JPH0222356B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a subcriticality monitoring method and apparatus for a nuclear fuel reprocessing system (hereinafter referred to as a reprocessing system).

Spent fuel contains U-, a fissile nuclide.
235, Pu-239, etc., nuclides that emit spontaneous neutrons, such as Cm-242, Cm-244, Pu-238, and Pu-240, are mixed, as well as fission-produced nuclides that emit even stronger gamma rays. In the reprocessing system, spent fuel is dissolved in nitric acid and liquefied to form a treatment liquid, which contains hydrogen, which serves as a moderator, along with the above-mentioned nuclides. Spontaneous neutrons induce nuclear fission absorbed by U-235, Pu-239, etc. In other words, the processing liquid has a neutron multiplication system. Therefore,
When the concentration of Pu-239 or U-235 is high, or when the piping or processing tank is large, it is extremely important for safety to maintain the processing liquid inside the subcritical state.

By the way, in addition to fission products (FP), the spent fuel reprocessing solution contains fissile nuclides U-235,
It contains Pu-239, Pu-241, and Pu-238, Pu-240, which hardly cause nuclear fission. Also,
It contains elements such as Cm-242 and Cm-244, although in extremely small amounts. These include those that significantly affect the k _eff value (U-235, Pu-239, Pu-241) and those that have _a high spontaneous neutron emission rate (Cm-242, Cm −244, Pu−240, Pu−238).

When measuring the neutrons emitted from the reprocessing solution system, a value proportional to the product of a factor dependent on k _eff and a factor proportional to the magnitude of the spontaneous neutron emission rate is obtained. However, since the latter has virtually no effect on k _eff , the degree of subcriticality cannot be correctly monitored unless the latter effect is canceled out. For example, even if the k _eff value is large, Cm−242, Cm−244, Pu−240,
When the concentration of Pu-238 etc. is low, the neutron count rate becomes relatively low, and even if it appears safe at first glance, there is a possibility that it may approach criticality.

The present invention was made based on the above circumstances, and an object thereof is to provide a subcriticality monitoring method and device for a reprocessing system that can monitor subcriticality with high accuracy regardless of the concentration of spontaneous neutron-emitting nuclides. It is said that

In the present invention, a flow path whose neutron multiplication factor is particularly small is provided in parallel or in series with the piping necessary for the reprocessing system, and the neutron counting rate of the piping is performed using the neutron count rate in the flow path. The above objective is achieved by removing from the rate the effect of changes in the concentration of spontaneous neutron-emitting nuclides flowing through the pipe during processing.

The details of the present invention will be explained below. Now, when processing liquids with the same composition are used, there is a channel in which the neutron multiplication effect can be almost ignored (hereinafter referred to as channel A), and a channel in which the neutron multiplication effect cannot be ignored (hereinafter referred to as channel B).
It is assumed that the flow path) and the flow path are respectively flowing.

Let the neutron count rates measured in the A channel and the B channel be φ ₀ and φ, respectively, and the effective multiplication factors are k ₀ and k, respectively.
When the proportionality constants are α ₀ and α, respectively, φ ₀ =α ₀ S ₀ /1−k ₀ (1) φ ₀ =αS/1−k (2). Note that S ₀ and S in equations (1) and (2) are neutron emission rates per unit volume. As mentioned above, since the processing liquid with the same composition flows through both flow paths, S=S ₀ , and from the ratio of equations (1) and (2), k=1−(α/α ₀ )(φ ₀ /φ) (1−k ₀ ) …(3) is obtained.

φ ₀ and φ in equation (3) are determined by measurement. Also,
Experiments and analyzes will be conducted using treatment liquids of various compositions by reducing the cross-sectional area of channel A and placing a neutron absorber in the channel so that k ₀ <<1. Find α/α ₀ by As mentioned above, k ₀
If <<1 is realized, equation (3) becomes k≒1−(α/α ₀ )(φ/φ ₀ ) …(4), and φ ₀ , φ, α/α obtained as above ₀
k can be obtained by substituting into equation (4).

Next, the multiplication factor k _x of the treatment tank is also given by k _x = 1 - (αx / α) (φ / φx) (1 - k) ... (5) In equation (5), k is given by equation (4) is given by
Further, φ and φx are determined by measurement, and αx/α is determined by calculation.

In the present invention, the subcriticality of the piping or treatment tank is monitored as described above.

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

In FIG. 1, a branch pipe 2 is provided in parallel with a pipe 1 of a reprocessing system, and this branch pipe 2 is provided with a U-shaped bent portion 2a. In addition, both leg pieces of the U-shaped bent part 2a are neutron shielding bodies 3 arranged between the pipe 1 and the straight part of the branch pipe corresponding to the bottom of the U-shape.
penetrates through. Note that neutron detectors 6 and 7 surrounded by gamma ray shields 4 and 5 are opposed to the pipe 1 and the straight portion.

In the above configuration, if the cross-sectional area of the branch pipe 2 is made small so that the multiplication factor when the processing liquid flowing in the pipe 1 is divided is particularly small, the outputs of the detectors 6 and 7 can be used to The multiplication factor k of the pipe 1 can be determined in the same manner as explained for the above equations (3) to (4), and regardless of the concentration of spontaneous neutron-emitting nuclides in the processing liquid, the subcriticality of the pipe 1 can be determined. Accurately monitor the degree of
can be evaluated.

FIG. 2 shows another embodiment of the invention. In this embodiment, instead of the branch pipe 2, a hollow cylindrical multiplication suppressor 8 made of a neutron absorbing material such as B ₄ C is installed inside the pipe 1. Further, the detector 6 is opposed to the pipe 1 at a position where the multiplication suppressor 8 is not present, and the detector 7 is opposed to the pipe 1 at a position where the multiplication suppressor 8 is present. The piping 1 is provided with flange-shaped neutron shields 9 and 10 at positions facing both ends of the multiplication suppressor 8.
is installed.

In this embodiment, since the multiplication suppressor 8 is provided in a part of the pipe 1, the multiplication factor k ₀ in that part is smaller than that in other parts. The size, thickness, composition, etc. of the multiplication suppressor 8 are appropriately selected, and k<<1.
If so, the part where the multiplication suppressor 8 is arranged can be handled in the same way as the branch pipe 2 of the embodiment shown in FIG.
The subcriticality of the pipe 1 can be monitored in the same manner as in the previous embodiment.

FIG. 3 shows a third embodiment of the invention. In this figure, the piping 1 in the embodiment of FIG. 1 opens into a processing tank 11 at one end, and a neutron detector 13 surrounded by a gamma ray shield 12 is opposed to the processing tank 11.

In this embodiment, the multiplication factor k x of the processing tank 11 is calculated by the above equation (5) using the multiplication factor _k of the piping obtained in the embodiment shown in FIG. 1 and the output of the detector 13. The degree of subcriticality of the processing tank 11 can be accurately monitored and evaluated.

[Brief explanation of drawings]

FIG. 1, FIG. 2, and FIG. 3 are schematic diagrams of the first, second, and third embodiments of the present invention, respectively. 1... Piping, 2... Branch pipe, 3, 9, 10...
Neutron shield, 4, 6, 12... Gamma ray shield, 6, 7, 13... Neutron detector, 8... Multiplication suppressor, 11... Processing tank.

Claims (1)

[Claims] 1. Neutron flux emitted from nuclear fuel reprocessing system piping through which a processing liquid flows, and neutron flux emitted from a flow path in which a processing liquid having the same composition as the processing liquid flows and has a particularly low neutron multiplication factor. A subcriticality monitoring method for a nuclear fuel reprocessing system, characterized in that the dependence of the neutron count rate on the concentration of spontaneous neutron emitting nuclides is removed by measuring the neutron fluxes and the neutron fluxes. 2 A branch pipe with a small diameter is provided in the nuclear fuel reprocessing system piping so that the neutron multiplication factor is particularly small, a neutron shield is provided between this branch pipe and the above piping, and the neutron shield is installed on the side of the branch pipe. A subcriticality monitoring device for a nuclear fuel reprocessing system, characterized in that a neutron detector surrounded by a gamma ray shield is disposed on the opposite side of the neutron shield and on the side of the piping opposite to the neutron shield. 3 A multiplication suppressor made of a neutron absorber is placed in a part of the nuclear fuel reprocessing system piping, and a neutron detector surrounded by a gamma ray shield is placed in the piping at a position without the multiplication suppressor and at a position where the multiplication suppressor is located. 1. A subcriticality monitoring device for a nuclear fuel reprocessing system, characterized in that neutron detectors are faced to each other and a neutron shield is placed between each neutron detector.