Patent Number: 051456354
Section: description

DETAILED OF THE PREFERRED EMBODIMENTS Referring firstly to FIG. 1, a fuel assembly 1 for a nuclear reactor core, comprises 151 fuel rods 3 arranged axially vertically in a channel box 2 having a hexagonal outer shape. The rods 3 are packed extending side-by-side in the channel box 2 in a close-packed hexagonal lattice. They are mounted by conventional mounting means. Eighteen lattice sites are occupied by control rod guide tubes 4, again in a conventional manner. The outer diameter of each fuel rod is 11.8 mm and the fuel rods are spaced apart by 1.3 mm, giving a geometrical water:fuel volume ratio of 0.5. The reactor is a boiling water reactor, and the aim is to achieve an effective water:fuel volume ratio preferably not more than 0.4, with a view to achieving a conversion ratio approaching unity. With the configuration described, a void fraction of 20% will give an effective water:fuel volume ratio of about 0.4, while this latter ratio falls to about 0.24 when the void fraction rises to 55%. The part of the fuel assembly charged with uranium-plutonium mixed fuel, or enriched uranium fuel, is referred to hereinafter as the "effective fuel region". FIG. 2 shows schematically the axial extent of this region for a first embodiment. The effective fuel region is divided into a top half and a lower half. The uranium-plutonium mixed oxide fuel 15 making up the lower half has an enrichment of 7.0% (enrichment is given here and hereinafter as weight %) of fissile plutonium. The fuel 11 making up the upper half of the effective fuel region has a fissile plutonium enrichment of 6.0%. Thus, the average enrichment of the fuel assembly is 6.5% fissile plutonium, but the upper half has a lower average enrichment than the lower half. In this embodiment, the conceptual boundary between upper and lower halves in fact coincides with an actual boundary between two regions of different plutonium enrichment. That is, the average enrichment throughout the upper half region is 6.0% and throughout the lower half region is 7.0%. In this embodiment, natural uranium enriched with plutonium is used. However, it will be appreciated that recovered uranium from spent fuel reprocessing, depleted uranium from enriching operations, slightly enriched uranium, or a mixture of any of these might be used for enrichment with plutonium. Furthermore, oxide sintered bodies of natural or depleted uranium may be placed at the end peripheries of the fuel region ends to decrease neutron leakage from the reactor core and to provide neutron shielding, in a known manner. FIG. 3(a) shows a nuclear reactor core made up from six hundred and one of the fuel assemblies 1. In the reactor core 5, the fuel assemblies are positioned axially vertically and coolant flows through them axially upwardly from the bottom (inlet) end to the top (outlet) end. FIG. 3(b) shows how, in a nuclear reactor, the core 5 is arranged horizontally, with the assemblies 1 axially vertical, between coolant supply 25 and coolant take-off 26 of the reactor (shown schematically). In an embodiment, the reactor core had the following specification. TABLE 1 ______________________________________ Thermal output 2700 MW Electrical output 900 MW Number of fuel assemblies 601 Height of reactor core 2.00 m Coolant flow rate 2.25 .times. 10.sup.4 t/h Core outlet quality 27% Specific power 17.5 kW/kg Power density 85.2 kW/l ______________________________________ Under reactor conditions of low water:fuel volume ratio e.g. 0.24, maintained with a view to achieving high conversion ratio, a relationship between plutonium enrichment, burn-up, and void coefficient is as shown in FIG. 4. FIG. 4 shows how a fuel having a lower enrichment of plutonium gives a smaller void coefficient than a highly enriched fuel at the same degree of burn-up. The difference becomes smaller as burning of fuel proceeds, but in a normal burn-up range for a light water reactor fuel, i.e. about 45 GWd/t, there is always a difference as shown and the lines do not cross. FIG. 5 shows a relation between burning average void fraction (an average void fraction value during the fuel burning period) and void coefficient. The curve assumes a constant burn-up value and water:fuel geometrical ratio of 0.5. It is seen that, even when two fuels have the same enrichment of plutonium, a larger void coefficient is achieved when a fuel has a higher burning average void fraction, owing to a larger rate of change in neutron infinite multiplication factor with change in void fraction. In operation of the reactor, coolant at the fuel assembly boils, with heat generated from nuclear fission, particularly at the upper region of the effective fuel region so that coolant void in this upper region is relatively large. Accordingly, this upper region--considered from halfway up the longitudinal axis--is a region with a high burning average void fraction and hence a large void coefficient. Use of fuel assemblies as shown in FIG. 2 reduces the relative enrichment of fissile plutonium in this upper region, and hence suppresses its contribution to void coefficient. Because the burning average void fraction is high here, this measure is particularly effective. Conversely, the relatively high enrichment of the fuel in the lower region increases reactivity in this region, where burning average void fraction is relatively small and hence the effective water:fuel volume ratio is larger than in the upper region. Thus, burning of fuel proceeds easily in this region and compensates for any lowering of upper-region reactivity owed to the relatively reduced fissile plutonium enrichment in the upper region. The void coefficient contributions from a FIG. 2-type assembly are indicated schematically by the bold portions of the curves in FIG. 5. The left-hand end of the graph corresponds to the low average void fraction, i.e. the bottom of the core, and this has the high plutonium enrichment as shown by the bold line. Half-way up, there is a transition to lower plutonium enrichment for the upper half which has high void fraction. It should be noted that as void fraction increases, the difference between the void coefficients of high and low enrichment fuels gradually increases too. The reduction in void coefficient by a transition to lower enrichment is therefore especially marked. To consider the advantage provided by the present embodiment, reactor conditions may be compared with a reactor core otherwise similar but loaded with fuel assemblies having a uniform 6.5% plutonium enrichment, all the way down the effective fuel region. Compared with this comparative example, the void coefficient suppression effect and reactivity increment effect of the fuel assemblies embodying the invention provided, when integrated and consolidated in the reactor core, a void coefficient reduced by about 0.4.times.10.sup.-4 .DELTA. k/k/% void and a reactivity increased by about 0.1% .DELTA. k/k. Thus the void coefficient is relatively low while reactivity is maintained. FIG. 6 shows a second embodiment of fuel assembly. This corresponds to the first embodiment except as regards the distribution of plutonium enrichment in the effective fuel region. FIG. 6 shows extreme top and bottom regions of the assembly, each taking up one tenth of the total axial length of the effective fuel region. These comprise uranium-plutonium mixed oxide fuel 9 having 5.1% average fissile plutonium enrichment. The four-tenths of length up from the bottom end tenth 9, i.e. the segment up to the half-way line, consists of fuel 16 having 7.1% enrichment. The remaining four-tenths, i.e. the region from the 5/10 to the 9/10 line (considering the bottom of the assembly as a base line) comprises uranium-plutonium mixed oxide fuel 12 having 6.1% enrichment. The average enrichment over the effective fuel region of this assembly is 6.3%. In the upper half, the average enrichment is 5.9%. In the lower half, the average enrichment is 6.7%. The low-enrichment portions 9 at the extreme ends of the effective fuel region give small power output and slow burning, while the relatively more enriched fuel with large neutron infinite multiplication factor is in the central region where the contribution to reactivity is large. Overall, this gives increased reactivity. It is found that a reactor core (as previously described), loaded with such fuel assemblies has a reactivity larger by 0.5% .DELTA.k/k than a comparative reactor core having the same average enrichment (6.3%) but distributed uniformly along the axial length of the assembly. Indeed, this embodiment with average 6.3% enrichment achieves the same reactivity as a core using 6.5% average enrichment distributed uniformly. Also, we find that in this embodiment the void coefficient is less by about 0.4.times.10.sup.-4 .DELTA.k/k/% void than that achieved with a core using a uniformly distributed 6.5% enrichment, i.e. of the same reactivity. FIG. 7 shows schematically a third embodiment. Apart from the distribution of enrichment, the make-up of the fuel assembly is the same as in the first embodiment. From the base line up to 1/10, and from the top down to 8/10, the effective fuel region has end regions containing 4.8% enriched fuel 6. Thus, the low-enrichment region at the top is twice as long as at the bottom. Between these regions 6, the region from 1/10 to 8/10 comprises uniformly 6.8% enriched fuel 13. Accordingly the average plutonium enrichment overall is 6.2%, that in the upper half of the effective fuel region is 6.0% and in the lower half 6.4%. This embodiment differs from the first two embodiments in that the half-way line does not correspond with any actual boundary between regions of different fuel enrichment. We find that a reactor core loaded with fuel assemblies according to this embodiment has a void coefficient less by about 0.2.times.10.sup.-4 .DELTA.k/k/% void than that of a corresponding core loaded with fuel assemblies having the 6.5% enrichment uniformly axially distributed through the effective fuel region. The advantage is therefore clear. Furthermore, the reactivity is greater by about 0.7% .DELTA.k/k relative to the comparative case (6.2% uniformly), and is comparable with that achievable using 6.5% enriched fuel with a uniform axial distribution. A particular advantage of this embodiment is that the fuel assembly comprises fuels of only two different degrees of enrichment. In particular, the aim is to improve the void coefficient, while forming the downstream (upper) part of the effective fuel region (where the fuel has high burning average void fraction and a large reactivity change with change in void fraction) with as few different enrichment grades as possible. FIG. 8 shows a fourth embodiment. As in the third embodiment, the half-way position at 5/10 does not coincide with any actual boundary between regions of different enrichment. As in the second embodiment the assembly has one-tenth end regions of relatively low enrichment, in this case of 4.85% enrichment fuel 7. A region from 2/10 to 7/10, i.e. most of the bottom half and part of the top half, is of 6.85% enriched uranium-plutonium mixed oxide fuel 14, while a region from 7/10 to 9/10 is 5.85% enrichment fuel 10. The average fissile plutonium enrichment over the effective fuel region is 6.25%, in the upper half 6.05% and in the lower half 6.45%. While the effective fuel region is divided into four different enrichment sections, as in the second embodiment, the present embodiment contains a relatively larger region of high enrichment fuel with more fissile substance in that region. Consequently, the power load per unit weight of high enrichment fuel is less in this embodiment than in the second embodiment for the same whole-core operation power. Thus the burn-up of high enrichment fuel is relatively lower than in the second embodiment so that the increment of void coefficient due to increased burn-up (see FIG. 4) is relatively suppressed. Compared with a loaded reactor core of similar fuel assemblies having an overall average enrichment of 6.5% distributed axially uniformly, the present embodiment gives a void coefficient smaller by about 0.6.times.10.sup.-4 .DELTA.k/k/% void. This is a substantial improvement. As before, the low enrichment fuels positioned at the ends of the effective fuel region lead to an increased overall reactivity--a relative increase of about 0.6% .DELTA.k/k relative to the comparative example (6.25% uniformly). This is the same reactivity as requires 6.5% average enrichment when that enrichment is uniformly axially distributed. Finally, FIG. 9 shows a fifth embodiment of fuel assembly. As in the third and fourth embodiments, the half-way mark of the effective fuel region does not coincide with any actual boundary between two regions of different plutonium enrichment. As before, the general construction of the fuel assembly is similar to that described for the first embodiment. The enrichment distribution, however, is as follows. As in the second embodiment the assembly has end regions of relatively low enrichment fuel 17, in this case 5.9% enrichment, each occupying one tenth of the axial extent of the fuel region. From 2/10 up to 4/10 i.e. a minor proportion of the central region at the bottom, the assembly comprises uniformly 6.9% enrichment uranium-plutonium mixed oxide fuel 18 while the remaining, upper, region from 5/10 to 9/10 i.e. a major proportion of the central axial length comprises uniformly 6.4% enrichment fuel 19. The average enrichment over the effective fuel region is 6.45%, in the upper half 6.3% and in the lower half 6.6%. This is a relatively small difference in enrichment between the two halves. Consequently the burn-up of higher enrichment fuel is less than in the second embodiment above and there is relatively a suppression of the increment of void coefficient, based on the relation between burn-up and void coefficient shown in FIG. 4. A reactor core loaded with fuel assemblies of this embodiment obtains the same reactivity as a reactor core using assemblies of 6.5% enriched fuel distributed uniformly, even though the average enrichment is 6.45% in the embodiment, and this reactivity is largely due to the placement of lower enrichment fuels at the ends of the effective fuel region. Furthermore, the reactor core has a void coefficient smaller by about 1.4.times.10.sup.-4 .DELTA.k/k/% void than a core of equal reactivity having uniformly-distributed enrichment (i.e. of 6.5%). The improved effect is easy to perceive. From the description above it will be seen that the assemblies described enable a reduction of a positive void coefficient in a high conversion ratio reactor core, without any decrease in reactivity, and hence enable improvement of power coefficient and safety margin values in such a reactor.