According to one aspect of the invention there is provided a method of disposing of spent nuclear fuel elements which includes the step of embedding the spent fuel elements in a matrix which comprises a mixture of particulate graphite and a binder.

According to another aspect of the invention there is provided consolidation apparatus for use in the disposal of spent fuel elements which apparatus includes, a mould defining a cavity into which spent nuclear fuel elements and a particulate matrix within which the fuel elements are to be embedded are receivable, the mould including at least one displaceable element which is displaceable to permit the size of the cavity to be adjusted and to permit the contents of the cavity to be placed under pressure; and thermostatically controlled heating means for heating the contents of the cavity.

The Inventor is aware of a nuclear reactor of the high temperature gas cooled type which makes use of spherical fuel elements referred to as pebbles. Each fuel sphere is approximately 60 mm in diameter and contains approximately 15 000 coated fuel particles. Fuel particles are generally uniformly distributed throughout an inner spherical volume of about 50 mm in diameter, surrounding which is a 5 mm unfueled layer or shell of graphite.

Typically, each fuel sphere may contain approximately 9 g of uranium, ie

each fuel particle contains about 0*6, ug of uranium. The coated particles are TRIBO particles, ie triple coated U02 partides with a UO2 kernel of 0.5 mm diameter and a density of approximately 10.5 g/cm3 ; and a fuel enrichment of about 8%. It will be appreciated that the number of fuel particles within a fuel sphere, the fuel enrichment and the amount of heavy metal may be varied and may be adjusted to achieve desired power outputs and peak fuel temperature. Each fuel particle consists of a UO2 kernel with four coatings applied thereto being, from an inner layer to an outer layer : a layer of buffer carbon, a pyrolytic carbon layer, a silicon carbide layer and a second layer of pyrocarbon.

Although retention of fission products is almost complete, small quantities of fission product can escape through the coating during the lifetime of the fuel element when retained in the graphite matrix. The escaped fission product can be leached out during the disposal times.

Although not limited thereto the Inventors believe that the invention will find application particularly in the disposal of these spherical fuel elements.

The binder with which the particulate carbon is mixed may be an inorganic binder.

In a preferred embodiment of the invention, the binder is sulphur.

The particulate graphite and sulphur may be mixed in a ratio of 4: 1 by mass.

The graphite powder may have a tap density of between 0.34 and 0.46 g/cm3. Preferably the tap density is 0.4 g/cm3.

The graphite powder may have a grain density of between 2 and 2.5 g/cm3. Preferably the grain density is 2.26 g/cm3.

The graphite powder may have a BET surface of between 1.8 and 2.2 rri/g. Preferably the graphite powder has a BET surface of 2 m2/g.

The graphite powder may have a crystal size of between 850 and 1150 A. Preferably the graphite powder has a crystal size of 1000 A.

The graphite powder may have a mean grain diameter of between 10 and 20, um.

The graphite powder may have an ash content of between 160 and 240 ppm. Preferably the ash content is 200 ppm.

When the fuel elements include a shell of graphite the method may include removing the graphite shells and using the graphite from the removed shells in the matrix.

The method may include grinding the graphite from the graphite shells.

The ground graphite may have an average grain size between 25 and 35, um. Preferably the average grain size is 30, um.

The particulate graphite may comprise a mixture of graphite powder and ground graphite from the graphite shells. The ground graphite and graphite powder may be mixed in a ratio of 3: 1.

The method may include packing at least one layer of fuel elements in a bed of the mixture of particulate graphite and binder in a mould and consolidating the contents of the mould into a solid body.

Consolidating the contents of the mould into a solid body may include heating the contents thereof.

Consolidating the contents of the mould into a solid body may include placing the contents of the mould under pressure.

In one embodiment of the invention, consolidating the contents of the mould into a solid body includes heating the contents of the mould to a temperature of 200°C and subjecting them to a pressure of 100 bar.

In another embodiment of the invention, consolidating the contents of the mould into a solid body includes heating the contents of the mould to a temperature of 150°C and subjecting them to a pressure of 120 bar.

The method may include packing a plurality of layers of the particulate graphite and binder mixture and fuel elements into the mould prior to consolidating the contents of the mould. The method may include compressing each layer prior to the introduction of a subsequent layer.

The method may include arranging the fuel elements such that the fuel elements in adjacent layers are staggered relative to one another.

The mould may be prismatic and may include a pair of oppositely disposed elements which are relatively displaceable to vary the size of the mould cavity and place the contents of the mould under pressure. In a preferred embodiment of the invention the mould is hexagonal in cross- section.

According to another aspect of the invention there is provided a spent nuclear fuel storage product which includes a plurality of spent nuclear fuel elements embedded in a consolidated matrix comprising a mixture of particulate graphite and a binder.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings: Figures 1 to 5 of the drawings show sequentially, steps involved in the disposal of nuclear fuel elements in accordance with the invention; Figure 6 shows a layout of the fuel elements in a first row thereof; Figure 7 shows the positions of the fuel elements in a second row thereof; and Figure 8 shows the second row of fuel elements superimposed on the first row of fuel elements.

In the drawings, reference numeral 10 refers generally to consolidation apparatus in accordance with the invention.

The consolidation apparatus 10, which is shown schematically in the drawings, includes a mould, generally indicated by reference numeral 12 which defines a cavity 14 which, as can best be seen in Figures 6 to 8 of the drawings, is hexagonal in cross-section. Naturally, however, other forms of the cavity 14 are envisaged.

The mould 12 includes a sleeve 16 which is hexagonal in cross- section and within which upper and lower die elements 18,20 are mounted to define the top and bottom of the cavity 14.

In the embodiment shown, the lower die element 20 is displaceable within the sleeve 16. However, it will be appreciated that, instead the upper die element 18 may be displaceable or both dye elements may be displaceable within the sleeve 16.

Naturally, the various dimensions of the consolidation apparatus 10 can be varied depending upon the intended application. However, when the apparatus 10 is intended for use with fuel spheres having a diameter of about 60 mm, the sleeve 16 will have a width W (Figure 6), measured across the flats, of about 360 mm.

In use, as illustrated in Figures 1 and 6 of the drawings, in order to dispose of spent fuel spheres 22, a first layer, generally indicated by reference numeral 24, of fuel spheres 22 and a particulate graphite/binder mixture 26, described in more detail herebelow, is introduced into the mould cavity 14, eg by removing the upper die element 18. As can best be seen in Figure 6 of the drawings, twenty one fuel spheres 22 are provided in the first layer 24. The first layer 24 is then compressed by displacing the lower die element 20 towards the upper die element 18 so that the fuel spheres are embedded in the compressed mixture 26.

The lower die element 22 is then displaced downwardly and a second layer, generally indicated by reference numeral 28 of fuel spheres 22 and the mixture 26 is introduced into the cavity 14 on top of the first layer 24.

As can best be seen in Figure 7 of the drawings, the second layer 28 includes nineteen fuel spheres 22. As can best be seen in Figure 8 of the drawings, the fuel spheres in the second layer 28 are staggered relative to the fuel spheres 22 in the first layer 24. The lower die element 20 is then displaced towards the upper die element 18 to compress the second layer 28.

This procedure is repeated with the fuel spheres 22 in a third layer, generally indicated by reference numeral 30 (Figure 3) corresponding in position to the fuel spheres in the first layer 24.

Once the desired number of layers have been formed, the lower die element 20 is displaced towards the upper die element 20 to place the contents of the cavity 14 under pressure. In addition, the contents of the cavity are heated in order to consolidate them as described in more detail herebelow.

The mixture 26 typically comprises particular graphite and an inorganic binder in the form of sulphur.

In one example of the invention, natural graphite powder with a tap density of 0.4 g/cm3 ; and grain density of 2.26 g/cm3 ; BET surface of 2m2/g, a crystal size of 1000 A, a mean grain diameter of between 10 and 20, um and an ash content of 200 ppm was mixed with sulphur of standard quality in a ratio of 4: 1 by mass. Both components were dry mixed.

The mould cavity 14 was filled with ten layers of fuel spheres and the graphite/sulphur mixture in the manner described above. Each layer typically has a thickness of about 14.5 cm.

Once the ten layers have been formed, the contents of the cavity 14 were heated to 150°C and subjected to a pressure of 120 bar. This pressure was maintained whilst the contents of the cavity 14 were allowed to cool down to 70°C. The consolidated mass or spent nuclear fuel storage product was then removed from the mould.

Subsequent examination revealed that the packing density of the spheres was 38% by volume and the density of the graphite/sulphur matrix was 1.99 g/cm3 which corresponds to 99% theoretical density.

In a second example, prior to introducing the fuel elements into the mould cavity 14, the graphite shell was removed from the spheres and ground down to an average grain size of 30, um.

The graphite which is used in the mixture 26 was in turn formed as a mixture of the ground graphite from the fuel sphere shells and fresh graphite powder as described above in Example 1 mixed in a ratio of 3: 1. The graphite mixture was mixed with sulphur in the ratio of 4: 1 as described above.

All other procedural steps correspond to those set out in Example 1.

When the consolidated mass removed from the mould was inspected, it was found that the packing density of the spheres was 40% by volume. The

matrix density was 1.98 g/cm3 ; corresponding to 98.7% of theoretical density.

The volume reduction obtained by using the graphite of the shell was 1.6.

The Inventor believes that the invention provides a method of safely disposing of spent nuclear fuel elements. Further, the Inventor believes that the consolidated mass will have a high resistance to leaching/corrosion, high mechanical integrity, and a high radiation stability.