JPS62150194A - Control element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Technical Part 5'f of the Invention] The present invention relates to a control element, and more specifically, the present invention relates to a control element, and more particularly, it is used in a control system for emergency shutdown of a high-temperature waste furnace, a so-called backup shutdown system, and is This invention relates to a control element that does not deteriorate in quality.

[Technical background of the invention and its problems] In high-temperature gas reactors, there is a back-up shutdown system separate from the control body, for example, for shutting down the furnace in an emergency.

That is, it is common that an emergency stop control system is provided. This backup shutdown system has the function of injecting all the control elements (hereinafter referred to as control elements) consisting of neutral f-absorbers installed in the upper part of the reactor core into the reactor core all at once. Generally, it consists of a control element, a hopper that accommodates the control element f, a guide pipe for guiding the control element into the reactor core, and a control mechanism that controls the charging operation of the control element. For such backup stoppages, the control element in the system can be introduced by bursting and opening the Lubutture disk installed at the bottom end of the hopper under gas pressure, or by inserting it into the hole at the bottom of the hopper. There is a method of charging by pulling out the plug. In this case, whichever method is used, it is necessary to smoothly insert the control elements all at once.

Note that this control element generally contains boron (B), which has a large neutron absorption cross section,
For example, a mixed powder sintered body (B4C/C type) of boron carbide (CB4G) and graphite is widely used.

By the way, He gas is used as a coolant in a high-temperature gas furnace, and the two-leg backup shutdown system installed in the furnace is placed in a He gas atmosphere at about 400°C. Although He gas is originally an inert gas, it may actually act as a carrier for various impurities, such as moisture generated from graphite, which is a core structural material, and reactions between this moisture and graphite. H2, Co, produced by
Co2. Contains CH4, etc., and forms a special corrosion environment such as a low-boiling atmosphere or a carburizing atmosphere. If such He gas enters the hopper of the system during a backup shutdown, the control elements will be exposed to the corrosive environment described above, the surface of the control elements will change, and in extreme cases, the control elements may condense against each other. I put it on and built a bridge in the hopper,
A problem arises in that the closing operation of the control element is hindered.

Purpose of C9i Akira The present invention solves these conventional problems, and the surface does not change in quality even during furnace operation, and is chemically stable.
The purpose of the present invention is to provide a control element that can smoothly perform a closing operation in an emergency.

[Summary of the Invention] The present inventors first conducted the following experiment in order to investigate the cause of the above-mentioned deterioration of the control element. That is, a mixed powder sintered body of boron carbide and graphite (spheres with a diameter of 10 mm, theoretical density ratio of 98.5% or more, hereinafter referred to as 84C/C element), which is commonly used as a control element, was prepared by hot pressing. Manufactured. A plurality of these B, C/C elements are tightly packed in a quartz container, and water vapor is
It was heated at 500°C for 2000 hours in an oxidizing He gas stream containing 0 locator ATM. Adhesion between elements was observed in the B4C/C element after heating was completed.

Further, as a result of examining the surfaces of these elements using the xvj diffraction method, the formation of B2O3 was confirmed. On the other hand, by performing the same heat treatment in place of the atmosphere described in L, CH4 was heated at 100 μatm.
As a result of carrying out the test in a gas flow containing carburizing He, not only no adhesion between the elements but also no deterioration of the element surface was observed.

Based on the above, the present inventors discovered that the deterioration of the 84C/C element occurs in an oxidizing atmosphere, and that the B2O3 generated on the element surface at this time is the cause of adhesion between the elements. Ta. Based on this knowledge, various tests #ft1 were repeated.

In order to cut off the contact between the surface of the control element containing oxidation and the oxidizing atmosphere, we confirmed that an excellent effect can be obtained by coating the surface with an oxidation-resistant film, and completed the process of I Ta.

That is, one control wife and child of the present invention is made of a neutron absorber containing boron. It is a T11J element, and its surface is coated with a # oxide film.

The oxidation-resistant film formed on the surface of the side control element of the present invention has good adhesion to the control element, exists stably in the above-mentioned oxidizing and carburizing atmosphere, and is compatible with such an atmosphere and control element. There is no particular limitation on the material as long as it completely cuts off contact with the element surface, but materials made of oxides or substances that form a dense liquefied layer upon oxidation are particularly preferred.

First, the oxide that provides oxidation resistance 1!2 is chemically stable in the atmosphere mentioned above, and has a high melting point,
It is preferable to use aluminum oxide (AM 203
), W'A chromium (Cr203), zirconium oxide (Zr02).

Titanium oxide (Ti02), hafnium oxide (Hf02)
, tantalum oxide (Ta20s), niobium oxide (Nb20
S), silicon dioxide (Sin2), beryllium oxide (B e O), or two or more selected from these. Further, the thickness of the skin 11 made of this oxide is not particularly limited, but it is usually preferably about 50 to 200 gm.

The method for forming such an oxide layer on the surface of the control element is as follows:
There are no particular restrictions, for example, molten! A thermal spraying method in which oxide powder of E is sprayed onto the surface of the control element at high speed.
Examples include physical vapor deposition (PVD) such as sputtering, chemical vapor deposition (CVD), etc. Among them, CVD is suitable for obtaining a dense film.

Next, the substance that is oxidized to form a dense oxide layer, which becomes the oxidation-resistant film, has a high chemical affinity with oxygen, and when operating the high-temperature gas furnace as described above, i.e., the oxidizing property at about 500°C. under the atmosphere.

It is preferable to use a material that quickly forms a chemically stable and dense oxide layer.1. In terms of r4 body.

Aluminum (An, chromium (Cr), zirconium (
Zr), titanium (Ti), beryllium (Be), silicon (Si), nickel-chromium (Ni-Cr) alloy, nickel-aluminum (Ni-AfL) alloy, nickel-chromium-aluminum (Ni- Cr
-AIL) alloy, stainless steel, or two or more selected from these. The oxides of these metals or alloys have a volume ratio of 1.2 to 2.5 with respect to the metal or alloy matrix, so they can completely cover the matrix surface and have no defects such as pores or cracks. Forms a dense film. The thickness of these metal or alloy layers is preferably about 10 to 100 gm.

The layer made of such a metal or alloy does not necessarily have to become an oxide over the entire layer during furnace operation, but if an oxide layer of a predetermined thickness is formed near the outer surface, it can be used as an oxidation-resistant film for the control element. It becomes possible to perform sufficient functions.

[Embodiments of the Invention] Example 1 First, a spherical 8 similar to the above was prepared by applying a hot press method.
4 A C/C element was produced, and a coating layer was formed on the outer surface of the element by thermally spraying Cr in a reduced pressure inert atmosphere. That is, this thermal spraying process was a low-pressure plasma spraying method, using thermal spray powder with a uniform particle diameter of about 7° ILm, and was carried out in an Ar gas atmosphere at a pressure of 100 Torr. The average thickness of the resulting Cr coating layer was about 100 pm.

A plurality of Cr-covered mB4C/C elements obtained in this way were densely filled into a quartz tube container, and 200% of H2O was added.
Heat treatment was performed at 500° C. for 2000 hours and then at 500° C. for 500 hours in a He gas flow containing GAT.

When we observed the state of the Cr-treated Bac/C elements after the heat treatment, we found that the adhesion between the elements was as follows. When the cross-sectional structure of the Cr-coated B and C/C elements was observed with an optical microscope, it was found that the surface was B4 C/
(No change was observed in the structure near the interface between the 41 body and the Cr coating layer.

Furthermore, we used an EPMA (X-ray microanalyzer) to investigate the distribution of oxygen in the cross section of the B a C/C element after heat treatment, from the outer surface of the Or coating layer to the center of the element. No significant difference was observed in any of the 5,000-hour treatments compared to the element before heat treatment, except that the oxygen concentration was higher in the extreme surface layer of the Cr coating layer.

From the above, B4C/C2+:,
It was confirmed that the oxidation of the particles, that is, the production of B2O3, was inhibited. Furthermore, under the heat treatment conditions mentioned above, that is, the conditions to which the backup shutdown system is exposed during normal operation of the high-temperature gas reactor, the surface layer of the Cr coating layer is rapidly oxidized, resulting in dense and chemically stable oxidation. generate a physical layer,
1 It was found that the oxide layer suppressed the diffusion of oxidation into the interior.

Example 2: CVD was applied to the surface of the same B, C/C element as in Example 1.
A pattern A203 coating layer was formed by applying the method. This process uses aluminum trichloride (All CfL) as a reactant gas.
3) and H2 and CO2 mixed gas, the precipitation temperature (
84C/C element temperature) 1050°C. The average layer thickness of the resulting Al 203 coating layer was about 70 gm.

A plurality of the thus obtained Ai, 03 fiB4C/C elements were filled into a quartz tube container and heat treated for 2000 hours and 5000 hours under the same conditions as in Example 1 above. When the state of the Au203-coated B4C/C element after the heat treatment was observed, no adhesion between the elements had occurred at all. Also, A before and after heat treatment! ;L2o3 coating B4
As a result of observing the cross-sectional structure of the C/C element with an optical microscope, B
No change was observed in the structure near the interface between the aC/C matrix and the A-texture 203 coating layer.

From this, the oxidation of B,
That is, it was confirmed that the deterioration of the BaC/C element due to the generation of B2O3 was effectively prevented.

[Effects of the Invention] As is clear from the above description, the control element of the present invention, particularly the control element used in the backup shutdown system of a high-temperature gas reactor,
Since its surface is coated with #oxidation wave 11, even if it is exposed to an oxidizing atmosphere during furnace operation, its surface will not change in quality, and therefore adhesion between elements due to the production of B2O3 etc. will be prevented. It never occurs. As a result, smooth operation is ensured when using the control element.

In addition, since the control J11 element of the present invention can be manufactured by simply subjecting a conventionally used control element to surface coating treatment, for example, design conditions such as the material, shape, and characteristics of the control element itself can be manufactured. It is not necessary to make any major changes to the system, and it has a great effect. Therefore, its industrial value is extremely large.

Claims (4)

[Claims] (1) A control element comprising a neutron absorber containing boron, the surface of which is coated with an oxidation-resistant film. (2) The control element according to claim 1, wherein the oxidation-resistant film is made of an oxide or a substance that forms a dense oxide layer when oxidized. (3) Claims in which the oxide is at least one selected from the group consisting of aluminum oxide, chromium oxide, zirconium oxide, titanium oxide, hafnium oxide, tantalum oxide, niobium oxide, silicon dioxide, and beryllium oxide. Control element according to item 2. (4) The substance that is oxidized to form a dense oxide is made of aluminum, chromium, zirconium, titanium, beryllium, silicon, nickel-chromium alloy, nickel-aluminum alloy, nickel-chromium-aluminum alloy, and stainless steel. The control element according to claim 2, which is at least one type selected from the group.