GB1598474A

GB1598474A - Neutron detector

Info

Publication number: GB1598474A
Application number: GB2169578A
Authority: GB
Original assignee: Kernforschungsanlage Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 1977-05-28
Filing date: 1978-05-23
Publication date: 1981-09-23
Also published as: DE2724315C3; FR2392399B1; DE2724315A1; JPS53149080A; DE2724315B2; FR2392399A1

Description

(54) NEUTRON DETECTOR (71) We, KERNFORSCHUNGSAN LAGE JÜLICH GESELLSCHAFT MIT BESCHRÄNKTER HAFTUNG, of Postfach 1913, 5170 Jülich, Federal Republic of Germany, a Body Corporate organised according to the Laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a self-powered neutron detector suitable for use at high temperatures and is concerned with p-- or y-induced neutron monitors operating without auxiliary energy.

Neutron detectors for use in water-cooled nuclear reactors are intended for operating temperatures of up to about 400"C. Such neutron detectors include an emitter which can be excited by neutrons to emit other particles or rays. A distinction is drawn between slowly responding emitters in which a (N, ss) reaction takes place, and promptly responding emitters which emit y rays when excited by neutrons. The emitter is surrounded by a collector with the interposition of an insulating cylinder. The emitter is connected to an inner measuring conductor within a sheathed line.

In high-temperature nuclear reactors, more stringent requirements must be met by neutron detectors, because they must operate at temperatures of about 800"C over a relatively long period. At such high temperatures, the electrical connection between the emitter and the inner measuring conductor of the line has been found to be a critical point, because this connection is subject to thermal stress including alternating stress. A particular cause of this is that the materials used for the emitter, the connecting line, the collector and the ceramic body have differing thermal expansion coefficients. The usual kinds of connection between the inner conductor and the emitter, vis. hard soldering, welding or pinching have been found to be inadequate.

The object of the invention is to provide an improved connection between the inner conductor of a connecting line and the emitter in a high-temperature neutron detector. Broadly the invention resides in putting a helical coil of the inner conductor before its attachment to the emitter.

According to this invention there is provided a self-powered high-temperature neutron detector having an emitter (which may suitably be cylindrical) connected to an inner measuring conductor of a sheathed line, characterised in that the emitter has a connecting pin and in that the inner measuring conductor extends in a helical coil around the connecting pin, through a transverse bore in the pin and is made fast to the pin, preferably by a bent-over section of the conductor being welded to the outside surface of the pin.

By virtue of the fact that the inner measuring conductor is wound helically around the connecting pin, changes in length due to temperature fluctuations can readily be balanced out, relieving the mechanical load on the weld. In this way, the weld is not subjected to any substantial shearing load.

The end portion of the inner measuring conductor may be bent over at the outlet of the transverse bore to lie parallel to the axis of the connecting pin. It is thus laid along a generatrix of the terminal connecting pin and welded thereto, and in this way a particularly reliable and durable connection can be obtained.

Preferably a an end portion of the sheathed line the connecting pin and the emitter are surrounded by a ceramic insulating cover, the whole assembly being sealingly enclosed by a cylindrical collector having a fluid-tight end seal. This provides an enclosed and sealed form of construction which is particularly stable and is therefore particularly suitable for many applications.

An embodiment of the invention is hereinafter described with reference to the accompanying drawing, which is a section through a high-temperature neutron detector according to the invention.

The high-temperature neutron detector is mounted on a sheathed measuring line 1 which comprises an outer (i.e. sheath) conductor 2 and two inner conductors 3 and 4, one of which is used as a compensating conductor 3 and the other as an inner measuring conductor 4. The conductors consist of a nickel chromium alloy known under the name "Inconel" (Registered Trade Mark), which is distinguished by high resistance to heat and corrosion and which is suitable for neutron detectors. The sheathed measuring line 1 further comprises insulating means consisting of magnesium oxide.

In the particular embodiment of the invention shown the outer conductor 2 has an external diameter of 2.0 mm and a wall thickness of 0.24 mm. The diameter of the inner conductors 3 and 4 is 0.36 mm. The length of the sheathed line depends, of course upon the particular conditions of installation and may be a few metres. In the figure, there is merely shown the end portion of the sheathed line 1, on which the actual neutron detector is built up. The opposite end of the sheathed line is provided with a plug connector which permits connection to further electrical switching circuits. At the end shown the outer conductor 2 is removed over a length of about 50 mm. The compensating conductor 3 projects beyond the outer conductor 2 by an end piece 3' having a length of about 5 mm.

The inner measuring conductor 4 has an end portion 4' projecting beyond the outer conductor 2 by the aforesaid length of 50 mm and on assembly is treated in the manner described below.

Pushed on to the outer conductor 2 is an intermediate member 5 which has a length of about 20 mm and has a step between parts with differing internal diameters. The external diameter is uniformly 3.1 mm and corresponds to the internal diameter of a collector 6 which encloses the whole neutron detector assembly. The said intermediate member provides a conductive bridge between the outer conductor 2 of the sheathed line and the collector 6. The intermediate member 5 fits on the outer conductor 2, the end which has the smaller internal diameter being directed towards the end of the outer conductor 2. The intermediate member 5 is sealingly connected to the outer conductor 2 by means of a circumferential seam weld 7. The intermediate member also consists of the aforesaid nickel-chromium alloy.

Cylindrical insulating block 8 of aluminium oxide has an external diameter of 2.2 mm and a length of 3 mm. This insulating block 8 is a ceramic part and is formed with two bores 9 having a diameter of 0.5 mm, which serve to guide the inner conductors, the end section 3' of the compensating conductor 3 terminating in one bore 9 and the end portion 4' of the inner measuring conductor 4 extending through the other bore. The insulating block 8 is mounted on the outer conductor 2 and thus also serves as a distance piece.

There is provided as the measuring member an emitter 10 of cylindrical form. It is of vanadium, which gives delayed neutron indication. The emitter has a diameter of 2 mm and a length of 200 mm. Situated at that end of the emitter which is nearer to the sheathed line there is an axial bore 11 having a depth of 3.5 mm and a diameter of 1.0 mm. This bore 11 is crossed by a transverse bore 12, of which the ends have conically widened portions 13. The emitter 10 is electrically connected to the inner measuring conductor 4 with the aid of a cylindrical connecting pin 14 which has a blunt end received within the axial bore 11 in the emitter, and has a conical tip 15 at its opposite end (i.e. the end nearer to the line 1, facing the block 8). The connecting pin is 11 mm long, has a diameter of 1.0 mm and is also made of the aforesaid nickel-chromium alloy.Near the blunt end of the pin 14 there is a transverse bore 16 having a diameter of 0.4 mm, which is aligned with the transverse bore 12 of the emitter 10. Situated substantially midway along the length of the connecting pin, at a distance of exactly 6 mm from the blunt end, is a further transverse bore 17.

Extending through the aligned transverse bores 12 and 16 in the emitter 10 and the connecting pin 14 is a pin-form key 18 consisting of the aforesaid nickel-chromium alloy. This key has at one end a head and is deformed at the other end by mechanical working to form a rivet head, so that the key is positively held in the transverse bores 16 and 12 and thereby gives a mechanically and electrically reliable connection between the emitter 10 and the connecting pin 14. The conically widened portions 13 of the transverse bore 12 ensure satisfactory seating of the rivet heads of the key 18.

The end portion 4' of the inner measuring conductor extends in the form of a helical coil 19 around the connecting pin 14, then has a bent-over part 20 extending through the transverse bore 17 and has a further section 21 bent over at the outlet to the transverse bore 17 to lie parallel to the axis of the connecting pin 14 (i.e. along a generatrix thereof). It lies against the outside surface of the pin. A number of laser spot welds 22 provide a mechanical and electrical connection between the bent-over section 21 and the connecting pin 14. Since the connecting pin 14 and the inner measuring conductor 4 consist of the same material, they have equal coefficients of expansion, so that the part 20 and the further bent-over section 21 of the inner measuring conductor 4 undergo no displacement in relation to the connection pin 14.Consequently, the spot welds 22 undergo no mechanical loading due to temperature stresses. In this way, loading in shear of the spot welds is avoided.

Consequently, these spot welds are very durable and give a satisfactory mechanical and electrical connection. Any thermal expansions occurring are taken up by elongations and displacements of the helical winding 19. In this region, unhindered elongation is possible. Hence, unavoidable thermal expansions and displacements of the inner measuring conductor in relation to the remaining parts of the high-temperature neutron detector are withheld from the spot welds 22, so that they are relieved of load.

An insulating cover 23 in the form of a solid ceramic consisting of pure aluminium oxide has a length of 220 mm with an internal diameter of 2.3 mm and an external diameter of 2.9 mm. It surrounds the emitter 10, the connecting pin 14 and the insulating cylinder 8, and extends over an end portion of the outer conductor 2.

The collector 6 which encloses the neutron detector assembly is a cylindrical body having an external diameter of 3.5 mm, an internal diameter of 3.1 mm and a length of 240 mm, and it also consists of the aforesaid nickel-chromium alloy. The collector 6 is mounted on the intermediate member 5 and is fixedly connected thereto by a weld seam 24. Its distal end is closed by a cover 25 having a central bore 26, which receives a plug 27. Satisfactory sealing of the distal end is ensured by weld seams 28 and 29 respectively. The cover 25 and the plug 27 also consist of the aforesaid nickel-chromium alloy.

The assembly of the neutron detector from the individual parts has already been partially explained in the foregoing. The individual parts are each carefully cleaned and baked out. When the insulating block 8 has been connected to the sheathed measur ing line 1 and the connecting pin 14 has been connected to the emitter 10, the end 4' of the inner measuring conductor is pushed through the bore 17 so that about 5 mm for the bent-over section 21 project. The helical winding 19 is then firmly wound upon the connecting pin 14. The winding is continued until the conical tip 15 of the connecting pin 14 bears against the insulating block 8 and pushes it towards the outer conductor 2 so that the block 8 forms a closure for the end of the sheathed line 1.The section 21 of the inner measuring conductor is then bent over towards the axis of the connecting pin 14 and made fast by spot welds 22.

After the mounting of the insulating covering 23 and of the collector 6 with the cover 25, and after the weld seams 24 and 28 have been made, the arrangement is carefully baked out. The central bore 26 is then closed by the plug 27 and hermetically sealed by a further weld seam 29. The neutron detector is then complete. Its various parts are sealed within the collector 6.

The thermal expansion coefficient of the nickel-chromium alloy is about twice as great as that of vanadium. Consequently, considerable thermal stresses may arise in high-temperature operation and more particularly at alternating temperature loading.

However, these stresses are taken up by the helical winding 19, so that the spot welds 22 are relieved of the effects of thermal expansions. This prolongs the useful life of hightemperature neutron detector.

Of course, the emitter 10 may alternatively consist of a material other than vanadium.

It may consist of another material in which a (N, ,e) reaction takes place. It is also possible to use a material in which a (N, y) reaction can be induced.

The neutron detector shown can be used at high operating temperatures under heavy alternating thermal loading. The construction used gives a strong and reliable connection between the connecting pin and the emitter as well as a reliable and durable connection between the pin and the inner measuring conductor 4.

WHAT WE CLAIM IS: 1. A self-powered high-temperature neutron detector having an emitter connected to an inner measuring conductor of a sheathed line, characterised in that the emitter has a connecting pin and in that the inner measuring conductor extends in a helical coil around the connecting pin through a transverse bore in the pin and is made fast to the pin.

2. A neutron detector according to claim 1 wherein, for making the conductor fast with the pin a bent-over section of the conductor is welded to the outside surface of the pin.

3. A neutron detector according to claim 2 wherein a section of the inner measuring conductor adjoining its end is bent over at the outlet of the transverse bore to lie parallel to the axis of the connecting pin.

4. A detector according to claim 2 or claim 3 wherein the bent-over section of the inner measuring conductor is attached to the connecting pin by a number of spot welds.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

generatrix thereof). It lies against the outside surface of the pin. A number of laser spot welds 22 provide a mechanical and electrical connection between the bent-over section 21 and the connecting pin 14. Since the connecting pin 14 and the inner measuring conductor 4 consist of the same material, they have equal coefficients of expansion, so that the part 20 and the further bent-over section 21 of the inner measuring conductor 4 undergo no displacement in relation to the connection pin 14. Consequently, the spot welds 22 undergo no mechanical loading due to temperature stresses. In this way, loading in shear of the spot welds is avoided.

Consequently, these spot welds are very durable and give a satisfactory mechanical and electrical connection. Any thermal expansions occurring are taken up by elongations and displacements of the helical winding 19. In this region, unhindered elongation is possible. Hence, unavoidable thermal expansions and displacements of the inner measuring conductor in relation to the remaining parts of the high-temperature neutron detector are withheld from the spot welds 22, so that they are relieved of load.

An insulating cover 23 in the form of a solid ceramic consisting of pure aluminium oxide has a length of 220 mm with an internal diameter of 2.3 mm and an external diameter of 2.9 mm. It surrounds the emitter 10, the connecting pin 14 and the insulating cylinder 8, and extends over an end portion of the outer conductor 2.

The collector 6 which encloses the neutron detector assembly is a cylindrical body having an external diameter of 3.5 mm, an internal diameter of 3.1 mm and a length of 240 mm, and it also consists of the aforesaid nickel-chromium alloy. The collector 6 is mounted on the intermediate member 5 and is fixedly connected thereto by a weld seam 24. Its distal end is closed by a cover 25 having a central bore 26, which receives a plug 27. Satisfactory sealing of the distal end is ensured by weld seams 28 and 29 respectively. The cover 25 and the plug 27 also consist of the aforesaid nickel-chromium alloy.

The assembly of the neutron detector from the individual parts has already been partially explained in the foregoing. The individual parts are each carefully cleaned and baked out. When the insulating block 8 has been connected to the sheathed measur ing line 1 and the connecting pin 14 has been connected to the emitter 10, the end 4' of the inner measuring conductor is pushed through the bore 17 so that about 5 mm for the bent-over section 21 project. The helical winding 19 is then firmly wound upon the connecting pin 14. The winding is continued until the conical tip 15 of the connecting pin

14 bears against the insulating block 8 and pushes it towards the outer conductor 2 so that the block 8 forms a closure for the end of the sheathed line 1.The section 21 of the inner measuring conductor is then bent over towards the axis of the connecting pin 14 and made fast by spot welds 22.

After the mounting of the insulating covering 23 and of the collector 6 with the cover 25, and after the weld seams 24 and 28 have been made, the arrangement is carefully baked out. The central bore 26 is then closed by the plug 27 and hermetically sealed by a further weld seam 29. The neutron detector is then complete. Its various parts are sealed within the collector 6.

The thermal expansion coefficient of the nickel-chromium alloy is about twice as great as that of vanadium. Consequently, considerable thermal stresses may arise in high-temperature operation and more particularly at alternating temperature loading.

However, these stresses are taken up by the helical winding 19, so that the spot welds 22 are relieved of the effects of thermal expansions. This prolongs the useful life of hightemperature neutron detector.

Of course, the emitter 10 may alternatively consist of a material other than vanadium.

It may consist of another material in which a (N, ,e) reaction takes place. It is also possible to use a material in which a (N, y) reaction can be induced.

The neutron detector shown can be used at high operating temperatures under heavy alternating thermal loading. The construction used gives a strong and reliable connection between the connecting pin and the emitter as well as a reliable and durable connection between the pin and the inner measuring conductor 4.

WHAT WE CLAIM IS: 1. A self-powered high-temperature neutron detector having an emitter connected to an inner measuring conductor of a sheathed line, characterised in that the emitter has a connecting pin and in that the inner measuring conductor extends in a helical coil around the connecting pin through a transverse bore in the pin and is made fast to the pin.
2. A neutron detector according to claim 1 wherein, for making the conductor fast with the pin a bent-over section of the conductor is welded to the outside surface of the pin.
3. A neutron detector according to claim 2 wherein a section of the inner measuring conductor adjoining its end is bent over at the outlet of the transverse bore to lie parallel to the axis of the connecting pin.
4. A detector according to claim 2 or claim 3 wherein the bent-over section of the inner measuring conductor is attached to the connecting pin by a number of spot welds.
5. A neutron detector according to any

one of the preceding claims wherein the connecting pin has a conical tip at its free end, which bears on an insulating block, which in turn provides a closure at the end of the sheathed line.
6. A neutron detector according to any one of the preceding claims wherein the connecting pin is received in a bore in the emitter.
7. A neutron detector according to claim 6 wherein the emitter and the connecting pin have aligned transverse bores which receive a pin-form key.
8. A neutron detector according to claim 7 wherein the ends of the pin-form key are enlarged to form rivet heads.
9. A neutron detector according to claim 8 wherein the rivet heads of the pin-form key lie in conically widened portions of the transverse bore of the emitter.
10. A neutron detector according to any one of claims 6 to 9 wherein the emitter is cylindrical and the bore in it is axial.
11. A neutron detector according to any one of the preceding claims wherein an end portion of the sheathed line the connecting pin and the emitter are surrounded by a ceramic insulating cover, the whole assembly being sealingly enclosed by a cylindrical collector having a fluid-tight end seal.
12. A neutron detector suitable for use at high temperatures, substantially as herein described with reference to the accompanying drawing.