Computing model of complex gamma dose field radioactive source

Abstract

The invention discloses a calculation model of a complex gamma dose field radioactive source, aiming at the problem that the contribution of a body source and a line source is difficult to distinguish in the existing monitoring, a natural background, a uniform dispersion body source and a leakage channel line source are simultaneously incorporated into a gamma dose rate measurement model in a cuboid space; establishing a body source volume fraction and line source line integral dose formula according to nuclide activity, a kerma constant and geometric parameters; 2 or more measuring points are arranged to construct a linear equation set, MCNP5 Monte Carlo simulation is used for optimizing the size, the position and the direction of the lead shielding body, equation morbidity is reduced, and contribution of each source item is obtained through solving. The experimental verification deviation is less than 33%, and reliable technical support can be provided for radiation source discrimination and protection after nuclear power station and nuclear facility accidents.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of radiation protection, and in particular to a calculation model of a complex gamma dose field radioactive source. BACKGROUND

[0002] In the existing radiation environment monitoring practice, the gamma dose rate meter is usually arranged in a single point and single angle mode, and can only give the integrated dose rate and cannot distinguish the contribution of different radioactive source terms. In nuclear power plants, reprocessing plants and nuclear accident scenarios, radionuclides can be in the form of dispersion to fill the entire space to form a volume source, or along the pipeline cracks, ventilation shafts or core melt channels to form a line source. The two are superimposed in the spatial dose field, making it difficult for traditional monitoring methods to identify the source terms.

[0003] The International Atomic Energy Agency (IAEA) technical document TECDOC-1162 and the domestic standard GB / T 15446-2011 both point out that when the dose field simultaneously exists volume source, line source and natural background, the existing analytical or empirical formula needs to introduce strong assumptions (such as uniform infinite volume source, infinitely thin line source), so that the calculation result deviates from the actual distribution by more than 50%. In recent years, although some people have tried to use multi-probe arrays or mobile platforms to obtain spatial distribution, but due to the high cost, long deployment period, and variable environmental conditions, it is difficult to promote on site. In addition, the existing numerical simulation is mostly focused on a single source term or simple geometry, and the coupling of volume source and line source, shielding optimization and ill-conditioned equation system solving problems are not enough, making it difficult to directly apply the calculation model in actual complex environments. Therefore, it is urgent to establish a complex gamma dose field source term identification technology that takes into account the theoretical accuracy, experimental feasibility and on-site adaptability, to break through the bottleneck that traditional monitoring cannot distinguish the contribution of volume source and line source, and to provide reliable support for nuclear facility operation, emergency and decommissioning. SUMMARY

[0004] The present application aims to provide a calculation model of a complex gamma dose field radioactive source, to solve the problem that the prior art cannot accurately distinguish and identify the contribution of different radioactive source terms, especially volume source and line source, to the total dose rate in a complex gamma dose field.

[0005] To achieve the above-mentioned purpose, the present application provides the following technical solution: a calculation model of a complex gamma dose field radioactive source, comprising the following steps:

[0006] S1, establishing a gamma dose rate measurement model containing natural environmental background, volume source and line source, the volume source being formed by radionuclides dispersed in space, and the line source being formed by radionuclides distributed along the leakage channel;

[0007] S2, constructing a dose rate calculation model of the volume source and the line source based on the activity concentration of the radionuclide, the air kerma rate constant and the spatial geometric parameters;

[0008] S3, establishing a linear equation group about the body source and the line source contribution by setting multiple measurement points;

[0009] S4, optimizing the measurement condition by using the Monte Carlo simulation method to reduce the ill-condition of the equation group;

[0010] S5, solving the equation group to obtain the contribution value of the body source and the line source to the gamma dose rate.

[0011] Specifically, the body source model regards the space as a cuboid, and the radionuclides are uniformly distributed, and the net dose rate of the measurement point is the total gamma dose rate minus the natural background determined in advance.

[0012] Specifically, the line source model regards the line source as a finite length uniform distribution source, and the dose rate at the measurement point is calculated by a line integral formula, and is approximately simplified when the measurement point is close to the center of the line source.

[0013] Specifically, the establishment of the linear equation group includes: setting the measurement point position, so that the body source and the line source contribution coefficient satisfy a certain relationship; calculating the equation group coefficient through the geometric parameter; and constructing the equation group by using the measurement values of not less than two measurement points.

[0014] Specifically, the Monte Carlo simulation method includes: using the MCNP5 software to establish a space radiation transmission model; setting a lead shielding body, optimizing the size, position and direction thereof to increase the difference between the contributions of different source terms; and determining the optimal measurement condition through simulation calculation.

[0015] Specifically, the optimization target of the lead shielding body is to maximize the dose rate ratio of the line source and the body source inside and outside the shielding body.

[0016] Principles and beneficial effects of the technical solution:

[0017] By constructing a gamma dose rate calculation model coupled with the body source and the line source, three types of contributions of the natural background, the diffusely distributed body source and the line source along the leakage channel are separated, a dose rate equation is established by using the radionuclide activity concentration, the air specific energy release rate constant and the space geometric parameter, and a linear equation group is formed by arranging detectors at multiple measurement points; for the possible ill-condition problem of the equation group, the size, position and direction of the lead shielding body are optimized by introducing the MCNP5 Monte Carlo simulation, the dose rate difference between the line source and the body source under different measurement conditions is artificially enlarged, so as to reduce the condition number of the coefficient matrix, and finally the independent contributions of the body source and the line source to the total gamma dose rate are accurately obtained by solving the optimized equation group, the discrimination of different source terms in a complex radiation field is realized, the beneficial effects are embodied in that the limitation that the traditional single detector cannot distinguish multiple source terms is broken through, the accuracy and reliability of the radiation source identification in a complex environment such as a nuclear accident are significantly improved, the experimental verification shows that the calculation deviation is less than 33%, and effective technical support is provided for the radiation protection and safety monitoring of nuclear power plants and nuclear facilities. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the spatial coordinates of the measurement point;

[0019] Figure 2 This is a schematic diagram showing the location of the experimental radon chamber. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments:

[0021] 1. Calculation model of γ dose rate for different radiation sources

[0022] When a radionuclide diffuses throughout the entire space, the entire space becomes equivalent to a volume source. At this time, there are other radionuclide leakage channels within the space. If this radioactive source is assumed to be a line source, and the environmental gamma dose rate is monitored at a certain location in the space, both the volume source and the line source will contribute to the gamma dose rate at that point.

[0023] The gamma dose rate measurement D at a point in space mainly consists of three parts:

[0024] (1) Natural environmental background D b

[0025] (2) Contribution of body source D t

[0026] (3) Contribution D of the line source x

[0027] Right now:

[0028] D = D b +D t +D x

[0029] The dose rate meter measurement results are the sum of the three sources mentioned above, and the contribution of each individual radiation source term cannot be distinguished. The gamma dose rate value D from the natural environmental background. b One is relatively fixed and can be determined by measurement before the experiment, while the other two are calculated through a theoretical model.

[0030] 1.1 Body Source Model

[0031] Approximating the space as a cuboid, and assuming that the radioactive nuclides are uniformly distributed, such as... Figure 1 As shown.

[0032] The dose rate generated by the body source at the measurement point P(x0, y0, z0) is expressed as:

[0033]

[0034] In the formula, D tD is the air kerma rate produced by the volume source at the measuring point, unit Gy / s; C i C is the activity concentration of the i-th radionuclide in the volume source, Bq / m3; Γ ti Γ is the air kerma rate constant of the i-th radionuclide in the volume source, Gy.m2 / (Bq.s); m is the number of radionuclides in the volume source.

[0035] 1.2 Line source model

[0036] Assuming that the line source is uniformly distributed in space, the length of the line source is 2L, and the γ dose rate meter is placed at a position that divides the line source into two segments Δ and 2L-Δ, and the vertical distance from the line source is r0, as shown in FIG. 1, the dose rate produced by the line source at the measuring point P(x0, y0, z0) is: Figure 1

[0037]

[0038] In the formula, D is the air kerma rate produced by the line source at the measuring point, unit Gy / s; C is the activity concentration of the i-th radionuclide in the line source, Bq / m3; Γ is the air kerma rate constant of the i-th radionuclide in the line source, Gy.m2 / (Bq.s); and m is the number of radionuclides in the line source.

[0039] The distance from the measuring point to the line source is small compared to the length of the line source, and when the γ dose rate meter is placed close to the center of the line source (i.e., Δ≈L), the inverse tangent in the above formula can be considered as a constant value π / 2, and can be simplified as follows:

[0040]

[0041] 1.3 Numerical estimation equation set of dose rate contribution of line source and volume source

[0042] When γ dose rate measurement is performed at a point in space, the dose rate D at the point is:

[0043]

[0044] Assuming that there are S measuring points in space, when there are two parallel line sources 1 and 2, the γ dose rate measurement value at a measuring point j is:

[0045]

[0046] j = 1, 2, …, s

[0047] In the formula, D j is the measurement value of the γ dose rate meter at the j-th measuring point; D bj is the background value of the γ dose rate at the j-th measuring point; x 0j , y 0j , and z 0j ​The coordinates of the jth measuring point; r 1j The distance of the jth measuring point from the line source 1; r 2j The distance of the jth measuring point from the line source 2; Δ 1j The distance of the jth measuring point from one end point of the line source 1; Δ 2j The distance of the jth measuring point from one end point of the line source 2; A L1i The linear activity concentration of the ith gas in the line source 1; A L2i The linear activity concentration of the ith gas in the line source 2; Γ x1i The air kerma rate constant of the ith gas in the line source 1; Γ x2i The air kerma rate constant of the ith gas in the line source 1.

[0048]

[0049] j = 1, 2, …, s

[0050]

[0051] Wherein, X, Y, Z are three position-independent unknowns, the geometric position of the measuring point can be set by oneself, therefore when the measuring point is not less than 3, X, Y, Z can be solved, and then the dose rate generated by the line source and the volume source at the measuring point can be respectively solved.

[0052] Due to the gamma dose rate generated by the volume source at the measuring point, Y and Z can be combined into one unknown by setting a special measuring position so that the coefficients of Y and Z are equal. The more intuitive setting condition is:

[0053]

[0054] Therefore:

[0055]

[0056] j = 1, 2, …, s

[0057]

[0058] At this time, at least 2 measuring points are needed, that is, S = 2, two equations are established according to the following formula, and the values of X and (Y+Z) can be solved as long as the parameters of space, line source and measuring point are known:

[0059] a1X+b1(Y+Z) = D1-D b1

[0060] a2X+b2(Y+Z) = D2-D b2

[0061] Wherein,

[0062]

[0063] The equation group coefficients are parameters only related to the geometry of the measurement space, so in actual measurement, the values of a1, a2, b1, b2 can be determined according to the size of the volume source space, the position and height of the line source, the spatial position of the measurement point, and other parameters. Then the contributions of the line source and the volume source can be obtained by solving the equation group.

[0064] 2Optimization of equation group coefficients

[0065] In actual problems, the space size is usually large, which will result in small differences in measurement conditions, large condition numbers of the equation group coefficient matrix, and a sick state feature, so that the contributions of each radiation source term cannot be accurately obtained. To solve this problem, the measurement scheme needs to be optimized before actual measurement, so that the differences in the equation group coefficients are as large as possible, so that a reasonable solution can be obtained.

[0066] To solve the above problems, the following methods are adopted:

[0067] ① A lead shielding body is placed in the space by using the Monte Carlo radiation transport software MCNP5 to establish a space model. In order to change the contributions of different radiation source terms to the γ dose rate, a lead shielding body is designed in the program and placed in the space, and γ dose rate simulation calculation is carried out outside and inside the lead shielding body;

[0068] ② The size of the lead shielding body is preliminarily set according to the size of the γ dose rate detector, and the ratio of the γ dose rate outside and inside the lead shielding body when the line source and the volume source exist in the space alone is simulated and calculated. Assuming that the ratio of the γ dose rate outside and inside the lead shielding body when the line source exists in the space alone is θ1, and the ratio of the γ dose rate outside and inside the lead shielding body when the volume source exists in the space alone is θ2;

[0069] ③ The size, position, and direction of the lead shielding body are optimized by the MCNP5 program, so that θ1 / θ2 in step ② is as large as possible;

[0070] ④ When the dose rate inside and outside the lead shielding body is actually measured in the space, the size, position, and direction of the lead shielding body obtained in step ③ are used as the measurement conditions;

[0071] ⑤ According to the ratio θ1 / θ2 obtained in step ③ and the dose rate measured in step ④, the equation group is established to obtain X and (Y+Z), and the contributions of different radiation source terms to the dose rate are obtained.

[0072] Based on the Monte Carlo radiation transport software MCNP5, a calculation model for γ dose rate detector measurement under shielded and unshielded conditions is established. By optimizing the size, position, and direction of the shielding body, the difference between the two measurement conditions is increased, thereby reducing the ill-conditioning of the equation group (10).

[0073] Embodiment:

[0074] The experiment was verified in the radon chamber space. The radon chamber was used as a volume source, and the experiment was simulated by multiple measurements of point sources to simulate a line source. The radon chamber used in the experiment had a size of 1.575 m x 1.1 m x 1.215 m, and the radon chamber was adjusted to have a radon concentration of about 65,000 Bq / m3. After waiting for more than 4 h, the radon and daughter equilibrium was started, and the gamma dose rate was measured. The point source used in the experiment was a 1.7 x 106 Bq activity 137Cs source.

[0075] The experimental radon chamber position diagram is shown in Figure 2 A coordinate system was established with the center of the front face of the radon chamber as the origin, and the coordinates of the two measurement point positions designed were P1(0, 0.27, 0) and P2(0.54, 0.27, 0). The line source position was on the horizontal center line of the front face of the radon chamber, as shown by the red line in the figure.

[0076] In the experiment, the line source was simulated by point sources, and the dose rate values of multiple points uniformly spaced on the line source were averaged to obtain the gamma dose rate value of the line source.

[0077] The gamma dose rate values of the measurement points P1 and P2 were measured for the environmental background, the environmental background plus the line source, the environmental background plus the volume source, and the environmental background plus the line source and the volume source. The results are shown in Table 1. The measurement results of the environmental background plus the line source and the environmental background plus the volume source after deducting the environmental background can be used as the actual measurement results for comparison, and the measurement results of the environmental background plus the line source and the volume source are used to solve the equation set.

[0078] Table 1 Gamma dose rate measurement values under different radioactive source terms (μGy / h)

[0079]

[0080] The equation set coefficients were calculated from the geometric position parameters of the measurement points P1 and P2 according to the above formula, and the results are shown in Table 2.

[0081] Table 2 Equation set coefficients for different measurement positions

[0082]

[0083] Substituting the above results into the equation set, we get:

[0084] 5.91X + 6.02Y = 1.495 - 0.230

[0085] 5.25X + 5.19Y = 1.351 - 0.230

[0086] The solution is X = 0.196, Y = 0.017.

[0087] Table 3 Comparison of calculation results and actual measurement results

[0088]

[0089]

[0090] Thus, the line source and volume source contributions can be calculated, which are 92% and 8%, respectively.

[0091] The comparison with the measured results is shown in Table 3. From the results of the experimental verification, the deviation of the calculation results from the measured results is <33%.

[0092] 3. Conclusion

[0093] The application proposes a dose rate calculation model of line source and volume source coupling, derives a coefficient-optimized dose rate contribution numerical estimation equation group, adopts a method of combining Monte Carlo simulation and experiment, establishes a complex gamma dose field radiation source item discrimination technology, and provides important technical support for complex radiation field safety monitoring. Laboratory verification and field application show that the deviation of the calculation results from the laboratory verification measurement results is <33%, indicating that it is feasible to use the application to discriminate the gamma dose rate contribution of different radiation source items.

[0094] The above is only an embodiment of the application, and the well-known specific technical solutions or characteristics in the scheme are not described in detail. For those skilled in the art, without departing from the technical solutions of the application, a number of modifications and improvements can be made, which should also be considered as the protection scope of the application, and these will not affect the effect and practicality of the application. The protection scope of the present application should be subject to the content of its claims, and the specific implementation mode and the like recorded in the specification can be used to explain the content of the claims.

Claims

1. A computational model for a complex gamma-ray dose field radiation source, characterized in that, Includes the following steps: S1. Establish a gamma dose rate measurement model that includes the natural environmental background, volume sources, and line sources. The volume sources are formed by radioactive nuclides diffusely distributed in space, and the line sources are formed by radioactive nuclides distributed along the leakage channel. S2. Based on the activity concentration of radionuclides, the air kerma rate constant, and spatial geometric parameters, a dose rate calculation model for volume sources and line sources is constructed. S3. By setting multiple measurement points, establish a system of linear equations concerning the contributions of volume sources and line sources; S4. Optimize measurement conditions using Monte Carlo simulation methods to reduce the ill-conditioned nature of the equation system; S5. Solve the system of equations to obtain the contribution values ​​of the body source and the line source to the γ dose rate.

2. The calculation model for a complex gamma-ray dose field radiation source according to claim 1, characterized in that: The model of the source treats the space as a cuboid with a uniform distribution of radionuclides, and the net dose rate at the measurement point is the total γ dose rate minus the pre-determined natural background.

3. The calculation model for a complex gamma-ray dose field radiation source according to claim 1, characterized in that: The model of the line source treats the line source as a finite-length uniformly distributed source, and the dose rate at the measurement point is calculated using the line integral formula, with approximate simplification when the measurement point is close to the center of the line source.

4. The calculation model for a complex gamma-ray dose field radiation source according to claim 1, characterized in that: The establishment of the linear equation system includes: setting the location of the measurement points so that the contribution coefficients of the volume source and the line source satisfy a specific relationship; calculating the coefficients of the equation system through geometric parameters; and constructing the equation system using the measurement values ​​of no less than two measurement points.

5. The calculation model for a complex gamma-ray dose field radiation source according to claim 1, characterized in that: The Monte Carlo simulation method includes: establishing a space radiative transfer model using MCNP5 software; setting up a lead shield and optimizing its size, position, and orientation to increase the difference in contributions from different source terms; and determining the optimal measurement conditions through simulation calculations.

6. The calculation model for a complex gamma-ray dose field radiation source according to claim 5, characterized in that: The optimization objective of the lead shield is to maximize the dose rate ratio between the linear source and the bulk source inside and outside the shield.

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