JPH06281775A - Core of fast reactor - Google Patents

Abstract

PURPOSE:To secure soundness of a core fuel by suppressing effects of a sudden fall in the coolant inlet temperature peculiar to a modular type reactor core. CONSTITUTION:In a modular type core of a fast reactor having a plurality of divided core regions 33 in a single atomic reactor vessel, void assemblies 25 each having a gas cavity container 12 having a hinged lid 48 and a sealed gas 13 in a modularized core region 33 or its peripheral region and a thermal expansion/contraction element connected from the bottom of this container 12 and having the function of opening by changes in coolant temperature of more than a predetermined change rate of temperature, are laid out.

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 breeder reactor (hereinafter referred to as a fast reactor) using liquid metal as a coolant.

[0002]

2. Description of the Related Art A fast reactor using liquid metal as a coolant constitutes a core by loading a large number of fuel assemblies filled with a nuclear fuel material. Sodium is mainly used as a coolant for removing heat from fuel. used.

In this fast reactor, if the flow rate of the coolant decreases for some reason, the coolant temperature rises, the coolant density decreases, and the reactivity is injected. The reactivity of fast reactors depends on the reactor power scale, and becomes negative in small reactors, but becomes positive in medium-sized reactors and large reactors, which may increase the reactor power due to the decrease in coolant density. .

In a fast reactor, sodium, which is usually a coolant, does not evaporate to form voids. However, in the unlikely event of an accident, even if sodium forms voids, the reactor can be safely shut down and the core It is designed to maintain safety.

As a response of the reactor when sodium, which is a coolant, is voided, when the core is small, neutrons leak from the core so much that the reactivity becomes negative as described above. It can be stopped safely.

On the other hand, when the core of the fast reactor becomes large due to the increase in output, the neutron leakage from the core decreases, and the reactivity when sodium, which is the coolant, becomes void becomes positive. In this positive reactivity state, it is necessary to confirm the safety of the core by performing a detailed analysis including other reactivity factors to determine whether or not the core will be safely stopped.

Therefore, in the prior art, safety is maintained by active / passive means. But,
It is important to reduce the sodium void reactivity in the core, and measures have been taken to make the sodium density coefficient of the coolant as a component of the normal power coefficient and temperature coefficient from positive value to zero or negative. .

Therefore, an island type modular core concept has been proposed in which the core size is reduced and the core is divided into a plurality of parts. As another measure, the positive reactivity of sodium as a coolant can be reduced by increasing neutron leakage from the core and softening the neutron spectrum.

Conventionally, in order to suppress and reduce the reactivity, the neutron absorbing material is loaded in the core, the height of the core is lowered to flatten and increase the neutron leakage, or beryllium or hydrogen is used. Means have been taken to soften the neutron spectrum by loading neutron moderators such as zirconium oxide.

Further, conventionally, as an apparatus for shutting down the reactor by utilizing the characteristics peculiar to the reactor even at an extremely low frequency event such as a scrum failure flow reduction type event, the output accompanying the decrease in the flow rate of the coolant is used. A technique for providing a thermal expansion element by utilizing the fact that the coolant temperature rises with an increase in the / flow rate ratio has been proposed.

On the other hand, there is almost no known device for applying a negative reactivity without using an external power source or a signal by paying attention to the rate of temperature change when the inlet temperature falls, which is the object of the present invention.

[0012]

If the void reactivity of sodium, which is a coolant, can be reduced, the safety and reliability of the core will be further improved, which is very effective in the safety design of the nuclear reactor. However, in a nuclear power plant such as a fast reactor, it is not preferable from the viewpoint of core safety in designing the plant that the core power is designed to be extremely small in terms of power generation cost.

Further, even if sodium, which is a coolant, should be voided, in order to make the reactivity of the core negative, a small reactor with a core output of, for example, about 100 MWe must be used.
If this core power is exceeded, the problem that the voiding reactivity of sodium becomes positive arises.

An example of a modular core is shown in FIG. This core has a modularized small core of A, B, C divided into three core regions by a boron shield 6. The coolant whose temperature has risen in the three cores is heat-exchanged through the intermediate heat exchanger, and the coolant whose temperature has been lowered through the pump again flows from the lower part of the core.

When the number of core modules is increased to 7, for example, it is often impossible to design the plant if the cooling system for each core is installed independently. Therefore, the outlet and inlet temperatures of a plurality of cores are normally set as a whole. A cooling system that is averaged is used.

In FIG. 9, reference numeral 1 is an internal fuel, 2 is an external fuel, 3 is a control rod, 4 is an internal blanket, 5 is an external blanket, 6 is a boron shield, and 7 is an external shield.

In the core having a plurality of divided core regions, for example, among the cores indicated by A, B, and C in FIG. 9, the control rod 3 in the core A is erroneously dropped. Assuming that the output of A is drastically reduced, if the amount of heat removed has the capacity as in the rated output, the outlet temperature of the intermediate heat exchanger will be lower than in the rated operation. Then, due to this, the reactivity is injected into the cores B and C due to contraction of the core support plate accompanying the decrease in inlet temperature.

Generally, since the core supporting plate has a large heat capacity, the positive reactivity of the core is gradually added due to shrinkage.
In the cores B and C as shown in 10, an event of reactivity injection type occurs. Therefore, the heat output increases in the cores B and C.

FIG. 10 shows a case where a reactivity of 40 ¢ is input (1
25% output), the output / flow rate ratio at the time of reactivity injection due to a decrease in coolant inlet temperature is shown. B, C
If the core output increases and the secondary system heat removal capacity is constant, the coolant inlet temperature slightly shifts to the inlet temperature side during rated operation due to the B and C core output increasing, and therefore decreases slightly. It will show a tendency and settle.

In the case where the modular cores A and B both have a power drop due to an accidental fall of the control rods due to some event, there is no operation such as inserting a control rod of only one modular core (core C). However, even if it is a short time, it becomes a factor of large reactivity input. Then, the output increases, and the integrity of the fuel element of the core C is threatened. In addition, in the plant design, the deviation of the temperature distribution of the coolant becomes large.

In the modularized core arrangement as described above, each core is provided with the boron shield 6 between the cores A, B and C as shown in FIG. 9 in order to facilitate the operation. It is necessary to install and eliminate neutron coupling between cores, but thermal coupling is inevitable in relation to the entire plant.

In such a situation, in a low void reactivity core composed of a plurality of modular cores, the frequency of occurrence is low, but the problem becomes the most severe when all but one module is shut down for some reason. There is.

The present invention has been made to solve the above-mentioned problems, and by suppressing the void reactivity of the core due to the coolant, when the flow rate of the coolant is reduced, when the core is overpowered, and bubbles are introduced into the core. In order to provide a fast reactor core that can improve the safety and reduce the amount of positive reactivity input factor in the core in the case of abnormality such as time, while improving the safety. There is.

In particular, in the present invention, as an effect of reducing the coolant void reactivity in a modular core in which the core is divided into a plurality of cores, the effect due to a sudden decrease in the coolant inlet temperature peculiar to the modular core is suppressed. It is to provide a core of a fast reactor capable of ensuring soundness.

[0025]

DISCLOSURE OF THE INVENTION The present invention provides a modular core of a fast reactor having a plurality of divided core areas in a single reactor vessel, wherein each modularized core area or a periphery of this core area is provided. In the region of (1), at least one voided assembly having a function of opening the opening / closing lid by incorporating a gas cavity container having an opening / closing lid and having a function of opening the opening / closing lid in response to a change in the coolant temperature above a predetermined temperature change rate is arranged. It is characterized by

[0026]

In the core of a fast reactor, a voided assembly is inserted between a plurality of fuel assemblies filled with fissile material. A gas cavity container having an opening / closing lid operates in response to a predetermined temperature difference and a coolant temperature change rate equal to or higher than a temperature change rate.

The opening / closing lid is carried out when a predetermined temperature change rate occurs within a certain time, and is opened by an opening mechanism. Simultaneously with this opening, the pressurized filling gas is opened. The opened gas pushes down the liquid surface of the coolant in the voided assembly. It is adjusted so that the pressed coolant liquid level is at least lower than the upper surface of the core region.

The principle of following the changes in the coolant temperature and the changes in the structural material temperature is determined by the balance of energy transfer in the concentrated capacity system as shown in FIG. If the change from the temperature at t ₀ to t ₂ is stepwise as indicated by reference numeral 1 in the figure,
It is expressed by the following equation, where τ is a time constant.

[0029]

[Equation 1]

T ₁ in the figure is 63% of the maximum temperature change of the substance.
Corresponding to. τ is the time from the beginning of this event to t ₁ .

Therefore, as shown in FIG. 8, when the temperature of the coolant sodium changes abruptly, by combining substances having different time constants (lines 2 and 3 in the figure) within a certain time, the substance is changed. The temperature rise can be changed.

For a simple cylindrical rod, if the rod has radius r and R,
For the same substance, τ ₁ = C ₁ P ₁ r / 2h, τ ₂ = C ₁ P ₁ R / 2h, τ ₂ / τ ₁ = R / r.

As a result, as shown in FIG. 7, the temperature that governs the axial thermal expansion and contraction of the rods having different diameters indicated by the large-diameter element 9 and the small-diameter element 10 which are fixed to the support 8 serving as a common fulcrum. However, there is a difference in a certain range, and the force applied to the lower surface of the fixed gas cavity container 12 is different in the surfaces fixed by the large diameter and small diameter elements 9 and 10.

As a result, an outlet for the enclosed gas 13 in the gas cavity container 12 is produced by creating a place where the pressure-concentrated portion of the V-shaped groove indicated by numeral 11 is easily broken when a stress difference is applied.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the core of a fast reactor according to the present invention will be described with reference to the drawings. FIG. 1 is a sectional view showing an example of a fast reactor having a core structure according to the present invention. The fast reactor has a safety container 15 provided inside a cavity wall 14a of a reactor building 14, and a reactor container 16 is accommodated in this safety container 15 to form a double container structure.

The reactor vessel 16 has a core portion loaded with nuclear fuel.
17 are housed. The core part 17 is installed in a state of being immersed in sodium 18 which is a liquid metal coolant filled in the reactor vessel 16, and is supported by a core support structure 19.

The reactor vessel 16 is divided into an upper plenum 20 and a lower plenum 21 by the core support structure 19. Above the core portion 17, a core upper mechanism 22 accommodating a control rod drive mechanism and the like (not shown) is installed.

Meanwhile, an intermediate heat exchanger 23 for exchanging heat between the primary coolant and the secondary coolant and a circulation pump 24 for forcibly circulating the primary coolant in the reactor vessel 16 are installed around the core portion 17. It

The core portion 17 constitutes an island type modular core as shown in an enlarged view in FIG. In FIG. 2, the same parts or parts having the same functions as those in FIG. 9 are designated by the same reference numerals.

That is, the central portion is divided into regular hexagons to form a core region, and a modular core is constructed in the core region. This modular core is composed of an internal fuel 1, an external fuel 2, a control rod 3 and an internal blanket 4.

Boron shields 6 are arranged on the outer side of this core, and are extended outward from six positions of the corners of the hexagon so as to be partitioned by the boron shields 6. Is a modularized core 31 of.

Therefore, FIG. 2 has a total of seven modular cores. It is to be noted that three voided aggregates 25, which will be described later, are inserted into the outer six modular cores 31, respectively.

The intermediate heat exchanger 23 and the circulation pump 24 are supported in a suspended state by an upper lid 26 and a roof slab 27 that cover the upper portion of the reactor vessel 16. A rotary plug 28 is rotatably provided at the center of the roof slab 27, and a small rotary plug 29 is rotatably supported at an eccentric position of the rotary plug 28. A cover gas space 30 is formed above the upper plenum 20. The space 30 is filled with a cover gas that is an inert gas.

2 and 3 are core structure diagrams showing a first embodiment of the core of the fast reactor according to the present invention. FIG. 3 is FIG.
3 is a cross-sectional view showing a core of a fast reactor which is one module shown in FIG.

The core portion 17 of the fast reactor comprises a small core 31 having the core structure shown in FIG. This core 31 has a core region 33 in which a large number of fuel assemblies 32 filled with fissile material are loaded.
Is formed in the central portion, a radial blanket region 34 is formed on the outside of the core region 33 in the radial direction, and a neutron shield 35 is formed outside the radial blanket region 34.
A neutron shielding region loaded with is formed.

An upper shaft blanket region 36 having a parent substance loaded on top of the fuel assembly 32 loaded in the core region 33 is
Lower shaft blanket regions 37 are formed in the lower portions thereof, and a gas plenum region 38 is provided above the upper shaft blanket regions 36. Below the lower shaft blanket region 37 is a gas plenum region or a neutron shielding region 39.

A plurality of voided assemblies 25 are inserted between the fuel assemblies 32 in the core region 33. Further, in the core area 33, the control rods 3 shown in FIG.

FIG. 4 is a structural diagram showing the relationship between the fuel assembly 32 and the voided assembly 25 in the core of the fast reactor shown in FIG.
In FIG. 4, UP represents the core upper end position (upper surface position of the core region), and LP represents the core lower end position.

The fuel assembly 32 and the voided assembly 25 are
A high pressure plenum 43 is supported on an upper support plate 41 of the core support plate 40, and between the upper support plate 41 and the lower support plate 42. The high pressure plenum 43 is supplied with sodium 18, which is the primary coolant discharged from the circulation pump 24 shown in FIG.

Inside the container body 44 of the voided assembly 25, a cylindrical lower open container 45 having an opening at the lower end is incorporated. A coolant passage 46 is formed inside the container body 44 and the lower open container 45. This lower open container 45
A gas cavity 47 is formed in the upper part of the.

The gas cavity 47 is formed by being housed in the gas cavity container 12. A pressurized gas is enclosed in the gas cavity container 12. An inert gas such as argon is suitable as the filling gas 13, but there is no particular limitation.

The gas cavity container 12 is provided with large-diameter elements and small-diameter elements 9 and 10 having different sizes. These elements 9 and 10 are fixed to a support plate 8 (having a hole through which sodium flows) serving as the same fulcrum, one of which is installed in the gas cavity container 12 and the other is installed in the opening / closing lid 48. . Around the opening / closing lid 48, there is a V-shaped groove 11 so that the thickness of the opening / closing lid 48 is thinner than that of the gas cavity container 12.

The coolant flow path 46 of the voided assembly 25 communicates with the high pressure plenum 43 shown in FIG. 1 through a coolant communication hole 50 formed in an entrance nozzle 49 which is a leg portion of the container body 44. There is. Further, the coolant flow path 46 is formed by the voided aggregate 25.
A coolant outflow hole 51 formed in the upper end plenum 20 communicates with the upper plenum 20.

As shown in FIG. 5, the gas cavity container 12
When the enclosed gas 13 in the gas cavity container 12 is opened to the inside of the lower open container 45, the coolant liquid level in the lower open container 45 is the upper surface of the core region 33 (U
It is set to be at least lower than P). In addition, the coolant communication hole 50 of the voided assembly 25 is a fuel assembly.
It is formed vertically above the 32 coolant communication holes 52.

By loading the voided assemblies 25 between the fuel assemblies 32 in the core region 33, the neutrons generated in the fuel assemblies 32 enter the gas cavities 47 in the axially upper part of the voided assemblies 25. This gas cavity to facilitate leakage
The neutron absorber 53 may be arranged on the upper portion of 47 as shown in FIG.

As a result, when a core shutdown event occurs in a modular core having a plurality of coolant inlet temperatures and one core scrum is not performed, the core filled with the above-mentioned enclosed gas is continuously output to the core. By expanding it, negative reactivity can be injected and the reactor output is prevented from rising. Therefore, the soundness of each module can be maintained.

In FIG. 7, materials having different heat capacities and specific heats of the large-diameter element 9 and the small-diameter element 10 can be combined, and the selection range increases depending on the shapes of these elements. Therefore, in the core of the fast reactor according to the present invention, it is important to eliminate the release of the enclosed gas at the time of normal start-up, stop, etc. according to the principle described in FIG.

Therefore, when the force applied to the V-shaped groove in the stress concentration portion shown by reference numeral 11 in FIG. 7 is the same on the left and right, it is necessary to make it so that it does not operate.
Since it is necessary to reduce the step-like temperature t ₂ so that the temperature ΔT shown in (1) becomes smaller than the predetermined range, it is necessary to keep the Na temperature rise and fall rates during operation within the range. This can be dealt with by combining such driving methods.

Next, a second embodiment of the present invention will be described. That is, the neutron absorber region 54 is arranged in the central portion, and the upper and lower blankets 55, 55 are formed on the upper and lower surfaces of the neutron absorber region 54, respectively.
56 is provided, and the upper core fuel part 57 is provided above the upper blanket 55.
And the lower core fuel portion 58 is arranged below the lower blanket 56. The lower blanket 56 and the lower core fuel portion 58 are provided with a pin bundle portion 59.

A support body 8 having the upper end of the upper blanket 55 as a fulcrum is provided, and the support body 8 is shown in FIG.
The large-diameter element 9 and the small-diameter element 10 shown in 1 and the gas cavity container 12 connected to the upper ends of these are provided.

In the vertically divided modular core, the decrease in the inlet temperature to the upper core due to the drop of the control rod in the lower core is a factor for the reactivity input. Therefore, the voided assembly 25 is connected to the upper core fuel part 57. It is located in. Thus, according to this embodiment, the voided aggregate 25 can be used in combination with fuel.

Next, the embodiments of the present invention will be listed. (1) In a modular core of a fast reactor having a core region divided into a plurality in a single reactor vessel, a gas cavity having an opening / closing lid in each of the modular core regions or in the region around the core region Disposing at least one voided assembly including a container and having a thermal expansion / contraction element having a function of performing an opening operation when the temperature of the coolant changes by a predetermined temperature change rate or more.

(2) The thermal expansion and contraction elements are connected to the gas cavity container by independent rod-shaped elements having the same material but different diameters, and the expansion of the elements opens the lid of the container to release gas. (3) The thermal expansion and contraction element should be configured so that the lid can be opened by using independent rod-shaped elements having different products of density and specific heat.

(4) The rate of rise and fall of the coolant temperature for starting and stopping the nuclear reactor should be below a predetermined value. (5) Independent cores are arranged at upper and lower locations, and coolant channels are made axially modular so that they flow from the bottom to the top in the same assembly. Place the voided assembly with the cavity container at the bottom and the fuel pin at the bottom.

[0065]

According to the present invention, in a plurality of nuclear decoupling partitioned modular cores, the thermal coupling characteristic of the core with reduced coolant void reactivity results. It is possible to maintain the integrity of the core even in the transient state of the reactivity injection type in which some of the modular cores accompanied by the decrease of the coolant inlet temperature are stopped and some of the cores are operated. Further, since the response is quicker than that of the core support plate having a large heat capacity, it is possible to reach the stop of the reactor in a state where the output increase is small.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing a tank type fast reactor for explaining an example of the core of the fast reactor according to the present invention.

FIG. 2 is a plan view showing a modular core in FIG.

FIG. 3 is a vertical cross-sectional view schematically showing the single modular core shown in FIG.

FIG. 4 is a vertical cross-sectional view showing the voided aggregate in FIG.

5 is a vertical cross-sectional view for explaining the action of the voided aggregate in FIG.

FIG. 6 is a conceptual diagram showing another example of the voided aggregate in FIG.

FIG. 7 is a conceptual diagram for explaining the operation of the voided aggregate in FIG.

FIG. 8 is a characteristic diagram showing different temperature rises in the modular core shown in FIG.

FIG. 9 is a plan view showing an island modular core of a conventional fast reactor.

10 is a characteristic diagram showing an output and a flow rate ratio when a reactivity is charged due to a decrease in coolant inlet temperature in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Internal fuel, 2 ... External fuel, 3 ... Control rod, 4 ... Internal blanket, 5 ... External blanket, 6 ... Boron shield, 7 ... External shield, 8 ... Support body, 9 ... Large diameter element, 10 ...
Small-diameter element, 11 ... V-shaped groove, 12 ... Gas cavity container, 13
… Filled gas, 14… Reactor building, 14a… Cavity wall, 15… Safety container, 16… Reactor container, 17… Reactor core part, 18…
Sodium, 19 ... Core support structure, 20 ... Upper plenum,
21 ... Lower plenum, 22 ... Upper core mechanism, 23 ... Heat exchanger,
24 ... Circulation pump, 25 ... Voided assembly, 26 ... Upper lid, 27
… Roof slab, 28… swivel plug, 29… small rotation plug,
30 ... Cover gas space, 31 ... Small core, 32 ... Fuel assembly,
33 ... Core region, 34 ... Diameter blanket region, 35 ... Neutron shield, 36 ... Upper shaft blanket region, 37 ... Lower shaft blanket region, 38 ... Gas plenum region, 39 ... Neutron shielding region, 40 ... Core support plate, 41 ... Upper support plate, 42 ... Lower support plate, 43 ... High pressure plenum, 44 ... Container body, 45 ... Lower open container, 46 ... Coolant passage, 47 ... Gas cavity, 48 ... Open / close lid, 49 ... Entrance nozzle, 50 ... Coolant communication hole, 51 ...
Coolant outflow hole, 52 ... Coolant communication hole of fuel assembly 32, 53 ...
Neutron absorber, 54 ... Neutron absorber region, 55 ... Upper blanket, 56 ... Lower blanket, 57 ... Upper core fuel part, 58
… Lower core fuel part, 59… Pin bundle part.

Claims (1)

[Claims] 1. A modular core of a fast reactor having a plurality of divided core regions in a single reactor vessel,
A function of incorporating a gas cavity container having an opening / closing lid in each of the modularized core areas or the area around the core area, and opening / closing the opening / closing lid by a change in the coolant temperature at a predetermined temperature change rate or more. A core of a fast reactor, wherein at least one voided aggregate having the above is arranged.