METHOD FOR LOCALIZING AREAS OF INCREASED RADIOACTIVITY IN A MEASURED SUBSTANCE

Description

[0001] The present invention relates to a method for locating areas of increased radioactivity within a material to be measured using a measuring chamber surrounded by a plurality of detectors for detecting radioactive radiation.

[0002] During the dismantling of potentially radiation-contaminated facilities, such as decommissioned nuclear power plants, large quantities of dismantling material are generated, which must be tested for radiation exposure before disposal or reuse. For this purpose, the material to be tested (hereinafter also referred to as the measured material) is introduced into a measuring system of the type described above, and its activity is determined. Only if certain limit values ​​regarding the activity of the measured material are not exceeded can the tested material be released under radiation protection law. The measuring chamber surrounded by detectors mentioned above is therefore also referred to as a clearance measuring system.

[0003] In practice, the demolition material to be examined is often sorted by material type and placed in a container, such as a wire mesh box. The filled container can then be placed in the clearance measurement system and measured there.

[0004] The current state of the art often presents the problem that, upon detection of a certain activity in the sample, it is initially unknown whether this activity is evenly distributed throughout the sample or whether there are areas of increased activity, so-called hotspots, within the sample. Therefore, in such cases, it was often necessary to conduct further investigations to locate the hotspots, if any. These further investigations are complex, as they require the use of additional measuring devices with greater local resolution.

[0005] Based on this prior art, the object of the present invention is to provide a method which enables a simpler and more economical localization of areas of increased radioactivity within a material to be measured.

[0006] This object is achieved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0007] First, some of the terms used in the context of the invention will be explained. Within the scope of the invention, a measured material can in particular be composed of dismantling material that accrued during the dismantling of a nuclear facility. The dismantling material is preferably sorted so that the measured material essentially consists of a single material. The method determines the material of the measured material. During this material determination, it is not necessary to determine all of the materials present in the measured material with high accuracy. Rather, it is sufficient if the materials of which the measured material consists are known to a certain extent. The accuracy of the material determination based on the mass is more than 85%, preferably more than 95%.

[0008] An inaccurate material determination can occur, for example, if it is assumed during the determination that the measured material contains 100 wt.% iron, but the measured material actually consists of 80 wt.% iron and 20 wt.% copper. Although such inaccuracies can be taken into account as systematic uncertainties within the scope of the invention, it has been shown that excessive inaccuracies make the localization of hotspots significantly more difficult. The measured material is introduced into the measuring chamber. For this purpose, the measured material can be filled into a container (e.g. a wire mesh box) and then placed in the container in the measuring chamber.

[0009] The measuring chamber is surrounded by a plurality of detectors. The term "detector" is assigned the index i.

[0010] The index is used to number the available detectors. The index i can therefore be an integer between 1 and the available number of detectors.

[0011] Within the framework of the method, to simulate the counting rate ZSIM_i,j, it is assumed that a point source is located at at least one coordinate of the sample. The term "coordinate" is assigned the index j. The index j numbers the coordinates assumed in the simulation and can thus assume integers ranging from 1 to the number of assumed coordinates.

[0012] The count rate Z_i is the count rate measured by detector i in step d. The count rate ZSIM_i,j is the count rate simulated for detector i, which is caused by the point source of activity A_j assumed at coordinate j. For example, the count rate simulated for detector "3," which is caused by a point source of activity A1 located at coordinate "1," is referred to as the count rate ZSIM 3,1.

[0013] The method according to the invention makes it possible to locate an area within the sample material that exhibits increased activity without the need for complex additional measurements. Rather, a conventional measurement in the measuring chamber surrounded by detectors can be used to locate an area of ​​increased activity. To do this, the activity and the coordinates (i.e., the assumed location) of the point source are modified so that the simulated count rate is adapted to the measured count rate. The core idea behind this is that a "hotspot" in the sample material, depending on its position within the sample material, leads to a count rate of the detectors that is characteristic of that position. In particular, the count rate of a detector will be higher the closer it is to the hotspot and the less shielding material is located between it and the hotspot.Since, according to the invention, the material and density of the measured material are taken into account in addition to the coordinate and the activity, an optimum can be achieved by changing the coordinate and the activity of the assumed point source, at which the simulated counting rate corresponds as closely as possible to the actually measured counting rate.

[0014] In principle, it is possible that a single assumed point source is sufficient (in this case, j = 1) to adjust the measured count rate sufficiently accurately to the simulated count rate. It can then be concluded that at the assumed coordinate j = 1 there is an essentially point-shaped area which has the activity A 1 assumed in the simulation. However, a randomly composed sample will regularly have a plurality of hotspots or hotspots with a larger extent. In this case, it can be provided that the stepe. is repeated for at least one further point source or for a plurality of point sources, wherein in step f. the simulated count rates originating from different point sources are taken into account in the adaptation. For example, it can be provided that the simulated count rates ZSIM_i,j obtained for the plurality of point sources are summed for each detector and in step f. the sum of the simulated count rates obtained for each detector i is adapted to the count rates Z_i measured by the detector i in each case by changing the plurality of assumed activities A_j and / or the plurality of assumed coordinates j. The method according to the invention thus makes it easy to localize several areas of increased activity or even extended areas of increased activity.

[0015] Even if a single assumed test source is sufficient to adapt the measured count rate to the simulated count rate with sufficient accuracy, it may nevertheless be provided that further test sources are added as described above for simulation and adaptation according to steps e. and f. in order to improve the statistical accuracy of the determined activity distribution.

[0016] In a preferred embodiment, a predetermined efficiency W_i,j is used for each detector i and for each coordinate j to carry out the simulation according to step e. The efficiency W_i,j of detector i describes the ratio between the particles emitted from coordinate j into the detector volume of detector i and the particles actually counted by detector i. The determination of such an efficiency is necessary because usually a certain proportion of the particles emitted into the detector volume do not trigger a counting pulse in the detector, for example because an electrical pulse triggered by the particle in the detector does not exceed a required threshold. The efficiency takes this into account so that the actual activity can be deduced from the counting rate measured by the detector.

[0017] It is advantageous if the specified efficiencies W_i,j are highly accurate. In this case, the accuracy of the efficiencies is defined by a test setup in which a 200-liter barrel is three-quarters full of water (density 1 g / cm3), with a Co-60 point source with a minimum activity of 25,000 Bq (e.g. suspended from a thread) being placed exactly in the middle of the water volume. The accuracy of the efficiencies in this test setup is preferably high enough that a 3 cm deviation of the position of the point source from the center can be detected with statistical significance. Further preferably, the accuracy of the efficiencies is high enough that a 1 cm deviation of the positioning of the point source from the center can be detected with statistical significance.Within the framework of the aforementioned experimental setup, the uncertainty of the efficiency is preferably less than 15%, more preferably less than 10%. The uncertainty of the efficiency is defined here as the deviation between the count rate simulated for a detector i and the actual count rate measured by detector i. This can significantly increase the accuracy achievable in locating the hotspots.

[0018] In an advantageous embodiment, the detectors have detector surfaces that surround the measuring chamber essentially without interruption. The measuring chamber preferably has a total efficiency of more than 40% if a Co-60 point source is placed in the center of the otherwise empty measuring chamber. A total efficiency of more than 40% is achieved if at least 40% of the particles actually emitted are detected by all detectors. This ensures that almost all particles escaping from the sample pass through a detector surface, which also contributes to the desired accuracy in locating the hotspots.

[0019] The detectors are preferably scintillation counters and more preferably plastic scintillation counters. The number of detectors is greater than 12, preferably greater than 16, and more preferably greater than 20. With this number of detectors, good spatial resolution of the detected particles can be achieved, which further increases the accuracy of localizing hotspots. To improve the measurement accuracy of a clearance measurement system, it is generally known to increase the number of detectors and reduce the detector volume accordingly. This increases the spatial sensitivity of the clearance measurement system. However, as explained below, it has been shown within the scope of the invention that a reduction in the detector volume is not necessary to achieve high localization quality.

[0020] During the dismantling of nuclear power plants, quantities of material to be released arise on an industrial scale (> 10,000 t). For economic reasons, it is therefore not possible to consistently ensure that the material to be measured has a homogeneous density before being introduced into a clearance measurement system. Since, for economic reasons, it is not possible to determine the density in every individual case, the simulation carried out by the present invention assumes an average (i.e. homogeneous) density for the purpose of simplification. It has now been shown that, due to these circumstances, reducing the detector volume within the scope of the present invention does not produce any improvement in the localization quality. In a preferred embodiment, the detectors therefore have a detector volume of more than 1,000 cm 3 , preferably more than 1,500 cm 3 , more preferably more than 2,000 cm 3 .In addition, the detectors have detector areas that are preferably larger than 300 cm 2 , more preferably larger than 400 cm 2 . Detectors of this size allow large quantities of material to be examined in a particularly economical manner, while at the same time, the method according to the invention can achieve a high localization quality despite the detector size.

[0021] The measured count rates depend on the filling level of the measured material. The filling level describes the proportion of the volume filled by the measured material relative to the volume of an envelope of the measured material. If the measured material is provided in a container (e.g. a wire mesh box, a metal crate or a barrel), an inner surface of the container usually forms the envelope of the measured material. A preferred application of the invention is the localization of regions of increased activity in metals that are provided in a wire mesh box. With an average density of, for example, 2 g / cm3, a preferred filling level is 50%. If the density is lower, higher filling levels can also be used. Preferably, a filling level of the measured material is determined, wherein the filling level is used in step e. to simulate the count rate ZSIM_i,j.Furthermore, the determined degree of filling is preferably not greater and at most 25% smaller than the actual degree of filling of the material being measured.

[0022] Since metals have a low natural activity, the method according to the invention delivers particularly accurate results when the sample has a high metal content. This low natural activity leads to a low radiation background, so that areas of increased activity stand out more clearly from the background. It is therefore advantageous if the metal content is greater than 90%, preferably greater than 95%, and more preferably greater than 99%.

[0023] In the simulation according to step e., in a preferred embodiment, it is assumed that the point source is the isotope Co-60. The energy of the particles emitted by Co-60 is relatively high compared to the particle energies of other radionuclides commonly encountered during the dismantling of nuclear facilities. The higher the particle energies, the more likely the particles can penetrate the material and hit the detectors, where they are more likely to deposit greater energy. If the sample material actually emits particles with lower energy (e.g., Cs-137), the assumption of Co-60 as the point source for the purposes of the simulation means that the location of a hotspot can be considered "conservative." In this case, "conservative" means that the actual activity in the sample material is overestimated by the activity calculation performed as part of the method.

[0024] The device according to the invention can be further developed by the features already explained above in connection with the method according to the invention.

[0025] A preferred embodiment of the invention is explained below by way of example with reference to the accompanying drawings. They show: Figure 1: a three-dimensional view of a clearance measurement system that can be used to carry out the method according to the invention; Figure 2: a schematic representation of an inventive device for locating areas of increased radioactivity within a sample; Figure 3: an exemplary localization result of the method according to the invention; Figure 4: another exemplary localization result of the method according to the invention.

[0026] Figure 1shows a three-dimensional view of a device for locating areas of increased radioactivity within a material to be measured 16, comprising a measuring chamber 14 surrounded by a plurality of detectors 13 for detecting radioactive radiation. The measuring chamber 14 can be used to carry out the method according to the invention. A grid box 12 filled with a material to be measured 16 is arranged in front of the measuring chamber 14. In this case, the material to be measured 16 is a plurality of metal discs that originate from a decommissioned nuclear power plant and are contaminated with radioactive material. The grid box 12 is 76% filled with the metal discs.

[0027] The grid box 12 can be introduced into the measuring chamber 14 via a roller conveyor. 32 large-area detectors are arranged on the walls of the measuring chamber (in Figure 1not visible), which surround the measuring chamber 14 almost continuously. The coverage of the detectors is 98%. The radioactive particles emitted by the sample 16 are therefore highly likely to hit one of the detectors 13.

[0028] Figure 2shows a schematic representation of the device according to the invention for locating areas of increased radioactivity within the material to be measured 16. In this simplified two-dimensional representation, eight detectors 13 are shown, which are arranged in the region of the walls 15 of the measuring chamber 14. The detectors are numbered from 1 to 8 by the index i. The detectors 13 are plastic scintillation counters designed to detect gamma radiation. The detectors are shielded against alpha and beta radiation. The detectors have a detector volume of 2200 cm 3 with a detector area of ​​approximately 400 cm 2 . The term detector area refers to the area of ​​the detector 13 facing the radiation source. In addition, each detector 13 comprises a photomultiplier, a discriminator, and a counting unit (not shown).

[0029] The sample 16 is located in the center of the measuring chamber 14. Within the sample 16 is an area 17 of increased radioactivity, a so-called hotspot. A gamma particle emitted by the hotspot 17 first passes through a portion of the sample 16, then exits the sample 16, and subsequently strikes one of the detectors 13, in this case the detector with index i = 1. The trajectory of the gamma particle is illustrated by a dashed line 22. The gamma particle interacts with the electrons of the detector material within the detector volume. This interaction generates so-called scintillation photons, which are converted into an electrical pulse by the photomultiplier using the photoelectric effect. The electrical pulse is transmitted to the discriminator, which transmits the electrical pulse to the counting unit when a certain threshold is exceeded.The counting unit records the electrical pulse as a counting event and thus forms a counting rate z2_1 over the measurement period.

[0030] Since gamma particles are emitted from hotspot 17 in a statistically uniform manner in all directions, a counting rate Z_i can be determined for each detector i over a certain measurement period. The counting rates are forwarded to an adaptation module 19 via a signal line 18.

[0031] The device according to the invention also comprises a simulation module 20, which is designed to calculate a counting rate ZSIM_i,j for each detector i. For this calculation, it is initially assumed that a point source of the material Co-60 (cobalt-60) with an activity A_1 is arranged at a coordinate j = 1 of the measuring material. For the calculation, a previously determined mass density and a previously determined material of the measuring material are also taken into account. Furthermore, for each detector i, an efficiency W_i,1 specified for the coordinate j = 1 is taken into account to simulate the counting rate ZSIM_i,1. The efficiency takes into account the fact that not every particle that passes through the volume of a detector 13, starting from the measuring material 16, triggers a corresponding counting pulse, but that the triggering of a counting pulse only occurs with a certain probability.The efficiency has an uncertainty of less than 15%. Using efficiencies of this high accuracy, the simulated count rates ZSIM_i,1 for each detector can be calculated with high reliability.

[0032] After a simulated count rate ZSIM_i,1 has been determined for each detector, the adaptation module 19 adapts the simulated count rate ZSIM_i,1 to the measured count rate Z_i. To do this, the adaptation module 19 changes the activity A_1 assumed in the simulation and / or the coordinate j = 1 of the point source. In the present case, it is shown that by assuming a single point source at coordinate j = 1, sufficient agreement cannot be found between the measured count rate Z_i and the simulated count rate ZSIM_i,1. Therefore, it is assumed here that a plurality of point sources with different activities A_j are arranged at different coordinates j (for example j = 1 to 10) of the measured material.During the simulation, the point sources are preferably added successively, and the coordinate j of the added point source as well as the activities A_j of the added and existing point sources are changed in order to adapt the simulated count rates ZSIM_i,j to the measured count rates. If multiple point sources (for example, a number of n point sources) are included in the simulation, the sum of the simulated count rates must match the count rate actually recorded by the detector. Therefore, for each detector i, the following must apply: . Z i = ∑ j = 1 n ZSIM i , j .

[0033] If this condition is met with sufficient accuracy, the procedure is completed and it can be assumed that there are areas of increased activity A_j at the determined coordinates j.

[0034] Figure 3shows an exemplary result of the localization method according to the invention. The grid box 12 described above is shown, whereby in this case, the measured object is not shown, but a diagonal plane is drawn on which the activity determined by the method is illustrated by a plurality of contour lines. Based on the contour line, the Figure 2 Hotspot 17 already marked is visible.

[0035] Figure 4 shows another exemplary result of the localization method according to the invention, which was carried out on a different sample. In this view, the result is shown in a two-dimensional sectional view through the grid box 12. The sample was divided into a plurality of coordinates for the implementation of the method. In the Figure 4In the section plane shown, a total of 63 of these coordinates are labeled with corresponding numbers. The coordinates j shown are designated accordingly by the numbers 1 to 63. Each coordinate is assigned a square area in the plane. By the method according to the invention, each of the coordinates was assigned an activity A_j with j = 1 to 63, whereby the Figure 4 The brightness shown represents a measure of the level of activity. It shows that areas of increased activity are present at the top (j = 1 to 9), right (j = 9, 18, 27, 26, 45, 54, and 63), and bottom (j = 55 to 63) edges of the grid box, as well as to the left of center (j = 20, 21, 29, 30, 38, and 39).

Claims

1. A method for localizing areas (17) of increased radioactivity within a material to be measured (16) using a measuring chamber (14) surrounded by a plurality of detectors (13) for detecting radioactive radiation, comprising the following steps: a. Provision of a material to be measured (16), b. Determining an average mass density of the material to be measured (16) and a material present in the material to be measured, c. Inserting the material to be measured (16) into the measuring chamber (14), d. Determining a count rate Z_i caused by the material to be measured (16) for each detector i (13), e. Simulating a count rate ZSIM_i,j for each detector i (13) under the assumption that a point source with an activity A_j is arranged at at least one coordinate j of the material to be measured (16), and taking into account the determined average mass density and the material of the material to be measured (16), wherein a degree of filling of the material to be measured (16) is determined and used in step e. to simulate the count rate ZSIM_i,j, f.Adaptation of the simulated count rates ZSIM_i,j to the measured count rates Z_i by changing the assumed activity A_j and / or of the assumed coordinate j, characterized in that the number of detectors (13) is greater than 12 and that during the material determination in step b up to a certain degree of more than 85% the material of which the material to be measured consists of is determined.

2. Method according to claim 1, in which step e. is repeated for at least one further point source, wherein in step f. the simulated count rates originating from different point sources are taken into account in the adjustment.

3. Method according to claim 1 or 2, in which in the simulation according to step e. a predetermined efficiency W_i,j is used for the detectors i (13) and for the coordinates j in each case.

4. Method according to claim 3, wherein the predetermined efficiencies W_i,j have an uncertainty of less than 15%, preferably less than 10%.

5. Method according to any one of claims 1 to 4, wherein the detectors (13) comprise detector surfaces surrounding the measuring chamber substantially without interruption.

6. Method according to one of claims 1 to 5, in which the detectors (13) are designed as scintillation counters and preferably as plastic scintillation counters.

7. Method according to any one of claims 1 to 6, wherein the number of detectors (13) is greater than 16, and further preferably greater than 20.

8. Method according to any one of claims 1 to 7, wherein the detectors (13) have a detector area of more than 300 cm2, preferably more than 400 cm2.

9. Method according to any one of claims 1 to 8, wherein detectors (13) have a detector volume of more than 1000 cm3, preferably more than 1500 cm3, more preferably more than 2000 cm3.

10. Method according to any one of claims 1 to 9, wherein the determined degree of filling is not greater than an actual degree of filling of the material to be measured (16).

11. Method according to any one of claims 1 to 10, wherein the material to be measured (16) has a metal content which is greater than 50%, preferably greater than 70%, more preferably greater than 90%.

12. Method according to any one of claims 1 to 11, wherein in the simulation according to step e. it is assumed that the point source is Cobalt-60.

13. Method according to one of claims 1 to 12, in which the material to be measured (16) is provided in a container, preferably in a mesh box (12), wherein in step c. the container filled with the material to be measured (16) is introduced into the measuring chamber (14).

14. Device for localizing areas of increased radioactivity within a material to be measured (16) by a method according to one of the preceding claims, comprising a plurality of detectors (13) for detecting radioactive radiation, a measuring chamber (14) surrounded by the plurality of detectors (13), the number of detectors (13) being greater than 12, a simulation module (20) and an adaptation module (19), having the following properties: a. the detectors i (13) are each designed to determine a count rate Z_i caused by a material to be measured (16) introduced into the measuring chamber (14), b. the simulation module (20) is designed to simulate a count rate ZSIM_i,j assigned to a respective detector i (13), it being assumed for the simulation that a point source with an activity A_j is arranged at a coordinate j of the material to be measured (16), and wherein for the simulation a previously determined mean mass density and a previously determined material of the material to be measured (16) are taken into account, wherein in the material determination it is determined to a certain degree of more than 85% which materials the material to be measured consists of, and wherein a filling degree of the material to be measured (16) is determined and in step e. is used to simulate the count rate ZSIM_i,j, c. the adaptation module (19) is designed to adapt the simulated count rates ZSIM_i,j to the measured count rates Z_i by changing the assumed activity A_j and / or of the assumed coordinate j.