GB2171185A

GB2171185A - Operating travelling charge furnaces

Info

Publication number: GB2171185A
Application number: GB08504356A
Authority: GB
Inventors: Peter Flatley; Alan Geoffrey Macdonald
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1985-02-20
Filing date: 1985-02-20
Publication date: 1986-08-20
Also published as: GB2171185B; GB8504356D0

Abstract

A charge is conveyed through a furnace, e.g. for sintering nuclear fuel pellets, by indexing a train of abutting skids (10) supporting charge-containing boats (12). The skids and boats emerging from the exit end of the furnace are recycled back to the entrance of the furnace with a fresh charge. If there is an interruption in the supply of the charge, the charge train continues to be indexed through the furnace but using dummy members in place of the skid/boat combinations, the dummy members being designed to simulate a skid/boat plus charge in terms of weight and thermal mass. The skids (10) and boats (12) may be made from sheet molybdenum or molybdenum alloy, and the dummy members from a slab of such material. The dummy members may incorporate shields for screening radiant heaters of the furnace from one another. <IMAGE>

Description

SPECIFICATION Furnace Charge Train This invention relates to furnaces of the type in which the charge to be subjected to heat treatment is conveyed through the furnace by means of a socalled charge train comprising a plurality of end-toend located carriers each supporting a respective charge container. The carriers and containers are normally termed skids and boats respectively. Such furnaces are used in the sintering of ceramic powder compacts, for example urania powder pellets for use as nuclear fuel, and the skids and boats are typically composed of molybdenum or molybdenum alloy.

In practice, to maintain thermal equilibrium within the furnace and continuity of the charge train, the charge train is maintained operational even when the supply of charge material is interrupted, for example when a change of charge material is required, as may be necessary in the case of nuclear fuel pellets when the level of enrichment is to be altered. The charge train is maintained in motion by pushing the train to index it through the furnace by means of the trailing skid and, as each skid/boat combination, emerges from the furnace the skids and boats are transferred from the leading end of the train back to the trailing end. Thus, when the supply of charge is interrupted, the skids and boats continue to be fed into the furnace but with a charge of scrap molybdenum so as to simulate the thermal mass and weight of a normally charged skid and boat.

In general, the working life of a skid or boat is determined by the number of passes it makes through the furnace since the high temperatures prevailing results in distortion by creep deformation and buckling. It will be appreciated that a certain amount of the useful working life of a skid or boat may therefore be lost as result of passes made while loaded with scrap material.

The object of the present invention is to enable the productive working lives of skids and boats to be lengthened with consequent cost savings in replacement.

According to one aspect of the present invention there is provided a method of operating a furnace in which the charge to be subjected to heat treatment is conveyed through the furnace by means of a charge train comprising a plurality of carriers disposed end-to-end and a plurality of charge containers each supported on a respective carrier, said carriers and containers, after emerging from the furnace, being transferred from the leading end to the trailing end of the charge train for reintroduction into the furnace with a fresh charge to be heat treated, said method being characterised in that movement of the charge train through the furnace is maintained upon interruption in the supply of charge to the charge train by inserting a dummy member at the trailing end of the charge train in place of each carrier/container pair withdrawn, said dummy member having a mass greater than the carrier/container combination it replaces.

According to a second aspect of the invention there is provided in or for a furnace a plurality of components forming, or adapted to form, a charge train for conveyance of charge material to be heat treated through the furnace, said components comprising a plurality of carriers which can be located end-to-end for lengthwise collective displacement of the carriers through the furnace, a plurality of charge containers adapted to be supported on the carriers in such a way that the containers do not touch when assembled into the charge train with the carriers, and at least one dummy member for insertion into the charge train in place of a carrier/container combination, the mass of the dummy member being in excess of that of a carrier/container combination.

The invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic plan view of a charge train; Figures 2-4 are side, end and plan elevations of a conventional skid; Figures 5-7 are similar elevations of a conventional boat; Figure 8-10 are similar elevations of a dummy skid in accordance with the invention; and Figures 11-13 are similar elevations of a shield structure for use with the dummy.

Referring to Figure 1, a furnace charge train comprises a series of skids 10 disposed in end-toend abutment to form a conveyor for movement along a duct (not shown) in the furnace. The skids are indexed in the direction A by pushing the trailing skid. As the train advances into the furnace, the skids which have emerged at the exit end of the furnace are recycled back to the trailing end of the train. Each skid supports a boat 12 which, in normal use, may contain a charge of pellet precursors composed of powdered nuclear fuel material.

Figures 2-4 illustrate one form of skid 10 currently in use. The skid is of elongated configuration, generally rectangular in plan view and of channel shape in transverse section, so as to form a carrier in which a boat 12 can be located so that the boats are supported in a stable manner and in spaced relation to each other. Figures 3-5 illustrate one form of boat construction, the boat having a bottom 14for seating on the skid and an open top 16 for charging and discharging of the pellets. The skids 10 and boats 12 are fabricated from molybdenum alloy in sheet form.

As mentioned previously, to maintain uniform furnace conditions when the supply of charge material is interrupted, the practice hitherto has been to continue recycling the skids and boats to the trailing end of the train and to load the boats with scrap molybdenum so asto simulate a normal charge.

This practice results in a reduction in the useful working lives of the skids and boats. To avoid this, the present invention employs dummy skids 20 (see Figures 8-10) which, in terms of their length and width dimensions (as seen in plan) are of substantially the same configuration as the skids 10 so that they may be used in the charge train in place of the skids 10. The dummy skids 20 are of substantially thicker section than the skids 10 so that they undergo less distortion due to creep deformation. Thus, as shown, the skids 20 may comprises a generally rectangular slab of molybdenum (with chamfered ends 22) whereas the skids 10 are fabricated from plate material.

The dummy skids 20 may be provided with a shield structure 30 if desired for use in circumstances where reliance is normally placed on the presence of the boats to screen radiant heaters on opposite sides of the path of travel of the charge train from each other. The shield 30 is mounted on top of the dummy skid and is located by upstanding pegs 32 which engage within the corners of the shield structure. The latter may also be fabricated from molybdenum sheet.

The mass of the dummy skid (or, where applicable, the combined mass of the dummy skid and associated shield structure) is such that it exceeds the combined thermal and inertial mass of the conventional skid and boat, preferably by an amount corresponding to the designed load of the boat when carrying the charge to be sintered. In this way, the overall weight of the train remains substantially unchanged whether or not the charge train includes dummy skids. Moreover, the thermal mass remains substantially unchanged thereby maintaining uniform thermal conditions within the furnace.

Claims

1. A method of operating a furnace in which the charge to be subjected to heat treatment is conveyed through the furnace by means of a charge train comprising a plurality of carriers disposed end-toend and a plurality of charge containers each supported on a respective carrier, said carriers and containers, after emerging from the furnace, being transferred from the leading end to the trailing end of the charge train for reintroduction into the furnace with a fresh charge to be heat treated, said method being characterised in that movement of the charge train through the furnace is maintained upon interruption in the supply of charge to the charge train by inserting a dummy member at the trailing end of the charge train in place of each carrier/container pair withdrawn, said dummy member having a mass greater than the carrier/container/charge combination it replaces.

2. In or for a furnace a plurality of components forming, or adapted to form, a charge train for conveyance of charge material to be heat treated through the furnace, said components comprising a plurality of carriers which can be located end-to-end for lengthwise collective displacement of the carriers through the furnace, a plurality of charge containers adapted to be supported on the carriers in such a way that the containers do not touch when assembled into the charge train with the carriers, and at least one dummy member for insertion into the charge train in place of a carrier/container combination, the mass of the dummy member being in excess of that of a carrier/container combination.

3. A method or charge train as claimed in Claim 1 or 2 in which the or each dummy member is essentially composed of the same material as the carriers and containers.

4. A method or charge train as claimed in Claim 3 in which said material is molybdenum or a molybdenum alloy.

5. A method or charge train as claimed in any one of Claims 1 to 4 in which the length dimension of the or each dummy member is substantially the same as said carriers.

6. A method or charge train as claimed in any one of Claims 1 to 5 in which the or each dummy member includes a shield structure.

7. A method or charge train as claimed in any one of Claims 1 to 6 in which each carrier is of channel shape in transverse section and the or each dummy member has an external configuration similay to that of each carrier but is of substantially thicker section.

8. A method or charge train as claimed in Claim 7 in which the or each dummy member comprises a slab of material whose external contour, at least in terms of length and width dimensions, is substan tidily the same as said carriers.

9. A method as claimed in Claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

10. Afurnace charge train substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.