Apparatus for grinding nuclear fuel pellets

Abstract: Particulates (42) from a source (12) fall into a tundish (14) which discharges to a collection chamber (18). The build-up of particulates (42) in the chamber (18) is sensed, and a valve (22) opened at the bottom of the chamber (18) when a pre-determined level of the particulates (42) is sensed in the chamber (18). A valve (17) at the top of the chamber (18) may be closed at the same time so that pressurized air can be injected into the chamber (18) to expel the particulates (42) through the bottom of the chamber (18) through the valve (17). A filter circuit (52) may be connected to the tundish (14) to assist in removing particulates (42) from the tundish (14).

Description: This invention relates to an apparatus for grinding nuclear fuel pellets. During the manufacture of nuclear fuel, for example uranium oxide fuel in which pellets comprising uranium oxide are ground to size, particulate residue needs to be collected so that it can be returned for further processing. FR-A-2 088 366 discloses an apparatus for grinding nuclear fuel pellets comprising wheels for grinding the pellets. During grinding a liquified inert gas is injected, and the particulate residue is sucked off and collected on a filter for further recycling. This document also refers to the prior art which describes the use of water during grinding, giving rise to several drawbacks such as criticality problems (water acting as a nuclear moderator for the fissile material in the nuclear fuel) and the need for a closed circuit to purify the sludge resulting from the grinding operation. US-A-4 348 279 deals with an apparatus for cleaning drilling mud, provided with a vessel having declivitous sides and a bottom outlet, a collection chamber having an upper inlet communicable with the bottom outlet and a lower outlet closable by a lower obturating means, sensing means for sensing an accumulation of the solid particles at a predetermined density in the chamber, and upper obturating means between the sensing means and the bottom outlet, the upper obturating means and the lower obturating means being responsive to the sensing means. According to the present invention, an apparatus for grinding nuclear fuel pellets comprises parallel grinding wheels for grinding the pellets, a trough located below the grinding wheels for collecting liquid and particulates from the grinding wheels, an outlet from the trough arranged to discharge the liquid and the particulates into a vessel having declivitous sides and a bottom outlet, a collection chamber having an upper inlet communicable with the bottom outlet and a lower outlet closable by a lower obturating means, sensing means for sensing an accumulation of the particulates at a predetermined level in the chamber, upper obturating means between the sensing means and the bottom outlet, and air pressure means between the upper obturating means and the sensing means, the upper obturating means, the lower obturating means, and the air pressure means being responsive to the sensing means such that the particulates are ejected when the predetermined level has been reached, a recirculation circuit for liquid connected at one end to the declivitous sides of the vessel and having an outlet at the grinding wheels, and pump means for circulating said liquid in the vessel through the recirculation circuit. Desirably, a filter circuit is connected to the vessel for passing liquid in the vessel through a filter means, preferably a cross-flow filter means. Advantageously, a particulate coating comprising U₃O₈ is applied to the ingress side of the cross-flow filter. The invention will now be further described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation of grinding and collection apparatus in the manufacture of nuclear fuel; Figure 2 shows a schematic representation of a modified form of the apparatus of Figure 1, and Figure 3 shows a fragmentary sectional representation of a filter element of Figure 2. In the above Figures, like parts have like numerals. Referring now to Figure 1, a collection apparatus 10 is shown for collection of particulates from a grinding apparatus 12. The collection apparatus 10 comprises a vessel 14 having declivitous sides 15 descending to an outlet 16 connected to one side of a top valve 17. A tubular upright collection chamber 18 is connected at its upper end to the other side of the top valve 17, and is closable at its lower end by a lower valve 22. An ultrasonic sensor 24 is positioned just below the top valve 17 to monitor particulate level at that position in the chamber 18, and a compressed air inlet 25 into the chamber 18 is located between the sensor 24 and the top valve 17. An overflow 26 extends near the top of the vessel 14, and a recirculation circuit 28 including a pump 30 is connected between the declivitous sides 15 to a position above the grinding apparatus 12. The grinding apparatus 12 comprises parallel grinding wheels 34, 35 respectively located above a trough 36 which has a bottom outlet 38 extending so as to discharge into the vessel 14. In operation, nuclear fuel pellets 40 (eg uranium oxide) are fed between and along the grinding wheels 34, 35 to provide a required diameter of pellet 40. The recirculation circuit 28 discharges water over the grinding wheels 34, 35 and the water together with particulates 42 (eg 1 to 4 µm) drop into the trough 36 and from there through the outlet 38 into the vessel 14. With the top valve 17 open and the lower valve 22 closed, the particulates 42 fall and settle in the chamber 18, whilst the water is recirculated by the recirculation circuit 28, any water flowing through the overflow 26 being collected and subsequently returned to the vessel 14. The particulates 42 gradually collect as a sludge 44 in the chamber 18 until they reach a level where they are detected by the sensor 24. The top valve 17 is then closed, the lower valve 22 opened, and compressed air introduced for a short period through the inlet 25 to eject the sludge 44 from the bottom of the chamber 18. The lower valve 22 is then closed, and the top valve 17 opened, to repeat the collection cycle in the chamber 18. Referring now to the modification shown in Figure 2, the collection apparatus 50 shown is in many respects identical to the collection apparatus 12 of Figure 1, except that in place of the recirculation circuit 28 of Figure 1, a filter circuit 52 is used. The filter circuit 52 comprises an outlet pipe 54 from the vessel 14 connected to a pump 56 arranged to discharge to a conventional cross-flow filter 58. A clean water filtrate outlet 60 from the filter 58 is arranged to discharge between the grinding wheels 34, 35, whilst a through-flow outlet 62 from the filter 58 is returned into the vessel 14 above the outlet pipe 54. As an option, a flush-down pipe 66 (shown in broken line) may be connected between the through-flow outlet 62 and the side of the trough 36 to assist in flushing the particulates 42 from the trough 36 into the vessel 14. In order to inhibit rapid blocking of the filter 58 from the particulates 62, an initial feed of particulate U₃O₈ may be introduced at 68 to form a permeable coating 70 (see Figure 3) at the ingress side of filter element 72 of the filter 58. For example, particulate U₃O₈ having a size distribution of between 45 and 90µm at a loading of about 229g in 30 litres of water mixed with U₃O₈ having a size distribution of between 1 and 45µm at a loading of about 50g in 30 litres of water has been found suitable to provide an effective coating. An example of a suitable cross-flow filter 58 may be obtained from: Fairy Microfiltrex Limited Fareham Industrial Park Fareham Hampshire PO16 8XO United Kingdom An example of a suitable sensor 24 to sense a sludge interface may be obtained from: Bestobell Mobrey 190-196 Bath Road Slough Berkshire SL1 4DN United Kingdom It will be appreciated that the sensor may form part of a circuit for automatically operating the top valve 17, the lower valve 22, and the air inlet 25 in response to signals from the sensor. Although the invention has been described in relation to the manufacture of nuclear oxide fuels, the invention has applications with alternative nuclear fuels. It will be understood that although obturating means in the form of valve means (eg ball valve means) have been described in relation to Figures 1 and 2, alternative suitable obturating means may be used preferably to provide a relatively clear axial path for the discharge of the sludge 44 therethrough.

Claims: Apparatus for grinding nuclear fuel pellets (40) comprising parallel grinding wheels (34,35) for grinding the pellets (40), characterised by a trough (36) located below the grinding wheels (34,35) for collecting liquid and particulates (42) from the grinding wheels (34,35), an outlet (38) from the trough (36) arranged to discharge the liquid and the particulates (42) into a vessel (14) having declivitous sides (15) and a bottom outlet (16), a collection chamber (18) having an upper inlet communicable with the bottom outlet (16) and a lower outlet closable by a lower obturating means (22), sensing means (24) for sensing an accumulation of the particulates (42) at a predetermined level in the chamber (18), upper obturating means (17) between the sensing means (24) and the bottom outlet (16), and air pressure means (25) between the upper obturating means (17) and the sensing means (24), the upper obturating means (17), the lower obturating means (22), and the air pressure means (25) being responsive to the sensing means (24) such that the particulates are ejected when the predetermined level has been reached, a recirculation circuit (28,52) for liquid connected at one end to the declivitous sides (15) of the vessel (14) and having an outlet at the grinding wheels (34,35), and pump means (30,56) for circulating said liquid in the vessel (14) through the recirculation circuit (28,52). Apparatus as claimed in Claim 1, wherein the recirculation circuit (52) includes cross-flow filter means (58), the filter means (58) having a filtrate outlet (60) arranged to discharge between the grinding wheels (34,35). Apparatus as claimed in Claim 2, wherein the filter means (58) has a through-flow outlet (62) connected so as to discharge into the vessel (14). Apparatus as as claimed in Claim 2 , wherein the filter means (58) has a through-flow outlet (62) arranged to discharge into the trough (36). Apparatus as claimed in any one of Claims 2 to 4, wherein a particulate coating (70) has been applied to the ingress side of the filter means (58) to inhibit rapid blocking of the filter means (58) by the particulates (42) from the vessel (14). Apparatus as claimed in Claim 5, wherein the particulate coating (70) comprises U₃O₈. Apparatus as claimed in Claim 6, wherein the particulate coating (70) comprises particulate U₃O₈ having a size distribution between 45 and 90 microns mixed with U₃O₈ having a size distribution of between 1 and 45 microns. Apparatus as claimed in Claim 7, wherein the particulate coating (70) has been applied with the U₃O₈ having a size distribution between 45 and 90 microns at a loading of about 229 g in 30 litres water mixed with the U₃O₈ having a size distribution between 1 and 45 microns at a loading of about 50 g in 30 litres of water. Apparatus as claimed in Claim 8, wherein an inlet (68) for the water and the U₃O₈ mixture is provided between the vessel (14) and the filter means (58).

Inventors: MACALPINE PETER BRUCE, MOORE FREDERICK JOHN, SNELSON BARRY, MACALPINE, PETER BRUCE, MOORE, FREDERICK JOHN, SNELSON, BARRY
Applicants: BRITISH NUCLEAR FUELS PLC
Application year: 1991
Main classification: B01D21
IPC codes: B24B55, B01D37, G01B17, G21F9, G21C3, B01D29, B01D21