Fuel spacer and fuel assembly

Abstract

During normal operation of a boiling water reactor, the thermal margin of the fuel assembly can be improved, and during shutdown and in the event of a reactivity-injected accident at cold temperatures, the void condensation effect can be reduced, thereby suppressing the rise in fuel enthalpy and reducing the risk of fuel rod damage. [Solution] A fuel spacer 15M is a component of a fuel assembly loaded into the core of a boiling water reactor, in which a plurality of fuel rods 11, each supported at both ends by an upper tie plate and a lower tie plate, are bundled together at intervals from one another. The spacer has a swirling vane 18 that generates a swirling flow in the coolant W flowing from the lower tie plate to the upper tie plate. The swirling vane is configured to be able to change direction so that it is tilted toward the fuel rods 11 during normal operation of the boiling water reactor, and faces in a vertical direction A along the lower tie plate to the upper tie plate when the boiling water reactor is shut down or when it is cold.

Description

Technical Field

[0001] Embodiments of the present invention relate to a fuel spacer and a fuel assembly.

Background Art

[0002] In a void behavior experiment simulating a reactivity insertion accident, an effect has been confirmed that voids generated in the coolant are condensed by a fuel spacer. That is, due to the presence of the fuel spacer, when voids in the coolant flow into the fuel spacer, they are rectified and agitated, and in combination with the cooling effect of the coolant flowing in from the upstream side, the voids condense and disappear immediately downstream of the fuel spacer. The fuel spacer structure used in this experiment was a round cell type spacer simulating a conventional 9×9 fuel assembly.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] In order to further improve the thermal margin (limit output) of a fuel assembly during normal operation of a boiling water reactor, in order to promote the effect of attaching droplets flowing in voids in the coolant to fuel rods by agitation of the coolant by a fuel spacer, a fuel spacer with swirling blades may be employed. Patent Document 1 describes that by generating a swirling flow in the coolant by a fuel spacer with swirling blades, more droplets are attached to the fuel rods, improving the thermal margin (limit output) of the fuel assembly. Thus, during normal operation of a boiling water reactor, the fuel spacer contributes to improving the thermal limit.

[0005] However, in the event of a reactivity-injection accident during shutdown or cooling of a boiling water reactor, the presence of a fuel spacer with swirling vanes causes the coolant to flow into the fuel spacer, swirl, and agitate. Combined with the cooling effect of the coolant flowing in from upstream, this causes voids (vapor phase) in the coolant to condense and disappear immediately downstream of the fuel spacer. In a fuel spacer with swirling vanes, the condensation effect of voids during the aforementioned reactivity-injection accident is considered to be greater, which may lead to a greater increase in fuel enthalpy compared to a fuel spacer without swirling vanes, potentially increasing the number of damaged fuel rods.

[0006] Embodiments of the present invention have been made in consideration of the above circumstances, and aim to provide a fuel spacer and fuel assembly that can improve the thermal margin of the fuel assembly during normal operation of a boiling water reactor, and reduce damage to fuel rods by suppressing the rise in fuel enthalpy by reducing the void condensation effect during shutdown and reactivity input accidents at cold temperatures. [Means for solving the problem]

[0007] The fuel spacer in the embodiment of the present invention is a component of a fuel assembly loaded into the core of a boiling water reactor, and is a fuel spacer that bundles a plurality of fuel rods, each supported at both ends by an upper tie plate and a lower tie plate, at intervals from each other, and has swirling vanes that generate a swirling flow in the coolant flowing from the lower tie plate to the upper tie plate, and is configured to be able to change direction such that it is inclined toward the fuel rods during normal operation of the boiling water reactor, and faces vertically along the lower tie plate from the upper tie plate when the boiling water reactor is shut down or when it is cold.

[0008] The fuel assembly in the embodiment of the present invention is characterized in that the fuel spacer in the embodiment is applied to the fuel spacer located on the upper part of the upper tie plate side of the fuel assembly. [Effects of the Invention]

[0009] According to embodiments of the present invention, the thermal margin of the fuel assembly can be improved during normal operation of a boiling water reactor, and during reactivity input accidents at shutdown and cold temperatures, the void condensation effect can be reduced and the rise in fuel enthalpy can be suppressed, thereby reducing damage to the fuel rods. [Brief explanation of the drawing]

[0010] [Figure 1] A perspective view showing a fuel assembly to which fuel spacers according to the first and second embodiments are applied, with some parts omitted. [Figure 2] Figure 1 is a plan view showing the core of a boiling water reactor into which the fuel assemblies are loaded. [Figure 3] Plan views showing fuel spacers of the first and second embodiments. [Figure 4] A perspective view showing a portion of the fuel spacer in Figure 3. [Figure 5] View from arrow V in Figure 4. [Figure 6] A perspective view showing a portion of a comparative fuel spacer configuration. [Modes for carrying out the invention]

[0011] Hereinafter, embodiments for carrying out the present invention will be described based on the drawings. [A] First Embodiment (Figures 1-5) Figure 1 is a perspective view showing a fuel assembly to which fuel spacers according to the first and second embodiments are applied, with some parts omitted. Multiple fuel assemblies 10 shown in Figure 1 are loaded together with control rods 2 into the core 1 of a boiling water reactor shown in Figure 2. As shown in Figure 1, multiple fuel rods 11 are supported at both ends by an upper tie plate 12 and a lower tie plate 13, and furthermore, multiple fuel spacers 15 are arranged at predetermined distances in the axial direction of the fuel assembly 10, bundling the fuel rods 11 apart from each other.

[0012] In other words, the fuel assembly 10 is composed of an upper tie plate 12 with a handle, a lower tie plate 13, a plurality of fuel rods 11 whose ends are supported by the upper tie plate 12 and the lower tie plate 13, water tubes 16 arranged between the fuel rods 11, and a plurality of fuel spacers 15 arranged in the axial direction of the fuel assembly 10. The fuel spacers 15 maintain a constant horizontal distance between the plurality of fuel rods 11 and water tubes 16.

[0013] Furthermore, the bundle of fuel rods 11 and water tubes 16, which are bundled together by fuel spacers 15, is surrounded by a rectangular channel box 17. This channel box 17 is attached to the upper tie plate 12. In addition, the water tubes 16 are provided with a coolant inlet 16A at the lower end and a coolant outlet 16B at the upper end.

[0014] During normal operation of a boiling water reactor, the coolant W flows vertically upward A from the lower tie plate 13 to the upper tie plate 12 between the fuel rods 11 in the channel box 17 of the fuel assembly 10. The coolant boils due to the reaction heat of the fuel rods 11 controlled by the control rods 2, becoming a two-phase flow (liquid phase, vapor phase) which removes heat to the outside and cools the fuel rods 11.

[0015] Among the multiple fuel spacers 15 arranged in the axial direction of the fuel assembly 10, a fuel spacer 15M having a swirling vane 18 (Figure 3) configured to change direction is applied to the fuel spacer 15 located in the upper part of the upper tie plate 12 side of the fuel assembly 10 where the thermal margin is low (i.e., the downstream side in the flow direction of the coolant W). Here, the fuel assembly 10 in which the fuel spacer 15M is located in the upper part in the axial direction will be referred to as "fuel assembly 10M" below. As shown in Figures 3 and 4, the fuel spacer 15M has, for example, a grid-like spacer body 19 that forms a large number of cells that individually house the fuel rods 11, and the swirling vane 18 that is provided protruding from the upper end of the spacer body 19 on the upper tie plate side (i.e., the downstream side in the flow direction of the coolant W) and whose direction can be changed.

[0016] Here, the fuel spacer 100 shown in FIG. 6 is a fuel spacer of a comparative form. In this fuel spacer 100, the swivel blade 101 is attached to the upper end of the spacer body 102 while being inclined toward the fuel rod 11 without its orientation being changed. Therefore, even when a reactivity insertion accident occurs during the shutdown and cold temperature periods of a boiling water reactor, the swivel blade 101 rotates and stirs the coolant W, and the void condensation effect in the coolant W cannot be reduced. For this reason, the negative reactivity feedback amount of the void cannot be expected, and there is a risk of causing an increase in fuel enthalpy.

[0017] On the other hand, as shown in FIG. 3, the swivel blade 18 in the fuel spacer 15M is made of a shape memory alloy and, during normal operation (e.g., 300°C) of a boiling water reactor when the temperature is above the transformation temperature, it is deformed so as to be inclined at an inclination angle φ (φ = a°) in the direction of the fuel rod 11 as shown by the solid lines in FIGS. 4 and 5. For this reason, the swivel blade 18 generates a swirling flow in the coolant W flowing through the fuel spacer 15 from the lower type plate 13 toward the upper type plate 12, and promotes the void (vapor phase) condensation effect in the coolant W. The above a° is, for example, 45°.

[0018] Also, during the shutdown and cold temperature periods (e.g., 20°C) of a boiling water reactor when the temperature is below the transformation temperature of the shape memory alloy, the swivel blade 18 is deformed so as to face in the vertical direction A (φ = 0°) from the lower type plate 13 toward the upper type plate 12 as shown by the two-dot chain lines in FIGS. 4 and 5. For this reason, even when a reactivity insertion accident occurs during the shutdown and cold temperature periods, the swivel blade 18 does not generate a swirling flow in the coolant W flowing through the fuel spacer 15, and reduces the void (vapor phase) condensation effect in the coolant W. Note that the swivel blade 18 below the transformation temperature returns to an inclination angle φ of φ = a° when the temperature becomes above the transformation temperature.

[0019] Here, the swivel blade 18 is a shape memory alloy composed of titanium, nickel, etc., which has excellent spring elasticity and rich corrosion resistance. For example, a shape memory alloy of about 50 atomic% nickel and about 50 atomic% titanium (for example, trade name: Nitinol manufactured by National laboratory of NAVY) is preferred. Further, the spacer body 19 is made of a zirconium alloy (for example, trademark: Zircaloy) or a nickel alloy (for example, trademark: Inconel).

[0020] The fuel spacer 15M is constituted by welding the above-described swivel blade 18 to the spacer body 19 in a plate state. Examples of the welding method include resistance welding, laser welding, electron beam welding, and friction stir welding.

[0021] Also, although the fuel spacer 15M has been described as being disposed above the upper type plate 12 on the side where the thermal margin is low in the fuel assembly 10M, it may be disposed at other locations in the fuel assembly 10M other than the upper part.

[0022] Furthermore, when the fuel assembly 10M provided with the fuel spacer 15M is loaded into the core 1 of a boiling water reactor, it is necessary to have a thermal-hydraulic coexistence with other fuel assemblies 10 mixed in the core 1. For this reason, the number of fuel spacers 15M arranged in the fuel assembly 10M is set such that the pressure loss in the core 1 of the fuel assembly 10M provided with the fuel spacer 15M becomes a value equivalent (for example, within an error range of ±1%) to the pressure loss in the core 1 of other fuel assemblies 10 loaded into the core 1.

[0023] Due to being configured as described above, according to the first embodiment, the following effects (1) and (2) are achieved. (1) The swirl vanes 18 of the fuel spacer 15M are set to be inclined toward the fuel rods 11 during normal operation of the boiling water reactor. Also, during the above-mentioned normal operation, the coolant W is heated by the heat generated in the fuel rods 11 and becomes a two-phase flow (liquid phase, vapor phase), and flows through the flow path formed between adjacent fuel rods 11. Therefore, the two-phase coolant W flowing through the fuel assembly 10M from the lower tie plate 13 toward the upper tie plate 12 is subjected to a swirling force by the swirl vanes 18 downstream of the fuel spacer 15M, and this swirling force generates a large swirling flow with a small twist shape in the region of high flow velocity.

[0024] This swirling flow promotes the condensation effect of voids (vapor phase) in the coolant, making it easier for droplets flowing through the vapor phase to adhere to the surface of the fuel rod 11 downstream of the fuel spacer 15M. As a result, the cooling effect of the fuel rod 11 is increased, delaying the occurrence of boiling transition of the fuel rod 11 downstream of the fuel spacer 15M. This allows for an improvement in the thermal margin (limiting power) of the fuel assembly 10M while suppressing an increase in pressure loss.

[0025] (2) The swirling blades 18 of the fuel spacer 15M are set to face in a vertical direction A along the lower tie plate 13 to the upper tie plate 12 when the boiling water reactor is shut down or cold. Therefore, even if a reactivity injection accident occurs when the reactor is shut down or cold, no swirling flow will be generated in the coolant W downstream of the fuel spacer 15M by the swirling blades 18. As a result, the condensation effect of the voids (vapor phase) generated during a reactivity injection accident is reduced, and a negative reactivity feedback amount of the voids can be expected. Consequently, the rise in fuel enthalpy can be suppressed, and thus damage to the fuel rods 11 can be reduced.

[0026] [B] Second embodiment (Figures 1-5) The difference between the fuel assembly 10N of this second embodiment, shown in Figures 1 to 3, and the fuel assembly 10M of the first embodiment is that a fuel spacer 15N, corresponding to the fuel spacer 15M of the first embodiment, is arranged in the fuel assembly 10N, and the swirling blades 21 of this fuel spacer 15N are configured with a bimetallic structure.

[0027] In other words, the swivel blade 21 is made of SiC (thermal expansion coefficient 4-5 × 10⁻⁶), which has a low thermal expansion coefficient. -6 K) and tin (thermal expansion coefficient 23 × 10) which has a large thermal expansion coefficient (thermal expansion coefficient 23 × 10 -6 It is constructed by bonding dissimilar materials such as K) together, or by bonding dissimilar materials such as a martensitic stainless steel sheet with a low coefficient of thermal expansion and an austenitic stainless steel sheet with a high coefficient of thermal expansion together. The swivel blade 21 is welded to the spacer body 19 in the same manner as in the first embodiment to form the fuel spacer 15N.

[0028] As shown in Figures 4 and 5, the swivel vanes 21 of the fuel spacer 15N change orientation within a predetermined temperature range (temperature range from normal operation to shutdown and cold temperatures of the boiling water reactor) based on the difference in thermal expansion coefficients of the dissimilar materials. In other words, during normal operation of the boiling water reactor (e.g., 300°C), the swivel vanes 21 deform to tilt toward the fuel rods 11 at an angle φ (φ=a°), as shown by the solid lines in Figures 4 and 5. Furthermore, during shutdown and cold temperatures of the boiling water reactor (e.g., 20°C), the swivel vanes 21 deform to face in a vertical direction A (φ=0°) from the lower tie plate 13 toward the upper tie plate 12, as shown by the dashed lines in Figures 4 and 5.

[0029] In the bimetallic swivel vane 21, during the deformation process described above, there exists a tilt angle φ(0°≦φ≦a°) between the tilt angle φ(φ=0°) during shutdown and cooling in a boiling water reactor and the tilt angle φ(φ=a°) during normal operation. However, in the relatively low temperature range where the tilt angle φ of the swivel vane 21 is close to 0°, the thermal margin (limiting power) of the fuel assembly 10N does not become a problem. Furthermore, in the relatively high temperature range where the tilt angle φ of the swivel vane 21 approaches a°, even if a reactivity-injection accident occurs, saturation boiling occurs rapidly, and the impact on the core 1 is low due to the negative feedback amount of voids in the coolant W.

[0030] Furthermore, the number of fuel spacers 15N in fuel assembly 10N is determined as follows. Specifically, since the swivel vanes 21 of the fuel spacer 15N are bimetallic structures made of different materials bonded together, their plate thickness is greater than that of the spacer body 19, and the pressure loss of the fuel spacer 15N tends to be higher. Also, the pressure loss of the fuel spacer 15 including the fuel spacer 15N is proportional to the projected area. Based on these considerations, the projected area of ​​the fuel spacer 15N is compared with the projected area of ​​the fuel spacers 15 of other fuel assemblies 10 loaded into the core 1, and the number of fuel spacers 15N in fuel assembly 10N is tentatively determined so that these projected areas are equal. Finally, the number of fuel spacers 15N in fuel assembly 10N is determined by confirming that the pressure loss in the core 1 of fuel assembly 10N equipped with fuel spacers 15N is equivalent to the pressure loss in the core 1 of other fuel assemblies 10 loaded into the core 1.

[0031] As configured as described above, this second embodiment also produces the same effects as those of the first embodiment (1) and (2).

[0032] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be carried out in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention, and such substitutions, modifications, and combinations are included in the scope and spirit of the invention, as well as in the claims and their equivalents.

[0033] For example, although the shape of the swivel vanes 18 and 21 in the first and second embodiments was described as being trapezoidal, they do not necessarily have to be trapezoidal and may be set to any shape that satisfies the required thermal margin (limit power) and pressure loss in the fuel assembly. Also, although the fuel spacers 15, 15M, and 15N were described as being grid-type spacers in the first and second embodiments, they may also be round cell-type spacers. [Explanation of Symbols]

[0034] 1...Core, 10, 10M, 10N...Fuel assemblies, 11...Fuel rods, 12...Upper tie plate, 13...Lower tie plate, 15, 15M, 15N...Fuel spacers, 18, 21...Swivel vanes, A...Vertical direction, W...Coolant, φ...Incline angle.

Claims

1. A fuel spacer is a component of a fuel assembly loaded into the core of a boiling water reactor, in which multiple fuel rods, each supported at both ends by an upper tie plate and a lower tie plate, are bundled together at intervals from one another. The coolant has swirling vanes that generate a swirling flow in the coolant flowing from the lower tie plate toward the upper tie plate, This fuel spacer is characterized in that the swirling blades are configured to be inclined toward the fuel rods during normal operation of the boiling water reactor, and to be oriented vertically along the upper tie plate from the lower tie plate when the boiling water reactor is shut down or cooled.

2. The fuel spacer according to claim 1, characterized in that the swivel blades are provided protruding from the upper end of the upper tie plate side of the spacer body and are made of a shape memory alloy or a bimetallic structure.

3. A fuel assembly characterized in that the fuel spacer described in claim 1 or 2 is applied to the fuel spacer positioned on the upper part of the upper tie plate side of the fuel assembly.

4. The number of fuel spacers according to claim 1 or 2 arranged in the fuel assembly is determined such that the pressure loss in the core of the fuel assembly equipped with the fuel spacers is equivalent to the pressure loss in the core of other fuel assemblies loaded into the core.

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