Reactor core for a light water cooled nuclear reactor and control rod for the same

Abstract: The present invention relates to a control rod (2) to be inserted between fuel assemblies loaded to a reactor core for a boiling water reactor, wherein wings (5) are arranged such that each spacing between the wings is 120 degrees and center lines extending from the respective wings in a horizontal cross section of the control rod constitute a regular triangle. Further the invention relates to a reactor core for a boiling water reactor having hexagonal fuel assemblies (1; 6; 9) containing fuels comprising uranium enriched with plutonium or plutonium and actinoid nuclides and Y-type control rods, wherein the Y-type control rod is a control rod according to claim 1, and the hexagonal fuel assembly has fuel rods (3) arranged in a regular triangular lattice pattern so as to form three sets of fuel rod rows, each set of fuel rod rows are parallel with a pair of opposing sides of the hexagonal fuel assembly, and the number of rows in two sets of fuel rod rows is equal to each other and is greater by one row than that of the remaining one set of fuel rod rows among the three sets of fuel rod rows.

Description: BACKGROUND OF THE INVENTION The present invention concerns a reactor core for a light water cooled nuclear reactor and a control rod intended for Pu multi-recycling at a breeding ratio of about 1.0 or slightly greater than 1.0 while keeping the economical and safety performance to the same level as in BWR now under operation, that is, while minimizing change of incore structures and maintaining a negative void coefficient. In a nuclear reactor, consumption of fissionable material such as uranium-235 and plutonium-239 and conversion of fuel fertile materials such as uranium-238 and plutonium-240 into fissionable material takes place by nuclear reaction. The ratio between the amount of fissionable material contained in fuel taken out of a reactor core and the amount of fissionable material contained in fuel to be loaded in the reactor core is referred to as a breeding ratio, and the breeding ratio is about 0.5 in existent light water cooled nuclear reactors. As a method of effectively utilizing uranium resources, it has been considered to increase the breeding ratio. JP-A-55-10591 and Nuclear Technology, vol. 59, 212 - 227 pp, 1982, both disclose that the breeding ratio can be improved by densely arranging fuel rods in a triangular lattice pattern to reduce a water-to-fuel volume ratio in pressurized water type nuclear reactors. However, the breeding ratio is about 0.9 at maximum and fissionable material has to be supplemented for continuous operation without lowering the power. For further increasing the breeding ratio, it may be considered to make the fuel rod gap narrower to further decrease the water-to-fuel volume ratio, but this suffers from a limit, for implementation, in view of manufacture of fuel assemblies and ensurance of thermal margin and is difficult to attain. On the other hand, JP-A-1-227993 discloses a method of effectively reducing the water-to-fuel volume ratio by utilizing steam voids generated in a reactor core, which is a feature of a boiling water type nuclear reactor. However, it has been shown in the prior art to make the plutonium breeding ratio (the ratio between the amount of fissionable plutonium contained in fuel taken out of a reactor core and the amount of fissionable plutonium contained in fuel loaded in the reactor core; breeding ratio relative to fissionable plutonium) to about 1, but it is not shown to make the breeding ratio (the value is smaller by about 4 to 5% than the plutonium breeding ratio in case of enriching natural uranium with plutonium) to about 1 or more. If the plutonium breeding ratio is about 1, it is necessary to enrich natural uranium with plutonium for continuing operation without lowering the power and the uranium resources can not be used up thoroughly. According to the present invention, a breeding ratio of about 1 means a value of 0.98 or more. JP-A-4301792 discloses a reactor core wich is constructed with combinations of hexagonal fuel assemblies in Y-shaped control rods. On the periphery of the core blanket fuel assemblies are arranged to increase the conversion rate to plutonium. SUMMARY OF THE INVENTION It is the object of the present invention to provide a reactor core and a control rod, for attaining a breeding ratio of 1.0 with a Pu-enriched uranium fuel by reducing the water-to-fuel volume ratio, being capable of maintaining the power generation cost, thermal margin and safety at about the same level as in light water cooled reactors now under operation, thereby contributing to stable long time energy supply. In accordance with the present invention, this object is accomplished with a control rod and a reactor core according to claims 1 and 2, respectively. Dependent claims are directed on features of preferred embodiments of the invention. The amount of natural uranium resources in the world is estimated to be about 15,000,000 tons, which corresponds to an amount capable of operating 1,000 units of existent light water cooled reactors with 1,000,000 kW of electric power for about 100 years. As a result nearly about 15,000,000 tons of degraded uranium and 15,000 tons of fissionable Pu will be left. Accordingly, a power reactor (RBWR) with a breeding ratio of 1.0 can continue nuclear fission for degraded uranium using fuel containing Pu-enriched degraded uranium under the catalyst-like effect of Pu with inside and outside inventory of fissionable Pu at 10 tons per 1,000,000 kW electric power. Since uranium generates a heat energy of about 1 MWD per 1 g, 1,500 units of RBWR can be operated for 10,000 years and the entire uranium resources can be used up thoroughly so that it can contribute to the stable long time supply. According to studies of the present inventors, for the relationship between the breeding ratio and the effective water-to-fuel volume ratio in the reactor core of a light water cooled reactor, the following has been found: The effective water-to-fuel volume ratio [(Vm/Vf)eff] is extended from the geometrical water-to-fuel volume ratio [(Vm/Vf)geo; water-to-fuel volume ratio not generating steam void] considering generation of steam void in the reactor core. Assuming the decreasing ratio of hydrogen density due to generation of the steam void as F, there is the following relationship between both of them. (Vm/Vf )eff = F(Vm/Vf )geo Further, F has the following relationship with the average steam void fraction V (%) for reactor core. F = (100 - V)/100 + f V/100 where f: ratio of saturated steam density to saturated water density. Generally, f is a value as small as about 1/20 and F can be approximated as below. F = (100 - V)/100. Fig. 2 shows a conversion ratio defined from the effective water-to-fuel volume ratio and neutron balance, and relationship between each of three factors constituting the conversion ratio. Conversion ratio = α (1 + β) - (1 + γ) where α : number of new neutrons generated when a neutron is absorbed in a fissionable material and one fissionable material is annihilated. β: additional contribution by nuclear fission of fuel fertile material in a high speed energy region. γ: ratio of wasteful neutron capture relative to neutron absorption amount by fissionable material (including neutron leakage). In light water cooled reactors now under operation, the effective water-to-fuel volume ratio is about 2.0 and the breeding ratio is about 0.5. For attaining a breeding ratio of about 1, it is necessary to raise the conversion ratio to about 1. According to the studies of the present inventors, it has been found that the breeding ratio of about 1 at a conversion ratio of 0.85 or more can be achieved by increasing the enrichment of fissionable Pu within a range to be described later and increasing the neutron leakage of the blanket. The effective water-to-fuel volume ratio for this purpose is 0.6 or less. On the other hand, for obtaining an effective water-to-fuel volume ratio of 0.1 or less, the average steam void fraction for the reactor core has to exceed 70%, so that two phase flow states can no more be maintained at the exit of the reactor core upon transient event. The effective water-to-fuel volume ratio of 0.1 to 0.6 can be attained by densely arranging fuel rods, utilizing the steam void generated in the reactor core or, if the control rod is not inserted, by inserting a follower at a control rod insertion position to exclude the moderator, or by the combination of such three means. Fig. 3 shows an embodiment of a relationship between the fuel rod gap and the geometrical water-to-fuel volume ratio. In Fig. 3, the diameter of the fuel rod is within a range of about 9.5 to 12.3 mm which is used at present in light water cooled reactors and a regular triangular fuel rod lattice is adopted. If the fuel rod gap is reduced to 2 mm or less, (Vm/Vf)geo of the fuel rod lattice is about 0.9 or less. In the case of a fuel assembly having fuel rods arranged densely in a regular triangular lattice pattern, (Vm/Vf)geo of the fuel assembly takes a value greater by 0.1 to 0.2 than (Vm/Vf)geo of the fuel rod lattice, in view of a gap region or a control rod insertion region between the fuel assemblies. Accordingly, for attaining the effective water-to-fuel volume ratio of 0.6 or less under the geometrical water-to-fuel volume ratio, it is necessary to increase the average steam void fraction for the reactor core to 45% or more in view of equation 1 (steam weight ratio at the exit of the reactor core is 20% or more in view of the relationship shown in Fig. 10). On the other hand, at a fuel rod gap within a range from 0.7 (minimum value for the fuel rod gap considering manufacture of the fuel assembly and ensurance for the thermal margin) to 1.0 mm (it can be more than 1.0 mm in a case where the diameter of the fuel rod is greater than 9.5 mm), (Vm/Vf)geo can be reduced to about 0.6 or less at a steam void fraction of 0%. Further, as a way for controlling the reactivity, it may be considered a method of inserting a Y-type control rod at the periphery of the hexagonal fuel assembly. A reactor core having the breeding ratio of 1.0 can be attained by the combination of the foregoing means. According to the studies of the present inventors, a reactor core constitution in which fuel rods are arranged densely in a regular triangular lattice pattern can be attained also by combining a hexagonal fuel assembly and a Y-type control rod. The Y-type control rod and the hexagonal fuel assembly can be combined by a method of making a fuel assembly with a regular hexagonal shape and by a method of constituting a regular hexagonal shape with one wing of the Y-type control rod and a fuel assembly. The former has the merit to simplify the constitution of the fuel assembly and the latter has the merit of bringing the central position of the assembly in the reactor core in a regular triangular shape. In the following, effects of the present invention in accordance with above-mentioned features of the present invention are explained. According to the present invention, degraded uranium, natural uranium, depleted uranium or low concentrated uranium can be burnt under a catalyst-like effect of Pu by attaining a breeding ratio of about 1.0 or more using fuel formed by adding Pu to degraded uranium, natural uranium, depleted uranium or low concentrated uranium, thereby enabling to contribute to stable long time energy supply. Further, since the effective water-to-fuel volume ratio of 0.1 to 0.6 is provided by the combination of the dense hexagonal fuel assembly comprising fuels formed by adding Pu to degraded uranium, natural uranium, depleted uranium or low concentrated uranium, coolants at high void fraction of 45% to 70% and the cluster-type, Y-type control rods, a breeding ratio of about 1.0 or more can be attained, thereby enabling to contribute to the stable long time energy supply. Further, since the diameter of the pressure vessel, the operation conditions such as power and the materials used are substantially identical with those in BWR now under operation, the power generation cost can be kept to about an identical level as that in existent BWR although the performance is greatly improved. The reactor core comprising the hexagonal fuel assembly and the Y-type control rod inserted between the fuel assemblies can utilize the technique in existent BWR of inserting from the lower portion of the reactor core as it is. Further, in the hexagonal or eclipsed hexagonal fuel assembly, constitution of multiple regions, particularly, 2 to 5 regions ranging from a region in the vicinity of the Y-type control rod to a region apart from the control rod with 2 to 5 kinds of fuel rods having varied fissionable Pu enrichments can reduce the power peaking in the fuel assembly and enhance the thermal margin. Further, the increase of the reactor core power density from 100 to 300 kW/l can reduce the Pu inventory amount per unit power and increase the capacity of the power generation facilities of the present invention that can be operated for a predetermined Pu, thereby contributing to the stable long time energy supply. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a horizontal cross sectional view of a first embodiment of a reactor core according to the present invention. Fig. 2 is a characteristic graph representing a relationship between a fuel rod lattice constant and an effective water-to-fuel volume ratio required for representing the conversion ratio. Fig. 3 is a characteristic graph representing the relationship between a fuel rod gap and a geometrical water-to-fuel volume ratio. Fig. 4 is a horizontal cross sectional view of a fuel assembly loaded in the reactor core shown in Fig. 1. Fig. 5 is a distribution of the enrichment of fissionable Pu in the axial direction of the outer reactor core region Fig. 6 is a horizontal cross sectional view of a reactor core in a second embodiment according to the present invention. Fig. 7 is a horizontal cross sectional view of fuel assemblies loaded in the reactor core shown in Fig. 6. Fig. 8 is a characteristic chart illustrating a relationship between the average void fraction in the reactor core and a steam weight ratio at a reactor core exit. 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. 4. 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 assemblies are shown. Fig. 4 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. 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. 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 assembly of 55 cm for the reactor core portion irrespective of the dense arrangement, Pu inventory is as small as 4.4 tons being converted as the amount of fissionable Pu per 1,000,000 kWe power and it is 10 tons 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. This embodiment can attain a 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 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 a 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 a BWR under operation, about 85% of nuclear reactions occur in a thermal neutron region at 0.6 eV or less, whereas the central energy value causing from nuclear reactions in this embodiment is about 1 keV, and the reaction ratio in the resonance region is extremely high. Therefore, while a BWR now under operation has a Doppler coefficient of 1.6 x 10 -5 Δ k/k/°C, the value in this embodiment is 3.7 x 10 -5 Δ k/k/°C which is about twice. The existent BWR now under operation has a void coefficient of -7.0 x 10 -4 Δ k/k/% void, whereas an absolute value in this embodiment is set as low as -0.5 x 10 -4 Δ k/k/% void. As a result, a thermal margin is relatively increased, for embodiment, in an event of pressure elevation or temperature lowering of coolants. 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 giving the reactor core a height of 55 cm, so that stable long time energy supply can be attained by the 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 assemblies 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, the distillation function by boiling can be maintained to confine radioactivated materials within the 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 to the constitution, the function and the effect to fuel 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 further 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. Description will be made to the second embodiment according to the present invention with reference to Fig. 6 and 7. Fig. 6 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. 7 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. 6 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. In this embodiment, the fuel assembly has 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 the space for 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 an upper and a lower region at 8/12 from the lower end of the reactor core, but it is not restrictive. Fig. 5 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 a 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.

Claims: A control rod (2) to be inserted longitudinally between hexagonal fuel assemblies loaded to a reactor core for a light water cooled nuclear reactor, comprising longitudinal wings (5) which include angles of 120 degrees in a plane perpendicular to the longitudinal axis of the control rod (2), characterised in that the wings are arranged such that prolongations of the wings constitute a regular triangle in said plane in the central section of the control rod (2). A reactor core for a light water cooled nuclear reactor having hexagonal fuel assemblies (1; 6; 9) containing fuels comprising uranium enriched with plutonium or plutonium and actinoid nuclides and Y-type control rods, wherein each of the Y-type control rods is a control rod according to claim 1, and each of the hexagonal fuel assemblies (1; 6; 9) has fuel rods (3) arranged in regular triangular lattice pattern so as to form three sets of fuel rod rows, each set of fuel rod rows being parallel with a pair of opposing sides of a hexagonal fuel assembly, and the number of rows in two sets of fuel rod rows being equal to each other and being greater by one row than that of the remaining one set of fuel rod rows among the three sets of fuel rod rows. A reactor core according to claim 2, wherein each wing (5) of the Y-type control rod is arranged to be parallel with each remaining one set of fuel rod rows of three hexagonal fuel assemblies (1; 6; 9) adjacent to the Y-type control rod (2). A reactor core according to claim 2 or 3, wherein the effective water-to-fuel volume ratio is between 0.1 and 0.6. A reactor core according to claim 2 or 3, wherein an average void ratio for the reactor core during operation at 50 % or more rated power is from 45 % to 70 %.

Inventors: AOYAMA MOTOO, MORIWAKI MASANAO, TAKEDA RENZO, AOYAMA, MOTOO, MORIWAKI, MASANAO, TAKEDA, RENZO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1995
Main classification: G21C7
IPC codes: G21C3, G21C7