JPH083539B2 - Core structure of pressure tube reactor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a fuel structure of a nuclear reactor.

[Conventional technology]

In a conventional pressure tube type reactor, as shown in FIG. 2, a plurality of fuel rods 1 are bundled to form a fuel assembly, and the fuel assembly is loaded in a pressure tube 2. The outside of the tube 2 is surrounded by a calandria tube 4 via a gap region 3 filled with a gas (CO ₂ gas or the like) as a heat insulating material, and the outside is filled with a neutron moderator 5 (heavy water, etc.). There is. In order to remove the heat generated by the nuclear fission in the fuel rod, a coolant (light water or the like) 6 is flown inside the pressure pipe 2. In the heavy water moderated boiling light water cooling pressure tube reactor, heavy water is used as a moderator 5 outside the calandria tube 4. Light water, which is the coolant 6, flows into the pressure tube 2 from the lower part of the core, receives heat from the fuel rods, boils and flows out from the upper part, and the average axial void fraction of the fuel assembly is about 40%. is there. In such a pressure tube type nuclear reactor, as shown in FIG. 3, a large number of the pressure tubes 2 are arranged in a lattice to form a core, and heavy water as a moderator 5 is a calandria tank 7
Stored in.

In the pressure tube reactor, the above structure enables the fuel exchange for each pressure tube even during operation at the rated output, thereby improving the plant operation rate and effectively utilizing the fuel. Further, the coolant region inside the pressure pipe 2 and the moderator (heavy water) region outside the calandria pipe 4 are separated. Then, the method of controlling the output by adjusting the amount of the neutron absorbing material (liquid water poison) mixed in the heavy water can be easily applied, and the core operation can be simplified. At the same time, since the fuel rods 1 are bundled in the pressure tube 2, the self-sustaining effect of resonance energy for neutrons decelerated in the external heavy water region is increased, the ratio of thermal neutrons is increased, and the neutron economy is increased. There are advantages such as improvement in fuel efficiency and fuel utilization.

As described above, the coolant (light water) in the pressure pipe 2 boils and forms a void distribution in the axial direction of the fuel assembly (void ratio 0% in the lower part, about 70% in the upper part), but the void reactivity coefficient Since the absolute value of is extremely small, the axial power distribution is distorted by the void distribution in a small proportion.Therefore, as in the boiling water type LWR, the fuel assembly is axially distributed to flatten the axial power distribution. It is divided into two regions and no measures such as changing the fuel enrichment and the filling ratio of combustible neutron absorbing material (gadolinia) above and below (raising the upper part) are required, and it is loaded in a pressure tube reactor. In the fuel assembly, it is usual to make the axial distribution of fuel enrichment and gadolinia filling ratio uniform.

[Problems to be solved by the invention]

The above-mentioned prior art is a reactor in which a reflector having a neutron moderating ability stronger than the moderator in the core is installed in the upper or lower part of the core, and a large output peak occurs in the upper part or the lower part of the fuel assembly. From the point of view of soundness, there was a problem that it was necessary to reduce the power output of the core for operation.

An object of the present invention is to simultaneously improve the fuel utilization rate and core safety in a nuclear reactor equipped with such a reflector.

[Means for solving problems]

A first means for achieving the above object has a gas moderator as a neutron moderator in a reactor core loaded with a reactor fuel and surrounded by a reflector, and the reactor fuel is a fuel in a vertical region of the fuel. A low enrichment region of the fissile material at the lowermost end and the uppermost end is formed in the vertical direction by 5 cm to 10 cm, and a middle enrichment region of the fissile material second from the lowermost end and second from the uppermost end is 5 cm to 10 cm. 15 cm is formed, in the region above and below the middle enrichment region, a high enrichment region of fissile material is formed, the enrichment of the low enrichment region is 2 w / o or less, the middle enrichment region Is set to 3w / o to 5w / o, and the enrichment in the high enrichment region is set to an enrichment exceeding the enrichment in the medium enrichment region. The second means is a structure in which a reactor core loaded with a reactor fuel and surrounded by a reflector is provided with an inner region and an outer region. The core is provided with means for partitioning each region into an intermediate region located in the middle of both outer regions, a gas moderator as a neutron moderator in the inner region, and a poison-containing neutron moderator in the intermediate region. Heavy water, heavy water as a neutron moderator in the outer region, respectively, respectively, the reactor fuel, the lower enrichment region of the fissile material at the lowermost and uppermost ends of the vertical region of the fuel, the vertical direction 5 to 10 cm from the bottom and the second and the second from the top and the middle enrichment region of the fissionable material is formed from 5 to 15 cm, and the fissionable region is located above and below the middle enrichment region. A high enrichment region of the substance is formed, the enrichment of the low enrichment region is 2 w / o or less, the enrichment of the medium enrichment region is 3 w / o to 5 w / o, and the high enrichment is The enrichment of the area is set to an enrichment exceeding that of the medium enrichment area. And it has a core structure of a pressure tube reactor.

[Action]

According to the first means, the moderator is a gas moderator,
Compared with conventional heavy water moderators, the neutron spectrum in the core becomes harder, and the effect of improving the conversion ratio of the fuel in the core is obtained.By this effect, the fuel utilization rate is increased and the uranium resource saving aspect is improved. Great effect can be obtained with. However, since the neutron spectrum in the core becomes hard at the same time, the neutron spectrum becomes harder in the core near the reflector than in the case where heavy water is used as a moderator, and a large output peak appears. Therefore, the power peak is reduced by gradually changing the degree of enrichment of the fissile material in the fuel in the core near the reflector. That is, as reflected by the reflector, the neutron spectrum becomes strong near the reflector,
From the part having the strongest spectrum, the degree of enrichment of the fissile material in the fuel is set to low, medium and high. According to this deployment, as shown on the right side of the solid line graph in Fig. 4 of the drawing attached to this specification, it is possible to obtain the effect of reducing the output sharply at multiple points. If you lower this in one place, the amount of rise after lowering will be
Alternatively, the amount of increase before lowering becomes large and the output flattening is not sufficient, but the neutron spectrum gradually becomes stronger as it gets closer to the reflector. In other words, although the output curve has a sawtooth shape, its average is flat, and the degree of output flattening is improved compared to the case where the relative output at the upper and lower ends of the fuel assembly is lowered at one place. , The effect of spatial spectral shift along the axial direction can be achieved with the optimal number of region divisions, region widths and fuel enrichment for each region, and even if the output is increased, extreme output peaks do not occur simultaneously,
It is possible to obtain an effect of improving fuel economy without causing fuel damage. At the same time, since the moderator is a gas rather than a liquid that causes voids, the void reactivity coefficient becomes negative, and the effect of ensuring the safety of the core can be obtained.

Further, according to the second means, not only the function of the first means is exhibited, but also a plurality of types of neutron moderators having different degrees of neutron moderating effect are used to form a hard spectrum area in the inner area and an outer area in the hard spectrum area. Soft spectrum region,
An intermediate spectrum region, which is an intermediate spectrum between the inner and outer spectra, is created in one core in the core, and the fuel initially loaded is sequentially loaded from the inner region to the intermediate region and from the intermediate region to the outer region. It becomes possible to operate the moving nuclear reactor fuel, which causes fissile plutonium to be produced in a large amount in the hard spectrum region, then to the intermediate spectrum region, and then to the soft spectrum region. The effect of more efficiently burning the fissile plutonium generated by sequentially transferring the fuel can be obtained, the fuel staying period in the core can be lengthened, and the generated output amount can be increased, which further increases the fuel utilization rate. can get.

〔Example〕

In the reactor where the moderator region outside the pressure tube is filled with gas (heavy water gas, He gas, etc.) and reflectors of heavy water are installed in the upper and lower parts of the core to prevent neutron leakage, The operation of the present invention will be described based on FIG. 4 showing an example of the axial output distribution. This figure shows the results when the upper half of the fuel assembly is divided into three regions, and the infinite multiplication factor is adjusted by lowering the fuel enrichment (content ratio of fissile nuclides) in the region closer to the upper end. ing. As can be seen from the figure, in a conventional fuel assembly having a uniform enrichment in the axial direction, a large amount of thermal neutrons (low-energy neutrons) decelerated by heavy water, which is a strong neutron moderator in the upper reflector. As the fuel flows into the core as if reflected, the output of the fuel assembly increases as it goes to the upper part as shown by the dotted line in Fig. 4, and the maximum at the upper end is about 3 times that of the central part of the core. Become. Contrary to this, in the present invention, since the fuel enrichment is lowered toward the upper part, the output peak is also reduced to the same level as in the central part of the core, and the axial power distribution is flattened.

 Specific examples of the present invention will be described below.

1 (a) and 1 (b) show a fuel rod 1 in a pressure pipe 2.
In the first embodiment, each fuel rod 1 is divided into 5 regions in the axial direction, and the fuel is bundled in the central region in the axial direction. Packed with high-enrichment fuel 8 with an enrichment of fissile plutonium (Pu) of 11 w / o on average in the radial direction of the aggregate, the upper and lower regions adjacent to this region were enriched with 4 w / o. The medium enriched fuel 9 is filled, and the upper and lower regions adjacent to these regions are filled with the low enriched fuel 10 of natural uranium (enrichment 0 w / o). The axial length of each region is 5 cm in the low enriched fuel region, 5 cm in the medium enriched fuel region, and 350 cm in the high enriched fuel region.

Next, a pressure tube reactor to which the fuel assembly of the present invention is applied will be described with reference to FIGS. 5 to 7.

FIG. 5 is a vertical sectional view of the core of the reactor, and FIG.
A horizontal sectional view of the core is shown. This core has a gas region 11 in which heavy water vapor of a moderator gas circulates in the radial center, a poison-containing heavy water region 12 in which heavy water containing a boron solution circulates outside thereof, and a heavy water region 13 in further outside thereof. Have As shown in FIG. 5, the heavy water area 13 is a gas area 11
And spreads above and below the poison-containing heavy water region 12 and acts as a neutron reflector. The above-mentioned areas are separated by the partition plate 14 so that the moderators do not mix with each other. A plurality of pressure pipes 2 vertically pass through the core, and these pressure pipes are placed in the calandria pipe 4 in the heavy water region so as not to directly contact heavy water as a moderator. Carbon dioxide, which is a heat insulating material, circulates between the pressure pipe 2 and the calandria pipe 4 from the bottom to the top. In the case of a pressure tube penetrating the gas region, the calandria tube is placed only in the upper and lower heavy water regions, which act as neutron reflectors, and this gap serves as the inlet and outlet for heavy water vapor.

Next, FIG. 7 shows a circulation flow of heavy water used as a moderator material in the core. As shown in the figure, the heavy water area 13
And heavy water coming out of the poison-containing heavy water region 12 passes through a pump 15 and a cooler 16, respectively, and a control rod guide pipe 17
Is returned to the core from the upper part of. The boron concentration in the poison-containing heavy water region is adjusted by the boron concentration adjuster 21.
Heavy water vapor circulating in the gas region flows in and out through the gap between the calandria pipe and the pressure pipe. After passing through the pump, the heavy water vapor coming out of the gas region 11 is cooled by the cooler and returned to the gas region again through the gap of the calandria pipe. That is, the heavy water in the vapor state is cooled by the cooler and liquefied, and the liquefied heavy water is temporarily stored in the heavy water tank 19, and the steam generator 20 is timely stored.
After being vaporized by, it is returned to the core. These circulatory systems are replenished as needed from the heavy water tank 21 when there is a shortage of heavy water.

The fuel assembly, which is one embodiment of the present invention, is first loaded in the innermost region (gas region) of the core and sufficiently burned, and then sequentially in the intermediate region (poison-containing heavy water region) and the outermost region ( It will be moved to the heavy water area). The effect of the present invention will be described based on FIG. 8 showing the relationship between the neutron infinite multiplication factor and the burnup of this example (fuel assembly) when this fuel exchange method is applied to the core. The fuel assembly loaded in the innermost region using heavy water gas as a moderator has a high neutron energy spectrum (high average energy), so the fuel conversion ratio is high (about 0.8), and we are staying in this region. Can accumulate a large amount of new fuel (fissile Pu). After staying in this region for as long as possible, when the infinite multiplication factor of the fuel assembly drops below the required value in this region, a region with a softer spectrum using heavy water containing poison (moderate region) ),
Operate by increasing the infinite multiplication factor of the fuel assembly. Similarly, when burning in this region and the infinite multiplication factor falls below the required value, it is moved to the heavy water moderator region (outermost region) with a softer spectrum, and the infinite multiplication factor of the fuel assembly is further increased. The operation is performed at a higher temperature, the fuel stay period in the core is lengthened, and the generated output amount (takeout burnup) is increased. In this way, in this core, a large amount of fissile Pu is generated in the hard spectrum region where the fuel conversion ratio is high, and then it is sequentially transferred to the soft spectrum region to efficiently generate the fissile Pu that is generated. It burns well. With this spectrum shift method, the take-out burnup can be increased by 20% or more compared to the conventional single-spectrum core loaded with fuel assemblies with the same fuel enrichment. The effect is great in terms of reducing fuel cycle costs.

In addition, since a gas moderator region with a negative void reactivity coefficient is provided in the central part of the core, the void reactivity coefficient of the entire core also shifts to the negative side, improving core safety during a transient or accident. .

In the core structure, in addition to the heavy water gas as the gas moderator, various gases such as He gas and CO ₂ gas can be used, and as the moderator and the reflector, in addition to the heavy water and the heavy water containing poison, Various neutron moderators such as light water, mixed liquid of heavy water and light water can be used, and solids such as graphite and compounds of Li and Be can be used instead of liquid.

Next, another example of the core to which the fuel assembly of the present invention is applied will be described with reference to FIGS. 9 to 11.

FIG. 9 shows a vertical sectional view of the core, and FIG. 10 shows a horizontal sectional view. This core has a structure in which a plurality of pressure tubes 2 loaded with the fuel assembly of the present invention are bundled, and a gas region 11 filled with a gas (heavy steam) as a moderator is provided outside the pressure pipe, and the axis of the core region is provided. Heavy water layers 22 are provided as neutron reflectors in the upper and lower parts in the direction and on the peripheral part in the radial direction. In order to flatten the radial power distribution, the enrichment of only the outermost one-layer fuel assembly 24 is reduced. The circulation flow of heavy water vapor as a moderator and heavy water as a reflector is shown in FIG. Compared with the circulation flow (Fig. 7) in the case of the spectrum shift core,
The loop that circulates the heavy water containing poison is reduced and the system is simplified.

In this core, the moderator is gas and the spectrum is hard, so the conversion ratio of fuel is 0.8 or more, which is more than 30% higher than that of the conventional pressure tube reactor using heavy water as moderator. The utilization rate of uranium is increased, and it has a great effect in terms of uranium resource saving. In addition to being a high conversion core, this core is also a highly safe core that excels in safety during a transient or accident because the void reactivity coefficient is negative.

The spectrum shift core and the high conversion core having the above-mentioned great improvement effect can be realized only by combining with the fuel assembly of the present invention. In these cores,
Since a large amount of thermal neutrons decelerated in the reflector regions above and below the core flow into the core, the neutron energy spectra of the upper and lower ends of the fuel assembly and the central part are significantly different, and the central part Hard, but suddenly softer as the upper and lower ends are approached (average energy of neutrons is low)
Continue to grow. Therefore, in the case of the conventional uniform enrichment fuel assembly, the output at the upper and lower ends is about three times that at the center, and the output peaking coefficient increases, so the output density is lowered from the aspect of fuel integrity. Inevitably, the above-mentioned improvement effects of these cores disappear. The fuel assembly determines the optimal number of zone divisions, zone widths, and fuel enrichment to cancel the influence of such spatial spectral shifts along the axial direction based on reactor physical considerations and analysis results. ing. According to the results of the examination, the optimal number of regions is 5 as a whole, as shown in Fig. 1, but other parameters may have optimal values due to differences such as the average enrichment of fuel assemblies and the number of fuels. The range has a width, as shown in the following table.

The above study results are the results of studies conducted on uranium-plutonium mixed oxide fuel, but even when uranium oxide fuel is used, the axial power distribution can be flattened by selecting a value within the same range. it can.

Another embodiment of the present invention will be described with reference to FIG. Similar to the above-mentioned example, two small small areas are provided at each of the upper and lower ends of the fuel rod, and a flammable absorbent having a different concentration (such as gadolinia) is added to each small area. High concentration gadolinia region closest to the top and bottom
25, low concentration gadolinia area 26 adjacent to those areas,
And, the gadolinia-added fuel rods composed of the gadolinia-free region 27 in the central portion are arranged alternately with the gadolinina-free fuel rods in the second layer, which is the second layer counting from the outer layer as shown in the horizontal cross section of the figure. It is a fuel assembly. Combustible absorbers such as gadolinia have a very large absorption cross section for thermal neutrons, so by adjusting the concentration in two steps at the upper and lower ends, they flow in from the reflectors above and below the core. It is possible to effectively absorb thermal neutrons and realize a flat power distribution in the axial direction. Further, in this example, if the concentration of gadolinia is appropriately adjusted, when applied to a spectrum shift core, gadolinia can be burned at the time of extraction from the gas moderator region, and when loaded in the subsequent region. Regarding flattening of the axial power distribution, it is superior to the embodiment using the enrichment difference described above. In other cases, the same improvement effect as in the above-mentioned embodiment can be obtained.

Various modifications can be made to this embodiment. For example, as shown in FIG. 13, the gadolinia-added fuel rod 28 may be provided in the outermost layer or may be provided in two or more layers. Also, do not change the gadolinia concentration in each of the two regions of the upper and lower ends, and properly combine two types of gadolinia-added fuel rods with different gadolinia addition region lengths (one region length and two region lengths, respectively). It is also possible to construct a fuel assembly together. It is also possible to add gadolinia to the central region of the gadolinia-added fuel rod to control the combustion characteristics of the excess reactivity of the core and improve the operating performance.

Another embodiment will be described with reference to FIG. This embodiment has a function of flattening the axial power distribution further enhanced in comparison with the above two embodiments, and when loaded in the gas moderator region, the void reactivity coefficient has a relatively large negative value. , The lower part of the coolant with a low void ratio has a higher output than the upper part with a high void ratio, so in order to suppress this tendency, a low / high enrichment fuel (or a high concentration gadolinia-added fuel) is provided in the lower part of the center part Void reaction when flattened by a 6-region fuel rod filled with 31 and filled with high / high enrichment fuel (or low-concentration gadolinia-added fuel) 32, and then loaded in the liquid moderator region thereafter. Since the absolute value of the degree coefficient is small and the output peak increases in the center of the axial distribution, 7-region fuel rods filled with medium / high enrichment fuel (high concentration gadolinia-added fuel) 30 in the axial center part of the fuel rod Due to axial output The flattening. The spectrum
The shift core requires the 6-region and 7-region fuel rods, but for the high conversion reactor, only the 6-region fuel rods are required.

In the embodiments described above, the length of each region in the upper and lower regions, the fuel enrichment, the combustible absorbent concentration, etc. are the same without distinction between the upper and lower regions. For different thicknesses or different reflector materials, these parameters can be varied at the top and bottom and optimized in each region to increase the flattening function of the axial power distribution.

In order to obtain the effect of the present invention, in addition to the method of adjusting the fuel enrichment and the concentration of the combustible absorbent in the upper and lower end regions, the method of changing the packing density of the fuel and the area of the hollow portion of the hollow pellet are changed. The method, changing the number of fuel rods (changing the length of the fuel rods) is also effective.

In the above embodiment, the case where the upper portion and the lower portion are divided into three regions has been described, but the effect of the present invention can be obtained even if the number of divided regions is four or more.

Further, the present invention can be applied to a nuclear reactor in which the pressure tubes are arranged in the horizontal direction and the neutron reflector is provided in the lateral direction of the core, and the same effects as the above can be realized.

〔The invention's effect〕

According to the present invention, the axial power distribution in the core having the reflector in the periphery can be flattened as much as possible, so that the utilization efficiency of fuel is improved and it is also safe.

[Brief description of drawings]

FIG. 1 (a) is a sectional view of the fuel assembly in the first embodiment of the present invention, FIG. 1 (b) is a view taken along the line AA 'in FIG. 1 (a), and FIG. FIG. 3 is a horizontal sectional view of the fuel assembly, and FIG. 3 is a horizontal sectional view of the conventional core. FIG. 4 is a characteristic diagram of the present invention, FIG. 5 is a vertical sectional view of a nuclear reactor core using the fuel assembly of the present invention, FIG. 6 is a horizontal sectional view of the same, and FIG. 7 is the core of FIG. Fig. 8 is a moderator circulation flow chart of Fig. 8, Fig. 8 is a graph showing the change of the infinite multiplication factor of the fuel assembly of the present invention in the core of Fig. 5, and Fig. 9 is a second embodiment of the present invention.
A vertical cross-sectional view of the core when the fuel assembly is applied to another reactor, Fig. 10 is a horizontal cross-sectional view of the same, Fig. 11 is a moderator circulation flow diagram of the core of Fig. 9, Fig. 12 and Fig. 13. And FIG. 14 shows the second, third and fourth embodiments of the present invention. FIG. 12 (a) is a longitudinal sectional view of a fuel assembly showing a third embodiment of the present invention, FIG. 12 (b) is a sectional view taken along the line AA 'in FIG. 12 (a), and FIG. Is a plan sectional view of a fuel assembly showing a fourth embodiment of the present invention, and FIG. 14 is a vertical sectional view of a fuel assembly showing a fifth embodiment of the present invention. 1 ... fuel rod, 2 ... pressure tube, 3 ... gear region, 4
...... Calandria tube, 5 ...... Moderator, 6 …… Coolant, 7
...... Calandria tank, 8 ...... Highly enriched fuel, 9 ・ ・ ・
… Medium enrichment fuel, 10 …… Low enrichment fuel, 11 …… Gas region, 12 …… Poison-containing heavy water region, 13 …… Heavy water region, 14
Partition plate, 15 pump, 16 cooler, 17 control rod guide tube, 18 boron concentration controller, 19 heavy water tank, 20 steam generator, 21 heavy water tank , 22 …… Heavy water layer, 23 …… High enrichment fuel assembly, 24 …… Low enrichment fuel assembly, 25 …… High concentration gadolinia region, 26 …… Low concentration gadolinia region, 27 …… No gadolinia addition Area, 28 …… Gadolinia-added fuel rods, 29 …… Gadolinia-free fuel rods, 30
…… Medium / high enrichment fuel, 31 …… Low / high enrichment fuel, 32…
… High and high enrichment fuel.

Claims (2)

[Claims] 1. A reactor core loaded with a reactor fuel and surrounded by a reflector has a gas moderator as a neutron moderator, and the reactor fuel is a nuclear fission at a lowermost end and an uppermost end of a vertical region of the fuel. Area of low enrichment of organic substances is 5 cm to 10 cm vertically
Formed, the middle enrichment region of the fissile material second from the lowermost end and second from the uppermost end is formed by 5 cm to 15 cm, and the high enrichment of the fissile material is formed in the region above and below the middle enrichment region. Area is formed, the enrichment of the low enrichment area is 2 w / o or less, and the enrichment of the medium enrichment area is 3 w / o to 5 w.
/ o, the enrichment of the high-enrichment region is set to an enrichment exceeding the enrichment of the medium-enrichment region. 2. A means for partitioning a reactor fuel-loaded reactor-enclosed core surrounded by a reflector into an inner region, an outer region, and an intermediate region located between the inner-outer region and the inner-outer region. A gas moderator as a neutron moderator in the inner region, poison-containing heavy water as a neutron moderator in the intermediate region, heavy water as a neutron moderator in the outer region, respectively, the reactor fuel Is a lower-enrichment region of the fissile material at the lowermost and uppermost ends of the fuel in the vertical direction, which is formed in the vertical direction by 5 cm to 10 cm, and is the second fissionable from the lowermost end and second from the uppermost end. A medium enrichment region of the substance is formed from 5 cm to 15 cm, a high enrichment region of fissile material is formed between the upper and lower regions of the middle enrichment region, and the enrichment of the low enrichment region is 2 w / Below, the enrichment of the above-mentioned medium enrichment range is 3 w / o ~ The core structure of the pressure tube reactor, wherein the enrichment in the high enrichment region is set to 5 w / o, which is higher than the enrichment in the medium enrichment region.