JPH07229986A - Core structure for boiling water reactor - Google Patents

Abstract

PURPOSE:To suppress a change in a gap between channel boxes in order to moderate the rise of a neutron flux at the time of an earthquake. CONSTITUTION:A channel spacer 6 for maintaining a water gap A between channel boxes 3 is provided between the channel boxes 3 on the side of insertion of a control rod 2. The width of the water gap A between the channel boxes is made larger in the lower part of a core than in the upper part along the axial direction of a fuel assembly, while the width of a water gap B between the channel boxes on the side of non-insertion of the control rod is made smaller in the lower part of the core than in the upper part along the axial direction of the fuel assembly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a boiling water reactor (hereinafter referred to as BWR) and a core internal structure.

[0002]

BWR cores are arranged in a square lattice and hold fuel rods and water rods at the upper and lower tie plates,
The surrounding structure is surrounded by a channel box to form a fuel assembly, and a plurality of the fuel assemblies are loaded in a square lattice. The channel boxes form a flow path for the coolant, and the cross control rods are inserted into the water gap between the adjacent channel boxes at a ratio of one fuel assembly to four fuel assemblies.

BWR cores are roughly classified into two types from the viewpoint of core lattice shape. The first is a C-lattice core having a so-called C-lattice shape in which the water gap width is the same at the insertion position and the non-insertion position of the cross control rod, and the second is the water at the control rod insertion position where the two water gap widths are different. A "D-lattice core" having a D-lattice shape in which the gap is wider than the non-inserted position is known.

The technology targeted by the present invention relates to a core structure of a BWR having this D-lattice core.

FIG. 7 is a plan view of the D-lattice core, showing four fuel assemblies 1 surrounding the cross-shaped control rod 2. In the figure, reference numeral 3 is a channel box, 4 is a channel fastener, 5 is an upper tie plate, 6 is a channel spacer, A is a control rod insertion side inter-channel water gap, and B is a control rod non-insertion side inter-channel water gap.

The D-lattice core has a shape in which the inter-channel water gap A on the control rod insertion side shown in FIG. 7 is larger than the water gap B on the side where the control rod 2 is not inserted. In addition, as shown in FIG. 8, the fuel assembly 1 in the core has a lower portion of the fuel assembly 1 supported by a fuel support fitting (not shown) arranged on the core support plate 8 and an upper portion of which has a strip shape. Upper lattice plate 7
Supported by.

The position of the fuel support fitting is fixed for each integrated fuel assembly, but the upper lattice plate 7 is divided into four fuel assemblies. Therefore, the gap between the fuel assemblies 1 in the upper part of the core is pushed by the channel fasteners 4 which are leaf springs attached to the upper part of the channel box 3 so that the fuel assemblies 1 are pressed against each other while maintaining a constant interval. It has a structure to be placed.

In the BWR fuel assembly, when comparing the neutron spectrum inside the fuel assembly with the portion close to the water gap between the channel boxes, the neutrons tend to be easily heated near the channel box wall where the moderator density is large, and the output tends to be generated. At the same time, inside the fuel assembly, neutrons are less likely to be decelerated due to the generation of voids due to boiling, which makes it difficult to output power.

Therefore, the fuel rod enrichment in the central portion of the fuel assembly is made larger than that of the fuel rods in the vicinity of the water gap so that the output distribution is not excessively distorted.
Further, in the C-lattice core, the water gaps on the control rod insertion side and the non-insertion side are the same, so that the fuel concentration distribution can be designed with symmetry.

On the other hand, in the D-lattice core, since the width of the water gap is different between the control rod insertion side and the non-insertion side, the neutron moderating effect is larger on the wide water gap side than on the narrow side. Therefore, in the fuel assembly for the D-lattice core, a design is adopted in which the enrichment of the fuel rod near the wide side of the water gap is relatively lower than the enrichment of the fuel rod near the narrow side of the water gap. The concentration distribution is set so that the local output of does not become excessively large.

[0011]

As described above, in the D-lattice core, the fuel rod enrichment on the side with a wide water gap is relatively lower than the enrichment on the fuel rod with a narrow water gap. For example, when the size of the water gap changes in a short time due to an earthquake, etc., and the wide side of the water gap is narrower than the initial size and the narrow side becomes wider, the moderator distribution changes and the fuel rod with the narrower water gap becomes The output decreases, and conversely the output that has become wider tends to increase.

[0012] In this case, comparing the increase in power due to the increase in the deceleration effect due to the expansion of the water gap on the narrow side in the D-lattice core and the increase in the output due to the decrease in the deceleration effect due to the reduction of the water gap on the wide side, the relative The narrower the initial water gap in which the highly concentrated fuel rods are arranged, the greater the sensitivity of the output change, and the greater the effect of increasing the output.

Therefore, in a nuclear reactor plant having a D-lattice core, when an earthquake occurs, the neutron flux may rise and may lead to a scrum.

In general, the upper part of the core having a high void fraction and the lower part of the core having a low void fraction have different effects on the reactivity due to the fluctuation of the water gap width. FIG. 9 shows the rate of increase in reactivity when the water gap width on the control rod non-insertion side increases. In the figure, a solid line S indicates a void ratio of 0%, a broken line T indicates a void ratio of 40%, and a dashed line U indicates a void ratio of 70%.

As is clear from FIG. 9, the higher the void ratio in the channel box, the greater the influence on the reactivity change. This is because the higher the void ratio is, the more the deceleration is insufficient, so that the relative change in the deceleration density is large if the same change in the water gap width is assumed. Therefore, the neutron flux increase is larger in the upper part of the core with a higher void fraction than in the lower part of the core.

In particular, during an earthquake, if the water gap width oscillates while maintaining the initial gap, the reactivity is unlikely to increase, but considering the upper support structure of the fuel assembly, the upper lattice plate 7 is a rigid body, so The change on the grid plate 7 side, that is, on the side where the water gap is narrow is small, but the change on the side where the water gap is wide is large because the channel fastener 4 side has a spring structure. Therefore, the inter-channel gap during an earthquake becomes wider when it is narrower and narrower when it is wider.

Due to the influence of the void fraction, the asymmetry of the upper support structure and the fuel rod enrichment distribution, the influence of the water gap fluctuation tends to promote the increase of the neutron flux. On the other hand, in the C-lattice core, since the fuel rod enrichments facing the respective water gaps have symmetry, the power increase of the fuel rod with the wider water gap and the fuel rod with the narrower water gap are increased. The decrease in output will be offset, and it will be difficult to reach a scrum.

Further, in the BWR fuel assembly, the axial power distribution tends to have a peak in the lower core due to the influence of the axial void distribution, so that the enrichment in the upper core is made larger than that in the lower core. The design may be adopted.

An example of the fuel rod enrichment distribution of the fuel assembly 1 in the D-lattice core is shown in FIGS. 3 and 10. Figure 3 and Figure
10 is an example of a fuel assembly 1 in which fuel rods are arranged in an 8 × 8 lattice and two water rods W are arranged. The average enrichment distribution of the fuel rods below the central portion in the axial direction of the core is shown in FIG. Fig. 10 shows the average enrichment distribution of fuel rods above the central portion in the axial direction of the core.

3 (a) and 10 (a) are plan views of the fuel assembly, and FIG. 3 (b) is a table showing the uranium enrichment of fuel rod numbers 1-12. Since it is necessary to secure the enrichment distribution in the vertical direction while providing the enrichment distribution in the cross section direction of the fuel assembly in the D lattice, it is necessary to make a complicated design as shown in FIGS. 3 and 10. There is a constraint on manufacturing.

The present invention has been made to solve the above-mentioned problems, and avoids scrum by mitigating an increase in neutron flux, that is, an increase in output due to fluctuations in the water gap of the D-lattice core during an earthquake, etc. It is an object of the present invention to provide a boiling water reactor core structure for the purpose of simplifying the concentration distribution of the assembly and improving manufacturing.

[0022]

According to the present invention, a large number of fuel assemblies each having a rectangular cylindrical channel box surrounding a fuel rod and a water rod arranged in a square lattice are regularly arranged, and each of the channels is arranged. Of the four sides of the box, the water gap between the channel boxes on the two sides facing the insertion position of the cross control rod is larger than the water gap between the channel boxes on the two sides not facing the insertion position of the cross control rod. In a boiling water nuclear reactor core structure having a lattice core, a channel spacer for maintaining a gap between the channel boxes is provided between the channel boxes on the control rod insertion side, and the water gap between the channel boxes is provided. The width of the lower part of the core is larger than that of the upper part of the core along the axial direction of the fuel assembly. The width of the gap, towards the reactor core lower along the axial direction of the fuel assembly is characterized by smaller than core upper.

Further, in the core structure for a boiling water reactor according to the above invention, the present invention has two side surfaces facing the control rod insertion position of the channel box of each fuel assembly arranged in the core and in the horizontal direction. It is divided into a first region surrounded by a diagonal line, two side faces that do not face the control rod insertion position, and a second region surrounded by the diagonal line, and from a central portion in the axial direction of the core of the fuel rods included in the first region. The average enrichment of the lower part is e, lw, the average enrichment above the central part of the core is e, uw, the average enrichment of the fuel rods included in the second region below the central part in the axial direction of the core e, ln , Average enrichment e, un above the central part in the axial direction of the core e, lw
<E, uw, e, ln <e, un, and e, un / e, uw ≤ e, ln / e, lw are loaded.

[0024]

The present invention suppresses fluctuations in the inter-channel gap, that is, fluctuations in moderator distribution, in order to mitigate the increase in neutron flux during an earthquake.

As a means for reducing the fluctuation of the water gap width at the time of an earthquake, it is desirable to provide a structural material for suppressing the amplitude due to the vibration in the central part of the core, but it is necessary to secure the insertability of the control rod and the exchangeability of the fuel. It is not a good idea to secure it.

In view of this, the present invention focuses on the correlation between the increase in neutron flux and the shape of vibration caused by the conventional reactor internal structure, and the void fraction, and makes a minimum structural change and makes a large modification of the existing plant. Without solving the above problems.

The present invention emphasizes that it is more effective to suppress vibrations in the upper core, and makes the water gap between channels on the control rod insertion side smaller in the upper core than in the lower core. The water gap between channels on the control rod non-insertion side should be larger in the upper core than in the lower core.

Since the water gap between channels on the control rod insertion side is smaller in the upper part of the core than in the lower part of the core, the channel fasteners provided on the control rod insertion side are smaller than those in the conventional D-lattice core. This makes it possible to reduce the fluctuation of the water gap due to vibration due to the pressing force. Therefore,
The rise in neutron flux during vibration is mitigated.

[0029]

EXAMPLE An example of a core structure for a boiling water reactor according to the present invention will be described with reference to the drawings. FIG. 1 shows a core structure of a first embodiment according to the present invention, in which a cross-shaped control rod 2 is surrounded by 4
The body fuel assembly 1 is shown in a plan view and FIG. 2 is a side elevational view of FIG.

In FIGS. 1 and 2, of the four side surfaces of each channel box 3, the other two side surfaces that do not face the insertion position of the cross-shaped control rod 2, that is, the upper lattice plate 7 side, have a protrusion 9 An example in which is provided is shown.

That is, the projection 9 is provided at a height position as shown in FIG. 2 at a position in contact with the upper grid plate 7. In the figure, A indicates a water gap between channels on the control rod insertion side, and B indicates a water gap between channels on the control rod non-insertion side.

Therefore, the inter-channel water gap A on the control rod insertion side is narrowed from the core support plate 8 to the upper lattice plate 7, and the channel fastener 4 is smaller than that in the conventional D lattice core. It will be pressed further in the upper lattice plate 7. As a result, the vibration to the control rod insertion side at the time of an earthquake becomes small.

In this embodiment, the protrusions 9 are provided at two places on each side surface of the channel box 3, but the protrusions 9
The number of is not limited to two, and the same effect can be obtained by providing a single number or three or more.

Next, a second embodiment of the present invention will be described. This second embodiment is to be subordinate to the first embodiment. That is, FIG. 3 and FIG.
Is an example of a fuel assembly in which two water rods W are arranged in a grid of 8 × 8 fuel rods.

The average enrichment distribution of the lower fuel rods from the central portion in the axial direction of the core in the second embodiment is shown in FIG. 3, and the average enrichment distribution of the fuel rods from the central portion in the axial direction of the core is shown in FIG. 3 and 4 (a) are plan views of this embodiment, and FIG. 3 (b) shows the uranium enrichment of fuel rod numbers 1-12 and 1-9.

According to the second embodiment, the distance between the fuel rods is the same as that of the conventional D-lattice core shown in FIG. 10 in the lower portion of the fuel rods, so that the distance from the central portion in the axial direction of the core is increased. The average enrichment of the lower part is as shown in FIG.

That is, the fuel rod of fuel rod No. 12 located at the corner on the control rod insertion side and the fuel rod of fuel rod No. 10 located at the corner on the control rod non-insertion side are connected to each other on a diagonal line. The average enrichment (e, lw) of the fuel rods belonging to the region on the control rod side from the diagonal line C with the diagonal line C as the boundary
Is lower than the average enrichment (e, ln) of the fuel rods belonging to the region on the control rod non-insertion side from the diagonal line C.

On the other hand, in the upper part of the fuel rod, the inter-channel water gap on the control rod insertion side becomes narrower than that of the conventional D-lattice core, and the inter-channel gap on the control rod non-insertion side becomes smaller than that of the conventional D-lattice core. Since it has spread, the difference between the control rod insertion side and non-insertion side of the water gap in the upper core is
It is less than the conventional D-lattice core, and is, for example, approximately equal.

Therefore, as shown in FIG. 4, the average enrichment from the central portion to the upper portion in the axial direction of the core is the average enrichment of fuel rods belonging to the region on the control rod insertion side from the diagonal D with the diagonal D as the boundary (e, uw) to the average enrichment (e, un) of the fuel rods belonging to the control rod non-insertion side from the diagonal line D, e,
It is possible to set un / e, uw to be equal to or less than the ratio e, ln / e, lw between the control rod insertion side and the control rod non-insertion side in the lower portion of the fuel rod.

That is, e, un / e uw ≦ e, ln
/ E, lw ... As a result, an increase in neutron flux when the water gap changes can be suppressed more than in the conventional D lattice core.

In the upper part of the fuel rod, the enrichment types are reduced, and in the conventional example, there are 12 enrichment types as shown in FIG. 10, whereas in this embodiment the enrichment types are shown in FIG. There are 9 types. Therefore, according to this embodiment, the fuel production can be simplified.

The enrichment relationship between the upper portion of the fuel rod and the lower portion of the fuel rod is larger in the upper portion than in the lower portion, that is, e, lw <
e, uw, e, ln <e, un. as a result,
The peak of the axial power distribution in the lower part of the core due to the influence of the axial void distribution will be relaxed.

The inter-channel water gap on the control rod insertion side is larger in the lower core than in the upper core, and the inter-channel water gap in the control rod non-insertion side is larger in the lower core than in the upper core. The method for doing so is not limited to this embodiment.

Next, a third embodiment according to the present invention will be described with reference to FIGS. The third embodiment is based on the first embodiment, and FIG. 5 is a conceptual view of the fuel assembly 1 as viewed from the side like FIG. That is, in the third embodiment, the thick wall portion 3a of the channel box 3 is provided on the two side surfaces that do not face the cross-shaped control rod insertion position.

FIG. 6 shows a sectional view of the channel box at the height G of the lattice plate shown in FIG. In FIG. 6, E
Indicates the control rod insertion side, and F indicates the control rod non-insertion side. The side surface of the channel 3 facing the control rod non-insertion side F has a thick portion 3a.

Similar to the first embodiment, the effect of the third embodiment is that the increase in neutron flux can be mitigated at the time of earthquake and the fluctuation of the gap between the channel boxes can be suppressed.
Vibration to the control rod insertion side is reduced.

The same effect can be obtained by providing the upper lattice plate 7 with projections instead of providing the projections 9 on the side surfaces of the channels 3 in the first embodiment. Furthermore, by substituting and adding the second embodiment to the first and third embodiments, the effect can be further enhanced.

[0048]

According to the present invention, since the amplitude at the time of earthquake becomes small, the fluctuation of the water gap width from the initial value becomes small and the rise of the neutron flux is alleviated. As a result, the probability of avoiding scrum increases, which contributes to improved driving performance.

Further, since the design for increasing the enrichment on the non-insertion side of the control rod, which is required in the fuel design of the conventional D core lattice, is relaxed, even if the water gap width changes during an earthquake, Since the enrichment of the fuel rod facing the insertion side water gap can be reduced, reactivity changes and neutron flux changes are reduced,
The unnecessary scrum probability is reduced.

[Brief description of drawings]

FIG. 1 is a plan view showing a first embodiment of a boiling water nuclear reactor core structure according to the present invention.

FIG. 2 is a conceptual elevational view of FIG. 1 seen from the side.

3 (a) is a cross-sectional view showing a lower portion of a fuel assembly in the second embodiment, and FIG. 3 (b) is a table showing a fuel rod average enrichment distribution below a central portion in a core axis direction in FIG. 3 (a). Fig.

FIG. 4A is a cross-sectional view showing an upper portion of the fuel assembly in the second embodiment, and FIG. 4B is a table showing a fuel rod average enrichment distribution above the central portion in the axial direction of the core in FIG. 4A. Fig.

FIG. 5 is a conceptual elevational view of a third embodiment of the boiling water nuclear reactor core structure according to the present invention as seen from the side.

6 is a cross-sectional view showing the channel box at the height G in FIG.

FIG. 7 is a plan view showing four fuel assemblies surrounding a control rod of a conventional D-lattice core.

FIG. 8 is a perspective view showing a core structure of a conventional D-lattice core partially cut away.

9 is a curve diagram showing the relationship between the variation of the water gap width and the void rate and the reactivity increase rate in FIG. 7.

10 (a) is a cross-sectional view showing a fuel assembly of a conventional D-lattice core, and FIG. 10 (b) is a table showing a fuel rod average enrichment distribution above the central portion in the axial direction of the core in FIG. 10 (a).

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 2 ... Cross control rod, 3 ... Channel box, 3a ... Channel thick part, 4 ... Channel fastener, 5 ... Upper tie plate, 6 ... Channel spacer, 7 ... Upper lattice plate, 8 ... Core Support plate, 9 ... Protrusion, 10 ...
Lower tie plate, A ... Water gap between control rod insertion side channels, B ... Water gap between control rod non-insertion channels,
C, D ... Diagonal line, E ... Control rod insertion side, F ... Control rod non-insertion side, G ... Height of upper lattice plate.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G21C 5/00 GDB A 8908-2G 7/113 GDB G21C 3/34 GDB K 9117-2G 7/10 GDB J

Claims (6)

[Claims] 1. A fuel assembly having a square tubular channel box surrounding a fuel rod and a water rod arranged in a square lattice is regularly arranged, and a cross is formed among four side surfaces of each channel box. Boiling water forming a D-lattice core in which the water gap between the two side channel boxes facing the insertion position of the mold control rod is larger than the water gap between the two side channel boxes not facing the insertion position of the cross control rod In the core structure for a nuclear reactor, a channel spacer for maintaining a gap between the channel boxes is provided between the channel boxes on the control rod insertion side, and the water gap width between the channel boxes is the axis of the fuel assembly. Along the direction, the lower core is larger than the upper core, and the water gap width between the channel boxes on the control rod non-insertion side is the fuel assembly. A core structure for a boiling water reactor characterized in that the lower part of the core is made smaller than the upper part of the core along the axial direction of the body. 2. The inner surface distance of the channel box of the fuel assembly is constant in the axial direction, and the axis of the fuel assembly is not perpendicular to the fuel support plate below the core. Item 1. A boiling water reactor core structure according to Item 1. 3. The channel box of the fuel assembly has most of the other channel boxes at a height position where the channel box thickness on the side not facing the cross control rod insertion position is in contact with the upper lattice plate. The core structure for a boiling water reactor according to claim 1, characterized in that the channel box is formed thicker than that of the control rod, and the thickness of the channel box on the side facing the control rod insertion position is constant in the axial direction. . 4. The side surface of the channel box of the fuel assembly that does not face the cross-shaped control rod insertion position has a protrusion outside the channel box at a height position where it contacts the upper lattice plate and faces the control rod insertion position. The core structure for a boiling water reactor according to claim 1, wherein the side channel box has no protrusion. 5. The boiling water nuclear reactor core structure according to claim 1, wherein at least a part of the upper lattice plate contacting the channel box has a protrusion. 6. A channel box of each fuel assembly arranged in the core has two side faces facing the control rod insertion position and a first region surrounded by a diagonal line in the horizontal direction, and two regions not facing the control rod insertion position. It is divided into a second region surrounded by the side surface and the diagonal line, and the average enrichment of the fuel rods included in the first region below the central portion in the axial direction of the core is e, l
w, average enrichment above the central part of the core e, uw, average enrichment below the central part of the axial direction of the fuel rods included in the second region e, ln, above the central part of the axial direction of the core If the average enrichment of e, un is e, lw <e, uw,
2. A fuel assembly in which e, ln <e, un and e, un / e, uw≤e, ln / e, lw are loaded.
A core structure for a boiling water reactor according to any one of claims 1 to 5.