Patent Number: 063013207
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The fuel rod illustrated (1) in FIG. 1 in cross section comprises a casing (2), commonly of a neutron transparent material such as zirconium alloy with removable end plugs (4, 6). The fuel rod is commonly cylindrical and fuel or other material pellets (8) of a snug fit are provided in its interior. The pellets, which may be of fuel or other materials are slid into the fuel rod from cans during assembly. A typical fuel rod assembly, in this case a pressurised water assembly, as illustrated in FIG. 2, consists of an array of such fuel rods (1) assembled in a lattice. Suitable bracing frameworks (10) are provided to securely mount the individual fuel rods within the assembly. The spaces between the fuel rods provide for the flow of water through the reactor in use. The water is essential in transferring the heat generated within the reactor to external locations where power can be extracted and converted into electricity. The present invention in a specific embodiment makes use of two particular types of fuel rod. Firstly there is the MOX fuel type and secondly a MOX fuel doped with gadolinia type. The first type rod contains fuel pellets made up of plutonium and depleted uranium. The composition of such fuel rods is 9% plutonium with the balance being made up of UO.sub.2. The pellets in such a fuel rod are consistent in their plutonium levels. The second type of fuel rod provided in the invention consists of mixed oxide fuel made up of plutonium mixed with depleted uranium in conjunction with a neutron poison. A slightly lower level of plutonium is employed in this second type of rod, 6% plutonium being used. The provision of 1.5% gadolinia in these doped fuel rods, with the balance being made up depleted uranium, provides significant advantages as discussed below. The plutonium, gadolinia and depleted uranium in this second type fuel rod are intimately mixed during manufacture and formed into pellets in the conventional way. The production of gadolinia alone, suitable for use in the present invention, is known in the art. The provision of the poison in this way and at the levels provided ensures that the reactivity throughout an 18 to 24 month cycle is provided at the desired level. The correct level of poisoning is provided at the commencement of the cycle, whilst burn up of the poison at the desired rate so as not to interfere with reactivity at the later part of the cycle is enabled. Additionally, by providing the poison in the fuel rods, rather than as discrete units, "holes" in the fuel rod assembly are avoided. Maximum transfer rates for the heat and minimal power peaking is thus provided. Furthermore, as only two levels of plutonium enrichment are provided fabrication costs are reduced compared with previous assemblies where a variety of enrichments and fuel rod configurations had to be provided The provision of gadolinia at the level set out in this invention is also advantageous as gadolinia is a poor thermal conductor relative to the fuel itself. Thus at the levels of poisoning required by the prior art thermal conductivity was severely hampered. In the present invention on the other hand the level of gadolinia is sufficiently low as to make negligible impact on the thermal conductivity of the rod. The intimate mixing also assists in this regard in avoiding localised poor conducting volumes. Beyond the provision of gadolinia in the MOX rod itself; the invention offers further advantages in terms of the configurations to which such fuel rods can be used. FIG. 3 illustrates one embodiment of a fuel assembly distribution, suitable for a PWR for instance, in which the fuel rod assembly is shown in a schematic plan. The assembly shown is of a conventional size, namely 17 fuel rods by 17 fuel rods. In reality the fuel rods are cylindrical and as a consequence significant gaps exist between fuel rods allowing the passage of water. As illustrated in FIG. 3 the squares provided with a circle (50) are available to allow incorporation of control rods into the fuel assembly. Conventional control rods such as those formed from Silver-Indium-Cadmium (Ag--In--Cd) can be used to this end. The parts of the lattice left blank (52) correspond to fuel rods of the first type provided with MOX fuel. Around the periphery of the array the crossed elements (54) correspond to the poisoned MOX fuel rods, the second type. As can be seen the majority of the poisoned MOX fuel rods are either provided at the periphery of the assembly or as rods adjoining peripheral rods. This assembly style has significant advantages, particularly with regard to PWR reactors. Firstly the distancing of the doped rods from the central part of the fuel assembly allows for control rods to be introduced if desired, and yet still have a substantial affect on the assembly. If the poisoned rods are provided in proximity to the control rods then the neutron absorbing effect of the control rods will be significantly diminished. Full control over the fuel assembly is thus provided by this configuration. The provision of the poisoned rods at the periphery also avoids the need for the fuel assembly to contain a significant number of different grades of fuel rod. Conventional systems employ the highest grade at the core of a fuel assembly with medium enriched rods around this and towards the perimeter and with a lower grade still provided at the corner locations. This configuration has previously been used in order to avoid net neutron currents between assemblies. With the present invention this is not necessary as full control over the assembly is provided by the two types of fuel rod. FIG. 4 of the invention illustrates an alternative embodiment for the fuel rod configuration. In this configuration the majority of peripheral rods are of the second type (58) but the invention's benefits still accrue with some peripheral first type rods (52) also. The assemblies need not be symmetrical. FIG. 5 illustrates a fuel rod configuration for a fuel assembly more suited to a BWR. In this structure the areas designated (56) correspond to the water conduits. As the control blades in BWR's are employed in a different manner to PWR's the doped MOX configuration is somewhat different. The same symbols are applied to this array as used in FIGS. 3 and 4. By placing the second type rods (54) inside the assembly the full effect of the external control blades is maintained. FIG. 6 illustrates a comparison between the average power and peak power fuel assembly within a reactor using entirely MOX fuel without Gadolinia doping and also for MOX fuel together with Gadolinia doped MOX according to the present invention. Within reactors certain assemblies, and even certain rods, can have a higher power level due to the conditions that they are under. Any given reactor has a maximum level at which any one assembly can run and as a consequence a safety margin must be maintained between this and the peak power level in practice. By its very nature the average power is some way below the peak power. The closer the average power can be run to the maximum the greater the energy extracted from the reactor. Breeching of the maximum power level, however, can cause thermal distortion and mechanical damage within the assembly causing significant delays and costs during any re-loading scheme. Other problems also follow from exceeding the power limit. FIG. 6 represents the power limit line (110) as being provided at 1.6 power against time in hours. When the average power line for unpoisoned MOX (100) and the peak power for unpoisoned MOX (102) are compared against the average power for the doped MOX according to the present invention (104) and the peak power for the doped MOX according to the present invention (106) significant differences are clear. Whilst in the latter stages of the cycle the peak and average power levels for the undoped MOX are comparable with, although lower, than the doped MOX, during the initial stages of the fuel cycle both the average and peak power are significantly higher. Indeed for the given average power at the end of the cycle the peak power (102) at the beginning exceeds the threshold (110). Thus if the essential compensation is made to keep the peak level (102) below the threshold (110) during the early part of the cycle by the end of the cycle the average power is consequently ever lower than with the doped MOX. In the early part of the cycle, on the contrary, the doped MOX peak and average power level are always well below the threshold line (110) and indeed a still higher grade of plutonium, with increased reactivity, could be used whilst still keeping the level well below the threshold value (110). A far greater average power over the entire fuel cycle is thus possible using the present invention as compared with undoped MOX. The longer the fuel cycle in question the more significant this difference becomes. FIG. 7 illustrates a similar comparison between the doped MOX peak level (106), doped MOX average power level (104) and the threshold value (110) as against UO.sub.2 doped with Gadolinia average power level (112) and peak power level (114). In this comparison whilst the average power from the doped UO.sub.2 line (112) is comparable with, although lower, than the average power with the MOX doped according to the present invention (104) very significant differences occur in the peak value. As can be seen the peak line (106) for the MOX doped according to the present invention remains relatively low and well below the threshold line (110). On the contrary the peak line (114) for the UO.sub.2 doped with Gadolinia relatively quickly exceeds the threshold value (110). This fuel regime gives rise to a number of rods which are at a significantly higher power output than the average. As a consequence, to maintain the peak rods below the threshold value the average power output of the peak assemblies must be significantly reduced. Once again, therefore, the present invention offers significant advantages in terms of the average power output of the peak assemblies which can be maintained over the entire fuel cycle. The manner in which the neutron poison is provided in the fuel rod is also significant to its effectiveness. FIG. 8 illustrates K-Infinity values for fuel assemblies incorporating a variety of fuel designs. The values are plotted against burn up cycle time in months. Line 200 illustrates the variation and the time for an undoped MOX assembly and clearly shows the tailing off as the fuel is consumed. Line 202 illustrates a fuel assembly according to a preferred form of the invention with both MOX rods and around its periphery MOX rods with Gadolinia poisoning being provided. The Gadolinia is provided at 1 weight per cent in these rods. The neutron poison is provided intimately mixed with the MOX fuel. As is desired the performance clearly indicates the depression of the reactivity during the initial part of the burn up cycle. This allows higher levels of plutonium enrichment to be provided as the increased reactivity arising therefrom is inhibited during this initial peak part of the cycle. By 18 months, dashed vertical line, the reactivity has risen to reach its peak value. This reflects the efficient burn up of the neutron poison during this first cycle period. Neutron poisoning is not needed, and indeed is highly detrimental, beyond the first cycle as the overall reactivity is now decreased. Beyond the first cycle the majority of Gadolinia has burnt out so the reactivity of the assembly behaves in a similar manner to an unpoisoned assembly 200. Alternative ways of providing the neutron poison in MOX fuel assemblies function less successfully. Lines 204, 206, 208 respectively detail the variation for a MOX fuel rod plus a poison containing MOX fuel rod incorporating assembly. In this case, however, the poison rods are formed from a discreet Gadolinia core around which annular pellets of MOX are provided. The lines indicate a 1 mm Gadolinia core at 100% (204); 50% (206); and 25% (208) loading. As can be seen the provision of neutron poisoning has the effect of reducing initial reactivity but to a far lesser extent than for line 202. Because the reduction is far less this would mean that the overall reactivity levels have to be kept lower to avoid those levels exceeding the threshold limit during the early part of the cycle. This has the effect of reducing reactivity during the latter stages of the cycle still further and as a result reduced power output arises. The presence of neutron poisoning also prevents the conventional technique of moving the assemblies inward within a core at the end of the first cycle. The negative effects would be even greater if still partially poison rods are present in a fuel assembly move towards the centre of a reactor core. The lower amount of depression is seemingly due to the reduced access of the Gadolinia core to the thermal neutrons. Less interaction therefore occurs. This reduction is both due to shielding by the neutron absorbing MOX and also due to the lower chance of neutron incidents, where the Gadolinia is presented in restricted locations. Not only does poisoning in this manner not work as efficiently and successfully in the early part of the fuel cycle, but it is also problematical later on. Whilst 202 begins to pick up significantly during the latter part of the first 18 month fuel cycle the assemblies poisoned by a core remain depressed. This effect continues well into the second cycle. This means that the activity depression due to the poisoning occurs long after it is needed so decreasing reactor performance. This problem again stems from the Gadolinia positioning as poor access leads to a low burner rate and hence long term presence of neutron poisons. Intimate mixing of the Gadolinia, therefore, clearly gives further benefits over and above those achieved by providing poisoning in the MOX rods themselves. Reactivity depression only when desired is provided in this way. The physical performance characteristics (both structural and heat transfer) are also far better for intimately provided poisons than for discreetly provided poisons. This is especially so as the poison becomes burnt up.