JPH0627274A - Nozzle under nuclear fuel assembly in pressurized water reactor - Google Patents

Abstract

PURPOSE:To realize a structure capable of absorbing irradiation growth by decreasing length without damaging the strength of a lower nozzle and the flowing-through state of cooling water, to improve the inflow state of cooling water in the lower nozzle, and to take a proper measure so as not to let a free solid arrive in the position of a fuel rod by obstructing it by the lower nozzle to catch even if it occurs within a core by any chance. CONSTITUTION:The under surface of a supporting base B of a nozzle under a nuclear fuel assembly of a PWR is formed in an arch form convex toward the center. A flow-through port in which a trap structure G is provided around the base B, is disposed. The structure G comprises large caliber drill holes shifting the central axes respectively to connect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a pressurized water reactor (PW).
PW which is one member of the PWR nuclear fuel assembly provided for R)
It relates to the lower nozzle of the R nuclear fuel assembly.

[0002]

2. Description of the Related Art The core of a pressurized water reactor (PWR) is
Several hundreds of PWR nuclear fuel assemblies are arranged in a lattice, and are fixed to the space between the upper and lower support plates of the core while being spring-biased downward. Then, during the core operation, the cooling water pressurized and maintained in the non-boiling state flows through the PWR nuclear fuel assembly and flows from the lower part of the core to the upper part of the core, and is arranged in the flow-through direction in the PWR nuclear fuel assembly. The cooling water rises at high speed along the long and narrow fuel rods that extend for 4 m, and heat is removed by exchanging heat with the fuel rods. Further, when the PWR nuclear fuel assemblies are regularly replaced, for example, one-third of the whole PWR nuclear fuel assemblies are taken out from the core, the remaining 2/3 of the PWR nuclear fuel assemblies are rearranged, and the empty space is replaced. 1/3 taken out
Will replace the new PWR nuclear fuel assembly.

In a PWR nuclear fuel assembly, an upper nozzle and a lower nozzle are connected by a plurality of thimble tubes, and a plurality of stages of support grids are installed at intermediate positions of the thimble tubes to form a grid-like skeleton structure. A large number of fuel rods are held in an aligned state in a lattice-like skeletal structure, which constitutes a fuel rod exchange unit and forms a heat exchange channel by pressurized water when assembled in the core. .

The lower nozzle of the conventional PWR nuclear fuel assembly is supported by a supporting substrate which has a flat surface on the side facing the lower end portion of the fuel rod in close contact or with a space and having a uniform thickness from the central portion to the outer peripheral portion. And a leg portion which is disposed at four corners of the lower surface of the substrate and supports the supporting substrate, and on the surface of the supporting substrate, thimble pipe connecting holes are arranged so as to be adapted to each of the plurality of thimble pipes. By filling the gap of No. 1, a large number of cooling water flow-through holes having a predetermined one or two kinds of diameters are regularly arranged. Each cooling water through hole straightly penetrates the support substrate with a uniform diameter.

[0005]

[Problems to be Solved by the Invention] Pressurized water reactor (PW)
In order to improve the facility utilization rate of R) and reduce the cost of nuclear fuel, the enrichment of nuclear fuel is increased to extend the life of fuel rods, and the exchange life of PWR nuclear fuel assemblies is extended as compared to the conventional one, and the exchange life period is extended. It is desired to increase the burnup of the nuclear fuel inside. However, if the exchange life of the PWR nuclear fuel assembly is extended, the total amount of radiation exposure increases, and the irradiation growth of the skeletal structure that holds the fuel rod cannot be sufficiently absorbed by the conventional core structure. Conventional P growth
The structure of the WR nuclear fuel assembly cannot be sufficiently absorbed. Therefore, on the premise of the conventional core structure and fuel rod length, the overall length of the PWR nuclear fuel assembly is slightly shortened to the extent that the irradiation growth until the end of life of the PWR nuclear fuel assembly can be absorbed, and the end of life of the fuel rod is shortened. It is necessary to slightly increase the distance between the upper nozzle and the lower nozzle to absorb the irradiation growth up to. Here, there is no room to reduce the length on the upper nozzle side, and there is no room to reduce the length of the legs of the lower nozzle from the mounting structure with the support plate on the core side. However, there has been a demand for reducing the thickness of the cooling water without impairing its strength and the flow-through state of the cooling water.

Further, during the core operation, the cooling water flowing from the lower part of the core through the lower nozzle rises along the fuel rods at a high speed and exchanges heat, but maintenance work of the cooling water circulation system and the core structure is performed. If free solids are generated in the core due to wires or chips being left unattended during the operation, the free solids are caught in the flow of cooling water and rise to the position of the fuel rod, which slows down the core and When stopped, it moves up and down repeatedly, and there is a possibility of repeatedly colliding with and rubbing against the fuel rod for a long period of time and damaging the fuel rod. Therefore, it is necessary to provide a trap structure in the lower nozzle so that even if such loose solids are generated in the core, they are blocked by the lower nozzle and do not reach the position of the fuel rod. In the lower nozzle of the conventional PWR nuclear fuel assembly, for example, the cooling water through hole is reduced in diameter. However, in this method, the resistance of the lower nozzle is increased and the gap between the adjacent PWR nuclear fuel assemblies is increased. There is a high possibility that the proportion of cooling water flowing through the fuel cell will increase and free solids will rise through this gap and reach the position of the fuel rod.

The present invention reduces the thickness of the support substrate of the lower nozzle without impairing the strength and the flow-through state of the cooling water, and, in the constraint of the reduced thickness, free solid matter is deposited on the support substrate. An object of the present invention is to provide a lower nozzle for a PWR nuclear fuel assembly, which is provided with a trap structure, and which further reduces the resistance of the lower nozzle and keeps the proportion of cooling water flowing through a gap between adjacent PWR nuclear fuel assemblies to be low.

[0008]

A lower nozzle of a PWR nuclear fuel assembly according to claim 1, wherein a plurality of cooling water through holes are formed at intervals between a plurality of thimble pipe connecting holes arranged on a flat surface thereof. And a lower nozzle of the PWR nuclear fuel assembly having four legs arranged at four corners of the lower surface of the supporting substrate to support the supporting substrate, the lower surface of the supporting substrate is formed in a shape higher toward the center, The thickness of the support substrate is thin in the central portion and thick in the peripheral portion of the central portion.

The lower nozzle of the PWR nuclear fuel assembly according to claim 2 is the same as the lower nozzle of the PWR nuclear fuel assembly according to claim 1, wherein the cooling water through holes have a plurality of different diameters, and a large diameter of the supporting substrate is used. It is arranged in a peripheral portion, and a small diameter one is arranged in an inner portion of the supporting substrate.

The lower nozzle of the PWR nuclear fuel assembly according to claim 3 is the lower nozzle of the PWR nuclear fuel assembly according to any one of claims 1 and 2, wherein the cooling water through-holes arranged in the peripheral portion of the supporting substrate are A trap structure is provided in which the opening width of the through-hole is reduced midway.

A lower nozzle of the PWR nuclear fuel assembly according to a fourth aspect is the lower nozzle of the PWR nuclear fuel assembly according to the third aspect, wherein the trap structure has a drill hole formed from the lower surface side of the supporting substrate in the supporting substrate. In the form of connecting to a plurality of drill holes formed from the front surface side, a drill hole formed from the lower surface side of the support substrate and a drill hole formed from the front surface side of the support substrate at an intermediate position of the thickness of the support substrate. It is formed by shifting the central axes of the two and connecting them.

[0012]

In the lower nozzle of the PWR nuclear fuel assembly according to claim 1, the load transmitted to the supporting substrate through the thimble tube is 4
The lower nozzle has a structure in which the load is distributed to the individual legs and transmitted to the core side, and the lower nozzle supports the load distributed on the support substrate by the four legs. Therefore, a larger shear load and bending moment act on the supporting substrate as it approaches the four corners. However, since the lower surface of the support substrate is formed in a high arch shape toward the center and the thickness of the support substrate is made thicker toward the four corners, the stress is reduced and the load is smoothly transmitted to the legs. Is. Therefore, the supporting substrate can be thinned while maintaining the rigidity of the supporting substrate, the flow-through resistance of the cooling water becomes low in the central portion where the thickness is remarkably reduced, and the ratio of the cooling water flowing through the space between the adjacent PWR nuclear fuel assemblies is reduced. And the proportion of cooling water flowing through the central part of the lower nozzle increases.

In the lower nozzle of the PWR nuclear fuel assembly according to claim 2, since the small-diameter cooling water through-holes arranged in the inner portion of the supporting substrate have a short length, the through-flow resistance of the cooling water is small, which results from the formation of the openings. The ratio of strength loss of the supporting substrate is small. On the other hand, the large-diameter cooling water through-holes arranged in the peripheral portion of the supporting substrate have a small through-flow resistance due to the large diameter, and the strength loss ratio of the supporting substrate is lower than that of the small-diameter cooling water through-holes. However, there is no problem because the peripheral portion of the supporting substrate has a large thickness.

In the lower nozzle of the PWR nuclear fuel assembly according to claim 3, of the solid solid matter of the core which is entrained in the cooling water and flows into the PWR nuclear fuel assembly, the opening which is smaller than the diameter of the through hole and is reduced in the middle Those larger than the width are trapped in the trap structure and cannot reach the fuel rod side. In the central part of the support substrate where the thickness is remarkably reduced, the resistance of the through hole of the cooling water is sufficiently low even with the through hole of the conventional small diameter, and the free solids are distributed along the arched ceiling of the support substrate. , And maintains a stable supplemental state at the center even if the flow velocity of the cooling water changes slightly. On the other hand, in the peripheral portion, it is easy to provide an effective trap structure because the thickness in the flow-through direction can be secured, and since resistance increases with the conventional small-diameter through-holes, the through-holes can be made larger and inside the through-holes. The free solids were able to be trapped so that the free solids did not flow out into the space between the adjacent PWR nuclear fuel assemblies even if the flow rate of the cooling water was slightly changed.

In the lower nozzle of the PWR nuclear fuel assembly according to claim 4, the trap structure is composed of large-diameter perforated holes which are connected by shifting the central axes of the supporting substrates at an intermediate position of the thickness of the supporting substrate, and the lower surface side of the supporting substrate. The cooling water that has flowed in through the support substrate flows through the support substrate in such a manner that it is dispersed in a plurality of drill holes formed from the surface side of the support substrate.

[0016]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic view of the lower nozzle of the PWR nuclear fuel assembly of the embodiment, and FIG. 2 is an explanatory view of the lower nozzle of FIG. Here, the lower surface of the support substrate is formed in a high arch shape toward the center, a through-flow port provided with a trap structure is arranged in the peripheral portion of the support substrate, and the trap structure has a large diameter connected by shifting the central axis. It is composed of drill holes of
(a) is a 1/4 plan view of the lower nozzle, (b) is X- of (a)
It is an X sectional view.

In FIGS. 1 and 2, the lower nozzle of the PWR nuclear fuel assembly is a 17 × 17 type PW having a side of about 220 mm.
It is applied to the R nuclear fuel assembly, and the four corners of the support substrate B are supported by the feet A, respectively, and the load transmitted to the support substrate B through the thimble tube attached to the support substrate B is 4
It is distributed to the legs A of the book and transmitted to the support plate on the core side. The lower surface of the support substrate B is formed in a shape of a high arch toward the center, and the arch is composed of a spherical surface having a radius of 220 mm, and the thickness of the central portion is 3/4 of the conventional thickness. At this time, the thickness of the outermost peripheral portion of the support substrate B was about three times that of the central portion, and the entire length of the lower nozzle including the foot A could be shortened by 5 mm as compared with the conventional one.

The upper surface of the supporting substrate B facing the lower end of the fuel rod is flat, and a total of 21 thimble pipe connecting holes are regularly arranged in the supporting substrate B, and the intervals of the thimble pipe connecting holes are filled. A large number of water flow holes E1, E2, E3 are regularly arranged. Of the water flow holes E1, E2, E3, a water flow port E3 having a diameter of 3 mm is provided at the center of the support substrate B, and a diameter of 10 mm is provided at the outer edge.
The water outlet E1 and the water outlet E2 having a diameter of 5 mm are arranged at the intermediate position, and the gap between the water outlet E1 and the water outlet E2 is filled with the water outlet E3 having a diameter of 3 mm to make the ratio of the opening area the same as the conventional one. ing. A trap structure G shown in FIG. 2 (b) is provided at the water outlet E1 at the outer edge.

The trap structure G is a type in which a perforation hole formed from the lower surface side of the support substrate B is connected to a plurality of perforation holes formed from the front surface side of the support substrate B, and is formed from the lower surface side of the support substrate B. The center hole of the bore hole and the bore hole of the large diameter formed from the front surface side of the support substrate B are displaced from each other at the middle position of the thickness of the support substrate B so that ¼ of the bore overlaps in the radial direction. It is formed by connecting.

On the supporting substrate B of the lower nozzle of the PWR nuclear fuel assembly constructed as described above, due to the weight from the thimble tube, a large shear load and bending moment act toward the four corners. However, since the lower surface of the supporting substrate B is formed in a high arch shape toward the center and the thickness of the supporting substrate B is made thicker toward the four corners, the result of strength calculation shows that the compressive strength of the lower nozzle is It was confirmed that it was equivalent to the conventional one even though the length was shortened. As a result of water flow resistance calculation, it was confirmed that the pressure loss of the lower nozzle was equivalent to that of the conventional type.

In addition, among the free solids of the core that are entrained in the cooling water and flow into the PWR nuclear fuel assembly during the core operation, the trap structure G of the large-diameter flowing water port E1 is smaller than the medium-diameter flowing water port E2. Which is larger than the opening width of No. 2 is captured by the inside of the large-diameter water flow port E1 and the arch-shaped ceiling of the supporting substrate B and cannot flow through the lower nozzle. Even if it changes a little, the stable trapped state is maintained at the center of the arched ceiling of the trap structure G and the support substrate B, and the trap structure G and the support substrate B do not flow into at least the space between the adjacent PWR nuclear fuel assemblies.

In this embodiment, the supporting board B and the four legs A are integrally formed including the arch-shaped ceiling portion. However, a structure in which a plurality of parts are screwed and assembled together, for example, the supporting board is used. B is separated at the height of the trap structure G, and is composed of a member composed of four legs A and a part of an arch-shaped ceiling, and a plate member having a uniform thickness screwed onto the member. You may comprise. Further, the arch-shaped ceiling portion does not have to be a continuous smooth curved surface, and may include a combination of several planes or a step.

[0024]

According to the lower nozzle of the PWR nuclear fuel assembly of the first aspect, it is possible to reduce the thickness of the supporting substrate of the lower nozzle without impairing the strength and the flowing state of the cooling water. Of the cooling water flowing through the gap between the adjacent PWR nuclear fuel assemblies can be suppressed to be low. Therefore, the flow state of the cooling water flowing through the PWR nuclear fuel assembly is improved, and even if free solid matter is generated in the core, the free solid matter in the core is bound under the support substrate and reaches the position of the fuel rod. Hard to do.

According to the lower nozzle of the PWR nuclear fuel assembly of claim 2, the strength of the supporting substrate is not significantly impaired,
The flow resistance of the cooling water in the support substrate can be reduced.

According to the lower nozzle of the PWR nuclear fuel assembly of the third aspect, an effective trap structure for free solids can be provided on the supporting substrate within the reduced thickness constraint of the lower nozzle, and the free core is removed. The solid matter is restrained by the trap structure, making it difficult to reach the position of the fuel rod.

According to the lower nozzle of the PWR nuclear fuel assembly of claim 4, the trap structure can be easily formed in the lower nozzle, and even if the trap structure is provided, the flow passage area of the through-hole of the cooling water by the trap structure is reduced. The lower nozzle does not have a large resistance (pressure loss).

[Brief description of drawings]

FIG. 1 is a schematic view of a lower nozzle of a PWR nuclear fuel assembly of an example.

FIG. 2 is an explanatory diagram of a lower nozzle of FIG.

[Explanation of symbols]

 A foot B support substrate D thimble tube connection hole G trap structure E1 water flow hole E2 water flow hole E3 water flow hole

Claims (4)

[Claims] 1. A support substrate having a large number of cooling water through holes formed at intervals between a large number of thimble pipe connection holes arranged on the flat surface, and the support substrate arranged at four corners of the lower surface of the support substrate. In a lower nozzle of a PWR nuclear fuel assembly having a supporting leg, a lower surface of the support substrate is formed in a shape higher toward a center, and the thickness of the support substrate is thin at a central portion, and a peripheral portion of the central portion is formed. P characterized by thickening
Lower nozzle of WR nuclear fuel assembly. 2. The lower nozzle of the PWR nuclear fuel assembly according to claim 1, wherein the cooling water flow-through holes have a plurality of diameters, and those having a large diameter are arranged in the peripheral portion of the support substrate, and those having a small diameter are arranged. A lower nozzle of a PWR nuclear fuel assembly, wherein the lower nozzle is disposed on an inner portion of the supporting substrate. 3. The lower nozzle of the PWR nuclear fuel assembly according to claim 1, wherein the opening width of the through hole is reduced in the middle of the through hole of the cooling water arranged in the peripheral portion of the support substrate. A lower nozzle of a PWR nuclear fuel assembly, which is provided with a trap structure comprising 4. The lower nozzle of the PWR nuclear fuel assembly according to claim 3, wherein the trap structure has a plurality of drill holes formed from a lower surface side of the support substrate and a plurality of drill holes formed from a front surface side of the support substrate. In a form of connection, a perforation hole formed from the lower surface side of the support substrate and a perforation hole formed from the front surface side of the support substrate are connected by shifting their central axes at an intermediate position of the thickness of the support substrate. A lower nozzle of a PWR nuclear fuel assembly characterized in that: