JPH02296190A - Nuclear fuel assembly - Google Patents

Abstract

PURPOSE:To achieve a higher working efficiency with safe and easier disassembling work by inserting upper end plugs of a plurality of nuclear fuel elements into a plurality of upper end plug holes provided in an upper tie plate. CONSTITUTION:A plurality of core water flow holes 11 are formed at a top of an insertion hole 8 for an upper end plug of a corner fuel element so that a core water infiltrating at the insertion hole 8 and a gap of an upper end plug 3 of the fuel element 2 flows out of a passage hole 11. The arrangement of the plurality of passage holes 11 allows a core water entering from one of the passage hole 11 to flow out of the other passage hole 11 so that the core water infiltrating into the insertion hole 8 will not retain because the core water is provided with a flow created. This permits inflow of the core water with a density of impurities always controlled to a gap between the insertion hole 8 of the fuel element 2 and an end plug 3 of the fuel element 2 thereby preventing sticking due to corrosion with the suppression of electrochemical corrosion.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to nuclear fuel assemblies for light water reactors, and in particular in radiation control areas such as nuclear power plants or the three irradiation facilities mentioned below, by remote control. The present invention relates to a nuclear fuel assembly suitable for smoothly removing a fuel plate.

[Prior Art] A conventional upper tie plate upper part of a nuclear fuel assembly and a procedure for disassembling the nuclear fuel assembly will be described with reference to FIGS. 2 and 3.

FIG. 2 is a schematic cross-sectional view of a nuclear fuel assembly, and FIG. 3 is an explanatory view of the fitting of a conventional upper tie plate and a fuel element upper end plug.

The configuration of FIG. 2 is as follows: 1: upper tie plate, 2: fuel element, 3: fuel element upper end plug, 4: spacer for holding the fuel element, 5: lower tie plate, 6: channel box , 7 is a tie rod nut.

The configuration of FIG. 3 is as follows: 1 to 3 are the same as those in FIG. 2, 8 is an insertion hole for a corner fuel element end plug, 9 is a water passage hole, and 10 is an insertion hole for a normal fuel element end plug. It is.

Next, the procedure for disassembling the fuel assembly will be explained.

When inspecting a fuel assembly used in a nuclear reactor in units of nuclear fuel elements, as shown in FIG. 2, first the tie rod nut 7 is removed and then the upper tie plate 1 is removed. Next, the upper end plug 3 of the nuclear fuel element 2 is grasped with tongs or the like and lifted upward, and successively pulled out from the fuel assembly. In this case, in the conventional upper tie plate, during use in a nuclear reactor, the contact surface between the corner fuel element end plug insertion hole 8 of the upper tie plate 1 and the fuel element upper end plug 3 shown in FIG. 3 corrodes. Sometimes it stuck. Due to this sticking, it became impossible to remove the two-part tie plate 1, and it was sometimes impossible to inspect each fuel element. What is the cause of this corrosion?

Since the material of the tie plate 1 is stainless steel and the material of the fuel element upper end plug 3 is a zirconium-based alloy, corrosion due to electrode reactions caused by dissimilar metals, @gas chemical corrosion, etc. are considered. The place where the upper tie plate and the fuel element end plug are fixed is the contact surface between the above-mentioned corner fuel element end plug insertion hole 8 and the fuel element upper end plug 3, and is usually the contact surface of the fuel element end plug insertion hole 180. No adhesion was observed. The difference between the corner fuel element end plug insertion hole 8 and the normal fuel element end plug insertion hole 10 is that the corner fuel element end plug insertion hole 8 is not a through hole, as is clear from FIG.

Therefore, when the fuel element upper end plug 3 is inserted into the corner fuel element end plug insertion hole 8, the impurity concentration in the reactor cooling water (reactor water) confined within the insertion hole 8 increases.
This caused electrochemical corrosion. In addition, there were some examples in which a water passage hole 9 was formed in the hole 5, but since the diameter of the water passage hole was small and it was a jli-hole, the water passage effect was small and it was not useful for corrosion prevention. On the other hand, since the insertion hole 10 for the normal fuel element end plug is a through hole, the cooling water of the reactor always flows through and is not confined within the hole, so no fixation due to corrosion was observed.

Now, when the fuel assembly is disassembled and the upper tie plate is removed, if a stuck state due to corrosion occurs as described above, it may become impossible to inspect the fuel element alone. Also,
To release the fixation, special equipment is required, and it has been desired to create a fuel assembly that is safe and can be dismantled reliably.

In the case of a conventional nuclear fuel assembly, Japanese Patent Application Laid-Open No. 60-202394 proposes a method of heating the lower tie plate and utilizing a thermal expansion method to pull out the fuel element from the stuck state. It is believed that this method can be applied to the removal of the upper tie plate.

[Problems to be Solved by the Invention] The above-mentioned prior art is an improved method that utilizes the difference in thermal expansion, but it is difficult to implement this method in a special environment such as inside a pool (underwater). This caused problems in terms of equipment costs and labor costs required for heating, as well as work efficiency.

An object of the present invention is to prevent the insertion hole of the fuel element upper end plug and the fuel element upper end plug from sticking due to corrosion when removing the upper tie plate from the fuel assembly, thereby making the disassembly work safe and easy. An object of the present invention is to provide a fuel assembly capable of improving efficiency.

[Means for Solving the Problems] In order to solve the above problems, the structure of the nuclear fuel assembly according to the present invention is such that the upper end plugs of a plurality of nuclear fuel elements having upper and lower end plugs are provided on the upper tie plate. The upper ends of these nuclear fuel elements are held by inserting them into a plurality of upper end plug holes, and some of these upper end plug holes have a non-penetrating structure. , a plurality of small holes for reactor water circulation are provided above the upper end plug hole having the non-through-hole structure.

[In the working co-fuel assembly, a plurality of reactor water flow holes are formed at the tip of the upper end plug insertion hole of the corner fuel element of the upper tie plate. Because reactor cooling water with controlled impurities is constantly circulated in the plug insertion hole, there is no accumulation of impurities, and electrochemical corrosion that tends to occur at the contact area between the end plug insertion hole and the upper end of the fuel element is suppressed. Therefore, a stuck state can be prevented.

[Example] An embodiment of the present invention will be described below with reference to FIG.

FIG. 1 is an explanatory diagram of a reactor water flow hole according to an embodiment of the present invention.

In the configuration shown in FIG. 1, 11 is a reactor water flow hole. Note that 1 to 3, 8, and 10 are the same as those shown in FIGS. 2 and 3.

The embodiment will be explained in more detail with reference to FIG. 1. The figure shows the structure of the upper tie plate 1, in which a plurality of 3 m diameter holes are installed at the top of the insertion hole 8 for the corner fuel element upper end plug. A reactor water flow hole 11 is formed, and the reactor water that enters through the gap between the insertion hole and the upper end plug 3 of the fuel element 2 flows through the reactor water flow hole 1.
1 will flow out.

Furthermore, by providing a plurality of reactor water flow holes 11, the reactor water that entered through one reactor water flow hole 11 flows out from the other reactor water flow hole 11. Water flows and does not stagnate.

In this way, since the reactor water that has entered the insertion hole 8 flows out from the reactor water flow hole 11, there is always a gap between the end plug insertion hole 8 of the corner fuel element and the upper end plug 3 of the corner fuel element. Since reactor water with a controlled impurity concentration flows in, electrochemical corrosion can be suppressed, which has the effect of preventing sticking due to corrosion.

[Effect of the invention] According to the present invention, the following effects can be obtained.

(1) By providing a plurality of small holes at the top of the corner fuel element end plug insertion hole of the upper tie plate, the flow of reactor water that enters between the end plug insertion hole and the upper end plug is improved;
Since no corrosion occurs, it no longer sticks.

(2) Therefore, when disassembling a nuclear fuel assembly, the upper tie plate can be easily removed before taking out the nuclear fuel elements.

(3) Since the dismantling work of nuclear fuel assemblies can now be carried out safely and reliably, the efficiency of the dismantling work has improved and has also had the effect of improving economic efficiency.

[Brief explanation of drawings]

FIG. 1 is an explanatory view of the end plug insertion hole of the upper tie plate of the embodiment according to the present invention, FIG. It is an explanatory view of an end plug insertion hole of an upper tie plate. <Explanation of symbols> 1... Upper tie plate, 2... Nuclear fuel element, 3... Nuclear fuel element upper end plug, 4... Spacer, 5... Lower tie plate, 6... Channel box, 7... Tie rod Nut, 8... Corner fuel element upper end plug insertion hole;
9... Water passage hole, 10... Normal fuel element upper end plug insertion hole, 11... Reactor water circulation hole.

Claims (1)

[Claims] 1. By inserting the upper end plugs of a plurality of nuclear fuel elements having upper and lower end plugs into the plurality of upper end plug holes provided in the upper tie plate, the nuclear fuel elements can be In the case where some of the plurality of upper end plug holes have an impenetrable structure at their upper ends, a hole for reactor water circulation is provided above the upper end plug hole having the impermeable structure. A nuclear fuel assembly characterized by having a plurality of small holes. 2. The nuclear fuel assembly according to claim 1, wherein the upper end plug holes having the impermeable hole structure are provided at the four corners of the upper tie plate.