WO2000025152A1

WO2000025152A1 - Device for inspecting radioactive drums

Info

Publication number: WO2000025152A1
Application number: PCT/NL1999/000661
Authority: WO
Inventors: Willem Frederik André Rogier VERBAKEL
Original assignee: Stichting Energieonderzoek Centrum Nederland
Priority date: 1998-10-26
Filing date: 1999-10-26
Publication date: 2000-05-04
Also published as: NL1010392C2; AU1188300A

Abstract

Device (1) for inspecting drums (4) which can contain highly radioactive material. Said device is placed in the path between the store (2) where the drums (4) are located and a receiving container (5) by means of which the drums (4) are further transported. The drums (4) are placed in the device (5) and rotated. Detection means (17) are present a considerable distance away from the drums (4) and collimators (32, 33) are arranged in the path between the detection means (17) and the drums (4), the whole being provided with all-round means (9, 10) for shielding against radiation.

Description

DEVICE FOR INSPECTING RADIOACTIVE DRUMS

The present invention relates to a device according to the preamble clause of Claim 1. Such drums can be filled with radioactive waste. In this context a distinction is made between material of low, moderate and high radioactivity. Material of low and moderate radioactivity is detected with the aid of a passive gamma scanning technique. If drums are present in a store for a prolonged period the activity thereof can change dramatically depending on the half-life of the isotopes contained therein. Therefore it is important to measure the activity, certainly if the contents are not known. For waste of low and moderate radioactivity a detector can be placed a relatively short distance away from a drum. No other special measures are needed for this. In this case the drum can be measured point by point, but it is also possible to make the drum rotate with respect to the detector. By coupling this rotary movement with a vertical movement of the container, the wall of the container can be scanned in a spiral manner. One example of such an device is disclosed in EP 0 178 069 A2. In the known device a radiation shield is present which serves to remove interfering effects of a radiation from the environment. That is to say the device is intended in particular for waste of very low radioactivity.

The method proposed in EP 0 178 069 A2 is suitable for use for waste of moderate and low radioactivity, but a measurement method of this type will not work in the case of highly radioactive waste. Firstly, there are no detectors in existence which yield an adequately usable measurement result with the set-ups as known from the prior art. After all, with known setups a conventional germanium detector will be overloaded, that is to say the count rate is much too high. Moreover, such a measurement would be impossible because of inadequate protection for the personnel. The aim of the present invention is to avoid these disadvantages and to be capable of being able to subject even highly radioactive drums or other containers for radioactive material to an inspection, the results obtained being a specific indication of both the nuclide specific activity in the drum and the presence of leaks.

This aim is achieved with an device as described above having the characterising features of Claim 1. According to the invention the shield is all-round because the product on which measurements are to be carried out can contain highly radioactive material. The radiation protection serves not to restrict interference from the environment but to protect personnel. Apart from this protection of the personnel, the detection means are arranged some distance away from the drum. This distance is preferably between 100 and 200 cm and is more particularly approximately 150 cm.

In a number of applications distance alone is inadequate or the distance becomes impracticably great. In that case it is desirable to arrange a collimator between the (germanium) detector and the drum. According to the invention the collimator preferably consists of a combination of a vertical and a horizontal collimator. More particularly, each collimator consists of a collimator slider in which a number of collimator openings of different sizes have been made. One collimator slider contains a series of vertical openings and the other collimator slider contains a series of horizontal openings. An adjustment optimally suited to the conditions of the drum can be obtained by applying a specific combination in the path between drum and detector. According to the invention, at least one of the collimators has a tapered section in the direction from the product on which measurements are to be carried out towards the detector. A divergent section can optionally adjoin the tapered section. If two collimators are used for the detector, use of a divergent section will have an effect, in particular, in the case of the collimator which is located closest to the detector. According to the invention, the collimators are shaped such that the intensity of radiation on the detector can be controlled by the choice of collimator combinations. The measurement is always carried out on the same complete section of the drum with each combination. Such an adjustment can be carried out on the basis of measurements by auxiliary detectors which are located very close to the drum. The auxiliary detectors can be ionisation chambers. Optionally, and in combination with the above, it is also possible to subject the detector to the radiation starting from the smallest combined collimator opening and to open the collimator until the detector receives the maximum permissible signal. Radioactivity of the drum concerned can be determined on the basis of this. Preferably the receptor means for the drums are arranged such that these are movable between a number of positions; one in which the drum can be inserted and one in which the drum is "clamped in position".

As indicated above, the device according to the present invention will be able to be used between an outlet from of a store and a container for further transport of the drums concerned. Preferably, the relevant coupling means are present in order to produce, in particular, a connection to the container for further transport. It will be understood that the device according to the invention is preferably of mobile construction, so that said device can be arranged above the relevant outlet from the relevant store. The invention will be explained in more detail below with reference to an illustrative embodiment shown in the drawing. In the drawing:

Fig. 1 shows, diagrammatically, in side view, a store for radioactive waste with the device according to the invention arranged thereon; Fig. 2 shows the device according to the present invention in more detail in the view according to Fig. 1 ;

Fig. 3 shows a plan view of the device according to Fig. 2; Fig. 4 shows a first detail of the device according to Figs 2 and 3 in plan view; and Fig. 5 shows a second, exposed, detail of the device according to Figs 2 and 3. The device according to the present invention is indicated by 1 in the figures. As can be seen from Fig. 1, the device is arranged above a store 2 in which there are a number of shafts 3. One or more drums 4 are arranged in each shaft. Such drums must, for example, be moved. Because these drums can contain highly radioactive waste, it is not permitted to move these freely through treatment areas. The use of a storage container 5, which is provided with a lifting device 6, which is not shown in detail, is needed for this purpose. As can be seen from Fig. 1, the storage device 5 is placed on the device 1 according to the invention and after coupling lifting cable 37 to the relevant drum 4 the drum can be removed from its place and raised. Storage container 5 is provided with a lid 7 that can be opened and closed and during further transport said lid 7 will, of course, be closed. In the position shown in Fig. 1 the lid is open.

Details of the device according to the present invention are shown in Figs 2 and 3. The part on the right in Fig. 2 that is indicated by 8, and a detail of which can be seen in Fig. 4, supports on the upright edge of store 2. Adjustable legs 39 are provided for positioning the device as desired. The device is of mobile construction in order thus to be able to be placed above the various shafts 3 of the store 2. The device consists of a steel casing 9 in which lead shielding 10 has been installed. Hatching is used to indicate what part is made of steel and what part is made of lead. The lead material provides the actual shielding, whilst the steel casing serves to support the lead material and as a safety feature in the event of possible melting of the lead material concerned. The general shape of the casing is such that the weight is as low as possible whilst the detection level is essentially constant. This is achieved by means of the tapered shape. device 1 consists of two parts. As indicated, a drive part 8, in which the drum 4 is accommodated and is rotated, which part is discussed in more detail with reference to Fig. 4. The second part is the detection part, which is indicated by 11. Detection is achieved with the aid of a nitrogen-cooled germanium detector 17 (counter) and an optical camera 31. This camera is placed behind a screen 40 consisting of a steel plate 30 in which an opening 42 filled with lead glass 41 has been made. Said opening 42 extends downwards and is indicated by 43. Opening 43 is open, that is to say no material has been fitted therein. This steel plate can, for example, have a thickness of 20 cm. Two collimators 31 and 32 are present, which will be discussed in more detail with reference to Fig. 5.

As can be seen from Figs 2 - 4, the drum 4 is accommodated between wheels 21-23. With this arrangement the wheels 22 and 23 are rotatably and hingeably mounted with the aid of supports. Wheel 21 is mounted on an arm 18 which is hingeable about fixed hinge point 19. Hinging of arm 18 is produced by moving tube 29 in the direction of the axis thereof, which tube 29 is, on the other hand, hingeably connected to arm 41 in the manner of a type of parallelogram construction. Movement of tube 29 along the axis thereof is controlled by a motor 12. These motor drives threaded spindle 14 via a right-angle transmission, which spindle 14 engages in a screw thread made in bush 15. Bush 15 is coupled via spring 13 to bush 16 which, in turn, is connected to hollow tube 29. That is to say, with the aid of motor 12 a resilient rotary force is applied to wheel 21 which is sufficient to clamp drum 4. If drum 4 has to be released, the motor is then operated such that wheel 21 swings away from the drum, after which the drum is again suspended solely by the lifting cable 37 described above. Apart from arm 18, ring 44 is also rotated with the aid of motor 12, by which means the wheels 22 and 23 are moved inwards and outwards, respectively, by a cam mechanism that is not shown in more detail.

Rotation of the drum is achieved by means of motor 28. Via a chain 26, this motor 28 brings chain 25 into movement, which chain 25, in turn, causes chain 24, connected to wheel 21, to turn. A transmission 27 is mounted between motor 28 and chain 26. In this way the drum 4 can be rotated after it has been engaged.

In addition to drive components, component 8 is also provided with four detectors 38 of the so-called ionisation chamber type. These serve for continually carrying out a very general determination of the radioactivity in the drum 4. The detection part 11 will be described in more detail on the basis of the combination of Figs 2 and 5. Two collimators are arranged in front of detector 17, that is to say in the path between detector 17 and the drum 4 to be detected. As can be seen from Fig. 5, these collimators consist of two collimator sliders 32 and 33. Collimator openings have been made in each of the sliders. Vertical openings 35 have been made in collimator slider 33 and horizontal collimator openings 34 in collimator slider 32. Said openings all have different dimensions. The nearest of each of the collimator sliders 32 and 33 is shown in exploded view. It can be seen from this that the collimator openings 34 are tapered, whilst the collimator openings 35 in slider 33 taper and then diverge. Optimum shielding against the radiation originating from the drum can be provided by positioning the relevant collimator openings suitably in front of one another. Collimator sliders can, for example, be made of lead and have a thickness of approximately 70 mm, the collimator opening being delimited by an approximately 1 cm thick tungsten insert. It will be understood that transmission through tungsten is appreciably less than that through lead. The design of the vertical collimator is of the 'tapered slit' type, so that the likelihood of detection from any point in a horizontal plane is the same. A smaller collimator provides for a narrower vertical section of the detector that is irradiated. The horizontal collimator is narrow on the detector side and becomes broader (higher) towards the front, so that radiation from the full 20 cm of vertical plane has an equally high likelihood of detection. Beyond this 20 cm the likelihood of detection decreases to 0 within 1.5 cm.

The dimensions of the vertical collimator openings 34 can be, for example, 0-0.4 - 2-8 mm. The dimensions of the horizontal collimator openings are, for example 2-8-20 mm. These collimators can be moved simply from the outside by hand without this giving rise to any risk for the staff concerned. Detector 17 is provided with a nitrogen cryostat, indicated by 42.

During operation of the device described above the combination of horizontal and vertical collimator openings will be optimised such that measurements can be carried out under optimum conditions in respect both of the sensitivity of the detector 17 and of the measurement time. Detector 17 will perform energy-sensitive measurements of the radiation, so that both the γ-radiating nuclide and its activity are determined from the measured spectrum. The correct position of highly radioactive waste can be determined with the aid of electronics/software. By this means it is possible, if necessary, to repack the drum effectively, with highly radioactive waste and waste of low radioactivity being separated from one another.

The light-sensitive camera 31, which is likewise placed behind lead glass, serves to detect any visible defects in the drum. Light sources, which are not shown, are provided close to the drum. Because of the appreciable distance from the drum and the shielding by the lead glass, the camera has an appreciable life. The distance between the drum and the various detectors is preferably approximately 155 cm.

Because the detector 17 cannot be moved in the vertical direction into the centre of the radiation path from the drum because of the presence of the camera 31 , as shown in Fig. 2 the detector has been shifted downwards somewhat, in particular the horizontal collimator and more particularly the top half thereof must have a greater divergence than the bottom half thereof.

The device described above provides optimum protection for operating personnel coupled with a relatively low weight (less than 4 tonnes). As a result of this relatively low weight, said device is easy to move and position above the relevant shaft.

By rotating the drum it is possible to obtain a very accurate impression both of the contents and of the surface condition thereof. It is possible for drums to be attacked, for example by acids which are produced in the drum, which, for example, is possible as a result of the decomposition of polyvinyl chloride during the period for which the drum remains in the store.

Although the invention has been described above with reference to a preferred embodiment, it must be understood that numerous modifications can be made thereto without going beyond the scope of the present application.

Claims

1. device (1) for inspecting drums (4) which can contain highly radioactive material, comprising receptor means (21-23) for said drums, said receptor means being equipped to allow said drums to rotate, as well as detection means (17, 31) arranged some distance away from the drums, the receptor means and the detection means being arranged in a casing (9, 10) which shields against radiation, characterised in that said casing (9, 10) which shields against radiation extends all round said receptor means and said detection means in order to provide radiation shielding for radiation originating from said receptor means.

2. device according to Claim 1, wherein at least one collimator (32-35) is arranged between said detection means and said drum.

3. device according to one of the preceding claims, wherein two collimators (32-35) extending peφendicularly with respect to one another are arranged between said detection means and said drum.

4. device according to Claim 3, wherein said collimators comprise a part which tapers in the direction of the radiation.

5. device according to Claim 4, wherein said collimators comprise a divergent part adjoining said tapering part.

6. device according to one of the preceding claims, wherein said collimators have a series of spaced collimator openings (33, 35) apart and of different size, it being possible to insert the desired collimator opening in the path between detection means and drum.

7. device according to one of the preceding claims, wherein the receptor means (21-23) can be matched to the cross-sectional dimension of the drum.

8. device according to one of the preceding claims, wherein the casing can be closed/opened close to and above said receptor means.

9. device according to Claim 8, wherein the casing is provided with coupling means for connection to a container (5) for accommodating said drum.

10. device according to one of the preceding claims, wherein said detection means comprise an optical camera (31).