JPH067182B2 - Fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to an improvement in a fuel assembly of a boiling water reactor (hereinafter referred to as BWR).

(Prior Art) A fuel assembly used in a BWR generally has the following configuration. That is, a plurality of channels (eg, 8 rows × 8 columns = 64) are provided in a channel box having a substantially square cross section.
Book) fuel rods are loaded in a grid pattern, and the upper and lower ends are fixed by the upper tie plate and the lower tie plate. Further, spacers are installed at several points in the axial direction, and the spacers maintain the distance between the fuel rods. The fuel rod has a structure in which a plurality of fuel pellets obtained by sintering uranium dioxide (UO ₂ ) is loaded in a cladding tube, and the upper end and the lower end thereof are sealed by an upper end plug and a lower end plug.

In order to ensure the integrity of the above fuel rods during the operation of the nuclear reactor, it is necessary to suppress the output per unit length of the fuel rods (hereinafter referred to as linear power density) to below the design limit value. The design is as shown.

First, in the case, the thermal neutron flux distribution in the cross section of the fuel assembly is highest at the outermost fuel rods facing the water gap and low at the inner side. Therefore, in order to flatten the output distribution and reduce the output peaking, a water rod is arranged in the center of the fuel assembly to circulate the coolant to increase the thermal neutron flux in the central part of the fuel assembly. In addition, four types of fuel rod enrichment are used, and low enrichment fuel rods are placed at the positions where the thermal neutron flux is high (the corners and outermost periphery of the fuel assembly), and the thermal neutron flux is It is practiced to arrange fuel rods having a higher enrichment toward the inner side which becomes gradually lower.

Next, in the case of BWR, since the voids are generated, the output distribution in the axial direction has a downward peak. Therefore, in order to flatten the axial output distribution, the axial enrichment distribution design and the gadolinia design as shown in FIG. 5 are performed. That is, the fuel enrichment is increased upward, and conversely, the gadolinia concentration is increased downward. This promotes combustion in the axial upper direction to reduce the linear power density (10 to 20%). In the diagram on the left side of Fig. 5, the horizontal axis represents the enrichment and the vertical axis represents the axial position of the core (the length of the shaded portion in the figure is the active fuel length) to show the axial distribution of the enrichment. It is the figure shown. The diagram on the right side of FIG. 5 shows the gadolinia concentration in the axial direction with the gadolinia concentration on the horizontal axis and the active fuel length on the vertical axis. As described above, in the conventional case, in order to suppress the linear power density to be equal to or less than the design limit value, a complicated nuclear design with low power peaking is performed.

Also, instead of the fuel assembly of (8 rows x 8 columns), adoption of a fuel assembly of (9 rows x 9 columns) is considered. This is a method in which the output per fuel rod is reduced by increasing the number of fuel rods, that is, the linear power density is reduced. According to this method, the linear power density can be reduced by about 20%.

By the way, recently, there is an increasing tendency of fuel reprocessing and a tendency of prolonging the cycle, and it is required to improve the fuel cycle cost by increasing the enrichment of the replacement fuel to reduce the number of replacements. Also, with the development of zirconium liner fuel rods (zirconium is lined inside the cladding of fuel rods to absorb the difference in thermal expansion between pellets and cladding),
It is required to design the fuel with the lowest possible enrichment and high reactivity within the linear power density limit to reduce the fuel cycle cost. In response to these recent design requirements, a fuel assembly of the nuclear design concept (8 rows × 8 columns) as shown in FIG. 6 has been put into practical use. However, there is a problem in that the optimum concept in combination with the zirconium liner fuel rod has not been established for the (9 rows × 9 columns) fuel assembly. More specifically, for example, as a reactor operating method for improving fuel cycle cost, the reactor is operated with a lower power distribution from the beginning of the cycle to the end of the line power density limit until the end of the cycle, and at the end of the cycle, the output distribution is at the central peak Alternatively, an operation method is adopted in which the peak is set to the upper peak, and thereby the core void fraction is reduced to obtain the core reactivity (hereinafter referred to as BSO operation). However (8 ×
8) In the case of the fuel assembly, the axial power distribution is controlled by complicating the axial design as described above.
The reactivity gain due to SO operation is small. On the other hand, (9
× 9) In the case of the fuel assembly, the power density limit is satisfied without complicating the axial design. However (9x9)
By this, the resonance absorption increases, and the void coefficient becomes a negative value and the absolute value becomes large, which is disadvantageous in terms of reactivity as compared with the case of (8 × 8). Resonance absorption means that the distance between the fuel rods becomes shorter by increasing the number of fuel rods (since the size of the fuel assembly is the same even if the number of fuel rods is increased), as a result, the fuel rods are absorbed. It is a phenomenon in which fellows interfere with each other. Therefore, in order to make the fuel cycle cost more advantageous than the (8 × 8) fuel assembly, in the above BSO operation,
The void reactivity must be reduced by making the end-stage axial power distribution have an upper peak than in the case of the (8 × 8) fuel assembly. However, in the case of the (9 × 9) fuel assembly, since the void coefficient increases in the negative direction as described above, the axial power distribution has an upper peak than that in the case of the (8 × 8) fuel assembly, As a result, the scrum curve gets worse. That is, even if the control rod is inserted, since the combustion portion is located above, it takes time to scram. At the same time, when the pressure rise transient phenomenon (for example, generator load cutoff or turbine trip), ΔMCPR (change amount of limit output ratio) becomes larger than that in the case of (8 × 8) fuel assembly, and therefore CPR during normal operation (Limit output ratio) There is a problem that the limit value (OCMCPR) must be increased.

(Problems to be Solved by the Invention) Thus, the conventional (8 × 8) fuel assembly and (9 ×)
9) There are various problems in the fuel assembly, and the present invention has been made on the basis of such a point, and an object of the invention is to limit the linear power density in the (9 × 9) fuel assembly. Among them, it is to provide a fuel assembly with higher fuel economy.

[Configuration of Invention] (Means for Solving Problems) That is, in the fuel assembly according to the present invention, the fuel rods are arranged in a lattice in 9 rows × 9 columns in the channel box, and the upper and lower ends thereof are of the upper type. And a lower tie plate, and in a fuel assembly in which spacers are provided at a plurality of positions in the axial direction for maintaining the distance between the fuel rods, the fuel rods with high fuel enrichment are arranged at the outermost peripheral position, and It is characterized in that the fuel enrichment at the lower part in the direction is made higher than the fuel enrichment at the upper part in the axial direction.

(Operation) First, focusing on the fact that the adoption of the (9 × 9) fuel assembly increases the thermal margin as compared with the case of the (8 × 8) fuel assembly,
Fuel with high fuel enrichment is placed at the outermost position to improve fuel economy. That is, the number of fuel rods in the (9 × 9) fuel assembly is increased by about 20% as compared with the (8 × 8) fuel assembly, and as a result, the average linear power density is reduced by about 20%. This is as described above. Therefore, the output peaking coefficient (local peaking coefficient) similar to the conventional one
The maximum linear power density of the (9 × 9) fuel assembly decreases about 20% and the thermal margin increases. Conversely speaking, if the same maximum linear power density is adopted, the local output peaking coefficient can be increased up to 1.2 times. Further, by making the fuel enrichment on the lower side in the axial direction higher than that on the upper side in the axial direction, the axial power distribution in the (9 × 9) fuel assembly has a lower peak.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 to 4. FIG. 1 is a cross-sectional view of a fuel assembly 101 according to this embodiment. In the figure, reference numeral 102 is a channel box, and the channel box 102 has a substantially square cross section. A plurality of (9 × 9) fuel rods 103 are arranged in a lattice in the channel box 102. The upper and lower ends of the plurality of fuel rods 103 are fixed by an upper tie plate and a lower tie plate (not shown). Spacers are installed at a plurality of locations in the axial direction to prevent vibration. Reference numeral 104 in the drawing denotes a control rod, and one control rod 104 is arranged at the center of the four fuel assemblies 101 to form a unit lattice.

The fuel rods 103 are made of a plurality of types and are arranged at predetermined positions. First, the fuel rod 1 indicated by reference numeral G in the drawing
No. 03 is a stack of fuel pellets containing gadolinia (Gd ₂ O ₃ ) as a burnable poison in uranium dioxide (uo ₂ ). This fuel rod 103 has (2
3), (2,7), (3,2), (3,5), (3
8), (5,3), (5,7), (7,2), (7,
5), (7,8), (8,3), and (8,7). The horizontal axis and the vertical axis are numbered to indicate the position in FIG. 1, and the position is specified by the numbers on the vertical axis and the horizontal axis. Also this fuel rod 10
3 is a blanket area (N) at the upper end as shown in FIG.
have. This is because the fuel pellet containing gadolinia has a slightly low thermal conductivity, so the temperature of the pellet rises,
As a result, the amount of fission product gas (hereinafter referred to as FP gas) is increased as compared with the fuel rod containing no gadolinia,
This is because the internal pressure tends to increase. The lower blanket area is then applied to the gas plenum. If the internal pressure is high, apply the upper blanket area to the gas plenum. The enrichment region is divided into two, and the enrichment region in the upper part (1/12 to 1/6 of the active fuel length) has a concentration of 3.0 wt% and a gadolinia concentration of 4.5 wt%. In the lower concentrated region, the concentration is 4.6 wt% and the gadolinia concentration is 4.5 wt%.

Further, the fuel rod indicated by the symbol P in the figure is a partial length fuel rod. This partial length fuel rod 103 is used for other fuel rods 1 as shown in FIG.
Its axial length is shorter than that of No. 03. This partial length fuel rod 103 has (2, 2), (2, 5), (2
8), (5,2), (5,8), (8,2), (8,
5) and (8, 8). Further, it is desirable that the partial length fuel rod 103 projects slightly above the spacer grid in consideration of hydraulic vibration. Therefore, the length of the portion of the partial length fuel rod where the fuel is missing has a value depending on the spacing of the spacer grid. The partial length fuel rods 103 have the effects of improving the reactor shutdown allowance, reducing the coolant pressure loss, and improving the void coefficient. In the design having seven spacer grids like the current BWR, one spacer interval or It is desirable to have a space between two spacers. In this embodiment, the space between the two spacers is set.
The concentration is 4.85 wt%.

In addition, reference numeral W in the drawing denotes a water rod, which is hollow and through which the coolant flows.

The fuel rod 103 indicated by reference numeral 1 in the drawing has a fuel effective length of 1/24 to
It has a 1/12 natural uranium or depleted uranium blanket region (indicated by symbol N in Fig. 2), and its enrichment region is separated into two upper and lower stages, and it is approximately from the upper end of the active fuel length.
It has a boundary at 1/3 to 1/2. The upper concentration is 4.60 wt%, while the lower concentration is 4.85 wt%. Further, the position of the boundary is the partial length fuel rod 10
Aligned with the upper end of the active fuel length of No. 3.

The fuel rod 103 shown by reference numeral 2 in the drawing has natural uranium or depleted uranium blanket regions N at the upper and lower ends similarly to the fuel rod 103 shown by the above reference symbol 1, but the enrichment region is not separated, Its enrichment is 4.00 wt%. The fuel rod 103 indicated by reference numeral 3 is also the same, and its enrichment is 3.00.
wt%.

As described above for each type, the number of each is
As shown in the figure, the number and each enrichment, etc. are determined depending on the operation length that employs the (9 × 9) fuel assembly 101, and in the present embodiment, for 15 to 18 months. Effective Output (EFPM: Effective Full)
For Power Per Month). The cladding of each fuel rod 103 is made of zirconium alloy,
There is a thin PCI barrier layer on the inside. In this embodiment, pure Zr
It is the liner layer.

With the above configuration, the following effects can be achieved.

First, in the case of the fuel assembly 101 according to the present embodiment, the outermost fuel rod 1 facing the water gap having a high neutron flux distribution 1
Due to the high concentration of 03, the thermal neutron utilization rate is improved,
The reactivity gain can be obtained, and the fuel economy is improved.

Next, the fuel rods 103 indicated by reference numerals 1, 2, and 3 have blanket regions (N) installed at the upper and lower ends thereof, so that the leakage of neutrons in the vertical direction can be reduced, and the central portion having a high neutron importance. Since the enrichment of the fuel assembly 101 is increased, the reactivity of the fuel assembly 101 is high.

And 1/12 to 1/6 from the upper end in the axial direction of the concentration region
Region is the low enrichment region (upper end of the fuel rod indicated by G), the unburned residue of the fuel is small and the reactor shutdown margin is improved. That is, in the case of the (9 × 9) fuel assembly 101 , the resonance absorption is larger than that of the (8 × 8) fuel assembly,
In addition, since the accumulation of Pu in the upper part is large and the void coefficient is large, the output ratio in the upper part is slightly smaller than that of the (8 × 8) fuel assembly, and the unburned residue of U ²³⁵ is large. As a result, in the low temperature state where combustion progressed, the axial reactivity distribution tended to have an upper peak. This point could be improved by this embodiment.

In the fuel rod indicated by reference numeral 1, the enrichment degree in the enrichment region below the boundary of about 1/3 to 1/2 from the upper end of the active fuel length is about 0.1 to 0.3 wt. It is raised by about%. This concentration region corresponds to a coolant void fraction of 0 to 40%, and since the water-to-fuel ratio is larger than that of the 40 to 70% void fraction portion above that, the reactivity tends to be large. Since this further increases the concentration of this region, more U ²³⁵ is arranged in the region of high importance, which is extremely effective in improving the reactivity gain.

Further, since the void coefficient of the (9 × 9) fuel assembly is larger than that of the (8 × 8) fuel assembly, the axial power distribution naturally becomes (8 × 8) fuel assembly as shown in FIGS. 3 and 4. The lower peak is stronger than the aggregate. The shaded portions in FIGS. 3 and 4 are control rods, and FIGS. 3 (c) and 4 (c) show the state in which the control rods are fully withdrawn. Further, since the degree of enrichment is high below the enrichment region, a lower peak output distribution can be obtained without making the control rod pattern an extreme new insertion control rod pattern when performing BSO operation. Therefore, in order to secure the reactivity adjustment margin by the control rod, the control rod pattern can be assembled with the control rod pulled out by about 1/12 to 1/6 of the effective core length, which facilitates the operation of the control rod plan. . Also, the concentration of the lower part of the concentration area is about 0.3w than that of the upper part.
Even if the BSO operation is performed, the reactivity of the lower part is still high at the end of the cycle even when the BSO operation is performed, so that the shape of the axial power distribution becomes an upper peak shape that is gentler than the conventional shape. As a result, the scrum curve is improved and the ΔMCPR in the event of an abnormal transient event is smaller than in the conventional axial uniform enrichment design.

Since the axial power distribution at the end of the cycle has a gentler upper peak than the conventional uniform axial enrichment design, the average void fraction in the core is slightly large, and the void reactivity is slightly large on the core Keff, and there is a loss. However, by partially rotating U ²³⁵ to a part with a high void ratio in the upper part of the core and a part with a high importance in the upper part of the core, the U ²³⁵ is effectively burned from the beginning of the cycle to the middle of the cycle, and the unburned residue of the upper part of U ²³⁵ remains Since it is less than the fuel design, there is no loss in the cycle burnup increase. Incidentally, FIG. 3 is a view showing a change in the axial output distribution during the cycle operation of the conventional (9 × 9) fuel assembly, and FIG. 4 shows a change in the axial output distribution in the case of the present embodiment. It is the figure shown.

The present invention is not limited to the above-mentioned embodiment, and various embodiments can be considered. For example, the partial length fuel rod is not limited to the partial length including the cladding tube as in the above-described embodiment, but the cladding tube is a normal cladding tube, and only the contents may be partial length.

[Advantages of the Invention] As described in detail above, according to the fuel assembly of the present invention,
In the (9 × 9) fuel assembly, the fuel economy can be improved, and the axial output distribution can have a downward peak, so that the BSO operation can be easily applied and the scram curve is deteriorated. The effect is great because it can be effectively suppressed.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a fuel assembly according to an embodiment of the present invention, FIG. 2 is a view showing the axial configuration of a fuel rod, and FIGS. 3 (a), (b) and (c) are conventional. (9 × 9) fuel assembly of BSO operation in the axial direction,
4 (a), (b), and (c) are characteristic diagrams showing the axial output distribution when the fuel assembly according to the embodiment of the present invention is operated in BSO, FIG. 5, and FIG. The figure is a characteristic diagram showing an axial configuration of a conventional (8 × 8) fuel assembly. 101 ... Fuel assembly, 102 ... Channel box, 1
03 ... Fuel rod.

Claims (3)

[Claims] 1. Fuel rods in 9 rows × 9 in a channel box.
The outermost circumference of a fuel assembly in which the spacers are arranged in rows in a grid pattern, the upper and lower ends of which are fixed by the upper tie plate and the lower tie plate, and spacers that hold the fuel rods at intervals are installed at multiple locations in the axial direction. A fuel assembly characterized in that a fuel rod having a high fuel enrichment is arranged at a position, and a fuel enrichment in an axially lower portion is made higher than an axially upper fuel enrichment. 2. The fuel assembly according to claim 1, wherein the fuel rod is a liner cladding tube with zirconium lining inside the cladding tube. 3. A fuel rod having an axial length shorter than an active fuel length, which is located at a lower portion in the axial direction, and has a fuel rod having a low enrichment region in an upper portion in the axial direction. The fuel assembly according to claim 1.