JPS6346396B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reactor core exposure monitoring system that immediately detects and issues an alarm when the reactor core is exposed due to a drop in the reactor water level.

In nuclear power plants, a large number of neutron detectors are usually installed in the reactor core to measure local neutron density in various parts of the reactor.

For example, a boiling water reactor is equipped with a large number of local power range monitors, intermediate range monitors, neutron source monitors, and mobile neutron detectors, which are used for core control.

When the water level inside the reactor drops below a certain level in the event of an accident, the reactor protection system is activated, the reactor control rods are fully inserted, the reactor is brought to an emergency shutdown, and the neutron flux level within the reactor core drops rapidly. .

Even if the reactor water level drops further, as long as the reactor water level gauge is operating normally, by monitoring this it is possible to prevent the core from being exposed by activating the emergency water injection system, etc. If the gauge does not operate normally, the water level inside the reactor cannot be accurately determined, and there is a risk that appropriate countermeasures will be delayed, leading to secondary disasters.

The temperature of the cooling water in the furnace during normal operation varies depending on the furnace type, but is approximately 280°C to 320°C, and therefore, if the cooling water level is normal, the temperature will not exceed this temperature.

However, if for some reason the cooling water level drops and the core is exposed, even if the control rods are fully inserted, heat removal will not be sufficient and the temperature inside the core will exceed 400℃. It turns out.

Therefore, by measuring and monitoring the temperature within the core, exposure of the core can be monitored.

By the way, the output of the neutron detectors mentioned above is drawn out from the reactor vessel through lead wires and guided to the measuring device, but these lead wires require excellent heat resistance, so inorganic insulated wires are usually used. used.

This electric wire has the property that the insulation resistance between the wires changes greatly depending on the temperature.

The present invention has been made with attention to this point,
By measuring and monitoring the insulation resistance of the neutron detector and its lead wires, it is intended to monitor the temperature inside the reactor and thus the exposure of the reactor core.

The present invention will be explained in detail below with reference to the illustrated embodiments.

Figure 1 conceptually shows the loading state of in-core neutron detectors in a boiling water nuclear power plant. A mobile neutron detector _S1 is placed at the upper end of the reactor core 1, and Neutron detectors 7 ₂ to S ₅ as monitors are sequentially installed at appropriate intervals in the vertical direction.

These neutron detectors are each connected to a local power range monitor amplifier 3 via a lead wire 2, and to an insulation measuring device 4 via a changeover switch Sw.

The output of this insulation measuring device is sent to the alarm circuit 5.
It is compared with a reference value, and if it is less than the reference value, the alarm 6 is activated.

In the above, the lead wires 2 are all made of inorganic insulated wires.

This electric wire has one or more core wires arranged within a sheath made of a copper tube or stainless steel tube with an insulating material of magnesia or alumina interposed therebetween.

The insulation resistance value between the core wires or between the core wire and the sheath of this inorganic insulated wire changes widely with changes in ambient temperature.

That is, as illustrated in FIG. 2, the insulation resistance at 300° C. is about 9×10 ¹⁰ Ω, but at 400° C., this becomes about 1.5×10 ⁹ Ω, which is reduced to about 1/40.

Therefore, as shown in FIG. 3, a power source E is connected via a resistor R between the core wire 2a and sheath 2b at the other end of the inorganic insulated wire 2 connected to the neutron detector S, and the voltage between the terminals of the resistor R is If you measure V with a voltmeter Vm,
The resistance r of the neutron detector S and the inorganic insulated wire 2 is calculated from the following equation.

r=R(E/V-1) When the water level in the core changes as shown in Figure 4A, the insulation of neutron detectors _S1 to _S5 and their lead wires must be removed before the water level reaches the top of the core. The resistance in each case is about 6× ¹⁰ Ω, as shown in Figure B.
When the water level falls below the top of the core at time t ₁ , the insulation resistance of the neutron detector S ₁ and its lead wire will change to B in the same figure.
decreases rapidly.

The water level further decreases to level A at time _t2 .
(See Figure 1), the insulation resistance of the neutron detector _S2 and its lead wires decreases as shown in Figure B.

In this way, in the device of the present invention, the drop in the core water level can be measured and monitored based on the change in the insulation resistance value of the lead wire of the neutron detector, so if the core is exposed, an alarm is immediately issued. It is possible to activate the emergency water injection system and take other measures.

Although the illustrated example shows only five neutron detectors, in an actual nuclear reactor, 20 to 150 neutron detectors are arranged, so if they are installed at different heights, the core water level can be adjusted. Levels can be determined in fine ranges.

In addition, in the present invention, the lead wire connecting the neutron detector and the local power range monitor amplifier is commonly used for water level detection, but neutron calibration is mainly performed during normal operation of the reactor. On the other hand, since water level detection is performed when the reactor is shut down, there is no inconvenience due to the above-mentioned common use.

However, if it is desired to perform neutron calibration and water level measurement simultaneously, this can be easily achieved by introducing a known time division method.

[Brief explanation of drawings]

Figure 1 is a circuit diagram conceptually showing the relationship between a neutron detector installed in a nuclear reactor and its attached circuit, and Figure 2 shows the relationship between ambient temperature and insulation resistance of the inorganic insulated wire used in the present invention. FIG. 3 is a circuit diagram explaining the principle of measuring insulation resistance in the present invention, and FIG. 4 is a graph showing the relationship between core water level and change in insulation resistance. 1...Reactor core, 2...Lead wire (inorganic insulated wire), 2
a...core wire, 2b...sheath, S, _S1 to _S5 ...neutron detector.

Claims (1)

[Claims] 1. Measures the insulation resistance of a neutron detector inserted into the reactor core and its lead wires, compares this with a predetermined reference value, and issues an alarm when the measured value falls below the reference value. A nuclear reactor core exposure monitoring device characterized in that it is configured to emit. 2. The reactor core exposure monitoring device according to claim 1, wherein the lead wire is made of an inorganic insulated wire.