JPH0650359B2 - Method and device for monitoring cooling status of light water reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline of the Invention] An apparatus for monitoring a cooling state in a core of a light water reactor includes a probe (2), which is introduced into a reactor and is made of a material having low neutron absorption and water resistance. And a resistor (12) hermetically sealed in the case (12). The resistor (11) is connected to the power supply lead (5). The resistance of the resistor (11) depends on the temperature of the resistor. The device further comprises detection means for producing an output signal, said output signal being dependent on the resistance of the resistor (11) when a current flows through the resistor.

A monitoring device is used to detect a reduction or disappearance of the cooling capacity of the coolant around the probe (2) in the first operating mode. Next, the heat is supplied to the resistor of relatively high power, and the heat generated in the resistor as a result of supplying the power is removed by the refrigerant around the probe (2) so that the supplied power is adjusted to correspond to the cooling capacity. However, if the cooling capacity decreases for some reason, such as in the case of a refrigerant deficiency accident, the temperature of the resistor (11) rises. Resistor (1
Since the resistance of 1) strongly depends on the temperature, the resistance changes and, as a result, the output signal output from the detection means changes.

[Industrial application field]

The present invention relates to a method and an apparatus for monitoring a cooling state of a core in a light water reactor. In this method, at least one probe equipped with a power supply means is introduced into a nuclear reactor.

[Conventional technology]

Through the Three Mile Island accident, the need for a reliable core cooling monitor became apparent, and core damage would probably be avoided if the reactor was equipped with monitoring equipment that could provide direct information on the cooling situation in the core. It is suggested that you can. Currently, in some countries, reactor inspectors are instructing that core cooling monitoring equipment be installed inside the LWR.

Such equipment needs to be able to provide a quick and clear indication in case of insufficient core cooling, such a situation occurring when the water level in the reactor falls below the water level at which it becomes unable to cover the core. To do. In particular, it is very important that the response of the monitoring device is large enough to avoid any false positives, otherwise the operator may perform undesired operations. This type of error determination can occur, for example, by a sudden pressure drop with a significant LOCA (Refrigerant Deficiency Accident), in which case the pressure in the BWR, for example, from 70 bar to 4-5 bar in a relatively short time. As it falls, the corresponding saturation temperature of the refrigerant drops from 258 ° C to about 150 ° C.

Different views have been reported on core cooling monitoring. A system based on pressure drop measurement has been proposed by Westinghouse and Babcock and Wilcox. The output signal obtained from this type of device can be difficult to determine in the abrupt transitional situations that occur in accident situations. Therefore, the interpretation of the signal requires the use of a computer program, which makes it impossible to directly observe the series of refrigerants in the reactor reactor with high reliability.

Another method used by Conservation Engineering is based on heating-coupled thermocouple probes. The probe incorporates two thermocouples, one for heating connection and one for non-heating connection. Thus, the output from the probe directly correlates to the cooling capacity of the surrounding fluid. The output voltage from the thermal element indicates that there is a temperature difference between the heated connection and the unheated connection, and that the cooling capacity of the ambient refrigerant is insufficient to cool the heated connection. This will also indicate that the core is not sufficiently cooled. This principle has the disadvantage that the output signal is only on the order of a few millivolts. For this reason, the output signal is easily disturbed by other phenomena occurring within the reactor.

I would also like to mention the method proposed by General Electric (DE 3327047Al). This method uses a small electric heater as the detector. When the water level in the reactor drops and the heater is no longer surrounded, the temperature of the heater rises because the coefficient of thermal conductivity decreases. However, the heater consists of a coaxial cable only, the inner conductor acts as a heat generating resistor and the outer shroud is used to close the electrical circuit. It is clear that the power density of this detector is small due to the small surface area of the inner conductor. Therefore, the temperature rise of the detector due to not being surrounded is small, and the required increase in ohmic resistance due to not being surrounded is actually smaller than the decrease in ohmic resistance. This is due to the decrease in the refrigerant saturation temperature during the LOCA. To do. The latter temperature drop is 13 within the BWR.
Up to 170 ° C in the PWR up to ℃ and also reduces the ohmic resistance of cables up to 15 meters long, which connects the coaxial axial sensor located in the core to the electrical system outside the reaction pressure vessel It is necessary for this.
Therefore, during the course of certain accidents, this type of monitoring device may give an unclear signal and lead to erroneous decisions regarding the state of the core.

It should be considered that the temperature rise in the core when inadequate cooling occurs cannot be traced by any of the devices described above.

[Problems to be Solved by the Invention]

It is an object of the present invention to provide a method and a device for monitoring core cooling in a light water reactor, wherein the occurrence of an output signal clearly and clearly indicates the presence of a core cooling disturbance and what is this output signal. This has the advantage that even if there is an inhibitor, it can be ignored, and this output signal can also be used to measure the core temperature during the heating period after the cooling capacity loss. This object is achieved by the adoption of a high power density electric heater, which consists of an electric heating coil wound tightly on an electric wire whose ohmic resistance varies strongly with temperature. The resistance coil is embedded inside a metal housing and the space between the coil and the housing is filled with an electrically insulating material. Therefore, the coil is electrically isolated from the outer housing. The power of the coil is supplied by two conductors, which are welded to the ends of the resistance wire.
Since only the ohmic resistance of the conductor as a heater is measured,
And to eliminate the resistance of the long conductors, the conductors couple the monitoring equipment to the equipment outside the pressure vessel and two additional conductors are welded to the heater coil. The voltage applied to the monitoring device is
It is measured by the latter two conductors. The above-mentioned four conductors are all part of the commercially available four-conductor armored cable.

When the monitoring equipment is no longer surrounded during the reactor accident,
The coefficient of heat transfer between the monitor and the refrigerant decreases dramatically and the voltage across the monitor increases sharply due to the high power density. The average response over the first 30 seconds of the transition process for a constant current in the range of 3-5 A would be between 20-60 mV / S. This signal clearly indicates that the surveillance device is not enclosed. Therefore, it becomes clear that the core is not surrounded and it is obviously desirable to measure the temperature in the core when the temperature rise of the core starts. The current input to the monitor is then reduced to the mA range and the monitor acts as a resistance thermometer.

[Means for Solving the Problems]

According to the invention, the object is to introduce at least one probe provided with power supply means into a nuclear reactor, said probe comprising at least one electric heating element containing a resistor, said electric heating element comprising: The output signal is connected to the power supply unit and the resistance of the probe is dependent on the temperature of the resistor, and the probe is connected to the resistor to generate an output signal. In order to detect the reduction of the cooling condition around the probe in the first operation mode by bringing the resistor into a heat conducting state when a current flows through the resistor and bringing it into contact with a medium around the probe. When the first electric power of high electric power is supplied to the resistor and the core is normally cooled, the heat generated in the resistor by the first electric power is generated by the probe. When the cooling capacity of the medium around the probe is reduced or disappears, the temperature inside the resistor rises as a result of the heat, and the resistance changes and the output changes, thereby cooling the light water reactor. In the method of monitoring, the coil of the electric conductor that is concentric with the probe is used as a resistor, and the coil as the resistor is enclosed in the probe case in the probe, and the output signal is the resistor. Supplying a second power to the resistor in a second mode of operation measuring the temperature around the probe through separate pairs of wires connected to each end of A light water reactor characterized in that the power is made lower than the first power and a switch means is provided so that the power supply of the resistor can be changed by the switch means. It can be solved by cooling condition monitoring methods.

The method for monitoring the cooling state of a light water reactor includes a probe (2) that can be inserted into a nuclear reactor and that includes a power supply means (5), and the probe (2) is made of a water resistant material (12). And a resistor (3) hermetically sealed in the case (12) and connected to the power supply means (5), the resistance of the resistor (3) depends on the temperature of the resistor, Furthermore, a heat conducting medium is provided between the inner wall of the case (12) and the resistor (3),
Further, a detection means for generating an output signal is provided, and when a current flows through the resistor, it is the resistor (3).
In a cooling condition monitoring device for a light water reactor configured to depend on the resistance of, the resistor is a coil of an electric wire, which is concentric with the probe, and separate pairs of wires have the output through the wire only. Connected to each end of the coil for measuring a signal and arranged for power to be supplied to the coil via another pair of conductors, further comprising switch means (7) for providing the switch means ( 7)
With this, the resistor (3) can be connected to different power sources, and can be realized by a cooling condition monitoring device for a light water reactor.

[Action effect]

The present invention provides the following advantages over the prior art: (1) In the event of a refrigerant deficiency, the signal compared with the signal achievable by a monitoring device based on the heating thermocouple connection method. The response of the monitoring device reaches more than 200 times. Due to the large response, the risk of misjudging the signal can be considered negligible.

(2) This device is especially
It can be used to measure the temperature not only outside the core but also inside the core.

(3) The device occupies very little space and consists of a very small number of parts, which is reliable in operation and thus reduces the occurrence of errors. Especially the probe
It can be designed to be built into a metrology guide tube that already exists in the reactor.

(4) When the probe is installed in the core, the device functions as a core cooling monitoring device, water level indicator or thermometer. However, when the probe is placed at the top, bottom or sides of the core, the device functions as a water level indicator or thermometer.

(5) The output signal from the monitoring device can also be used to activate any safety device during the heating process that occurs after the refrigerant deficiency.

(6) The probe does not require any shielding and the output signal is strong, so the effect of water droplets striking the probe is negligible. Heating-coupled thermocouple probes are often shielded against this type of interference, especially against splashing that occurs just above the surface of a two-phase water mixture flow.

Laboratory tests using a water flow lube operating at pressures in the range between 1-160 bar show the expected effect of splashing liquid and water drop deposition when normal refrigerant levels disappear and the probe is no longer enclosed. It was found to have a negligible effect on the response from the monitoring device.

〔Example〕

Other advantages and features of the present invention will become more apparent from the following description of the embodiments of the present invention with reference to the accompanying drawings.

In the circuit diagram of FIG. 1, three measuring devices 1a, 1b, 1c are provided, each consisting of a probe 2a, 2b, 2c, which are introduced into a suitable location of the reactor. Each probe 2a, 2b, 2c has a resistor 3a, 3 respectively.
b and 3c are incorporated and their resistance depends on the temperature of the resistor. Each resistor is a signal conductor 6a, 6b
And 6c are connected to a recorder 4, which here records the voltage across the respective resistor. The resistors 3a, 3b, 3c are connected to the power supply leads 5a, 5b, 5c.
And switches 7a, 7b, 7c can be connected to any of power sources 8a, 8b, 8c and 9a, 9b, 9c.

In the first operating mode, the resistors 3a, 3b, 3c are respectively connected to the first power sources 8a, 8b and 8c to supply a relatively high power, eg 4A, to the corresponding resistor. In normal operation, the heat generated in the resistors 3a, 3b, 3c by the supplied power is removed by the surrounding refrigerant in the reactor,
The result is a slight increase in temperature within the resistor.
However, when the cooling capacity of the surrounding medium is reduced due to, for example, a lack of the refrigerant, the corresponding resistors 3a, 3b, 3c are
Will be insufficiently cooled and the temperature inside the corresponding resistor will rise. The resistance of the resistor changes as the temperature inside the resistor rises. When a constant current is supplied to the monitoring device, the output voltage increases when the core cooling is lost. At a pre-measured output signal level, which can be on the order of a few volts, the alarm is released to indicate core cooling deficit in the reactor and the necessary safety steps are released or implemented.

After that, the switches 7a, 7b, 7c are switched so that the resistors 3a, 3b, 3c are respectively switched to the second power source 9a,
Connect to 9b and 9c. This second power source supplies a relatively low current, for example on the order of 0.1 A, to the corresponding resistor. In this second operation mode, the resistor 3a,
The device functions as a resistance thermometer by supplying extremely low electric power to 3b and 3c. Therefore, in this second mode of operation, the system is used to monitor the temperature rise in the core.

FIG. 2 shows a cross section of the probe 2 of the apparatus. For example, a resistance wire 11 made of a suitable material, such as Kanthal, is wound around an insulator 10 made of a solid material of Al ₂ O ₃ or another material coated with an electrically non-conductive material. To do. The resistance wire 11 is connected to the power source by the cable 5. The resistor coil 11 is laser- or electric-welded to, for example, stainless steel or Inconel (I
(nconel), and is hermetically sealed in a case 12 made of a material having low neutron absorption and water resistance. Thermal conductivity, such as for example M _{g O} Powder space between the resistor 11 and the case 12 and filled with electrically insulating and neutron-low-absorbent material.

[Brief description of drawings]

FIG. 1 shows a circuit diagram of three devices according to the invention, FIG.
The drawing is a cross-sectional view of a probe which is a part of the measuring apparatus shown in the drawing of FIG. 1a, 1b, 1c ... Measuring device, 2a, 2b, 2c ...
Probes 3a, 3b, 3c ... Resistors, 4 ... Recorders 5a, 5b, 5c ... Power supply lead wires 6a, 6b, 6c ... Signal lead wires 7a, 7b, 7c ... Switches, 8a, 8b, 8c ... …
First power source 9a, 9b, 9c ... Second power source 10 ... Insulator, 11 ... Resistance wire 12 ... Case

Claims (5)

[Claims] 1. At least one probe provided with a power supply means is introduced into a nuclear reactor, and the probe is provided with at least one electric heating element having a built-in resistor, and the electric heating element is provided in the power supply means. Connecting the resistor and making its resistance dependent on the temperature of the resistor, further comprising detection means for connecting the probe to the resistor to generate an output signal, and making the output signal dependent on the resistance of the resistor, When a current flows through the resistor, the resistor is brought into a heat conducting state, is brought into contact with a medium around the probe, and a high power is applied to the resistor to detect the reduction of the cooling condition around the probe in the first operation mode. When the core is normally cooled, the heat generated in the resistor body by the first power is generated by the refrigerant around the probe. When the cooling capacity of the medium around the probe is reduced or disappears, the heat causes the temperature inside the resistor to rise, resulting in a change in the resistance and a change in the output, thereby monitoring the cooling status of the light water reactor. In, while using a coil of an electric wire that is concentric with the probe as a resistor, the coil as the resistor in the probe is enclosed in a probe case, and the output signal is applied to each end of the resistor. Supplying a second power to the resistor in a second operating mode in which the temperature around the probe is measured through separate pairs of conductors connected to the first power and the second power to the second power. A cooling condition monitoring method for a light water reactor, which is further lowered, and a switch means is provided so that the power supply of the resistor can be changed by the switch means. . 2. A method according to claim 1, characterized in that at least one probe is arranged inside one of the measuring guide tubes of the core. 3. A case (12) comprising a probe (2) insertable into a nuclear reactor and comprising a power supply means (5), said probe (2) being made of a water resistant material, and said case (12). A resistor (3) that is hermetically sealed in and connected to the power supply means (5), the resistance of the resistor (3) depends on the temperature of the resistor, and the case (12) A heat conducting medium disposed between the inner wall of the resistor and the resistor (3), and a detecting means for generating an output signal. When a current flows through the resistor, this is the resistor (3). In a cooling condition monitoring device for a light water reactor configured to depend on the resistance of, the resistor is a coil of an electric wire, which is concentric with the probe and separate pairs of wires through only the wire. Output signal Connected to each end of the coil for measuring the voltage, and configured to supply power to the coil via another pair of conductors, further comprising a switch means (7) to provide the switch means ( 7)
The cooling condition monitoring device for a light water reactor, characterized in that the resistor (3) can be connected to different electric power sources. 4. A device according to claim 3, characterized in that the heat transfer medium is MgO, Al ₂ O ₃ or the like, preferably in the form of a powder. 5. The resistor (3) is a Kansal wire (1).
Device according to claim 3 or 4, characterized in that it consists of 1) or its analogues.