JPH07111468B2 - Fuel assembly for nuclear reactor - Google Patents

Description

The present invention relates to a fuel assembly to be loaded into a nuclear reactor, and particularly, to improve fuel economy and reduce pressure loss of the fuel assembly. The present invention relates to a fuel assembly having improved thermal hydraulic characteristics.

(Prior Art) FIG. 10 shows a structure of a typical fuel assembly loaded in a conventionally known boiling water reactor.
FIG. 11 shows the structure of the fuel rods constituting the fuel assembly.

In the figure, a fuel rod 11 has a cladding tube 13 filled with pellets 12 made of uranium oxide as a ceramic, held by a spring 14 and a getter 15, and an upper end plug 16 and a lower end plug.
The structure is such that 17 is welded to the cladding tube 13 and sealed, and the inside is filled with helium. The space indicated by 18 is filled with helium and is called a plenum.

In the fuel assembly, the fuel rods 11 and the water rods 21 are arranged in a grid pattern of 8 rows and 8 columns, fixed by an upper tie plate 22, a lower tie plate 23 and a spacer 24, and the outside thereof is a channel box 25. It has an enclosed structure.

FIG. 12 (a) is a cross-sectional view taken along the line I-I of FIG. 10, and as described above, the fuel rod 11 containing uranium inside.
(In the figure, numbers and symbols G are written in circles) and water rods 21 containing no uranium and flowing a coolant are regularly arranged in an 8 × 8 lattice. Reference numeral 26 indicates a control rod for controlling the nuclear reaction of the nuclear reactor.

FIG. 12 (b) illustrates the uranium enrichment and gadolinia concentration of each fuel rod shown in FIG. 12 (a). The numbers below each fuel rod indicate the fuel rod number, e _{1 to} e ₁ . ₄ indicates the uranium enrichment, and g indicates the gadolinia (Gd ₂ O ₃ ) concentration. e ₁ ＞ e ₂ ＞ e
₃ > e ₄ .

A fuel assembly for a boiling water reactor generally uses several types of fuel rods having different enrichments to have an enrichment distribution. The method of distribution of enrichment depends on the design purpose of the fuel assembly. For example, as shown in FIG. 12 (a), a fuel rod of No. 1 containing highly enriched pellets (hereinafter referred to simply as fuel rod 1. The same applies hereinafter) is arranged in the center of the fuel assembly, A method is known in which fuel rods 3 and 4 containing pellets having low enrichment are arranged around the fuel assembly near the box.

In addition, gadolinia (Gd ₂ O
₃ ) is usually contained in a few%, and in the example shown in FIG. 12 (a), G is a fuel rod with gadolinia.

Among the fuel assemblies loaded in the reactor core, those that generate a predetermined amount of energy are replaced with new fuel assemblies at the annual periodic inspection. From the viewpoint of fuel economy, it is desirable to increase the thermal energy generated by one fuel assembly, that is, to increase the burnup of the extracted fuel as much as possible. In order to increase the burnup of the extracted fuel, it is necessary to increase the enrichment.

However, there are some technical problems in increasing the fuel enrichment and burnup. The main ones are the decrease of the reactor shutdown margin and the increase of the absolute value of the void coefficient caused by the hardening of the neutron spectrum by the thermal neutron absorption of uranium-235.

In other words, due to the higher enrichment of the fuel (increased uranium 235 content), the absorption of thermal neutrons into the fuel (uranium 235) increases, so the thermal neutron absorption into the moderator decreases relatively, Has a relatively small energy distribution in the thermal region, and the neutron spectrum becomes hard. When the neutron spectrum becomes hard, the resonance absorption in the epithermal region increases, so the void coefficient increases in the negative direction, reducing the margin of thermal characteristics, core stability and transient characteristics, and controlling The reduction in rod value will reduce the reactor shutdown margin, which will affect the safety of the reactor. The effect of neutron spectrum hardening due to this high concentration is remarkable especially in the upper part of the fuel. this is,
This is because, in a boiling water reactor, voids are generated, so that the moderator is small in the upper portion of the fuel and the neutron spectrum is originally hard.

In order to improve the hardening of the neutron spectrum, it is conceivable to increase the moderator / fuel ratio to relatively increase the thermal neutron absorption by the moderator and soften the neutron spectrum. In order to increase the moderator / fuel ratio, either the moderator is increased or the fuel is decreased.

As a design corresponding to the above problem, there has been proposed a fuel assembly in which the number of water rods or the thickness thereof is increased and the number of fuel rods is further increased so as to be arranged in 9 rows and 9 columns. In this case, the hardening of the neutron spectrum is suppressed, deterioration of various characteristics accompanying it is also suppressed, the average linear power density is also reduced and the thermal margin is increased, but on the other hand, the number of fuel rods is increased. Internal pressure loss increases and stability deteriorates.

As a remedy for this, a method has been considered in which a part of the fuel is made into a short type to reduce the pressure loss. For example, a water rod and a short type fuel rod that are shaped to expand upward are arranged at the bottom of the fuel assembly. There is one (JP-A-52-50498). When such a design is adopted, the above-mentioned object is achieved to some extent. However, when viewed in the axial direction of the fuel assembly, the amount of fuel is reduced even in the central area where the output is high.Therefore, the loss of fissionable material due to combustion is large and the reactivity decreases accordingly. Peaking at the bottom of the core becomes large. In addition, since there is a lack of fuel rods at the upper ends of short fuel rods, the thermal neutron flux in this portion becomes high, and the linear output of adjacent fuel rods locally rises, which may reduce the thermal margin.

(Problems to be Solved by the Invention) The present invention has been made in view of the above circumstances. The problem to be solved by the present invention is to reduce the thermal margin and the reactivity of the core in a fuel assembly. It aims at saving uranium and improving the shutdown margin without reducing the pressure loss of the coolant and improving the thermal-hydraulic characteristics.

[Structure of the Invention] (Means for Solving the Problems) The present invention has been made to achieve the above-mentioned object, that is, the present invention is an elongated fuel rod and an axial length longer than the elongated fuel rod. In a fuel assembly for a reactor in which a large number of short fuel rods and short fuel rods are bundled by a plurality of spacers, the upper end of the short fuel rod is located at the uppermost spacer position. And a fuel assembly for a nuclear reactor.

(Operation) According to the present invention, in the upper part of the fuel, the reactor shutdown margin is increased due to the decrease in the amount of fissile material and the increase in the water-to-uranium fuel ratio, and the flow passage area is increased in the upper part of the fuel. And the hydro-hydraulic characteristics and stability are improved. Also, although short fuel rods are used, their effective fuel length is close to that of long fuel (about 5/6 or more), so the reduction of total enriched uranium weight can be minimized. It is also possible to suppress the increase in the linear output generated above the short fuel rods within the limit range.

Further, when viewed in the radial cross section of the fuel assembly, by arranging the short type fuel at a position excluding the outer peripheral portion of the fuel assembly, it is possible to make the moderator distribution in the radial cross section above the fuel more uniform. By increasing the neutron moderating effect, the reactivity during power operation is increased to compensate for the decrease in fuel inventory, and the axial power distribution is flattened.

(Embodiment) FIG. 1 is a sectional view schematically showing the axial positional relationship between a long fuel rod and a short fuel rod according to one embodiment of the present invention. In the figure, the fuel rod 31 is a long fuel rod (having the same prescribed length as the conventional one), and the fuel rods 32 and 33 are short fuel rods. In the figure, the blank part 34 is the gas plenum,
What is indicated by the hatched portion 35 is a fuel pellet.

The upper end position of the fuel pellets of the short fuel rods 32 and 33 is lower than that of the long fuel rod 31, and at the same time, the tip positions of the fuel rods are also reduced in order to reduce the pressure loss of the coolant flowing through the fuel assembly. It's getting low. Further, the short fuel rod 32 is also provided with a gas plenum 34 in the lower part.

The fuel rods are bundled into a square lattice by a plurality of spacers to form a fuel assembly.
Has a structure held by the uppermost spacer.

FIG. 2 illustrates a fuel assembly (hereinafter referred to as “case A”) which is an embodiment of the present invention. (A) is a fuel rod layout diagram, and (b) is a long fuel rod. It is a schematic diagram which shows the axial direction cross section of a short type fuel rod. As shown in FIG. 2 (a), the fuel assembly of this embodiment is a fuel assembly having a 9-row by 9-column fuel rod arrangement in which a large diameter water rod 41 is arranged in the center, and is designated by a symbol P 24 It is composed of one short fuel rod 42 and 48 long fuel rods 43. Of these, the short fuel rods 42 are arranged at the fuel rod positions within the second row from the outermost periphery.

As shown in FIG. 2 (b), the axial cross section of the fuel rod is such that when the height (fuel active length) of the portion containing the fuel pellets of the long fuel rod 43 is 24 node length, the short fuel is The height of the rod tip is the 23rd node and the highest position of the effective fuel area is the 21st node. In the figure, a part surrounded by a broken line (a long type above the 24th node in height, the short type 22 nodes and 23 nodes are gas plenums 34. Short length fuel rods 42
Is held by the top spacer at the 23rd node height. It should be noted that the length of 2 nodes at the upper end and the length of 1 node at the lower end shown by the diagonal lines in the figure indicate that natural uranium pellets are enclosed.

In this embodiment, by using the short type fuel rod in this way, the pressure loss of the coolant of the fuel assembly is reduced, and the pressure loss is almost the same as that of the conventional 8 × 8 type shown in FIG. I was able to.

By the way, if only a pressure loss value similar to that in the above embodiment is obtained, it is possible to reduce the number of short fuel rods and shorten their length. Examples of fuel assemblies having pressure drop values similar to those of the above-described embodiment are compared and shown in FIGS. 8 and 9, and the effects of the present invention will be described in more detail by comparison with these. In each of FIG. 8 and FIG. 9, (a) is a fuel rod arrangement view, and (b) is a schematic view of an axial cross section of a long fuel rod and a short fuel rod. In FIG. 8 (hereinafter referred to as “case B”), twelve short fuel rods 44 are used, but the axial length thereof is shorter than that of the present invention, and the upper end position is the second spacer from the top. Position (20th node). Further, the example of FIG. 9 (hereinafter referred to as “case C”) also has eight shorter fuel rods shorter than the present invention.
45 is used, and the upper end position is the position of the third spacer from the top (16th node). FIG. 3 is a comparison of the pressure losses of the cases B and C with reference to the case A which is the embodiment of the present invention. In FIG.
The vertical axis represents the difference in pressure loss value of each example with reference to Case A.
As can be seen from this figure, there is no significant difference in pressure loss values between these three cases.

However, among these examples, Case A exhibits excellent effects in the following points.

(1) Although the weight of uranium per fuel assembly is reduced by using the short fuel rods, it is preferable to increase the number of the short fuel rods and increase the length thereof as in case A to reduce the number of fuel rods. It is possible to suppress a decrease in the loading amount of enriched uranium rather than shortening the length. FIG. 4 shows relative magnitudes of the weight of the natural uranium portion and the weight of the enriched uranium portion of Cases A, B, and C with reference to Case A. In case A, the weight of the natural uranium part is reduced,
It has been shown that the weight of the enriched uranium part, which occupies the major part, is high.

(2) Generally, when a short fuel rod is used, the fuel rod is lacking in the portion above the upper end of the short fuel rod.
The thermal neutron flux increases due to the decrease in thermal neutron absorption during power operation, and the linear output of adjacent fuel rods increases. By the way, since the magnitude of the linear power increase is larger as it is closer to the central part of the core, if the upper end position of the short fuel rod is in the upper part, that is, in the position away from the central part of the core, as in case A, The linear output increase due to this short type fuel rod is not so large and becomes a value in the range sufficiently smaller than the limit value. Rather, combustion is promoted by increasing the linear output in this portion, which is preferable in terms of fuel economy. On the other hand, in Cases B and C, since the line power increases due to the lack of fuel rods at a position near the central portion of the core where the line power is originally high, the line power increase in this portion is more remarkable than that in Case A.

5 (a) and 5 (b) are a fuel rod arrangement diagram showing a more detailed example of case A, and a uranium enrichment of each fuel rod and a schematic diagram of an axial cross section. As shown in the figure, 24 short fuel rods are placed at positions other than the outermost position along the water gap.
42 are arranged, and the long type fuel rod 43 is composed of three types of uranium enrichment having numbers 1, 2, and 3 in circles and gadolinia input fuel rods having G. Uranium enrichment is e ₁ , e
₂ and e ₃ (e ₁ > e ₂ > e ₃ ), and g is the gadolinia concentration. There are 12 fuel rods with gadolinia.
The fuel assembly average enrichment is about 4%.

In this embodiment, by using the short type fuel rods, the conventional type using only the long type fuel rods (No. 12
While reducing the required amount of natural uranium by about 1% from the figure) and making the core reactivity at the end of cycle power operation the same,
Moreover, the effect is that the furnace shutdown margin at low temperatures is improved by about 0.3% Δk. Further, as shown in FIG. 6, the power distribution in the axial direction of the core at the end of the cycle (in FIG. 6, the solid line indicates the example of the present invention, the broken line indicates the example of the prior art) is almost the same as the conventional example, and as can be seen from this example. In the present invention, the output distribution tends to be rather flattened.

FIGS. 7 (a) and 7 (b) are a fuel rod arrangement diagram showing another embodiment of the present invention, and a schematic diagram of uranium enrichment of each fuel rod and an axial cross section. As shown in the figure, in this embodiment, a short fuel rod 42 having a gas plenum at its lower end is used. This is especially aimed at reducing the pressure of each fissionable gas in a short fuel rod.

[Effects of the Invention] As described above, the fuel assembly of the present invention uses the short length fuel rod of a predetermined length to improve the fuel economy and improve the thermal-hydraulic characteristics. Can be minimized, and the linear output increase near the upper end of the short fuel rod can be reduced.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view schematically showing the positional relationship in the height direction of fuel rods having different lengths according to an embodiment of the present invention, FIG. 2 (a).
FIGS. 3 (b) and 3 (b) are schematic views showing the arrangement of fuel rods in a fuel assembly according to an embodiment of the present invention and axial cross sections of the long fuel rods and the short fuel rods, and FIG. The pressure loss values of the three fuel assemblies shown in FIGS. 2, 8 and 9 are compared, and FIG. 4 is a diagram comparing the uranium weights in the three fuel assemblies. (A) and (b) are more detailed fuel rod arrangement diagrams of the fuel assembly shown in FIG. 2 and diagrams schematically showing the uranium enrichment and axial cross section of each fuel rod, and FIG. 6 is Core-axis axial power distribution diagram of the fuel assembly shown in FIG. 5 and the conventional 8 × 8 fuel assembly during end-cycle power operation, FIGS. 7 (a) and (b)
FIG. 8 (a) and FIG. 9 (a) and FIG. 9 (b) are views schematically showing a fuel rod arrangement view of a fuel assembly which is another embodiment of the present invention and uranium enrichment and axial cross section of each fuel rod thereof. a) is the second
A fuel rod arrangement diagram of a fuel assembly (comparative example) having the same pressure loss value as that of the fuel assembly shown in FIGS. 8 (b) and 9 (b) is shown in FIGS. 8 (a) and 9 (). a) A diagram schematically showing an axial cross section of a fuel rod of the fuel assembly, FIG. 10 is a side view showing a partially broken view of a conventional fuel assembly used in a boiling water reactor, 11 is a side sectional view of the fuel rods constituting the fuel assembly of FIG. 10, and FIGS. 12 (a) and 12 (b) are
FIG. 10 is a cross-sectional view taken along the line I-I of FIG. 10 and a diagram showing the enrichment / gadolinia distribution in the fuel assembly. 26 …… Control rod 31,32,33 …… Fuel rod 34 …… Gas plenum 35 …… Fuel pellet 41 …… Water rod 42 …… Short fuel rod 43 …… Long fuel rod

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-60-125589 (JP, A) JP-A-61-281993 (JP, A) JP-A-63-127190 (JP, A) JP-A-63- 235889 (JP, A) JP 63-127190 (JP, A) JP 62-276493 (JP, A)

Claims (5)

[Claims] 1. A fuel assembly for a nuclear reactor, comprising a plurality of spacers bundled with a large number of fuel rods each comprising a long fuel rod and a short fuel rod whose axial length is shorter than that of the long fuel rod. A fuel assembly for a nuclear reactor, wherein the upper end of the short type fuel rod is located at the uppermost spacer position. 2. The fuel assembly for a nuclear reactor according to claim 1, wherein the active fuel length of the short fuel rod is 5/6 or more of the active fuel length of the long fuel rod. 3. A short fuel rod is arranged at a position excluding an outer peripheral portion of a fuel rod array of a fuel assembly.
A fuel assembly for a nuclear reactor according to the item. 4. The fuel assembly for a nuclear reactor according to claim 1, wherein the short fuel rod has a gas plenum portion arranged only at the upper end portion or at both upper and lower end portions. 5. The nuclear reactor according to claim 1, wherein natural uranium pellets are encapsulated in the upper ends of the long fuel rods, and natural uranium pellets are not encapsulated in the upper ends of the short fuel rods. Fuel assembly.