A design method for a light water cooled high flux reactor

Abstract

The present invention relates to a light water reactor, which is based on the core design concept of low self-shielding. The light water reactor uses light water as a coolant, and includes a core fuel zone with a plurality of fuel assemblies and a reflector layer surrounding the core fuel zone on the radial outer side, wherein a plurality of irradiation channels are formed between the fuel assemblies, and the core fuel zone is designed using the core design concept of low self-shielding, so that the proportion of the area occupied by the irradiation channels in the reactor within a migration length (approximately equivalent to the average penetration free path) that neutrons generated by any fuel zone can reach is not less than 20%, and does not pass through other irradiation channels before reaching the irradiation channels, wherein each of the irradiation channels is respectively filled with different moderators suitable for producing fast neutrons and thermal neutrons.

Description

Technical Field

[0001] The invention relates to a light water reactor in the technical field of nuclear reactor engineering, in particular to an ultra-high flux light water cooling research reactor. Background Art

[0002] The high flux reactor is an important engineering-level irradiation device, which mainly uses neutrons. There are usually multiple irradiation channels and other irradiation facilities in the reactor, which has important applications in the fields of nuclear fuel and nuclear material irradiation testing, radioactive isotope production, neutron scattering, neutron imaging, etc. Among the existing research reactors, high flux reactors account for about 17%. The types, reactor models and application fields of high flux reactors in various countries in the world are different, but they all aim to obtain sufficient irradiation space and maximize the neutron flux in the irradiation channels to meet the needs of nuclear fuel and material irradiation testing, isotope production and other tasks.

[0003] The goal of high-flux reactor core design is to achieve the highest possible neutron flux level at the lowest possible power density and total power level. High-flux reactors at home and abroad use different design concepts to improve the neutron flux level of the irradiation channel. The design concepts of high-flux reactors mainly include: 1) Directly increase the core power density. The fuel power density of this type of high-flux reactor is very high. A higher neutron flux can be obtained by directly constructing irradiation channels in the reactor. The maximum fast and thermal neutron fluxes appear in the reactor. Due to the extremely high power density, the core of this type of high-flux reactor is usually compact, such as the HFIR in the United States and the SM-3 in Russia. 2) Anti-neutron trap pool reactor. The core of this type of high-flux reactor is submerged at the bottom of a large reactor pool. The core area is highly under-moderated, the energy spectrum is hard, and the average fission cross section is small. The water in the reactor pool is used as both a moderator and a coolant, and the core has a high fast neutron flux. Light water, heavy water, beryllium, graphite and other materials are usually used as reflectors. Fast neutrons leaking from the core area are fully slowed down in the reflector. The reflector has a high thermal neutron flux, and the space of the high thermal neutron flux area is large. my country's CMRR adopts this design concept. 3) Slightly pressurized pool tank reactor. This type of high-flux reactor (such as my country's CARR) maintains the "anti-neutron trap" core design of fast and thermal neutron flux partitions, adds a small pressure tank to the core, and adopts a slightly pressurized design, which can increase the core power density. The reflector pool is still operated at normal pressure, which is convenient for operations such as replacing targets. 4) Pool shell high-pressure reactor. This type of high-flux reactor (such as the ATR in the United States) places the core in a pressure vessel, increases the pressure to about 2MPa, and uses forced circulation to cool the core, which can increase the allowable temperature of the fuel element surface, thereby further increasing the thermal load and neutron flux level of the fuel element surface.

[0004] However, it is difficult for existing high-flux reactors to strike a good balance between neutron flux levels, safety, and economy. The core power density of high-flux reactors that adopt the design concept of directly increasing the core power density is extremely high, which poses a great challenge to the thermal safety design of the reactor. High-flux reactors based on the "anti-neutron trap" design concept achieve fast and thermal neutron separation in the core and reflector positions, but peripheral thermal neutrons are difficult to converge, and it is difficult to achieve local high neutron flux levels. In order to obtain the highest possible irradiation channel neutron flux level at the lowest possible power level, it is of great engineering significance to propose a core design method for ultra-high flux research reactors. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a light water reactor based on the low self-shielding core design concept, wherein the light water reactor uses light water as a coolant, comprises a core fuel zone having a plurality of fuel assemblies and a reflective layer surrounding the core fuel zone on the radial outside, and is characterized in that a plurality of irradiation channels are formed between the fuel assemblies, and the core fuel zone is designed in such a way that the proportion of the area occupied by the irradiation channels in the pile within a migration length (approximately equivalent to the average penetration free path) that the neutrons generated by any fuel zone can reach is not less than 20%, and the neutrons do not pass through other irradiation channels before reaching the irradiation channels, wherein each of the irradiation channels is respectively filled with materials with different moderating properties suitable for producing fast neutrons and thermal neutrons.

[0006] Light water as coolant is located between fuel assemblies and in some gaps inside fuel assemblies. Since light water has a strong shielding effect on neutrons, the average penetration free path of neutrons in light water is short, and a large proportion of neutrons will be attenuated in the process of reaching the irradiation channel from the fission position, and the water layer in the thermal neutron channel also has a strong shielding effect on fast neutrons. Therefore, it is necessary to combine the core layout with low self-screening effect to better achieve a higher spatial separation of fast neutrons and thermal neutron fluxes. By properly designing the core size and the distribution position of the irradiation channel in the fuel area, the proportion of the area occupied by the irradiation channel in the pile can be no less than 20% of the area that the neutrons generated in any fuel area can reach within a migration length (approximately equivalent to the average penetration free path), reducing the penetration distance between each fission fuel area and the irradiation channel to the shortest possible, and making each neutron channel avoid the penetration path of other channels as much as possible, thereby reducing the mutual screening effect between irradiation channels. That is to say, the fuel assembly is arranged at the position where the fission neutrons contribute the most to each irradiation channel, and at the same time, the shielding effect of the moderator in the thermal neutron irradiation channel on the fast neutrons is minimized. The fast neutrons generated by fission enter either a fast neutron irradiation channel or a thermal neutron irradiation channel within a migration length and are moderated into thermal neutrons. In this way, a higher fission neutron utilization rate can be achieved under the premise of ensuring the critical requirements of the reactor.

[0007] According to a preferred embodiment of the light water reactor of the present invention, at least a portion of the irradiation channels are hollow or filled with weakly moderated materials such as aluminum. As an alternative or supplement, at least a portion of the irradiation channels are filled with light water as a moderator. The irradiation channels filled with weakly moderated materials have a higher fast neutron flux, while the irradiation channels filled with light water have a higher thermal neutron flux.

[0008] According to a preferred embodiment of the light water reactor of the present invention, the core fuel zone is divided into a central fuel zone located in the center and a peripheral fuel zone radially outside the central fuel zone, and the irradiation channels of the central fuel zone can be filled with aluminum or air. Aluminum, as a commonly used reactor structural material, has poor moderation ability, so the irradiation channels of the fuel assembly at the center of the core fuel zone have a very high fast neutron flux. It should be understood that the present invention is not limited to the technical solution of filling the irradiation channels of the central fuel zone with materials with poor moderation performance such as aluminum or air. Moderators or air or aluminum can be optionally filled in the irradiation channels in the central fuel zone or the peripheral fuel zone. The irradiation channels filled with moderators will become thermal neutron channels, and the irradiation channels filled with air or aluminum will become fast neutron channels. Obviously, each irradiation channel can also be designed to have different size ratios.

[0009] Preferably, there is only one irradiation channel in the central fuel zone, and several outer zone fuel assemblies in the peripheral fuel zone surround the central fuel assembly of the central fuel zone, which realizes a compact core structure and is beneficial to the core criticality and reactivity control. However, it is obvious that when the core is controlled to an appropriate size, multiple irradiation channels can also be provided in the central fuel zone, and these channels are filled with aluminum or air. In this way, each central irradiation channel can achieve the desired fast neutron flux. Similarly, the peripheral fuel zone can also have several irradiation channels distributed in several layers in the radial direction, that is, the irradiation channels of the peripheral fuel zone can also be arranged in multiple layers in the radial direction.

[0010] According to a preferred embodiment of the light water reactor of the present invention, the irradiation channels filled with a moderator suitable for producing fast neutrons and the irradiation channels filled with a moderator suitable for producing thermal neutrons are arranged alternately in the circumferential direction of the core fuel zone. Nevertheless, since the fuel content at various locations in the core and the neutron energy spectrum at various locations in the fuel zone are not much different, the irradiation channels at the circumferential direction of the core fuel zone can actually be filled with a moderator suitable for producing fast neutrons and a moderator suitable for producing thermal neutrons according to selection, without being restricted to a certain arrangement order. For example, the irradiation channels of the fuel assembly in one semicircle of the circular cross-section of the core fuel zone can be set to be mainly used for producing fast neutrons, while the irradiation channels of the fuel assembly in the other semicircle can be set to be mainly used for producing thermal neutrons. To this end, it is only necessary to fill the irradiation channels in one semicircle with weakly moderated material and fill the irradiation channels in the other semicircle with light water.

[0011] According to the present invention, neutrons generated at any fission position can directly reach at least two of the irradiation channels within a migration length, which determines the compact size structure of the core. Therefore, the reactivity of the light water reactor can be controlled only by a number of rotating control drums arranged inside the reflector layer without inserting control rods in the core area. The additional advantages of the design without control rods are firstly to avoid the shielding and obstruction of the path of fission neutrons reaching the irradiation channel by the absorber inserted into the core, thereby increasing the neutron flux of the irradiation channel, and secondly to flatten the axial power distribution of the core, because the control rods are inserted in the axial direction of the core, and their insertion depth will affect the axial power distribution of the core.

[0012] The fuel assemblies can be arranged in a ring layer form, which allows the coolant to flow inside the fuel assemblies. According to a preferred embodiment of the light water reactor of the present invention, each fuel assembly in the core fuel zone is respectively composed of multiple rings of fuel arc plates, and the gaps between adjacent fuel arc plates form coolant flow channels.

[0013] In a light water reactor, the average penetration free path of fission neutrons is short. Therefore, in the core design, the fuel content in various locations in the fuel zone and the neutron energy spectrum in various locations in the core fuel zone are not much different. Different neutron distributions are produced by utilizing the properties of different moderators in the irradiation channels. This allows thermal neutrons and fast neutrons to be separated in space, and both can be provided at a higher flux in the corresponding irradiation channels.

[0014] It should be understood that the structure composed of multiple circles of fuel arc plates is only an example of a fuel assembly that is easy to calculate, process and assemble. Obviously, it is also possible to consider using flat-plate fuel or forming a fuel assembly by stacking multiple full circles of fuel tube rings, or to construct the fuel assembly to have a spirally curled cross-section. These alternative implementation forms can also provide flow channels for coolant flow in the fuel assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The following is an explanation of the embodiments of the present invention in conjunction with the accompanying drawings. In the accompanying drawings:

[0016] Figure 1 is a schematic cross-sectional view of a core of a light water reactor according to the present invention;

[0017] Figure 2 is along Figure 1 A schematic diagram of an axial cross section along line AA;

[0018] Figure 3 It is a schematic cross-sectional view of a core of a light water reactor designed based on a low self-shielding core design concept according to an implementation form of the present invention;

[0019] Figure 4 Schematic diagram of a core cross section of a light water reactor designed based on a low self-shielding core design concept according to another embodiment of the present invention

[0020] Figure 5 is a schematic diagram of the distribution of thermal neutron flux (E<1 eV) in the core of a light water reactor according to the present invention;

[0021] Figure 6 It is a schematic diagram of the distribution of fast neutron flux (E>100eV) in the core of a light water reactor according to the present invention. DETAILED DESCRIPTION

[0022] Figure 1 The core cross section of the light water reactor according to the present invention is schematically shown. The core structure is axially symmetrical along the horizontal midplane. The fuel zone 1 of the core is surrounded by a reflector 2 on the radial outside, and is surrounded by an upper reflector 3 and a lower reflector 4 in the vertical direction. The radial reflector 2 is cylindrical, has an equivalent thickness of 37.5 cm, and is made of metal beryllium. In the core structure, the axial height of the fuel zone 1 is 50 cm, the axial height of the radial reflector 2 is 80 cm, and the thickness of the upper reflector 3 and the lower reflector 4 are both 40 cm.

[0023] exist Figure 1The central fuel area of ​​the core shown in the figure is composed of six central fuel assemblies 5 forming a closed ring and a central irradiation channel 8 formed by these central fuel assemblies 5. Each central fuel assembly 5 is composed of 23 circles of fuel arc plates and side plates on both sides. The inner radius of the central fuel assembly 5 is 6.55 cm, the outer radius is 14.8 cm, and the angle between the two side plates is 60°, thereby forming a fan ring with a central angle of 60° on the cross section. The thickness of each circle of fuel arc plates is 1.5 mm, and the width of the coolant flow channel between two adjacent fuel arc plates is 2.0 mm; the thickness of the fuel core is 0.7 mm, and U3Si2-Al dispersed fuel is used, and the U-235 enrichment is 19.75 wt%; the two sides of the fuel core are covered with T6061 aluminum alloy cladding with a thickness of 0.4 mm; the six central fuel assemblies 5 form a central irradiation channel 8 with a diameter of 13 cm, and the medium in the irradiation channel is aluminum.

[0024] The peripheral fuel zone is composed of six outer zone fuel assemblies 6 and six outer zone irradiation channels 9. Each outer zone fuel assembly 6 is composed of 19 circles of fuel arc plates and side plates on both sides. The thickness of each circle of fuel arc plates is 1.5mm, and the width of the coolant flow channel between two adjacent fuel arc plates is 2.0mm; the thickness of the fuel core is 0.7mm, and U3Si2-Al dispersed fuel is used, and the U-235 enrichment is 19.75wt%. The two sides of the fuel core are covered with T6061 aluminum alloy cladding with a thickness of 0.4mm. The inner radius of the outer zone fuel assembly 6 is 14.80cm, the outer radius is 21.65cm, and the angle between the two side plates is 30°, thereby forming a fan ring with a central angle of 30° on the cross section. Thus, in the other six 30° interval regions between the sector rings on the circumference, outer zone irradiation channels 9 are formed. The diameter of the outer zone irradiation channels 9 is 10 cm. Three of the irradiation channels are filled with beryllium or aluminum, and the remaining three are filled with light water. The irradiation channels filled with different moderation media are arranged alternately. Figure 1 This structural design enables neutrons generated at any fission position to directly reach at least two of the three irradiation channels, including the two adjacent outer zone irradiation channels 9 and the central irradiation channel 8, within one migration length, and the proportion of the irradiation channels in the area covered within one migration length is not less than 20%.

[0025] Figure 3 Combined with the core scheme introduced above, a schematic diagram describing this low self-shielding concept is given. The fuel assembly in the figure can be divided into three different fuel zones according to their relative positions to the duct. The circular area (such as Figure 3The area occupied by the irradiation channels in the pile is greater than 20%, so that the proportion of each fission neutron shielded by the fuel area is smaller, and the fission neutron has a higher probability of reaching the irradiation channels in the pile, and the neutron utilization rate is higher. This is the low self-shield core design referred to in the present invention.

[0026] According to the above-mentioned low self-shield core design concept, the core can also be similar to Figure 1 Other shapes only need to meet the above-mentioned low self-shield standard while ensuring the core criticality requirements. Figure 4 The core design shown also conforms to the design concept of low self-shield. Figure 3 The difference between the core cross-section structure shown is that the three outer zone irradiation holes 9 are replaced by three outer zone irradiation holes 9 which are closer to the radial inside and have a fan-shaped cross section. The two shapes of outer zone irradiation holes 9 are still arranged at intervals.

[0027] Figure 1 The radial reflection layer 2 shown is provided with 6 rotating control drums 7, which are composed of a beryllium matrix and an annular neutron absorber, the absorber is made of hafnium or B4C, the absorber thickness is greater than 1 cm, and the absorber coverage angle is 120°. The reactor control function can be realized by relying solely on the rotating control drum 7.

[0028] The core power is 75MW. In order to obtain the highest possible neutron flux level, the reactor adopts a pool shell reactor structure and the core operating pressure is 2.0MPa. At this power level, the average unperturbed fast neutron flux (E>100eV) of the central irradiation channel (8) is 2.09×10 15 n / cm 2 / s, and the average unperturbed fast neutron flux (E>100eV) of the irradiation channel (9) in the outer core filled with beryllium is 1.04×10 15 n / cm 2 / s, and the average undisturbed thermal neutron flux (E<1eV) of the irradiation channel (9) in the outer core filled with water is 1.97×10 15 n / cm 2 / s, with a very high neutron flux level, which can significantly improve the efficiency of radioisotope production and material irradiation testing. Figure 5 and Figure 6 The core thermal neutron flux (E<1 eV) distribution and the core fast neutron flux (E>100 eV) distribution of the light water reactor according to the present invention are schematically shown respectively.

[0029] As shown in Table 1, the maximum neutron flux in the core channel, the total core power, the maximum fuel surface heat flux density (which can approximately represent the maximum power density) and other parameters of the present invention are compared with those of typical high-flux reactors at home and abroad. From the results, it can be seen that the reactor core using the core design method proposed by the present invention can achieve ultra-high thermal neutron and fast neutron fluxes at a lower power density and total power level.

[0030] Table 1 Comparison of the core parameters of the present invention with those of the existing domestic high flux reactors

[0031]

[0032] The preferred embodiments of the present invention are described above, but the spirit and scope of the present invention are not limited to the specific contents disclosed herein. Those skilled in the art can make more embodiments and applications based on the teachings of the present invention, including any combination of the various technical features of the above embodiments. These embodiments and applications are all within the spirit and scope of the present invention. The spirit and scope of the present invention are not limited by the specific embodiments, but by the claims.

[0033] Reference numerals list

[0034] 1 Fuel Zone

[0035] 2 Radial reflection layer

[0036] 3 Upper reflection layer

[0037] 4 Lower reflection layer

[0038] 5 Center fuel assembly

[0039] 6 Outer fuel assembly

[0040] 7 Rotating control drum

[0041] 8 Central irradiation channel

[0042] 9 External zone irradiation channel.

Claims

1. A light water reactor, wherein the light water reactor uses light water as a coolant, comprises a core fuel region having a plurality of fuel assemblies and a reflector layer surrounding the core fuel region on the radial outer side, wherein: A plurality of irradiation channels are formed between the fuel assemblies, and the core fuel zone is designed so that the proportion of the area occupied by the irradiation channels in the core is not less than 20% of the area that the neutrons generated by any core fuel zone can reach within a migration length, and the neutrons do not pass through other irradiation channels before reaching each irradiation channel, wherein each irradiation channel is filled with materials with different moderating properties suitable for producing fast neutrons and thermal neutrons.

2. The light water reactor according to claim 1, characterized in that: At least a portion of the irradiation channels are filled with light water as a moderator so as to serve as thermal neutron channels.

3. The light water reactor according to claim 1, characterized in that: At least a portion of the irradiation channel is a hollow cavity so as to serve as a fast neutron channel.

4. The light water reactor according to claim 1, characterized in that: At least a portion of the irradiation channels are filled with weakly moderating material so as to serve as fast neutron channels.

5. The light water reactor according to claim 4, characterized in that: The weakly moderating material is aluminum.

6. The light water reactor according to any one of claims 1 to 5, characterized in that: The core fuel zone is divided into a central fuel zone located in the center and a peripheral fuel zone radially outside the central fuel zone.

7. The light water reactor according to claim 6, characterized in that: There is only one irradiation channel in the central fuel zone.

8. The light water reactor according to claim 6, characterized in that: The peripheral fuel zone has a plurality of irradiation channels distributed in a plurality of layers in a radial direction.

9. The light water reactor according to any one of claims 1 to 5, characterized in that: The light water reactor controls reactivity only by means of several rotating control drums arranged inside the reflector.

10. The light water reactor according to any one of claims 1 to 5, characterized in that: Each fuel assembly in the core fuel zone is respectively composed of a plurality of circles of fuel arc plates, and the gaps between adjacent fuel arc plates form coolant flow channels.

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