JPH08320380A - Method for monitoring subcriticality of nuclear fuel by gamma-ray measurement - Google Patents

Abstract

PURPOSE: To simply and rapidly measure the degree of subcriticality of a system containing nuclear fuel. CONSTITUTION: A neutron source 2 (having predetermined relation of neutron and gamma ray generating ratio) is placed near the center in a fuel tank 1 containing solution-like nuclear fuel, and a gamma ray detector 3 is installed at a suitable distance from the tank 1. The output of the detector 3 is processed by a measurement signal processor 4 including a pulse height analyzer. The gamma ray intensity is measured in terms of the case where the fuel concentration is zero and the case where the fuel exists. In order to avoid the large dependence of the attenuation characteristics of the gamma ray upon the fuel conditions in the system, the measured value of the high energy of several MeV or more is employed as the measured value of the intensity. The subcriticality is obtained from S/(P+S) of the ratio of the measured value S of the former = (proportional to the number of neutrons supplied from the source to the system) to the measured value (P+S) of the latter = (proportional to the sum of the number of the neutrons generated by the nuclear fission in the system and the number of the neutrons supplied from the source to the system).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a subcriticality monitoring method for nuclear fuel useful for criticality safety management in various facilities constituting a nuclear fuel cycle. More specifically, the risk of reaching criticality is reduced by monitoring the degree of subcriticality of nuclear fuel during handling of nuclear fuel at facilities that compose the nuclear fuel cycle, thereby rationalizing the method of criticality safety management of the facility. Especially about the technology to be used.

[0002]

2. Description of the Related Art In a subcritical system containing a nuclear fuel, if there is no neutron source inside or in the vicinity of the neutron source, or if the neutron source is weak, the amount of radiation emission does not increase significantly unless the neutron source approaches a critical state. Can't be seen. However, once the criticality is exceeded, slight systematic changes cause a rapid increase in fission reactions. Therefore, careful attention has been paid to the criticality safety control of nuclear fuel, such as limiting the amount of fuel handled to a sufficiently small range or frequently inspecting that the fuel concentration is in a sufficiently low range.

Therefore, in the criticality safety control of nuclear fuel,
If it is possible to measure and monitor the degree of subcriticality in real time, it will be possible to ease the restrictions on the handling amount and reduce the frequency of concentration inspection, and at the same time for workers, the system will actually be a nuclear reaction. Since it is guaranteed to be sub-critical in terms of balance, it is thought to contribute greatly to reducing the mental burden.

Therefore, various attempts have been made to develop a method for measuring the subcriticality. However, it is necessary that the configuration of the nuclear fuel system to be measured is well known in advance, that it is necessary to calibrate using the measured value by another method, or that the measuring device is too large and the measuring time is long. Because there are problems such as overshooting,
Even now, neither method has been put to practical use.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for easily and quickly measuring the subcriticality of a system containing nuclear fuel. In addition, through this, we will try to rationalize criticality safety management and reduce the mental burden on workers in the facilities that make up the nuclear fuel cycle.

[0006]

The present invention relates to the nuclear fission of a nuclear fuel and the intensity of gamma rays generated by a neutron source for a subcritical nuclear fuel system having a neutron source in which the generation ratio of neutrons and gamma rays has a constant relationship. The above technical problem is solved by measuring and separately determining the subcritical degree of the system from the ratio of the gamma ray intensity of only the neutron source.

The gamma ray measurement under each of the above conditions is preferably performed in a high energy region of several MeV or more in order to avoid that the attenuation characteristic of the portion from the gamma ray source to the detector largely depends on the fuel condition in the system. . Therefore, it is preferable to use, as the neutron source, one having a high gamma ray energy associated with neutron generation.

[0008]

[Function] To know the subcriticality of the subcritical nuclear fuel system, some kind of neutron source is necessary to induce the fission reaction in the system. When the steady state is formed by the stationary neutron source, the balance equation of the nuclear reaction per unit time is as follows. L + A = P + S (1) where S is the number of neutrons supplied to the system from the neutron source,
P is the number of neutrons generated by nuclear fission of fuel in the system, A is the number of neutrons absorbed in the system, and L is the number of neutrons leaking out of the system. Here, the ratio of P / (L + A) increases as the system approaches criticality, and becomes exactly 1 in the critical state.

Therefore, if the difference from 1 in this ratio is defined as the subcriticality, (subcriticality) = 1-P / (L + A) = S / (P + S) (2) from the equation (1). . If the value of the ratio on the right side of the equation (2) is obtained by some measurement, the value of the subcriticality on the left side is obtained.

Some conventional methods measure the value of the ratio similar to the equation (2) by neutron measurement. However, in neutron measurement, since the neutron flux or neutron density at the detector position is measured, in order to obtain P, for example, it is necessary to know the fission cross section of the system, the deceleration / diffusion characteristics of neutrons up to the detector position, etc. is there.

That is, the construction conditions of the system including the detector must be known. The present invention seeks to obtain the value of the ratio of equation (2) with minimum systematic information by measuring gamma rays associated with nuclear fission. That is, in the steady state, gamma rays with an intensity proportional to the nuclear fission rate of the fuel are emitted, and if gamma rays of high energy of several MeV or more are measured, the attenuation characteristic up to the detector is the fuel condition within the system. It does not depend much on.

However, it is necessary to use, as the neutron source, one in which the relationship between the number of neutrons generated and the number of gamma rays generated is constant. In the ratio of equation (2), P + S is obtained by measuring the gamma ray intensity from the nuclear fuel system with a neutron source, and S
Is determined by measuring gamma-ray intensities from a neutron-source-only analog system containing no fissile material.

In this case, the U fuel requires an external neutron source such as 252 Cf, but in the Pu fuel that can use its own spontaneous neutrons as the neutron source, the amount of Pu in the system is measured by low energy gamma ray spectrum analysis. Then, S can be obtained by using it and the literature values concerning the spontaneous neutron and Pu gamma ray generation rates.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram for explaining the arrangement for carrying out the present invention. As shown in the figure, a neutron source 2 is placed near the center of a fuel tank 1 containing a solution-type nuclear fuel,
The gamma ray detector 3 is installed at an appropriate distance from.
As the neutron source 2, one having a constant relationship between the number of neutrons and the number of gamma rays is used.

As the gamma ray detector 3, a gamma ray detector 3 having a semiconductor detecting element for outputting a detection pulse having a wave height substantially proportional to the energy of the detected gamma ray is used. The detection signal of the gamma ray detector 3 is processed by the detection signal processing device 4 including a well-known pulse height analyzer, and the gamma ray intensity above a certain energy is measured.

The fuel concentration, the soluble neutron absorbing material concentration, etc. in the fuel tank 1 fluctuate, and the performance of the fixed type neutron absorbing material such as a rod is deteriorated. There is a possibility of reaching.

The gamma ray detector 3 and the detection signal processing device 4 are used to measure the gamma ray intensity when the fuel concentration is zero and when the fuel is present. In order to prevent the attenuation characteristics from the inside of the fuel tank 1 to the gamma ray detector 3 to largely depend on the fuel condition in the system, the measured value of gamma ray intensity for high energy of several MeV or more is adopted. Is appropriate.

Therefore, the detection signal processing device 4 is set so as to output the gamma ray intensity for high energies of several MeV or more analyzed by the pulse height analyzer.

The gamma ray intensity obtained by the former measurement is proportional to the above-mentioned S (the number of neutrons supplied from the neutron source to the system). On the other hand, the gamma ray intensity obtained by the latter measurement is
It is proportional to P + S (the number of neutrons generated by fission in the system plus the number of neutrons supplied to the system from a neutron source). Therefore, the right side of equation (2) is calculated from the ratio of the two, and the subcriticality is obtained.

The subcriticality can be monitored in real time, and if it exceeds a certain value and approaches the criticality, the criticality can be prevented by taking measures such as discharging the fuel in the tank.

[0021]

According to the present invention, the degree of subcriticality of the system containing nuclear fuel can be quickly measured with a simple device without requiring detailed information on the system conditions. It is effective in reducing the mental burden on the person.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an arrangement for carrying out the present invention.

[Explanation of symbols]

 1 Fuel Tank 2 Neutron Source 3 Gamma Ray Detector 4 Detection Signal Processing Device

Claims (2)

[Claims] 1. A neutron source for a subcritical nuclear fuel system having a neutron source in which the generation ratio of neutrons and gamma rays is in a fixed relationship, the fission of the nuclear fuel and the intensity of gamma rays produced by the neutron source are measured separately. A method for monitoring subcriticality of nuclear fuel by gamma ray measurement, which determines the degree of subcriticality of the system from the ratio to the measured gamma ray intensity. 2. A nuclear fuel system in a subcritical state having a neutron source in which the generation ratio of neutrons and gamma rays has a constant relationship, and the fission of nuclear fuel and the intensity of gamma rays generated by the neutron source are measured in a high energy region of several MeV or more. The above-mentioned number Me of gamma rays of only the neutron source to be separately measured
A method for monitoring subcriticality of nuclear fuel by gamma ray measurement, which determines the degree of subcriticality of the system from the ratio with the intensity measurement value in the high energy region of V or higher.