JP2914801B2 - Fast reactor core - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a core of a fast reactor using liquid metal as a coolant.

[0002]

2. Description of the Related Art In a fast reactor using liquid metal as a coolant, a core is constituted by loading a large number of fuel assemblies filled with fissile material, and liquid sodium mainly removes heat from the fuel. Used as a coolant. In this liquid metal-cooled fast reactor, when the coolant flow rate decreases for some reason, the coolant temperature increases, the coolant density decreases, and the reactivity is charged. The reactivity of the fast reactor depends on the power scale of the nuclear reactor and is negative in small reactors, but becomes positive in medium and large reactors, and may increase the reactor power due to a decrease in coolant density.

[0003] In a fast reactor, liquid sodium, which is a coolant, usually does not evaporate and form voids. However, in the unlikely event of an accident, even if sodium is voided, the reactor can be shut down safely and the core can be kept safe. The design is maintained. As a response of the reactor when sodium, which is a coolant, is voided, if the core is small, neutrons leak from the core greatly, and as described above, the reactor has a negative reactivity and the core is safe. Can be stopped. On the other hand, when the core of the fast reactor becomes large, the leakage of neutrons from the core decreases, and the reactivity when sodium as the coolant is voided becomes positive. In this positive reactivity state,
Whether or not the core shuts down safely needs to be analyzed in detail, including other reactivity factors, to confirm the safety of the core.

[0004] Positive reactivity by the coolant sodium can be reduced by increasing neutron leakage from the core or softening the neutron spectrum. For this reason, conventionally, in order to suppress and reduce the reactivity, a neutron absorbing material is loaded in the core, the neutron leakage is increased by lowering the core height, and the neutron leakage is increased, or beryllium or hydrogenation is performed. Means have been taken to load a neutron moderator such as zirconium to soften the neutron spectrum.

[0005]

If the void reactivity of sodium, which is a coolant, can be reduced, the safety and reliability of the reactor core can be further improved, which is very valuable in the safety design of a nuclear reactor.

However, in a nuclear power plant such as a fast reactor, it is not preferable in terms of power plant cost to design the reactor core power to be extremely small from the viewpoint of core safety. Also, even if sodium as a coolant is voided, in order to make the reactivity of the core negative, the core power must be reduced to, for example, a small furnace of about 100 MWe. There will be a problem that the voiding reactivity of sodium becomes positive.

The present invention has been made in view of the above points, and suppresses the void reactivity of the core due to the coolant to reduce the flow rate of the coolant, the overpower of the core, It is an object of the present invention to provide a fast reactor core capable of obtaining a core with a higher output while improving safety by reducing the factor of positive reactivity input into the core in the case of an abnormality such as inflow of bubbles. The purpose is.

[0008]

In order to achieve the above object, the present invention relates to a core region in which a coolant containing fissile material and made of liquid metal flows upward from below in the core region, and the core region. A void region provided in parallel with the axial center of the core region.The void region has a gas region formed in an upper portion thereof, and a coolant inflow hole and a coolant inlet hole in a lower portion and an intermediate portion in the axial direction. Provided is a core of a fast reactor, wherein the core is formed by a cladding tube formed with a coolant outlet hole, and further, in the core of the fast reactor, an axial direction in the cladding tube constituting the voided region is provided. A core of a fast reactor characterized by providing a heating element below.

[0009]

In the core of the fast reactor constructed as described above, the liquid level of the coolant in the cladding tube forming the voiding region fluctuates due to the pressure change of the coolant flowing into the cladding tube. In addition, by disposing the heat generating substance in the lower part, the coolant in the cladding tube forming the voided region is voided at the time of overpower.

[0010] As a cause of sodium void as a coolant in the cladding tube, when the flow rate of the coolant decreases and the sodium temperature rises and boiling occurs, the core power abnormally rises, that is, overpower occurs and the sodium temperature rises. In some cases, a large amount of gas may be mixed into the coolant outside the core and flow into the core.

In the case of a decrease in the coolant flow rate, the enclosed gas expands due to a decrease in the coolant pressure of the voided assembly, and an enlarged cavity is formed. The neutrons generated in the fuel assembly leak from the voided assembly through the expanded cavity mainly upward in the core, and the reactivity becomes negative.

In the case of overpower, the amount of neutrons and gamma rays reaching the exothermic substance in the cladding tube constituting the voided region increases, and the exothermic quantity of the exothermic substance due to the absorption of neutrons and gamma rays increases. The coolant in the cladding tubes that make up the region boils. As a result, neutrons generated in the fuel assembly leak from the cladding tube forming the voided region mainly through the boiling coolant through the core, and the reactivity becomes negative.

When gas flows from outside the core, the gas flows more into the coolant flow path in the cladding tube forming the voiding region. As a result, as in the case of the decrease in the flow rate of the coolant, neutrons generated in the fuel assembly leak mainly from the cladding tube constituting the voided region upward in the core, and the reactivity becomes negative.

As described above, the void reactivity of sodium as a coolant can be reduced as compared with a core without a cladding tube constituting a voided region, and a core with a larger output can be obtained while improving safety. A fast reactor core can be provided.

[0015]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a longitudinal sectional view showing one embodiment of a fast reactor equipped with a core according to the present invention. The fast reactor has a reactor safety vessel 2 provided on a cavity wall 1a of a reactor building 1, and a reactor vessel 3 is accommodated in the safety vessel 2 to form a double vessel structure. The reactor vessel 3 contains a reactor core 4 loaded with nuclear fuel. The core 4 is immersed in sodium 5 which is a liquid metal coolant filled in the reactor vessel 3, and is supported by a support structure 6. The inside of the reactor vessel 3 is divided into an upper plenum 7 and a lower plenum 8 with the support structure 6 as a boundary.
Above the core 4, an upper core mechanism 10 containing a control rod drive mechanism (not shown) and the like is installed. Around the core 4, an intermediate heat exchanger 11 for exchanging heat between the primary coolant and the secondary coolant and a circulation pump 12 for forcibly circulating the primary coolant in the reactor vessel 3 are provided. The intermediate heat exchanger 11 and the circulating pump 12 are suspended from an upper lid 13 and a roof slab 14 that cover the upper part of the reactor vessel 3. Roof slab
A rotary plug 15 is rotatably provided at the center of the rotary plug 14, and a small rotary plug 16 is rotatably supported at an eccentric position of the rotary plug 15. A cover gas space 17 is formed above the surface of the sodium 5 of the reactor vessel 3, and the cover gas space 17 is filled with a cover gas which is an inert gas.

FIG. 2 is a core configuration diagram showing a first embodiment of the fast reactor core according to the present invention. FIG. 3 is a cross-sectional view showing the core of the fast reactor shown in FIG. The core 4 of the fast reactor
It is configured like a core 20 shown in FIG. The core 20 has a core region 22 in which a large number of fuel assemblies 21 loaded with fissile material are loaded at the center thereof.
A radial blanket region 24 in which a radial blanket aggregate 23 is loaded in an annular shape is formed on the outer peripheral side. On the outer peripheral side of the diameter blanket region 24, a neutron shielding region 26 loaded with a neutron shield 25 is formed. The fuel assembly 21 loaded in the core region 22 has an upper shaft blanket region 28 loaded with a parent material on the upper portion, and a lower shaft blanket region 28
29 are formed, and a gas plenum region 30 is provided above the upper shaft bracket region 28. Below the lower shaft blanket region 29 is configured as a gas plenum region or a neutron shielding region 31. The core region 22 is loaded with a plurality of voided assemblies 33 in addition to the fuel assemblies 21. Further, control rods 52 are arranged in the core region 22 so as to be dispersed in the horizontal direction.

FIG. 4 is a perspective view showing the relationship between the fuel assembly and the voided assembly of the core shown in FIG. UP indicates the core upper end position (upper surface position of the core region), CP indicates the core center position, and LP indicates the core lower end position. Fuel assembly
21 and voided assembly 33 are upper support plates of core support plate 34
The high-pressure plenum 37 is formed between the upper support plate 35 and the lower support plate 36. The high-pressure plenum 37 is supplied with sodium 5 which is a primary coolant discharged from a circulation pump. The voided assembly
In 33, a tubular coolant passage 40 is formed inside a cladding tube 53, and a gas cavity 41 in which a sealed gas is sealed is formed above the coolant passage 40. An inert gas such as argon is suitable for the filling gas, but is not particularly limited. The coolant flow passage 40 of the voided assembly 33 communicates with the high-pressure plenum 37 via a coolant inflow hole 44 formed in an entrance nozzle 43 as a leg thereof. This coolant inflow hole 44 is formed above a coolant inflow hole 46 formed in the entrance nozzle portion 45 of the fuel assembly 21. The coolant outlet hole 48 is formed below the vicinity of the axial center of the core 20. The liquid level (free liquid level) of the coolant flowing into the coolant channel 40 of the voided assembly 33 moves in the core axis direction due to the pressure of the coolant in the high-pressure plenum 37. The coolant level is
During normal operation of the fast reactor, the top level of the core (core area 22
Upper end) Set higher than UP. In the event of a coolant flow rate drop accident, the coolant level is reduced to the core top level U.
The internal shape of the coolant channel 40 and the amount of the filled gas are set so as to be lower than P. Specifically, the coolant level of the coolant channel 40 is set to be higher than the upper shaft blanket region 28 during normal operation, and to be located near the axial center of the core 20 when the coolant flow rate is reduced. In this way, the neutron leakage from the fuel assembly 21 to the voided assembly 33 is increased,
The negative reactivity effect can be increased.

Next, the operation of the first embodiment having such a configuration will be described. In the fast reactor, the liquid sodium as the coolant is voided when the sodium temperature rises due to a decrease in the coolant flow rate and boiling occurs, and a large amount of gas such as cover gas flows into the coolant outside the core. Flow into the reactor core 20.

If the coolant flow rate decreases, the high pressure plenum
The coolant pressure in 37 decreases, and with this pressure decrease, the coolant pressure in voided assembly 33 also decreases, and the charged gas expands. Due to the expansion of the charged gas, the liquid level of the coolant flow path 40 moves below the core top level UP, and the gas cavity 41
Increase. In this case, the coolant inside the voided assembly 33 first voids as the coolant pressure inside the voided assembly 33 decreases. Simultaneously with or subsequent to the void formation, the coolant in the fuel assembly 21 is also voided due to an increase in the coolant temperature. However, since the coolant in the coolant passage 40 in the voided assembly 33 is voided, neutrons generated in the fuel assembly 21 leak from the voided assembly 33 toward the upper part of the core through the gas cavity 41. And a negative reactivity is obtained.

Next, let us consider a case where gas flowing from outside the core mixes with the coolant and flows into the core. In this case, as shown in FIG. 4, the coolant inlet 44 of the voided assembly 33 is provided vertically above the coolant inlet 46 of the fuel assembly 21. On the other hand, the gas mixed into the coolant discharged from the circulation pump 12 to the high-pressure plenum 37 is lighter than the coolant, and is accumulated in a lower portion near the upper support plate 35. For this reason, the gas accumulated in the lower portion of the upper support plate 35 flows into the voided assembly 33 before flowing into the fuel assembly 21 and the coolant passage 40 inside the voided assembly 33.
Is voided. Subsequently, the gas is mixed into the coolant and flows into the fuel assembly 21, and the same operation and effect as in the case of the decrease in the coolant flow rate are obtained. If gas mixing into the coolant outside the reactor core is not considered, the coolant inlet hole 44 of the voided assembly 33
And the coolant inflow hole 46 of the fuel assembly 21 may be in any positional relationship.

Further, by loading the voided assemblies 33 between the fuel assemblies 21 in the core region 22, neutrons generated in the fuel assemblies 21 are transferred to the cavity channels 41 axially above the voided assemblies 33. The neutron absorber 49 may be arranged as shown in FIG.

As a result, the sodium void reactivity of the core of the present embodiment can be reduced to zero or negative as compared with the core having no voided aggregate. For example, in the case of a flat core having a core height of 60 cm and a core diameter of about 300 cm, the sodium void reactivity at the core height after 365 days of combustion without a cavity assembly is about 3 mm. When the voided assembly 33 is arranged as shown in FIG. 3, the sodium void reactivity in the core of the same size is determined from the height of the core from the upper end height of the upper axial blanket region (thickness 35 cm) 28. When the core is voided up to the center height and the core region 22 of the fuel assembly 21 is voided, it becomes about -0.5 mm. Cavity assembly
In the case where 33 is voided to 30 cm above the height of the upper shaft blanket region 28 and the core region 22 of the fuel assembly 21 is voided, the angle becomes about -2 °.

As described above, according to this embodiment, in the core of the fast reactor, a large number of fuel assemblies are loaded to form a core region, and a cylindrical cooling member is provided between the fuel assemblies in the core region. A voided assembly in which a material flow path is formed and a sealed gas such as an inert gas is sealed is loaded. The coolant flow path of this voided assembly is provided with a coolant inflow hole through which a coolant can enter and exit, and the coolant level of the coolant flow path is set to change in the core axis direction by a change in pressure of the coolant. I do. As the flow rate of the coolant circulating in the reactor vessel decreases, the coolant pressure decreases. Then, due to the decrease in the coolant pressure, the coolant pressure in the voided assembly decreases, and the sealed gas expands,
The voided assembly first becomes voided, and the liquid level in the coolant channel drops. Due to this lowering of the liquid level, neutrons generated in the fuel assembly leak mainly from the voided assembly upward in the core, and the sodium void reactivity can be kept low. Further, a coolant inflow hole into the coolant flow path of the voided assembly is provided above the coolant inflow hole into the fuel assembly. Therefore, even when a gas such as a cover gas outside the reactor core is mixed into the coolant, the mixed gas is accumulated upward due to a difference in density with the coolant, and flows from the fuel inlet through the coolant inlet of the voided assembly. Also flows in front. Therefore, the same operation and effect as in the case of the decrease in the coolant flow rate can be obtained. in this way,
The sodium void reactivity can be kept small, a core with a higher output can be obtained, and the safety or soundness of the furnace can be sufficiently maintained even with a large core. Next, a second embodiment of the present invention will be described with reference to FIG.

In FIG. 5, the same portions as those in FIG. 4 are denoted by the same reference numerals, and the description of the configuration of the portions will be omitted. In the cavity assembly 33, a heating substance 47 is arranged at a position corresponding to a position lower than the core lower end position LP of the adjacent fuel assembly 21. The heat generating substance 47 is desirably a substance that generates a large amount of heat due to neutron absorption or gamma ray absorption. For example, boron carbide B ₄ C and tungsten are suitable. The exothermic substance 47 is covered with a cladding tube made of stainless steel so as not to cause a chemical reaction with the coolant. The cooling flow rate during normal operation in the voided assembly 33 is
The exothermic material 47 is properly cooled and no boiling occurs in the voided mass 33. Further, at the time of over-output, the size and number of the coolant inflow holes 44 and the coolant outflow holes 48 are set such that the coolant in the voided aggregate 33 boils due to the increase in the heat generation amount of the heating substance 47. The values are adjusted to maintain an appropriate value.

The operation of the second embodiment will be described. Consider the case where over-power occurs and the sodium temperature rises. When an overpower occurs, the coolant temperature in the fuel assembly 21 naturally rises, but also in the voided assembly 33, the amount of neutrons and gamma rays generated in the fuel assembly 21 increases. The amount of neutrons and gamma rays reaching the pyrogen 47 also increases. Accordingly, the amount of heat generated by neutron absorption or gamma ray absorption of the heat generating substance 47 increases, and the coolant temperature rises. Here, the coolant temperature at the coolant outlet hole 48 of the voided assembly 33 during normal operation is set higher than the coolant outlet temperature of the fuel assembly 21 until the coolant in the voided assembly 33 is voided. Keep the margin of Therefore, the coolant in the voided assembly 33 is voided before the fuel assembly 21.

Therefore, before the coolant in the fuel assembly 21 is voided at the overpower, the coolant in the voided assembly 33 is voided, and neutrons are passed through the gas cavity 41 of the voided assembly 33 to the core. Leakage in the upward direction has a negative reactivity effect, improving the safety of the core.

Further, a third embodiment will be described with reference to FIGS. FIG. 6 shows the third core of the fast reactor according to the present invention.
It is a core structure figure showing an example. FIG. 7 is a cross-sectional view showing the core of the fast reactor shown in FIG. The core of the fast reactor shown in this embodiment is a core region constituted by loading a large number of fuel assemblies 21 in addition to the core structure shown in FIGS.
A plurality of voided aggregates 33 are collectively arranged in a central part of 22. The other core configurations are basically the same as those shown in FIGS. 2 and 3, and thus the same reference numerals are given and the description is omitted. In the core of this fast reactor, a voided assembly 33 is arranged at the center of the core in the axial direction of the core.
Thereby, the neutron leakage effect toward the center of the reactor core is promoted when sodium as a coolant is voided. Further, the voided assembly 33 is loaded adjacent to the control rod 52 disposed in the core region 22 of the flat core, or is loaded in an annular shape along the circumferential direction, so that neutron leakage in the control rod direction is positively performed. It can be used. At that time, if the plenum region 51 is arranged between the upper axial blanket region 50 and the core region 22 of the fuel assembly 21 or the axial blanket region 50 is deleted, neutrons leaking upwards are more likely to leak through the plenum region 51. Become. In the embodiment of the present invention, the example in which the voided assemblies are arranged between the fuel assemblies in the core region is shown. However, the arrangement of the voided assemblies 33 or the configuration in the voided assembly is limited to the above example. It is not something that can be done. The voided aggregates may be arranged so that the reactivity of sodium voids is reduced when sodium as a coolant is voided, and the shape and structure may be set so as to satisfy the above-described effects. In addition, the present invention can provide the same effect of improving safety not only in a large furnace but also in a small furnace having a small void reactivity.

[0029]

As described above, according to the present invention,
By reducing the reactivity of the core by the coolant,
By reducing the factors that increase the reactor's positive reactivity in the event of abnormalities such as when the flow rate of the coolant drops, when the core is overpowered, or when air bubbles flow into the core, it is possible to improve safety and improve power output. A fast reactor core from which a core can be obtained can be provided.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing one embodiment of a fast reactor equipped with a core according to the present invention.

FIG. 2 is a core configuration diagram showing a first embodiment of the core of the fast reactor according to the present invention.

FIG. 3 is a cross-sectional view showing a core of the fast reactor shown in FIG. 2;

FIG. 4 is a perspective view showing a relationship between a fuel assembly and a voided assembly of the core shown in FIG. 2;

FIG. 5 is a perspective view showing a second embodiment of the voided assembly according to the present invention.

FIG. 6 is a core configuration diagram showing a third embodiment of the core of the fast reactor according to the present invention.

FIG. 7 is a cross-sectional view showing a core of the fast reactor shown in FIG. 6;

[Explanation of symbols]

 5 ... Sodium 20 ... Core 21 ... Fuel assembly 22 ... Core region 24 ... Diameter blanket region 26 ... Neutron blocking region 28,50 ... Upper shaft blanket region 29 ... Lower shaft blanket region 30,51 ... Gas plenum region 31 ... Gas plenum region or Neutron shielding area 33 ... Voided assembly 40 ... Coolant flow path 41 ... Gas cavity 44 ... Coolant inflow hole 46 ... Coolant inflow hole 47 ... Heat generation substance 48 ... Coolant outflow hole 53 ... Clad tube

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁶ , DB name) G21C 5/00 GDF G21C 7/32 G21C 9/00

Claims (2)

(57) [Claims] 1. A core region in which a coolant made of a liquid metal and containing a fissile material flows through the inside from below to above, and a void is provided in the core region so as to penetrate in parallel with the axial center of the core region. The voided region is formed by a cladding tube having a gas region formed therein at an upper portion thereof and a coolant inlet hole and a coolant outlet hole formed at a lower portion and an intermediate portion in an axial direction. A fast reactor core. 2. The fast reactor core according to claim 1, wherein
A fast reactor core, wherein a heating element is provided axially below the cladding tube forming the voided region.