JPH06186340A - Radiation dose rate mapping unit - Google Patents

Abstract

PURPOSE:To evaluate the distribution of radiation dose rate in a working space conveniently with high accuracy by estimating the distribution of radiation sources highly accurately from measurements of radiation fewer in number than the radiation source distribution units, i.e., small region, through the use of known information concerning to the distribution of radiation source in addition to measurements of radiation. CONSTITUTION:A radiation dose rate, measurable by means of an input, a fixed type, and a mechanical scanning radiation measuring equipment, is employed as a radiation measurement. The radiation dose rate 1 is input, along with measuring position information thereof 2, from a radiation dose rate measurement input unit to a radiation source distribution estimating unit. When a detector has directivity, directivity performance and the direction of the detector are also input. The radiation distribution estimating unit determines the radiation distribution 3 in a working space based on the information of radiation dose rate measurements and outputs the radiation source distribution 3 to a radiation dose rate distribution generating unit. The radiation dose rate distribution generating unit determines a radiation dose rate at an arbitrary spatial position which is output along with the position thereof. An output unit comprising a CRT and a hard copy displays the radiation dose rate distribution visually on the CRT and prints the distribution on a paper, as required.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation work space dose rate map creation apparatus, and more particularly, to a radiation dose rate map creation method capable of highly accurately obtaining a dose rate distribution in the entire work space from a small number of measured radiation values. Regarding the device.

[0002]

2. Description of the Related Art When working in an environment where radiation is present, such as in a nuclear power plant, it is necessary to keep the external exposure dose of radiation of workers engaged in the work as low as possible. Therefore, the radiation dose rate in the actual work space must be known when planning a work plan.

Conventionally, the radiation dose rate is measured by (1) a method of directly measuring the dose rate at a plurality of positions in the work space, (2) measuring the radiation source distribution and indirectly evaluating the dose rate based on this. The two methods, i.e. Regarding (1), there are a method of manually using a portable radiation measuring instrument and a method of mechanically scanning a radiation detector as described in JP-A-59-122989. Regarding (2), JP-A-56
-30664 and Japanese Patent Laid-Open No. 61-139305, the radiation source intensity in the work space is evaluated from the radiation measurement values measured by the radiation detector having directivity, and work is performed based on this radiation source intensity information. There is a method of indirectly measuring the radiation dose rate distribution in space.

[0004]

In the case of the prior art shown in the above (1), the measured dose rate is reflected in the dose rate distribution as it is. Therefore, in order to obtain a fine dose rate distribution, it is necessary to obtain the dose rate distribution at many positions. Radiation dose rate measurement is required. In the case of regular inspections that are normally performed at nuclear power plants, the work space is large, so the number of measurement points becomes enormous, and in the case of manual measurement, the issue of radiation exposure of the measurer must be considered separately. I won't. Moreover, when mechanically scanning the radiation detector, a large investment must be made in the scanning device.

In the case of the prior art described in (2) above, since the estimation accuracy of the radiation source distribution is reflected in the evaluation accuracy of the dose rate distribution, it is necessary to estimate the radiation source distribution as finely as possible. The radiation source distribution is calculated as the radiation source intensity within this small region by virtually dividing the equipment, pipes, etc. having radioactivity into small regions. Therefore, in order to obtain the radiation source distribution with high accuracy, it is necessary to divide the region where radioactivity is present into the smallest possible units. In the case of a nuclear power plant, the piping where radioactivity is present is long and complicated, so
The number of small areas becomes enormous. In order to obtain the radiation source distribution, radiation measurement values at spatially independent positions of the number of divided small regions or more are required. As a result, the time and cost involved in radiation measurement increase.

In view of the above-mentioned problem of the prior art of (2), the object of the present invention is to accurately estimate the radiation source distribution from the number of radiation measurement values smaller than the number of small regions which are units of the radiation source distribution. The object of the present invention is to provide a method and apparatus for creating a radiation dose rate map capable of easily and accurately evaluating the radiation dose rate distribution in the work space.

[0007]

The above-described object is not to uniquely obtain the radiation source distribution from the radiation measurement value, but to positively utilize the information known in advance regarding the radiation source distribution to approximate the radiation source distribution. Can be achieved by asking for it.

[0008]

[Function] The dose rate distribution in the work space can be evaluated by virtually dividing this space into multiple meshes, and if one type of nuclide contributes to the air dose rate, It can be represented by 1.

[0009]

[Equation 1]

In equation 1, y is a vector of dose rate distribution (element y _i ), A is a response matrix (element a _ij ), x is a vector of source distribution (element x _j ), and i and j are respectively It is a mesh number related to the dose rate and the radiation source. Since the arrangement of structures, pipes, devices, etc. in the work space can be known in advance, the response matrix A can be calculated by a shielding calculation code that takes into account attenuation / scattering at these shieldings. Therefore, if all of the radiation source distribution x can be known, the dose rate distribution in the work space can be evaluated.

Taking the case of a nuclear power plant as an example, the location where the radiation source exists can be specified as a pipe or equipment that is in contact with the reactor water, and therefore the mesh number where the radiation source exists can be treated as known information in equation (1). However, the source intensity (source distribution) within the mesh number is generally unknown. Therefore, in order to obtain the dose rate distribution y in the entire work space, the source distribution x must first be obtained. In order to obtain the radiation source distribution x exactly from the equation 1, spatially independent dose rate measurement values equal to or larger than the mesh number (unknown number) of the radiation source distribution are required. In this case, one measurement must have the radiation contribution from the mesh in which at least one source is present.
Since the accuracy of the dose rate distribution to be obtained directly depends on the estimation accuracy of the radiation source distribution, it is necessary to obtain the radiation source distribution in small small area units. Therefore, as a practical matter, the number of measurement points becomes considerably large.

Attempts to obtain an unknown number from a smaller number of measurement data than the unknown number are carried out in the field of emission CT. This is an attempt to obtain an unknown number approximately by an iterative operation, not to obtain the unknown number exactly. An EM algorithm based on the maximum likelihood method is one of the methods of this repetitive calculation.

The calculation procedure of the EM algorithm is described in Reference (1): IEEE Transactions on Nuclear Sc
ience, Vol. 35, No. 1, p. 611-614 (1988)), and the procedure when this is applied to the radiation source distribution measurement is shown in Equation 2.

[0014]

[Equation 2]

In Equation 2, the superscript ￣ is the estimated value, the subscript m is the measured value, and k is the number of repeated calculations.

Further, in a nuclear power plant, there are cases where piping of a plurality of systems is complicated. In such a case, considering a pipe in contact with reactor water of the same system, it can be considered that the change in the radioactivity concentration adhering to the inner surface of the pipe in the reactor water route direction is gradual. The known information about the source distribution can be expressed as the smoothing process shown in Formula 3.

[0017]

[Equation 3]

Therefore, by adding the processing of the equation 3 to the calculation procedure of the equation 2, the estimation accuracy of the radiation source distribution can be dramatically improved. In Expression 3, s is a mesh number in which meshes in the same flow channel are arranged in order along the flow of the reactor water, p is the total number thereof, and r is a coordinate. α and β are obtained by the method of least squares from the coordinates of two adjacent meshes and the radiation source intensity estimated value.

If the estimated value of the source distribution can be known,
The evaluation value of the dose rate distribution in the work space can be obtained by the equation 4.

[0020]

[Equation 4]

In the above description, it was assumed that only one type of nuclide contributes to the air dose rate, but in reality, a plurality of nuclides contributes to the air dose rate. However, if the radioactivity ratios of multiple nuclides can be used as known information, it will be mathematically the same as the case of one type of nuclide. Since the source of radiation is a place in contact with the reactor water on the inner surface of pipes and equipment, when multiple nuclides are present, their radioactivity ratio can be considered to be less dependent on the place. Therefore, their ratios can be evaluated by another measure.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of the present invention. This is the case when the dose rate is selected as the radiation measurement value.
If a simple counting rate is selected, it can be converted into a dose rate by multiplying it by a coefficient that takes energy into consideration. The dose rate can be measured manually, or with a fixed radiation detector or a mechanical scanning radiation meter. The dose rate measurement value 1 is input from the dose rate measurement value input device to the radiation source distribution estimation device together with the information 2 of the measurement position (spatial coordinate). This input method is manual or automatic input via an interface. When the detector has directivity, the directivity performance and the spatial orientation of the detector are input together with the position information. In the radiation source distribution estimation device, which includes a collimator as a detector, the radiation source distribution 3 in the working space is obtained based on the information of the dose rate measurement value, and this is output to the dose rate distribution generation device. The dose rate distribution generation device obtains a dose rate at an arbitrary spatial position, and outputs the obtained dose rate 4 and its position (spatial coordinate). The output device is composed of a CRT and a hard copy, which visually displays the dose rate distribution on the CRT and outputs it on paper or the like as necessary.

FIG. 2 shows the configuration of the radiation source distribution estimating device. The structure data storage device stores arrangement data of pipes and equipment in the work space where radioactivity is present and arrangement data of structural materials and the like that contribute to shielding of radiation where radioactivity is not present. The mesh generation device reads out the arrangement data of the pipes and equipment having radioactivity from the structural data storage device, and virtually divides this region into small regions of arbitrary size. In this case, the size of the small area is set in advance in consideration of the accuracy of the dose rate map to be finally obtained. Further, whether to treat the radiation source in the small region as a uniform distribution or the point radiation source is set in advance in consideration of the accuracy of the dose rate map as in the case of the size of the small region. The response matrix generation device is a device that calculates how much the radiation source in the small region created by the mesh generation device contributes to the dose rate at an arbitrary position in space. The position of the space is calculated as the dose rate measurement position 2. The calculation is performed by the method normally used in the occlusion calculation. In the calculation, the directivity of the detector is taken into consideration.

The radiation source distribution calculation device obtains the radiation source distribution in the work space by the iterative calculation method shown in the equations (2) and (3). The response matrix of equation 1 corresponds to the output of the response matrix generator, and the dose rate measurement value of equation 1 corresponds to the measurement value of 1.
Further, the mesh generation device outputs the identification information of the system of the source distribution, and the system information of the source distribution is input to the source distribution calculation device via the response matrix generation device.

FIG. 3 shows the configuration of the dose rate distribution generator. Based on the outputs of the same structure memory device and mesh generation device as in FIG. 2, the response matrix generation device creates a response matrix for the dose rate evaluation position 5 input from the evaluation position input device. The calculation method is the same as in the case of the radiation source distribution estimating apparatus shown in FIG. In the dose rate distribution calculation device, the dose rate 4 at the evaluation position is obtained using the radiation source distribution estimated value 3 according to Equation 4.

The effects of this embodiment will be described with reference to FIGS. As shown in FIG. 4, a computer simulation was performed assuming a two-dimensional mesh as a work plane. The size of the system is 10 × 10, and it is assumed that there is no shield that blocks radiation in the work space. FIG. 5 shows the characteristics of the response matrix used in the simulation. Figure 6
Is the assumed radiation source distribution in the work space, and it is assumed that the point radiation source is located only in the center of the divided small areas of mesh numbers 1 to 10. FIG. 7 shows the dose rate evaluation value when the dose rate is measured at the centers of mesh numbers 1, 4, 7, and 10. Dose rate is when measured with a non-directional detector. The result of FIG. 7 shows that the number of small regions of the radiation source which is an unknown number is 1
While it is 0, it shows that the dose rate can be estimated with high accuracy within about 5% even from the four dose rate measurement values.

[0028]

According to the present invention, a detailed radiation source distribution can be obtained even from a small amount of radiation measurement values, so that the dose rate distribution in the working space can be obtained with high accuracy. Therefore, the cost related to radiation measurement when creating the dose rate map can be significantly reduced, and can be reduced to at least half or less.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram of a radiation source distribution estimation device.

FIG. 3 is a block diagram of a dose rate distribution generation device.

FIG. 4 is an explanatory diagram of mesh division of a computer simulation work space.

FIG. 5 is a characteristic diagram of a response matrix used in computer simulation.

FIG. 6 is a radiation source distribution map used in computer simulation.

FIG. 7 is an explanatory diagram of a dose rate evaluation result of computer simulation.

[Explanation of symbols]

1 ... Dose rate measurement value, 2 ... Measurement position information, 3 ... Source distribution estimated value, 4 ... Dose rate evaluation value, 5 ... Dose rate evaluation position information.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hiroshi Miyai 72-1 Omika-cho, Hitachi City, Ibaraki Prefecture Hitachi Energy Research Laboratory (72) Inventor Katsutoshi Sato 7-2, Omika-cho, Hitachi City, Ibaraki Prefecture No. 1 Inside the Hitachi Energy Research Laboratory

Claims (4)

[Claims] 1. An apparatus for creating a dose rate map of a radiation work space from radiation measurement values, wherein the presence of radioactivity is known in advance in the radiation measurement value input device and in the structures and devices in the work space. A certain area is virtually divided into a plurality of small areas, a radiation source distribution measuring device for obtaining the radiation source intensity in all the small areas divided from the radiation measurement value of the number smaller than the total number of the divided small areas, A radiation dose rate map creation device comprising: a dose rate distribution generation device that obtains a dose rate distribution of an arbitrary work space based on an output from a radiation source distribution measurement device; and an output device thereof. 2. The radiation dose rate map creating device according to claim 1, wherein the radiation measurement value input device is capable of inputting a radiation measurement position and a measurement value in a work space. 3. The radiation source distribution estimating device includes a storage device for storing arrangement data of a structure in the work space in which a radioactivity exists and a structure such as a shield having no radioactivity, and a radioactivity. A mesh generator that virtually divides a structure into small areas of arbitrary size, a response matrix generator that obtains the effect of radiation on the spatial position of the divided small area source, and a source distribution The radiation dose rate map creation device according to claim 1, comprising a calculation device for obtaining the radiation dose rate map. 4. The dose rate generating device includes an evaluation position input device for inputting a position of a space in which a dose rate is to be evaluated, a structure in the work space where radioactivity exists, a shield having no radioactivity, and the like. A storage device that stores the arrangement data of the structure, a mesh generator that virtually divides the structure containing radioactivity into small areas of arbitrary size, and the radiation source of the divided small areas is the arbitrary spatial position. 2. A response matrix generation device for determining the influence of radiation on the radiation and a calculation device for determining the dose rate.
The radiation dose rate map creation device described in.