JPS642236B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a system for detecting damaged fuel locations for detecting which nuclear fuel assembly is damaged in a nuclear reactor in which each fuel assembly housed in a reactor vessel is cooled by a coolant. This invention relates to a detection device.

Conventional devices for detecting damaged fuel of this kind introduce a portion of the coolant near the top of each nuclear fuel assembly out of the reactor through a sampling tube placed above the top of each nuclear fuel assembly. A means for investigating the presence of fission products contained in the coolant, a pipe inserted into the reactor is sequentially placed over the top of each nuclear fuel assembly, or a nuclear fuel assembly is pulled up into the pipe, By supplying carrier gas to the coolant near the top of the nuclear fuel assembly that has entered the pipe, the release of the fission products contained in the coolant is promoted, and the carrier gas containing the fission products is recovered and radiated. There was a means to measure ability.

However, with the above-mentioned known technical means, the concentration of fission products contained in the coolant sampled for sampling and measuring the coolant near the top of the nuclear fuel assembly is low, and the concentration of fission products near the top of the target nuclear fuel assembly is low. There is no guarantee that it is a coolant. In other words, in addition to the coolant near the top of the target nuclear fuel assembly, coolant mixed with coolant near the top of other adjacent nuclear fuel assemblies is being sampled and measured.
It was not possible to detect radioactivity with sufficient accuracy.

This invention was made in view of these points, and involves attaching a collection member to a nuclear fuel assembly, collecting coolant containing nuclear fission products at a high concentration, and then gas-pubbling it into the gas. By transferring the fission products and measuring the radioactive concentration in this gas, it is possible to measure each individual piece of nuclear fuel in the reactor with high precision, and the above measurements can be performed reliably without sampling the coolant outside the reactor. The purpose of this invention is to provide a highly reliable damaged fuel position detection device.

An embodiment of the present invention will be described below with reference to FIG. The figure shows the case where the device of the present invention is applied to a Na (sodium) cooled fast breeder reactor, and shows the device of the present invention positioned at the upper part of the reactor and inserted into the reactor.

In the figure, 1 is a guide tube whose surface is hard chrome plated to prevent Na from adhering to the inside of the furnace. Reference numeral 2 denotes a shipping port of a flexible member attached to the tip of the guide tube 1, and a bellows is used as a structure that allows it to be easily brought into close contact with the top of the fuel assembly. 3
is a spiral-shaped sodium sampling pipe that collects the coolant Na, 4a is a sodium tank that accumulates the collected sodium, 4b is a gas blowing nozzle, 5 is a gas supply pipe that sends gas into this sodium, and 6 is this circulating 7 is a gas sampling tube that guides the gas to the outside of the guide tube 1; 7 is a gas exhaust tube that returns the gas whose radioactivity concentration has been measured; and 8 is a downstream tube that inserts the guide tube 1 into the reactor. A seal flange 9 is a shaft sealing device around the guide tube 1, and 9 is a door valve 10 that constitutes a seal boundary between the device of the present invention and upper reactor equipment.
1 is a drive device for driving the guide tube 1 up and down into the reactor; 11 is a wire for lifting the guide tube 1;
Reference numeral 2 represents a radiation detector for measuring the radioactive concentration in the above-mentioned circulating gas, 13 represents a gas circulation unit, and 14 represents a control panel for operating and controlling these drive systems and gas circulation systems. As equipment around the reactor, which is the connection part with the above-mentioned damaged fuel position detection device,
15 is a door valve for a fuel exchange hole, 16 is a shield plug, 17 is a hold-down shaft, 18 is a cover gas in the reactor, 19 is a sodium coolant in the reactor, and 20 is a fuel assembly. The piping and tanks that come into contact with Na and Na paper, such as the sodium sampling tube 3 above, should be installed as shown in Figure 2.
A preheating heater 21 consisting of an MI sheath microheater and temperature control of this preheating heater 21 are provided.
A thermocouple 22 of the MI sheath is wound around the MI sheath. Also, in the sodium tank 4a, there is a gas blowing nozzle 4b for generating a gas valve, in which a double cylindrical punching metal is attached to the tip of the gas supply pipe, so that the punching metal is oriented horizontally as shown in FIG. installed on. This is to prevent Na from adhering to and sticking to the punching metal when drained from the Na tank.

The operation of the device of the present invention configured as described above is as follows.
This will be explained with reference to FIG. 4, which shows the state of detection of damaged fuel in the apparatus of the present invention. In FIG. 4, the same reference numerals as those in FIGS. 1 to 3 indicate the same components, so the explanation thereof will be omitted. In the figure, a guide tube 1 is positioned at a shipping port 2 at the top of a fuel assembly 20 in a nuclear reactor. Here, the fission products 30 dissolved in the fuel assembly 20 are introduced from the shipping port 2 through the sodium sampling pipe 3 into the sodium tank 4a, and into the sodium from the gas circulation unit 13 through the gas supply pipe 5. The gas bubbles 23 are brought to the radiation detector 12 via the gas sampling tube 6 circulation pump 24 and gas piping 25.

The operating procedure for the gas circulation unit 13 is to first open the valve B2 in front of the clean gas supply source 27.
If valve A26 and valve C29 are opened with valve 8 closed, the pressure of the reactor cover gas 18 and the gas pressure within the gas circulation unit 13 will be equally balanced, and the sodium level in the sodium tank 4a will be reduced. and the in-furnace sodium 19 level match. Next, the radioactivity concentration in the sodium can be measured by the radiation detector 12 by closing the valve C29 and starting the circulation pump 24 to circulate the gas (in the direction of the arrow in FIG. 4). The gas whose measurement has been completed is stored in advance in the sodium tank 4a.
After the sodium inside is discharged, the valve C29 is opened, the valve A26 is closed, and the valve 28 is opened, and the sodium is discharged from the gas exhaust pipe 7 into the cover gas 18.

By repeating the above steps for each fuel assembly, it is possible to measure the presence or absence of damaged fuel over all of the core fuel.

In addition, as for the driving conditions of the guide tube 1 itself, in order to detect damaged fuel with high accuracy, the fuel assembly 2
With the nuclear fission products 30 accumulated in the sodium tank 4a, the guide tube 1 is raised about 400 to 500 mm from the top of the fuel using a drive device, and the above-mentioned gas bubbling is performed. This is because when discharging sodium after measuring the radioactivity concentration in the gas, if the shipping port 2 remains in close contact with the top of the fuel, the gas will be blown directly into the fuel assembly 20, causing disturbance to the adjacent fuel. This is because the adjacent fission products to be measured next time will diffuse into the reactor coolant sodium 19, making it difficult to detect the position of the damaged fuel.

By discharging the measured sodium 400 to 500 mm above the top of the fuel as described above, there is a question of whether or not there will be any effect of the fission products that have diffused into the sodium 19 inside the reactor. Since the shipping port 2 of the fuel assembly 20 is in close contact with the top of the fuel assembly 20, the flow of sodium inside the fuel assembly is stopped at the shipping port 2, and the concentration of fission products 30 in the sodium 19 inside the reactor is reduced. Because it is higher than that diluted in
It becomes easy to distinguish it from these background levels.

In addition, the sodium 19 inside the furnace, such as the guide tube 1, the sodium sampling tube 3, and the sodium tank 4a, etc.
Tanks immersed in or in contact with sodium
The sodium temperature of the piping is approximately
Since the temperature is as high as 250°C, there is a risk of it breaking due to thermal stress if it is not sufficiently preheated in advance. is less,
In addition, the entire area is heated uniformly, and the spiral shape absorbs thermal expansion of piping, etc., making it safe. In addition, since a gas blowing nozzle 4b in which cylindrical punching metals are arranged in double rows is installed in the sodium tank 4a, many small gas bubbles 23 are generated, so that fission products 30 dissolved in the sodium 19 are generated. is actively transferred into the circulating gas, the resolution of the radiation detection device 12 is improved, and damaged fuel is detected effectively.

In the above explanation, the case where the present invention device is applied to a Na-cooled fast breeder reactor is described, but it goes without saying that it can also be applied to a light water reactor, a heavy water reactor, etc., and the same effects as in the above embodiment can be obtained. have In addition, the detection target is not limited to nuclear fuel assemblies in the reactor core.
Nuclear fuel assemblies in in-reactor storage racks can also be detected.

In addition, in Figure 4, if a sodium level meter (contact type or continuous type) is installed in the sodium tank, it is possible to grasp fluctuations in the sodium level in the tank, including during gas bubbling, and monitor abnormal changes in the sodium liquid level. , it is possible to increase the safety of the device.

As described above, this invention brings the flexible member into close contact with the top of the fuel assembly, collects the coolant from only the fuel assembly,
In addition, the gas injection nozzle generates many small gas vesicles and activates the transfer of the fission products dissolved in the coolant into the circulating gas, making it possible to detect the location of damaged fuel with high accuracy, and to eliminate the need for preheating heaters and thermoelectric The pair allows for appropriate preheating, reduces thermal stress, prevents breakage, and improves safety, among other effects.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an embodiment of the present invention, FIG. 2 is a side view showing how the preheater and thermocouple are installed in the sodium sampling pipe section of FIG. 1, and FIG. 3 is the sodium tank of FIG. 1. FIG. 4 is a cross-sectional view showing the operating state of the device of the present invention. In the figure, 1 is a guide pipe, 2 is a shipping port, 3 is a sodium sampling pipe, 4a is a sodium tank, 4b is a gas blowing nozzle, 5 is a gas supply pipe, 6 is a gas sampling pipe, 8 is a seal flange, 10 is a A drive device, 12 a radiation detection device, 13 a gas circulation unit, 14 a control panel,
21 is a preheating heater, and 22 is a thermocouple. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. The guide tube has a flexible member at its tip that is in close contact with the top of the nuclear fuel assembly housed in the reactor vessel, and the coolant is collected through the flexible member into a coolant tank provided inside the guide tube. A coolant sampling pipe for introducing a coolant, a blowing nozzle for supplying circulating gas into the coolant tank through a cylindrical punching metal and bubbling the gas, and a gas sampling pipe for collecting the gas bubbled circulating gas. , comprising a radiation detector for detecting the radiation concentration in the circulating gas sampled by the gas sampling pipe, and a heating means comprising a preheating heater and a thermocouple for preheating the coolant tank and the coolant sampling pipe to a predetermined temperature. A damaged fuel position detection device characterized by: