JPH0636045B2 - Fuel assembly - Google Patents

Description

TECHNICAL FIELD The present invention relates to a boiling water nuclear reactor, and more particularly to a fuel assembly of a boiling water nuclear reactor suitable for extending the life of fuel, saving uranium, and reducing pressure loss. .

[Prior Art] In a boiling water reactor, water acting as a moderator and a coolant is divided into two regions, a two-phase flow portion inside the channel box and a saturated water portion outside the channel box. To do. In the current limited space, the optimum values for the area ratios of these two regions differ depending on the evaluation target.

In boiling water reactors, in order to improve the burnup of fuel, extend the life of fuel, and improve fuel economy by taking measures such as uranium saving, in the fuel assembly, a non-boiling region, that is, a large number of water rods is used. A method of arranging the fuel cells, a method of increasing the uranium loading amount in the fuel assembly as compared with the conventional method, and a method of increasing the number of lattice constituents can be considered.

However, when a large number of water rods are arranged in the fuel assembly in the size of a conventional channel box,
There is a problem that the pressure loss greatly increases due to the increase of the wetted area and the decrease of the coolant passage area, and the cooling capacity decreases due to the proximity of the fuel rod intervals. Further, if the amount of uranium loaded is increased by the size of the conventional channel box, the pressure loss due to the coolant and the cooling capacity will be decreased.

Furthermore, when the number of grids is increased with the size of the conventional channel box, an increase in pressure loss and a decrease in cooling capacity are unavoidable.

In other words, in order to increase the loading amount of uranium, increase the non-boiling region in the channel box, increase the cooling capacity, and make the coolant flow passage area as wide as possible, the current size of the channel box is Not enough.

As a countermeasure against this, Japanese Patent Laid-Open No. 61-118689 discloses a technique in which the area of the saturated water region surrounding the fuel assembly is reduced, the inner area of the channel box is widened by the reduced amount, and the fuel lattice arrangement is increased. Is listed.

[Problems to be Solved by the Invention] In the above conventional technique, the pitch of the current fuel assembly lattice is about 15
It is trying to improve fuel economy without changing 2 mm, but the burnup is increased from the current 30 GWd / t to 7
In order to increase to 0 Gwd / t, it is necessary to further increase the water rod area and make the neutron spectrum even softer (to slow the average velocity of thermal neutrons). If the water rod area is increased with the conventional fuel assembly lattice pitch unchanged, the amount of uranium loaded per fuel assembly decreases, and there is a problem that improvement in fuel economy cannot be expected so much. It was

An object of the present invention is to provide a fuel assembly capable of enhancing the neutron moderating effect without reducing the uranium loading amount per fuel assembly and achieving a high fuel degree.

[Means for solving problems]

The above-mentioned object is a fuel assembly including a plurality of fuel rods, a plurality of pipes through which cooling water flows, and a channel box surrounding the pipes and the fuel rods, wherein the cross-sectional area of the pipes is at least two. The sum of the cross-sectional areas of
The number of fuel rods is 16 or more and 16 or less, and 55% or more and 75% or less of the fuel rods can be achieved by adjoining the pipes.

For example, the moderator in the fuel assembly is increased so that the number of water rods in the fuel assembly is increased after doubling the size of the fuel assembly so that substantially the same amount of non-boiling water is arranged in each fuel rod. This can be achieved by homogenizing (water) and fuel (uranium).

With regard to enlarging the fuel assembly, the size of the fuel assembly should be within the diagonal length of the control rod pitch, which is the standard specification of the current boiling water reactor, so that the fuel rod can be used without changing the control rod pitch. The aggregate can be about twice as large.

Specifically, the control rod pitch of the existing LWR core is about 30.
Since it is 5 mm to 310 mm, the diagonal length is about 215 m
It becomes m-219 mm. With this size, the clearance required for inserting the control rod is about 13 mm and the wall thickness of the channel box is about 2 mm.
Considering 3mm, the inner width of the channel box is about 1
It becomes 96 mm to 202 mm.

Further, preferably, the pipe through which the cooling water flows has an opening at the cooling water inflow portion, and the diameter of the opening is smaller than the inner diameter of the cooling water inflow portion.

Further, it is preferable that the pipe through which the cooling water flows has an upper end portion at a position lower than the upper end portion of the fuel rod by at least 10 cm or an upper portion from this position is thinner than its lower portion.

[Action]

According to the present invention, the cross-sectional area of the pipe through which the cooling water flows is equal to or larger than the sum of the cross-sectional areas of at least two fuel rods, and the pipe is
More than 16 and less than 16 installed, 55% of fuel rods
By constructing a structure in which not less than 75% is adjacent to the pipe, the water rod region, which is the non-boiling region during operation, is increased without reducing the uranium loading amount per fuel assembly, and the amount is approximately equal to each fuel rod. Since the non-boiling water can be homogenized so as to be placed therein, the neutron moderating effect can be enhanced. In addition, since unnecessary absorption of neutrons by hydrogen atoms is eliminated, it is possible to improve reactivity and achieve high burnup.

For example, in the fuel assembly having about twice the size as described above, the water region portion of the gap of the fuel assembly which occurs when the conventional fuel assembly is arranged in the core can be a portion within the fuel assembly. This portion can be utilized for increasing the amount of uranium loaded per fuel assembly or increasing the water rod area.

Further, it is preferable that the pipe through which the cooling water flows has an opening at the cooling water inflow portion, and the diameter of the opening is smaller than the inner diameter of the cooling water inflow portion, so that it is generated outside the water rod. Since the voids can be prevented from entering the water rod, the neutron moderating effect can be achieved more reliably.

Further, it is preferable that the pipe through which the cooling water flows has an upper end portion at a position at least 10 cm or more below the upper end portion of the fuel rod, or the upper portion from this position is thinner than the lower portion. It is possible to reduce the pressure loss due to the increase in the uranium loading amount and the increase in the water rod area.

This action is due to the following reasons.

To improve the hydraulic stability of the fuel channel,
It is important to reduce the pressure loss in the fuel channel. Friction pressure loss ΔPf due to gas-liquid two-phase flow such as in a boiling water reactor is
It is expressed by the following formula.

Here, ΔPf = friction pressure loss W = channel flow rate g = gravitational acceleration ρ = water density D = channel hydraulic diameter A = channel flow path area L = length f = friction pressure loss coefficient Φ = two-phase flow friction pressure loss multiplication factor fuel In the region above the effective length, there is almost no effect of fuel combustion in the core, and the void ratio in this region is large. In the above equation of friction pressure loss, the two-phase flow friction pressure loss multiplication factor Φ increases as the void ratio increases.
Therefore, if the large diameter water rod above the active fuel length is made thinner, the channel flow passage area A becomes larger as the water rod cross-sectional area becomes smaller, so the pressure loss becomes smaller, and if the water rod is made shorter. , L becomes smaller, and the pressure loss becomes smaller, and the effect thereof is large in combination with Φ being large at the upper portion, and thus contributes greatly to the pressure loss reduction.

[Embodiment] An embodiment of the present invention will be described below with reference to FIG. First
The figure shows a cross section of a fuel assembly. In FIG. 1, 1 is
Channel box, 2 is water rod, 3 and 3 '
Indicates a fuel rod.

Inner width 201 of the channel box of the fuel assembly of this embodiment
mm is the conventional fuel with an inner width of 134 mm And the fuel assembly of the present invention is about double.

In this embodiment, the fuel rods 3 are arranged in 14 rows and 14 columns. When the inner width of the channel box is 201 mm, in order to make the uranium loading amount in the fuel assembly according to the present invention equal to the uranium loading amount in the conventional fuel assembly per unit volume, the fuel rod diameter is about 10. It must be 6 mm. Further, in order to prevent the cooling characteristics of the fuel rods from deteriorating, when the fuel rod gap is set to about 3.5 mm which is equivalent to that of the conventional fuel, the arrangement of the fuel rods is determined to be 14 rows and 14 columns.

From FIG. 2 showing the relationship between the number of water rods and the amount of natural uranium saved, the number of water rods is about 9 in the 14-row, 14-column arrangement of this embodiment.

The water rods 2 are arranged so as to face all the fuel rods 3 ′ except the fuel rods 3 which face the channel box 1. Therefore, all fuel rods face either the water rods or the saturated water region outside the channel box, which has the effect of more homogenizing the moderator and fuel distribution within the fuel assembly.

Further, the inner diameter of the water rod 2 is about 29 mm, which is about 3 times the inner diameter of the fuel rod, about 9.3 mm, and the cross-sectional area is about 9 times. The water rod 2 is arranged by removing four fuel rods 3 '.

In this embodiment, the total number of fuel rods 3, 3'is 160, and the number of fuel rods 3'facing the water rod 2 is 10.
Since there are eight rods, the proportion of the fuel rods 3'facing the water rod 2 is 67.5%.

FIG. 3 shows the combustion change with infinite multiplication factor.

In FIG. 3, reference numeral 5 denotes the combustion change of the infinite multiplication factor of the present invention,
6 shows the combustion change of an infinite multiplication factor in the conventional fuel assembly lattice. From the infinite multiplication factor of the present invention, it can be seen that there is a large reactivity gain over the conventional lattice.

FIG. 4 shows a vertical cross section of the fuel assembly in this embodiment.

In FIG. 4, 1 is a channel box, 2 is a water rod, 3 is a fuel rod, and 4 is a spacer.

The water rod 2 of this embodiment has a thin lower portion from the viewpoint of strength. This is based on the fact that the smaller the diameter, the smaller the amount of deformation and the increase in mechanical strength with respect to the horizontal load applied to the water rod. This characteristic is especially remarkable for the lower end plug of the water rod and the first spacer. The diameter of this section is made smaller because it is important for the section to be held at. Further, by providing an opening in the cooling water inflow portion under the water rod 2 and making the diameter of the opening smaller than the inner diameter of the cooling water inflow portion, the voids generated on the outside of the water rod during the output operation can be prevented. It prevents you from getting inside. Therefore, the moderating effect of neutrons can be achieved more reliably.

The upper part of the water rod 2 has a shorter tube length than the fuel rod 3 in order to reduce pressure loss due to two-phase flow and increase stability. The length that can be shortened is about 10 cm up to just above the uppermost spacer.

Incidentally, FIGS. 4 and 5 of JP-A-47-649 show a structure of a fuel assembly similar to the present invention, which uses a control rod guide pin having a large diameter. And
Since it is premised on the insertion of the control rod, it has a different function from the water rod described in the present invention, and in the case of the control rod guide tube, unlike the water rod of the present invention, the diameter is made smaller at the lower portion of the fuel. Since the structure is not possible, the invention described in JP-A-47-649 can be said to be an invention different from the present invention.

Further, a second embodiment of the fuel assembly according to the present invention is shown in FIG. This embodiment has an array of 15 rows and 15 columns. This example provides a shape that further improves the neutron moderating effect in the central portion of the fuel assembly, where the neutron spectrum is generally harder. In this embodiment, the proportion of the fuel rods 3'facing the water rod 2 is 68.2.
%.

Further, FIGS. 6 and 7 show third and fourth modified examples. FIG. 6 shows a fuel assembly having an array of 12 rows and 12 columns, which has a short wetting length and therefore has a margin for pressure loss, and is a suitable arrangement for increasing the effective length. Further, FIG. 7 shows a fuel assembly having an array of 18 rows and 18 columns, which is an arrangement suitable for shortening the effective length because the uranium loading can be increased. In the embodiment of FIG. 6, the proportion of the fuel rods 3'facing the water rod 2 is 59.3%,
In the embodiment shown in FIG. 7, this ratio is 73.8%.

〔The invention's effect〕

According to the present invention, it is possible to increase the non-boiling region without reducing the uranium loading amount per fuel assembly, and to homogenize so that substantially the same amount of non-boiling water is arranged in each fuel rod, so the neutron moderating effect It is possible to increase the burnup and increase the burnup.

Further, by shortening or thinning the upper part of the water rod, it is possible to reduce the pressure loss due to the increase of the uranium loading amount and the increase of the water rod area.

[Brief description of drawings]

FIG. 1 is a horizontal sectional view of a fuel assembly according to the present invention, FIG. 2 is a view showing a relationship between the number of water rods and a natural uranium saving amount, FIG. 3 is a view showing a combustion change of infinite multiplication factor, and FIG. Is a longitudinal sectional view of a fuel assembly according to the present invention, FIG. 5, FIG.
FIG. 7 shows a horizontal sectional view of a fuel assembly according to a modification of the present invention. 1 ... Channel box, 2 ... Water rod,
3, 3 '... Fuel rod, 4 ... Spacer, 5 ... Infinite multiplication factor combustion change according to the present invention, 6 ... Infinite multiplication factor combustion change in conventional lattice

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Yamashita 3-1-1, Sachimachi, Hitachi City, Ibaraki Prefecture Hitachi Ltd. Hitachi Factory (56) Reference JP-A-55-13879 (JP, A) JP-A-61-159185 (JP, A)

Claims (6)

[Claims] 1. A fuel assembly comprising a plurality of fuel rods, a plurality of tubes through which cooling water flows, and a channel box surrounding the tubes and the fuel rods, wherein the cross-sectional area of the tubes is at least two. A fuel assembly in which the number of the tubes is 4 or more and 16 or less, and 55% or more and 75% or less of the fuel rods are adjacent to the tubes at a sum of cross-sectional areas of the rods or more. 2. The fuel assembly according to claim 1, wherein the pipe has an opening in a cooling water inflow portion, and the diameter of the opening is smaller than the inner diameter of the cooling water inflow portion. Is a fuel assembly. 3. The fuel assembly according to claim 1 or 2, wherein the tube is at least 10 cm from the upper end of the fuel rod.
A fuel assembly characterized in that it has an upper end portion at a position lower than the above, or an upper portion is thinner than the lower portion from the position. 4. The fuel assembly according to claim 1 or 2, wherein the lower portion of the pipe is thinner than the upper portion thereof. 5. The fuel assembly according to claim 1 or 2, wherein the channel box has a substantially rectangular cross section and an inner width of 196 mm or more and 202 mm or less. Fuel assembly. 6. The fuel assembly according to claim 1 or 2, wherein the fuel rods are arranged in 14 rows and 14 columns.