JPH0141959B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an output detection device for a power range in a nuclear instrumentation system installed in a nuclear reactor.

In general, the nuclear instrumentation of a nuclear reactor, for example, the nuclear instrumentation system of a boiling water reactor, includes a reactor power detection unit that includes a startup region neutron detector, an intermediate region neutron detector, a power region neutron detector, and a power region neutron detector. A traveling neutron detector is provided to calibrate the detector measurements.

Here, the startup region neutron detector and the intermediate region neutron detector are used from reactor startup to steady operation output, and are 10 nv to 10 ⁷ nv, respectively.
It is used in the low neutron flux region of 10 ⁶ nv to ^{10 12} nv, and is withdrawn from the reactor outside this region.

On the other hand, power range neutron detectors are used to monitor the reactor power under steady-state operating conditions, and the neutron range used is as high as 10 ¹¹ nv to ^{10 14} nv, and since they are not extracted from the reactor, a large amount is always generated. will be irradiated with neutrons.

Generally, the power output of a nuclear reactor is measured by measuring the number of neutrons produced by nuclear fission per unit time, but since it is not possible to directly measure the number of neutrons, for example, uranium-235 ( The number of neutrons can be indirectly measured using nuclear reactions such as n, f), boron-10 (n, α), rhodium (n, β), and platinum (n, γ). It is.

In other words, detectors that utilize the (n, β) reaction of rhodium or the (n, γ) reaction of platinum are generally called self-power neutron detectors. A detector using this is called a fission chamber type neutron detector.

This fission type ionization chamber type neutron detector generally has a pair of electrodes facing each other in the ionization chamber, and uranium-235 is attached to one of the electrodes, and the uranium-235 is When a nuclear fission reaction (n, f) is caused, gas in the ionization chamber is ionized by the ionization action of the fission fragments, and this is measured.

However, when such a nuclear fission chamber type neutron detector is used as a neutron detector for the output region, it will be placed in a high neutron region, so uranium-235 will be drastically reduced, and the neutron detector will be It is necessary to replace it periodically.

Moreover, even when used in a nuclear reactor, it is necessary to use a traveling neutron detector in order to calibrate the detection sensitivity.

In addition, a method of adding uranium-234 to the electrodes of the ionization chamber has been adopted to extend the replacement period.
Even in this case, the replacement period is at most 4
It will simply be doubled.

Here, we will discuss the problems with conventional reactor power detection devices in light water reactors such as boiling water reactors. A large number of fuel assemblies are installed in the core of a nuclear reactor, and these fuel assemblies are burned by a nuclear fission reaction. The output of a nuclear reactor is proportional to the number of nuclear fission per unit time. Furthermore, when fuel fission, three types of nuclear fission fragments, fast neutrons, and prompt gamma rays are generated. Therefore, if any one of these three is detected, the output of the nuclear reactor can be determined. Conventional reactor power detection equipment selects fast neutrons from among these three types as described above, and detects these fast neutrons after they are decelerated by the water around the fuel assembly and become thermal neutrons. .

However, with this detection method, it was not possible to accurately determine the local output of the fuel assembly because it was measured indirectly through the medium of water.

Therefore, for example, a neutron detector has been developed in which instead of uranium-235, uranium-238, which only reacts with fast neutrons, is attached to the electrode of the ionization chamber.
This neutron detector has a low reaction rate and has not yet achieved sufficient practical performance.

Furthermore, the self-powering type neutron detector described above also has the drawback that its detection sensitivity gradually decreases in the output region, requiring sensitivity calibration.

Therefore, a neutron detector using gamma rays was developed.

That is, in a nuclear reactor, in addition to neutrons produced by nuclear fission of fuel, gamma rays are produced by activation of reactor constituent materials, fissile organisms, or simultaneously with nuclear fission of fuel.

The amount of gamma rays that are generated simultaneously with nuclear fission of fuel is two to three orders of magnitude larger than other gamma rays, and is also generated in proportion to the number of nuclear fission, so by detecting these gamma rays, the degree of fuel combustion can be determined.
In other words, the output of the nuclear reactor can be detected.

However, since ionization chamber-type detectors are conventionally used as such detectors, ionized gases such as argon and helium may change in quality due to large amounts of irradiation with neutrons or gamma rays, and regular maintenance is still required. It needed to be replaced.

The present invention has been made in response to such circumstances, and its first object is to provide an output detector that uses gamma rays and has an extremely long life.

A second object is to provide an output detector that can directly measure the amount of gamma rays produced by nuclear fission.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to an embodiment shown in the drawings.

FIG. 1 is a cross-sectional view of a nuclear reactor core suitable for carrying out the present invention, FIG. 2 is a vertical cross-sectional view thereof, FIG. 3
It is an enlarged view of part A of the figure.

In FIG. 1, in the center of the reactor vessel 1,
Fuel assemblies 2 are arranged in parallel, and control rods 4 for controlling nuclear fission of four fuel assemblies 2 are each provided as a set.

The fuel assembly 2 is covered with cooling water filled in the reactor vessel 1, and a power range detector assembly 3 is disposed at the center of each of the four control rods 4.

This output area detector assembly is constructed as shown in FIGS. 3 and 4.

In FIG. 3, inside the protection tube 7 extending downward from the detection range 6 in contact with the fuel assembly 2,
A heating element 9 having a guide tube 8 is inserted into its center.

The heating element 9 is a cylinder made of, for example, stainless steel that generates heat by irradiation with gamma rays generated by nuclear fission of fuel, and the protection tube 7 and guide tube 8 are made of, for example, stainless steel.

Further, the protective tube 7 is provided so as to closely contact and surround the outer surface of the heating element 9, but in the detection range 6, there are one or several places where the protective tube 7 and the outer periphery of the heating element 9 overlap. Adiabatically isolated.

Furthermore, in this embodiment, the heating element 9 has a reduced diameter portion 10.
is formed, and a heat insulating space 11 is provided.

In the output range detector assembly 3 configured as described above, a temperature detector such as a thermocouple 13 is attached to the so-called code junction 12 where the heating element 9 is in contact with the protection tube 7. When a thermocouple 15 is attached to the so-called hot junction 14 of the diameter portion 10, the heating element 9 is heated by gamma rays.
When heat is generated, the cooling water 5 cools the cord junction 12 of the heating element 9 through the protection tube 7 to prevent the temperature from rising.
4 is not in direct contact with the protection tube, so it is difficult to cool down, so it generates heat corresponding to the irradiated gamma dose, and thermocouple 13
The heat generation increment of the heating element 9 can be determined from the temperature difference measured by the thermocouple 15 and the thermocouple 15.

On the other hand, a guide tube 8 provided at the center of the heating element 9
A traveling path 16 is provided on which a traveling gamma ray detector (not shown) travels.

This traveling type gamma ray detector directly measures the gamma ray dose of each part of the heating element 9 in a timely manner, and is pulled out of the reactor core each time the measurement is completed, so there is no need to worry about durability even with conventional ionization chamber type detectors.

If the relationship is determined in advance by measuring the heat generation increment of the heating element 9 corresponding to the gamma ray dose measured by the traveling gamma ray detector, the relationship can be determined in advance by continuously measuring the heat generation increment of the heating element 9. The furnace output, which is proportional to the gamma ray dose, can be continuously converted and displayed.

Although the arithmetic unit that performs this conversion is not shown, it is preferable to use a microcomputer that is generally used for various types of data analysis.

As described above, the output detection device of the present invention has the following features:
It has unique heat-generating properties without being altered by irradiation with neutrons or gamma rays, and has enough durability that it does not require replacement during the operating period of the nuclear reactor.

The traveling gamma ray detector used for calibrating the measured value by the heating element is used less frequently and therefore has less change in characteristics, so maintenance is easy.

Moreover, since the present invention directly measures gamma rays generated by nuclear fission, efficiency is high and fuel can be used more effectively.

Note that the output detection device of the present invention can be applied to a pressurized water reactor, in addition to the boiling water reactor shown in the above embodiment.
It can also be applied to new type converter reactors and fast breeder reactors.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of the core of a nuclear reactor suitable for carrying out the present invention, FIG. 2 is a vertical cross-sectional view thereof, FIG. 4 is an enlarged view of section A in FIG. 3. FIG. 1... Reactor vessel, 2... Fuel assembly, 3...
Output area detector assembly, 4... Control rod, 5... Cooling water, 6... Detection range, 7... Protection tube, 8... Guide tube, 9... Heating element, 10... Diameter reduction part, 11...
Heat insulation space, 12...Cold junction, 1
3, 15...Thermocouple, 14...Hot junction, 16...Running path.

Claims (1)

[Claims] 1. A heating element that generates heat due to gamma ray irradiation, a temperature detector that measures the heat generation increment of this heating element, a traveling gamma ray detector that measures the gamma ray dose in the vicinity of the heating element in a timely manner, and the temperature detector and the traveling gamma ray detector. In the output detection device for a nuclear reactor, the device includes an arithmetic unit that calculates a gamma ray dose irradiated to a measurement point of the heating element from detection data of a type gamma ray detector, wherein the heating element is in close contact with the outer surface of the heating element. The heat generating element is housed in a protective tube surrounding it, the outer periphery of one or several parts of the measured part is adiabatically isolated from the protective tube, and a guide tube is provided in the center of the heating element, An output detection device for a nuclear reactor, characterized in that a traveling path for a traveling gamma ray detector is provided in the guide tube.