JPH09304580A - Method for taking out core fuel and its simulated fuel assembly - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To secure the subcriticality of the core at the time of periodic inspection and reduce the number of fuel assemblies handled. SOLUTION: In a method of taking out a spent fuel assembly from a boiling water nuclear reactor core 1 in which a plurality of unit cells (cells) each having four fuel assemblies 4 arranged in a lattice pattern are used. After pulling out one of the integrated fuel assemblies 4, a simulated fuel assembly 8 including a neutron absorber is inserted into the pulled out cavity. By doing this for all unit cells of the core, subcriticality can be secured even if all control rods are pulled out. In addition, it is possible to reduce the handling quantity of the fuel assembly during the periodic inspection, and to shorten the handling time.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a periodical inspection of a boiling water reactor core in a nuclear power plant (hereinafter referred to as a regular inspection).
The present invention relates to a method for taking out core fuel at a time, and a simulated fuel assembly for use in this taking out method.

[0002]

2. Description of the Related Art A periodical inspection of a nuclear power plant basically replaces spent fuel with advanced combustion with new fuel, and various equipment is inspected during the period. Basically, the period of periodic inspection is controlled by the length of time for core work including refueling.

The core consists of fuel, control rods and neutron instrumentation. There is a possibility that a criticality accident may occur if the fuel is loaded or the control rod is operated carelessly at the time of regular inspection. For example, withdrawing a plurality of control rods from a core clogged with fuel may increase the reactivity of the core and possibly exceed the criticality. A criticality accident is an event that should never occur, and various regular measures are taken to avoid it during regular inspections.

One of the most important measures against this is a checkered fuel extraction method, which is widely used. Hereinafter, a conventional checkered fuel extraction method will be described.
The state of the core at the start of regular inspection is shown in FIG. This figure 3
(A) is a plan view schematically showing the state of the core viewed from above. Reference numeral 1 indicates a core for a boiling water reactor, and an outer circle indicates a core boundary 2. In the core 1, a large number of fuel assemblies 4 are partitioned by an upper grid plate 3 into a lattice shape.
This one division is referred to as a unit cell (hereinafter referred to as a cell). FIG. 3 (b) is an enlarged view of one cell extracted.

In one cell, there are four fuel assemblies 4 and one control rod 5 having a cross-shaped cross section inserted in the center of these fuel assemblies 4, and neutrons are present at the intersections of the upper lattice plates 3. An instrumentation detector 6 is installed. The lower part of the fuel assembly 4 is supported by the fuel support fitting, and the fuel support fitting is supported by the core support plate.

This checkered pattern fuel removal method is designed so that the fuel assembly 4 can be replaced before the fuel replacement and core work (control rod replacement, control rod drive mechanism replacement, neutron instrumentation detector replacement, etc.) are performed. As shown in FIG. 4 (a), half of the pieces are extracted in a checkered pattern. FIG. 4B shows one cell extracted in this state.

There are two fuel assemblies 4 in one cell on a diagonal line and one control rod 5 located between the two fuel assemblies 4, and a neutron meter is located at the intersection of the upper lattice plates. Equipment detector 6
Is installed. Reference numeral 7 is a hollow portion after the fuel assembly 4 is taken out.

When two fuel assemblies are diagonally removed from one cell, the reactivity increase is small even if the control rod 5 of this cell is withdrawn. Even if the same state as this cell is made in the whole core, the core 1 It has been confirmed by calculation that does not exceed the criticality.
In this core state, even if a plurality of control rods 5 or control rod drive mechanisms are randomly withdrawn for replacement of the control rod drive mechanism, the criticality is never reached.

Further, since the criticality is never reached, it is not necessary for the neutron instrumentation to monitor the core. Therefore,
Inspection of the neutron instrumentation or replacement of the neutron instrumentation detector 6 can be performed at any time. Furthermore, since there is no need to insert the control rod 5 in an emergency (scrum), inspection of the hydraulic control unit (HCU) that controls the control rod drive mechanism, inspection of the reactor protection system composed of the scrum function interlock, etc. It is possible at any time while the fuel is being taken out in a checkered pattern. As described above, the conventional checkered pattern fuel extraction method is a very excellent method for ensuring core safety during regular inspections, and has been widely used.

[0010]

Although the checkerboard fuel extraction method is a very excellent method for ensuring core safety during regular inspection, about half of the fuel assemblies in the core are taken out at the initial stage of regular inspection. The disadvantage is that it takes a considerable amount of time.
In an era when the need to improve the utilization rate of the plant is emphasized from the economical point of view, the checkered fuel extraction method becomes a cause of lengthening the regular inspection process. An important issue is how to shorten the number of regular inspection days.

Recently, a cell mask method using a core monitoring device has come into the spotlight in place of the checkerboard fuel extraction method. This method uses a computer to manage the fuel arrangement in the core, information on the control rod insertion status, and to prevent the withdrawal of control rods from cells containing fuel, and to prevent the loading of fuel into cells from which control rods are missing. It is a method to eliminate mistakes that increase carelessly.

However, this method is a mechanization of the method in which the human core is used to manage the core, and the probability of a critical accident is significantly reduced as compared with the human system management. If the control is impossible, the probability of a criticality accident increases because the core is not an absolutely subcritical core (a core that does not reach criticality even if all control rods in the core are pulled out). Therefore, this method has some anxious problems.

The present invention has been made in order to solve the above-mentioned problems, and reduces the number of fuel assemblies to be handled while ensuring the safety of the core during regular inspection, and the core-related work that is a critical path of the regular inspection process. (EN) A core fuel extraction method that can shorten the total number of regular inspection days by shortening the time and improve the utilization rate of a nuclear power plant, and a simulated fuel assembly for use in the extraction method. is there.

[0014]

According to a first aspect of the present invention, a fuel assembly is taken out from a boiling water nuclear reactor core formed by arranging a plurality of unit lattices each having four fuel assemblies in a lattice form. The method is characterized in that any one integral fuel assembly in the unit cell is extracted, and then a simulated fuel assembly including a neutron absorber is inserted into the extracted cavity. As a result, the number of fuel assemblies to be handled can be reduced, and core-related work, which is a critical path in the regular inspection process, can be shortened.

According to a second aspect of the present invention, a rectangular tubular stainless steel channel box, a neutron absorber provided on at least the inner surface of the channel box, a handle provided on the upper portion of the channel box, and a lower portion of the channel box. And a lower tie plate provided in. As a result, the structure is simple and the extraction work can be facilitated.

The invention of claim 3 is characterized in that the neutron absorber is made of a cadmium thin plate. Cadmium is cheaper than hafnium and boron carbide, has a workability that is easy to process, and has a manufacturing effect that is easily lined on stainless steel.

The invention of claim 4 is characterized in that a visual identification portion is provided on at least one of the upper surface of the channel box and the upper surface of the handle. By providing this visual identification part, it is possible to easily distinguish from other fuel assemblies from the upper part of the core.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION A method for extracting core fuel according to the present embodiment will be described with reference to FIGS. 1 (a) and 1 (b). In FIG. 1, the same parts as those in FIG. 4 are designated by the same reference numerals, and overlapping description will be omitted. The present embodiment is different from the conventional method of taking out the core fuel shown in FIG. 4, and one of the four fuel assemblies 4 loaded in one cell is extracted as the core fuel. The purpose is to insert the simulated fuel body 8 including the neutron absorber into the cavity after the removal.

That is, as shown in FIG.
Only one fuel assembly 4 is taken out of the four fuel assemblies 4 loaded for each cell, and the cavity formed after taking out the fuel assembly 4 has the same outer dimensions and weight as the fuel assembly 4. A simulated fuel assembly 8 having a thermal neutron absorption capacity equivalent to that of the cross control rod 5 and having a neutron absorber arranged in a plate shape on four side surfaces of a rectangular prism is loaded, that is, inserted. As a result, the multiplication factor per cell can be made equal to the multiplication factor per cell by the conventional checkerboard fuel extraction method.

Next, the difference in effect between the present embodiment and the conventional checkerboard fuel extraction method will be described. Quantitatively evaluating the difference between the number of work steps and the time during the critical path process of the conventional neutron absorber-containing simulated fuel assembly method according to the present embodiment with the checkered pattern fuel extraction method,

(1) Number of steps in the checkered fuel extraction method 1. Extraction of one fuel assembly: core → fuel pool 2. Return of fuel exchanger fuel pool → core 3. Diagonal fuel assembly of the first fuel assembly Body removal ... core →
Fuel pool 4. Refueling machine return 4 steps in total

(2) Fuel Extraction Method of this Embodiment 1. Extracting one fuel assembly ... Reactor → fuel pool 2.1 Inserting neutron absorber-containing simulated fuel assembly ... Fuel pool → Recore 2 steps in total The simulated fuel assembly with neutron absorber is temporarily placed in the fuel pool at a time not on the critical path of the regular inspection process.

As described above, this embodiment is about half the size of the conventional example. Calculating the time difference per cell from the speed of the current automatic refueling machine is about 10 minutes. In the BWR mainstream 1.1 million kw class BWR plant, about 1
Since there are 85 to 190 cells, this time difference is about 2000 minutes (about 1.4 days). Considering the restoration, about twice this
It will be 2.5 to 3 days shorter.

In the current 1.1 million kw class BWR plant, the number of regular inspection days has been shortened to around 50 days.
The shortening of the critical path on the 3rd is very large, and the economic effect will be quite expensive per regular inspection.

Next, an embodiment of the simulated fuel assembly according to the present invention will be described with reference to FIGS. 2 (a) and 2 (b). The simulated fuel assembly according to the present embodiment is for inserting the fuel assembly 4 in one cell into the hollow portion after extracting one fuel assembly 4 in the core fuel extraction method described above.

That is, the simulated fuel assembly 8 shown in FIGS. 2A and 2B is a square tubular stainless steel channel box 9, a neutron absorber 10 provided on the inner surface of the channel box 9, and a channel. It is composed of a handle 11 provided on the upper part of the box 9 and a lower tie plate 12 provided on the lower part of the channel box 9.

A visual identification portion 13 is provided on the upper surface of the handle 11 so that it can be easily distinguished from other fuel assemblies 4 from the upper part of the core. Since this simulated fuel assembly 8 is used in a state in which the reactor water temperature is low during regular inspection, paints are unlikely to undergo thermal decomposition, radiation decomposition, etc., and can be selected quite freely.

Further, the simulated fuel assembly 8 has the same outer diameter size and weight as the BWR fuel assembly 4, and is a low cost neutron absorber 10 having the same thermal neutron absorption capacity as the cross-shaped control rod. It uses cadmium (element No. 48 of the periodic table), which is easy to process. The neutron absorber 10 is provided on the inner surface of the channel box 9 in FIG. 2, but it can be provided on the outer surface,
Further, it can be contained in the stainless steel of the channel box 9.

Further, since the simulated fuel assembly 8 absorbs thermal neutrons, it is not necessary to uniformly disperse the neutron absorber in the channel box 9, and as shown in FIG. The amount of the neutron absorber 9 can be saved by only thinly concentrating on the four vertical faces of the rectangular column that is long in the direction.

Since the simulated fuel assembly 8 is used at atmospheric pressure (the head pressure depends on the water depth of the reactor water) and room temperature (at most 50 ° C.), the structure considering strength can be simplified.

The normal control rods used for the neutron absorber 9 during operation are hafnium or boron carbide, but the simulated fuel assembly 8 is inexpensive because there is no reason to select these expensive materials and structures. A plate-shaped absorber made of cadmium covered with stainless steel is arranged on four vertical surfaces of a long rectangular column.

[0032]

EFFECTS OF THE INVENTION According to the present invention, while ensuring core safety during regular inspection, the number of fuel assemblies handled is reduced and the core-related work time, which is a critical path in the regular inspection process, is shortened. The number of regular inspection days can be shortened and the utilization rate of the plant can be improved.

In other words, a 1.1 million kw class BWR plant will save about 2.5 to 3 days, but the current 1.1 million kw class BWR plant
Since the number of regular inspection days has been shortened to around 50 days at the plant, this shortening of the critical path for 2.5 to 3 days is extremely large and has a great economic effect.

[Brief description of drawings]

FIG. 1 (a) is a plan view schematically showing a core for explaining an embodiment of a method for taking out core fuel according to the present invention, and FIG. 1 (b) is an enlarged view showing a unit cell in (a).

FIG. 2A is an elevation view showing a simulated fuel assembly according to the present invention, and FIG. 2B is a transverse cross-sectional view taken along the line AA in FIG.

FIG. 3A is a plan view schematically showing a core for explaining a state at the start of a conventional core periodic inspection, and FIG. 3B is an enlarged view showing a unit cell in FIG. 3A.

4 (a) is a plan view schematically showing the core after the fuel is taken out from the state of FIG. 3 (a), and FIG. 4 (b) is an enlarged view showing a unit cell in (a).

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor core, 2 ... Core boundary, 3 ... Upper lattice plate, 4 ... Fuel assembly, 5 ... Control rod, 6 ... Neutron instrumentation detector, 7 ... Cavity part, 8 ... Simulated fuel assembly including neutron absorber , 9 ... Channel box, 10 ... Neutron absorber, 11 ... Handle, 12
… Lower tie plate, 13… Visual identification part.

Claims (4)

[Claims] 1. A method for taking out a fuel assembly from a boiling water reactor core comprising a plurality of unit grids each having four fuel assemblies arranged in a grid pattern, wherein any one of the unit grids is integrated. The method for extracting core fuel is characterized in that after extracting the fuel assembly, the simulated fuel assembly including the neutron absorber is inserted into the extracted cavity. 2. A rectangular tubular stainless steel channel box, a neutron absorber provided on at least the inner surface of the channel box, a handle provided on the upper portion of the channel box, and a lower type provided on the lower portion of the channel box. And a simulated fuel assembly. 3. The simulated fuel assembly according to claim 2, wherein the neutron absorber is made of a cadmium thin plate. 4. The simulated fuel assembly according to claim 2, wherein a visual identification portion is provided on at least one of the upper surface of the channel box and the upper surface of the handle.