JP2007139615A - Fuel assembly, and nuclear reactor having core loaded with fuel assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce required enrichment of a fuel assembly; to reduce a cladding tube temperature; and to uniformize a coolant temperature near a coolant outlet of the fuel assembly, in a supercritical pressure light water reactor. <P>SOLUTION: In this fuel assembly 1 for a light water reactor or the supercritical pressure light water reactor wherein fuel rods 3 are arrayed in a square grid shape in a channel box 2 and loaded on a core, the fuel rods 3 are arrayed in the square grid shape to the number of N×N (N is an integer of 12 or higher), and a square tube-shaped water rod 4 is arranged on a domain to the number of [(N+1)/2]×[(N+1)/2] on a center part in the channel box 2 of the fuel rods 3. In this case, [X] is the maximum integer not exceeding a real number X. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel assembly for a light water reactor, and more particularly to a fuel assembly for a supercritical pressure light water reactor in which supercritical pressure water is used as a coolant.

  Currently, development of supercritical light water reactors using supercritical pressure water as a coolant is underway. A supercritical light water reactor is a nuclear reactor that uses high-temperature and high-pressure supercritical water (500 ° C., 25 MPa) as a coolant. In supercritical light water reactors, fuel assemblies are used in a temperature range that is 200 degrees higher than conventional light water reactors. Therefore, in order to maintain the integrity of the fuel assemblies, it is required to reduce the temperature rise locally. It was.

Therefore, in order to flatten the power distribution of the fuel assembly in a supercritical light water reactor, a method has been proposed in which a water rod is arranged inside or outside the fuel assembly (see, for example, Patent Document 1).
JP 2003-294878 A

  To average the power distribution of fuel assemblies in light water reactors (especially supercritical light water reactors), optimize the shape of the fuel assemblies, set the enrichment distribution according to the shape of the assemblies, and the fuel assemblies in the reactor Placement is important, but there is currently no comprehensive study that combines them.

  The present invention has been made in view of the above problems, and in a light water reactor (particularly a supercritical pressure light water reactor), reducing the required concentration of the fuel assembly, reducing the cladding tube temperature, and the fuel assembly The purpose is to make the coolant temperature near the coolant outlet uniform.

  In order to solve the above problems, a fuel assembly according to the present invention is used for a light water reactor in which fuel rods are arranged in a square lattice in a channel box and filled into a reactor core as described in claims 1 and 2. In the fuel assembly for a supercritical light water reactor, the fuel rods are arranged in a square lattice of N × N (N is an integer of 12 or more), and [(N + 1) / 2] at the center of the fuel rod in the channel box. In a region of x [(N + 1) / 2] ([X] is the maximum integer not exceeding the real number X), a rectangular cylindrical water rod is arranged.

  Further, as described in claim 9, the nuclear reactor according to the present invention includes a set of four bodies arranged in a square lattice in a nuclear reactor loaded with the fuel assembly according to claim 1 or 2 on the core. One control rod is arranged for each group in the center of the fuel assembly, and the number of cycles of each fuel rod of the four fuel assemblies is approximately equal.

  According to the fuel assembly according to the present invention, in a light water reactor or a supercritical pressure light water reactor, the required enrichment of the fuel assembly is reduced, the cladding temperature is reduced, and the cooling in the vicinity of the coolant outlet of the fuel assembly is performed. It is possible to make the material temperature uniform.

  An embodiment of a fuel assembly according to the present invention will be described with reference to the accompanying drawings.

[First Embodiment]
1 to 5 are views showing a first embodiment of a fuel assembly according to the present invention.

  As shown in FIG. 1, the fuel assembly 1 according to the first embodiment includes a square tubular channel box 2. Inside the channel box 2, a number of fuel rods 3 extend in the core axis direction and are arranged in a square lattice shape, and a rectangular tube-shaped water that extends in the core axis direction in the center of the channel box 2 in the core axis direction. A rod 4 is arranged.

  In the first embodiment, for example, the fuel rods 3 are arranged in a 16 × 16 square lattice shape, and the water rods 4 are arranged so as to occupy a space of 8 × 8 sizes of the fuel rods 3. . That is, for example, 192 fuel rods 23 are arranged in the fuel assembly 1. 192 is the number obtained by removing 8 × 8, which is a space where the water rod 4 is disposed, from 16 × 16.

  Here, the number of the fuel rods 3 per side of the fuel assembly 1 (hereinafter referred to as the number of fuel rods) is N, and the fuel rods 3 in a state where the core is installed corresponding to the length of one side of the water rod 4. The equivalent number (hereinafter referred to as the number corresponding to the water rod) is M.

  FIG. 2 shows the number M of water rods corresponding to the number N of fuel rods and the initial enrichment necessary for performing a certain period of combustion (for example, take-out average burnup 45 GWd / t) (hereinafter referred to as required enrichment). Shows the relationship.

From FIG. 2, the relationship between the number of fuel rods N and the number M of water rods that minimize the required enrichment is
[Equation 1]
M = [(N + 1) / 2] (1)
(Here, [X] represents the maximum integer not exceeding the real number X.)
It can be seen that it is.

  Here, the cooling water in the water rod 4 has two contradictory effects. One is the effect of reducing the required enrichment caused by the action of a moderator that decelerates fast neutrons to thermal neutrons. The other is the effect of increasing the required concentration resulting from the function as an absorber that absorbs thermal energy from neutrons.

  While the effect of the neutron moderator as the water rod 4 increases in size, the effect as a neutron absorber increases in proportion to the increase in the size of the water rod 4. In other words, the water rod 4 has a greater effect due to its function as a moderator when the amount of water is reduced, and the effect due to its function as an absorbent is increased when the amount of water is increased.

  Therefore, from the viewpoint of minimizing the required enrichment, it can be seen that there is an optimal relationship between the number N of fuel rods and the number M corresponding to water rods, and the relationship is as shown in Equation (1).

  FIG. 3 shows the gap between the adjacent fuel rods 3 and the number of fuel rods for each required enrichment (7.4, 7.6, 7.8, 8.0, 8.2, 8.4 wt%). The relationship with N is shown. According to FIG. 3, the required enrichment decreases as the gap between adjacent fuel rods 3 increases, and the dependency of the required enrichment on the gap between adjacent fuel rods 3 decreases. . The gap between adjacent fuel rods 3 is held by a gap holding spacer.

  Considering that it is desired to reduce the gap between the adjacent fuel rods 3 and the required enrichment and to increase the number N of fuel rods, the gap between the fuel rods 3 is 1.3 mm from FIG. It can be seen that the number of fuel rods N is most preferably 16.

  FIG. 4 shows the ratio (0.8, 0.9, 1.0, 1.1, 1.2, 1.3) of the cladding tube surface temperature to the reference value of the temperature rise amount on the cladding tube surface as an index. ) Shows the relationship between the gap between adjacent fuel rods 3 and the number N of fuel rods. A range F in which the ratio of the temperature rise amount to the reference value is 1 or less (a range shaded in FIG. 4) is a range in which the cladding tube surface temperature can be reduced.

  According to FIG. 4, it can be seen that the cladding surface temperature can be reduced by setting the fuel rod gap to 1.5 mm or less.

  FIG. 5 shows a fuel assembly 1A as another example of the fuel assembly 1 of the first embodiment. Since it is difficult to round the corner A while maintaining the strength required when the channel box 2A of the fuel assembly 1A is molded, actually, as shown in FIG. 5, the corner of the channel box 2A The part A is often gently curved. Even when the corner portion A is curved as described above, the relationship between the number N of one side of the fuel rod 3 and the length M of one side of the water rod 4 can be applied.

  According to the fuel assemblies 1 and 1A according to the first embodiment, by determining the size of the water rod 4 from the number of the fuel rods 3, the cladding tube temperature is reduced, and the coolant outlet of the fuel assemblies 1 and 1A is reduced. The coolant temperature in the vicinity can be made uniform.

[Second Embodiment]
Next, a second embodiment of the fuel assembly according to the present invention will be described with reference to FIGS. In addition, the same code | symbol is attached | subjected to the structure same as 1st Embodiment, and the overlapping description is abbreviate | omitted.

  As shown in FIG. 6, the fuel assembly 1 </ b> B according to the second embodiment includes fuel rods 3 arranged in a square lattice shape in the core radial direction inside a rectangular tube box 2, and a rectangular tubular water rod. 4 is arranged in the center part in the core radial direction.

  In the second embodiment, as in the first embodiment, for example, the fuel rods 3 are arranged in a 16 × 16 square lattice shape, and the water rods 4 are formed in the size of 8 × 8 fuel rods 3. .

  FIG. 7 shows the concentration distribution in the core axis direction of the fuel rod 3 of the fuel rod types U1, G1, G2 of the fuel rod 3 used in the fuel assembly 1B. The fuel rod types U1, G1, and G2 shown in FIG. 6 correspond to the fuel rod types U1, G1, and G2 shown in FIG.

  In the fuel type U1, from the lower part in the core axis direction, the C region (the region within 1 to 18 when the fuel rod effective length of FIG. 7 is an enriched uranium enriched uranium enrichment of 9.5 wt%, and E region (an area of 23 to 24 fuel rod effective lengths in FIG. 7). 24) is filled with natural uranium (or depleted uranium) containing 0.71 wt% uranium 235.

  The fuel type G1 has a uranium enrichment with a concentration of 9.5 wt% uranium enriched in the C region containing 3.0 wt% gadolinia and a uranium enrichment with a D region containing 3.0 wt% gadolinia. 5 wt% enriched uranium, E region is filled with natural uranium (or depleted uranium) containing 0.71 wt% uranium 235.

  In addition, fuel type G2 has a uranium enrichment with a concentration of 9.5 wt% containing 15.0 wt% gadolinia in the C region and a uranium enrichment with a D region containing gadolinia with a concentration of 15.0 wt%. 5 wt% enriched uranium, E region is filled with natural uranium (or depleted uranium) containing 0.71 wt% uranium 235.

  In addition, the average value of the density | concentration (uranium enrichment) of the uranium 235 of each node is shown on the right end of FIG.

  In the second embodiment, gadolinium (Gd) is used as the flammable poison, but other flammable poisons such as europium (Eu) and americium (Am) may be used.

  The fissile material concentration distribution and the flammable poison concentration distribution in the core radial direction of the fuel assembly 1 </ b> B have a ¼ diagonal symmetry. If the fuel rods 3 are arranged with diagonal symmetry in this way, the power distribution in the fuel assembly 1B can be flattened, and a local increase in the cladding surface temperature can be suppressed.

  In the corner portion (corner portion) A in the core diameter direction of the fuel assembly 1B, at least three fuel rods G2 containing flammable poisons are arranged in an L shape so as to surround the outer periphery of the corner portion A. When the fuel rods G2 containing flammable poisons are arranged side by side, the period during which the reactivity suppression effect by the flammable poisons can be maintained can be extended by the mutual shielding effect, and the effect of improving the furnace shutdown margin and improving the power distribution can be achieved. can get.

  By increasing the concentration of the combustible poison in the combustible poison-containing fuel rod G2, it is possible to obtain the reactivity suppression effect by the combustible poison. However, increasing the concentration of flammable poisons has the disadvantages of deteriorating heat transfer characteristics and increasing the required concentration to compensate for the decrease in fissile material. It is not preferable to increase the concentration of toxic substances.

  In particular, in the supercritical light water reactor, as shown in FIG. 8, the coolant temperature in the core axis direction changes greatly. In the region where the coolant temperature exceeds the supercritical temperature (about 400 ° C. and pressure of 25 MPa) (in the vicinity of node positions 16 to 24 in the axial direction of the core in FIG. 8) It was found that the maximum surface temperature of the cladding tube can be lowered by lowering the output of.

  For this reason, the uranium enrichment in a range (core axis direction node positions 18 to 24) in which the fuel effective length is reduced by a quarter from the upper end in the core axis direction of the fuel rod 3 is lowered.

  In addition, by providing an upper blanket (region E in FIG. 7) of natural uranium (or deteriorated uranium) at the upper part of the fuel rod 3 in the core axis direction, the fissile material concentration is set to 0.2 to 0.71 wt%. It is possible to lower the surface temperature of the upper cladding tube, which tends to increase the cladding surface temperature.

  Although not shown in FIG. 8, when the length of the upper blanket is 1/24 of the effective fuel length, the cladding tube surface temperature becomes high at the center of the fuel rod 3 in the core axis direction. Is preferably 2/24 or more of the effective fuel length.

  However, on the other hand, it is necessary to consider that when the output in the upper part of the core axis direction decreases, the output of other parts increases and the surface temperature of the cladding tube of other parts increases.

  FIG. 9 shows the relationship between the upper blanket length and the maximum cladding surface temperature.

  According to FIG. 9, when the length of the upper blanket is 1/6 of the effective fuel length (the blanket length of FIG. 9 is approximately 0.16 times the effective fuel length), the decrease in the cladding tube surface maximum temperature is saturated. It was. Therefore, it is desirable that the length of the upper blanket is 1/6 or less of the effective fuel length.

  In the second embodiment, the length of the upper blanket is set to 2/24, that is, 1/12 of the effective fuel length, which is the length at which the decrease in the maximum temperature of the cladding tube begins to saturate.

  Moreover, the length of the upper blanket was set to 1/6 or less of the effective fuel length.

  According to the fuel assembly 1B according to the second embodiment, the temperature of the cladding tube is reduced by disposing the fuel rod G2 containing the flammable poison at the corner portion A of the channel box 2 in the core radial direction, and the outlet of the fuel assembly 1 is reduced. This makes it possible to make the coolant temperature uniform.

  Further, according to the fuel assembly 1B, the maximum temperature of the coating crown surface can be lowered by setting the length of the upper blanket to 1/6 or less of the effective fuel length.

[Third Embodiment]
A nuclear reactor loaded with the fuel assembly 1 according to the first embodiment of the present invention will be described as a third embodiment with reference to FIGS.

  FIG. 10 shows a quarter core of fuel loading of the reactor core of the third embodiment. In FIG. 10, the position of the control rod is indicated by a circle and the number of fuel cycles is symbolized as 1 to 4.

  At this time, the fuel assembly 1 of the initial fuel cycle having a high reactivity is arranged on the outer peripheral side of the core, and the fuel assembly 1 of the fuel cycle having a low reactivity and advanced combustion is arranged on the center side of the core. .

  As shown in FIG. 10, when the four fuel assemblies 1 around the control rods are loaded so that the number of fuel cycles is equal, the four rods 1 around the control rods are operated by operating the cross-shaped control rods. It is possible to adjust the output of the set of fuel assemblies 1 and maintain the power distribution of the entire core in a constant shape during the operation period.

  Further, by changing the flow rate classification according to the output distribution shape, the temperature distribution near the outlet of the coolant can be flattened and the cladding surface temperature can be lowered. FIG. 11 shows an example of the flow rate classification of the quarter core of the fuel loading of the reactor core. FIG. 11 shows the flow rate as three categories. The high flow rate category is shown as category 1, the middle flow rate category as category 2, and the low flow rate category as category 3.

  At this time, the flow rate may be low on the outer peripheral side of the core, and the flow rate may be high on the central side of the core. In addition, on the outer peripheral side (for example, the outermost two rows), there are two sections, a low flow section 3 and a medium flow section 2, taking into account that the power of the reactor is high at a position where the amount of neutron leakage is small. It is good to set so that it consists of.

  FIG. 12 shows the coolant outlet temperature in the reactor in which the fuel assembly 1 of the first embodiment is loaded in the core when the three flow rates are added (solid line) and when the flow rate is not added (broken line). Shows the transition of maximum temperature. It can be seen that the maximum temperature of the coolant outlet temperature is lowered by about 75 ° C. by attaching three flow rate sections.

  According to the reactor of the third embodiment, by arranging four pairs of fuel rods 3 around the control rods to have the same number of collection cycles, or by providing a flow rate section in the cooling water, It is possible to flatten the temperature distribution and lower the cladding tube surface temperature.

1 is a horizontal sectional view showing a first embodiment of a fuel assembly according to the present invention. The figure which showed the relationship between the magnitude | size of the water rod of 1st Embodiment, and required concentration. The figure which showed the relationship between the number of the fuel rod of 1st Embodiment, the gap | interval of a fuel rod, and required enrichment. The figure which showed the relationship between the number of the fuel rod of 1st Embodiment, the gap | interval of a fuel rod, and the cladding tube surface temperature rise amount. The horizontal sectional view which shows the other example of the suitable fuel assembly of 1st Embodiment. The horizontal sectional view (concentration distribution) which shows 2nd Embodiment of the fuel assembly which concerns on this invention. The figure which showed the enrichment distribution in the axial direction of the fuel rod of 2nd Embodiment. The figure which showed the core direction distribution of the fuel rod of coolant temperature and cladding tube surface temperature. The figure which showed the relationship between cladding tube surface maximum temperature and upper blanket length. The figure which showed the example of the fuel loading pattern of the 1/4 core of the nuclear reactor which loaded the fuel assembly which concerns on this invention into the core. The figure which showed the example of the flow volume division of the 1/4 core of the nuclear reactor which loaded the fuel assembly which concerns on this invention in the core. The figure which showed the temperature of the cooling-water exit vicinity in the case where the flow division is provided in the reactor which loaded the fuel assembly which concerns on this invention in the core, and when it does not provide.

Explanation of symbols

1 Fuel Assembly 2 Channel Box 3 Fuel Rod 4 Water Rod

Claims (11)

In a fuel assembly for a light water reactor in which fuel rods are arranged in a square lattice in a channel box and filled in the core,
The fuel rods are arranged in a square lattice of N × N (N is an integer of 12 or more),
In the region of [(N + 1) / 2] × [(N + 1) / 2] lines ([X] is the maximum integer not exceeding the real number X) in the center of the fuel rod channel box, a rectangular tube of water A fuel assembly, wherein a rod is arranged. In a fuel assembly for a supercritical light water reactor in which fuel rods are arranged in a square lattice in a channel box and filled into the core,
The fuel rods are arranged in a square lattice of N × N (N is an integer of 12 or more),
In the region of [(N + 1) / 2] × [(N + 1) / 2] lines ([X] is the largest integer not exceeding the real number X) in the center of the fuel rod channel box, water in the shape of a rectangular tube is used. A fuel assembly, wherein a rod is arranged. The fuel assembly according to claim 1 or 2, wherein a gap between each adjacent fuel rod is 1.5 mm or less. 4. The fuel assembly according to claim 3, wherein when the fuel rods are arranged in a 16 × 16 square lattice, a gap between adjacent fuel rods is 1.3 mm. The fuel assembly according to claim 1 or 2, wherein at least three of the fuel rods arranged in the vicinity of a corner on the outer peripheral side of the channel box are fuel rods containing a combustible poison. 3. The fuel assembly according to claim 1, wherein in each of the fuel rods, a fissile material concentration in a portion of a length of 1/6 of the effective fuel length from the upper end in the core axis direction is 0.2 to 0.71 wt%. . 3. The fuel assembly according to claim 1, wherein a fissile material concentration in a portion of a length of ¼ of the effective fuel length from the upper end in the core axis direction of the effective fuel length is 1 wt% or more lower than a fissile concentration in other portions. . 6. The fuel assembly according to claim 5, wherein the enrichment distribution of the fuel rods and the arrangement of the fuel rods containing combustible poisons have a 1/4 diagonal symmetry. 9. A nuclear reactor in which the fuel assembly according to claim 1 is loaded on a reactor core, and a control rod is arranged for each set at the center of a set of four fuel assemblies arranged in a square lattice. And the number of cycles of each fuel rod of the set of four fuel assemblies is substantially equal. Two rows of fuel assemblies composed of fuel in the initial fuel cycle with high reactivity are arranged on the outer peripheral side of the core, and fuel assemblies in the fuel cycle with low reactivity and advanced in the center of the core The reactor according to claim 9, wherein the fuel assemblies are arranged in a region extending from the outer peripheral side to the center of the core so that the fuel cycles are layered in two rows in the core circumferential direction. 10. The flow rate classification of the coolant flowing through the core is classified into at least three sections of high, middle and low in the core radial direction, and at least two flow classifications around the fuel assembly loaded on the outer peripheral side. Nuclear reactor.

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