JP2012220325A - Fast reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fast reactor in which the quantity of uranium required for electric energy is reduced rather than a light-water reactor by reducing uranium concentration and long-time operation is enabled without fuel exchange.SOLUTION: A fast reactor comprises a core fuel 25 containing as a main component condensed uranium of which the condensation rate is set within the range of 11% to 13%, a blanket 26 containing as a main component thorium, deteriorated uranium, natural uranium or a combination thereof, a fuel rod 22 in which the blanket 26 (26a, 26b) is disposed above and below the core fuel 25, a fuel assembly 20 in which a plurality of fuel rods 22 are disposed in such a manner that a product of a spatial occupancy ratio of the core fuel 25 in a horizontal cross section and a theoretical density ratio becomes 0.35 or more, and a core section 10 in which a plurality of fuel assemblies 20 are disposed and a ratio (D/H) of a core diameter D to a height H of the core fuel is settled within the range of 4 to 7.

Description

  The present invention relates to a core of a fast reactor.

  Coping with the increasing demand for uranium resources is an important issue in the global trend of nuclear power generation. In contrast to light water reactors that fission uranium with slow neutrons, fast breeder reactors grow plutonium with fast neutrons as nuclear fuel. The practical application of this fast breeder reactor is one of the solutions to the aforementioned problems.

  Unlike light water reactors that use water as a coolant, fast reactors use sodium as a coolant. As a result, neutrons generated by fission of nuclear fuel cause the next fission without reducing energy, and further propagate plutonium.

The following are fast reactors disclosed so far.
A fast reactor is disclosed in which neutrons are supplied from enriched uranium to depleted uranium to generate fissile material, and the fuel is replaced when burned up to the end of the core to enable long-term operation (for example, Patent Document 1). ).
In addition, a fast reactor has been disclosed that enables long-term operation by controlling neutron leakage from the reactor core using a reflector (for example, Patent Document 2).
Further, enriched uranium is disposed in the center of the core, and a blanket is disposed outside. When plutonium is accumulated in the blanket, the arrangement of the enriched uranium aggregate and the blanket is exchanged. Thus, a fast reactor has been disclosed that enables long-term operation by repeatedly exchanging aggregates of enriched uranium with reduced fissile material (eg, Non-Patent Document 1).

Japanese Patent No. 3463100 Japanese Patent No. 2892824

Tyler Ellis et al: “Traveling-Wave Reactors: A Truly Sustainable and Full-Scale Resource for Global Energy Needs” (Paper 10189), Proceedings of ICAPP '10, San Diego, CA, USA, June 13-17, 2010.

By the way, since the countries where plutonium can be used are limited, it is believed that the demand for enriched uranium will continue to grow significantly.
Therefore, if the fast reactor is put into practical use, it is considered possible to save uranium resources. However, the above-described conventional fast reactor has a problem that the required amount of natural uranium per power generation amount is higher than that of a light water reactor because the required uranium enrichment is high.
This is because in the fast reactor, the energy of neutrons is high, so that the uranium enrichment necessary for the criticality is significantly higher than in the light water reactor.

  The present invention has been made in consideration of such circumstances, and a fast reactor capable of operating for a long time without fuel replacement while lowering the uranium concentration to reduce the required amount of uranium per power generation compared to a light water reactor. The purpose is to provide.

  In a fast reactor, a core fuel mainly composed of enriched uranium whose enrichment is set in a range of 11% to 13%, a blanket composed mainly of thorium, deteriorated uranium, natural uranium or a combination thereof, and the core A fuel rod in which the blanket is disposed above and below the fuel, and a fuel assembly in which a plurality of the fuel rods are disposed so that a product of a space occupancy ratio and a theoretical density ratio of the core fuel in a horizontal section is 0.35 or more A plurality of the fuel assemblies, and a core portion having a ratio of a core diameter to a height of the core fuel in a range of 4 to 7.

  According to the present invention, a fast reactor is provided in which the uranium concentration is lowered to reduce the required amount of uranium per power generation compared to a light water reactor, and it can be operated for a long time without fuel replacement.

The 1/6 horizontal sectional view of the core part which shows 1st Embodiment of the fast reactor which concerns on this invention. (A) External view of fuel assembly in each embodiment, (B) Vertical sectional view of fuel rod, (C) Horizontal sectional view of fuel rod. The longitudinal cross-sectional view of the core part in 1st Embodiment. The graph which shows the effective multiplication factor with respect to the operation period in case the ratio of the core diameter D and the height H of the core fuel is 1.67 (comparative example) and 6.67 (example), respectively. The graph which shows the variation | change_quantity of an effective multiplication factor with respect to ratio (D / H) of a core diameter and the height of a core fuel. The graph which shows the uranium enrichment required in order to achieve initial criticality with respect to ratio (D / H) of a core diameter and the height of a core fuel. The longitudinal cross-sectional view of the fuel rod used with the fast reactor which concerns on 2nd Embodiment. The longitudinal cross-sectional view of the core part in the fast reactor of 2nd Embodiment. The longitudinal cross-sectional view of the core part in the fast reactor of 3rd Embodiment. (A) Graph showing the radial power distribution of the core in the fast reactor of the first embodiment, (B) Radial power when the uranium concentration outside the core is increased in the fast reactor of the first embodiment. The graph which shows distribution, (C) The graph which shows the radial direction power distribution of the core part in the fast reactor of 3rd Embodiment. (A) The longitudinal cross-sectional view of the core part in the fast reactor of 4th Embodiment, (B) The graph which shows radial direction power distribution of the core part in the fast reactor of 1st Embodiment and 4th Embodiment. Explanatory drawing of the movement method of the fuel assembly in the fast reactor of 5th Embodiment. The 1/6 horizontal sectional view of the core which shows the conventional form of a fast reactor as a comparative example.

Prior to describing the embodiment of the present invention, a conventional fast reactor will be described as a comparative example.
FIG. 13 shows a 1/6 horizontal section defined by a radius that intersects the center O of the core portion 35 of the conventional fast reactor. And the specification of this conventional fast reactor is shown in the column of Comparative Example 1 in Table 1.
This fast reactor is a prototype reactor-type fast reactor having an electrical output of about 300,000 kWe. The core 35 includes a fuel assembly 30 having different uranium enrichments on the inside and outside, a control rod 31 for power adjustment, a control rod 32 for stopping the reactor, a radial blanket 34, and a radial reflector 33. It is composed of Although not shown, axial blankets are provided above and below the core fuel disposed in the fuel assembly 30.

  The uranium enrichment of the core fuel disposed in the fuel assembly 30 is 24% and 30%, respectively, toward the center O on the inside and outside. Then, one fifth of the fuel is changed every operation cycle of about 5 months. The required amount of natural uranium per power generation in the conventional fast reactor is about four times that of the 1.3 million kWe light water reactor.

  This is compared to the 1.3 million kWe light water reactor (uranium enrichment of about 4%), and the conventional fast reactor (uranium enrichment is about 26% on the core average) has the natural nature required to achieve the required uranium enrichment. This is because uranium increases. This is because in a fast reactor, the neutron spectrum is hard, so the rate of fission is smaller than in a light water reactor, and more U-235 is required to make the reactor critical.

(First embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 shows a 1/6 horizontal section defined by a radius that intersects the center O of the core 10 of the fast reactor according to the first embodiment.
As shown in the column of Example 1 in Table 1, the heat output and electric output of this fast reactor are 750 MWth and 300 MWe, respectively.
The core section 10 includes a fuel assembly 20 loaded with enriched uranium, a control rod 11 for adjusting power, a control rod 12 for stopping the reactor, and a radial reflector 13.

2A is an external view of the fuel assembly in each embodiment, FIG. 2B is a longitudinal sectional view of the fuel rod, and FIG. 2C is a horizontal sectional view of the fuel rod. Yes.
As shown in FIG. 2, the fast reactor mainly includes core fuel 25 mainly composed of enriched uranium whose enrichment is set in the range of 11% to 13% and thorium, deteriorated uranium, natural uranium, or a combination thereof. The product of the blanket 26 as a component, the fuel rod 22 in which the blanket 26 (26a, 26b) is arranged above and below the core fuel 25, and the space occupation ratio and the theoretical density ratio of the core fuel 25 in a horizontal section is 0.35 or more. A fuel assembly 20 in which a plurality of fuel rods 22 are disposed so that a plurality of fuel assemblies 20 are disposed, and a ratio (D / H) between the core diameter D and the core fuel height H is 4-7. And a core portion 10 (FIG. 3) in the range of.

The fuel assembly 20 is obtained by bundling a large number (271) of fuel rods 22 in a hexagonal hollow tube 21 called a trumpet tube.
In the fuel rod 22, a core fuel 25 having a height of 60 cm is arranged at the center of a hollow cladding tube 23 having an outer diameter of 14 mm and an inner diameter of 12 mm, and blankets 26 (26a, 26b) having a height of 60 cm are arranged above and below the fuel. A gas plenum 27 is disposed on the upper blanket 26b, and the lower and upper ends of the fuel rods 22 are sealed with end plugs 24 (24a, 24b).
In addition, the outer diameter and inner diameter of the fuel rod 22 of the embodiment are twice that of the comparative example.

The core fuel 25 is an alloy fuel in which uranium enriched with a uranium enrichment of 12% (weight ratio) and zirconium in a weight ratio of 9: 1 are mixed.
Here, the preferable range of uranium enrichment is 11% to 13%. When the enrichment is lower than 11%, it becomes difficult to reach the initial criticality, and when the enrichment is higher than 13%, the uranium required amount is reduced. This is contrary to the purpose of reduction.

  The blanket 26 is obtained by mixing a metal form of depleted uranium having a concentration of 0.2% (weight ratio), which is lower than natural uranium, and zirconium in a weight ratio of 9: 1. The blanket 26 hardly causes fission, but captures neutrons and grows plutonium.

The weight ratio of zirconium to uranium may be set to be small on the radially outer side of the core portion 10 in addition to setting the horizontal section of the core portion 10 to be uniform with the above-described ratio.
That is, in Example 1, the weight ratio of zirconium in the uranium-zirconium composition was 10% in all the core fuels 25 and blankets 26, but the weight ratio of zirconium was 10% in the radially inner fuel assembly 20, In the radially outer fuel assembly 20, the weight ratio of zirconium may be set as low as 5%. Thereby, in the core fuel 25 on the radially outer side, uranium, which is a fissile material, is increased, and the output in that region is relatively increased. Thereby, the radial core power distribution can be flattened.

  The blanket 26 is not limited to the above-described composition, and may be thorium, a mixture of thorium and deteriorated uranium, or a mixture of thorium and natural uranium. Thorium does not fission, but it absorbs neutrons and produces fissile material, just like uranium.

Table 2 shows typical composition and physical property values of the core fuel 25.
The core fuel 25 and the blanket 26 are not limited to the case where uranium is in the metal form as described in the first embodiment, and may be any one of nitride, carbide, and silicide.

Table 1 shows the specifications of the core of a fast reactor using nitride fuel as Example 2.
A feature of nitride fuel is that, like metal fuel, uranium weight per unit volume of fuel is larger than that of oxide fuel. In Example 2, the uranium enrichment becomes higher than that in Example 1, and as a result, the required amount of natural uranium per power generation amount is slightly increased (0.5 → 0.6).
In Examples 1 and 2, the required amount of natural uranium per power generation can be reduced to about half compared to the case of a light water reactor. Furthermore, long-term operation of 50 years becomes possible.
Detailed specifications of the carbide fuel, silicide fuel, and oxide fuel are omitted, but since the uranium weight per unit volume of fuel is large, the core characteristics similar to those of metal and nitride can be obtained.

  As shown in the horizontal cross-sectional view of FIG. 2C, the fuel rod 22 has a larger inner diameter of the cladding tube 23 than the outer diameter of the rod-shaped core fuel 25. Thereby, a space for escape of the core fuel 25 that expands by irradiation is formed between the cladding tube 23 and the space. In the cross-sectional area defined by the inner diameter of the cladding tube 23, the core fuel 25 occupies about 75%, and the remaining 25% space is filled with sodium.

In the first embodiment, the space occupation ratio of the fuel rods 22 in the horizontal section is about 38%. In addition, since the core fuel 25 of Example 1 is a metal form, its theoretical density ratio is 1. The remaining 62% is occupied by the structural material of the cladding tube 23 and the trumpet tube 21, and sodium filled in the cladding tube 23 and sodium that cools the inside and outside of the trumpet tube 21.
In addition, the preferable range of the product of the space occupancy of the core fuel 25 and the theoretical density ratio in the horizontal cross section is 0.35 or more. If this value is less than 0.35, it becomes difficult to make the core critical with low uranium enrichment.

Here, when the comparative example and the example 1 are compared, the fuel composition is different in that the fuel composition is an oxide such as uranium dioxide in the comparative example, whereas the fuel composition is the metal uranium in the example 1.
Compared with oxide fuel and metal fuel, the weight of fuel per unit volume is 1.3 times that of metal fuel, and the product of space occupancy and theoretical density ratio is also about 1.3 times. The weight of uranium contained per unit cross-sectional area of the fuel assembly is about 1.7 times.

  In the first embodiment, the reason why the fuel space occupation ratio is large is related to the fact that the space occupation ratio of the structural material and the coolant is relatively small because the diameter of the fuel rod 22 is larger than that in the conventional example. ing.

Thus, by increasing the weight of uranium contained per unit cross-sectional area of the fuel assembly 20, the uranium enrichment necessary for making the core critical can be reduced.
If the fuel of the comparative example is simply changed from the oxide fuel to the metal fuel, the increase in the weight of uranium contained per unit cross-sectional area of the fuel assembly is only about 1.3 times.

Next, the relationship between the core diameter D and the core fuel height H in the core 10 will be examined.
FIG. 3 shows a longitudinal section of the core 10, and illustrations of structures such as the trumpet tube 21, the fuel rod 22, and the coolant other than the region of the core fuel 25 and the blanket 26 are omitted.
In each embodiment, the ratio (D / H) between the core diameter D and the core fuel height H is set in the range of 4-7. Here, if D / H is smaller than 4, the variation amount of the effective multiplication factor increases as will be described later (FIG. 5). If D / H is larger than 7, it is necessary to increase the uranium enrichment necessary to achieve the initial criticality as described later (FIG. 6).

In Example 1, since the diameter of the fuel rod 22 is larger than that of the comparative example, the size of the fuel assembly 20 is increased. Therefore, in Example 1, the number of fuel assemblies 20 is 198, which is the same as that of the comparative example, but the core diameter D is about 400 cm, which is larger than 200 cm of the conventional example.
For this reason, the ratio (D / H) of the core diameter D and the core fuel height H (D / H) is 200/100 = 2 in the comparative example, but is large at 400/60 = 6.67 in the first embodiment. Become.
Based on this difference, the fast reactor of Example 1 can be operated for 50 years without refueling for the reasons described later.

In the graph of FIG. 4, when the ratio of the core diameter D to the core fuel height H is 1.67 (Comparative Example 2) and 6.67 (Example 1), respectively, the operation period (year) is plotted on the horizontal axis. Is the result of calculating the change in effective multiplication factor on the vertical axis.
This graph is calculated under the condition that all control rods are pulled out. The effective multiplication factor of Example 1 can be said to be extremely small fluctuations that are slightly higher than the critical value 1 in 50 years of operation. For this reason, in Example 1, it is possible to drive for 50 years by a slight operation of the control rod.
On the other hand, in Comparative Example 2 (D / H = 1.67; D = 250, H = 150), the effective multiplication factor increases until 15 years of operation, and then decreases and the effective multiplication factor falls below 1 in 40 years of operation. Compared with Example 1, it shows a behavior that lacks stability in the critical state.

In the graph of FIG. 5, the ratio of the core diameter D to the height H of the core fuel (D / H) is taken on the horizontal axis under the condition that the core fuel volume is constant, and the effective multiplication factor in 50 years is taken on the vertical axis. It shows the amount of fluctuation.
From this graph, it can be seen that when the ratio (D / H) of the core diameter D to the core fuel height H is 4 to 9, the change in the effective multiplication factor becomes small. That is, the shape of the core fuel 25 in the core portion 10 of the first embodiment is a flat shape such as the core diameter D = 400 cm and the core fuel height H = 60 cm. Therefore, the blanket 26 (26a, The amount of neutrons going to 26b) increases. For this reason, in the blanket 26, the amount of conversion from U-238 to plutonium increases, and the decrease in the amount of combustion of U-235 in the core fuel 25 is offset. For this reason, in Example 1, a critical state is stably maintained over a long period of time.

  On the other hand, in Comparative Example 2, the reason why the stability in the critical state is less than in Example 1 is that the ratio (D / H) of the core diameter D to the height H of the core fuel is small. This is thought to be because the amount of neutrons toward the (upper and lower) blankets 26 (26a, 26b) decreases.

The graph of FIG. 6 shows the uranium enrichment necessary to achieve the initial criticality with respect to the ratio (D / H) between the core diameter D and the core fuel height H under the condition that the core fuel volume is constant. Is shown.
As shown in this graph, the uranium enrichment necessary to achieve the initial criticality decreases monotonically as the D / H ratio decreases.
In the case of Comparative Example 2 (D / H = 1.67), the uranium enrichment necessary to achieve the initial criticality is about 11%. The lower the uranium enrichment, the smaller the U-235 / U-238 ratio. Therefore, the amount of conversion from U-238 to plutonium in the core 10 relative to the amount of U-235 consumed in the core fuel 25 increases. .

  Thereby, in the comparative example 2 (D / H = 1.67), as shown in FIG. 4, the initial effective multiplication factor tends to increase. However, the amount of neutrons going to the axial blanket 26 is small, and plutonium production in this region is small. For this reason, in the long term, the amount of plutonium produced is insufficient, and the effective multiplication factor tends to gradually decrease after reaching a peak.

From FIG. 5, in order to minimize the fluctuation amount of the effective multiplication factor, the ratio (D / H) between the core diameter D and the core fuel height H is preferably about 4 to 9. On the other hand, from FIG. 6, in order to reduce the uranium enrichment, the ratio (D / H) between the core diameter D and the core fuel height H should be as small as possible.
Therefore, in order to achieve both the minimization of the fluctuation amount of the effective multiplication factor and the reduction of the uranium enrichment, the ratio (D / H ratio) between the core diameter and the core fuel height is set in the range of 4-7.
The reason for setting this range is that the upper limit is the ratio of the core diameter D to the height H of the core fuel (D / H = 7) that minimizes the fluctuation amount of the effective multiplication factor. This is because the smaller side (D / H = 4) of the ratio between the core diameter D and the core fuel height H, which is compatible with the reduction of the variation in magnification, is selected as the lower limit.

  As shown in Table 1, when the required amount of natural uranium per power generation is set to 1 in a 1.3 million kW class light water reactor with an electric output, it is reduced to 0.5 in Example 1. Further, even when the uranium enrichment necessary for the criticality is compared with Comparative Example 1 (core average 26%), it can be seen that Example 1 has a small value of 12%. Further, even when the operation cycles are compared, Comparative Example 1 is 0.5 years, while Example 1 is as long as 50 years.

  As described above, in the fast reactor according to the embodiment, natural uranium can be saved and the fuel cost can be greatly reduced as compared with the conventional light water reactor and fast reactor. Furthermore, since it can be operated for 50 years using only the fuel loaded first without changing the fuel, it contributes to an improvement in operating rate and simplification of operation.

(Second Embodiment)
FIG. 7 is a longitudinal sectional view of the fuel rod 22 used in the fast reactor according to the second embodiment. FIG. 8 is a longitudinal sectional view of the core 10 in the fast reactor according to the second embodiment. 7 and 8, the same or corresponding parts as those in FIGS. 2B and 3 are denoted by the same reference numerals, and redundant description is omitted.

In the second embodiment, as shown in FIG. 7, the fuel rod 22 has two stages in which each of the core fuels 25 (25a, 25b) is sandwiched between blankets 26 (26a, 26b, 26c) in the axial direction. ing.
As shown in FIG. 8, when the core 10 is viewed in a longitudinal section, a plurality of layers of core fuel 25 (25a, 25b) are formed (two layers in the drawing), and above and below each layer of the core fuel 25. A layer of the blanket 26 (26a, 26b, 26c) is formed.
The ratio (D / H) between the core diameter D and the height H of each core fuel 25 is set in the range of 4-7.

  In the fast reactor according to the second embodiment configured as described above, the required amount of natural uranium per power generation amount can be reduced to about half that of a light water reactor while enabling long-term operation for 50 years as in the first embodiment. . Furthermore, the output of the fast reactor can be doubled without increasing the core diameter D.

(Third embodiment)
FIG. 9 is a longitudinal sectional view of a core part in the fast reactor of the third embodiment. 9 that are the same as or correspond to those in FIG. 3 are denoted by the same reference numerals, and redundant description is omitted.
In the fast reactor of the third embodiment, the core fuel 25 is set high on the radially outer side of the core section 10.
That is, as shown in FIG. 9, the core fuel 25 c with the core fuel height H 1 = 70 cm is arranged outside the core portion 10, and the core fuel 25 d with the core fuel height H 2 = 50 cm is placed inside the core portion 10. Deploy.

According to the fast reactor of the first embodiment (FIG. 3), since neutrons emitted from the core fuel 25 on the radially outer side are likely to leak to the outside, the output on the radially outer side is lower than that on the radially inner side. Tend. On the other hand, according to the fast reactor of the third embodiment, the neutron emitted from the core fuel 25c on the radially outer side increases, so that the radial power distribution is flattened.
In the conventional example shown in Comparative Example 1, as shown in Table 1, the uranium enrichment was increased from the inner 24% to the outer 30%. In Comparative Example 1, the output is flattened at the early stage of combustion, but when the operation is prolonged, the consumption of fissile material increases outside the uranium enrichment. As a result, the output on the outer side of the core 10 is reduced, the output on the inner side is relatively increased, and the output distribution is greatly changed.

  On the other hand, in the third embodiment, the uranium enrichment is one kind, but the outer power is increased by making the height H1 of the core fuel outside in the radial direction higher than the height H2 of the inner core fuel. Can be relatively increased. As a result, the power distribution of the entire core can be flattened. Further, since the uranium enrichment is uniform, the consumption of fissile material is uniform, and the power distribution does not fluctuate much even if the operation is prolonged.

The graph of FIG. 10A shows the radial power distribution of the core 10 in the fast reactor of the first embodiment, and the graph of FIG. 10B is more than the radially inner side of the fast reactor of the first embodiment. The radial output distribution when the uranium concentration is increased on the outer side is shown.
Here, the number 1 on the horizontal axis indicates the position of the control rod 11 at the center O (FIG. 1) of the core 10, and the numbers after the number 2 indicate the position of the fuel assembly 20.

  As shown in FIG. 10 (A), in the first embodiment, it can be said that the power distribution of the core 10 hardly changes between the early stage of combustion and the end stage of combustion. On the other hand, in FIG. 10B, as a result of the uranium enrichment being in two stages, the output distribution at the beginning of combustion is flattened, but at the end of combustion, the inner output decreases and the outer output increases to flatten. Disruption is observed.

The graph of FIG. 10C shows the radial output distribution of the fuel assembly in the fast reactor (FIG. 9) of the third embodiment.
As shown in FIG. 10C, in the third embodiment, both the flattening of the radial power distribution and the suppression of the change in the power distribution from the initial stage of combustion to the end stage are achieved. Thus, in the third embodiment, the output distribution can be flattened over the entire operation period. As a result, the maximum line output and burnup that affect the fuel life and fuel integrity can be reduced, and the maximum temperature of the cladding tube and fuel can be lowered, contributing to the prolongation of the operation period.

(Fourth embodiment)
FIG. 11A is a longitudinal sectional view of a core portion in a fast reactor according to the fourth embodiment. Note that in FIG. 11A, portions that are the same as or correspond to those in FIG.
In the fast reactor according to the fourth embodiment, the height H of the core fuel is constant, and the blanket 26 is set high on the radially outer side of the core portion 10.
Thus, the amount of neutrons leaking from the region outside the core fuel 25 can be reduced by making the radially outer blankets 26a, 26b higher than the inner blankets 26d, 26e. For this reason, the power density in this outer region can be increased.

The graph of FIG. 11 (B) is the result of overwriting the result of the first embodiment on the radial power distribution of the fuel assembly in the fast reactor of the fourth embodiment.
Thus, in the fourth embodiment, it can be seen that the aggregate output distribution in the radial direction is flattened as compared with the case of the first embodiment.

(Fifth embodiment)
With reference to FIG. 12, the movement method of the fuel assembly in the fast reactor of 5th Embodiment is demonstrated. In FIG. 12, parts that are the same as or correspond to those in FIG.
In the fast reactor of the fifth embodiment, after the combustion of the core fuel proceeds, the fuel assembly 20 with a high burnup is radially outward of the core 10 and the fuel assembly 20 with a low burnup is radially inward. Move.

As shown in FIG. 12, the core 10 is classified into the zones (1) to (4) concentrically from the center O in the horizontal sectional view. Then, based on a predetermined shuffling plan, the fuel assemblies 20 in the same zone are collectively moved to another zone.
In general, since the power output of the core portion 10 is higher toward the inner side in the radial direction, the degree of burnup and the amount of irradiation that determine the fuel life are higher as the fuel assembly 20 is positioned at the inner side in the radial direction. For this reason, after a certain period of combustion has elapsed, the fuel assembly 20 of zone 1 is moved to zone 2, the zone 2 is moved to zone 3, the zone 3 is moved to zone 4, and the zone 4 is moved to zone 1. Move like this. By periodically repeating such movement, the burnup and irradiation amount of all the fuel assemblies 20 are made uniform.

  The present invention is not limited to the above-described embodiments, and can be appropriately modified and implemented within the scope of the common technical idea.

  DESCRIPTION OF SYMBOLS 10 ... Core part, 11 ... Control rod for power adjustment, 12 ... Control rod for reactor stop, 13 ... Radial reflector, 20 ... Fuel assembly, 21 ... Trumpet tube (hollow tube), 22 ... Fuel rod , 23 ... cladding tube, 24 ... end plug, 25 (25a, 25b, 25c, 25d) ... core fuel, 26 (26a, 26b, 26c, 26d) ... blanket, 27 ... gas plenum, 30 ... fuel assembly, D ... Core diameter, H: Height of core fuel.

Claims (7)

A core fuel whose main component is enriched uranium, the enrichment of which is set in the range of 11% to 13%,
A blanket based on thorium, depleted uranium or natural uranium or a combination thereof;
A fuel rod for placing the blanket above and below the core fuel;
A fuel assembly in which a plurality of the fuel rods are arranged so that a product of a space occupation ratio of the core fuel and a theoretical density ratio in a horizontal section is 0.35 or more;
A fast reactor comprising: a plurality of the fuel assemblies, and a core portion in which a ratio of a core diameter to a height of the core fuel is in a range of 4 to 7. The fast reactor according to claim 1,
When the longitudinal section of the core is viewed,
A plurality of core fuel layers are formed, and the blanket layers are formed above and below each of the core fuel layers;
The fast reactor, wherein a ratio between the core diameter and the height of each core fuel is in the range of 4 to 7. In the fast reactor according to claim 1 or 2,
The fast reactor, wherein the core fuel and the blanket are made of at least one of metal, nitride, carbide and silicide. In the fast reactor according to any one of claims 1 to 3,
The fast reactor is characterized in that a height of the core fuel is set high on a radially outer side of the core portion. In the fast reactor according to any one of claims 1 to 4,
The fast reactor, wherein a height of the blanket is set high on a radially outer side of the core portion. In the fast reactor according to any one of claims 1 to 5,
A fast reactor, wherein after the combustion of the core fuel proceeds, the fuel assembly having a high burnup is moved radially outward of the core portion, and the fuel assembly having a low burnup is moved radially inward. In the fast reactor according to any one of claims 1 to 6,
The core fuel and the blanket are fast reactors characterized in that uranium is an alloy of zirconium and the weight ratio of the zirconium is small outside in the radial direction of the core portion.

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