are withdrawn slowly, the rate of change of the coolant temperature is small, and thus there has been a problem that an abnormal increase in the output of the fast reactor cannot be detected.

[0005] Therefore, the present invention has been made in view of these points, and an object thereof is to be able to detect an abnormality accompanied by an increase in the reactor output in a wide range regardless of the magnitude of the rate of change of the coolant temperature.

[0006] The reactor protection device for a fast reactor according to the first aspect of the present invention includes a temperature measurement unit that measures the temperature of the primary coolant of the fast reactor reactor, a calculation unit that calculates the rate of change of the temperature of the primary coolant measured by the temperature measurement unit, and a signal output unit that outputs a trip signal when the temperature of the primary coolant measured by the temperature measurement unit exceeds a first threshold value or when the rate of change of the temperature calculated by the calculation unit exceeds a second threshold value.

[0007] The calculation unit may calculate the rate of change by inputting the temperature signal indicating the temperature of the primary coolant measured by the temperature measurement unit into a low-pass filter that removes frequency components corresponding to temperature fluctuations due to turbulence in the flow of the primary coolant, which have been specified in advance, and then differentiating it.

[0008] The calculation unit may calculate the rate of change by inputting the temperature signal indicating the temperature of the primary coolant measured by the temperature measurement unit into an imperfect differentiator that outputs the rate of change from which frequency components corresponding to temperature fluctuations due to turbulence in the flow of the primary coolant, which have been predetermined, have been removed.

[0009] The second threshold may be set to a value higher than the highest rate of change in temperature that can occur when the output of the fast reactor is changed as part of the normal operation of the fast reactor. The first threshold may be set to a value higher than the highest temperature of the primary coolant corresponding to the current target operating output as part of the normal operation of the fast reactor, and lower than the primary coolant temperature permitted in the fast reactor.

[0010] The temperature measuring unit may measure the temperature of the primary coolant by acquiring a temperature signal indicating the temperature measured by a thermometer installed at the primary side inlet or primary side outlet of a heat exchanger that performs heat exchange between the primary coolant and the secondary coolant of a fast reactor.

[0011] A second aspect of the present invention relates to a fast reactor reactor protection method comprising: a step of measuring the temperature of the primary coolant of the fast reactor reactor, performed by a computer; a step of calculating the rate of change of the measured primary coolant temperature; and a signal step of outputting a trip signal when the measured primary coolant temperature exceeds a first threshold or when the calculated rate of change of the temperature exceeds a second threshold.

[0012] According to the present invention, anomalies accompanied by an increase in reactor output over a wide range can be detected regardless of the magnitude of the rate of temperature change of the coolant.

[0013] This figure shows the schematic configuration of the reactor. This figure shows the configuration of the reactor protection device according to this embodiment. This flowchart shows the processing flow in the reactor protection device until a trip signal is output. This figure shows the simulation results showing the relationship between the reactivity addition rate to the reactor core, the time until a trip signal corresponding to the reactivity addition rate is output, and the time until the reactor core reaches the upper limit temperature at which fuel integrity can be ensured.

[0014] [Outline of the Fast Reactor Structure] In describing the reactor protection device 100 of the fast reactor according to this embodiment, first, the structure of the reactor 1 will be described with reference to Figure 1. Figure 1 is a diagram showing the schematic configuration of the reactor 1. In Figure 1, the flow of sodium as a coolant is indicated by arrows. Also, in the following description, the reactor protection device 100 of the fast reactor will also be simply referred to as the reactor protection device 100.

[0015] The reactor 1 is, for example, a tank-type fast reactor. Inside the reactor vessel 2 of the reactor 1, as shown in Figure 1, there is a core 3, an intermediate heat exchanger 4, a pump 5, and internal piping 6. The reactor 1 is also equipped with a thermometer 7. Liquid sodium is contained inside the reactor vessel 2 as a coolant.

[0016] The reactor vessel 2 is, for example, a vessel with a diameter of about 15 to 20 meters. The reactor core 3 is supported horizontally inside the reactor vessel 2. The reactor core 3 contains core fuel containing fissile material and control rods for controlling the core reactivity. The control rods are driven by a control rod drive mechanism. The control rod drive mechanism controls the amount of insertion of the control rods between the core fuel. This controls the fission of the core fuel and controls the thermal output in the reactor core 3. The reactor core 3 heats up sodium, which is a liquid metal used as the primary coolant. In the following description, sodium before heating is also called low-temperature sodium, and sodium that has been heated to a high-temperature state is also called high-temperature sodium.

[0017] In reactor vessel 2, a free liquid level is set within the sodium loop, and a cover gas such as argon gas is sealed above the liquid level. As a result, reactor vessel 2 absorbs the volume increase due to the thermal expansion of sodium, which acts as a coolant, in the gas space above the liquid level.

[0018] The intermediate heat exchanger 4 performs heat exchange between sodium, which is the primary coolant circulating in the reactor vessel 2, and sodium, which is the secondary coolant circulating in the steam generator 8. The intermediate heat exchanger 4 has an inlet window on the primary side for introducing high-temperature sodium as a coolant, and an outlet window for releasing the low-temperature sodium after heat exchange. The intermediate heat exchanger 4 performs heat exchange between the high-temperature sodium that flows in through the inlet window and the sodium that functions as a coolant in the secondary system.

[0019] Specifically, the action of pump 5 causes high-temperature sodium, whose temperature has risen to approximately 550°C in the reactor core 3, to flow into the inlet window of the intermediate heat exchanger 4. The incoming high-temperature sodium undergoes heat exchange with the sodium in the secondary system, and its temperature drops to approximately 400°C, becoming low-temperature sodium. The low-temperature sodium flows out through the outlet window to the lower part of the reactor vessel 2. The sodium in the secondary system flows into the steam generator 8, where it heats water and generates steam to drive the turbine.

[0020] Pump 5, although not shown in the diagram, is located on the circumference where the intermediate heat exchanger 4 is installed, as viewed from above the reactor vessel 2. Pump 5 pumps the low-temperature sodium that has flowed out of the intermediate heat exchanger 4 into the in-core piping 6. The in-core piping 6 guides the low-temperature sodium pumped by pump 5 to the reactor core 3.

[0021] The thermometer 7 is installed at the primary side inlet of the intermediate heat exchanger 4 and measures the temperature of the high-temperature sodium (primary system sodium) flowing into the intermediate heat exchanger 4. Specifically, a thermometer guide tube filled with guide tube gas is provided near the inlet window of the intermediate heat exchanger 4. The thermometer guide tube is provided near the inlet window inside the intermediate heat exchanger 4, but is not limited to this. The thermometer guide tube may also be provided near the inlet window on the outside of the intermediate heat exchanger 4. The thermometer 7 is installed inside this thermometer guide tube and has a sheathed thermocouple for measuring temperature. The thermometer 7 measures the temperature of the high-temperature sodium flowing into the intermediate heat exchanger 4 from the inlet window using the sheathed thermocouple and outputs a temperature signal indicating the measured temperature to the reactor protection device 100. Note that the thermometer 7 is provided at the primary side inlet of the intermediate heat exchanger 4, but is not limited to this. The thermometer 7 may also be provided at the primary side outlet of the intermediate heat exchanger 4. In this case, the thermometer 7 is installed inside a thermometer guide tube located near the outlet window.

[0022] The reactor protection device 100 is a device that, in the event of an abnormal increase in the output of the reactor 1, outputs a trip signal to initiate protective operation of the reactor 1 to prevent the abnormality from spreading, and causes the reactor 1's shutdown system to perform an emergency shutdown of the reactor 1. The reactor protection device 100 is, for example, a computer.

[0023] The reactor protection device 100 outputs a trip signal based on the rate of change of the primary system sodium temperature measured by the thermometer 7. The temperature of the high-temperature sodium measured by the thermometer 7 changes slowly due to a first-order lag response caused by the transport delay due to circulation from the core 3 to the inlet window, as well as heat transfer in the thermometer guide tube. Therefore, if a control rod is mistakenly withdrawn and the withdrawal speed of the control rod is high, causing a rapid increase in reactor power, it will take time to detect the rise in the high-temperature sodium temperature as an abnormality and perform protective action. In contrast, the reactor protection device 100 outputs a trip signal based on the rate of change of the primary system sodium temperature measured by the thermometer 7. This allows the reactor protection device 100 to quickly detect an abnormality in the reactor 1's shutdown system, such as an increase in reactor power caused by a mistaken withdrawal of a control rod at a high withdrawal speed.

[0024] On the other hand, if the control rods are withdrawn slowly, the rate of change in the sodium temperature of the primary system is small, and a trip signal is not output based on the rate of change in the sodium temperature. In response to this, the reactor protection device 100 outputs a trip signal not only based on the rate of change in the sodium temperature of the primary system measured by the thermometer 7, but also based on the sodium temperature of the primary system measured by the thermometer 7. As a result, the reactor protection device 100 can detect abnormalities in the shutdown system of the reactor 1 that involve an increase in reactor power over a wide range, regardless of the magnitude of the rate of change in the sodium temperature of the primary system. The configuration of the reactor protection device 100 will be described below.

[0025] [Configuration of the reactor protection device 100] Figure 2 is a diagram showing the configuration of the reactor protection device 100 according to this embodiment. The reactor protection device 100 is, for example, a computer and comprises a communication unit 110, a storage unit 120, and a control unit 130.

[0026] The communication unit 110 is, for example, a communication interface for the reactor protection device 100 to communicate with the thermometer 7 and the shutdown system device 200 that constitutes the shutdown system of the reactor 1. The storage unit 120 is, for example, ROM (Read Only Memory) and RAM (Random Access Memory). The storage unit 120 stores various programs for making the reactor protection device 100 function. For example, the storage unit 120 stores a reactor protection program that makes the control unit 130 of the reactor protection device 100 function as a temperature measuring unit 131, a calculation unit 132, and a signal output unit 133.

[0027] The control unit 130 is, for example, a CPU (Central Processing Unit). The control unit 130 functions as a temperature measurement unit 131, a calculation unit 132, and a signal output unit 133 by executing a reactor protection program stored in the memory unit 120.

[0028] The temperature measuring unit 131 measures the temperature of the primary system sodium, which is the primary coolant of the reactor 1. Specifically, the temperature measuring unit 131 measures the temperature of the primary system sodium by acquiring a temperature signal indicating the temperature measured by a thermometer 7 installed at the primary side inlet or primary side outlet of the intermediate heat exchanger 4, which performs heat exchange between the primary system sodium and the secondary system sodium.

[0029] The calculation unit 132 calculates the rate of change of the primary system sodium temperature measured by the temperature measurement unit 131. For example, the calculation unit 132 calculates the rate of change of the primary system sodium temperature by inputting the temperature signal indicating the primary system sodium temperature measured by the temperature measurement unit 131 to an imperfect differentiator that outputs the rate of change of sodium temperature from which frequency components corresponding to temperature fluctuations due to turbulence in the flow of primary system sodium in the reactor 1 have been removed.

[0030] Here, the frequency corresponding to the temperature fluctuations due to turbulence in the flow of sodium in the primary system of the reactor 1 is assumed to be predetermined based on the time change of the temperature of the sodium in the primary system, which has been measured in advance by the temperature measurement unit 131.

[0031] Specifically, the calculation unit 132 receives the temperature signal acquired by the temperature measurement unit 131 as input to the transfer function shown in equation (1) below, which represents an imperfect differentiator. The calculation unit 132 acquires the signal output from the transfer function as a signal indicating the rate of change of temperature, and calculates the rate of change of temperature of the sodium in the primary system.

[0032] In equation (1), T Ｄ T is the derivative time, and η is the reciprocal of the derivative gain. Ｄ η is set to a value corresponding to a period longer than the period of the lowest frequency component among the frequency components corresponding to temperature fluctuations due to turbulence in the sodium flow in reactor 1.

[0033] The calculation unit 132 calculates the rate of change of the sodium temperature in the primary system by inputting the temperature signal measured by the temperature measurement unit 131 into an imperfect differentiator, but is not limited to this. For example, instead of an imperfect differentiator, a low-pass filter that removes frequency components corresponding to temperature fluctuations due to turbulence in the flow of the primary coolant and a differentiator that differentiates the signal that has passed through the low-pass filter may be provided. The calculation unit 132 may then calculate the rate of change of the sodium temperature in the primary system by inputting the temperature signal measured by the temperature measurement unit 131 into the low-pass filter and then differentiating it using the differentiator.

[0034] The signal output unit 133 outputs a trip signal to the shutdown system 200 for protecting the reactor 1 when the temperature of the primary system sodium measured by the temperature measurement unit 131 exceeds a first threshold, or when the rate of change of the primary system sodium temperature calculated by the calculation unit 132 exceeds a second threshold.

[0035] The first threshold is set to a value higher than the maximum sodium temperature of the primary system at the operating power of the fast reactor as performed under normal operating conditions, and lower than the allowable sodium temperature of the primary system in the fast reactor. The signal output unit 133 identifies the current target operating power as performed under normal operating conditions of the fast reactor. The signal output unit 133 sets the first threshold to a value higher than the maximum sodium temperature of the primary system corresponding to the identified operating power, and lower than the allowable sodium temperature of the primary system in the fast reactor. As a result, the reactor protection device 100 can output a trip signal early in the event of abnormal operation due to accidental withdrawal of a control rod at a slow withdrawal speed, before the temperature exceeds the upper limit that ensures the integrity of the fuel in the reactor core in the fast reactor.

[0036] The second threshold is set to a value higher than the highest rate of change in the primary system sodium temperature that can occur when the output of the fast reactor is changed as part of the normal operation of the fast reactor. The signal output unit 133 sets the second threshold to a value higher than the highest rate of change in the primary system sodium temperature that can occur when the output of the fast reactor is changed as part of the normal operation of the fast reactor. As a result, the reactor protection device 100 can output a trip signal early in the event of abnormal operation in the fast reactor due to the accidental withdrawal of a control rod with a high withdrawal speed, before the temperature exceeds the upper limit that can ensure the fuel integrity of the reactor core in the fast reactor.

[0037] The signal output unit 133 outputs a trip signal to the shutdown system 200 for protecting the reactor 1 when the temperature of the primary system sodium measured by the temperature measurement unit 131 exceeds a first threshold set in accordance with the current target operating output, or when the rate of change of the primary system sodium temperature calculated by the calculation unit 132 exceeds a set second threshold.

[0038] [Flowchart] Next, we will explain the processing flow until a trip signal is output in the reactor protection device 100. Figure 3 is a flowchart showing the processing flow until a trip signal is output in the reactor protection device 100.

[0039] First, the temperature measuring unit 131 measures the temperature of the sodium in the primary system of the reactor 1 (S1). Subsequently, if the temperature of the sodium in the primary system measured by the temperature measuring unit 131 exceeds a first threshold, the signal output unit 133 outputs an ON signal as a trip signal to initiate the protective operation of the reactor 1 (S2).

[0040] In parallel with the processing in S2, the calculation unit 132 performs the incomplete derivative of the temperature of the primary system sodium measured by the temperature measurement unit 131 and calculates the rate of change of the temperature of the primary system sodium (S3). The signal output unit 133 outputs an ON signal if the rate of change of the temperature of the primary system sodium measured by the temperature measurement unit 131 exceeds a second threshold (S4).

[0041] The signal output unit 133 outputs the logical sum of the signal output in S2 and the signal output in S4 (S5). As a result, when the temperature of the primary sodium measured by the temperature measurement unit 131 exceeds the first threshold value, or when the rate of change of the temperature of the primary sodium calculated by the calculation unit 132 exceeds the second threshold value, the signal output unit 133 outputs an on signal.

[0042] [Simulation Results] A simulation was performed on the elapsed time from when a control rod was accidentally withdrawn until a trip signal was output by the reactor protection device 100 in the fast reactor according to this embodiment. FIG. 4 is a diagram showing the simulation results showing the relationship between the reactivity addition rate corresponding to the accidental withdrawal, the time until a trip signal is output corresponding to the reactivity addition rate, and the time until the temperature reaches the upper limit at which the fuel integrity of the reactor core can be ensured in the reactor 1. The vertical axis in the figure shown in FIG. 4 indicates the reactivity addition rate corresponding to the withdrawal speed of the control rod. The reactivity addition rate increases as the withdrawal speed of the control rod increases. The horizontal axis in the figure shown in FIG. 4 indicates the elapsed time from the occurrence of the output change, that is, the elapsed time since the accidental withdrawal of the control rod occurred.

[0043] In FIG. 4, the solid line shows the relationship between the reactivity addition rate and the elapsed time until the temperature reaches the upper limit at which the fuel integrity of the reactor core can be ensured. In FIG. 4, the broken line shows the relationship between the reactivity addition rate and the elapsed time until a trip signal is output when the temperature of the primary sodium exceeds the first threshold value. In FIG. 4, the dashed-dotted line shows the relationship between the reactivity addition rate and the elapsed time until a trip signal is output when the rate of change of the temperature of the primary sodium exceeds the second threshold value.

[0044] As shown in FIG. 4, when the reactivity addition rate is higher than r, the elapsed time until a trip signal is output due to the rate of change of the temperature of the primary sodium exceeding the second threshold is shorter than the elapsed time until the upper limit temperature that can ensure the fuel integrity of the reactor core is reached. Further, it can be confirmed that the elapsed time until the trip signal is output is shorter than the elapsed time until the trip signal is output due to the temperature of the primary sodium exceeding the first threshold.

[0045] Also, as shown in FIG. 4, when the reactivity addition rate is lower than r, it can be confirmed that the elapsed time until a trip signal is output due to the temperature of the primary sodium exceeding the first threshold is shorter than the elapsed time until the upper limit temperature that can ensure the fuel integrity of the reactor core is reached. Further, it can be confirmed that the elapsed time until the trip signal is output is shorter than the elapsed time until the trip signal is output due to the rate of change of the temperature of the primary sodium exceeding the second threshold. Therefore, the reactor protection device 100 according to the present embodiment can output a trip signal earlier than reaching the upper limit temperature that can ensure the fuel integrity of the reactor core and perform a protection operation of the reactor 1 both when the control rod is rapidly withdrawn and the rate of change of the temperature of the primary sodium is large, and when the control rod is gently withdrawn and the rate of change of the temperature of the primary sodium is small.

[0046] [Effect in the present embodiment] As described above, the reactor protection device 100 according to the present embodiment measures the temperature of the primary coolant of the fast reactor, calculates the rate of change of the measured temperature of the primary coolant, and outputs a trip signal when the measured temperature of the primary coolant exceeds the first threshold or when the calculated rate of change of the temperature exceeds the second threshold. By doing so, the reactor protection device 100 can detect abnormalities accompanied by an increase in reactor power in a wide range regardless of the magnitude of the rate of change of the coolant temperature.

[0047] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments, and various modifications and changes are possible within the scope of its gist. Furthermore, the specific embodiments of the distribution and integration of the device are not limited to the above embodiments, and all or part thereof can be configured by functionally or physically distributing and integrating them in any unit. In addition, new embodiments resulting from any combination of multiple embodiments are also included in the embodiments of the present invention. The effects of the new embodiments resulting from the combinations are combined with the effects of the original embodiments.

[0048] 100 Fast reactor reactor protection device 110 Communication unit 120 Memory unit 130 Control unit 131 Temperature measurement unit 132 Calculation unit 133 Signal output unit

Claims

1. A reactor protection device for a fast reactor, comprising: a temperature measuring unit for measuring the temperature of the primary coolant of the reactor of a fast reactor; a calculation unit for calculating the rate of change of the temperature of the primary coolant measured by the temperature measuring unit; and a signal output unit for outputting a trip signal when the temperature of the primary coolant measured by the temperature measuring unit exceeds a first threshold, or when the rate of change of the temperature calculated by the calculation unit exceeds a second threshold.

2. The calculation unit calculates the rate of change by inputting the temperature signal indicating the temperature of the primary coolant measured by the temperature measurement unit into a low-pass filter that removes frequency components corresponding to temperature fluctuations due to turbulence in the flow of the primary coolant, which has been specified in advance, and then differentiating it, according to claim 1.

3. The calculation unit calculates the rate of change by inputting a temperature signal indicating the temperature of the primary coolant measured by the temperature measurement unit to an imperfect differentiator that outputs the rate of change from which frequency components corresponding to temperature fluctuations due to turbulence in the flow of the primary coolant, which have been predetermined, have been removed, the reactor protection device for a fast reactor according to claim 1.

4. The reactor protection device for a fast reactor according to any one of claims 1 to 3, wherein the second threshold is set to a value higher than the highest rate of change of temperature that can occur when the output of the fast reactor is changed as part of the normal operation of the fast reactor.

5. The reactor protection device for a fast reactor according to any one of claims 1 to 3, wherein the first threshold is set to a value higher than the maximum temperature of the primary coolant corresponding to the current target operating power performed as part of the normal operation of the fast reactor, and lower than the temperature of the primary coolant permitted in the fast reactor.

6. The reactor protection device for a fast reactor according to any one of claims 1 to 3, wherein the temperature measuring unit measures the temperature of the primary coolant by acquiring a temperature signal indicating the temperature measured by a thermometer provided at the primary side inlet or primary side outlet of a heat exchanger that performs heat exchange between the primary coolant and the secondary coolant of the fast reactor.

7. A method for protecting a fast reactor, comprising: a step of a computer measuring the temperature of the primary coolant of the fast reactor; a step of calculating the rate of change of the measured primary coolant temperature; and a signal step of outputting a trip signal when the measured primary coolant temperature exceeds a first threshold or when the calculated rate of change of the temperature exceeds a second threshold.

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