Patent Number: 044774117
Section: description

Conveniently the helium gas pressure alternates between a relatively high value and a relatively low value so that in a square lattice arrangement of rods, for example, a rod with a helium gas pressure at one of the values has as its closest neighbours up to four rods with a helium gas pressure at the other of the values. Such an arrangement of a 4.times.4 array of fuel rods is shown diagrammatically in the accompanying FIG. 1. The rods 1 are filled with helium at a relatively high pressure and the rods 2 which alternate with the rods 1 are filled with helium at a relatively low pressure. By arranging for the differing helium gas pressures in a fuel element, in accordance with the invention, the result of a transient at a temperature below that at which the high pressure rods would be unstable (and therefore liable to rupture) can be swelling of the high pressure rods without interference from their low pressure neighbours (which should neither swell significantly nor rupture). On the other hand in a transient at higher temperatures the high pressure rods would be expected to rupture during the rise in temperature following loss of coolant so that they would not provide any restraint to significant deformation of their low pressure neighbours if the temperature subsequently rose high enough for this to occur. Blockage of coolant channels should therefore not be significant because, owing to the lack of restraint in either case, the blockage will not reach the maximum otherwise attainable. The actual pressures selected will, of course, depend on the design of the fuel element and, in particular, on the ductility of the fuel rod sheath material which, with the pressure, will be a determining factor in the temperature and dilation at which the high pressure rods fail. It will be appreciated that, as well as taking into account the factors which are significant to the present invention, the optimising of a fuel element design has to consider the effect of such matters as the cost of materials, their absorption of neutrons and the physics of reactor control e.g., the spacing of the fuel rods. Thus there are practical limits to the specification of the sheath which is currently of a zirconium/niobium alloy containing 1/2-21/2% niobium or one of the zirconium/tin alloys known as Zircaloy 2 and Zircaloy 4. With these alloys and a sheath thickness such that the sheath has the capacity to strain to touch the sheaths of neighbouring fuel rods--to obtain maximum advantage from the invention--a typical high helium pressure on filling is 2-3 MPa, leading to failure of high pressure rods above about 750.degree. C. A corresponding typical low helium pressure on filling is 0.5-1.5 MPa but the low pressure value may be as high as 1.75 MPa. Variations of this order in internal pressure do not unduly affect the performance of a fuel element during normal reactor operation. The invention may therefore be introduced in existing fuel element designs without difficulty. In general the wider apart the high and low pressure values are the greater the benefit for the purpose of the invention but the values to be chosen for a particular fuel rating are a matter of technical and economic judgement in each case; they have to be commensurate with both safe and economic operation of the reactor in which the fuel is to be used. They may be determined in conjunction with computer models of fuel performance in normal situations. Thus the high pressure value should not be greater than the reactor coolant pressure and there should be a safety margin allowed for this. Allowance should also be made for increase in pressure with operating temperature and for fission product gas release, for example, and the eventual high pressure value will be calculated accordingly. Similarly in calculating the low pressure value consideration has to be given to the effect of a lower pressure on such parameters as creep of the sheath material on to the fuel and fission product gas release, both of which will be modified further by the rise in fuel temperature which is another consequence of lowering the helium gas pressure. Thus, from experiments performed on fuel rods of a particular design at a range of initial internal filling pressures to predict the accident conditions under which the fuel rods will be dilated to the point of rupture a curve such as curve A in FIG. 2 may be constructed. A second curve B may be constructed from calculations of the lower limits to the conditions under which the rods would strain to produce axially extending deformations (swelling) leading eventually to rupture. The high and low level pressures are then so selected that whatever the temperature the high and low pressure rods would not both be swelling, irrespective of the changes in pressure occurring during the lifetime of the rods in a reactor, these changes being calculated using a computer modelling code. A typical fuel element to which the invention may be applied is shown diagrammatically in FIG. 3 and an example of the fuel rods 1, 2 is shown in FIG. 4. In FIG. 3 there is a cluster of fuel rods 1, 2 arranged in a square array between a base plate 3 and a top member 4. The fuel rods are supported intermediately by grids 5 fitted with springs (not shown). In each individual fuel rod 1, 2 (as shown in FIG. 4) there is a column of uranium dioxide fuel pellets 6. The pellets are contained in a sheath 7 closed by end plugs 8, 9. At each end of the column of pellets 6 is an insulating pellet 10, 11. A spring 12 extends through the gas plenum 13 between the upper insulating pellet 10 and the top end plug 8.