JP5112265B2 - Fuel assembly, manufacturing method thereof, and uranium powder - Google Patents

Description

  The present invention relates to a fuel assembly loaded in a nuclear reactor, a manufacturing method thereof, and uranium powder.

  It is desirable to increase the enrichment of uranium in the fuel in order to increase the output of the nuclear power plant and extend the operating cycle, and to improve the economy by suppressing the number of spent fuel assemblies. . Increasing uranium enrichment, that is, increasing the concentration of 235U in uranium, reduces the number of new fuel assembly replacements and the amount of spent fuel assemblies required to obtain the same amount of power generation. , Greatly contribute to reducing fuel cycle costs.

  However, light water reactor fuel assembly processing facilities put to practical use on a commercial scale are generally designed to ensure criticality safety with uranium enrichment as an upper limit of 5%. Safety reviews of such facilities are conducted based on the “Uranium Processing Facility Safety Review Guidelines”, and it is confirmed by the review that critical safety has been secured, and the installation of the facility is permitted. . On the other hand, facilities that handle uranium with an enrichment level exceeding 5% are subject to stricter regulations based on the “Safety Review Guidelines for Specific Uranium Processing Facilities”.

Therefore, in order to use a fuel assembly using uranium with an enrichment exceeding 5% in a nuclear reactor, from the viewpoint of criticality management, it is necessary to make a major design change and modification of the processing facility, which increases costs. there is a possibility. In addition, when uranium with an enrichment level exceeding 5% is used, design changes and facility modifications are required in each process of new fuel transportation and new fuel storage, which may increase costs. In this case, the fuel cycle cost reduction effect due to the increase in the concentration of the reactor fuel may be offset.
Japanese Patent Laid-Open No. 2004-177241 Japanese Patent Laid-Open No. 4-212093 Published by Nikkan Shobo, October 31, 1988, "Critical Safety Handbook" edited by Nuclear Safety Regulation Division, Nuclear Safety Bureau, Science and Technology Agency

  In order to adopt a reactor fuel with an enrichment of uranium exceeding 5 wt%, the cost increases with design changes and facility modifications in each process from the viewpoint of criticality control, and the fuel due to the increase in the enrichment of reactor fuel. The cycle cost reduction effect may be offset. For this reason, measures corresponding to this are required. In adopting a reactor fuel having a uranium enrichment of more than 5 wt%, it is practical that the upper limit of the uranium enrichment of commercial light water reactor fuel is about 10 wt%.

  In order to handle nuclear fuel with uranium enrichment exceeding 5 wt% at the current processing facility, the neutron multiplication factor of the mixture is increased by adding a substance with a large neutron absorption cross section to uranium. There is a method to make it equal to 5% or less of uranium. When using a substance having a large neutron absorption cross section to reduce the neutron multiplication factor of the fuel, it is preferable that the amount used to minimize the influence of the neutron absorption effect on the neutron behavior of the core during the operation cycle is as small as possible.

Examples of the material having a large neutron absorption cross section include gadolinium (Gd), which is currently used for the purpose of controlling the reactivity during the operation cycle. Since gadolinium has a very large neutron absorption cross section, the neutron multiplication factor of the fuel can be reduced even with a small amount. However, when a small amount of gadolinia (Gd ₂ O ₃ ) is added to uranium, if the concentration distribution of gadolinia becomes non-uniform in the mixed powder with uranium dioxide or sintered pellets, There is a possibility that the suppression effect cannot be exhibited. On the other hand, if the concentration of a substance with a large neutron absorption cross section is increased to compensate for the reduction in the effect of suppressing the neutron multiplication factor due to such inhomogeneities, the reactivity during the operation cycle decreases and the fuel cycle cost increases. there's a possibility that.

Patent Document 1 discloses a method for ensuring critical safety by adding a small amount of erbium oxide (Er ₂ O ₃ ) having a smaller neutron absorption cross-section than gadolinia (Gd ₂ O ₃ ) to UO ₂ pellets. Has been. Also, critical safety can be ensured by a method of applying boron (B) or the like disclosed in Patent Document 2 to the surface of the UO ₂ pellet or the inside of the fuel cladding tube.

  However, the thermal neutron absorption cross section at room temperature (0.025 eV) is about 640 burns with Er-167, about 3,840 burns with B-10, and 254,080 burns with Gd-157, the isotope of Gd. Much smaller than For this reason, when erbium or boron is used as nuclear fuel, depending on the concentration of the flammable poisons, there is a possibility that the flammable poisons remain at the end of the operation cycle and the reactivity loss of the core occurs. When flammable poisons remain at the end of the operation cycle, it becomes difficult to exert the cost reduction effect of the fuel cycle due to the high enrichment of uranium.

  In view of the above, an object of the present invention is to enable criticality management without significant cost increase when uranium having a enrichment of more than 5 wt% is used for nuclear fuel.

In order to achieve the above-mentioned object, the present invention provides a fuel assembly having a fuel element, wherein the fuel element has a chemical formula A _1-X Gd _X O _2-0 with gadolinium and a rare earth element A excluding gadolinium _{. 5X} or a Gd composite oxide represented by the chemical formula A _1-X Gd _X O _1.5 and a uranium oxide having a concentration of more than 5% but not more than 10% , and the content ratio of gadolinium is the Gd composite Converted to the case where gadolinium in the oxide is present as a single oxide, it is equal to or higher than the lower limit obtained by multiplying an increment from 5 wt% of the enrichment of the uranium by 61 × 10 ⁻⁴ and less than 0.1%. It is characterized by that.
Further, the present invention provides a fuel assembly having a fuel element, the fuel element has the formula _{A 1-X Gd} and rare earth element A, except for gadolinium and gadolinium _{X O 2-0.5X} or formula _{A 1-} Containing a Gd composite oxide represented by _X Gd _X O _1.5 and an oxide of uranium with a concentration exceeding 5%, wherein the rare earth element A is either lanthanum or cerium, and the oxide of uranium The ratio of the Gd composite oxide to the total of the Gd composite oxide is 0.1 wt% or more and 0.3 wt% or less, and the ratio of gadolinium in the Gd composite oxide is 5 wt% or more and 33 wt% or less. It is characterized by that.

In addition, the fuel assembly manufacturing method according to the present invention has a chemical formula A _1-X Gd _X O of uranium oxide having a concentration of more than 5% and not more than 10% and rare earth element A excluding gadolinium and gadolinium. _2-0.5X or the chemical formula A _1-X Gd _X O _1.5 , and the content ratio of gadolinium is calculated when gadolinium in the Gd composite oxide is present as a single oxide. A step of mixing a Gd composite oxide which is equal to or higher than a lower limit value obtained by multiplying an increment from 5 wt% of the enrichment of the uranium by 61 × 10 ⁻⁴ and less than 0.1%. .
A method of manufacturing a fuel assembly according to the present invention, the chemical formula _{A 1-X Gd} X _{O 2-0.5X} or Formula _{A 1-X} Gd _{X O 1.5} of a rare earth element A, except for gadolinium and gadolinium The rare earth element A is either lanthanum or cerium, and the ratio of the uranium oxide to the sum of the uranium oxide is 0.1 wt% or more and 0.3 wt% or less, and 5 wt% or more and 33 wt% or less. It is characterized by comprising a step of mixing a Gd composite oxide containing a proportion of gadolinium and a uranium oxide having a concentration exceeding 5%.

Moreover, uranium powder according to the present invention, Gd complex represented by the chemical formula _{A 1-X Gd} X _{O 2-0.5X} or Formula _{A 1-X} Gd _{X O 1.5} of a rare earth element A, except for gadolinium and gadolinium beyond oxide and 5% containing an oxide of less than 10% enrichment of uranium, the content of the gadolinium in terms when gadolinium in the Gd compound oxide exists as an oxide alone It is not less than the lower limit obtained by multiplying the increment from 5 wt% of the enrichment of the uranium by 61 × 10 ⁻⁴ and less than 0.1% .
Further, the uranium powder according to the present invention is a Gd compound represented by the chemical formula A _1-X Gd _X O _2-0.5X or the chemical formula A _1-X Gd _X O _1.5 with rare earth element A excluding gadolinium and gadolinium. The rare earth element A is either lanthanum or cerium, and the Gd with respect to the sum of the uranium oxide and the Gd composite oxide. The ratio of the composite oxide is 0.1 wt% or more and 0.3 wt% or less, and the ratio of gadolinium in the Gd composite oxide is 5 wt% or more and 33 wt% or less.

  According to the present invention, when uranium having a concentration of more than 5 wt% is used for nuclear fuel, criticality management can be performed without a significant cost increase.

  An embodiment of a fuel assembly according to the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same or similar structure, and the overlapping description is abbreviate | omitted.

[First Embodiment]
FIG. 2 is a longitudinal sectional view of the fuel rod used in the first embodiment of the fuel assembly according to the present invention.

  The fuel rod in the present embodiment has a cladding tube 21, a pellet 23, a lower end plug 22, an upper end plug 25, and a plenum spring 24. The cladding tube 21 is a tube formed of a zirconium alloy into a cylinder. The pellet 23 is obtained by baking and hardening a uranium oxide or the like into a cylindrical shape having a height of about 1 cm, for example. A plurality of pellets 23 are accommodated inside the cladding tube 21. A lower end plug 22 is welded to the lower end of the cladding tube 21. An upper end plug 25 is welded to the upper end of the cladding tube 21. A plenum spring 24 is disposed between the upper end plug 25 and the end face of the pellet 23, and the pellet 23 is pressed and supported.

  FIG. 1 is a cross-sectional view of the fuel assembly of the present embodiment. FIG. 1 also shows the positional relationship with the control rod 40 when this fuel assembly is loaded in a nuclear reactor.

  The fuel assembly 10 of the present embodiment is loaded into a boiling water reactor (BWR) with the channel box 41 attached. The fuel assembly 10 includes fuel rods 13, 14, 15, 16, 17, 18, 19 and a water rod 11. The water rod 11 is a tube formed of, for example, a zirconium alloy so that water flows inside. This fuel assembly 10 is a design example of a replacement fuel assembly with an average uranium enrichment of about 6.2 wt%, assuming an operation cycle of 2 years and an average removal burnup of about 70 GWd / t.

  The 74 fuel rods 13, 14, 15, 16, 17, 18, 19 and the two water rods 11 are bundled, for example, in a 9 × 9 square lattice. The two water rods 11 are disposed in the vicinity of the center of the cross section of the fuel assembly 10 at positions that occupy seven square lattices. The 74 fuel rods 13, 14, 15, 16, 17, 18, 19 as fuel elements are arranged at the positions of the remaining square lattice. The fuel rods 13, 14, 15, 16, 17, 18, 19 and the water rod 11 are supported at their upper and lower ends by tie plates, and by spacers provided at, for example, seven locations between the tie plates. Movement in the cross-sectional direction is restricted.

  The 74 fuel rods 13, 14, 15, 16, 17, 18, 19 are divided into uranium fuel rods 13, 14, 15, high gadolinia fuel rods 19, and low gadolinia fuel rods 16, 17, 18. Can be classified. The uranium fuel rods 13, 14, and 15 are fuel rods containing pellets 23 formed of uranium oxide having a concentration of 5 wt% or less. In this embodiment, as the uranium fuel rods 13, 14, and 15, the uranium fuel rod 13 having a uranium enrichment of 3 wt%, the uranium fuel rod 14 having a uranium enrichment of 4 wt%, and the uranium enrichment of 5 wt%. Three types of uranium fuel rods 15% are used.

The high gadolinia fuel rod 19 is a fuel rod containing a pellet 23 containing a relatively high concentration of gadolinia (Gd ₂ O ₃ ) in uranium oxide. The gadolinia contained in the pellets 23 accommodated in the high gadolinia fuel rods 19 is introduced for the purpose of flattening the excess reactivity of the core during the operation cycle of the nuclear reactor. The content ratio in the gadolinia pellets introduced for such a purpose is generally 1 wt% or more. Further, uranium with a concentration of 6 wt% is used for the pellets 23 stored in the high gadolinia fuel rods 19.

The low gadolinia fuel rods 16, 17, and 18 contain pellets 3 containing uranium oxide having a concentration exceeding 5% and Gd composite oxide of rare earth elements excluding gadolinium and gadolinium. The gadolinium contained in the low gadolinia fuel rods 16, 17 and 18 is introduced mainly for the purpose of ensuring critical safety until the reactor is loaded, such as fuel processing and transportation. It is generally less than 0.1 wt%. In the present embodiment, as the low gadolinia fuel rods 16, 17, 18, the fuel rod 16 using UO ₂ having a concentration of 6 wt% added with 53 ppm of complex oxide in gadolinia conversion, and Gd in gadolinia conversion of 110 ppm. A fuel rod 17 using UO ₂ with a concentration of 7 wt% added with a complex oxide and a fuel rod 18 using UO ₂ with a concentration of 8 wt% added with a Gd complex oxide of 170 ppm in gadolinia conversion are used. ing.

Here, the composite oxide refers to an oxide that is two or more kinds of metal oxides and maintains a stoichiometric composition even in a microstructure. The Gd composite oxide is an oxide of gadolinium Gd and another rare earth element A, and the chemical formula when the ratio of Gd to the number of atoms of the rare earth element A is X is A _1-X Gd _X It is an oxide represented by O _2-0.5X or A _1-X Gd _X O _1.5 . As the rare earth element A, for example, cerium Ce or lanthanum La can be used. Also, the gadolinia terms and to refer to terms in a state separated gadolinia (Gd 2 _{O 3)} from Gd composite oxide. The equivalent gadolinia concentration is a concentration when Gd in the Gd composite oxide exists as a single gadolinia.

In the criticality safety handbook (Non-patent Document 1), “complete submersion” is assumed as the severest case in terms of criticality safety. This “completely submerged” state is a state in which water penetrates into the gaps between the nuclear fuel materials and is surrounded by water. In such a “completely submerged” state, the “minimum estimation” is used as the limit value in “mass management” or “geometry management” in the entire uranium concentration range for a homogeneous UO ₂ / H ₂ O system. "Critical value" and "Minimum estimated critical lower limit value" are defined. “Mass management” is management that does not handle reactor fuel exceeding a certain mass, which is a critical safety limitation. “Shape and dimension management” refers to management that does not handle reactor fuel that exceeds a certain shape and dimension. These limit values in the process of handling UO ₂ powder are shown in Table 1.

  Here, the estimated critical value is a value that is determined to be critical if the mass, shape, or dimension is the value, and the estimated critical lower limit value is determined to be subcritical if the mass, shape, or dimension is less than the value. Each value in Table 1 is shown as the minimum value in the entire uranium concentration range.

In the present embodiment, for UO ₂ powder handled in a fuel molding processing facility, a small amount of Gd composite oxide, for example, Gd composite oxide corresponding to gadolinia of less than 0.1 wt% is added to UO ₂ powder having a uranium enrichment of more than 5 wt%. Is added uniformly. As a result, the nuclear fuel is set so as to be equal to or smaller than the maximum value of the effective neutron multiplication factor in the limit value of mass management or shape dimension control that is a critical safety limit of UO ₂ powder with a uranium enrichment of 5 wt%. Perform criticality management of cycle facilities. That is, each neutron effective multiplication factor of mass management or shape dimension management of UO ₂ powder having a uranium enrichment level of more than 5 wt% corresponds to each of mass management or shape dimension management of UO ₂ powder at uranium enrichment level of 5 wt% in Table 1. The critical safety of UO ₂ powder with enrichment exceeding 5 wt% by adding a small amount of gadolinia to UO ₂ powder with enrichment exceeding 5 wt% so that it is equal to or less than the maximum neutron effective multiplication factor Is set equal to the critical safety limit of UO ₂ powder having a concentration of 5 wt%.

  It should be noted that the enrichment of uranium, which is used as a criterion for the effective neutron multiplication factor as a critical safety limiting condition, is not limited to 5 wt%, and may be 5 wt% or less. For example, the neutron effective multiplication factor with the uranium enrichment of 4.5 wt% may be used as a reference in consideration of manufacturing tolerance of uranium enrichment. However, it should be noted that as the standard uranium enrichment becomes smaller than 5 wt%, the limit on critical safety becomes excessive and the amount of gadolinia added becomes excessive.

FIG. 3 is a graph showing the effective neutron multiplication factor when a UO ₂ powder having a metal amount of 33 kg is made into a sphere and has a water reflection thickness of 30 cm.

FIG. 3 shows that the amount of metal which is the minimum estimated critical lower limit value of 5 wt% enrichment among the uranium enrichments (3 wt%, 4 wt%, 5 wt%, 10 wt%, 20 wt%) shown in Table 1 is 33 kg. using UO ₂ powder is an example of calculating the equivalent gadolinia concentration related to mass control. The maximum value of neutron effective multiplication factor of UO ₂ powder having a uranium enrichment of 5 wt% and a metal uranium amount of 33 kg in the entire uranium concentration range shown in Table 1 is defined as kmax. FIG. 3 shows the relationship between the uranium enrichment and the equivalent gadolinia concentration when the effective neutron multiplication factor of UO ₂ powder with a uranium enrichment of more than 5 wt% to which this kmax is added and a small amount of gadolinia is equivalent or small. It is shown.

The relationship between the uranium enrichment and the equivalent gadolinia concentration is calculated using the uranium concentration (or sphere volume) as a parameter in a spherical homogeneous system in which the gap of UO ₂ powder is filled with water and water reflection conditions. When performed, the concentrations of equivalent gadolinia added to the UO ₂ powder for the cases of uranium enrichment 6 wt%, 7 wt%, 8 wt% and 10 wt% are 53 ppm, 110 ppm, 170 ppm and 305 ppm, respectively. Here, 1 ppm = 1 × 10 ⁻⁴ wt%.

Further, under the same equivalent gadolinia concentration conditions, an infinite column diameter of 24.4 cm, an infinite plate thickness of 11.2 cm, and a spherical radius of 24.0 cm of UO ₂ powder having a uranium concentration of 5 wt% shown in Table 1 were given. When the neutron transport calculation is performed using the neutron as a parameter, the effective neutron multiplication factor of UO ₂ powder with a uranium enrichment of over 5 wt% containing a small amount of gadolinia is the maximum of the effective neutron multiplication factor of UO ₂ powder with a uranium enrichment of 5 wt% The value is smaller than the value, and the restriction condition is satisfied similarly.

The fuel pellets produced following the UO ₂ powder process in the fuel fabrication facility performs neutron transport calculations with the conditions of the same equivalent gadolinia concentration for each stage of the nuclear reactor fuel rods assembled and fuel assembly assembled in UO ₂ powder process It was confirmed that the effect of suppressing the effective neutron multiplication factor was the smallest value. That is, by using the equivalent gadolinia concentration set in the UO ₂ powder process, the arrangement of fuel pellets using reactor fuel with a uranium enrichment of more than 5 wt%, and the bundle of reactor fuel rods in the fuel assembly assembly stage The neutron effective multiplication factor of the UO ₂ system for the bundle of reactor fuel rods and the assembled fuel assembly is suppressed below the neutron effective multiplication factor of fuel pellets, reactor fuel rods and fuel assemblies with 5 wt% uranium enrichment, respectively. As well as satisfying the limiting conditions.

FIG. 4 is a diagram showing the relationship between the equivalent gadolinia concentration to be added to UO ₂ powder having a concentration level exceeding 5 wt% and the uranium concentration level in the present embodiment.

As described above, when the enrichment of uranium is 6, 7, 8, 10 wt%, if the concentration of equivalent gadolinia added is 53, 110, 170, 305 ppm, the effective neutron multiplication factor is uranium with a enrichment of 5 wt%. Less than or equal to In addition, the equivalent gadolinia concentration to be added is 5 ppm for UO ₂ powder having a concentration of 5 wt%. When these points are connected, as shown in FIG. 4, the relationship between the increment from 5 wt% of the uranium enrichment of the UO ₂ powder having a concentration exceeding 5 wt% and the concentration of equivalent gadolinia to be added is almost proportional.

Therefore, if this relationship is proportional, the proportionality constant with respect to the increment from 5 wt% of the uranium enrichment of the UO ₂ powder with the enrichment exceeding 5 wt% is the equivalent to be added when the uranium enrichment is 10 wt%. The gadolinia concentration of 305 × 10 ⁻⁴ wt% (= 305 ppm) is divided by an increment from 5 wt% of the uranium enrichment of 10 wt%, that is, 5 wt%, to be 61 × 10 ⁻⁴ . That is, the equivalent gadolinia concentration (wt%) to be easily added can be obtained by multiplying the increment (wt%) from 5 wt% of the uranium enrichment of the UO ₂ powder with the enrichment exceeding 5 wt% by 61 × 10 ^−4. Can be sought.

As shown in FIG. 4, the gradient of the equivalent gadolinia concentration to be added to the UO ₂ powder having a concentration exceeding 5 wt% increases as the concentration of uranium increases. For this reason, the method using the approximate straight line in this way calculates the equivalent gadolinia concentration higher than the method of calculating the equivalent gadolinia concentration corresponding to the enrichment of each uranium by performing neutron transport calculation. . Therefore, it is a management method on the safe side from the viewpoint of criticality management.

As described above, according to the present embodiment, a small amount of Gd composite oxide is uniformly added from the stage of UO ₂ powder to uranium having a concentration of more than 5 wt%. It can be handled in the same manner as UO ₂ powder having a uranium enrichment of 5 wt%. Then, not only the handling of the UO ₂ powder in the fuel fabrication facility, the fuel pellets fabrication, 5 wt% reactor fuel equivalent critical even in nuclear reactor fuel rod fabrication and fuel assembly assembly and the manufacturing process of storage, etc. Management can be performed. As a result, the neutron effective multiplication factor is suppressed below the limit value that ensures subcriticality in the processes of fuel molding, new fuel transportation, and new fuel storage. An increase in costs can be suppressed.

  FIG. 5 is a graph showing the infinite multiplication factor during the output operation of the fuel assembly of the present embodiment. This graph shows a case where the void ratio is 40%, and FIG. 5 shows that the average uranium enrichment without using a small amount of gadolinia of 0.1 wt% or less is about 6.2 wt% for comparison. The infinite multiplication factor of the design example of the fuel assembly is also shown.

  In FIG. 5, line A shows the burnup and infinite multiplication factor in a fuel assembly not using a small amount of gadolinia. Line B shows the burnup and infinite multiplication factor in the fuel assembly of the present embodiment.

In FIG. 5, line C represents the equivalent gadolinia concentration contained in the reactor fuel rod using UO ₂ powder with a uranium enrichment of more than 5 wt% among the reactor fuel rods provided in the fuel assembly of the present embodiment. For example, the burnup and infinite multiplication factor in the fuel assembly doubled are shown. That is, the fuel assemblies to be infinite multiplication factor of the line C is equivalent gadolinia concentration and fuel rods with uranium enrichment 6 wt% of UO ₂ is at 106 ppm, equivalent gadolinia concentration uranium enrichment 7 wt% of UO ₂ with 220ppm And a fuel rod using UO ₂ having an equivalent gadolinia concentration of 340 ppm and a uranium enrichment of 8 wt%.

In FIG. 5, a line D represents a fuel in which the equivalent gadolinia concentration contained in a fuel rod using UO ₂ having a uranium enrichment of more than 5 wt% is tripled among the nuclear fuel rods provided in the fuel assembly of the present embodiment. The burnup and infinite multiplication factor in the assembly are shown. That is, the fuel assemblies of the line D includes a fuel rod equivalent gadolinia concentration using uranium enrichment 6 wt% of UO ₂ in 159 ppm, the fuel rods equivalent gadolinia concentration using uranium enrichment 7 wt% of UO ₂ with 330ppm And a fuel rod using UO ₂ having an equivalent gadolinia concentration of 510 ppm and a uranium enrichment of 8 wt%.

  As shown in FIG. 5, the difference of the infinite multiplication factor (k) of the fuel assembly of line B with respect to the infinite multiplication factor of the fuel assembly not using a small amount of gadolinia shown by line A is about 1% Δk at the beginning of combustion. The effect on the reactivity of the core is small. That is, in the fuel assembly of the present embodiment, the difference in infinite multiplication factor due to the use of a small amount of gadolinia is about 1% Δk, and it is necessary to change the conventional design specially because a small amount of gadolinia is used. Absent.

In addition, as shown by lines C and C, in the fuel assemblies in which the concentration of gadolinia contained in fuel rods using UO ₂ with a uranium enrichment over 5 wt% is doubled and tripled, the difference in infinite multiplication factor Are about 2% Δk and about 3% Δk, respectively, and it is not necessary to change the conventional design as in the fuel assembly of the present embodiment, or the number, concentration or the number of high gadolinia fuel rods 19 It is possible to cope with minor design changes such as placement.

  Further, the infinite multiplication factor of the fuel assembly according to the present embodiment quickly decreases with combustion for a fuel assembly not using a very small amount of gadolinia, and has a cycle burnup of about 5 GWd / t (1/2 year operation) Equivalent) Above, there is almost no difference. Therefore, the loss of reactivity due to the addition of a small amount of gadolinia at the end of the operation cycle can be ignored.

  FIG. 6 is a graph showing an equivalent gadolinia concentration range to be added to the low gadolinia fuel rod in the present embodiment.

  As can be seen from FIG. 5, the uranium enrichment is 5 if the minute amount of the equivalent gadolinia added to the low gadolinia fuel rods 16, 17, 18 is between three times the value of the present embodiment. It is possible to handle processing, transportation, etc. at the same facilities and equipment as fuel rods of less than%, and the reactivity loss during the operation cycle is negligible. Therefore, by using uranium with enrichment exceeding 5% mixed with a small amount of gadolinia in the range shown by the diagonal lines in FIG. 6, it can be processed and processed at the same facilities and equipment as fuel rods with uranium enrichment of 5% or less. Handling such as transportation is possible, and the reactivity loss during the operation cycle is negligible. For this reason, uranium with a concentration exceeding 5% can be used without significant changes in processing facilities, transportation equipment, and the like. Further, since the reactivity loss during the operation cycle can be ignored, the fuel cycle cost can be further reduced.

  For example, the equivalent gadolinia concentration at a uranium enrichment of 10 wt% can range from 305 ppm to 915 ppm, which is three times that value. Even in each enrichment in which the uranium enrichment exceeds 5 wt%, it can be in a range up to three times the concentration of gadolinia of the present embodiment. Assuming that the upper limit of uranium enrichment in commercial light water reactor fuel is practically around 10 wt% when adopting a reactor fuel with a uranium enrichment over 5 wt%, there is a trace amount in the reactor fuel with uranium enrichment exceeding 5 wt%. The concentration of gadolinia added to is about 0.1 wt%.

The nuclear fuel according to the present embodiment can be treated as having an enrichment of 5 wt% or less for criticality management even if the enrichment is uranium having an enrichment exceeding 5 wt%. That is, a fuel rod using UO ₂ with a uranium enrichment of 5 wt% or more added with a small amount, for example, less than 0.1 wt% gadolinia, can be assembled by the same assembly process as a conventional fuel assembly, so Thus, a fuel assembly using uranium having a uranium enrichment exceeding 5 wt% can be obtained.

  In addition, since Gadolinia added in a small amount burns out quickly in the early stage of combustion, as a nuclear characteristic of the fuel assembly, by introducing a reactor fuel with a uranium enrichment exceeding 5 wt% without causing a loss of reactivity at the end of the operation cycle. The fuel economy can be improved at the same time.

  In the present embodiment, the low gadolinia fuel rods 16, 17, and 18 contain gadolinium, which is a flammable poison, as a complex oxide with other rare earth elements. Gadolinium has a larger neutron absorption cross section than other rare earth elements. For this reason, other rare earth elements that form composite oxides with gadolinium are used as diluents that dilute the effect of gadolinium as a neutron absorber. For example, the thermal neutron absorption cross section of cerium Ce or lanthanum La is about 10 burns and much smaller than that of Gd. Therefore, Gd is dominant in the neutron absorption characteristics of the Gd composite oxide containing cerium or lanthanum.

Such low gadolinia fuel rods 16, 17, 18 can be manufactured, for example, as follows. First, for Gd and lanthanum or cerium having a small neutron absorption cross section, ammonium carbonate is added to a mixed solution of each nitrate aqueous solution and coprecipitated to be thermally decomposed. Thereby, a composite oxide powder of Gd and another rare earth element is obtained. An oxide powder that is uniformly mixed can be obtained by diluting and mixing a complex oxide powder of Gd and another rare earth element with a predetermined amount of UO ₂ powder.

When gadolinium is contained in the pellet 23, if the content is small, the concentration distribution of the flammable poison in the UO ₂ powder or in the pellet 23 may be non-uniform. However, in the present embodiment, since Gd composite oxide particles in which Gd is diluted with a rare earth element having a small neutron absorption cross section are used, the number of powder particles for obtaining the same gadolinium concentration increases, and gadolinium Can be easily dispersed uniformly.

  Further, since Gd has a very large neutron absorption microscopic cross section, the larger the gadolinia particles, the more the self-shielding effect of neutron absorption increases and the neutron absorption efficiency decreases. For this reason, it is preferable that the diameter of the gadolinium-containing particles be as small as possible. In this embodiment, Gd composite oxide particles in which Gd is diluted with a rare earth element having a small neutron absorption cross section are used. Therefore, the density of Gd in the particles containing gadolinium is reduced to reduce the self-shielding effect. Therefore, the decrease in neutron absorption efficiency is suppressed.

FIG. 7 is a diagram schematically showing the number of Gd composite oxides when the equivalent gadolinia concentration in the pellet is the same and the Gd densities in the Gd composite oxides are different. FIG. 7A shows a high Gd density. In the case (b), the Gd density is low. FIG. 8 is a graph schematically showing the relationship between the total neutron absorption effect by Gd and the particle diameter of the Gd composite oxide in UO ₂ having a constant amount of Gd contained per unit volume. FIG. 8 shows two types of cases where the Gd density ρ in the Gd composite oxide particles is small and large, corresponding to (a) and (b) of FIG. 7.

  As schematically shown in FIG. 7, when the diameter of the Gd composite oxide particles is the same and the equivalent gadolinia concentration is the same, the number of the Gd composite oxide particles 51 having a low Gd density in the pellet 23 is equal to that of the Gd having a high Gd density. The number is larger than the number of composite oxide particles 52. That is, when the Gd composite oxide particles have the same diameter, if the amount of Gd contained per unit volume is constant, the number of powder particles increases as the Gd density ρ in the Gd composite oxide particles decreases. Are more uniformly dispersed.

  When the Gd density ρ in the Gd composite oxide particles is the same, the smaller the diameter of the Gd composite oxide particles, the greater the neutron absorption effect. When the diameters of the Gd composite oxide particles are the same, the neutron absorption effect increases as the Gd density ρ in the Gd composite oxide particles decreases.

For example, when the atomic ratio of Gd is X, the chemical formula of the complex oxide of Gd and Ce is Ce _1-X Gd _X O _2-0.5X . The chemical formula of the complex oxide of Gd and La is La _1-X Gd _X O _1.5 . When the Gd addition concentration is constant, the smaller the Gd ratio X in the Gd composite oxide, the lower the neutron absorption effect per powder particle and the higher the efficiency. On the other hand, the amount of Gd composite oxide increases. The total number of powder particles also increases. For this reason, it is extremely useful from the viewpoint of uniformly dispersing the flammable poison particles. On the other hand, in order not to reduce the inventory of UO ₂ in the fuel assembly as much as possible, it is preferable that the ratio of the Gd composite oxide is as small as possible.

In order to make the Gd dilution effect effective in the Gd composite oxide, the dilution is preferably about 3 times or more. That is, the maximum weight ratio of Gd in the Gd composite oxide is preferably about 33 wt%. On the other hand, if the enrichment of uranium is equal to or less than 10 wt%, because a low gadolinia gadolinia concentration required to enter the fuel rod 16, 17 and 18 is less than 0.1 wt%, it is mixed with the mixed powder or UO ₂ and UO ₂ The concentration of the Gd composite oxide in the pellets is preferably about 0.1 / 0.33 = 0.3 wt% at the maximum. Further, when excessively dilute, the amount of Gd compound oxide will reduce the inventory of too many UO _2, Gd weight ratio of Gd compound oxide is preferably set to 5 wt% or more.

In the present embodiment, since gadolinium is used together with the diluent, the number of particles of the Gd composite oxide introduced into the UO ₂ powder is larger than that when used as a single oxide of gadolinium. Become. For this reason, the uniformity can be easily increased upon mixing with UO ₂ powder. In addition, since gadolinium is a rare earth element, even if a complex oxide is formed with other rare earth elements, chemical characteristics and the like do not change significantly compared to gadolinium.

  The oxide powder thus obtained is treated in the same manner as uranium having a concentration of 5% even if the concentration of uranium exceeds 5 wt%, as long as Gd of a predetermined concentration is contained. be able to.

  In addition to cerium (Ce) and lanthanum (La), praseodymium (Pr) and yttrium (Y) can be used as rare earth elements having a small neutron absorption cross section. Further, scandium (Sc), neodymium (Nd), promethium (Pm), thulium (Tm), holmium (Ho), and ytterbium (Yb) may be used. In addition, erbium (Er), samarium (Sm), and europium (Eu), which are neutron absorbers, have a smaller neutron absorption cross section than gadolinium, and thus can be used as other rare earth elements.

  As described above, according to the present embodiment, a flammable poison having a large neutron absorption cross section is uniformly mixed with uranium having a concentration of more than 5 wt% as a Gd composite oxide with other rare earth elements. Addition reduces the impact on criticality control measures at nuclear fuel cycle facilities. For this reason, when uranium having a concentration of more than 5 wt% is used for nuclear fuel, criticality management can be performed without a significant cost increase. Therefore, it is possible to effectively improve the economy by effectively utilizing the fuel cycle cost reduction effect due to the increase in uranium enrichment.

[Second Embodiment]
FIG. 9 is a cross-sectional view of the fuel assembly according to the second embodiment of the present invention.

  The fuel assembly of the present embodiment is loaded into a pressurized water reactor. In this fuel assembly, 264 fuel rods 31 and 32 are bundled in a 17 × 17 square lattice, and control rod guide thimble 33 is arranged at the lattice position of 34, and is used for in-core instrumentation at the central lattice position. A guide thimble 34 is arranged. The fuel rods 31 and 32 are divided into a low gadolinia fuel rod 31 and a high gadolinia fuel rod 32.

  The low gadolinia fuel rod 31 accommodates pellets 23 (see FIG. 2) obtained by adding 53 ppm equivalent gadolinia as a Gd composite oxide with rare earth elements A except Gd to uranium having a concentration of 6 wt%. As the rare earth element A, for example, cerium Ce or lanthanum La can be used. The high gadolinia fuel rod 32 accommodates pellets 23 in which 7 wt% gadolinia is added to uranium having a concentration of 5 wt%.

  As shown in FIG. 3, the addition of Gd composite oxide with an equivalent gadolinia concentration of 53 ppm to uranium with a concentration of 6 wt% is less than or equal to the effective neutron multiplication factor of uranium with a concentration of 5 wt%. . Therefore, in the manufacturing process of the fuel assembly of the present embodiment, after adding and mixing the Gd composite oxide to the uranium powder, criticality safety is maintained by facilities and equipment that handle uranium with a concentration of 5 wt%. It can be handled as it is.

When the fuel assembly of the present embodiment is applied, the influence on the initial reactivity of the core is negligible, and no unburned Gd remains in the fuel rods containing the trace concentration Gd composite oxide at the end of the operation cycle. , Reactivity loss can be reduced. In addition, high quality fuel is manufactured by uniformly adding a small amount of Gd composite oxide from the stage of handling UO ₂ powder to uranium with enrichment exceeding 5 wt%, and equipment modification of the fuel molding processing facility related to criticality safety It is possible to suppress an increase in costs and associated molding costs. For this reason, the significant reduction effect of the number of new fuel replacement bodies and the reduction of the fuel cycle cost due to the increase of the reactor fuel enrichment, which is the purpose of applying the reactor fuel with the enrichment of uranium exceeding 5%, can be achieved.

  Some fuel assemblies loaded in pressurized water reactors do not use high-concentration Gd mainly for the purpose of adjusting the reactivity during the operation cycle. Even in such a case, Gd, which is a flammable poison with a large neutron absorption cross-section, is added to uranium with a concentration of more than 5 wt% uniformly as a Gd composite oxide with other rare earth elements. Thus, the same effect can be obtained. Further, a nuclear fuel rod having an enrichment of 5 wt% or less may be arranged at the corner portion or the peripheral portion of the fuel assembly to further flatten the local power distribution. A fuel assembly using borosilicate glass or the like as the flammable poison may be used.

[Third Embodiment]
FIG. 10 is a cross-sectional view of the fuel assembly according to the third embodiment of the present invention.

  The fuel assembly of the present embodiment is a fuel assembly that is loaded into a pressurized water reactor. In the present embodiment, one type of low-gadolinia fuel rod 35 is used as the fuel rod. The low gadolinia fuel rod 35 accommodates pellets 23 (see FIG. 2) obtained by adding a Gd composite oxide of gadolinium and erbium to uranium with a concentration of 6 wt%. The equivalent gadolinia concentration of the composite oxide of gadolinium and erbium is 50 ppm. The equivalent erbia concentration in the pellet is 0.3 wt%. Here, the equivalent erbia concentration is a concentration when erbium in the composite oxide exists as a single oxide.

  If the Er concentration in the pellet is high, Er may remain unburned at the end of the operation cycle, resulting in a burnup penalty. However, in this embodiment, by reducing the amount of Er by using Gd together, it is possible to reduce such burn-up penalty while ensuring critical safety. In addition, the mixing ratio can be selected in consideration of the neutron absorption effect of Gd and Er. In the case of uranium having a concentration of 10 wt% or less, the amount of Gd composite oxide to be added is 0.1 wt% or less in terms of equivalent gadolinia concentration. Therefore, considering that erbium mainly controls the reactivity of the core, for example, equivalent gadolinia The concentration range is preferably 1/10 or less than 0.01 wt%.

  In this way, when a small amount of Gd and Er composite oxide are formed, a neutron absorber in which Gd is uniformly diluted with Er as a base material can be obtained. By using such high-quality fuel, not only criticality safety but also the reactivity control of the core can be smoothly performed.

[Other embodiments]
The above description is merely an example, and the present invention is not limited to the above-described embodiments, and can be implemented in various forms. Moreover, it can also implement combining the characteristic of each embodiment.

It is a cross-sectional view of the first embodiment of the fuel assembly according to the present invention. It is a longitudinal cross-sectional view of the fuel rod used for 1st Embodiment of the fuel assembly which concerns on this invention. Metal amount is a UO ₂ powder of 33kg and a sphere is a graph showing the effective neutron multiplication factor of the presence of water reflecting the thickness of 30 cm. In a first embodiment of a fuel assembly according to the present invention, enrichment is a diagram showing the relationship between 5 wt% more than UO ₂ powder equivalent gadolinia concentration and uranium enrichment should be added. It is a graph which shows the infinite multiplication factor at the time of the output driving | operation of 1st Embodiment of the fuel assembly which concerns on this invention. 4 is a graph showing a concentration range of equivalent gadolinia to be added to a low gadolinia fuel rod in the first embodiment of the fuel assembly according to the present invention. FIG. 6 is a diagram schematically showing the number of Gd composite oxides when the equivalent gadolinia concentration in the pellet is the same and the Gd densities in the Gd composite oxide are different, and FIG. ) Indicates a case where the Gd density is low. Gd content contained per unit volume is a graph schematically showing the relationship between the particle diameter of Gd compound oxide neutron absorption effect of the total by Gd in certain UO _2. It is a cross-sectional view in the second embodiment of the fuel assembly according to the present invention. It is a cross-sectional view in the third embodiment of the fuel assembly according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Fuel assembly, 11 ... Water rod, 13 ... Fuel rod, 14 ... Fuel rod, 15 ... Fuel rod, 16 ... Fuel rod, 17 ... Fuel rod, 18 ... Fuel rod, 19 ... Fuel rod, 21 ... Cladding tube 22 ... Lower end plug, 23 ... Pellet, 24 ... Plenum spring, 25 ... Upper end plug, 31 ... Fuel rod, 32 ... Fuel rod, 33 ... Control rod guide thimble, 34 ... Guide thimble for in-core instrumentation, 35 ... Fuel rod 40 ... Control rod 41 ... Channel box 51 ... Gd composite oxide particle 52 ... Gd composite oxide particle

Claims (11)

A fuel assembly having a fuel element,
The fuel element is
Beyond Gd compound oxide and 5% of Formula _{A 1-X Gd} X _{O 2-0.5X} or Formula _{A 1-X} Gd _{X O 1.5} of a rare earth element A, except for gadolinium and gadolinium 10% Contains uranium oxide with the following enrichment :
The content ratio of the gadolinium is a lower limit value obtained by multiplying the increment from 5 wt% of the enrichment of the uranium by 61 × 10 ⁻⁴ in terms of the presence of gadolinium in the Gd composite oxide as a single oxide. Or more and less than 0.1%,
A fuel assembly characterized by that. 2. The content ratio of gadolinium in the fuel element is 3 times or less of the lower limit value in terms of gadolinium in the Gd composite oxide as a single oxide. The fuel assembly as described. The fuel assembly according to claim 1 , wherein the rare earth element A is one or more elements selected from erbium, lanthanum, and cerium . The rare earth element A is erbium, and the content ratio of gadolinium in the fuel element is 0.01 wt% or less in terms of gadolinium in the Gd composite oxide as a single oxide. The fuel assembly according to claim 1 or 2 , wherein the fuel assembly is characterized in that: The high-gadolinia fuel-containing element containing gadolinium oxide at a rate higher than a concentration converted when gadolinium in the Gd composite oxide of the fuel element is present as a single oxide. The fuel assembly according to any one of claims 1 to 4. A fuel assembly having a fuel element,
The fuel element is
Gadolinium and rare earth element A, except for gadolinium formula _{A 1-X Gd} X _{O 2-0.5X} or Formula _{A 1-X} Gd _{X O 1.5} in represented by Gd complex oxide and 5% beyond enrichment Contains uranium oxide,
The rare earth element A is either lanthanum or cerium, and the ratio of the Gd composite oxide to the sum of the uranium oxide and the Gd composite oxide is 0.1 wt% or more and 0.3 wt% or less. The ratio of gadolinium in the Gd composite oxide is 5 wt% or more and 33 wt% or less .
Fuel assemblies shall be the characterized in that. The high-gadolinia fuel-containing element containing gadolinium oxide at a rate higher than a concentration converted when gadolinium in the Gd composite oxide of the fuel element is present as a single oxide. 7. The fuel assembly according to 6 .   Uranium oxide having a concentration of more than 5% and not more than 10%;
  Chemical formula A of gadolinium and rare earth element A excluding gadolinium A _1-X Gd _X O _2-0.5X Or chemical formula A _1-X Gd _X O _1.5 The gadolinium content is 61 × 10 6 in increments from 5 wt% of the uranium enrichment in terms of gadolinium in the Gd composite oxide as a single oxide. ^-4 A Gd composite oxide that is not less than the lower limit multiplied by and less than 0.1%,
  A method for producing a fuel assembly, comprising a step of mixing. Represented by the chemical formula _{A 1-X Gd} X _{O 2-0.5X} or Formula _{A 1-X} Gd _{X O 1.5} of a rare earth element A, except for gadolinium and gadolinium, the rare earth element A lanthanum and cerium A Gd composite oxide containing gadolinium in a proportion of 0.1 wt% or more and 0.3 wt% or less and 5 wt% or more and 33 wt% or less of the sum of the uranium oxide and the uranium oxide ,
Uranium oxide with a concentration of more than 5% ,
Method for producing a fuel assembly, characterized in that it has a more mixed Engineering. Beyond Gd compound oxide and 5% of Formula _{A 1-X Gd} X _{O 2-0.5X} or Formula _{A 1-X} Gd _{X O 1.5} of a rare earth element A, except for gadolinium and gadolinium 10% Contains uranium oxide with the following enrichment :
The content ratio of the gadolinium is a lower limit value obtained by multiplying the increment from 5 wt% of the enrichment of the uranium by 61 × 10 ⁻⁴ in terms of the presence of gadolinium in the Gd composite oxide as a single oxide. Or more and less than 0.1%,
Uranium powder characterized by that. Gadolinium and rare earth element A, except for gadolinium formula _{A 1-X Gd} X _{O 2-0.5X} or Formula _{A 1-X} Gd _{X O 1.5} in represented by Gd complex oxide and 5% beyond enrichment Contains uranium oxide,
The rare earth element A is either lanthanum or cerium, and the ratio of the Gd composite oxide to the sum of the uranium oxide and the Gd composite oxide is 0.1 wt% or more and 0.3 wt% or less. The ratio of gadolinium in the Gd composite oxide is 5 wt% or more and 33 wt% or less.
Uranium powder characterized by that .

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