GB1583188A

GB1583188A - Containers for storing irradiated nuclear fuel elements

Info

Publication number: GB1583188A
Application number: GB2748977A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1978-05-12
Filing date: 1978-05-12
Publication date: 1981-01-21

Description

(54) CONTAINERS FOR STORING IRRADIATED NUCLEAR FUEL ELEMENTS (71) We, UNITED KINGDOM ATO MIC ENERGY AUTHORITY, London, A British Authority do hereby declare the invention, for which we pray that a patent may be granted to us. and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention concerns containers for storing irradiated nuclear fuel elements and in particular the invention concerns end seal assemblies for such containers.

It is established practice to store irradiated nuclear fuel elements dry, or as near dry as possible, in sealed containers which are submerged under water in a so-called cooling pond in which the activity of the fuel elements lessens with time. A tubular stainless steel container closed and sealed at one end is fitted over an irradiated fuel element, the operation taking place under water. and then the opposite end of the container is closed and sealed.

At present. sealing is effected using an elastomer O-ring which is trapped between an end closure cap for the container and a seat or abutment surface formed on the interior of the container. However, with time and irradiation such an O-ring hardens and becomes brittle and can no longer provide an effective seal. and replacement is difficult since remote handling has to be employed because of the activity of the container and its contents.

According to the present invention. a container for storing a nuclear fuel element has an end seal assembly comprising a soft metal sealing ring located at the periphery of a carrier disc which is rockably mounted on screw means operable to urge the ring into sealing engagement with a seat on the wall of the container. there being included coarse pitch screw means for initial positioning and tightening of the seal ring on its seat and fine pitch screw means for final tightening of the seal ring on its seat.

The invention will be described further, by way of example, with reference to the drawings accompanying the Provisional Specification, in which: Figure 1 is a diagrammatic fragmentary section of part of an existing container for a nuclear fuel element, Figure 2 is a similar view to that of Figure 1 and showing a first construction of container embodying the invention, and Figure 3 is a view like that of Figure 2 and illustrating a second construction embodying the invention.

Figure 1 illustrates a knowri form of container for a nuclear fuel element, such container I being tubular and of stainless steel and formed with a rolled thread 2 at its lower open end. The upper end (not shown) of the container is closed and sealed. The lower end of the container is closed by a tubular cap 3 having a rolled thread to engage the thread on the container. An elastomer O-ring 4 is located between the cap 3 and a seat 5 formed by a reduced diameter portion of the wall of the container. The cap 3 is provided with a star ring 6 at its lower end and the star ring is engaged by tightening apparatus (not shown) which serves to screw the cap 3 on to the thread 2 of the container 1 sufficiently to trap the 0-ring between the end of the cap and the seat 5.An irradiated fuel element (not shown) within the container can rest on the domed end wall of the cap when the container is in an upright position such as is shown in Figure 1.

The arrangement shown in Figure 1 can be unsatisfactory because the elastomer O-ring hardens and becomes brittle with time and under irradiation. and this can result in leakage. Furthermore as the container becomes radioactive, it is difficult to change the O-ring employing remote hand ling.

Figure 2 shows one construction of container to which the invention has been applied. Such container I() is tubular and of stainless steel and is formed with a rolled thread 11 to co-operate with a similar thread on a sleeve 12. The sleeve 12 has a star ring 14 at its lower end for c()-()pcrati()n with tightening apparatus (not shown). The opposite end of the sleeve 12 has an inwardly directed peripheral flange 14 which supports a Belleville washer 15.The washer 15 is engaged by a flange of a nut 16 having a co-operating screw 17.'l'he upper end of the screw 17 terminates in ;I domctl he;id IX ind a carrier dise I 9 sits on the head 18. l'he carrier dise 19 has a peripheral flange which engages the Belleville washer 15.A ring 2() of soft metal is located within a groove or recess about the upper periphery of the carrier disc I 9. Conveniently, the soft metal ring 2() is lead-tin-alloy (soft solder) which is soldiered iii position. In this way the ring 2() is both fixedly secured and sealed to the carrier disc 19.

To facilitate fitting by remote control the parts are assenbled as a unit. The Belleville washer 15 can be temporarily secured to the flange 14 hv solder or adhesive and the domed head 18 of the screw 17 is retained by a skirt 21 depending from the carrier disc 19 and having an inwardly directed lip to engage beneath the head of the screw 17.

In use, with an irradiated fuel element within the container, the sleeve 12 is screwed in on the rolled threads 11 until the sealing ring 20 of soft metal contacts a seat 22 formed by a reduced diameter portion of the wall of the container. The carrier dise 19 can rock on the domed head of the screw and hence the seal is self-aligning and can accomodate any swash of the seat.

The screw 17 is then rotated, again by remote control. The screw thread of the nut 16 is small in diameter and pitch compared to the rolled thread and consequently for any given torque will exert a great axial force on the seal. As the screw 17 is rotated it moves the nut 16 downwardly (as shown in Figure 2) which exerts upward pressure on the disc 19 via the Belleville washer 15.

This upward axial force applied to the soft metal sealing ring 2() causes the sharp edge thereof which contacts the seat 22 to deform plastically to contact the seal 22 over an enlarged area. At the same time Belleville washer 15 distorts elastically towards flatness and the seal carrier disc 19 can also deform elastically to assume a slightly domed shapc. Further the seal pressure gives rise to elastic strain (hoop stress) in the container wall. These factors provide a reservoir of stored elastic strain to accommodate long term creep of the soft metal sealing ring 20 thereby ensuring that it rem;iins under compression and in sealing engagement with its seat.

Th embodiment of Figure 2 utilises cooperating rolled threads as in the prior art arrangement of Figuer 1. The sleeve 12 in Figure 2 is identical to the cap 3 in Figure 1 minus the domed cntl.

In the Figure 3 embodiment the assembly is produced by machiiiiiig A cylindrical extension 3() which is butt welded at 31 the lower end of a tubular, stainless steel container, is formed with a scal 32 to co-operate with the sealing ring. similar to the sealing ring 2(1 of the Figure 2 cmbodiment. The seat 32 can take a number of configurations and examples of two alternatives are shown in Figure 3, see the enlarged detail views in dot-and-dash circles. The sleeve, which corresponds to sleeve 12 of the Figure 2 construction, is provided with a machined screw thread 33 t() co-operate with corresponding threads on the cylindrical extension 30.The free end of the exten- sion 3() has castellations 34 engageable with tightening apparatus (not shown). Similar components to the nut 16, screw 17 with domed head 18, Belleville washer 15, carrier disc 19. seal 20 and retaining skirt 21 of the Figure 2 construction are provided as shown, and the mode of operation is also similar.

WHAT WE CLAIM IS: 1. A container for storing a nuclear reactor fuel element having an end seal assembly comprising a soft metal sealing ring located at the periphery of a carrier dise which is rockably mounted on screw means operable to urge the ring into sealing engagement with a seat on the wall of the container, there being included coarse pitch screw means for initial positioning and tightening of the seal ring on its seat and fine pitch screw for final tightening of the seal ring on its seat.

2. A container according to claim 1, wherein there is interposed between the seal ring and the screw means, means for providing a reservoir of stored elastic strain for accommodating long term creep of the seal ring and ensuring continued compression thereof.

3. A container for storing a nuclear reactor fuel element, substantiallv as hereinbefore described with reference to Figures 2 or 3 of the drawings accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

ling.

Figure 2 shows one construction of container to which the invention has been applied. Such container I() is tubular and of stainless steel and is formed with a rolled thread 11 to co-operate with a similar thread on a sleeve 12. The sleeve 12 has a star ring

14 at its lower end for c()-()pcrati()n with tightening apparatus (not shown). The opposite end of the sleeve 12 has an inwardly directed peripheral flange 14 which supports a Belleville washer 15.The washer

15 is engaged by a flange of a nut 16 having a co-operating screw 17.'l'he upper end of the screw 17 terminates in ;I domctl he;id IX ind a carrier dise I 9 sits on the head 18. l'he carrier dise 19 has a peripheral flange which engages the Belleville washer 15.A ring 2() of soft metal is located within a groove or recess about the upper periphery of the carrier disc I 9. Conveniently, the soft metal ring 2() is lead-tin-alloy (soft solder) which is soldiered iii position. In this way the ring 2() is both fixedly secured and sealed to the carrier disc 19.

To facilitate fitting by remote control the parts are assenbled as a unit. The Belleville washer 15 can be temporarily secured to the flange 14 hv solder or adhesive and the domed head 18 of the screw 17 is retained by a skirt 21 depending from the carrier disc 19 and having an inwardly directed lip to engage beneath the head of the screw 17.

In use, with an irradiated fuel element within the container, the sleeve 12 is screwed in on the rolled threads 11 until the sealing ring 20 of soft metal contacts a seat 22 formed by a reduced diameter portion of the wall of the container. The carrier dise 19 can rock on the domed head of the screw and hence the seal is self-aligning and can accomodate any swash of the seat.

The screw 17 is then rotated, again by remote control. The screw thread of the nut 16 is small in diameter and pitch compared to the rolled thread and consequently for any given torque will exert a great axial force on the seal. As the screw 17 is rotated it moves the nut 16 downwardly (as shown in Figure 2) which exerts upward pressure on the disc 19 via the Belleville washer 15.

This upward axial force applied to the soft metal sealing ring 2() causes the sharp edge thereof which contacts the seat 22 to deform plastically to contact the seal 22 over an enlarged area. At the same time Belleville washer 15 distorts elastically towards flatness and the seal carrier disc 19 can also deform elastically to assume a slightly domed shapc. Further the seal pressure gives rise to elastic strain (hoop stress) in the container wall. These factors provide a reservoir of stored elastic strain to accommodate long term creep of the soft metal sealing ring 20 thereby ensuring that it rem;iins under compression and in sealing engagement with its seat.

Th embodiment of Figure 2 utilises cooperating rolled threads as in the prior art arrangement of Figuer 1. The sleeve 12 in Figure 2 is identical to the cap 3 in Figure 1 minus the domed cntl.

In the Figure 3 embodiment the assembly is produced by machiiiiiig A cylindrical extension 3() which is butt welded at 31 the lower end of a tubular, stainless steel container, is formed with a scal 32 to co-operate with the sealing ring. similar to the sealing ring 2(1 of the Figure 2 cmbodiment. The seat 32 can take a number of configurations and examples of two alternatives are shown in Figure 3, see the enlarged detail views in dot-and-dash circles. The sleeve, which corresponds to sleeve 12 of the Figure 2 construction, is provided with a machined screw thread 33 t() co-operate with corresponding threads on the cylindrical extension 30.The free end of the exten- sion 3() has castellations 34 engageable with tightening apparatus (not shown). Similar components to the nut 16, screw 17 with domed head 18, Belleville washer 15, carrier disc 19. seal 20 and retaining skirt 21 of the Figure 2 construction are provided as shown, and the mode of operation is also similar.

WHAT WE CLAIM IS: 1. A container for storing a nuclear reactor fuel element having an end seal assembly comprising a soft metal sealing ring located at the periphery of a carrier dise which is rockably mounted on screw means operable to urge the ring into sealing engagement with a seat on the wall of the container, there being included coarse pitch screw means for initial positioning and tightening of the seal ring on its seat and fine pitch screw for final tightening of the seal ring on its seat.
2. A container according to claim 1, wherein there is interposed between the seal ring and the screw means, means for providing a reservoir of stored elastic strain for accommodating long term creep of the seal ring and ensuring continued compression thereof.
3. A container for storing a nuclear reactor fuel element, substantiallv as hereinbefore described with reference to Figures 2 or 3 of the drawings accompanying the Provisional Specification.