JPH0472907B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method and apparatus for manufacturing a guide tube for a nuclear reactor measurement element.

[Background technology and its problems]

For example, in a boiling water reactor, a fuel assembly 12 is placed in a reactor pressure vessel 11 as shown in FIG.
A plurality of sets of neutron detector assemblies 14 (only one set is shown) are installed to detect neutrons or reactor temperature in each part.
Therefore, the aggregate 14 includes a plurality of neutron detectors 1.
5, 15... are inserted. That is, aggregate 1
4 is a protection tube 16 as shown in FIGS. 2 and 3.
A plurality of neutron detectors 15, 15, . . . and their output leads 17, 17, . This guide tube 18 is made of a long stainless steel pipe with a small diameter, and a calibration measuring element (not shown) is installed inside it for the purpose of calibrating the detection performance of each neutron detector during operation of the nuclear reactor. It is for passing through. Since the calibration measuring element is repeatedly inserted and extracted, a layer 19 of nitrided steel is formed on the inner surface of the measuring element as shown in FIG. This nitrided layer 19 must have sufficient hardness over the entire length of the guide tube and, in particular, must have a uniform layer thickness over the entire length. This is because if the layer is too thin in whole or in part, the lifetime will be shortened in terms of wear resistance, and if it is too thick, it will be brittle, and in either case it will be unreliable. The guide tube is formed of a long stainless steel pipe with a small diameter, for example, having an inner diameter of 7 mm, a wall thickness of 1.5 mm, and a total length of 14 m. Ammonia gas is then passed through the guide tube while heating it to a high temperature to form a nitrided layer. The thickness of the nitride layer produced depends on the processing temperature, time,
and depends on the degree of dissociation of ammonia gas. As mentioned above, since the guide tube is small in diameter and long, it is difficult to maintain a uniform temperature throughout and to bring ammonia gas into uniform contact with the entire area.
Furthermore, it is not easy to maintain a uniform degree of gas dissociation. Due to these circumstances, it has been difficult to form the inner nitrided layer of the guide tube with high hardness and uniform thickness.

[Purpose of the invention]

The present invention provides a method for manufacturing a highly reliable guide tube for a nuclear reactor measurement element by efficiently forming a nitride layer of a desired thickness and uniform quality over the entire inner surface of the guide tube, and It provides manufacturing equipment for direct use in carrying out the method.

[Summary of the invention]

The manufacturing method of the present invention is characterized in that the guide tube is heated to a high temperature and ammonia gas is introduced and discharged from one of the open ends of the guide tube to the other by alternating the flow direction multiple times to form a nitrided layer. It is in.
Furthermore, while maintaining the guide tube at a predetermined high temperature, the degree of dissociation of ammonia gas during that time is reduced by 20%.
Another feature is that it can be controlled within a range of 45%.

Further, the manufacturing apparatus of the present invention includes a heating furnace that accommodates a plurality of small-diameter and long guide tubes to be treated, a pair of manifolds commonly connected to both ends of the guide tubes, and a plurality of manifolds connected to each of these manifolds. A gas introduction pipe, a gas discharge pipe, a pair of switching valves connected to switch gas introduction and discharge to each of these pipes, and an ammonia gas source connected upstream of the gas introduction switching valve via a pressure control valve. and a gas analyzer connected downstream of the gas discharge switching valve to detect the degree of dissociation of ammonia gas.

As a result, a nitrided layer having a desired thickness and being relatively uniform over the entire area can be formed, and a highly reliable guide tube can be obtained.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to FIGS. 5 and 6. Note that the same parts are represented by the same symbols.

First, the configuration of a manufacturing apparatus directly used for carrying out the manufacturing method of the present invention will be explained with reference to FIG. This apparatus has a long electric heating furnace 21, and a pair of manifolds 22, 23 at both ends thereof, to which a plurality of long stainless steel guide tubes 18 to be treated are connected. The manifolds 22 and 23 have gas introduction pipes 24a and 24b and discharge pipes 25a and 25.
b are connected to each other. Regarding the gas introduction path, an ammonia gas (NH ₃ ) gas source 26 is connected to electromagnetic switching valves 32 and 33 via a valve 27, an electromagnetic valve 28, a pressure control valve 29, a flow meter 30, and a check valve 31. There is. Also,
An inert gas source 34, such as nitrogen _N2 , is supplied to the valve 3.
5. Also connected to switching valves 32 and 33 via an electromagnetic valve 36, a flow meter 37 and a check valve 38. One of the switching valves 32 is connected to one manifold 22, and the other valve 33 is connected to the other manifold 23 via gas introduction pipes 24a and 24b, respectively. Note that the symbol G in the figure represents a pressure gauge, and the arrow shown on the gas pipe path represents the flow direction of gas. Each manifold 22, 2
The gas exhaust pipes 25a, 25b of No. 3 are connected to another set of electromagnetic switching valves 39, 40, and the outlets thereof are connected to an electromagnetic valve 41, a flow meter 42, a gas analyzer 43,
It is connected via a solenoid valve 44 to a gas exhauster or scrubber 45 . Also, switching valve 2
The outlets 9 and 40 communicate from the bypass path to the scrubber 45 via a bypass valve 46,
Furthermore, it is also connected to a scrubber 45 via a valve 47 and a brewet 48. Note that the reference numeral 53 represents a water tank and 49 represents a water valve, both of which are attached to the brewet 48.
This filter 48 and gas analyzer 43
is for measuring the degree of dissociation of ammonia gas coming out through the guide tube to be treated, and the output signal of the analyzer 43 is automatically recorded on the recorder 50, and this output signal is also given to the pressure controller 51. is,
This controller 51 is designed to automatically control the pressure of the pressure control valve 29 in the NH ₃ gas source path. The switching valves 32, 33, 39, and 40 automatically switch the direction in which the introduced gas flows from the manifold to the treated guide tube 18 to the left or right in the figure by the switching controller 52. That is, from the switching valve 32 to the introduction pipe 24a, the manifold 22 on the left side of the figure.
From the guide pipe, through the manifold 23 on the right side of the figure, through the discharge pipe 25b, and through the switching valve 40 to the exhaust route, and while blocking this route, from the switching valve 33 to the inlet pipe 24b, on the right side of the figure. The route passing through the manifold 23, the guide pipe, the left manifold 22, the discharge pipe 25a, and the switching valve 39 is switched by controlling the energization of the electromagnetic coil of each switching valve.

Next, the processing procedure will be explained with reference to FIG. First, in the step shown by symbol a in Fig. 6, both ends of the guide tube to be treated are connected to a manifold and installed in an electric heating furnace, and before raising the furnace temperature, the inside of the guide tube is
Start by flowing sufficient NH ₃ gas to exhaust the internal air and replace the gas. In the figure, the solid line curve (NH ₃ ) represents the flow rate of ammonia gas. After step a, proceed to step b, in which the flow rate of NH ₃ gas is reduced, the temperature of the electric furnace is increased, and the temperature T of the guide tube is increased. During this temperature rise, beads of vinyl chloride resin are introduced into the manifold while a small amount of HN ₃ gas is flowed in order to remove the oxidized layer on the inner surface of the guide tube, and the vaporized gas is passed through the guide tube. At the latter stage of step b, the temperature inside the furnace is kept constant to make the temperature of the guide tube uniform. Next, in step c, the temperature is further increased.
The flow rate of NH ₃ gas is increased all at once and the nitriding process begins. That is, while maintaining the NH ₃ gas at a temperature suitable for high-temperature decomposition, the flow rate of the NH ₃ gas is controlled in step d, and each switching valve is operated by the switching controller 52 of the device to change the flow direction of the NH ₃ gas at predetermined time intervals. Switch alternately. For example, the flow direction of NH ₃ gas is reversed every 5 minutes, and the process is automatically repeated 5 to 20 times. Meanwhile, the device's gas analyzer 43
Alternatively, the discharge route by the brewet 48
The degree of dissociation of NH ₃ gas (2NH ₃ →3H ₂ +2N) is detected, and the pressure controller 51 and pressure control valve 29 are controlled according to the degree of dissociation to control the flow rate of the introduced NH ₃ gas. This control area is represented by a hatched area P in FIG. For example, when the degree of dissociation is low, such as 20% or less, the pressure control valve 29 is tightened to reduce the flow rate, thereby controlling the degree of dissociation to increase. Conversely, if the degree of dissociation of the discharged NH ₃ gas is as high as 45% or more, the flow rate of the NH ₃ introduced gas is increased to control the degree of dissociation to decrease. During this step, the degree of dissociation is preferably controlled within the range of 20% to 45%. After processing in this way for a predetermined period of time, the furnace temperature is lowered and at the same time NH ₃
The introduction of gas is stopped, and then the process moves to step e, in which _N2 gas is introduced to prevent the guide tube from oxidizing, and the guide tube is slowly cooled.

Through the above treatment, the inner surface of the guide tube is preferably
A layer of nitrided steel with a thickness of 60 μm to 70 μm can be formed. In the device of this invention, each valve etc. is controlled to perform the above processing, and the bypass path provided in parallel to the gas analyzer is set to an amount that optimizes the measurement accuracy of the gas flow rate to the analyzer. It is a route for Biurets are also used to calibrate measurements and are effective in increasing and maintaining overall gas analysis accuracy. Of course
Control of the flow rate from the NH ₃ source can be done manually or automatically as described above.

〔Effect of the invention〕

The manufacturing method of this invention decomposes ammonia gas at high temperature and performs nitriding treatment by alternating the flow direction in a small diameter and long guide tube and repeating the process several times. Even in the case of a guide tube, the variation in the thickness of the nitrided steel layer is minimized, and it can be manufactured with good reproducibility. In particular, by controlling the degree of dissociation within the range of 20% to 45%, a nitrided layer with excellent hardness and a predetermined thickness can be obtained by nitriding for at most 30 minutes to 1 hour (step d). .

Further, with the apparatus of the present invention, the above manufacturing method can be carried out automatically and reliably, and processing can be performed with high efficiency and stability.

In this way, a highly reliable guide tube can be manufactured as a member for a nuclear reactor.

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram of the reactor, Figure 2 is a vertical cross-sectional view of its detector assembly, and Figure 3 is 3-3 in Figure 2.
FIG. 4 is an enlarged cross-sectional view of the guide tube, FIG. 5 is a structural diagram showing an embodiment of the apparatus of the present invention, and FIG. 6 is a diagram showing the steps of an embodiment of the manufacturing method of the present invention. It is a temperature and gas flow rate control curve diagram. 18... Guide tube, 19... Nitrided layer, 21... Electric heating furnace,
22, 23...manifold, 32, 33, 39,
40...Switching control valve, 26...Ammonia gas source, 29...Pressure control valve, 43...Gas analyzer,
48...Biuret, 51...Pressure controller.

Claims (1)

[Claims] 1. A manufacturing method for forming a nitrided layer on the inner surface of a small diameter and long guide tube for a nuclear reactor measurement element made of stainless steel, in which the guide tube is heated to a high temperature and ammonia gas is added to the guide tube. A method for manufacturing a guide tube for a nuclear reactor measurement element, characterized by forming a nitrided layer by introducing and discharging the flow direction from one of both open ends to the other by alternating the flow direction a plurality of times. 2 While maintaining the guide tube at a predetermined high temperature,
The degree of dissociation of ammonia gas between 20% and 45%
Claim 1 of controlling and processing within the scope of
Manufacturing method described in section. 3. A heating furnace that accommodates a plurality of small-diameter and long guide tubes for reactor measurement elements, a pair of manifolds commonly connected to both ends of the guide tubes, and gas introduction pipes connected to each of these manifolds, respectively. and a gas discharge pipe, a pair of switching valves connected to switch gas introduction and discharge to each of these pipes, and an ammonia gas source connected upstream of the gas introduction switching valve via a pressure control valve;
A manufacturing apparatus for a guide tube for a nuclear reactor measurement element, comprising: a gas analyzer connected downstream of the gas discharge switching valve for detecting the degree of dissociation of ammonia gas.