Patent Number: 051456354
Section: summary

FIELD OF THE INVENTION This invention relates to nuclear fuel assemblies and nuclear reactors. In particular, it is concerned with light water cooled, light water moderated reactors, having a high conversion ratio of fertile to fissile substances with uranium-plutonium mixed fuel. BACKGROUND OF THE INVENTION In a nuclear reactor, fissile substances such as uranium-235 are consumed by a fission reaction, new fissile substances such as plutonium-239 being yielded as uranium-238 undergoes neutron absorption. The conversion ratio of the reactor is the ratio, at the time of unloading a spent fuel assembly, of the amount of fissile substance yielded to the amount of fissile substance consumed. In a conventional light water cooled and moderated reactor the conversion ratio is about 0.5. With the general aim of conserving uranium resources, recently there has been interest in increasing the conversion ratio of reactors. In particular, recently published JP-A-1/227993 discloses a fuel assembly construction designed to achieve a conversion ratio approaching unity. The fuel assembly comprises an array of fuel rods arranged in a particularly dense configuration so that the effective volume ratio of water to fuel, as an average over the assembly, is not more than 0.4. The reactor is a boiling water reactor. So, this densely-packed fuel assembly construction will provide for recovery of nearly as much plutonium-239 etc. as the amount of fissile substance (uranium-235, plutonium-239) consumed. The recovered plutonium can be used to enrich uranium from any suitable source, and this can be burned in a nuclear reactor. In a reactor, however, there are many factors other than conversion ratio which are important. In particular, reactors must be safe not only during normal operation but also should some abnormal transient condition arise. In a conventional boiling water reactor, the effective volume ratio of water to fuel is usually about 2.0; much higher than in JP-A-1/227993. With a soft neutron spectrum, the void coefficient of uranium-plutonium mixed fuel (void coefficient=change of reactivity with change of void fraction of coolant) is much less (more negative) than the corresponding void coefficient for an enriched uranium fuel. When the fuel rod configuration is made more dense to raise the conversion ratio, as disclosed in JP-A-1/227993, we observe that the void coefficient of uranium-plutonium mixed fuel tends to increase and approach positive values. Indeed, the prior art core having effective water to fuel volume ratio of 0.4 has a positive void coefficient. This has important implications as regards safety. The safety of a reactor in the event of some abnormal transient or accident can be assessed with reference to a power coefficient. The power coefficient is the rate of reactivity change with unit power change, and is expressed as a sum of the void coefficient and a Doppler coefficient which is a component indicating reactivity change with temperature. In fact, the particular construction described in JP-A-1/227993 does have a negative power coefficient and hence is safe in principle, because the Doppler coefficient is sufficiently negative to compensate for the positive value of the void coefficient. For increased control of safety, however, it would be desirable not to have to rely on the Doppler coefficient, but to be able to reduce (i.e. make less positive or more negative) the void coefficient contribution to the power coefficient. It is known that void coefficient of a reactor core can be kept down by constructing the core so that electrons can leak easily, since void coefficient in a core depends on a sum of the changes of neutron infinite multiplication factor and neutron leakage value. Leakage of neutrons can suppress neutron infinite multiplication factor since although increased void fraction increases the number of neutrons in the core, it also increases the amount of leakage. However, a core which allows neutrons to leak easily has serious disadvantages, namely a lowering of reactivity with the leakage of neutrons at steady state. SUMMARY OF THE INVENTION In view of the above, it is a primary object herein to enable a high conversion water reactor using uranium-plutonium mixed oxide fuel to have a power coefficient commensurate with safety, and preferably improved safety, without a disadvantageous loss in reactivity. This is a new object which we have perceived in relation to these new, high conversion reactors. To achieve this object would contribute to the practical usefulness of a high conversion reactor. According to the invention, surprisingly we can reduce the void coefficient by using a fuel assembly comprising a plurality of uranium-plutonium mixed oxide fuel rods extending axially between a water coolant intake end (upstream) and a water coolant discharge end (downstream), the rods being densely packed to give a high conversion ratio, and in which the plutonium enrichment is axially non-uniform in an effective fuel region of the assembly, such that an upstream half of the effective fuel region has a higher average plutonium enrichment than the corresponding enrichment in the downstream half. In other aspects, the invention provides a reactor core consisting of a plurality of such fuel assemblies, a nuclear reactor loaded with the assemblies, and a method of operating such a reactor at a high conversion ratio. It should be noted that the invention applies only to high conversion reactors, in particular to those which operate with an effective water to fuel volume ratio which is usually less than 1.5 and most preferably 0.4 or less. Operating such a reactor is desirably so as to achieve a conversion ratio of at least 0.6, more preferably greater than 0.8 and most preferably about 1.0. The difference in average plutonium enrichment between the upstream and downstream halves of the effective fuel region should be at least 0.05% but usually less than 1.5%; a larger void coefficient reduction may be obtained in the range of 0.1 to 1.1% enrichment difference. The overall average plutonium enrichment in the effective fuel region is not usually more than about 10% by weight. The fuel region should preferably contain uranium-plutonium mixed oxide fuel for substantially its entire axial extent so that a good output is achieved. The effect achieved by the invention is a surprising one. In the prior art, a fuel assembly having a non-uniformity of plutonium enrichment has already been disclosed in JP-A-60/66187. This is a conventional, low-conversion reactor in which the fuel rods of the assembly are not densely packed. These fuel rods have a downstream portion of uranium fuel, and an upstream portion--which may be as much as two-thirds of the fuel region--of plutonium-uranium mixed oxide fuel. Consequently, the average enrichment of plutonium is greater upstream. However, in JP-A-60/66187 the higher upstream plutonium enrichment is disclosed as increasing the void coefficient of the reactor which, though perfectly safe, has a void coefficient so low as to be inadequate. The effective volume ratio of water to fuel in this prior art document is the conventional value; about 2.0. The present inventors have discovered that in a high conversion reactor, where the fuel rods are relatively densely packed, quite the opposite effect can be achieved. That is, by having a higher plutonium enrichment at the upstream half a void coefficient which otherwise might be dangerously high can be reduced. This is based on the appreciation that, when fuel rods are densely packed and the reactor is operated at a low effective water:fuel volume ratio e.g. less than 0.4, the fuel gives a positive void coefficient all over the range of voids fraction; fuel of lower plutonium enrichment gives a smaller void coefficient than a highly enriched fuel at the same degree of burn-up and, for a given plutonium enrichment, a larger void coefficient is achieved for a fuel operating at a higher burning average void fraction. By reducing the relative enrichment in the upper region where the void fraction is high, the contribution of this upper region to void coefficient can be reduced. Relatively high enrichment in the lower region, however, contributes less seriously to void coefficient while maintaining good reactivity. These effects will be described below in more detail. The fuel assembly may have portions of relatively low enrichment at both ends of the effective fuel region. These can increase the overall reactivity by limiting the large neutron infinite multiplication factor towards the central part of the effective fuel region, where the contribution to reactivity is large. Preferred aspects of the invention provide particular distribution patterns for regions of uniform enrichment in the axial dimension, which provide variously for different effects on the void coefficient, ease of construction, good reactivity etc. according to choice. Fuel assemblies embodying the invention are constructed so as to operate at high conversion ratios. To achieve the necessary low effective water:fuel volume ratio in operation, the geometrical coolant space:fuel volume ratio will also generally be substantially smaller than conventional. For example, it may be less than 1.5, more particularly less than 1.0.