GB2238504A

GB2238504A - Transport flask

Info

Publication number: GB2238504A
Application number: GB9024507A
Authority: GB
Inventors: Donald Orr
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1989-12-02
Filing date: 1990-11-12
Publication date: 1991-06-05
Anticipated expiration: 2010-11-12
Also published as: GB9024507D0; GB8927321D0; GB2238504B

Abstract

A shielded transport flask (10) comprises a hollow container (12) closed at one end by a door assembly comprising a single gamma gate (38) or two opposed gamma gates (78) slidable in a transverse duct (42) in a door frame (36). With the gate(s) closed, the ends of the transverse duct are sealed by removable cover plates (46) which abut the outer end faces of the gate(s). When the gates (38 or 78) are to be opened the cover plates are removed, and the cover plates may be replaced by duct extensions (88) which incorporate a mechanism (92, 94, 96) for pulling out the gates (38 or 78). Because the duct extensions and the mechanism do not form an integral part of the transport flask, the flask can both be comparatively cheap to manufacture and also able to experience impacts without loss of integrity. The compactness of the flask (10) allows relatively inexpensive outer packaging to be provided, enclosing and protecting the flask, so as to meet the International Regulations for off-site transport of radioactive materials. <IMAGE>

Description

Transport Flask This invention relates to a flask for transporting radioactive material or other hazardous materials.

Radioactive materials such as nuclear fuel rods or highly active waste may need to be transported, and to avoid any risk during transport they are enclosed in flasks of suitable shielding material. The nature of the shielding will depend on the nature of the emissions from the material to be transported; if gamma emissions are to be shielded then a dense material such as lead, tungsten, or steel may be used, of a thickness determined by the intensity of the emitted radiation and which may for example be 0.15 m. Transport flasks can therefore weigh several tonnes. They are also required to be able to withstand severe impacts without their integrity being compromised.

According to the present invention there is provided a transport flask comprising a shielded hollow cylindrical container with at one end a door assembly comprising a door frame defining a doorway aligned with the cylindrical container and a duct extending transverse to the longitudinal axis of the container, at least one shielding gate slidable in the duct so as to open and to close the doorway, removable cover means to seal the end of the duct when the gate is in the closed position, said cover means substantially abutting the outer end of the gate.

There might be two shielding gates which engage each other when closed, and which slide in opposite directions in the duct. After removal of the cover means from the end or ends of the duct the gate or gates can be slid out to open the door. The gates may be supported by low friction metal rubbing strips to facilitate their displacement, or by roller bearings. After removing the cover means, it may be replaced by a duct extension incorporating means for engaging with and withdrawing the gate.

Because the door assembly does not project far beyond the periphery of the container, the transport flask can more readily withstand an impact than a conventional design which incorporates a duct extension and gate opening mechanism as an integral part of the flask; any such integral protruding component must be capable of withstanding the severe forces involved in decelerating the massive flask on impact at any possible orientation. The absence of an integral opening mechanism also makes the flask cheaper and simpler to manufacture; where a removable door-opening mechanism is provided, this can be of much lighter construction than an integral mechanism (as it does not have to withstand impacts), and hence is cheaper.

Consequently the transport flask is both economical and sturdy.

The compact shape of the transport flask also makes it comparatively simple and cheap to provide outer packaging to enclose and protect the flask, so as to enhance its impact and thermal integrity under accident conditions and so to comply with the IAEA Regulations for the Safe Transport of Radioactive Materials and to obtain B(U) certification by the Department of Transport for off-site movement of radioactive materials.

The invention will now be further described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a longitudinal sectional view of a transport flask; Figure 2 shows a perspective view of the door frame of the flask of Figure 1; Figure 3 shows a longitudinal sectional view of part of an alternative transport flask; and Figure 4 shows a longitudinal sectional view of the port part of the flask of Figure 3 with the doors open.

Referring to Figure 1, a transport flask 10 is of generally cylindrical shape, of length 1.33 m and diameter 0.58 m, and of mass about 3.5 tonnes. It comprises a hollow cylindrical tube 12, double-walled of steel filled with lead, the inner wall being of stainless steel defining a chamber 14 of diameter 280 mm and length 850 mm. At the top end (as shown) the bore of the tube 12 is outwardly stepped and is closed by a plug 16 provided with seals and secured by bolts 18. The plug 16 defines an axial stepped bore 20 closed by a blanking-off cap 21 at the inside, and by an inner plug 22 covered by a cover plate 23, both of which are sealed to the plug 16 by O-rings and secured to it by bolts. These features enable the flask 10 to be used in conjunction with conventional posting ports.At the lower end of the tube 12 is an external flange 24 and a tubular boss 26 defined by a downward extension of the inner wall. The lower end of the tube 12 is closed by a door assembly 30, secured by bolts 28 through the flange 24, and which defines a circular recess in its upper surface into which the boss 26 engages. The flask 10 is provided with four equally spaced lifting lugs 32 (only two are shown) welded to the tube 12 near the top, and with two trunnions 34 bolted into corresponding supports 35 welded to the tube 12 on opposite sides at the level of the centre of gravity of the flask 10.

The door assembly 30 comprises a cast and machined steel door frame 36, octagonal in cross-section (see Figure 2) and of width 750 mm, which defines both a circular passage 40 aligned with the bore of the tube 12, and also a rectangular transverse duct 42.

Referring again to Figure 1, a steel gamma gate 38 locates in the transverse duct 42, extending from one end of the duct 42 to beyond the far side of the circular passage 40. The remainder of the duct 42 is occupied by a removable shielding plug 48 which is held in position by two locking plates 50 slidable in a T-shaped groove at the end face of the plug 48 and which engage in slots on the top and bottom surfaces of the duct 42. Each end of the transverse duct 42 is closed and sealed by a cover plate 46 which locates in the end portion of the transverse duct leaving a clearance of 2 mm from the outer end face of the gate 38 or of the plug 48. The lower surface of the gate 38 defines an annular groove in which locates a corresponding annular rim 52 of a seal plate 54 which is bolted and sealed to the outside of the door frame 36.

Along each edge of each gate 38 an aluminium bronze rubbing strip (not shown) is inset which supports the weight of the gate 38. To the outer end face of the gate 38 is attached a plate 62 at the centre of which is a slot 63; behind the slot 63 is a shaped recess 64. The end face is also provided with a safety catch (not shown).

When it is desired to open the transport flask 10 the seal plate 54 holding the gate 38 and the cover plate 46 closing the end of the duct 42 are removed from the door frame 36. The gate 38 is then free to slide sideways along the transverse duct 42 after an operator has released the safety catch. A mechanism for opening the door 38 is described below in relation to Figure 4. While the gate 38 is open, radioactive materials can be inserted into or removed from the chamber 14.

Referring now to Figure 3 there is shown a door assembly 80 of an alternative transport flask 70 with many similarities to the flask 10, identical components being referred to by the same reference numbers. It comprises a double-walled steel lead-filled tube 12 defining a chamber 14, closed and sealed at its upper end by a plug 16 (not shown in Figure 3) and at its lower end by the door assembly 80. The door assembly 80 comprises a door frame 36 as described above, in the transverse duct 42 of which are located two gamma gates 78 with zig-zag mating end faces plated with nickel. The gates 78 are held closed by the rim 52 of a seal plate 54 engaging in an annular groove 81 defined in the lower surfaces of the gates 78, and cover plates 46 seal the ends of the transverse duct.To the outer end face of each gate 78 is attached a plate 62 at the centre of which is a slot 63; behind the slot 63 is a shaped recess 64.

When it is desired to open the flask 70, the seal plate 54 and the cover plates 46 are removed from the door frame 36. Referring now to Figure 4 (which shows the lower part of the flask 70 with both the gates 78 open), two duct extensions 88 are bolted to the frame 36, one at each end of the transverse duct 42. At the outer end of each duct extension 88 is an end plate 90 carrying a captive nut 92 engaging a threaded rod 91. The inner end of each rod 94 forms a T piece 96, which is inserted through the slot 63 to engage in the recess 64 in the end of the gate 78. By rotation of the captive nut 92 the rod 94 and hence the gate 78 is pulled out into the position shown, with the gate 78 extending into the duct extension 88.

To open the door 38 of the flask 10 of Figure 1 the same mechanism would be used, differing only in requiring a single duct extension 88 which is longer than those shown in Figure 4.

It should be appreciated that where such duct extensions 88 are provided the gate opening mechanism may differ from the bayonet-fitting threaded rod 94 and captive nut 92 described above. For example a threaded rod might engage with a nut held in a hole through the gate. Yet again a rod might be connected to a threaded recess at the end of the gate, and the rod then pulled by a rack-andpinion arrangement. In every case the mechanism enables a more accurately controlled and/or a greater pulling force to be exerted on the gates 38 or 78 than an operator could apply by hand, so that more massive gates 38 or 78 can be opened. The mechanism may be provided with interlocks linked with safety procedures, for example linked with the safety catch, or to ensure that a crane cannot lift the flask 70 if the gates 78 are open.The mechanism may be operated manually (using for example a suitable spanner to turn the nut 92 in Figure 4) or by a motor, for example an electric motor (not shown).

In both flask 10 and flask 70, when the gates 38 and 78 are closed and the seal plate 54 and the cover plates 46 are bolted on, no component protrudes very far from the external surface of the tube 12 or 72. It will also be observed that although the plug 76 and the door assembly 30 or 80 are both held to the tube 12 by bolts 18 or 28, shear forces due to accidental impact are also resisted by the interengaging shapes of these components; the same applies to the engagement between the door frame 36 and the seal plate 54 and cover plates 46. The gates 38 and 78 are held closed by the rim 52 of the seal plate 54 so the bolts holding the cover plates 46 do not have to provide the force to hold the gates 38 and 78 on impact; similarly the plug 48 is held in position by the locking plates 50, rather than the cover plate 46.In the absence of the trunnions 34 the parts which protrude furthest from the longitudinal axis of the flask 10 are the cover plates 46, which extend about 150 mm beyond the surface of the tube 12. This arrangement minimises the extent to which the door mechanism protrudes: the lengths of the gates 38 and 78 are governed by shielding considerations, while the cover plates 46 are as close as practicable to the outer end faces of the gates 38 and 78 or of the plug 48, allowing only for any play in the means holding those components, and are as thin as can be achieved bearing in mind the requirement that they must provide a seal which is not damaged by an accidental impact for example from a drop through one metre.

It will be appreciated that the flask may differ in size to those described above, and that the nature and thickness of the shielding material will depend upon the nature of the emissions which are to be shielded. It will also be appreciated that although the door frame 36 is described as being of machined cast steel, it might instead be machined from forged steel, or made from steel parts welded together, or held together by bolts or dowels. The most appropriate method of formation depends upon the size of the flask and the loads it has to withstand.

Claims

1. A transport flask comprising a shielded hollow cylindrical container with at one end a door assembly comprising a door frame defining a doorway aligned with the cylindrical container and a duct extending transverse to the longitudinal axis of the container, at least one shielding gate slidable in the duct so as to open and to close the doorway, removable cover means to seal the end of the duct when the gate is in the closed position, said cover means substantially abutting the outer end of the gate.

2. A transport flask as claimed in Claim 1 provided with a duct extension incorporating means for engaging with and withdrawing the gate, the duct extension being connectable to the flask after removal of the cover means.

3. A transport flask as claimed in Claim 1 or Claim 2 comprising mechanical locking means engagable in a corresponding recess in the gate to further prevent movement of the gate even after removal of the cover means.

4. A transport flask substantially as hereinbefore decribed with reference to, and as shown in, Figure 1 and Figure 2, or Figure 3 and Figure 4 of the accompanying drawings.