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Title:
A REFERENCE SYSTEM FOR WASTE CONTAINERS
Document Type and Number:
WIPO Patent Application WO/2020/070497
Kind Code:
A1
Abstract:
A reference system is provided for muon scattering tomography of hazardous contents of a waste container having a container body (408). The reference system comprises a support member (400). The support member is configured to support one or more fiducial marks (402, 404). The support member may be part of the container body (including the lid), or fitted inside the container, or in a frame configured to fit to the waste container. The fiducial marks have a higher atomic number than the container body. The fiducial marks are fixed to the support member to fix the one or more fiducial marks' location in use in relation to the hazardous contents of the container. The fiducial marks are rotationally asymmetric in relation to a corresponding rotationally symmetric axis of the container body.

Inventors:
KAISER RALF BERND (GB)
Application Number:
PCT/GB2019/052788
Publication Date:
April 09, 2020
Filing Date:
October 03, 2019
Export Citation:
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Assignee:
LYNKEOS TECH LIMITED (GB)
International Classes:
G01V5/00; G01T7/00
Foreign References:
US20130108017A12013-05-02
JP2018036156A2018-03-08
US3733889A1973-05-22
US20160104290A12016-04-14
US9851311B22017-12-26
Other References:
DURHAM J M ET AL: "Verification of spent nuclear fuel in sealed dry storage casks via measurements of cosmic ray muon scattering", ARXIV.ORG, CORNELL UNIVERSITY LIBRARY, 201 OLIN LIBRARY CORNELL UNIVERSITY ITHACA, NY 14853, 5 October 2017 (2017-10-05), XP081310212, DOI: 10.1103/PHYSREVAPPLIED.9.044013
Attorney, Agent or Firm:
LEAN IP (GB)
Download PDF:
Claims:
CLAIMS

1 . A reference system for muon scattering tomography of hazardous contents of a waste container having a container body, the reference system comprising:

- a support member configured to support one or more fiducial marks;

- one or more fiducial marks having a higher atomic number than the container body, wherein the one or more fiducial marks are fixed to the support member to fix the one or more fiducial marks' location in use in relation to the hazardous contents of the container; and

- the one or more fiducial marks are rotationally asymmetric in relation to a corresponding rotationally symmetric axis of the container body.

2. The system of claim 1 , comprising a waste container having a container body that comprises the support member.

3. The system of claim 2, wherein the one or more fiducial marks are fixed to the container body when the waste container is empty of hazardous waste.

4. The system of claim 2 or claim 3, wherein the container body comprises a container lid that comprises the support member.

5. The system of claim 1 , wherein the support member is configured to fit inside the waste container.

6. The system of any of claims 2 to 5, further comprising one or more reference marks configured and located on the container body to indicate a spatial relationship between the reference system and the waste container.

7. The system of claim 1 , comprising a frame that comprises the support member, wherein the support member is configured to fit to the waste container.

8. The system of claim 7, wherein the support member is configured with a marking guide for guiding marking of a reference mark on the waste container, to indicate a spatial relationship between the reference system and the waste container.

9. The system of any preceding claim, wherein the fiducial marks comprise a plurality of fiducial marks having a range of different atomic numbers higher than the container body.

10. The system of any preceding claim, wherein the one or more fiducial marks are at least partially embedded in the support member.

1 1 . The system of any preceding claim, wherein the support member is shaped to receive the one or more fiducial marks.

12. The system of any preceding claim, wherein the one or more fiducial marks comprise lead glass.

13. The system of any preceding claim, wherein the one or more fiducial marks comprise steel.

14. The system of any preceding claim, wherein the one or more fiducial marks are relatively more corrosion-resistant compared to the hazardous contents of the waste container.

15. The system of any preceding claim, wherein the waste container comprises a nuclear waste container and the hazardous contents comprise radioactive contents.

16. A waste container comprising the reference system of any preceding claim.

17. A method for muon scattering tomography of hazardous contents of a waste container having a container body, the method comprising the steps:

- providing one or more fiducial marks having a higher atomic number than the container body wherein the one or more fiducial marks are rotationally asymmetric in relation to a corresponding rotationally symmetric axis of the container body; and - fixing the one or more fiducial marks to a support member configured to support the one or more fiducial marks and to fix the one or more fiducial marks' location in use in relation to the hazardous contents of the container.

18. The method of claim 17, wherein fixing the one or more fiducial marks to the support member comprises fixing the one or more fiducial marks to the container body.

19. The method of claim 18, wherein the one or more fiducial marks are fixed to the container body when the waste container is empty of hazardous waste.

20. The method of claim 18 or claim 19, wherein fixing the one or more fiducial marks to the container body comprises fixing the one or more fiducial marks to a container lid.

21 . The method of claim 17, further comprising the step of fitting the frame inside the waste container.

22. The method of claim 17, further comprising the step of fitting the frame to the waste container.

23. The method of claim 22, further comprising the step of marking the waste container using the frame as a guide, to indicate a spatial relationship between the reference system and the waste container.

24. The method of any of claims 17 to 23, further comprising the step of using one or more reference marks located on the container body to determine a spatial relationship between the reference system and the waste container.

25. The method of any of claims 17 to 24, further comprising the steps:

- storing information about the one or more fiducial marks;

- placing the waste container containing hazardous contents and the one or more fiducial marks in a muon tomography detector;

- recording muon tomography data obtained by the detector; - identifying fiducial data related to the one or more fiducial marks in the muon tomography data;

- retrieving the stored information about the one or more fiducial marks; and

- calculating coordinates of the hazardous contents using the retrieved

information and the fiducial data.

Description:
A REFERENCE SYSTEM FOR WASTE CONTAINERS

Field of Invention

The present invention relates to a reference system for muon scattering tomography of hazardous contents of a waste container. The present invention may be used for example for inspection of nuclear waste containers.

Background Art

Muon scattering tomography (MST) is an imaging technique, which uses cosmic-ray muons to image and inspect the internal composition of shielded containers. Cosmic- ray muons occur naturally from the decay of high-energy pions and kaons produced in the Earth’s upper atmosphere. Cosmic-ray muons interact with matter via ionising interactions with atomic electrons and Coulomb scattering off nuclei. Compared to electrons, cosmic-ray muons are massive and much heavier, making them highly- penetrating particles.

A typical MST imaging technique has at least two sets of position sensitive detectors, preferably placed above and below an inspection object, to measure the trajectories of incoming and outgoing muons, respectively. When a muon is traveling through contents of a shielded container, its direction of movement will be altered by multiple Coulomb scattering events from nuclei. Magnitudes of muon scattering vary with the radiation length of the matter under inspection. With the measured trajectories, the scattering density l, which is defined as the inverse of the radiation length, can be determined. The scattering density l is known to exhibit an inherent dependence on atomic number Z of the matter, and the higher the atomic number of the matter, the larger the resulting scattering density. This makes MST a competitive technique for differentiating contents with different atomic numbers, which are otherwise difficult to interrogate using conventional forms of imaging radiation such as X-rays.

With vast quantities of nuclear waste stored globally, there is a growing need to be able to characterise contents of nuclear waste containers to improve safety and reduce storage costs. However, conventional 3D tomographic imaging of shielded containers with cosmic-ray muons results in a data set that on its own does not use a reference system. For practical applications, therefore, these data cannot be referenced to the actual container. It is difficult to locate a piece of matter with a high atomic number (i.e. reactor fuel, uranium, etc.) that has been found in such a 3D image inside the actual container. It is not possible to be able to inspect outside of the container for any visible signs (e.g. bulging) and to combine the data with complimentary data sets obtained from the imaging technique. In the event of repackaging of the container, for example, it is difficult to know where exactly to expect to find the targets.

United States Patent US3733889A discloses a reference marker for use in conjunction with volumetric inspection techniques on nuclear reactor pressure vessels. A block of material having a sharp corner geometry is provided at particular locations on the vessel structure for inspection. The block of material then serves as a physical location reference marker for use in calibration of the volumetric inspection device. In one embodiment of the patent, the reference marker is permanently affixed to a wall of the pressure vessel.

However, such an arrangement is not suitable for MST because the container walls are effectively transparent to muons and so such reference markers are practically invisible to the imaging system.

United States Patent Publication US2016/0104290A1 discloses techniques, systems and devices for analysing a reconstructed charged particle image of a volume of interest from charged particle detector measurements to determine a location and boundaries of one or more objects or an orientation of the same. In one embodiment, a muon tomography system is used to perform tomography of a target object under inspection, such as cargo in a truck, wherein the hitch of a truck container is extracted from the reconstructed image as a fiducial to determine the bottom end of a container. Image processing techniques were applied to find the edges of the container, the wheels and the truck to localise the corners, boundary and orientation of the container. Fiducial marks can also be located as objects by determining the centre of mass of the container.

Patent US985131 1 B2 discloses a reference system where one or more fiducial markers are placed at known positions along the travel path of positionally-aligned, moveable upper and lower detection units to provide calibration data, which can be used to identify false positives or false negatives in the analysed data of the container or other target object. In some examples, the fiducial markers can be configured as a steel block located on or beneath the floor or on or above the ceiling of the warehouse or other storage facility. In one embodiment, the station includes one or more rails to position the target vehicle and/or target object in the detection region. The one or more rails can be positioned on the bottom plane within the detection region to ensure the target vehicle conforms to a particular alignment with the upper and lower detection units and within the detection region.

However, when placing fiducial marks in the detector or outside the container, the spatial relation of the marks to the inspection object is not reproducible on re inspection of the object. In the aforementioned case where the truck’s hitch is used as the fiducial mark, when the container is reloaded onto to another truck with a different configuration, consistent inspections of the cargo container would be difficult. This is particularly true for containers with rotational symmetry, such as rectangular prisms including cubes, or cylinders such as Intermediate Level Waste (ILW) containers.

For fiducials that are determined as the centre of mass of the container, the problem with rotational symmetry remains. Furthermore, nuclear waste naturally decays by radioactivity over time, which gives rise to changes in atomic number and likely mass number too. Therefore, such a marker is not invariant in relation to the radioactive contents of the container over long periods of time (decades or more) and introduces errors in coordinates because the marker varies with the contents.

Placing fiducial marks in radioactive contents would complicate the filling process and adversely affect safety.

Furthermore, with the aforementioned arrangements of fiducial marks, a direct determination of atomic numbers of the radioactive contents in 3D tomographic images is difficult. Summary of invention

It is desirable to provide a reference system that allows unambiguous and consistent inspection of hazardous contents of a waste container, such as nuclear waste containers, to assist in mitigating risks inherent with the long-term storage of hazardous contents, such as radioactive contents. It is further desirable that the reference system supports a more direct determination of atomic numbers of the hazardous contents in 3D tomographic images.

According to a first aspect of the present invention, there is provided a reference system for muon scattering tomography of hazardous contents of a waste container having a container body, the reference system comprising:

- a support member configured to support one or more fiducial marks;

- one or more fiducial marks having a higher atomic number than the container body, wherein the one or more fiducial marks are fixed to the support member to fix the one or more fiducial marks' location in use in relation to the hazardous contents of the container; and

- the one or more fiducial marks are rotationally asymmetric in relation to a corresponding rotationally symmetric axis of the container body.

Preferably, the system comprises a waste container having a container body that comprises the support member.

Preferably, the one or more fiducial marks are fixed to the container body when the waste container is empty of hazardous waste.

Preferably, the container body comprises a container lid that comprises the support member.

Preferably, the support member is configured to fit inside the waste container.

Preferably, the system further comprises one or more reference marks configured and located on the container body to indicate a spatial relationship between the reference system and the waste container. Preferably, the system comprises a frame that comprises the support member, wherein the support member is configured to fit to the waste container.

Preferably, the support member is configured with a marking guide for guiding marking of a reference mark on the waste container, to indicate a spatial relationship between the reference system and the waste container.

Preferably, the fiducial marks comprise a plurality of fiducial marks having a range of different atomic numbers higher than the container body.

Preferably, the one or more fiducial marks are at least partially embedded in the support member.

Preferably, the support member is shaped to receive the one or more fiducial marks.

Preferably, the one or more fiducial marks comprise lead glass.

Preferably, the one or more fiducial marks comprise steel.

Preferably, the one or more fiducial marks are relatively more corrosion-resistant compared to the hazardous contents of the waste container.

Preferably, the waste container comprises a nuclear waste container and the hazardous contents comprise radioactive contents.

According to a second aspect of the present invention, there is provided a waste container comprising the reference system of the first aspect.

According to a third aspect of the present invention, there is provided a method for muon scattering tomography of hazardous contents of a waste container having a container body, the method comprising the steps:

- providing one or more fiducial marks having a higher atomic number than the container body wherein the one or more fiducial marks are rotationally asymmetric in relation to a corresponding rotationally symmetric axis of the container body; and

- fixing the one or more fiducial marks to a support member configured to support the one or more fiducial marks and to fix the one or more fiducial marks' location in use in relation to the hazardous contents of the container.

Preferably, fixing the one or more fiducial marks to the support member comprises fixing the one or more fiducial marks to the container body.

Preferably, the one or more fiducial marks are fixed to the container body when the waste container is empty of hazardous waste.

Preferably, fixing the one or more fiducial marks to the container body comprises fixing the one or more fiducial marks to a container lid.

Preferably, the method further comprises the step of fitting the frame inside the waste container.

Preferably, the method further comprises the step of fitting the frame to the waste container.

Preferably, the method further comprises the step of marking the waste container using the frame as a guide, to indicate a spatial relationship between the reference system and the waste container.

Preferably, the method further comprises the step of using one or more reference marks located on the container body to determine a spatial relationship between the reference system and the waste container.

Preferably, the method further comprises the steps:

- storing information about the one or more fiducial marks;

- placing the waste container containing hazardous contents and the one or more fiducial marks in a muon tomography detector;

- recording muon tomography data obtained by the detector; - identifying fiducial data related to the one or more fiducial marks in the muon tomography data;

- retrieving the stored information about the one or more fiducial marks; and

- calculating coordinates of the hazardous contents using the retrieved

information and the fiducial data.

Brief description of drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the drawings, in which:

Figure 1 illustrates, in schematic form, an application of a known muon tomography detector to a test container containing a lead cube, a uranium cylinder and a brass cylinder;

Figure 2 shows a xy-plane slice image through the uranium cylinder of the test container of Figure 1 in a reconstructed 3D tomographic image;

Figure 3 shows a xy-plane slice image through the lead cube of the test container of Figure 1 in a reconstructed 3D tomographic image;

Figure 4a illustrates, in schematic form, a plan view of a reference system, in accordance with an embodiment of the present invention, with fiducial marks fixed to a lid of a waste container;

Figure 4b illustrates, in schematic form, a vertical cross section of a waste container, with fiducial marks fixed to a lid of a waste container;

Figure 4c illustrates, in schematic form, a vertical cross section of a waste container, wherein a support member for the fiducial marks is fitted inside the container;

Figure 5a illustrates, in schematic form, a plan view of a reference system, in accordance with an embodiment of the present invention, with fiducial marks fixed to a frame having a guide for marking a waste container; Figure 5b illustrates, in schematic form, a vertical cross section of a waste container, where the frame of Figure 5a is arranged on the top of the container;

Figure 6a illustrates, in schematic form, a plan view of a reference system, in accordance with an embodiment of the present invention, wherein the fiducial marks are fixed to a support member in the structure of the muon tomography detector;

Figure 6b illustrates, in schematic form, a vertical cross section of the support member of Figure 6a;

Figure 7a illustrates, in schematic form, a support member, in accordance with an embodiment of the present invention, shaped with recesses in it to receive fiducial marks;

Figure 7b illustrates, in schematic form, a vertical cross section of a support member, wherein example configurations of fiducial marks are shown;

Figure 7c illustrates, in schematic form, a vertical cross section of a waste container, wherein fiducial marks are fixed inside the wall of the container;

Figure 7d illustrates, in schematic form, a vertical cross section of a waste container, wherein fiducial marks are fixed inside the base of the container;

Figure 7e illustrates, in schematic form, a vertical cross section of a double-walled waste container, wherein a fiducial mark is fixed via connection blocks inside the hollow space between double walls of the container;

Figure 7f illustrates, in schematic form, a vertical cross section of a double-walled waste container, wherein a fiducial mark is wedged between double walls of the container;

Figure 8 is a flow chart illustrating known steps involved in a process of using muon tomography data to reconstruct a 3D image of a waste container; and Figure 9 is a flow chart illustrating steps involved in a process using an embodiment of the present invention, from the provision of fiducial marks, to the identification of the fiducial marks in the reconstructed 3D image of the container, to the mapping of the 3D image onto hazardous contents of the container, and re-imaging the container later and identifying changes.

Description of embodiments

Embodiments provide a reference system for muon scattering tomography of hazardous contents of a waste container having a container body. The container body is the structure that contains the hazardous waste, keeping the hazardous contents safely inside the container. In a nuclear waste container, the container body may have a radiation shielding function. In embodiments, for example as shown in Figures 4 to 7, the reference system comprises one or more fiducial marks having a higher atomic number than the container body, wherein the one or more fiducial marks are fixed to a support member configured to support the one or more fiducial marks and to fix the one or more fiducial marks' location in use in relation to the hazardous contents of the container and the one or more fiducial marks are rotationally asymmetric in relation to a corresponding rotationally symmetric axis of the container body.

The reference system provides an unambiguous connection between the spatial coordinates of the container in a reconstructed image space and the container in real physical space. This allows reproducible measurements of the hazardous contents of the container, increasing the accuracy of comparisons made over very long time periods (decades or more) without being subject to changes in them.

In embodiments, for example as shown in Figures 4 to 7, the one or more fiducial marks have a higher atomic number than the container body. This allows their detection in MCT images acquired through the container body when they are in the container, or in the container body itself, or in the muon path but outside the container. They may have an atomic number lower than the hazardous contents.

The fiducial marks comprise a plurality of fiducial marks having a range of different atomic numbers higher than the container body. The one or more fiducial marks characterised by atomic numbers provide reference points for the hazardous contents in atomic number Z, allowing a direct comparison in the muon tomography data between the fiducial mark features and hazardous waste features. This allows a direct determination of atomic numbers of the hazardous contents.

Preferably, the one or more fiducial marks are relatively more corrosion-resistant compared to the hazardous contents of the waste container. This ensures that the reference system is suitable for use in place over long periods of time. The one or more fiducial marks may comprise lead, lead glass and/or steel.

In embodiments, for example as shown in Figures 4 and 7, the one or more fiducial marks are at least partially embedded in, and/or attached to, the container body. This promotes safety by allowing advance installation of the fiducial marks while avoiding interaction with the hazardous contents of the container. The container body may comprise a container lid as well as wall, base and handle. For example, as illustrated in Figures 7c and 7d, the fiducial marks 732 and 734 are embedded in the wall and base of the container 730, respectively. In some embodiments, the support member, such as the container body, is shaped to receive the one or more fiducial marks.

In embodiments, for example as shown in Figures 4a and 5, the reference system comprises a frame configured to support the one or more fiducial marks, wherein the one or more fiducial marks are at least partially embedded in, and/or attached, to the support member. In an embodiment, for example as shown in Figure 5, the support member is made with rigid material that is low-Z in relation to the hazardous contents of the container. A sturdy plastic, such a POM or acrylic will fulfil this requirement.

In embodiments, one or more reference marks are configured and located on (e.g. at and/or outside) the container body to indicate a spatial relationship between the reference system and the waste container. For example, the one or more reference marks may be in a fixed position with respect to one or more fiducial marks that are themselves fixed to the container body. Preferably, the one or more reference marks are permanently applied and located without imposing any impact on the structural integrity of the container. The one or more reference marks may comprise 1 ) a visible physical mark 2) a mark such as a bar-code or a QR-code; and/or 3) a magnetic mark detectable from outside the container. The one or more reference marks may be put in place via methods including, but not limited to, laser etching or adhesive attachment.

In embodiments, for example as shown in Figures 4 to 7, important information, for example,“no fuel fragments in this container”,“3cm diameter fuel fragment at (x,y,z)” and/or“fiducial mark type X” is marked on (e.g. at and/or outside) the container body to ensure the use and storage of a container’s data for long periods of time.

In the Figures, elements labelled with reference numerals found in the preceding Figures represent the same elements as described for the respective preceding Figure. For example, feature 800 in Figure 9 corresponds to the same feature 800 as described with reference to Figure 8.

Figure 1 illustrates, in schematic form, a known muon tomography detector 100 applied to test container 104 without any fiducial marks. The detector 100 is described in“Application of muon tomography to encapsulated nuclear waste”, Yang et al., IEEE 12 th International Conference on Electronic Measurement & Instruments (ICEMI 2015). The muon tomography detector 100 uses cosmic rays with sensor arrays above 103 and below 107 the container 104 to image high-Z contents of the container. The sensor arrays comprise four modules 102, 104, 106 and 108, each of which is made of two orthogonal layers of scintillating fibres with 128 fibres per module. Saint Gobain (TM) BCF-10 fibres with a diameter of 2mm are used. A multi anode PMT H8500 from Flamamatsu (TM) was used to read the signal from the fibres. In order to reduce the electronics channel number and reduce cost, two fibres are coupled to the same PMT channel. The container 104 is concrete-filled and constructed with 12mm-thick stainless steel, with a height of 255mm and a diameter of 175mm. In the container, a lead cube 1 14 with an edge of length 40mm is located approximately 40mm above a uranium cylinder 1 16; and a 20mm-thick brass cylinder 1 18 with a diameter of 40mm is located approximately 40mm below the uranium cylinder 1 16. The container 104 has a hollow supporting arm 1 12, which is made of a 12mm-thick steel pipe with a diameter of 70mm. Figure 2 and Figure 3 are xy-plane slice images through the centre of the uranium cylinder 1 16 and the centre of the lead cube 1 14 of the test container of Figure 1 respectively, in an image space reconstructed from muon tomography data. The figures demonstrate that high atomic number contents, such as the uranium cylinder 1 16 and the lead cube 1 14, can be clearly distinguished from low atomic number matter, such as the wall of the stainless-steel container 202.

In use of embodiments, fiducial marks of the reference system play a clear role in actual measurements of the container, so that the reference system and the container should be arranged within the direct vertical acceptance of the muon tomography detector and parts that are not relevant to the actual measurements may be left outside the vertical acceptance of the container.

Embodiments may be applicable to different types of containers, such as ILW containers (both new and used), half-height ISO containers and containers with nuclear or non-nuclear toxic contents. Applying the reference system to new containers before hazardous contents are added is a safe process, which keeps the imaging sample preparation (i.e. loading of the container) separate from the loading of the one or more fiducial marks. The new containers are thus empty of hazardous waste. The requirements of the reference system for new containers may be different from those for existing, legacy containers. When in use with new containers, the reference system may have one or more of the following features:

• The one or more fiducial marks are made of matter which is economically moderate to be used in every new container;

• The one or more fiducial marks are corrosion resistant; and

• A placement of the reference system inside the container may be considered.

In an embodiment, the support member is a container lid. An exemplar container lid 400 with two fiducial marks 402 and 404 fixed to it is given in Figure 4a.

Figure 4b illustrates, in schematic form, a vertical cross section of a waste container, with fiducial marks fixed to a lid of a waste container. The container lid 400 has two fiducial marks 402 and 404 fixed to it. It also has one or more reference marks 406 configured and located on the container body to indicate a spatial relationship between the reference system and the waste container.

Figure 4c illustrates, in schematic form, a vertical cross section of a waste container, with the support member for the fiducial marks is fitted inside the container. In this embodiment, the support member 400 comprises a carrier plate configured to fit inside the waste container 408.

Fiducial marks 402 and 404 in Figure 4c (and optionally in Figures 4a, 4b and 5 to 7) comprise lead glass, which is corrosion resistant and will not be affected by the placement inside the waste container.

In another embodiment, the fiducial marks are affixed to a frame. The frame allows the fiducial marks to be fixed at predetermined locations in use in relation to the hazardous contents of a legacy ILW container. As illustrated in Figures 5a and 5b, the frame 504 with fiducial marks 500, 502 and 510 is arranged on the top of the container 514. In use of the embodiment, the frame 504 optionally lines up with one or more unambiguous reference marks previously provided on (e.g. at and/or outside) the container body. The reference marks have a known correspondence to the fiducial marks with respect to the waste container, and may indicate the location of the fiducial marks 500, 502 and 510, as well as the x- and y-axes. In another embodiment, a placement of the frame under the container is possible, but a placement on top of the container may be acceptable and more plausible in practice. A circular groove 512 is machined into the frame 504 from the lower side, with a depth of 1 cm. The groove 512 is designed to fit on top of the upper rim 506 of the container 514. In this way the bottom of the fiducial marks 500, 502 and 510 may be at the same height as the rim 506 of the container and this provides the zero-point reference for the z-coordinate in both real and data space.

Where there are no reference marks on the container body, in one embodiment, a slit 508 is machined into the frame 504, in this example, along the diagonal (e.g. along the x-axis with a direction from fiducial marks 502 to 510). This embodiment allows addition of a minimum appropriate mark to the upper rim 506 of the container 514 by scraping a tool through the slot, using the slot as a guide. In this way, the reference mark is configured to mark the container permanently without affecting the integrity of the container. Thus, the frame is configured with a marking guide for guiding marking of the reference mark on the waste container, to indicate a spatial relationship between the reference system and the waste container.

A single, unambiguously defined reference mark would be sufficient for identifying the container’s orientation. A single reference mark located at the axis of rotational symmetry of the container is however not unambiguous, for example. Examples where an unambiguous reference mark is added to the container comprise, but are not limited to:

• A reference mark does not exist already;

• A single reference mark exists, but when used alone, it is not sufficient for identifying the container’s orientation; or

• Two or more reference marks exist, but they are rotationally symmetric in

relation to the axis of rotational symmetry of the container.

In another embodiment, the fiducial marks are fixed to a support member that is part of the structure of the muon tomography detector, as illustrated in Figures 6a and 6b. In this embodiment, the frame 608 is a low-Z support member (e.g. acrylic) that holds two fiducial marks made of steel 600 and two fiducial marks made of lead 602 and 604. One of the two lead fiducial marks 604 with a size of 3 cm x 3cm x 2cm is larger than the other one 602. The other lead fiducial mark 602 and the steel fiducial marks 600 are cubes with 2 cm long sides. The fiducial marks (e.g. 600, 602 and 604) are oriented so that the corners of the fiducial marks are on the diagonals of the support member. In use of the embodiment, the diagonal from the smaller fiducial mark 602 to the bigger fiducial mark 604 defines the direction of the x-axis 610 of the container. In principle the other two axes are also automatically defined, assuming a right-handed Cartesian coordinate system. The diagonal between the two steel fiducial marks 600 defines the y-axis 612, and the axis of rotational symmetry of the container defines the z-axis of the container, both in real and in data space. The overall position of the container may be fixed by consistently putting the container onto the same marked circle on the table in the muon tomography detector. In this way, the container can be handled entirely remotely.

ILW containers may have two engraved identifiers on the rim of the containers, which could serve as permanent reference marks. However, when there are two of them diametrically opposed this is not unambiguous and an additional marker is introduced to serve as a spatial reference point in connection with a reference system. For reference system embodiments used with existing ILW containers (where the support member is not part of or inside the container), an unambiguously defined reference mark may be used, which, in combination with the two existing identifiers, forms a set of reference marks that are rotationally asymmetric in relation to the axis of rotational symmetry of the container.

In an embodiment, the one or more fiducial marks are held in a 2 cm deep recess machined into the frame. As illustrated in Figure 7a, the support member 700 contains machined recesses (e.g. 704) for fiducial marks (e.g. 702). In these embodiments, the one or more fiducial marks, which may be different sizes (e.g. the fiducial mark 702 is bigger than the fiducial mark 706), may be glued in place.

Figure 7b illustrates examples of fiducial marks partially embedded and/or attached to a support member 707 such as a frame or the container body (such as the wall, lid, base or handle), comprising:

• fiducial mark 708 attached to the top of the support member 707 via a

connection block 726;

• fiducial mark 710 attached to the top of the support member 707 wherein the bottom of the fiducial mark is flush with the top of the frame;

• fiducial mark 712 partially embedded in the support member 707;

• fiducial mark 714 fully embedded in the support member 707 wherein the top of the fiducial mark is flush with the top of the support member;

• fiducial mark 716 fully embedded in the support member 707 ;

• fiducial mark 718 embedded in the support member 707 wherein the bottom of the fiducial mark is in contact with the bottom of the frame; • fiducial mark 720 partially embedded in the support member 707;

• fiducial mark 722 attached to the bottom of the support member 707 wherein the top of the fiducial mark is in contact with the bottom of the frame; and

• fiducial mark 724 attached to the bottom of the support member 707 via a connection block 726.

In an embodiment, as illustrated in Figures 7c and 7d, the fiducial marks 732, 734 are embedded in the wall and base of the container 730, respectively.

In an embodiment, as illustrated in Figures 7e and 7f, the container is a double- walled structure with a hollow space 740 between the two walls 728 and 730. In use of this embodiment, fiducial marks can be additionally configured in the hollow space between the walls. In Figure 7e, the fiducial mark 736 is configured between the walls 728 and 730 via connection blocks 726. In Figure 7f, the fiducial mark 738 is wedged between the walls 728 and 730, wherein one side of the fiducial mark 738 is in contact with the inner surface 731 of the outer wall 730 and one other side of the fiducial mark 738 is in contact the outer surface 729 of the inner wall 728.

Figure 8 is a flow chart illustrating known steps involved in an acquisition of muon tomography data of hazardous contents of a waste container from the muon tomography detector, as follows:

800: Placing the waste container in the muon tomography detector for a certain period of time.

802: Determining scattering positions of each muon with muon tracking analysis software.

804: Reconstructing muon trajectories from the muon scattering positions. The muon tomography data obtained from the steps of 800 to 804 enables the visualisation of the hazardous contents of the container in a 3D image using statistical image reconstruction software. The software may utilise the Maximum Likelihood

Expectation maximisation algorithm. The image reconstruction software can be used to determine the most likely estimation of the scattering density of each pre-divided voxel of the reconstructed image of the container. Different voxel sizes may have an impact on the resolution of the reconstructed image, which is that the smaller the voxel size, the larger a data set is required, and a smoother image is derived.

Figure 9 is a flow chart illustrating a method for muon scattering tomography of hazardous contents of a waste container having a container body, according to an embodiment of the present invention.

The method has the following steps, which can be performed in different orders or concurrently.

900: Storing information about one or more fiducial marks.

902: Providing the one or more fiducial marks, wherein the fiducial marks have a higher atomic number than the container body, and fixing the fiducial marks to a support member configured to support the one or more fiducial marks and to fix the one or more fiducial marks' location in use in relation to hazardous contents of the waste container. The one or more fiducial marks are rotationally asymmetric in relation to a corresponding rotationally symmetric axis of the container body. The support member may be part of the container body. A frame may be provided as the support member, or the support member may be part of a frame. The frame may be fitted to the waste container, where the waste container may be marked using the frame as a guide, as described with reference to Figure 5a. Alternatively, a carrier plate may be placed inside the waste container, as described with reference to Figure 4c.

800 - 804: Recording muon tomography data obtained by the muon tomography detector, as described with respect to Figure 8.

904: Identifying fiducial data related to the one or more fiducial marks in the muon tomography data. 906: Retrieving the stored information about the one or more fiducial marks. Digital reference marks such as a bar-code or QR-code system could be used as a means for data retrieval. 908: Calculating coordinates of the hazardous contents using the retrieved information and the fiducial data.