Patent Number: 063013207
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is concerned with improvements in and relating to fuel rods and assemblies for nuclear reactors, principally of the mixed oxide fuel (MOX) type and in particular, but not exclusively, for pressurised water reactors (PWR). 2. Present State of the Art Operators of nuclear reactors throughout the world increasingly wish to run their reactors on a longer fuel cycle than that previously entertained. Eighteen or even twenty four month cycles are now preferred between reloads. Whilst such runs call for more expensive fuel to be provided in the first place, cost savings are made in terms of the overall power generation from the fuel cycle as the shut down period for re-fuelling occurs less frequently. A re-fuelling cycle can take up to six weeks during which time the reactor is off line. The procedure also involves an experienced and consequently expensive team to be involved on site. In addition, re-fuelling is normally followed by having to obtain regulatory approval before the reactor can be brought on line once more. Where longer fuel cycles are employed, to achieve the desired level of activity and hence power output from the reactor at the end of the cycle, the fuel must provide a higher reactivity level to start with. To provide even output over the cycle despite this higher initial reactivity a form of neutron poison, such as boron-10, must be provided at start up to inhibit the reactivity at this stage. The poisons involved, which may be of a variety of types, commonly act as neutron absorbers and consequently depress the reactivity whilst present. Whilst these poisons are essential during the initial part of the cycle, towards its end they must no longer be present as this would defeat the object by lowering the reactivity at the end of the cycle too. Control of reactivity has previously been addressed in a number of ways in conjunction with solely uranium fuel reactors. Control has been provided in the past by providing poison materials in the reactor at start up and during the initial part of the cycle and subsequently physically removing them as the cycle progresses. Clearly such an arrangement is mechanically and methodically complex. Other control techniques have involved the provision of discrete poison rods which are inserted into the guide thimbles of a fuel assembly. Thus the fuel material is provided in one or more grades in rods and the poison is provided in a separate set of rods. Unfortunately, the provision of such discrete rods in the right amount to provide the desired level of reactivity control at the beginning of the cycle and yet not interfere with the end of the cycle is very difficult indeed. In addition, even if the poisons burn out during the subsequent part of the cycle the overall assembly is effectively unbalanced where the poison rod used to be. This lack of balance can present itself as hot spots which are highly undesirable and can lead to stresses which deform the assembly in that region. Additionally, these "empty" spaces formerly occupied by the poison rods are not available for water passage and consequently the transfer rate is diminished. In a further prior art system UO.sub.2 fuel rods only have been provided in conjunction with lower grade UO.sub.2 which contains the poison; 6 wt % poison in 1.8 wt % U.sub.235. The poison is provided as a part of the fuel or alternatively as a coating around the circumference of the pellets. Construction of such assemblies is complex and time consuming as different levels and contents are frequently employed for different parts of the reactor fuel cell. Even longitudinally, within a given rod the enrichment grade and poison presence may vary. The prior art problems are particularly acute in MOX reactor cores as MOX itself is a bigger absorber of neutrons than UO.sub.2. As a consequence poison provision using discrete rods is even harder to control than in other fuel types as the burn out rates for poisons in such MOX reactors is lower. If combined UO.sub.2 poison rods are employed in such reactor systems a further problem occurs where the poison does eventually burn out due to the differential absorption properties of MOX and UO.sub.2. As a consequence power peaking around the formerly poison containing UO.sub.2 rod occurs at a later date. Once again, this can lead to undesirable hot spots within the core leading to undesirable stresses and potential melting of the fuel rods. Similar problems occur where discrete poisoning is provided within fuel rods or pellets, for instance as a central core. Neutron access for the poison is poor in such cases and once burnt out gives significant variations with location. Poison systems have not successfully been employed in MOX fuelled reactors to date. At present at least 3 plutonium grades are provided in the fuel rods. OBJECTS AND BRIEF SUMMARY OF THE INVENTION Amongst the aims of the present invention it attempts to provide an improved fuel assembly, fuel rods, a method of reactor control and improved fuel regime. According to a first aspect of the invention we provide a fuel rod, said fuel rod containing mixed oxide fuel and a neutron poison. The provision of the neutron poison in the fuel rod with the mixed oxide fuel has advantages in simplifying the production technique. Preferably the neutron poison is gadolinia (Gd.sub.2 O.sub.3) or substantially consists of gadolinia. The gadolinia may be present at between 0.25 and 3 weight per cent, or 0.5 and 2.5 weight per cent compared with the total fuel rod contents. The provision of between 0.75 and 1.75, and most preferably 1 and 1.5 weight per cent is particularly preferred. The reduced level of gadolinia employed, whilst effectively providing the desired degree of neutron absorbtion, does not significantly detract from the thermal conductivity of the fuel rod. Preferably the mixed oxide fuel accounts for the remainder of the fuel contents. The plutonium content of the fuel rod may be between 3% and 12% and most preferably 6% and 10%. The balance of the fuel rod contents preferably comprise depleted UO.sub.2. Preferably the neutron poison is mixed with the mixed oxide fuel, most preferably intimately. Homogenous dispersion of the poison in this way in the MOX is highly advantageous. The possibility also exists to coat the mixed oxide fuels around its circumference with the neutron poisons. The fuel rod may contain a middle segment containing mixed oxide fuel and the neutron poison with a mixed oxide only portion being provided at one or either end of the fuel rod. The portion provided with the neutron poison, and in particular gadolinia, may comprise between 40% and 95% of the fuel rod length measured from end cap to end cap. In a particularly preferred form the portion containing the gadolinia is between 75 and 85% of the length. The fuel rod may be for a pressurised water or boiling water reactor. According to a second aspect of the invention we provide a fuel assembly comprising a multiplicity of fuel rods wherein the assembly comprises a first fuel rod type provided with mixed oxide fuel and a second fuel rod type provided with mixed oxide fuel in conjunction with a neutron poison. A MOX fuel assembly provided in this way offers controlled reactivity during the initial phase of the cycle and yet offers longer cycles than previously possible. Preferably the neutron poison is gadolinia. The gadolinia is preferably present as between 0.5 and 2.5 weight per cent compared with the total fuel content of a second type rod. Preferably the first type rod contains mixed oxide fuel only. Preferably the plutonium content of such first type rods is between 3% and 12%, most preferably 6 to 10%, by weight compared with the total fuel content of the first type rod. The remaining material in the first type rod is preferably depleted UO.sub.2. Preferably the mixed oxide of the second type rod contains between 1.5% and 8%, preferably 3 to 6% plutonium. The plutonium content of the second type is preferably between 20% and 75% that of the first type, for instance 25 to 50%. The plutonium content of the second type may be up to 50% lower than the first type. Preferably the poison, plutonium, oxide and uranium oxide are intimately mixed with one another. Preferably the fuel rods of such a fuel assembly consist of first and second,type fuel rods only. Preferably only two different fuel enrichments, most preferably plutonium enrichments, are provided in the fuel assembly. Preferably the corner fuel rods, with only two fuel enrichments in the assembly, do not exceed the maximum power threshold, for instance a power of less than 1.6 and more preferably 1.55. A reduced number of fuel and/or Pu enrichments is achieved in this way with reduced fabrication costs over the prior art as downtime between enrichment change-overs is reduced, as is waste. Control rods or spaces to accommodate control rods may be provided in the fuel assembly. Fuel rods of the second type may comprise between 10% and 40% of the fuel rods in an assembly. Most preferably 20% to 30% of the fuel rods are of the second type. Between 5% and 15% of fuel rod locations in the assembly may be provided so as to accommodate control rods. The remainder of the fuel rod assembly is preferably made up of first typesfuel rods. The fuel rod assembly may comprise a multiplicity of fuel rods in an array of 6.times.6 or more fuel rods. Thus for example, 7.times.7, 9.times.9, 14.times.14 and 17.times.17 arrays may be provided. The fuel assembly may be provided for a pressurised water reactor. The second type fuel rods may be provided at, or adjacent, the periphery of the fuel assembly. Most preferably the second type fuel rods are provided as the edge rods in a given fuel assembly. The provision of the second type rods at the periphery and/or immediately adjacent to the peripheral fuel rods is also envisaged. Preferably between 60% and 90% of the second type fuel rods are provided as edge fuel rods in the assembly. Between 50 and 10%, and more preferably between 65 and 100% of the rods at the periphery may be second type rods. Between 10% and 20% of the second type fuel rods may be provided as non-peripheral fuel rods. Most preferably all or substantially all of these second type non-peripheral fuel rods are provided adjacent to peripheral fuel rods. Between 0 and 50% of the rods adjacent to the periphery may be second type rods. More preferably between 5 and 35% are so provided. Between 0 and 15%, and more preferably between 3 and 10% of the non-peripheral fuel rods may be of the second type. Alternatively the fuel assembly may be provided for a boiling water reactor. For boiling water reactors, in particular, different second type fuel rod configurations may be provided within the array. The second type fuel rods may be provided as intermediate rods. Preferably first type rods are provided as the, or substantially all, of the peripheral rods. Preferably first type rods are provided as the, or substantially all, of the rods around the water passage. In this way the full effect of the control blade is maintained. According to a third aspect of the invention we provide a nuclear reactor core incorporating one or more fuel assemblies according to the second aspect of the invention and/or one or more fuel rods according to the first aspect of the invention. Preferably the peak rod power to average assembly power ratio for such a reactor core is between 1.65:1 and 1.4:1. A ratio of less than 1.60:1, more preferably 1.55:1 and most preferably 1.5:1 and ideally less than 1.45:1 is desired. Preferably the aforementioned peak to average power ratios are maintained throughout the fuel cycle, or at least following the first 150 hours. Preferably the average power of a given assembly increases relative to the initial value. The peak power value of the same assembly may increase relative to the initial value. Preferably the peak to average power ratio reduces during the cycle, or at least during the first 5000 hours. Preferably the desired peak to average ratios exist between 3000 and 10000 hours of the fuel cycle.