JPS59138984A - Seal element for helium gas cooled reactor - 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 The present invention relates to a sealing structure for a coolant between core structural blocks constituting a core structure of a gas-cooled nuclear reactor.

Generally, the core structure of a high-pressure gas-cooled reactor, which is a type of gas-cooled nuclear reactor, has a structure in which regular hexagonal blocks are closely arranged on a flat surface.

In other words, in the reactor core, as shown in FIG. 1, a set of six standard fuel blocks FB surrounding one control block CB form a fuel handling area. In addition, in the outer peripheral part where the control block CB and the standard fuel block FB are arranged in combination, the outer periphery of the movable reflector block MR shown in FIGS. 1 and 2 is surrounded by a fixed reflector block SR, and These fixed reflector blocks SR
is surrounded and held by a side shield SM, and a core barrel B is provided by forming a gap of a required length around the outer periphery of this side shield SM, and the fixed reflector block SR is fixed to the core barrel B by a core restraining arm CBA. , furthermore, the whole is a pressure vessel P
It is surrounded by V.

In addition, standard fuel block FB, movable reflector block MR
and the fixed reflector block SR are each made of graphite, and the side shield SM is made of boro-doped graphite.

In the core structure having such a configuration, fuel in the fuel handling area is taken in and taken out through a standpipe SP (see FIG. 1) provided at the center of each block. In addition, the fuel blocks FB in the area are stacked one to several levels high to form a column as shown in Fig. 1 (A column formed by stacking blocks is called a column. ).

As is clear from FIGS. 1 and 2, in the conventional core structure, cooling gas is introduced from the inlet pipe IP connected to the bottom of the pressure vessel PV, and the cooling gas is introduced around the core, i.e., from the control block C8% standard fuel. Core block and pressure vessel P consisting of block FB, reflector block MR, 8 ratios, etc.
(11), and then flows from the upper part of the core block through the gap between the control block and heats up to a high temperature of approximately 1,000°C, which is supported by the still-temperature plenum block PB below the core. It is configured to collect in a high temperature plenum HP formed by a high temperature plenum HP, and then to an outlet pipe OP connected to the hearth part.

As mentioned above, in a conventional high-temperature gas-cooled furnace, the temperature of the cooling gas is increased by flowing down particularly through the control block CB and the standard fuel block FB, but the cooling gas does not flow through these blocks but instead flows through other blocks. If a large amount of bypass flows through the gaps between the core blocks and directly into the high-temperature plenum HP below the core without raising the temperature, the core will not be cooled sufficiently and there is a risk that the fuel will reach an abnormally high temperature.

In order to prevent such dangers, conventionally, a high-temperature plenum block PB made of graphite was constructed to form a high-temperature blennium HP and support the reactor core in order to reduce the bypass flow rate of cooling gas flowing down the gap between the core blocks. A seal is placed in between.

As shown in FIGS. 3(a) and 3(b), this conventional Yuzu seal has a sealing element Se made of graphite, a heat-resistant alloy, or a plate made of a combination of these placed over the gap between adjacent high-temperature plenum blocks PB. and is arranged to close the gap. Figure 3(b) is c of Figure 3(a).
-C cross section is shown. In addition, the solid reflector block S is also connected to the adjacent fixed reflector block SR and the side mB.
A sealing structure can be provided by sealing elements 8e in the horizontal and vertical directions on the joint surface with the shield 8M.

However, the conventional seal structure described above has the following problems. That is, the sealing element 8e made of graphite
When using 4th plenum block PB, due to the difference in thermal expansion caused by the temperature distribution inside the furnace
This results in a difference in level as shown in Figures (a) and (b), and the sealing element 8e is tilted, resulting in insufficient contact between the sealing surfaces and a reduction in sealing performance. Particularly, as shown in FIG. 4(b), if there is a difference in level between adjacent high-temperature plenum blocks PB, a portion of the seal element Se will be lifted from the surface of each block of the high-temperature plenum blocks PB, resulting in a seal. sex becomes significantly inferior. On the other hand, when a sealing element made of a heat-resistant alloy is used, the ability to follow the step difference between the above-mentioned still-temperature plenum blocks is better than that of the sealing element Be made of graphite. This is because graphite and alloys have different deformability with respect to temperature, and from the point of view of strength, graphite seal elements require a thickness of several tens of thickness, whereas heat-resistant alloy seal elements require a thickness of 0. It only takes about 5 kakus, and the thickness can vary by a wide range. However, the sealing element of heat-resistant alloy is CH in the cooling gas helium at high temperature.
Since it is corroded and deteriorated by impurities such as Co, CO2, H2,02, H,0, etc., the sealing performance is lowered compared to that of graphite elements. In particular, the seal element made of heat-resistant alloy is made as thin as possible, with the thickness of the seal element being about 0.5 mm, in order to improve its ability to follow steps as described above, so corrosion cannot be expected. The above problem is caused by the reflector block, that is, the movable reflector block M.
The same can be said of the seal structures of R and fixed reflector block S.

Nickel is used as a heat-resistant alloy for sealing elements.

There are materials whose main components are chromium and iron, and whose high-temperature strength has been improved by molybdenum, aluminum, titanium, etc., and representative materials include Ni-based heat-resistant alloy N'CF600, iron-based heat-resistant alloy NCF300, which is specified by JIS. Another example is Hastelloy X, a trade name of a Ni-based heat-resistant alloy. These heat-resistant alloys usually contain 16 to 23% by weight of chromium to improve corrosion resistance, so that a stable oxide film is formed on the alloy surface in the atmosphere, providing sufficient corrosion resistance. However, in the cooling gas helium of a gas-cooled nuclear reactor, oxidizing impurities O, , H,0,
Since the concentration of CO, etc. is extremely low, it is difficult to form a stable oxide film on the surface of these heat-resistant alloys.On the contrary, the heat-resistant alloys are carburized by carburizing impurities such as CH, CO, etc. in the cooling gas helium. In some cases, the material becomes brittle and unusable.

Methods to prevent carburization of heat-resistant alloys placed in such reducing or weakly oxidizing atmospheres include adding aluminum to the alloy, or melting and dipping aluminum on the alloy surface, sputtering, or vapor deposition. ,CVD
It is considered that coating methods such as the following are used. However, when aluminum is added to a heat-resistant alloy, corrosion progresses inside the alloy due to internal oxidation, and if the aluminum content is too large, the alloy becomes brittle and its mechanical properties are impaired. On the other hand, when aluminum is coated on the surface of a heat-resistant alloy, the aluminum and alloy elements will interdiffuse at temperatures above 700°C, but due to the Kirkendall effect, pores will form at the interface between the aluminum coating layer and the alloy, and aluminum will further diffuse into the alloy. An inconvenient phenomenon occurs.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a sealing element of a heat-resistant alloy with high corrosion resistance for sealing a bypass flow of a cooling gas helium containing corrosive impurities flowing through a gap between core blocks in a gas-cooled nuclear reactor, eliminating the above-mentioned drawbacks. It's about doing.

The inventor of the present invention has conducted various studies in order to overcome all the problems of the seal structure in conventional gas-cooled nuclear reactors, and as a result, coated the surface of the seal element made of a heat-resistant alloy with ceramics such as alumina. The authors found that this prevents the heat-resistant alloy in the helium cooling gas from being carburized or oxidized, and that interdiffusion with the alloy is slow and stability is improved compared to when aluminum is added to the alloy. The purpose of the invention has been achieved.

Since the diffusion coefficient of oxygen and metal ions in alumina is smaller than that of other oxides such as chromium oxide, corrosion resistance is significantly improved when alumina is formed on the alloy surface. Also, the equilibrium partial pressure of oxygen with alumina is, for example, 10 at i, ooo°C.
Atm, which is significantly lower than that of other alloying elements, it is thought that oxidation reactions of other alloying elements hardly proceed inside the alumina coating layer.

Possible methods for coating heat-resistant alloys with alumina include plasma spraying and chemical vapor deposition, but according to the inventor's experiments, the alumina coating obtained by plasma spraying is porous and relatively thick. Therefore, the corrosion resistance is insufficient, the ability to follow the deformation of the base alloy is poor, and it is easily damaged by mechanical force. On the other hand, the alumina film coated by chemical vapor deposition is dense and has sufficient corrosion resistance against corrosive impurities in helium gas. The deformation imposed on the sealing element was sufficiently followed, and a favorable result was obtained without mechanical damage.

The present invention will be explained below with reference to Examples.

Example 1 A plate material having a thickness of 0.5 tan and having the trade name Hastello This material is methane 200μat
m, i in helium gas containing 200 μatm of hydrogen.
As a result of leaving it for 30θ hours at , ooo°C, no change was observed in the metal structure, and no carburization reaction was observed. On the other hand, without rolling up the alumina surface coating, Hastelloy
When exposed for a long time, intense carburization progressed. In order to compare the metal structures of the two, Fig. 5 (a) and (b) are shown at a magnification of 2.
Shown as a micrograph at 00x magnification. As shown in Figure 5(a), the No-STEROY As shown in Figure 5(b), alloy X does not contain charcoal, indicating that it has a mining effect that is lagging behind in reducing or weakly oxidizing atmospheres.

Example 2 Using NCF300, which is specified by JIS as a heat-resistant alloy for the seal element material, the same surface treatment as in the case of Hastelloy A test with the same atmosphere effect as in Example 1 was conducted, and the metal structures of both were shown in Figure 6(a).
(b) shows a micrograph at 200x magnification. Figure 6 (a
) is up to -1 of NCF300 without an alumina coating layer, and NCF300 with an alumina coating layer of 0)) in Figure 6
As is clear from the comparison with the case shown in FIG. 6(b), it is clear that the alumina coating layer shown in FIG. 6(b) has a carburization prevention effect and also has no internal oxidation of the aluminum contained in the material, resulting in superior corrosion resistance.

If the operating temperature of the heat-resistant alloy sealing element is 700°C, then the test temperature in Example 1 and Example 2! 1000℃
Considering the diffusion rate of carbon, this is an accelerated test of about 10'. Therefore 1,000℃.

Conditions Fi for 300 hours, 30. That's equivalent to 10,000 hours. However, this is not limited to the operating temperature and operating time of the heat-resistant alloy sealing element provided with the alumina coating layer, and the present invention can be expected to be put to practical use even under even harsher conditions. .

Example 3 A 0.5-tone thick plate of the trade name Hastelloy Impurity gas in helium gas H2200μatm,
CO100μatm, CO22μatm, CH45
Exposure to an atmosphere containing μatm at 900°C.

A 1000 hour standing test was conducted. The results are shown in FIG. 7 as a relationship between the test time and the weight increase of the oxide.

In FIG. 7, curve (a) shows the case where Hastelloy X is not subjected to surface treatment, and curve (b) shows the case where an alumina coating layer is formed on the surface of Hastelloy X by chemical vapor deposition.゛As is clear from the comparison between curve (a) and curve (b), the test piece coated with alumina shows almost no weight increase due to corrosion over time, and is suitable for use in gas-cooled nuclear reactors. This shows that it has excellent corrosion resistance in typical atmospheres.

As explained above, in the high-temperature plenum block and movable and fixed reflector blocks of a gas-cooled nuclear reactor, the sealing element made of a heat-resistant alloy used to prevent the bypass flow of the cooling gas helium into the reactor is prevented by impurities in the helium gas. In order to prevent oxidation or carburization, the present invention uses chemical vapor deposition to provide an alumina coating layer on the sealing element, resulting in a sealing element that is extremely corrosion resistant and highly resistant to long-term use at high temperatures. Ta.

[Brief explanation of the drawing]

Figure 1 is a vertical cross-sectional view showing the core structure of a conventional high-temperature gas-cooled reactor. Figure 2 is a diagram showing the core structure shown in Figure 1 in different displacement states of the conventional seal structure. Figure 5 is an illustration of the core structure shown in Figure 1. FIG. 6 is a metallurgical microscope photograph of Example 1, FIG. 6 is a metallurgical microscope photograph of Example 2, and FIG. 7 is a diagram showing the relationship between test time and weight increase of the test piece in Example 3. CB...Ill hook, FB...standard fuel block, M box...movable reflector block, S R...
Fixed reflector plot), PB...high temperature plenum block, se-seal element. T 3 Figure f go 1 □ 2 T + Mouth (phantom δ8 piece r Figure C) size bu pu //3) sai Δ taste (,4) size 6 country c'3)

Claims (1)

[Claims] 1) A core structure formed of a column-shaped unit core body in which a control block is surrounded by a plurality of standard blocks, and a movable reflector block and a fixed reflector block that further surround the outer periphery of the core body, A sealing element for blocking a bypass flow of helium gas flowing through the gaps between the blocks of a helium gas-cooled nuclear reactor supported by a plurality of adjacent high-temperature plenum blocks, the sealing element having an alumina coating layer on its surface. A sealing element for a helium gas-cooled nuclear reactor, characterized in that it is made of a heat-resistant alloy. 2. A sealing element for a helium gas-cooled nuclear reactor, characterized in that the alumina coating layer according to claim 1 is formed to a thickness of 30 μm or less by a chemical vapor deposition method.