A method for determining the minimum critical accident source term

Abstract

The present invention relates to a method for determining the minimum critical accident source term, belonging to the field of critical accident source term setting. By separately determining the minimum number of fissions V min , the minimum average number of fission neutrons N min , and the minimum leakage rate D of neutrons or photons of each device in the plant min , the number of leaked neutrons or photons DN corresponding to the minimum critical accident is obtained. Combining the number of leaked neutrons or photons DN corresponding to the minimum critical accident and the energy spectrum of the minimum critical accident source term, the leakage source term corresponding to the minimum critical accident is obtained. By using the method for determining the minimum critical accident source term disclosed in the present invention, a relatively enveloping minimum critical accident source term can be determined, which can cover the critical accident source terms under more working conditions, reduces the false negative rate of critical identification, is a prerequisite for accurately identifying and alarming critical accidents, and provides a favorable guarantee for the health and safety protection of staff.

Description

Technical Field

[0001] The present invention belongs to the field of setting critical accident source terms, and particularly relates to a method for determining the minimum critical accident source term. Background Art

[0002] According to regulations and standards such as GB15146.9, in an independent area, for any operation involving a total amount exceeding 700 g 235 U, 520 g 233 U, 450 g of fissile isotopes of plutonium or any combination of 450 g of these isotopes, it is necessary to evaluate the necessity of setting up a critical accident alarm system. In the area where critical accident alarm coverage is required, means for detecting excessive radiation dose or dose rate and sending out a signal for personnel evacuation must be provided.

[0003] When designing a critical alarm system, it is required that the critical alarm system can detect the smallest critical accident. Then, a very important question is how large the smallest critical accident is. In other words, what is the source term of the smallest critical accident? Due to the complex occurrence mechanism and diverse working conditions of critical accidents, there has been no definite conclusion on what the smallest critical accident is. In GB15146.9, the smallest critical accident of concern is defined as the total absorbed dose of neutrons and γ radiation in free air at a distance of 2 m from the reaction object within 60 s under unshielded conditions being 0.2 Gy. It can be seen that this definition evaluates the size of an accident through the consequences of the accident caused within a certain period of time.

[0004] According to research, some critical accidents with relatively small reactivity introduction and slow power growth are not covered by the definition of the smallest critical accident in the standard GB15146.9, but long-term irradiation will still cause harm to the human body. Therefore, in actual analysis, the definition of the smallest critical accident may be made more stringent, that is, the absorbed dose in GB15146.9 is set lower.

[0005] Due to different fission numbers, different average fission neutron numbers, different leakage rates, and different energy spectra of critical accidents under different working conditions, different combinations of these parameters may all lead to the same accident consequences, but the dose rate at the detector is different.

[0006] Therefore, determining the relatively enveloping minimum critical accident source term is a prerequisite for accurately identifying and alarming critical accidents, which can reduce the false negative rate of critical accidents and provide a favorable guarantee for the health and safety protection of staff. Summary of the Invention

[0007] Aiming at the defects existing in the prior art, the purpose of the present invention is to provide a method for determining the minimum critical accident source term, which can determine the relatively enveloped minimum critical accident source term, can include the critical accident source terms under more working conditions, reduces the false negative rate of critical identification, is a prerequisite for accurately identifying and alarming critical accidents, and provides a favorable guarantee for the health and safety protection of staff.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A method for determining the minimum critical accident source term, the method comprising the steps of:

[0010] S1. According to the overall characteristics of the equipment in the plant, and accordingly determine the minimum number of fissions V corresponding to the minimum critical accident that meets the standard regulations, min the minimum average number of fission neutrons N, min and the minimum leakage rate D of neutrons or photons of each piece of equipment in the plant. min According to the formula DN = V min × N min × D min obtain the number of leaked neutrons or photons DN corresponding to the minimum critical accident;

[0011] S2. Calculate the neutron-photon energy spectra corresponding to each critical condition, and calculate the radiation absorption dose caused by a single neutron or photon under the corresponding energy spectrum, and use the neutron-photon energy spectrum corresponding to the minimum radiation absorption dose as the energy spectrum of the minimum critical accident source term;

[0012] S3. Combine the number of leaked neutrons or photons DN corresponding to the minimum critical accident and the energy spectrum of the minimum critical accident source term to obtain the leaked source term corresponding to the minimum critical accident.

[0013] Further, the overall characteristics of the equipment in the plant in step S1 include the shape of the equipment in the plant and the maximum boundary of the fuel solution concentration.

[0014] Further, step S1 includes sub-steps:

[0015] S11. According to the shape characteristics of the equipment in the plant, calculate a series of critical concentration values within the range of the concentration boundaries of the fuel solutions U and Pu;

[0016] S12. According to the series of critical concentration values, calculate the number of fissions and the average number of fission neutrons corresponding to each critical condition that meet the standard regulations for the minimum critical accident, and count the minimum number of fissions V min and the minimum average number of fission neutrons N. min ;

[0017] S13. Analyze the neutron or photon leakage rate of each device in the plant and find the minimum leakage rate D. min ;

[0018] S14. According to the formula DN = V min × N min × D min obtain the corresponding leakage neutrons or photons DN under the minimum critical accident, where V min is the minimum number of fissions, N min is the minimum average number of fission neutrons, and D min is the minimum neutron or photon leakage rate.

[0019] Furthermore, the standard described in step S1 is the GB15146.9 standard, and the dose rate specified by the GB15146.9 standard is 0.2 Gy / min at a distance of 2 m from the device.

[0020] Furthermore, the standard-specified time t is 60 s.

[0021] Furthermore, the shapes of the devices in the plant described in step S1 include cylinders, spheres, and cuboids.

[0022] The effect of the present invention is as follows: A method for determining the minimum critical accident source term disclosed by the present invention, by respectively determining the minimum number of fissions V min , the minimum average number of fission neutrons N min , and the minimum leakage rate D of neutrons or photons of each device in the plant min , to obtain the corresponding leakage neutrons or photons DN under the minimum critical accident, and combining the corresponding leakage neutrons or photons DN under the minimum critical accident and the energy spectrum of the minimum critical accident source term, the corresponding leakage source term under the minimum critical accident is obtained. It can be seen that the minimum number of fissions, the minimum average number of fission neutrons, and the minimum leakage rate in the present invention may come from different devices and working conditions. Therefore, by using the method for determining the minimum critical accident source term disclosed by the present invention, a relatively enveloping minimum critical accident source term can be determined, which can include the critical accident source terms under more working conditions, reduces the false negative rate of critical identification, is a prerequisite for accurately identifying and alarming critical accidents, provides a favorable guarantee for the health and safety protection of staff, and has the advantages of a wide application range, being able to include critical accidents under more working conditions, a low false negative rate, and a strong critical accident identification ability. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a flowchart of a method for determining the minimum critical accident source term shown in an embodiment of the present invention;

[0024] Figure 2To analyze the critical concentration of spheres within the concentration boundary range, a diagram showing the distribution of the critical concentrations of spheres with different diameters is obtained;

[0025] Figure 3 To calculate the distribution diagram of the number of fissions corresponding to different critical concentrations under the conditions specified in GB15146.9;

[0026] Figure 4 To calculate the distribution diagram of the average number of fission neutrons corresponding to different critical concentrations under the conditions specified in GB15146.9. Specific implementation manners

[0027] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation manners.

[0028] Example 1

[0029] Although a critical accident has different numbers of fissions, different average numbers of fission neutrons, different leakage rates, and different energy spectra under different working conditions, different combinations of these parameters may all lead to the same accident consequences, and the dose rate at the detector is different. However, the number of leaked neutrons is DN = V min × N min × D min , where DN is the number of leaked neutrons, V min is the minimum number of fissions. When the volume is smaller and the Pu concentration is higher, the number of fissions is smaller. N min is the minimum average number of fission neutrons. When the Pu concentration is lower and the volume is larger, the average number of fission neutrons is smaller. D min is the minimum leakage rate, which is mainly related to the geometric shape of the equipment. Generally speaking, the larger the surface area / volume ratio, the smaller the leakage rate.

[0030] Therefore, by determining the minimum number of fissions V min , the minimum average number of fission neutrons N min and the minimum leakage rate D of neutrons or photons of each equipment in the plant min , the number of leaked neutrons DN can be calculated. Combining with a series of neutron energy spectra, the source term causing the minimum critical accident can be determined. The technical solutions disclosed in the embodiments of the present invention will be described below based on this theory.

[0031] As Figure 1 shown, an embodiment of the present invention discloses a method for determining the source term of a minimum critical accident. The method includes the steps:

[0032] S1. Determine the overall characteristics of the equipment in the plant

[0033] Determine the overall characteristics of the equipment in the plant. The overall characteristics of the equipment in the plant include the shape of the equipment in the plant and the maximum boundary of the fuel solution concentration.

[0034] The shapes of the equipment in the plant are mainly cylinders, spheres, or cuboids, etc. The maximum boundaries of the concentrations of the fuel solutions U and Pu in the plant are determined according to the design requirements, which belong to the design input.

[0035] Suppose there are 8 pieces of equipment in a certain plant, all of which are spheres of different sizes. This plant is mainly configured with U and Pu solutions of different concentrations. Although the concentration ratios of U and Pu are uncontrollable, it can be known by querying the design input that the highest U concentration is 1000 g / L and the highest Pu concentration is 150 g / L.

[0036] S2. For the shape characteristics of the equipment in the plant, within the range of the concentration boundaries of the fuel solutions U and Pu, calculate a series of critical concentration values, and the range included should be as wide as possible within the concentration boundaries.

[0037] As Figure 2 shown, analyze the critical concentration of the spheres within the range of the concentration boundaries to obtain the distribution of the critical concentrations of spheres with different diameters.

[0038] S3. According to the series of critical concentration values obtained in step S2, calculate the corresponding fission numbers and average fission neutrons for each critical condition that meets the requirements of GB15146.9 for the minimum critical accident. And count the minimum fission number V min and the minimum average fission neutron number N min .

[0039] As Figure 3 shown, under the condition of meeting the requirements of GB15146.9, calculate the fission numbers corresponding to different critical concentrations. Based on this, the minimum fission number can be counted as 6.97E+13 fissions / s. As Figure 4 shown, under the condition of meeting the requirements of GB15146.9, calculate the average fission neutrons corresponding to different critical concentrations. Based on this, the minimum average fission neutron number can be counted as 2.52 neutrons / fission.

[0040] S4. According to the series of critical concentration values obtained in step S2, calculate the corresponding neutron-photon energy spectra for each critical condition. And calculate the dose caused by a single neutron or photon under the corresponding energy spectrum, and take the neutron-photon energy spectrum corresponding to the minimum dose as the energy spectrum of the minimum critical accident source term.

[0041] According to the series of critical concentration values obtained in step S2, calculate the corresponding neutron-photon energy spectra for each critical condition, and obtain the minimum critical accident neutron energy spectrum table shown in Table 1. At the same time, the minimum dose caused by a single neutron is calculated.

[0042] Table 1 Minimum critical accident neutron energy spectrum

[0043]

[0044]

[0045] S5. Analyze the neutron or photon leakage rate of each device in the plant and find the minimum leakage rate D min 。

[0046] In this embodiment, the minimum leakage rate among the 8 devices is 25%.

[0047] S6. Multiply the obtained minimum number of fissions by the minimum average number of fission neutrons and the leakage rate to obtain the corresponding number of leaked neutrons or photons under the minimum critical accident, that is, the number of leaked neutrons or photons DN = V min ×N min ×D min 。Meanwhile, combined with the energy spectrum obtained in step S4, the corresponding leakage source term under the minimum critical accident is obtained.

[0048] According to steps S3 and S5, the minimum number of leaked neutrons in the critical accident is

[0049] 6.97E+13 fissions / s × 2.52 neutrons / fission × 25% = 4.39E+13 neutrons / s. Combining with Table 1, the leakage source term of the minimum critical accident can be obtained.

[0050] It is worth mentioning that according to research, some critical accidents with less introduced reactivity and slow power growth are not covered by the definition of the minimum critical accident in GB15146.9, but long-term irradiation will still cause harm to the human body. Therefore, in actual analysis, the definition of the minimum critical accident may be made more stringent, that is, the absorbed dose in GB15146.9 is set lower. When the absorbed dose standard for evaluating the minimum critical accident changes, in the analysis, it will affect the calculation result of the number of fissions in the minimum critical accident and has no impact on the rest of the analysis. Therefore, these changes should also be included within the scope of the present invention and should not be considered as departing from the present invention.

[0051] As can be seen from the above embodiments, a method for determining the minimum critical accident source term disclosed by the present invention can determine a relatively comprehensive minimum critical accident source term, can cover the critical accident source terms under more working conditions, reduces the false negative rate of critical identification, is a prerequisite for accurately identifying and alarming critical accidents, provides a favorable guarantee for the health and safety protection of staff, and has the advantages of a wide application range, being able to cover critical accidents under more working conditions, a low false negative rate, and a strong ability to identify critical accidents.

[0052] The method described in the present invention is not limited to the embodiments described in the specific implementation manners. Other implementation manners obtained by those skilled in the art based on the technical solution of the present invention also fall within the scope of the technical innovation of the present invention.

Claims

1. A method for determining the minimum critical accident source term, the method comprising the steps: S1. According to the overall characteristics of the equipment in the plant, and accordingly determine the minimum number of fissions V corresponding to the minimum critical accident that meets the standard regulations min , the minimum average number of fission neutrons N min and the minimum leakage rate D of neutrons or photons of each piece of equipment in the plant min , according to the formula DN = V min × N min × D min to obtain the number of leaked neutrons or photons DN corresponding to the minimum critical accident; S2. Calculate the neutron-photon energy spectra corresponding to each critical condition, and calculate the radiation absorption dose caused by a single neutron or photon under the corresponding energy spectrum, and use the neutron-photon energy spectrum corresponding to the minimum radiation absorption dose as the energy spectrum of the minimum critical accident source term; S3. Combine the number of leaked neutrons or photons DN corresponding to the minimum critical accident and the energy spectrum of the minimum critical accident source term to obtain the leaked source term corresponding to the minimum critical accident; The overall characteristics of the equipment in the plant described in step S1 include the shape of the equipment in the plant and the maximum boundary of the fuel solution concentration; Step S1 includes sub-steps: S11. According to the shape characteristics of the equipment in the plant, calculate a series of critical concentration values within the range of the U and Pu concentration boundaries of the fuel solution; S12. According to the series of critical concentration values, calculate the number of fissions and the average number of fission neutrons corresponding to each critical condition that meets the standard regulations for the minimum critical accident, and count the minimum number of fissions V min and the minimum average number of fission neutrons N min ; S13. Analyze the neutron or photon leakage rate of each piece of equipment in the plant, and find the minimum leakage rate D min ; S14. According to the formula DN = V min × N min × D min to obtain the number of leaked neutrons or photons DN corresponding to the minimum critical accident, where V min is the minimum number of fissions, N min is the minimum average number of fission neutrons, D min is the minimum neutron or photon leakage rate.

2. The method for determining the minimum critical accident source term according to claim 1, wherein: The standard described in step S1 is the GB15146.9 standard, and the dose rate specified by the GB15146.9 standard is 0.2 Gy / min generated at a distance of 2 m from the equipment.

3. The method for determining the minimum critical accident source term according to claim 2, wherein: The standard specifies that the time t is 60 s.

4. The method for determining the minimum critical accident source term according to claim 1, wherein: The shapes of the equipment in the plant described in step S1 include a cylinder, a sphere, and a cuboid.

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