Patent Number: 059404612
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are explained in detail with reference to the drawings. The following embodiments are directed to reactor cores of 1,350,000 kW electric power grade but the scale of the power is not restricted only thereto. It is applicable also to other power scales by changing the number of fuel assemblies. A first embodiment of the present invention will be explained with reference to FIG. 1 and FIG. 6 to FIG. 11. FIG. 1 shows a horizontal cross section for 1356 MWe electric power in this embodiment. 720 fuel assemblies 1 and 223 Y-type control rods 2, each per three fuel assembles are shown. FIG. 6 shows a cross section of a fuel assembly lattice. Fuel rods 3 each of 10.1 mm diameter are arranged in a regular triangular shape each at 1.3 mm fuel rod gap, and one row of fuel rods at the outermost circumference on one side of a regular hexagonal fuel assembly is not present so as to form a regular hexagonal fuel assembly lattice with a channel box 4 and one of wings 5 of the Y-type control rod. Namely, in the hexagonal fuel assembly, among three sets of fuel rod rows in parallel with the fuel rod rows of opposing outermost layers, two sets are equal with each other having 17 rows and the remaining one set has 16 rows. Stainless tubes filled with B4C are arranged in the wings of the control rod and wings are arranged each at a 120 degree spacing and so as to constitute a regular triangular shape with extensions from respective wings. Further, the control rod has at the top end a follower portion constituted with carbon which is a material having a smaller moderating function than light water. FIG. 7 shows a fuel arrangement for a equilibrium reactor core. The numbers indicated on the fuel assembly 1 show staying periods in the reactor core by the number of cycles. Fuels at the third cycle of the longest core staying period are loaded to the outermost circumference of the reactor core with low neutral importance. Fuels at the first cycle of the core staying period with the highest infinite medium neutron multiplication facter are loaded to the outer reactor core region at the inside thereof, to flatten the radial power distribution of the reactor core. In the inner reactor core, fuels at the second and the third cycle of the core staying period are loaded dispersively to flatten the power distribution in the inner region. FIG. 8 shows an orifice state in the equilibrium reactor core in which numbers indicated on the fuel assemblies 1 show that the opening/closing degree of the orifices disposed to a fuel support portion is different and they are divided into three regions. The orifice diameter of the outer reactor core region (numbers 1 and 2) of lower fuel assembly power is smaller than the orifice diameter of the inner region. FIG. 9 shows a distribution of the enrichment of fissionable Pu in the axial direction averaged along a horizontal cross section of fuel assemblies for the equilibrium reactor core. Pu-enriched uranium is degraded uranium. The height of the reactor core is 55 cm and it is divided into two regions at 8/12 from the lower end of the reactor core in which the enrichment in an upper portion is 12 wt % and that in an lower portion is 10 wt %. Degraded uranium blankets each of 25 cm and 20 cm are attached to upper and lower ends of the reactor core respectively. FIG. 10 shows a distribution of Pu-enrichment along a horizontal cross section in the lower portion of the fuel assembly. The fissionable Pu enrichment includes four kinds of 10.4 wt %, 9.4 wt %, 8.4 wt % and 7.4 wt %, with an average enrichment being 10 wt %. The distribution of the Pu enrichment along the horizontal cross section in the upper portion of the fuel assembly is identical with that in the lower portion, in which the fissionable Pu enrichment includes four kinds of 12.4 wt %, 11.4 wt %, 10.4 wt % and 9.4 wt %, with an average enrichment being 12 wt %. FIG. 11 shows a power distribution and a void fraction distribution averaged in the axial direction for the reactor core. The average reactor core void fraction is 61% and the steam weight ratio at the exit of the reactor core is 32 wt %. The combination of the dense hexagonal fuel assembly comprising a regular triangular lattice with 1.3 mm fuel rod gap, an average reactor core void fraction of 61% and a Y-type control rod can provide an effective water-to-fuel volume ratio of 0.27 to attain an incore breeding ratio of 0.90, a blanket breeding ratio of 0.11 and a total breeding ratio of 1.01. With the reasons as described above, a light water cooled reactor at a breeding ratio of 1.01 is achieved in this embodiment by decreasing an effective water-to-fuel volume ratio from about 2.0 as in the existent reactor to 0.27. The power of this reactor core is 1,350,000 kWe which is identical with the power of existent ABWR, and the circumscribing radius of the reactor core is 2.8 m which is substantially equal to the value for ABWR. The reactor core has a 55 cm height and has blankets of 25 cm and 20 cm attached to upper and lower ends respectively to constitute a short fuel assembly. However, since the fuel rods are arranged densely, the entire length of the fuel rod is substantially equal to that in ABWR, and MCPR is 1.32 which can sufficiently satisfy the standard thermal design value of 1.24. Since this is constituted as a short fuel of 55 cm for the reactor core portion irrespective of the dense arrangement, Pu inventory is as small as 4.4 toil being converted as the amount of fissionable Pu per 1,000,000 kWe power and it is 10 ton or less per 1,000,000 kWe even considering the outer core staying period of Pu such as in reprocessing. With the reasons described above, according to this embodiment at a breeding ratio of 1.01, 1,500 units of 1,000,000 kWe reactors can be operated continuously for 10,000 years using 15,000 tons of fissionable Pu and 15,000,000 tons of degraded uranium resulting from 15,000,000 tons of uranium deposits in the world, to attain a stable long time energy supply system. In this embodiment, identical power with that of ABWR under construction can be attained by a pressure vessel of a size substantially equal thereto and an identical burnup degree of 45 GWd/t with that in ABWR can also be attained by using identical zircalloy for the fuel cladding material. With the reasons described above, this embodiment can attain BWR capable of coping with stable long time energy supply at a substantially equal power generation cost to that in fuel once-through type light water cooled reactors under operation at present. While the height of the reactor core is about 370 cm in BWR under operation, it is 55 cm in this embodiment. Accordingly, it has a large neutron leakage effect of rendering the void efficient negative that represents increase of reactivity when the amount of steams generated in the reactor core is increased. Further, it comprises two upper and lower region fuels in which the fissionable Pu enrichment is different at 18.3 cm from the upper end in the axial direction of the fuel assembly in which the enrichment in the upper portion is 12 wt % and the enrichment in the lower portion is 10 wt %. Further, when the void amount in the reactor core is increased, the relative increment of the void fraction is greater by about 20% in the lower portion of the reactor core where the void fraction is low than in the upper portion of the reactor core which has already reached the saturation state, and, as a result, a neutron flux distribution swings from the upper portion of the reactor core of high neutron importance to the lower portion of the reactor core of low neutron importance to charge a negative void reactivity. Further, in this embodiment, since the steam weight ratio at the exit of the reactor core is 32%, entire coolants are not converted into steams even upon abnormal transient change but always kept in a two phase flow state, to confine radioactive materials such as corrosion products accumulated in the reactor core within the reactor core under the distillation effect by boiling, and prevent them from transferring to a turbine, like that in the existent BWR. With the reasons described above, this embodiment can achieve BWR capable of coping with stable long time energy supply under the same extent of safety as that of the fuel once-through type light water cooled reactors now under operation. In BWR under operation, about 85% of nuclear reactions occurs in a thermal neutron region at 0.6 eV or less, whereas the central energy value caused from nuclear reactions in this embodiment is about 1 keV, and the reaction ratio in the resonance region is extremely high. Therefore, while BWR now under operation has a Doppler coefficient of 1.6.times.10-5 .DELTA.k/k/.degree. C., the value in this embodiment is 3.7.times.10-5 .DELTA.k/k/.degree. C. which is about twice. The existent BWR now under operation has a void coefficient of -7.0.times.10-4 .DELTA.k/k/% void, whereas an absolute value in this embodiment is set as low as -0.5.times.10-4 .DELTA.k/k/% void. As a result, a thermal margin is relatively increased, for this embodiment, in an event of pressure elevation or temperature lowering of coolants. For the reasons described above, this embodiment can provide a BWR reactor core of greater safety margin for most of transient events than existent BWR now under operation. According to this embodiment, the breeding ratio at 1.01 can be attained by the combination of the dense hexagonal fuel assembly, the Y-type control rod and the average reactor core void fraction of 61%, using fuels comprising degraded uranium enriched with fissionable Pu of 10.5 wt % in average, and the Pu inventory is also reduced by making the reactor core height to 55 cm, so that stable long time energy supply can be attained by BWR capable of operating 1,500 units of 1,000,000 kW reactors for 10,000 years, with 15,000,000 tons of natural uranium deposits in the world. In addition, since the diameter of the pressure vessel, operation conditions such as power and the materials to be used are made substantially equal to those of BWR now under operation, the power generation cost can be suppressed about to the same extent as that for the existent BWR, with the performance being improved remarkably. Further, since the negative void coefficient is maintained by means of the short fuel assembly and two upper and lower region fuel assemblies and the steam weight ratio is kept to about 30% at the exit of the reactor core, a distillation function by boiling can be maintained to confine radioactivated materials within a pressure vessel, so that about the same extent of safety margin as that for the existent BWR can be obtained. In this embodiment, descriptions have been made as to the constitution, the function and the effect to fuels obtained by enriching Pu to degraded uranium produced as residues upon production of concentrated uranium used in existent light water cooled reactors, intended for stable long time energy supply. However, an equivalent or superior effect can be obtained also with fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium. In this instance, enrichment of the fissionable Pu can be lowered by 0.5 wt % or more as compared with the case of using degraded uranium, by the increase of the weight ratio of uranium-235 contained in the fuels. As a result, a breeding ratio to the fissionable Pu can be increased by about 3% or more and the void coefficient can be made further negative. Further, since the Pu inventory can be reduced, the number of RBWR reactor units to be operated can be increased further. The void coefficient is negative in this embodiment but the power coefficient including a Doppler coefficient can be made negative even if the void efficient is zero or slightly positive. Studies made by the present inventors, show that the positivity or negativity of the void coefficient causes no substantial problem so long as the power coefficient is negative from the result for the safety evaluation. Accordingly, the thermal margin can further be increased by making the reactor core portion longer than 55 cm. Further, the breeding ratio can be increased by narrowing the fuel rod gap to less than 1.3 mm. In this embodiment, descriptions have been made to fuels in which uranium is enriched only with Pu but other actinoid nuclides may also be enriched together with Pu. In this instance, since the neutron average energy is high in RBWR, Pu less transfers to actinoid nuclides of high mass number, as well as the actinoid nuclides can be annihilated by nuclear reactions. Further, although the fissionable Pu enrichment is divided into upper and lower regions at 8/12 from the lower end of the reactor core, this is not limitative. FIG. 28 shows an embodiment of a distribution in the axial direction for the fissionable Pu average enrichment along a horizontal cross section. Pu-enriched uranium is the same degraded uranium as in this embodiment, and degraded uranium blankets each of 25 cm and 20 cm are attached to upper and lower portions of the reactor core respectively. The reactor core height is 55 cm which is identical with that in this embodiment and it is divided into five regions at 1/12, 2/12, 7/12 and 8/12 from the lower end of the reactor core. The fissionable Pu enrichment is 12.5 wt %, 10.5 wt %, 9.5 w %, 10.5 wt % and 12.5 wt % from the upper portion, the fissionable Pu average enrichment for the fuel assembly is 11 wt %, the average enrichment in the upper half portion is 11.7% and the average enrichment in the lower half portion is 10.2 wt %. As shown in FIG. 29, the axial power distribution can be flattened further by increasing the fissionable Pu enrichment in the region near the lower end and disposing an intermediate enrichment (10.5 wt %) between the highest enrichment (12.5 wt %) and the lowest enrichment (9.5 wt %). In the embodiment shown in FIG. 28, the power peaking can be reduced further by 5% as compared with this embodiment. Further, for the axial direction of the fuel assembly, the average fissionable Pu enrichment in the upper half portion is higher than the average value for the lower half portion and an effect of reducing the void reactivity coefficient can be obtained like that in this embodiment. Further, since the axial power distribution is flattened, the amount of neutron leakage from the upper and the lower portions of the reactor core is increased. This increases the required fissionable Pu enrichment to greater than that in this embodiment, but it can provide an effect of further reducing the void reactivity coefficient. FIG. 30 shows a modification of FIG. 28 in which the intermediate enrichment (10.5 wt %) is saved. The embodiment in FIG. 28 has a greater effect for the flattening of the power distribution and the same effect can be obtained by two kinds of fissionable Pu enrichments like that in this embodiment. Description will be made to the second embodiment according to the present invention with reference to, FIGS. 18 to 20. FIG. 18 shows a horizontal cross section of a reactor core of 1356 MWe electric power of this embodiment. 720 fuel assemblies 9 and 223 Y-type control rods 10 each per three fuel assemblies are shown. FIG. 19 shows a cross section of a fuel assembly lattice. Fuel rods 3 each of 10.1 mm diameter are arranged in a regular triangular shape at 1.3 mm fuel rod gap to constitute a regular hexagonal fuel assembly of 10 rows of fuel rods. Then, Y-type control rods each per three fuel assemblies 3 are arranged as shown in FIG. 18 and a gap between each of the fuel assemblies not inserted with the control rod is made narrower than a gap between each of the fuel assemblies inserted with the control rod. Stainless tubes filled with B4C are disposed to the wings of the control rod and the wings are spaced each by 120 degree. Further, the control rod has at the top end a follower portion constituted of carbon which is a substance of smaller moderating function than that of light water. The arrangement of fuels in the reactor core, the state of the orifice and the distribution of the fissionable Pu enrichment in the axial direction averaged along the horizontal cross section of the fuel assembly for the equilibrium reactor core are identical with those shown in FIG. 7, FIG. 8 and FIG. 9 for Embodiment 1. FIG. 20 shows the distribution of the fissionable Pu enrichment along the horizontal cross section in the lower portion of the fuel assembly. The distribution of the fissionable Pu enrichment is symmetrical with respect to one of the wings of the Y-type control rod not adjacent to the regular hexagonal fuel assembly. The fissionable Pu enrichment comprises four kinds of 10.4 wt %, 9.4 wt %, 8.4 wt % and 7.4 wt %, with the average enrichment being 10 wt %. The distribution of the fissionable Pu enrichment along the horizontal cross section in the upper portion of the fuel assembly is identical with the distribution in the lower portion, and the fissionable Pu enrichment includes four kinds of 12.4 wt %, 11.4 wt %, 10.4 wt % and 9.4 wt %, with the average enrichment being 12 wt %. In this embodiment, the fuel assembly is in a regular hexagonal shape, the number of fuel rods per fuel assembly is increased by 10 as compared with that in Embodiment 1 and the thermal margin is improved since the average linear power density is reduced and the heat conduction area is increased. On the other hand, since a space of the Y-type control rod is increased to the outside of the fuel assembly, the circumscribing radius of the reactor core is increased to greater than that in Embodiment 1. Also in this embodiment, the combination of the dense hexagonal fuel assembly, the Y-type control rod and the average reactor core void fraction of 61% can attain an effective water-to-fuel volume ratio of 0.27. As a result, the reactor core property is equal to that in Embodiment 1 and can provide a similar effect. Further, also in this embodiment, an equal or superior effect can be obtained also with fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium. Further, other actinoid nuclides can also be enriched together with Pu. Further, in this embodiment, the fissionable Pu enrichment is divided into upper and lower two regions at 8/12 from the lower end of the reactor core but it is not restrictive. The same effect as that in Embodiment 1 can be obtained by adopting FIG. 28 or FIG. 30 as a modification of Embodiment 1. A third embodiment according to the present invention will be explained with reference to FIG. 15 to FIG. 17. FIG. 15 shows a horizontal cross section of a reactor core of 1356 MWe electric power in this embodiment. There are shown 720 regular hexagonal fuel assemblies 6 and 223 control rod driving mechanisms 7 each for operating cluster-type control rods each to be inserted into per three fuel assemblies. FIG. 16 shows a horizontal cross section of a fuel assembly lattice. Fuel rods 3 each of 10.1 mm diameter are arranged in a regular triangular shape at 1.3 mm fuel rod gap to constitute a regular hexagonal assembly comprising 10 rows of fuel rods. Among them, guide tubes 8 for housing the cluster-type control rods are arranged at 12 positions in the fuel rod lattice. The arrangement of the fuels in the reactor core, the state of the orifice and the distribution of the fissionable Pu enrichment in the axial direction averaged along the horizontal cross section of the fuel assembly for the equilibrium reactor core are identical with those in FIG. 7, FIG. 8 and FIG. 9 for Embodiment 1. FIG. 17 shows the distribution of fissionable Pu enrichment along the horizontal cross section of the lower portion in the fuel assembly. Since the distribution of the moderators is more homogeneous than in Embodiments 1 and 2, power peaking can be suppressed by two kinds of the fissionable Pu enrichment. The fissionable Pu enrichment in fuel rods 1 and 2 are 9.0 wt % and 10.1 wt %, respectively. The distribution of the fissionable Pu enrichment along the horizontal cross section is identical between the upper portion and the lower portion in the fuel assembly, and the fissionable Pu enrichment in the fuel rods 1 and 2 are 11.0 wt % and 12.1 wt % respectively. In this embodiment, since the control rod is inserted in the fuel assembly, the number of the fuel rods is decreased by two as compared with Embodiment 1 but a greater effect for reactivity control can be obtained and necessary reactivity can be controlled also by using natural boron as the absorbent. Also in this embodiment, the combination of the dense hexagonal fuel assembly, the cluster-type control rod and the average reactor core void fraction of 61% can provide an effective water-to-fuel volume ratio of 0.27. As a result, the reactor core property is equal to that in Embodiment 1 and the same effect can be obtained. Further, also in this embodiment, the equal or superior effect can be obtained also with fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium. Further, other actinoid nuclides can be enriched together with Pu. Further, in this embodiment, the fissionable Pu enrichment is divided into the upper and lower regions at 8/12 from the lower end of the reactor core but it is not limitative. The same effect as that in Embodiment 1 can be obtained by adopting FIG. 28 or FIG. 30 as a modification of Embodiment 1. Fourth Embodiment! A fourth embodiment according to the present invention will be explained with reference to FIG. 12 to FIG. 14. In this embodiment, the reactor core property is further improved based on the constitution of Embodiment 1, but a similar reactor core can be achieved also based on the constitution of Embodiment 2 or 3. This embodiment shows a case of a reactor core of 1356 MWe electric power in which fuel burnup is enhanced. The horizontal cross section of the reactor core, the cross section of the fuel assembly lattice and the orifice distribution in this embodiment are identical with those in FIG. 1, FIG. 6 and FIG. 8 for Embodiment 1. FIG. 12 shows a fuel arrangement for a equilibrium reactor core. The numbers indicated on the fuel assembly show the staying period in the reactor core by the number of cycles. Third cycle fuels of the longest core staying period are loaded to the outermost circumference of the reactor core with a low neutron importance. In the outer reactor core region at the inside thereof, fuels at the first cycle of core staying period with the highest infinite medium neutron multiplication factor are loaded to flatten the radial power distribution in the reactor core. In an inner reactor core region, fuels at the second and third cycles of core staying period are loaded dispersively to flatten the power distribution in the inner region. In this embodiment, since the burnup reactivity is reduced as compared with Embodiment 1 by providing a blanket portion to an axial central portion, the number of third cycle fuels is increased in a central region of the reactor core. FIG. 13 shows the distribution of the fissionable Pu enrichment in the axial direction averaged along the horizontal cross section of the fuel assembly for the equilibrium reactor core. Uranium to be enriched with Pu is degraded uranium. The reactor core height is 77 cm, which is divided into three regions 1, 2, 3 at 23 cm and 52 cm from the lower end of the reactor core, in which the fissionable Pu enrichment is 17 wt %, 0 wt % and 17 wt % respectively, with the average enrichment being 10.6 wt %. Further, degraded uranium blankets of 25 cm and 20 cm are attached to upper and lower portions of the reactor core respectively. FIG. 14 shows the power distribution and the void fraction distribution averaged along the direction of the reactor core height. The average reactor core void fraction is 60% and the steam weight ratio at the exit of the reactor core is 29%. The constitution of the fuel assembly is identical with that in Embodiment 1, and the fuel assembly is a dense hexagonal fuel assembly comprising a regular triangular lattice at 1.3 mm of fuel rod gap. The combination of the average reactor core void fraction of 60% and the Y-type control rod can attain an effective water-to-fuel volume ratio of 0.27, in which an incore breeding ratio is 0.87, a blanket breeding ratio is 0.14 and a total breeding ratio is 1.01. In this embodiment, the fissionable Pu enrichment comprises 17 wt % portions both in upper and lower portions along the axial direction of the fuel assembly and the central region therebetween comprises degraded uranium not containing the fissionable Pu. Upon power up or lowering of the reactor core coolant flow rate, the steam void fraction in the reactor core is increased in which the power distribution in the upper portion of the reactor core swings to the central region not containing the fissionable Pu. This can provide a greater effect of lowering the reactivity in the reactor core than that in Embodiment 1. As a result, the void coefficient could be kept at -0.5.times.10-4 k/k/% void which is identical with that in Embodiment 1 even if the fuel burnup is further increased to be greater than that in Embodiment 1. In this embodiment, the same power can be attained as that in ABWR now under construction with a pressure vessel of a size substantially equal thereto and 70 GWd/t can be attained. As compared with Embodiment 1, the reactor core portion is somewhat increased in the length as 77 cm but the Pu inventory is as less as 6.2 tons being converted as the amount of fissionable Pu per 1,000,000 kWe, and this is 10 tons or less per 1,000,000 kWe even when an outer core staying period of Pu such as in reprocessing is taken into consideration, to provide the same effect as in Embodiment 1. Further, also in this embodiment, an equal or superior effect can be obtained with fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium. Further, other actinoid nuclides can be enriched together with Pu. Further, in this embodiment, upper and lower portions identical with each other for the fissionable Pu enrichment are provided along the axial direction of the fuel assembly and degraded uranium not containing fissionable Pu is present therebetween. However, the fissionable Pu enrichment is not necessarily be equal between the upper and the lower portions. Further, in this embodiment, the region for degraded uranium is disposed somewhat higher than the central portion of the reactor core but this is not limitative. It is possible to attain an identical axial power peaking by the combination of the fissionable Pu enrichment in the upper and the lower portions and the position for the degraded uranium region. A fifth embodiment according to the present invention will be explained with reference to FIG. 21 and FIG. 22. In this embodiment, the reactor core performance is further improved on the basis of the constitution of Embodiment 1 but an identical reactor core can also be attained on the basis of the constitution of Embodiment 2 or 3. This embodiment is directed to a reactor core of 1356 MWe electric power, having a margin for the minimum critical power ratio and the maximum linear power density. The constitution of this embodiment along the horizontal cross section of the reactor core is identical with that in Embodiment 1. FIG. 21 shows the distribution in the axial direction of the enrichment of the fissionable Pu averaged along the horizontal cross section of the fuel assembly for the fifth embodiment. Pu-enriched uranium is degraded uranium. The reactor core height is 91 cm which is divided into three regions 1, 2 and 3 at 33 cm and 53 cm from the lower end of the reactor core in which the fissionable Pu enrichment in each of the regions is 11.7 wt %, 0 wt % and 11.7 wt % and 9.1 wt % in average. Further, degraded uranium blankets of 25 cm and 20 cm are attached to upper and lower ends of the reactor core respectively. FIG. 22 shows the power distribution and the void fraction distribution in average along the axial direction of the reactor core. The average reactor core void fraction is 57% and the steam weight ratio at the exit of the reactor core is 26%. The constitution of the fuel assembly is identical with that in Embodiment 1, and the combination of the dense hexagonal fuel assembly of a regular triangular lattice at 1.3 mm of fuel rod gap, the average reactor core void fraction of 57% and the Y-type control rod attained an effective water-to-fuel volume ratio of 0.28, and an incore breeding ratio of 0.93, a blanket breeding ratio of 0.08 and a breeding ratio of 1.01 in total were attained. In this embodiment, upper and lower regions along the axial direction of the fuel assembly have 11.7 wt % of the fissionable Pu enrichment, and a central region between them is composed of degraded uranium not containing the fissionable Pu. Upon power up or lowering of the reactor core coolant flow rate, the steam void fraction in the reactor core is increased, in which the power distribution in the upper region of the reactor core swings to the central region not containing the fissionable Pu. This can provide an effect of reducing the reactor core reactivity which is greater than that of Embodiment 1. As a result, the void coefficient could be kept to -0.5.times.10-4 k/k/% void which was identical with that in Embodiment 1 even if the reactor core height was increased to greater than that in. Embodiment 1. Further, since neutrons flow from the upper and the lower regions containing the fissionable Pu to the central region of the reactor core not containing the fissionable Pu, the breeding ratio can be increased. Accordingly, even if the reactor core flow rate is increased and the average reactor core void fraction is lowered to less than that in Embodiment 1, an equal or greater breeding ratio can be obtained. Further, as a result of increasing the reactor core flow rate, MCPR is 1.45 and a reactor core with an increased thermal margin as compared with that in Embodiment 1 can be attained. Further, also in this embodiment, an equal or superior effect can be obtained also with fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium, and other actinoid nuclides can be enriched together with Pu. Further, also in this embodiment, it is not always necessary that the fissionable Pu enrichment is equal between the upper and the lower regions, and the position for the degraded uranium at the central region of the reactor core is not restricted only to that described above. Although the length of the reactor core was somewhat increased as 91 cm as compared with that in Embodiment 1, the Pu inventory is as less as 6.3 tons being converted as the amount of the fissionable Pu per 1,000,000 kWe power and this is 10 tons or less per 1,000,000 kWe even if the outer core staying period of Pu such as in reprocessing is taken into consideration and the same effect as that in Embodiment 1 can be obtained. A sixth embodiment according to the present invention will be explained with reference to FIG. 23 and FIG. 24. In this embodiment, the reactor core performance is further improved on the basis of the constitution of Embodiment 1 but similar reactor core can also be attained on the basis of the constitution of Embodiment 2 or 3. This embodiment is directed to a reactor core of 1356 MWe electric power, with an increased Pu inventory and having a feature as a Pu storing reactor. The constitution of this embodiment along the horizontal cross section of the reactor core is identical with that in Embodiment 1. FIG. 23 shows the distribution in the axial direction of the enrichment of the fissionable Pu averaged along the horizontal cross section of the fuel assembly of the sixth embodiment. Pu-enriched uranium is degraded uranium. The reactor core height is 126 cm, which is divided into three regions 1, 2 and 3 at 42 cm and 82 cm from the lower end of the reactor core in which the fissionable Pu enrichment in each of the regions is 11.7 wt %, 0 wt % and 11.7 wt %, with 8.0 wt % in average. Further, degraded uranium blankets of 25 cm and 20 cm are attached to the upper and lower ends of the reactor core respectively. FIG. 24 shows the power distribution and the void fraction distribution in average along the axial direction of the reactor core. The average reactor core void fraction is 60% and the steam weight ratio at the exit of the reactor core is 31%. The constitution of the fuel assembly is identical with that in Embodiment 1, and the combination of the dense hexagonal fuel assembly of the regular triangular lattice at 1.3 mm of fuel rod gap, an average reactor core void fraction of 60% and the Y-type control rod can attain an effective water-to-fuel volume ratio of 0.27, and an incore breeding ratio of 0.95, and a blanket breed ratio of 0.07 and a breeding ratio, 1.02 in total were attained. In this embodiment, upper and lower regions along the axial direction of the fuel assembly have 11.7 wt % of the fissionable Pu enrichment, and a central region between them is composed of degraded uranium not containing the fissionable Pu. As compared with Embodiments 4 and 5, the degraded uranium region in the central portion of the reactor core was increased to 40 cm, so that the effect of reducing the void coefficient and the effect of increasing the breeding ratio could be enhanced. This can increase the region containing fissionable Pu to make the Pu inventory to 10.3 tons. Further, also in this embodiment, an equal or superior effect can be obtained also with fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium, although the Pu inventory is somewhat reduced. Further, other actinoid nuclides can be enriched together with Pu. Further, in this embodiment, upper and lower portions of identical fissionable Pu enrichment are provided in the axial direction of the fuel assembly and depleted uranium not containing the fissionable Pu is disposed therebetween. However, it is not always necessary that the fissionable Pu enrichment is equal to each other between the upper and the lower regions. Further, in this embodiment, although the region of the degraded uranium is disposed somewhat above the central region of the reactor core, this is not limitative. A seventh embodiment according to the present invention will be explained with reference to FIG. 25 and FIG. 26. In this embodiment, the reactor core performance is further improved on the basis of the constitution of Embodiment 1 but an identical reactor core can also be attained on the basis of the constitution of Embodiment 2 or 3. This embodiment is directed to a reactor core of 1356 MWe electric power, having increased negative void coefficient. The constitution of this embodiment along the horizontal cross section of the reactor core is identical with that in Embodiment 1. FIG. 25 shows the distribution in the axial direction of the enrichment of the fissionable Pu averaged along the horizontal cross section of the fuel assembly of the seventh embodiment. Pu-enriched uranium is degraded uranium. The reactor core height is 65 cm which is divided into three regions 1, 2 and 3 at 23 cm and 38 cm from the lower end of the reactor core in which the fissionable Pu enrichment in each of the regions is 13.5 wt %, 0 wt % and 13.5 wt %, with 10.5 wt % in average. Further, degraded uranium blankets of 25 cm and 20 cm are attached to upper and lower ends of the reactor core respectively. FIG. 26 shows the power distribution and the void fraction distribution in average along the axial direction of the reactor core. The average reactor core void fraction is 60% and the steam weight ratio at the exit of the reactor core is 29%. The constitution of the fuel assembly is identical with that in Embodiment 1, and the combination of the dense hexagonal fuel assembly of a regular triangular lattice at 1.3 mm of fuel rod gap, an average reactor core void fraction of 60% and the Y-type control rod can attain an effective water-to-fuel volume ratio of 0.27, and an incore breeding ratio of 0.90 and a blanket breeding ratio of 0.12, a breeding ratio of 1.02 in total were attained. In this embodiment, upper and lower regions along the axial direction of the fuel assembly have 13.5 wt % of the fissionable Pu enrichment, and a central region between them is composed of degraded uranium not containing the fissionable Pu. As compared with Embodiments 5 and 6, the upper and lower regions for the fissionable Pu in the reactor core are decreased and the along the axial direction of the fuel assembly an effect of reducing the void coefficient by the shortened length is further added. As a result, the void coefficient could be -1.8.times.10-4 k/k/%void. This enables power control or reactivity control by the flow rate control. Further, also in this embodiment, an equal or superior effect can be obtained also with fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium. Further, other actinoid nuclides can be enriched together with Pu. Further, in this embodiment, upper and lower regions of identical fissionable Pu enrichment are provided along the axial direction of the fuel assembly and depleted uranium not containing the fissionable Pu is disposed therebetween. However, it is not always necessary that the fissionable Pu enrichments are equal to each other between the upper and the lower regions. A modification of this embodiment for the fissionable Pu enrichment and the position for the degraded uranium region are shown in FIG. 31. Also in FIG. 31, the axial power peaking can be made equal to that in this embodiment. Further, FIG. 32 shows the distribution of the fissionable Pu enrichment in the axial direction averaged along the horizontal cross section of a fuel assembly, different from that in this embodiment. The Pu enriched uranium is degraded uranium like that in this embodiment, the reactor core height is 65 cm and degraded uranium blankets of 25 cm and 20 cm are attached to upper and lower ends of the reactor core respectively. It is divided into three regions 1, 2 and 3 at 25 cm and 35 cm from the lower end of the reactor core, in which the fissionable Pu enrichment are 13.5 wt %, 0 wt %, 13.5 wt % respectively, with 10.5 wt % in average. Since the degraded uranium region not containing the fissionable Pu is further displaced to the lower portion of the reactor core than that in this embodiment, the axial power distribution has an upper peak pattern as shown in FIG. 33, and the power fluctuation is increased upon increase of the void, to provide an effect of rendering the void reactivity coefficient more negative. In an eighth embodiment, actinoid nuclides taken out of spent fuels are recycled together with Pu in the seventh embodiment. The constitution of this embodiment as viewed along the horizontal cross section of the reactor core is identical with that in Embodiment 1. Also in the fuel assembly for this embodiment, the distribution of the fissionable Pu enrichment in the axial direction averaged along the horizontal cross section is identical with that in Embodiment 7. The Pu enriched uranium is uranium taken out of spent fuels together with plutonium, and actinoid nuclides taken out of the spent fuels are added simultaneously. The fissionable Pu enrichment is 13.5 wt %, 0 wt % and 13.5 wt %, with 10.5 wt % in average. Further, degraded uranium blankets of 25 cm and 20 cm are attached to the upper and the lower regions of the reactor core respectively. The constitution of the fuel assembly is identical with that in Embodiment 1, and the combination of the dense hexagonal fuel assembly of a regular triangular lattice at 1.3 mm of fuel rod gap, an average reactor core void fraction of 61% and the Y-type control rod can attain an effective water-to-fuel volume ratio of 0.27, and an incore breeding ratio of 0.91, and a blanket breed ratio of 0.10, a breeding ratio of 1.01 in total were attained. In this embodiment, upper and lower regions along the axial direction of the fuel assembly have 13.5 wt % of the fissionable Pu enrichment, and a central region between them is composed of degraded uranium not containing the fissionable Pu. As compared with Embodiments 5 and 6, the upper and lower regions for the fissionable Pu in the reactor core are decreased and an effect of reducing the void coefficient by the shortened length is further added. As a result, if the actinoid nuclides taken out of the spent fuels are recycled together with Pu, the void coefficient can be rendered negative. Further, by recycling the actinoid nuclides taken out of the spent fuels together with Pu repeatedly, the long life radioactive nuclides attain a equilibrated state in the reactor to reach a predetermined amount. Accordingly, in this embodiment, the amount of generation and the amount of annihilation of the actinoid nuclides are equilibrated, the increment becomes to zero, by which the entire generation amount of the long half-life actinoid nuclides, that particularly result in problems among the radioactive wastes, can be reduced remarkably, as well as actinoid nuclides containing Pu can be confined only within the nuclear reactor, the reprocessing facility and the fuel production facility. In a ninth embodiment, as shown in FIGS. 34 to 36 the present invention is applied to PWR. In this embodiment, a reactor core comprises the same cluster-type control rods as those in Embodiment 3 and regular hexagonal fuel assemblies in which fuel rods having an outer diameter of 14.3 mm greater than that of each existent PWR are arranged densely in a regular triangular lattice pattern at 1.0 mm of fuel rod gap. FIG. 34 shows the distribution of the fissionable Pu enrichment in the axial direction averaged along the horizontal cross section of the fuel assembly for the ninth embodiment. The Pu enriched uranium is degraded uranium. The height of the reactor core is 50 cm and the fissionable Pu enrichment is made uniform as 10.5 wt %. Degraded uranium blankets each of 30 cm are attached to upper and lower ends of the reactor core portion. The combination of the dense hexagonal fuel assembly with a regular triangular lattice at 1.0 mm of fuel rod gap, fuel rods of large diameter and the cluster-type control rods can attain a water-to-fuel volume ratio of 0.44. As a result, an incore breeding ratio of 0.90, a blanket breeding ratio of 0.11 and thus a total breeding ratio of 1.01 was attained. In this embodiment, the fissionable Pu enrichment is made uniform in the axial direction of the fuel assembly. However, the constitution of the fuels is not restricted only thereto. FIG. 35 and FIG. 36 show modifications of this embodiment regarding the axial distribution of the fissionable Pu enrichment. In FIG. 35, the fissionable Pu enrichment in the upper and lower ends of the reactor core is made greater than that in the central region for further flattening the axial power peaking. In FIG. 36, the reactor core height is 65 cm which is divided into three regions 1, 2 and 3 at 25 cm and 40 cm from the lower end of the reactor core in which the fissionable Pu enrichment for each of them is 13.0 wt %, 0 wt % and 13.0 wt % respectively and 10 wt % in average. Further, degraded uranium blankets each of 30 cm are attached to the upper and lower ends of the reactor core. Since the degraded uranium region not containing the fissionable Pu is present at the central portion of the reactor core, the charged reactivity upon generation of void can be made more negative. In this embodiment, the outer diameter of the fuel rod used is 14.3 mm which is greater than the outer diameter of the fuel rods in existent PWR but 9.5 mm in the existent PWR may also be used. In this instance, the combination of the dense hexagonal fuel assembly of a regular triangular lattice at 1.0 mm of fuel rod gap and the cluster-type control rod can provide a water-to-fuel volume ratio of 0.58. Since the water-to-fuel volume ratio is increased, the fissionable Pu enrichment has to be increased by about 0.5 wt % in this embodiment, but breeding ratio of 1.0 can be attained. Further, also in this embodiment, fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium can be used. Further, other actinoid nuclides may be enriched together with Pu. In a tenth embodiment, as shown in FIG. 37 the present invention is applied to the same square fuel assembly as that in existent PWR. FIG. 37 shows the constitution of the fuel assembly in this embodiment. For reducing the water-to-fuel volume ratio, fuel rods having an outer diameter of 13.8 mm greater than that of existent BWR were arranged densely in a regular triangular lattice pattern at 1.0 mm of fuel rod gap. The number of the fuel rods per fuel assembly is 85. Moderator at the outside of a channel box 11 is excluded by a follower portion disposed at the upper end of the cruciform control rod and a gap water region on the opposite side not inserted with the control rod is excluded by a water excluding plate made of a material having a smaller moderation function like that of the follower portion. This can attain an effective water-to-fuel volume ratio of 0.53 and a breeding ratio of 1.0 can be attained by the same fuel constitution in the direction of the reactor core height as in other embodiments. For flattening the fuel rod power peaking in the fuel assembly, the fissionable Pu enrichment of the fuel rods facing the channel box is made lower than that of fuel rods in other regions in this embodiment. Further, also in this embodiment, fuels formed by enriching Pu to natural uranium, depleted uranium recovered from spent fuels and low concentrated uranium instead of degraded uranium may be used. Further, other actinoid nuclides may be enriched together with Pu.