Gaseous radioactive iodine on-line monitoring equipment calibration method

Abstract

The invention belongs to the technical field of radiation monitoring and the technical field of metering, and particularly relates to a gaseous radioactive iodine online monitoring equipment calibration method. Comprising the following steps: step 1, establishing a model; and 2, verifying the radioactive source. The solid source calibration method has the beneficial effects that the accuracy of the calibration result is improved by 10-15% compared with that of the current solid source calibration method.

Description

Technical Field

[0001] The invention belongs to the technical fields of radiation monitoring and measurement, and in particular relates to a calibration method for gaseous radioactive iodine online monitoring equipment. Background Art

[0002] According to the relevant requirements of the Technical Specifications for Effluent Monitoring in Nuclear Power Plants, detectors used to measure gaseous effluents need to be calibrated regularly. Currently, the method commonly used on site is to test the detection activity response with a Ba-133 plane source to achieve the purpose of calibration. This method fails to fully consider the distribution of gaseous iodine in the sampling box during continuous sampling. According to existing foreign research data, the maximum deviation may be around 20%. In order to improve the reliability of the measured values ​​of the gaseous iodine detector of the gaseous effluent detection equipment of nuclear power plants, it is necessary to study its calibration scheme and correct the deviation of the calibration scheme at the factory. Summary of the invention

[0003] The purpose of the invention is to provide a calibration method for gaseous radioactive iodine online monitoring equipment, which can effectively solve the problem of large error in the calibration of a continuous gaseous radioactive iodine detector.

[0004] The technical solution of the present invention is as follows: A method for calibrating a gaseous radioactive iodine online monitoring device comprises the following steps:

[0005] Step 1: Model building;

[0006] Step 2: Radioactive source verification.

[0007] The step 1 comprises:

[0008] Using an impregnated activated carbon filter cartridge as the sampling medium for airborne I-131, the iodine inside the sampling filter cartridge is exponentially distributed with the sampling depth:

[0009] A(x)=A0·e -λx ,0<x<L (1)

[0010] Where A(x) represents the depth distribution function of I-131 in the filter box, A0 represents the activity of I-131 per unit depth in the shallow layer, x represents the sampling depth, L represents the depth of the sampling filter box, and λ represents the distribution parameter;

[0011] According to formula (1), the probability density function of the normalized radioactive iodine distribution in the sampling filter box is obtained:

[0012]

[0013] Use standard surface sources to calibrate the detection efficiency of different depth cross-section layers of the activated carbon sampling filter box;

[0014] When the sampling filter cartridge is placed upright, the detection efficiency curve is

[0015] ε1(x) = ax 2 + bx + c (3)

[0016] When the sampling filter cartridge is placed upside down, the detection efficiency curve is

[0017] ε2(x) = dx 2 + ex + f (4)

[0018] In equations (3) and (4), a, b, c, d, e, and f are fitting coefficients; x is the distance between the cross-sectional layer and the inlet end face, that is, the depth of the cross-sectional layer described above, and the value range is: 0 < x < L; L is the total depth of the sampling filter cartridge,

[0019] When the sampling filter cartridge is placed upright, there is

[0020]

[0021] When the sampling filter cartridge is placed upside down, there is

[0022]

[0023] In equations (5) and (6), A is the total activity of iodine in the sampling filter cartridge, and ε defined in equation (5) is the detection efficiency of the measuring instrument for the I-131 radionuclide. By dividing the two equations, an equation with only one unknown λ is obtained:

[0024]

[0025] In the formula, R represents the ratio of the net count rate of the full-energy peak when the sampling filter cartridge is placed upright and upside down.

[0026] The described step 1 includes: when the depth of the sampling filter cartridge is 20 mm, cutting a series of unused sampling filter cartridges into two pieces. From the inlet angle of the sampling filter cartridge, the position coordinates of the cutting surface in the depth direction are 0 mm, 2 mm, 4 mm,..., 20 mm. Place the sampling filter cartridge upright and upside down, then place the standard surface source at the cutting surface, close the two cutting bodies, and place them upright and upside down at the positions in the actual measurement respectively to calibrate the surface source detection efficiency of different depth cross-sectional layers.

[0027] The described step 1 includes: according to equation (7), draw the curves of the distribution parameter λ and the detection efficiency ε varying with R, and directly obtain the distribution parameter λ and the detection efficiency ε by interpolation calculation according to the ratio R of the forward and reverse count rates measured.

[0028] The described step 1 includes: using Ba-133 instead of I-131 to perform efficiency calibration on the measuring instrument.

[0029] Step 2 includes verifying the corrected calibration results, and the verification work uses a radiation source to carry out the primary source response verification.

[0030] The radiation source used in the verification work in Step 2 meets the following conditions:

[0031] (1) 131 I non-uniform standard body source, with a size specification of Φ57.7mm × 26.8mm, the matrix is an activated carbon filter cartridge, and the distribution inside the filter cartridge is non-uniform. The expected distribution parameter is λ = 0.35mm -1 , subject to the actual source parameters produced, the uncertainty introduced by the radiation source is less than or equal to 5%;

[0032] (2) 131 I uniform standard body source, with a size specification of Φ57.7mm × 26.8mm, the matrix is an activated carbon filter cartridge, and the distribution inside the filter cartridge is uniform. The uncertainty introduced by the radiation source is less than or equal to 5%.

[0033] The beneficial effect of the present invention is that the accuracy of the calibration result is improved by 10% - 15% compared with the current solid source calibration method. Specific embodiments

[0034] The following further elaborates on the present invention in conjunction with specific embodiments.

[0035] A calibration method for a gaseous radioactive iodine on-line monitoring device provided by the present invention includes the following steps:

[0036] Step 1: Model establishment

[0037] The impregnated activated carbon filter cartridge is the most common sampling medium for airborne I-131. Iodine shows an exponential distribution with the sampling depth inside the sampling filter cartridge:

[0038] A(x) = A0·e -λx , 0 < x < L (1)

[0039] In the formula, A(x) represents the depth distribution function of I-131 in the filter cartridge; A0 represents the activity of I-131 per unit depth in the shallow surface layer, Bq·mm -1 ; x represents the sampling depth, mm; L represents the depth of the sampling filter cartridge, mm; λ represents the distribution parameter, mm-1. The distribution parameter is related to the form of iodine, environmental temperature and humidity, sampling flow rate, etc. When λ tends to zero, it means that the distribution of I-131 in the filter cartridge tends to be uniform; when λ tends to infinity, it means that the distribution of I-131 in the filter cartridge tends to remain completely in the surface layer.

[0040] According to formula (1), the probability density function of the distribution of normalized radioactive iodine in the sampling filter cartridge can be obtained:

[0041]

[0042] Use a standard area source to calibrate the detection efficiency of different depth cross - section layers of the activated carbon sampling filter cassette. Taking the sampling filter cassette with a depth of 20 mm as an example, a series of unused sampling filter cassettes are cut into two pieces. From the perspective of the inlet of the sampling filter cassette, the position coordinates of the cutting surface in the depth direction are 0 mm, 2 mm, 4 mm, …, 20 mm respectively. Place the sampling filter cassette in the forward and reverse directions,

[0043] Then place the standard area source at the cutting surface, close the two cutting bodies, and place them in the forward and reverse directions at the positions in actual measurement to calibrate the area source detection efficiency of different depth cross - section layers.

[0044] When the sampling filter cassette is placed in the forward direction, the detection efficiency curve is

[0045] ε1(x) = ax 2 + bx + c (3)

[0046] When the sampling filter cassette is placed in the reverse direction, the detection efficiency curve is

[0047] ε2(x) = dx 2 + ex + f (4)

[0048] In equations (3) and (4), a, b, c, d, e, and f are fitting coefficients; x is the distance between the cross - section layer and the inlet end face, that is, the depth of the cross - section layer described above, in mm, and the value range is: 0 < x < L; L is the total depth of the sampling filter cassette, in mm.

[0049] Then, when the sampling filter cassette is placed in the forward direction, there is

[0050]

[0051] When the sampling filter cassette is placed in the reverse direction, there is

[0052]

[0053] In equations (5) and (6), A is the total activity of iodine in the sampling filter cassette, in Bq. There are two unknowns in these two equations, the distribution parameter λ and the total activity A. And ε defined in equation (5) is the detection efficiency of the measuring instrument for the I - 131 radionuclide. By dividing the two equations, an equation with only one unknown λ is obtained:

[0054]

[0055] In the formula, R represents the ratio of the net full - energy peak counting rates when the sampling filter cassette is placed in the forward and reverse directions.

[0056] According to Equation (7), the curves of the distribution parameter λ and the detection efficiency ε varying with R can be plotted in advance. In practical applications, the distribution parameter λ and the detection efficiency ε can be obtained by interpolation calculation according to the curve from the ratio R of the forward and reverse counting rates measured directly.

[0057] Similarly, Ba-133 can also be used to replace I-131 for efficiency calibration of the measuring instrument.

[0058] Step 2: Radioactive source verification

[0059] Verify the corrected calibration results. The verification work must be carried out using a radioactive source to conduct the primary source response verification. The radioactive source used for the verification work is the responsibility of the contractor to customize and purchase and must meet the following conditions:

[0060] (1) 131 I non-uniform standard body source (methyl iodide), with dimensions of Φ57.7mm×26.8mm, the matrix is an activated carbon filter cartridge, and the distribution in the filter cartridge space is non-uniform. The expected distribution parameter is about λ = 0.35mm -1 or so, subject to the actual source parameters produced. The uncertainty introduced by the radioactive source is less than or equal to 5%;

[0061] (2) 131 I uniform standard body source (methyl iodide), with dimensions of Φ57.7mm×26.8mm, the matrix is an activated carbon filter cartridge, and the distribution in the filter cartridge space is uniform. The uncertainty introduced by the radioactive source is less than or equal to 5%.

Claims

1. A calibration method for an on-line monitoring device of gaseous radioactive iodine, characterized in that, It includes the following steps: Step 1: Model establishment; Step 2: Radioactive source verification.

2. The calibration method of a gaseous radioactive iodine on-line monitoring device according to claim 1, wherein, The said Step 1 includes: Using an impregnated activated carbon filter cassette as the sampling medium for airborne I-131, and iodine shows an exponential distribution with the sampling depth inside the sampling filter cassette: A(x) = A0·e -λx , 0 < x < L (1) In the formula, A(x) represents the depth distribution function of I-131 in the filter cassette, A0 represents the activity of I-131 per unit depth in the shallow surface layer, x represents the sampling depth, L represents the depth of the sampling filter cassette, and λ represents the distribution parameter; According to formula (1), the probability density function of the distribution of normalized radioactive iodine in the sampling filter cassette is obtained: Calibrate the detection efficiency of different depth cross-sectional layers of the activated carbon sampling filter cassette using a standard surface source; When the sampling filter cassette is placed forward, the detection efficiency curve is ε1(x) = ax 2 + bx + c (3) When the sampling filter cassette is placed backward, the detection efficiency curve is ε2(x) = dx 2 + ex + f (4) In formulas (3) and (4), a, b, c, d, e, f are fitting coefficients; x is the distance between the cross-sectional layer and the inlet end face, that is, the depth of the cross-sectional layer described above, and the value range is: 0 < x < L; L is the total depth of the sampling filter cassette, When the sampling filter cassette is placed forward, there is When the sampling filter cassette is placed backward, there is In formulas (5) and (6), A is the total activity of iodine in the sampling filter cassette, and ε defined in formula (5) is the detection efficiency of the measuring instrument for the I-131 radionuclide. By dividing the two formulas, an equation with only one unknown λ is obtained: In the formula, R represents the ratio of the net count rates of the full-energy peaks when the sampling filter cassette is placed forward and backward.

3. The calibration method of a gaseous radioactive iodine online monitoring device according to claim 2, characterized in that, The said Step 1 includes: When the depth of the sampling filter cassette is 20 mm, a series of unused sampling filter cassettes are cut into two pieces. From the inlet angle of the sampling filter cassette, the position coordinates of the cutting surfaces in the depth direction are 0 mm, 2 mm, 4 mm,..., 20 mm. The sampling filter cassettes are placed forward and backward, and then the standard surface source is placed at the cutting surface. The two cut bodies are closed and placed forward and backward at the positions in actual measurement to calibrate the surface source detection efficiency of different depth cross-sectional layers.

4. A calibration method for an on-line monitoring device of gaseous radioactive iodine according to claim 2, characterized in that The said Step 1 includes: According to formula (7), draw the curves of the distribution parameter λ and the detection efficiency ε varying with R, and directly obtain the distribution parameter λ and the detection efficiency ε by interpolation calculation according to the ratio R of the forward and backward count rates measured.

5. A calibration method for an on-line monitoring device of gaseous radioactive iodine as claimed in claim 2, characterized in that, The said Step 1 includes: Using Ba-133 instead of I-131 to perform efficiency calibration on the measuring instrument.

6. The calibration method of an on-line monitoring device for gaseous radioactive iodine according to claim 1, characterized in that: The said Step 2 includes verifying the corrected calibration result, and the verification work uses a radioactive source to carry out the primary source response verification.

7. A calibration method for an on-line monitoring device of gaseous radioactive iodine according to claim 6, characterized in that: The radioactive source used in the verification work in the said Step 2 meets the following conditions: (1) 131 I Non-uniform standard body source, with dimensions Φ57.7mm × 26.8mm, the matrix is an activated carbon filter cartridge, non-uniformly distributed within the filter cartridge space, and the expected distribution parameter is λ = 0.35mm -1 , subject to the actual source parameters produced, the uncertainty introduced by the radiation source is less than or equal to 5%; (2) 131 I Uniform standard body source, size specification Φ57.7mm × 26.8mm, the matrix is an activated carbon filter cartridge, evenly distributed in the filter cartridge space, and the uncertainty introduced by the radiation source is less than or equal to 5%.

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