JPH07333382A - Core monitoring device for reactor - Google Patents

Abstract

PURPOSE:To obtain the method for easily detecting the event increasing the void fraction in the coolant in a part of channels in case of a flow blockage events. CONSTITUTION:With a fast neutron flux detector 11 placed in a guide tube 1 and an output calibration detector 12 placed in detection lead pipe 10 for output calibration, the fluctuation in the fast neutron flux level due to void fraction variation in the coolant generated from fuel element 1 can be detected. By this, such a coolant flow lowering event due to flow blockage can be detected early.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure tube type reactor capable of surely and easily detecting an event such as a flow path clogging event in which the coolant void ratio of a very small number of channels rapidly increases. Regarding

[0002]

2. Description of the Related Art FIG. 4 shows a conventional example of an apparatus for monitoring a core state by measuring a radiation level in a pressure tube reactor. The fuel element part 1 is surrounded by a pressure pipe 2 and further by a calandria pipe 3. The calandria pipe 3 penetrates the calandria tank 5. A thermal neutron flux detector 6 is provided to measure the radiation level (thermal neutron flux) generated from the fuel element unit 1 and monitor the reactor output. The thermal neutron flux detector 6 is surrounded by a guide tube 7, and a signal cable 8 transmits a signal to a detection circuit device. An output calibration detector 9 is provided in the output calibration detector conduit 10 for calibrating the output state.

[0003]

In a pressure tube reactor, the core is composed of a large number of independent channels,
The reliability of the reactor can be improved by detecting a local coolant flow rate drop event in which one channel is blocked.

As a method of detecting this flow rate decrease event,
A decrease in the flow rate raises the void ratio of the coolant and a decrease in the effective density of the coolant, which changes the radiation level around the pressure tube.By detecting this variation, the flow rate of the coolant can be decreased. It is possible to sense.

As a method of measuring the radiation level and monitoring the core state, there is a conventional example. In this case, the thermal neutron flux is measured, and the thermal neutron flux is the neutron moderator of the moderator outside the pressure tube. Because the ones that are caused by the effects are dominant,
The sensitivity of the fluctuation of the thermal neutron flux level to the fluctuation of the coolant flow rate is extremely low, and the reliability of the detector is low for the local coolant flow rate drop event in which one channel is blocked. there were.

An object of the present invention is to provide a highly reliable method capable of reliably detecting a local coolant flow rate drop event by detecting an increase in fast neutron flux associated with an increase in void ratio of the coolant. To provide.

[0007]

[Means for solving the problems] The void ratio of the coolant is increased due to a flow path clogging event and the effective density of the coolant is reduced,
Since the shielding effect of the coolant decreases, the fast neutron flux around the pressure tube increases. By detecting this increase in fast neutron flux with a fast neutron flux detector installed in a conventional neutron detector guide tube, an increase in the void fraction of the coolant is detected.

[0008]

FIG. 3 shows the relative attenuation distribution of the fast neutron flux with respect to the neutron transmission distance of the coolant (light water), using the void ratio of the coolant as a parameter. From the figure, it can be seen that as the void fraction of the coolant increases, the shielding effect of the coolant on fast neutrons decreases. The coolant void fraction in the pressure tube of the pressure tube reactor during normal operation is 0 to 70.
It is distributed between about%. In addition, when the coolant flow rate is extremely reduced due to a flow path blockage event, the void ratio is almost 1
Reach 00%. Fast neutrons are generated in the fuel region inside the pressure tube, penetrate the coolant layer surrounding the fuel region, and leak to the outside of the pressure tube. Therefore, the fast neutron flux level on the outside of the pressure tube rises due to the decrease in the coolant void fraction, and the increase degree is from FIG. 3, for example, when the average void fraction reaches 30% to 100%, With a penetration distance of about 10 cm, the fast neutron flux doubles, enabling significant detection.

Further, by sharing the guide tube of this fast neutron flux detector with the guide tube of the conventional detector, the increase in equipment can be suppressed and the maintenance becomes easy.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows one embodiment of the present invention. This is configured by installing a fast neutron flux detector 11 in a guide tube 7 of a conventional example (see FIG. 4) and an output calibration detector 12 in an output calibration detector conduit 10. The neutron flux level corresponding to the void fraction of the coolant during normal operation is set by the output calibration detector 12, then the fast neutron flux level is monitored by the fast neutron flux detector 11, and the fast neutron flux level The change in the void ratio of the coolant is detected by the change. In order to secure the neutron coolant penetration distance necessary to capture the change in the void fraction of the coolant as the change in the neutron flux level at the detector position, both detectors are located above the fuel element portion 1, that is, It is installed downstream of the coolant flow. Since the change in the void rate of the coolant propagates along the flow of the coolant in the initial stage, the installation at this position makes it possible to detect the change in the void rate of the coolant in a short time.

FIG. 2 shows another embodiment of the present invention. This is the fast neutron flux detector 11 of the above embodiment (see FIG. 1).
Also, the output calibration detector 12 is installed at a lower level than the fuel element unit 1, that is, at a position upstream of the flow of the coolant. At normal times, the void ratio of the coolant upstream of the fuel element part 1 is low, and the difference in void ratio before and after the event becomes large, so the fluctuation of the neutron flux level when the detector is installed at the same position becomes large. . Therefore, there is an advantage that the requirement for the detection accuracy of the detector is relaxed and the reliability of the device is improved.

[0013]

EFFECTS OF THE INVENTION According to the present invention, it is possible to easily detect a phenomenon in which the void ratio of the coolant in a part of the channel increases, such as a flow path clogging event in a pressure tube reactor. Further, by sharing the guide tube of the fast neutron detector with the guide tube of the conventional detector, increase in equipment can be suppressed and maintenance becomes easy.

[Brief description of drawings]

FIG. 1 is a cross-sectional view in which a fast neutron flux detector according to an embodiment of the present invention is installed on the downstream side of a coolant flow.

FIG. 2 is a cross-sectional view showing an embodiment of the present invention in which a fast neutron flux detector is installed on the upstream side of a coolant flow.

FIG. 3 is a relative attenuation distribution map of fast neutron flux with respect to a neutron transmission distance of a coolant.

FIG. 4 is a cross-sectional view of a conventional example of a device for monitoring a core state.

[Explanation of symbols]

1 ... Fuel element part, 2 ... Pressure tube, 3 ... Calandria tube, 4
... Coolant, 5 ... Calandria tank, 6 ... Thermal neutron flux detector, 7 ... Guide tube, 8 ... Signal cable, 9 ... Output calibration detector, 10 ... Output calibration detector conduit, 11 ... Fast neutron flux Detector, 12 ... Detector for output calibration.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsumi Hayashi 3-2-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi Engineering Co., Ltd. (72) Inventor Hitoshi Narita 2-3-2, Saiwaicho, Hitachi-shi, Ibaraki No. 1 within Hitachi Engineering Co., Ltd. (72) Inventor Noriaki Shimozaki 3-2-1, 3-chome, Saiwaicho, Hitachi City, Ibaraki Within Hitachi Engineering Co., Ltd.

Claims (1)

[Claims] 1. A pressure tube type comprising a fuel element section, a plurality of pressure tubes containing a coolant for cooling the fuel element section, and a neutron detector guide tube provided between the pressure tubes. In the nuclear reactor, a fast neutron detector is installed in the neutron detector guide tube, and a reactor core monitoring device for a nuclear reactor.