JP2892824B2 - Small reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a small nuclear reactor having a long life reactivity,
In particular, it relates to a fast reactor using liquid metal sodium as a coolant.

(Prior art) Reactivity control of a reactor adjusts the enrichment of the loaded fuel to compensate for the operation cycle length such as a predetermined decrease in combustion reactivity and the amount accompanying the rated output operation, For the excess reactivity for that purpose, the criticality is adjusted by changing the insertion state of the neutron absorbing substance such as a control rod. The arrangement of the control rods is optimized together with the flattening of the power distribution while satisfying the furnace shutdown conditions. The arrangement of the control rods is optimized together with the flattening of the power distribution while satisfying the furnace shutdown conditions. This method has been adopted over a wide core power range from small to large cores.

From the viewpoint of reactivity control, a method of controlling the amount of neutron leakage is used in addition to the method using the control rod.
In particular, it is effective in a small core where neutrons leak from the core a lot, and in the past, the experimental reactor SEFOR was operated (SEFOR,
Critical 1969, UO ₂ -PuO ₂ fuel, Na cooling, heat output 20MW, core size about 566l).

In the SEFOR furnace, there is a reflector area divided into sectors outside the core, and the divided reflector is moved up and down,
The neutron leakage is controlled by changing the relative positional relationship with the core.

As a design example of the concept of a small core using a reflector, a report on a 10MWe-class plant for military use in the United States is referenced (IECEC
−87 Intersociety Energy Conversion Engineering Co
nference TAMoss and EBBaumeister, “A. Liquid-M
etal Reactor / Air Brayton−cycle option for a Multi
megawatt Terrestrial Power (MTP) Plant "Proc.of IEC
EC-87.p1596).

This core aims at a long life (10 years) using enriched uranium oxide fuel. The core heat output is 55MWth (electric power 5500KW), a small power scale. However, the reactivity control of the core is performed by raising and lowering the reflector outside the furnace vessel. It has been shown that by adjusting the core length, the operation period can be adjusted to some extent, utilizing the obvious fact that the core power density is adjusted.However, the relationship between the void coefficient reduction and the reflector length is described. Not. In this report, the void coefficient of the coolant is easily negative because enriched uranium is used.

(Problems to be Solved by the Invention) By the way, in a fast reactor utilizing plutonium (Pu), the density coefficient of the coolant (Na) is set to be positive (the reactivity decreases with the decrease in the coolant density). This may help mitigate virtually possible ATWS (non-scrum transients) events. In particular, in the case of a flow reduction type event, a negative feedback reactivity accompanying a temperature change of a core component improves the natural reactor shutdown capability.

However, it is important but not easy to realize a core equipped with this feature in a system using Pu fuel from the viewpoint of improving the safety of the reactor. In particular, many studies have shown that it is difficult to achieve both long life of the reactivity (maximization of (reactor power) x (operating period)) and non-correction of the void reactivity. ing.

The general tendency of reducing the amount of combustion reactivity reduction in one cycle for a longer period and the general tendency of void reactivity are shown in Fig. 19 as the tendency obtained with the conventional cylindrical core. It has become. This is because, as shown in Fig. 20, the void coefficient is reduced, and the fuel enrichment is increased (in the void effect, the neutron leakage Is required by increasing the number of the components), and internal conversion is reduced. Increasing the core burnup also increases neutron absorption by fission products (FPs), hardens the spectrum and shifts the void coefficient to the positive side. It has been known from the comparison of the core spectrum and the nuclear cross-section to install a region using high-concentration uranium as fuel or to use it in the entire furnace region in order to reduce the void reactivity.

As described above, in the Pu-based fast reactor, the void coefficient is made negative (non-positive) during the reactivity life, and special measures are required to achieve a long reactivity life. In particular, when the life of a core with a small reflector is to be extended, the neutron leakage of the core is suppressed by surrounding the core with the reflector, which is contrary to making the void coefficient non-positive during the life. Direction.

The present invention has been made in consideration of such points,
When using plutonium as fuel to extend the life,
In any combustion state during the life, make sure that the core void coefficient is not positive, and even if the core flow rate decreases,
An object of the present invention is to provide a small nuclear reactor capable of enhancing the ability of a nuclear reactor to shut down naturally by a unique reactivity feedback action.

[Configuration of the invention]

(Means for Solving the Problems) The present invention utilizes plutonium as all or a part of fuel as means for achieving the above object,
In a small nuclear reactor that uses liquid metal as a coolant and controls the reactor reactivity by driving a reflector installed outside the core, the height of the core fuel part is H, the equivalent diameter of the core is D, When the effective length of the reflector is L, the core is H / D
> 1 and the L / H is 0.4 ~
The range is 0.6.

(Operation) In the small nuclear reactor according to the present invention, when the height of the core fuel portion is H, the equivalent diameter of the core is D, and the effective length of the reflector is L, the core has H / D> 1. It is formed so as to have a satisfactory vertically long cylindrical shape and L / H is in the range of 0.4 to 0.6. For this reason, even if the core void coefficient during the operation period becomes zero or less (negative) and a flow rate reduction event during rated output operation, which is an extremely low frequency event, occurs, It is possible to shut down and maximize (reactivity life of one batch core) x (furnace output)
Furnace reactivity life can be extended.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a small nuclear reactor according to a first embodiment of the present invention. In this small nuclear reactor, a reflector 2 is installed outside a reactor core 1 having a vertically long cylindrical shape. The reactivity of the furnace is adjusted mainly by moving the furnace in the vertical direction (moving in the radial direction and adjusting the distance to the reactor core 1 can be used together).

As shown in FIG. 1, an upper reflector region 3 and a lower reflector region 4 are formed at the upper and lower ends of the core 1, respectively. Core barrel 5, Na downcomer 6 and furnace vessel (safety vessel)
7 are provided, and a gap 8 is provided on the outer peripheral side of the furnace container 7.
The reflector region 9 is formed through the substrate. The reflector 2 moves up and down the reflector region 9.

This small reactor has a thermal output of 125 MWth and aims for a 10-year reactivity life. The height H of the core 1 is about 3 m, the equivalent diameter D of the core 1 is about 92 cm, and the effective length of the reflector 2 L is about
It is set to 1.7m.

In the early stage of the life, the upper end of the reflector 2 is in the lower reflector region 4 and is in a so-called “bare core” state. In order to satisfy certain requirements, the height H of the core 1, the fuel volume ratio, and the layout and type of the fuel body are determined. For example, since the core 1 has an axial size longer than the radial size, the subcriticality of the “bare core” is one factor that substantially determines the radial sizing fuel enrichment.

FIG. 2 is a horizontal cross-sectional view of the core 1.
Is constituted by arranging a normal assembly 10 and a special shape assembly 11 in a core barrel 5, and uses a liquid metal as a coolant 12. The core 1 flattens the radial distribution of the aggregate power, increases the neutron leakage in the radial direction, increases the controllability of the reflector 2, and increases the leakage of the entire core (the non-reflective portion). From the leak)
A radial enrichment distribution is given. That is, the center has a low enrichment, the outer periphery has a medium enrichment, and the outermost periphery has a high enrichment.

Further, as shown in FIGS. 3A and 3B, each of the assemblies 10, 11 is composed of a fuel pin 13 and a tie rod 14, and is intended to increase the control ability of the reflector 2 and reduce the void coefficient. Each of the assemblies 10 and 11 is a ductless assembly.

Next, in order to make the core void coefficient “non-positive (zero)”, the selection range of important and basic parameters in setting the basic specifications of the core design is explained. The range to be satisfied will be described.

FIG. 4 is a graph showing the relationship between the core diameter R, the amount of reactivity that can be applied by the reflector that determines the reactivity life, and the amount of combustion reactivity in a core where H / R> 1 is satisfied with respect to the core height H with respect to the core diameter R. In the figure, reference symbol A denotes a curve of the addition of the reflectivity by the reflector outside the reactor core, reference symbol B denotes a curve of the amount of combustible reactivity depending on the core size, and reference symbol C denotes a relationship between the reactor core and the reflector. Respectively show the curves of the reactivity applied to the reflector when the distance is reduced.

From the viewpoint of the reactivity, in the reflector control core, the reflectance compensation ΔK _refl by pulling up the reflector is ΔK _refl ΔK _Burn + ΔK (for temperature compensation)... (1) If it is possible to compensate for the decrease in reactivity associated with _Burn combustion, it can be said that the system has been optimized.

ΔK _refl is the reactivity applied by the reflector outside the core, which is large as the effect is small when the core diameter R is small, but the movement is sharply reduced when the core diameter R is large.

At the same power density, the combustion reactivity becomes higher as the core diameter R becomes smaller, and the operating cycle length becomes the core diameter R
Becomes small, the enrichment decreases and the enrichment decreases as the core diameter R increases.

If the furnace power is constant, there is also a power density reduction effect, and the combustion reactivity decreases as the core diameter R increases. 4th
From the viewpoint of the relationship between the curves A and B in the figure and the void coefficient, the optimization is performed so that the above-mentioned equation (1) is satisfied.

From the above, the lower limit of the core diameter R is the design target life,
The upper limit of the core diameter R is mainly governed by the criticality (subcritical requirement) of the "bare core" and the amount of reactivity applied by the reflector.

In the magnitude of the reflectivity adjustment amount by the reflector, the distance between the reactor core and the reflector, the width of the range covered by the reflector are important parameters, but in the present embodiment, the entire circumferential direction is covered. ing.

The structure between the core periphery and the reflector shields neutrons from the core, thus reducing the effectiveness of the reflector. Therefore, the thickness of the stainless steel of the structural material including the core, the components around the core, and the furnace vessel is set to about 10 cm.

When installing a reflector outside the reactor vessel, it is necessary to consider the requirements and restrictions from the neutron irradiation conditions on the reactor vessel for the core performance target (maximizing the precision calculation power of the reactor).
Generally, the relationship between the maximization of the furnace refining calculation power and the core diameter R is as shown in FIG.

Increasing the core diameter R reduces neutron leakage from the furnace per furnace power, and is effective in prolonging the reactivity life by reducing the combustion reaction within the range of the irradiation limit to the structure.

From the above, the outline regarding the core diameter R is determined. The fuel volume ratio will be described as the next basic fuel specification. This is mainly related to the target performance (void coefficient nullification) for the core void coefficient.

When the core shown in FIG. 1 is operated, the relationship between the approximate axial level of the control reflector during operation and the void coefficient during the life of the furnace is shown in FIG.

In the early stage in which combustion is not progressing, the overlap of the core fuel portion of the reactant is small, and as the combustion progresses, the overlap between the reflector and the core increases to compensate for the reflectivity.

In the example shown in Fig. 6 (reactor power 125 MWth), the reflector completely overlaps the reactor core approximately eight years after the start of operation.
For ~ 12 years, reflector lengths fall within the core height range.

FIG. 7 shows the axial power distribution of the central axis in order to show the relationship between the movement of the reflector and the core combustion state.
In FIG. 7, a curve BOC shows a case in the early stage of combustion, a curve MOC shows a case in the middle stage of combustion, and a curve EOC shows a case in the last stage of combustion.

As is clear from FIG. 7, according to the reflector position,
It can be seen that the axial output peak position changes in the axial direction. Since this peak value overlaps with the peak position of the neutron flux distribution for maintaining the fission reaction, it indicates that the axial range where the reflectors overlap is a substantial core portion. Further, as shown by the power distribution in FIG. 7, in the early stage when the fuel inventory is large, even in the area not covered by the reflector, the power share is relatively large. This means that when considering neutron leakage in the radial direction, there is a large amount of neutron leakage from a portion that does not overlap with the reflector, and as shown in the figure of the void coefficient shown in the lower part of FIG. The initial stage is “negative”.

In the state where the reflectors overlap in the middle stage of combustion, the substantial core substantially coincides with the overlapping area with the reflectors. In this case, high radial reflection efficiency substantially reduces neutron leakage in the radial direction. It is also understood from the comparison between the power distribution curves BOC and MOC that the neutron leakage in the axial direction is relatively reduced.

At the end of combustion (EOC) in which the reflector is arranged at the upper part of the core, the void coefficient becomes almost maximum as shown in FIG.
This is because at the end of combustion, where combustion has progressed, the fuel inventory in the actual core has decreased, and the recovery of reactivity by the reflector is particularly important. Length is important. The void coefficient shown in FIG. 6 also increases at the end of combustion. The reason is that the fission products (EP) are increasing, and there is also a spectral effect, which is essentially higher than the early stage of combustion (BOC). ), Although the output distribution is slightly steep, the void coefficients are almost the same. The fact that the void reactivity at the end of combustion becomes “non-positive” is related to the reflector length. Therefore, the length of the movable reflector is important as a basic specification of the core in terms of the life void coefficient after being determined at the reflector position.

The optimization of the length of the reflector in the long core in which the void reactivity is "non-positive" in the plutonium utilization core targeted by the present invention will be described in detail later, but first, it is related to the determination of the core diameter R. Fuel volumeization will be described from the viewpoint of void coefficient.

FIG. 8 shows the relationship between the core void coefficient and the fuel volume ratio (smear) at the end of combustion and the reactivity life when the core radius and height are set to the values described in FIG. 8th
In the figure, curve D shows the relationship between the fuel volume ratio and the void coefficient, and curve E shows the relationship between the fuel volume ratio and the reactivity life.

Smear fuel volume ratio, when the pin inner radius and d _i, pin number to N, the cross-sectional area of the core is S, a value defined by ^{_{π · d i 2 · N /}} S ... (2), the core Is the ratio of the cross-sectional area of the fuel portion to the cross-sectional area of. Then, the fuel effective smear density P _i fall within the radius d _i.

When the fuel volume ratio is large, the number of fuel sheets having a high density increases, so that the mean free path is reduced, the radial leakage is reduced, and the void coefficient is positive, but at about 35% shown in this example, , The void coefficient becomes “non-positive”. To extend the reactor reactivity life, the larger the fuel volume ratio, the better. However, to reduce the void reactivity, an upper limit of about 35% of the smear fuel volume ratio is estimated. As shown in the figure, the smear fuel volume ratio
It is suitable to set the range of 20 to 38%, preferably about 35%.

The results of the survey analysis in this example show the core void (EOC) for reflector length and fuel volume ratio (smear).
FIG. 9 shows the achievement range of the “non-corrected” coefficient target and the core reactivity life extension target. In FIG. 9, curve F
Indicates a range in which the void coefficient is zero, curve G indicates a curve having a reactivity life of 10 years, and curve H indicates a curve having a reactivity life of 12 years.

If the fuel volume ratio is large, the reflector length must be shortened in order to increase the leakage in the axial direction in order to make the void coefficient during the life "non-positive". On the other hand, a longer reflector tends to be more effective for increasing the reactivity life of the furnace, so the range in which both are satisfied is limited. In this embodiment, a fuel volume ratio of about 35% and a reflector length / core length ratio of about 0.6 are selected.

Next, FIG. 10 shows the effect of giving a radial distribution to the fuel enrichment, the reduction of the void coefficient, and the survey result of the reactivity life as shown in FIG.

Increasing the fuel enrichment on the side closer to the reflector increases the reactivity application effect of the reflector. At this time, it has been confirmed that the condition is satisfied within a range that ensures the subcriticality of the “bare core” at the beginning of combustion. Regarding the void coefficient, the component that makes the void coefficient negative due to neutron leakage decreases in the inner region of the core, but the neutron leakage component to the outside increases in the outer region where the enrichment is increased, and the overall combustion As shown in FIG. 10, even when the initial enrichment is slightly radially uniform and the void enrichment is slightly positive, the enrichment ratio (outer enrichment ε _{When the} average enrichment ε ₁ ) is about 1.2, the void coefficient is negative.

The flattening of the power distribution generally has a tendency to shift the void coefficient to the positive side. However, as described above, the compatibility between the prolongation of the reactivity life and the reduction of the void coefficient is generally caused by the flat core ( Difficult with H / D <1). As shown in FIG. 10, it is a characteristic of the small core that both the reactivity life increase and the void coefficient reduction are compatible.

Next, in order to increase the power of the furnace or increase the reactivity life, the relationship between the core length and the reflector length when the core length is set to 6 m and that of the example shown in FIG. Shown in the figure. In FIG. 11, curve I shows the reactivity life per 3 m core, curve J shows L / H for a 3 m core, and curve K shows L / H for a 6 m core.

In the case of the same output, it has been confirmed that the reactivity life is almost doubled when the radial size and the fuel specification are the same. The curve I in FIG. 11 was converted to the reactivity life per 3 m, but this ratio was almost the same even when the core length was increased.
The effect of plus α was not significant.

The goal of achieving a “non-negative” void coefficient at the end of combustion is
Due to the increase in the length of the core, the range of formation was as shown by a curve K in FIG. 11, and the value of the reflector length L / core height H was 0.4 to 0.6. Then, by adjusting the enrichment ratio in the radial direction, it was possible to slightly improve the prolongation of the service life and make L / H close to 0.6.

Therefore, in the present method, L / H = 0.4 to 0.6 is an optimization range of the reflector length and the core height.

As described above with reference to FIG. 4, a larger core radius tends to be generally advantageous for prolonging the service life. For example, when the core radius is reduced, it is advantageous for reducing the void coefficient. FIG. 12 shows the relationship with the reflector length at this time. As is clear from FIG. 12, when the diameter is small, if the condition for the reflector position is the same, the value of (reactivity life) × (output) tends to decrease, but the void coefficient is reduced. It is easy to make it negative.

Therefore, as shown in Fig. 13, even during the ATWS flow reduction type event, it has only negative feedback, the coolant temperature rise is small even at low flow rates, The furnace can be shut down.

FIG. 14 to FIG. 16 show a second embodiment of the present invention, which will be described below.

When the reflector is installed outside the furnace vessel, inserting a large amount of structural material between the reactor core and the reflector reduces the reactivity applied to the reflector and leads to a shorter life. Therefore, the aim is to reduce the amount of structural material, but the fluence of the furnace vessel is thereby increased. As a measure to reduce this, there is a method of setting an enrichment distribution in the axial direction as shown in FIG.

That is, in the prior art, as shown in FIG. 14 (a), the distribution of the enrichment in the axial direction of the fuel was uniform, but as shown in FIG. About 1/3 of
A natural uranium (blanket) area of about 50 cm in width will be provided centering on the position. This region may contain DU (depleted uranium) and NP (neptinium) in addition to NU (natural uranium).

By doing so, as shown in FIG. 14 (c), the maximum value of the fast neutron integral irradiation dose decreases, and the reactor vessel fluence can be reduced.

When the enrichment is changed in the axial direction, as shown in FIG. 15 (a), only the outermost aggregate (shown by a vertical line in the figure) near the reflector is shown in FIG. 14 ( 15 (b) and the method of making the installation position of the natural uranium region different between the outermost periphery (shown by oblique lines in the figure) and the inside as shown in FIG. 15 (b). There is. That is, in the case of FIG. 15 (b), the outermost aggregate is such that the natural uranium region is located below the central level as shown in FIG. 16 (a), and the other aggregates are: As shown in FIG. 16 (b), the natural uranium region is located above the central level.

17 and 18 show a third embodiment of the present invention, in which the neutron shield 21 is provided together with the core 1 in the core barrel 5.
Inside (see Fig. 17) or outside the core barrel 5 (see Fig. 18) and installed inside the furnace vessel, while the operating reflector for reactivity adjustment is installed outside the furnace vessel It is something to do.

The neutron shield 21 is provided to reduce the fluence to the furnace vessel, and as shown in FIGS. 17 (a) and 18 (a), a partial shield opening 22 is formed. Have
The outer reflector corresponding to the shield opening 22 is operated.
When the furnace vessel fluence of the shield opening 22 reaches the design limit value, as shown in FIGS. 17 (b) and 18 (b),
The neutron shield 21 is moved to change the shield opening 22, and the next operation is performed.

(Effects of the Invention) As described above, according to the present invention, when plutonium is used for fuel to extend the life, the core void coefficient is prevented from being positive in any combustion state during the life. Even if the flow rate of the core is reduced, the ability of the reactor to shut down the natural reactor by the inherent reactivity feedback action can be enhanced.

[Brief description of the drawings]

FIG. 1 is a conceptual view of a core showing a small nuclear reactor according to a first embodiment of the present invention, FIG. 2 is a horizontal sectional view of the core, FIG. 3A is a configuration diagram of a normal fuel assembly, and FIG. FIG. 4 is a graph showing the relationship between the core radius in a cylindrical core, the reactivity applied by the reflector, and the combustion reactivity, and FIG. 5 is a graph showing the relationship between the reactor integrated output and the reactor diameter. FIG. 6 is a graph showing the relationship between the position variation of the combustion control reflector and the Na void coefficient, FIG. 7 is a graph showing the axial output distribution at the beginning, middle, and end of combustion, and FIG. FIG. 9 is a graph showing the relationship between the fuel volume ratio, the void reactivity, and the reactivity life, FIG. 9 is a graph showing the relationship between the reflector effective length, the fuel volume ratio, the zero void region, and the reactivity life extension;
FIG. 10 is a graph showing the relationship between the radial enrichment and the void coefficient, FIG. 11 is a graph showing the relationship between the void reactivity and the ratio of the reflector length / core height, and FIG. FIG. 13 is a graph showing the relationship between L / H for the void coefficient “non-negative”, FIG. 13 is a graph showing the reactivity application at the end of the cycle, and FIG. 14 (a) to FIG. FIGS. 15 (a) and 15 (b) are explanatory views showing a second embodiment of the present invention, and FIGS. 15 (a) and 15 (b) are explanatory views showing a core arrangement of an assembly having an enrichment distribution in the axial direction, respectively, and FIGS. ) Is a schematic diagram showing the state of the enrichment distribution of the outermost aggregate in the core of FIG. 15 (b),
FIG. 16 (b) is a schematic diagram showing the state of the enrichment distribution of the central assembly in the core of FIG. 15 (b), and FIGS.
(B) and FIGS. 18 (a) and (b) are explanatory diagrams showing a third embodiment of the present invention, respectively, and FIG. 19 shows the relationship between the core void coefficient of a Pu-based fast reactor and the decrease in reactivity. Graph, 20th
The figure is a graph showing an example of the tendency of the core void coefficient and internal conversion ratio of a Pu-based fast reactor. 1 ... core, 2 ... reflector, 7 ... furnace vessel, H ... core height, D ... equivalent diameter of core, L ... effective length of reflector, R ... core diameter.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masaaki Iida 1 Toshiba, Komukai Toshiba-cho, Saiwai-ku, Kawasaki City, Kanagawa Prefecture (56) References JP-A-4-52593 (JP, A) JP-A Heisei JP-A-3-282396 (JP, A) JP-A-2-2222861 (JP, A) JP-A-63-269093 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G21C 1/02 G21C 7/28 G21C 5/00

Claims (2)

(57) [Claims] (1) using plutonium as all or a part of a fuel, using liquid metal as a coolant,
In a small nuclear reactor in which the core reactivity is controlled by driving a reflector installed outside the core, the height of the core fuel portion is H, the equivalent diameter of the core is D, and the effective length of the reflector is L.
The core was formed into a vertically long cylindrical shape satisfying H / D> 1, L / H was set in the range of 0.4 to 0.6, and the smear fuel volume ratio was set in the range of 20% to 38%. A small nuclear reactor characterized by the above. 2. The small nuclear reactor according to claim 1, wherein a region including a position that is one third of the effective length of the core from a lower end of the core is a natural uranium region.