Patent Number: 042241065
Section: description

FIG. 1 shows the frame 4 in which the grid 6 is fitted. Said frame 4 comprises two side plates 4a and 4b as well as two end plates 4c and 4d. In the example herein described, the grid 6 is constituted by two sets of thin wires 6a and 6b of Zircaloy which intersect at right angles and are joined together by electric welding at points such as those designated by the reference 7 so as to form a lattice in which the ceramic fuel wafers 8 of the fuel element are subsequently intended to be fitted. In the example shown in FIG. 1, the meshes of the lattice have a rectangular shape but could just as readily have a square shape for certain different applications. The different wires 6a and 6b of the grid 6 have a diameter which is substantially equal to the thickness of the fuel wafers 8. Said wires are subjected first to electric welding under pressure in order to reduce overthicknesses at each point of intersection 7, then to a trimming operation in order to ensure perfect calibration of the lattice openings. As a general rule, the different wafers of sintered fuel have a thickness within the range of 1 to 1.5 mm. The dimensions of each rectangular mesh are chosen so as to optimize ease of shaping at the moment of compression of each fuel wafer. By way of indication, good dimensions are of the order of 30.times.18 mm; as mentioned earlier, however, a square shape (17.times.17 mm, for example) appears to represent a particularly advantageous solution on technological grounds. In accordance with the invention, the assembly formed by the grid 6 and the fuel wafers 8 is finally placed between a top cladding plate 9 and a bottom cladding plate 10. In the alternative embodiment of FIG. 2, the grid 6 is provided with a framing wire 11 which surrounds said grid on all four sides and has the same composition and diameter as the wires 6a and 6b of the grid itself. Said framing wire 11 is welded to the wires 6a and 6b as well as to the side plates 4a and 4b and to the end plates 4c and 4d. One of the most attractive industrial applications of the thin fuel element in accordance with the invention lies in the possibility of converting the core of an existing pool reactor by fitting it with fuel element plates of low enrichment. In fact, in order to establish a neutron balance in the core of a reactor of this type, there are four essential factors to be taken into consideration and these are respectively as follows: a moderating ratio equal to the ratio of the volume of moderator (water) to the volume of fuel; a fuel enrichment which can attain 93% in U-235 in pool-type reactors of conventional design ; the mass of U-235 within the reactor core; the available reactivity which represents the capacity of the reactor for irradiation of materials. The ratios between the four parameters given above, two of which are related to each other (mass of uranium and enrichment) are illustrated by the set of curves of FIG. 3 which shows the variation of reactivity in respect of a certain number of enrichments respectively equal to 1.5%, 2.5%, 3%, 4%, 5%, 7.5% and 10%, as a function of the moderating ratio which is plotted as abscissae. The general shape of these curves (increasing function which passes through a maximum and then decreases) indicates that the moderating ratio must be placed in the vicinity of a value at least equal to 2 in order to derive maximum benefit from the core reactivity. In the case of the fuel elements commonly employed up to the present time in pool reactors (platetype elements of aluminum-uranium alloy enriched with 93% U-235), the moderating ratio usually adopted is in the vicinity of 2 as mentioned above. If plate fuel elements are replaced in a reactor of this type by plate elements of the fuel wafer type of relatively substantial thickness (4 to 5 mm, for example), it is possible in such a case: either to retain substantially the same number of plate elements within the internal space provided for the reactor core, thus resulting in a reduction of the moderating ratio and entailing the need for much higher enrichment in order to maintain the same reactivity, or to reduce the number of plate elements in order to retain the same moderating ratio, in which case the power level of the reactor core is also reduced. On the contrary, by making use of fuel elements in the form of thin plates which is made possible by the present invention, the number of plates can be increased while retaining a moderating ratio which is very close to 2 and consequently permitting a relatively low fuel enrichment in order to maintain a substantially identical power level in the pool-type reactor core. Referring again to FIG. 3, it is apparent, for example, that a reactor which is made up of plates of substantial thickness, which permits a moderating ratio of the order of 1, which is charged at an enrichment of 10% U-235 and discharged at 5% residual U-235 (path BC), operates with the same reserve of reactivity as a core which permits a moderating ratio of 2, which is charged at 4.5% U-235 and discharged at 3% U-235 (path AD).