Patent Number: 062721976
Section: description

Referring now to the drawings and where FIG. 1 shows a perspective view of a known fuel pin assembly 10, also called a fuel "element", for a PWR nuclear reactor core (not shown) and FIG. 2 shows a graph of average oxide layer thickness on fuel pin cladding. The assembly 10 comprises a plurality of fuel pins 12 arranged in a square array; top 14 and bottom 16 nozzles for locating the assembly in the reactor core; and, structural spacer grids 18 for aligning the pins 12 parallel to each other and to the assembly axis. The assembly shown in FIG. 1 is about 4 m in length, intervening parts 20 of the fuel pin length having been removed in the view shown. The structural spacer grids 18 have a plurality of apertures, each one accepting a fuel pin 12 or a thimble tube 22 (see FIGS. 3, 4 and 5) as appropriate and have resilient spring fingers (not able to be seen) in the apertures to grip the fuel pin to prevent any relative movement and consequent fretting damage: In operation, coolant water is pumped upwardly from the bottom nozzle 16 through the assembly 10 to exit via the top nozzle 14. Turbulence inducing vanes (not able to be seen) are provided on the grids 18 to promote mixing of the coolant and hence improve cooling of the fuel pins 12. The graph of FIG. 2 does not relate to the specific fuel assembly 10 of FIG. 1 which is merely exemplary of fuel pin assemblies. FIG. 2 shows a curve 30 showing average oxide layer thickness on the cladding against axial position on the fuel pin 12. In the curve 30, maxima 32 and minima 34 (only one of each indicated by arrows for the sake of clarity) are shown at various positions along the fuel pin length. The fuel pin assembly corresponding to the curve 30 has structural spacer grids located at positions which lie between a maxima and the preceding minima, e.g. at position 36 and other corresponding positions. The greatest average oxide layer thickness occurs at the maxima 38 and represents an oxide thickness which is effectively life-limiting for the fuel pin. The ability to reduce the oxide thickness at this position, and also possibly at the preceding maxima 40, would enable the full burn-up potential of the fuel per se to be utilised or would allow the fuel assembly to operate at a higher power level or a combination of these advantages. In the fuel pin assembly according to the present invention there is provided an additional grid 50 as shown in FIGS. 3 and 4, of which FIG. 3 shows a perspective view of part of the area of a short axial portion of a fuel pin assembly and FIG. 4 a plan view thereof. A plurality of fuel pins 12 are again shown, the fuel pin comprising an outer tubular sheath or cladding 22 which is filled with fuel material (not shown). Interspersed amongst the fuel pins 12 are thimble tubes 52 which receive control rods (not shown) to control the power output of the reactor. The grid 50 is stamped from sheet material such as Zircaloy (trade mark) and comprises a continuous framework 56 surrounding apertures 58 through which both the fuel pins 12 and thimble tubes 52 extend. However, the fuel pins 12 do not touch the framework 56 whereas the thimble tubes 52 are swaged outwardly at the location 60 where they pass through the apertures 58 so as to fixedly grip the framework 56 and thus locate the grid 50 in a desired position and orientation relative to the fuel pins 12. During the stamping operation, turbulence inducing means 61 in the form of vanes are integrally formed with the framework 56, the vanes being deflected away from the plane of the original sheet material during the forming operation to a desired configuration so as to optimise turbulence in the coolant water, indicated only schematically by the arrows 62, flowing through the fuel pin assembly 10. The grid 50 extends only up to the outer ring 64 of fuel pins since the outer ring of pins run at a lower temperature and have a consequently thinner oxide thickness layer. The grid 50 is positioned intermediate adjacent spacer grids 18 in the vicinity of the maxima 38 (and also possibly in the vicinity of maxima 40). The increased turbulence and improved coolant mixing caused by the grid 50 lowers the temperature of the fuel pins at this point and consequently also reduces the oxide thickness. The low volume or mass of the mixing grid 50 does not significantly increase the parasitic loss due to neutron capture. FIG. 5 shows a slightly modified grid 50 having turbulence inducing vanes 70 of different form. Other features remain essentially the same as in FIGS. 2 and 3. FIGS. 6 and 7 show a small part of the framework 56 of a grid 50. In this modification, some or all of the individual framework members 80 are twisted out of the plane of the original sheet from which the grid is stamped or pressed to an angle so as to promote turbulence in the coolant thereby. Vanes as described above with reference to FIGS. 3, 4 and 5 may or may not be employed depending upon the specific geometry and requirements of the fuel assembly 10 in question.