METHOD FOR DETERMINING A MINIMUM CRITICALITY ACCIDENT SOURCE TERM

Abstract

METHOD FOR DETERMINING A MINIMUM CRITICALITY ACCIDENT SOURCE TERM The invention relates to a method for determining a minimum criticality accident source term, comprising the following steps: obtaining a number of leaky neutrons or photons DN for a minimum criticality accident by determining a minimum number of fissions Vmin, a minimum average number of fission neutrons Nmin, and a minimum leakage rate of neutrons or photons Dmin from respective devices in an installation; obtaining, based on the number of leaky neutrons or photons DN for the minimum criticality accident and an energy spectrum of the minimum criticality accident source term, a leakage source term corresponding to the minimum criticality accident. [Fig. 1]

Description

Title of the invention: METHOD FOR DETERMINING A MINIMUM CRITICALITY ACCIDENT SOURCE TERM technical field

[0001] The present invention relates to the field of defining a criticality accident source term and, more particularly, to a method for determining a minimal criticality accident source term. State of the art

[0002] In accordance with the legislative provisions and standards relating to nuclear safety-criticality concerning plutonium (Pu), uranium (U) and other fissile materials, in particular standard GB / T15146.9, it is imperative, in an independent area wherever operational activities involve fissile isotopes with a total mass exceeding 700 g of 235U, 520 g of 233U, 450 g of plutonium or 450 g of any mixture of these isotopes, to assess the need to implement a criticality accident alarm system. In an area that must be covered by the criticality accident alarm, it is necessary to provide means of detecting an excessive cumulative radiation dose or cumulative dose rate (in other words, a cumulative radiation dose or cumulative dose rate that exceeds a threshold), and of issuing a signal to evacuate personnel.

[0003] When designing a criticality alarm system, it must be capable of detecting the minimum criticality accident. The important question that then arises is that of the severity of the minimum criticality accident, in other words, the value of a source term for the minimum criticality accident. Given the complexity of the mechanism by which the minimum criticality accident occurs and the diversity of working conditions (hereinafter referred to as "conditions"), the exact nature of the minimum criticality accident remains to be determined. The minimum criticality accident in question is defined in GB / T15146.9 as follows: within 60 seconds and in the absence of shielding, the total absorption dose of neutrons and gamma rays occurring in open air 2 meters from the surface of a reactive material is 0.20 Gy.As can be seen, this definition assesses the severity of an accident based on the consequences of the accident manifesting within a certain period of time.

[0004] Studies show that certain criticality accidents with low introduction reactivity and slow power ramp-up are excluded from the above definition of a minimal criticality accident in GB / T15146.9. However, long-term irradiation is just as harmful to the human body. In practical terms, it will be appropriate therefore to apply a stricter definition of the minimum criticality accident, in other words to lower the absorption dose in GB / T15146.9.

[0005] Insofar as criticality accidents are characterized by different fission numbers, different average fission neutron numbers, different leakage rates and different energy spectra under different working conditions, it is possible that different combinations of these parameters may cause the same accident but present different dose rates at the detector.

[0006] It follows that the determination of a minimum relative envelope criticality accident source term is a necessary prerequisite for the accurate identification and notification of a criticality accident, making it possible to reduce the rate of missed criticality accident reports and to advantageously guarantee the protection of personnel in terms of health and safety. Summary

[0007] To overcome the prior art problem mentioned above, an object of the present invention is to provide a method for determining a minimum criticality accident source term, capable of determining a minimum criticality accident source term with a relative envelope, of covering criticality accident source terms under various working conditions, and of reducing the rate of missed criticality identification reports. This method is therefore a necessary prerequisite for the accurate identification and notification of criticality accidents and advantageously guarantees the health and safety protection of personnel.

[0008] To achieve this goal, the present invention proposes a method for determining a source term of minimal criticality for an accident, comprising the following steps:

[0009] acquisition of overall characteristics of devices in an installation; determination, on the basis of the overall characteristics, of a minimum number of fissions V min and a minimum average number of fission neutrons Nmin corresponding to a minimum criticality accident stipulated by a standard, and of a minimum leakage rate of neutrons or photons Dmin of respective devices in the installation; and acquisition, according to the formula DN = Vmin x Mnm x Dmin, of a number of leakage neutrons or photons DN corresponding to the minimum criticality accident;

[0010] Calculation of a corresponding neutron or photon energy spectrum in each just critical condition (also referred to as a just criticality condition or critical condition); calculation of a radiation absorption dose caused by a single neutron or photon by virtue of the corresponding energy spectrum; adoption of the corresponding neutron or photon energy spectrum causing a minimum radiation absorption dose as the source term energy spectrum of minimal criticality accident; and

[0011] obtaining, on the basis of the number of leakage neutrons or photons DN corresponding to the minimum criticality accident title and the energy spectrum of the minimum criticality accident source term, a leakage source term corresponding to the minimum criticality accident.

[0012] Optionally, the overall characteristics of the devices in the installation include the shapes of the respective devices in the installation and the maximum and minimum limits for the concentration of a load.

[0013] Optionally, the step of acquiring the overall characteristics of devices in an installation; determining, on the basis of the overall characteristics, a minimum number of fissions Vmin and a minimum average number of fission neutrons Nmin corresponding to a minimum criticality accident stipulated by a standard, and a minimum leakage rate of neutrons or photons Dmin of respective devices in the installation comprises the following steps:

[0014] calculation, based on the characteristics of the shapes of the devices in the installation, of a series of critical concentration values ​​(also called criticality concentration or critical concentration) within the concentration limits of charges of U, Pu;

[0015] calculation, based on the series of just-critical concentration values, of fission numbers and average fission neutron numbers corresponding to the respective just-critical conditions responding to the minimum criticality accident, and determination of the minimum fission number Vmin and the minimum average fission neutron number Nmin; and

[0016] analysis of the leakage rate of neutrons or photons of the respective devices in the installation, and determination of the minimum leakage rate Dmin.

[0017] Possibly, the standard is GB / T15146.9, a dose rate stipulated in the standard is 0.2 Gy / min at 2 meters from a surface of a device.

[0018] Optionally, the time t stipulated in the standard is 60 s.

[0019] Optionally, the shapes of the devices in the installation include a cylinder, a sphere or a cuboid.

[0020] The method for determining a minimum criticality accident source term according to the present invention obtains a number of leaky neutrons or photons DN corresponding to the minimum criticality accident title by determining respectively a minimum number of fissions Vmin, an average number of minimum fission neutrons Nmin, and a minimum leakage rate of neutrons or photons Dmin from the respective devices in the installation, and obtains a leaky source term corresponding to the minimum criticality accident based on the number of leaky neutrons or photons DN corresponding to the minimum criticality accident title and the spectrum of energy of the minimum criticality accident source term. It follows that the minimum number of fissions, the average number of minimum fission neutrons, and the minimum leakage rate in the present invention can arise from different devices and working conditions. It follows that adopting the method for determining a minimum criticality accident source term according to the present invention makes it possible to determine a minimum criticality accident source term with a relative envelope; the method is capable of covering criticality accident source terms under diverse working conditions, reducing the rate of missed criticality identification reports, which is a necessary prerequisite for the accurate identification and notification of criticality accidents and advantageously guarantees the health and safety protection of personnel.The process offers the following advantages: a wide range of applications, coverage of criticality accidents in diverse working conditions, a reduction in the rate of missed reports of criticality accidents, and a robust ability to identify criticality accidents. Brief description of the figures

[0021] [Fig. 1] represents a flowchart illustrating a method for determining a source term of criticality of an embodiment of the present invention;

[0022] [Fig.2] represents a graph illustrating the distribution of concentrations just critical concentrations of spheres of different diameters obtained by analysis of the critical concentrations of the spheres within a concentration limit;

[0023] [Fig. 3] represents a graph illustrating the distribution of fission numbers corresponding to different just critical concentrations obtained by calculation in the case where the circumstances prescribed in GB / T15146.9 are met; and

[0024] [Fig.4] represents a graph illustrating the distribution of average numbers of Fissile neutrons corresponding to different just-critical concentrations obtained by calculation in the case where the circumstances prescribed in GB / T15146.9 are met. Detailed description of embodiments

[0025] The present invention is described in more detail below in connection with the accompanying drawings and embodiments.

[0026] The inventors of the present invention have observed that, although criticality accidents are characterized by different fission numbers, different average fission neutron numbers, different leakage rates, and different energy spectra under different operating conditions, it is possible for different combinations of these parameters to cause the same accident, albeit with different dose rates at the detector. The present invention proposes to calculate the leakage neutron number according to the formula: DN = Vmin x Almin x Dmin. In the formula, DN represents the number of leaky neutrons. Vmin represents the minimum number of fissions; the smaller the volume and the higher the Pu concentration, the lower the number of fissions. Nmin represents the average number of minimum fission neutrons; the lower the Pu concentration and the larger the volume, the lower the average number of fission neutrons. Dmin represents the minimum leakage rate, which is essentially related to the geometric shape of the device and, generally speaking, the larger the area-to-volume ratio, the higher the leakage rate.

[0027] It is therefore possible to calculate the number of leaky neutrons DN by determining the minimum number of fissions Vmin, the average number of minimum fission neutrons Nmin, and the minimum leakage rate of neutrons or photons Dmin from the respective devices in the installation. Furthermore, it is possible to determine, in conjunction with a series of neutron energy spectra, the source term causing the minimum criticality accident. The technical diagram according to an embodiment of the present invention will be described below in application of this principle.

[0028] Referring to [Fig.1], an embodiment of the present invention relates to a method for determining a source term of minimum criticality accident, which comprises the following steps SI to S6.

[0029] In the SI step, the overall characteristics of devices in an installation are acquired.

[0030] The installation referred to in the present invention designates a site or building where fissile materials are placed or where devices housing or containing fissile materials are placed. The installation of the present invention includes, but is not limited to, nuclear auxiliary facilities, nuclear fuel facilities (e.g., spent fuel reprocessing facilities), reactor fuel storage facilities, and the like. In this embodiment, devices (e.g., tanks) housing a charge (e.g., a liquid charge) are placed in the installation, and the charge contains fissile material.

[0031] In this embodiment, the overall characteristics of the devices in the installation include the shapes of the respective devices in the installation and the maximum (or minimum) limit of load concentration.

[0032] The shapes of the devices in the installation essentially include a cylinder, a sphere or a cuboid, etc. The maximum limits of concentration of the charges of U, Pu in the installation are determined according to design criteria and are part of the design inputs.

[0033] Assuming that the installation contains eight devices in the form of spheres of different sizes and that the installation is essentially supplied with U, Pu solutions of different concentrations, even if the ratio of the U, Pu concentrations is not not controllable, it is nevertheless possible to determine, from the design inputs, that the maximum concentration of U is 1000 g / l and that the maximum concentration of Pu is 150 g / l.

[0034] In step S2, a series of just-critical concentration values ​​are calculated within the concentration limits of the U and PU charges, based on the characteristics of the respective device shapes. The calculation may include as wide a range as possible within the concentration limits.

[0035] As shown in [Fig. 2], the just-critical concentrations of the spheres are analyzed within the concentration limits in order to obtain the distribution of just-critical concentrations of spheres of different diameters. In this figure, each line represents a series of just-critical concentrations corresponding to a device (sphere), each point represents a set of U and Pu concentrations corresponding to the criticality working condition, and the U and Pu concentrations each fall within their concentration limits.

[0036] In step S3, on the basis of the series of just critical concentration values ​​obtained in step S2, the number of fissions and the average number of fission neutrons corresponding to each just critical condition meeting the minimum criticality accident stipulated in the standard are calculated, and the minimum number of fissions Vmin and the minimum average number of fission neutrons Nmin are determined.

[0037] It should be noted that, since the criticality accident covers a certain range, the term "just critical concentration" in the present invention refers to a concentration value that corresponds to a situation in which the device has just reached the critical state as the concentration increases; and the corresponding critical state is designated as the "just critical condition".

[0038] In addition, in step S3, the respective just critical conditions for each device are calculated.

[0039] As shown in [Fig. 3], taking GB15146.0 as an example standard, the fission numbers corresponding to different just critical concentrations are calculated under the circumstances prescribed in GB15146.9. On this basis, the minimum fission number can be calculated to be 6.97E+13 fissions / s (see the dot in the bottom right corner of [Fig. 3]). As shown in [Fig. 4], the average number of fission neutrons corresponding to different just critical concentrations is also calculated under the circumstances prescribed in GB 15146.9. On this basis, the minimum average number of fission neutrons can be calculated to be 2.52 neutrons per fission.

[0040] In step S4, neutron or photon energy spectra corresponding to the respective just-critical conditions are calculated based on the series of just-critical concentration values ​​obtained in step S2; the dose oc is then calculated controlled by a single neutron or photon according to the corresponding energy spectrum, and the energy spectrum of the minimum criticality accident source term is taken as the corresponding neutron or photon energy spectrum that causes the minimum dose.

[0041] On the basis of the series of just critical concentration values ​​obtained in step S2, neutron or photon energy spectra corresponding to the respective just critical conditions are calculated to obtain a neutron energy spectrum of the minimum criticality accident, as shown in Table 1, and the minimum dose caused by a single neutron is calculated simultaneously.

[0042] [Table 1]

[0043] Neutron Energy Spectrum of the Minimal Criticality Accident Lower Energy Limit (MeV) Upper Energy Limit (MeV) Standardized Counting 1.00E-11 1.00E-08 1.50E-02 1.00E-08 3.00E-08 7.75E-02 3.00E-08 5.00E-08 7.73E-02 5.00E-08 1.00E-07 1.10E-01 1.00E-07 2.25E-07 5.35E-02 2.25E-07 3.25E-07 1.04E-02 3.25E-07 4.00E-07 5.27E-03 4.00E-07 8.00E-07 1.70E-02 8.00E-07 l.00E-06 5.16E-03 l.00E-06 l.13E-06 2.79E-03 l.13E-06 l.30E-06 3.13E-03 l.30E-06 l.77E-06 6.84E-03 l.77E-06 3.05E-06 l.18E-02 3.05E-06 l.00E-05 2.39E-02 l.00E-05 3.00E-05 2.36E-02 3.00E-05 l.00E-04 2.70E-02 l.00E-04 5.50E-04 3.99E-02 5.50E-04 3.00E-03 4.10E-02 3.00E-03 l.70E-02 4.41E-02 l.70E-02 l.00E-01 5.57E-02 l.00E-01 5.50E-01 9.73E-02 5.50E-01 9.00E-01 5.30E-02 9.00E-01 l.40E+00 4.54E-02 l.40E+00 l.85E+00 3.48E-02 l.85E+00 3.00E+00 6.39E-02 3.00E+00 6.43E+00 4.97E-02 6.43E+00 2.00E+01 5.33E-03

[0044] In step S5, the leakage rates of neutrons or photons of the respective devices in the installation are analyzed, and the minimum leakage rate Dmin is determined.

[0045] In this embodiment, it is assumed that the minimum leakage rate of the eight devices is 25%.

[0046] In step S6, the minimum number of fissions obtained is multiplied by the average number of minimum fission neutrons and by the leakage rate to obtain the number of leakage neutrons or photons corresponding to the minimum criticality accident, in other words, the number of leakage neutrons or photons DN = Vmin x Amin x Dmin. Furthermore, in conjunction with the energy spectrum obtained in step S4, the leakage source term corresponding to the minimum criticality accident is obtained.

[0047] According to steps S3 and S5, the number of leaky neutrons in the minimum criticality accident obtained is 6.97E+13 fissions / sx 2.52 neutrons / fission x 25% = 4.39E+13 neutrons / s, which then allows us to obtain, by referring to Table 1, the leaky source term of the minimum criticality accident.

[0048] It should be noted that certain criticality accidents with low introduction reactivity and slow power rise are excluded from the above definition of a minimal criticality accident in GB / T15146.9. However, long-term irradiation is just as harmful to the human body. In practical terms, it will therefore be necessary to apply a stricter definition of a minimal criticality accident, in other words, to lower the absorption dose in GB / T15146.9, and this lowered absorption dose will form the basis of a "minimal criticality accident as stipulated in the standard".

[0049] Any modification of the absorption dose criterion for the assessment of the minimum criticality accident will affect the result of the calculation of the number of fissions of the minimum criticality accident, without affecting the rest of the analysis. Modifications of this type are deemed to fall within the scope of the present invention and shall not be considered as falling outside the scope of the present invention.

[0050] It is apparent from the above embodiments that it is possible to use the method for determining a source term of minimal criticality for an accident according to the present An invention for determining a minimum criticality accident source term with a relative envelope; the method is capable of covering criticality accident source terms under diverse working conditions, reducing the rate of missed criticality identification reports, which is a necessary prerequisite for the accurate identification and notification of criticality accidents, and advantageously ensuring the health and safety protection of personnel. The method offers the following advantages: a wide range of applications, coverage of criticality accidents under diverse working conditions, a reduced rate of missed criticality accident reports, and a robust ability to identify criticality accidents.

[0051] Furthermore, an embodiment of the present invention further relates to a computer storage medium storing a computer program, which is configured to execute the method for determining the minimum criticality accident source term proposed in the above embodiments.

[0052] The process described in the present invention is not limited to the examples described in the particular embodiments. Those skilled in the art may derive from the technical diagram of the present invention other embodiments which also fall within the scope of the technical innovation of the present invention.

[0053] By way of example, while the device containing the charge is representatively described in the embodiments described above, the present invention is not limited to this, and the present invention can also be applied to a device containing fissile materials in solid form (the solid being considered as a particular example of a solution). Furthermore, while the minimum criticality accident criterion stipulated in GB15146.9 is taken as an example in the above embodiments, the method of the present invention can also be applied to nuclear safety-criticality standards in force in other countries, organizations, or companies.

Claims

Demands

1. A method for determining a source term for a minimum criticality accident, comprising the following steps: acquiring overall characteristics of devices in an installation; determining, on the basis of the overall characteristics, a minimum number of fissions Vmin and a minimum average number of fission neutrons Nmin corresponding to a minimum criticality accident stipulated by a standard, and a minimum leakage rate of neutrons or photons Dmin from the respective devices in the installation; and acquiring, according to the formula DN = Vmin x Mnm x Dmin, a number of leakage neutrons or photons DN corresponding to the minimum criticality accident term; calculating a corresponding neutron or photon energy spectrum in each just-critical condition; calculating a radiation absorption dose caused by a single neutron or photon by virtue of the corresponding energy spectrum;adoption of the energy spectrum of the neutrons or corresponding photons causing a minimum radiation absorption dose as the energy spectrum of the minimum criticality accident source term; and obtaining, on the basis of the number of leakage neutrons or photons DN corresponding to the minimum criticality accident title and the energy spectrum of the minimum criticality accident source term, a leakage source term corresponding to the minimum criticality accident.

2. A method according to claim 1, wherein the overall characteristics of the devices in the installation include the shapes of the respective devices in the installation and a maximum concentration limit of a load.

3. A method according to claim 2, wherein the step of acquiring the overall characteristics of devices in an installation; determining, on the basis of the overall characteristics, a minimum number of fissions Vmin and a minimum average number of fission neutrons Nmin corresponding to a minimum criticality accident stipulated by a standard, and a minimum neutron or photon leakage rate Dmin of the respective devices in the installation, comprises the following steps: calculating, based on the shape characteristics of the devices in the installation, a series of just-critical concentration values ​​in

4.

5. the concentration limits of charges of U, Pu; calculation, based on the series of just-critical concentration values, of fission numbers and average fission neutron numbers corresponding to the respective just-critical conditions meeting the minimum criticality accident, and determination of the minimum fission number Vmin and the minimum average fission neutron number Nmin; and analysis of neutron or photon leakage rates from the respective devices in the installation, and determination of the minimum leakage rate D min- A method according to claim 1, wherein a dose rate stipulated in the standard is 0.2 Gy / min at 2 meters from a surface of a device. A method according to claim 2, wherein the shapes of the devices in the installation include a cylinder, a sphere, or a cuboid.