DE2427662A1 - HEAT SIMULATOR - Google Patents

Description

9/9 7 RR Background of the Invention

The invention relates generally to thermal simulators for nuclear reactor fuel rods that enable the thermal properties of a Nuclear reactor fuel rod and its assembly can be simulated.

So far, thermal simulators for nuclear fuel rods are in the way was built in that an electrical heating element was supported in an electrically insulated manner within a stainless steel pipe jacket simulate the thermal properties of a nuclear reactor fuel rod. Such rods are approximately 6.35 mm (0.250 ") outside diameter and 2.4 to 6.2 m (8 to 20 feet) long; they point a hot zone or heated piece along part of the length of the rod that is approximately 91.5 cm (3 feet) long.

With these electrically heated fuel rod simulators existed the high resistance heating element made of a platinum alloy, the heating element being electrically insulating within the pipe jacket was supported with the help of boron nitride particles, which was very high Density compressed by applying relatively high pressure to the packing material of about 4550 to 7030 kgf / cm (65,000 to 100,000 PSI)

During operation, electrical current is sent through the resistance heating element in order to bring the temperature of the heating element to about 2300 ^° C. At this temperature the boron nitride must be very pure in order to prevent undesirable reactions between the hot heating element and impurities in the boron nitride. The boron nitride has a relatively high thermal conductivity and is used to conduct heat from the heating element to the stainless steel jacket. A relatively large number, up to 350 pieces, of such fuel rod simulators are packed relatively tightly in a fuel assembly subassembly and a coolant such as water, gas or liquid sodium is passed through the spaces between adjacent fuel rod simulators

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sent to cool them.-In the case of sodium as the coolant, the outer surface of the fuel rod simulator is at a temperature of at least 900 ^° C. The normal heat flow that is required by the fuel rod simulator, normally falls in the range of 300 W / cm ² to 1000 W / cm ² .

Some of the problems encountered in the manufacture and use of such resistance heater nuclear fuel rod simulators are that manufacturing yields are relatively low because it is difficult to precisely center the heating element relative to the surrounding jacket. This is necessary to avoid the formation of hot spots due to uneven spacing. If the distance is not precise, it is also possible that the heating element is electrically short-circuited to the surrounding jacket. Due to the relatively poor thermal connection between the heating element and the jacket surrounding it through the intermediate layer of boron nitride, there is also a relatively large thermal gradient between these parts, so that the heater has to work at the relatively high temperature of around 2300 ^° C., around 900 ^° C. on the outer surface of the surrounding stainless steel jacket.

It has also been suggested to use the outer jacket of a simulated one To heat the nuclear fuel rod by electron bombardment from a centrally located hot cathode emitter, the inside an evacuated piece of the fuel rod is included. A problem with this solution is that it has been shown has that significant amounts of hydrogen in the coolant circuit en around the outside of the fuel rod simulator in question to find oneself. When the fuel rods operate at higher temperatures, hydrogen diffuses through the walls of the fuel rod in the evacuated central area, so that the vacuum is created destroyed and the electron bombardment heater put out of operation. The hydrogen diffusion rate is so great and the elongated fuel elements have such a high resistance to the flow of gas that it is not possible to do so

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Pump out fuel rods with a vacuum pump in such a way that the vacuum is maintained.

Summary of the invention

The main aim of the invention is to provide an improved thermal simulator for nuclear reactor fuel rods, and in particular an improved heater for such simulators.

In accordance with one feature of the invention, the outer jacket becomes a thermal simulator for a nuclear reactor fuel rod heated by means of a glow discharge that is built up inside the tubular jacket in order to heat it to operating temperature.

According to a further feature of the invention, a glow discharge heater for a thermal simulator for a nuclear reactor fuel rod an electrode structure which is arranged within a tubular jacket in an electrically insulated manner from the latter, the jacket is hermetically sealed and filled with a gas carrying a glow discharge, for example hydrogen or helium.

According to a further feature of the invention, the glow discharge heater of a nuclear reactor fuel rod simulator has a central electrode structure and an outer electrode structure, and are these electrode structures are both made of tungsten or molybdenum or their alloys; for example Mo-Re, Pt, etc., where the outer electrode is arranged in heat exchange relationship with the outer shell of the fuel rod simulator.

Further features and advantages of the invention emerge from the following description in conjunction with the drawing; show it:

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Fig. 1 is a simplified side view of a nuclear reactor fuel

Rod unit as used in the core of a nuclear reactor;

Figure 2 is a cross-section through one of the nuclear reactor / fuel rod subassemblies according to Fig. 1;

Fig. 3 is a section through a thermal simulator for a

Nuclear fuel rod with the heating structure according to the ■ 'invention;

4 shows a schematic longitudinal section, partly as a block diagram, a thermal simulator for a nuclear reactor fuel rod according to the invention;

FIG. 5 shows details of the part enclosed by the line 5-5 in FIG. 4; FIG. and

6 shows the breakdown voltage in V plotted against the size pd in Torr centimeters for the glow discharge heater according to the invention, with hydrogen being the gas maintaining the glow discharge, ρ the pressure of the discharge in Torr and d the distance in cm between the anode and the Cathode are in the glow discharge area.

In Fig. 1, the fuel-rod unit of a nuclear reactor is shown in a greatly simplified form. In particular, the Fuel rod assembly 11 comprises a collection of elongated nuclear fuel rod subassemblies 12 which, for example, have a hexagonal cross-section, as shown in FIG. 2. The subunits 12 each have a plurality, up to 350, individual ones Fuel rods 13 on. The fuel rods 13 are in a tightly packed geometry within a hexagonal envelope 14 In a typical embodiment, there are about 100 fuel sub-units 12 within the fuel unit 11 of the core of the nuclear reactor. The subunits

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can be moved axially within the reactor core. The fuel rods 13 are typically between 2.4 and 9.2 m (8 to 20 feet) long and about 6.4 mm (1/4 ") in outside diameter when sodium-cooled reactors are considered (water-cooled reactors may be A flowing coolant, for example gas, water or sodium, flows axially in the bundle of fuel rods 13 in the spaces between adjacent rods. In a typical embodiment, the distance between adjacent fuel rods 13 at the point of smallest distance is 17 up to 32 % of the rod diameter to define the coolant channels in between.

In a typical reactor, only part of the length of the fuel rods is Located within the hot zone of the reactor so that a section of only about 92 cm (3 feet) the length of the fuel rods exposed to the intense heat of the nuclear reactor.

In Figures 3 and 4, the thermal simulator 13 is for a nuclear reactor fuel rod shown according to the invention. More specifically, the simulator stick 13 has a glow discharge heater structure 15 to heat the 'simulator rod 13 over part of its length corresponding to the hot zone of the reactor to be simulated. The simulator rod 13 has an outer, tubular jacket 16, for example made of stainless steel, with a wall thickness of 0.254 to 0.381 mm (0.010 to 0.015 ") and an outside diameter The stainless steel tube 16 has an overall length of 2.4 to 6.2 m (8 to 20 feet).

Two electrical insulator units 17 are used to create a central electrode " Structure 18 of the glow discharge heater 15 to electrically isolate. In addition, the isolator units 17 are hermetically sealed between the heater electrode 18 and the inner wall of the jacket tube 16 are inserted at the two ends of the hot zone 19. The.space between the inner electrode 18 and the jacket 16 surrounding it is evacuated

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and filled with a gas carrying a glow discharge, for example Hydrogen or helium.

A glow discharge power supply 21 is between the center electrode structure 18 and the outer jacket 16 are connected to the correct voltage and current to the glow discharge area of the heater 15 to deliver ·. One side of the power supply is earthed, as is the outer conductor or the stainless steel jacket 16 of the fuel rod simulator 13. The center conductor 18 of the glow discharge heater 15 is connected to the power supply via a insulated line 22 connected, which runs axially in the tubular fuel rod 13.

One or more thermocouples 23 are thermally conductive to the inside of the stainless steel jacket 16 within the hot zone area 19 arranged to measure the temperature of the fuel rod simulator 13. The lines 25 for the thermocouples extend axially within tube 16 and through upper insulator body 17 'to corresponding ones of thermocouples 23. One Thermocouple reading circuit 24 is connected to lines 25, to read the temperature of the nuclear fuel simulator 13.

The distance between the center electrode 18 and the inner wall of the tubular jacket 16 in glow discharge heater 15 is for operation dimensioned in the abnormal area of the glow discharge, as he is through the hatched area of the graphic representation in Figure 6 is indicated. This area of glow discharge is considered abnormal The area in which the glow discharge current increases when the applied voltage increases. In this area the discharge will be the upper limit of the glow discharge current by overheating a part of the glow discharge heater 15 or set by a transition of the glow discharge into an arc discharge. In a preferred Operating range, the product pd is in the range from 0.2 to 1.6 Torr centimeters at operating temperature.

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It is indeed hydrogen as the gas carrying the glow discharge provided in the illustration according to FIG. 6, but other gases are also suitable, in particular helium. Hydrogen and helium are preferred because they are relatively light atoms and these gases cause the least amount of spray. Spraying is undesirable because it erodes the electrodes and the sprayed material can cover the insulators 17 so that they are put out of operation. In a typical embodiment, the inner electrode 18 consists of a tungsten wire with about 1.52 mm (0.060 ") outside diameter, the inside wall has of the jacket tube 16 has a diameter of 5.33 mm (0.210 "), so that there is a distance of about 0.2 cm, and a pressure of the Hydrogen or helium as fill gas of 2.5 torr at operating temperature. The gas filling pressure is preferably between 1 and 10 Torr at the operating temperature of the glow discharge. The operating temperature of the glow discharge is generally between 3.5- and 5 times the room temperature. The gas filling pressure under normal conditions of temperature and pressure is then between the 3.5th and 5th part of the operating pressure. In this context the desired filling pressure preferably also takes this into account Outgassing of the metal parts of the pipe jacket. This outgassing serves as a source of gas, which has to be added to the original gas filling Time and temperature are added and depend on the history of the metal parts that make up the vessel. The outgassing contribution is best determined empirically and taken into account in the original gas filling.

In one mode of operation, the power supply is polarized so that the central electrode 18 becomes the cathode and the outer electrode 16 becomes the anode. In this way, high-energy electrons serve to to provide the heating of the outer jacket 16, while ions incident on the cathode 18 serve to discharge the discharge entertained by delivering secondary electrons to the cathode. Different types of glow discharges and their heating properties

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are in an article by R.A. Dugdale "The Application of the

Glow Discharge to Material Processing, "Journal of Material Science I.

(1966), pages 160-169.

Other modes of operation of a glow discharge are, for example Applying alternating voltage and current via the gas filling between the electrodes or reversing the polarity of the glow discharge supplied DC voltage, so that the middle electrode 18 to The anode and the outer jacket 16 become the cathode.

In Fig. 5 a detail of the upper seal of the glow discharge heater 15 is shown. To be more precise, the insulator body 17 ¹ , for example made of thorodioxide, sapphire, alumina or beryl alumina, is by means of a. Metal flange 31 is hermetically sealed to the center electrode 18. The flange 31 is inserted tightly, for example, in that it is metallized on the outer lip 32 of a central bore in the insulator body 17 ', through which the central conductor passes with noticeable play to the glow discharge area 34 of the glow discharge heater 15. The flange 31 is hermetically sealed to the center electrode 18, for example by Heliarc welding.

The outer periphery of the insulator body 17 is hermetically sealed with connected to the inner wall of the jacket tube 16, for example by metallizing the insulator body 17 and soldering it to the inner wall of the tube 16. In a preferred embodiment, the Pipe wall 16 has a lining 37 made of tungsten or molybdenum or their alloys with Re, Pt etc. on the inside. the In a typical embodiment, liner 37 has a wall thickness of 0.13 to 0.25 mm (0.005 to 0.010 ") and is closely connected to the inner wall of the stainless steel part of the piston 16, for example by the fact that the outer stainless Steel wall 16 is pulled onto the liner pipe 37. The lining 37 serves to have a more heat resistant lining available to withstand the particle bombardment

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that is received from the glow discharge to them heat up, and also has a very low hydrogen diffusion rate.

In another embodiment, helium or others are used Glow discharge carrying gases are used as gas filling, so that Hydrogen diffusion through the walls of the vessel is not so important is. In this case, the lining 37 is omitted, see above that the complexity of the simulator construction is significantly reduced and the heat transfer through the outer wall 16 is improved.

The insulator body 17 has an axially directed, cylindrical Part 38 protruding into the glow discharge area 34 to to obtain a long leakage current path, so that a short circuit of the inner electrode 18 with the outer electrode or the jacket 16 is prevented. In addition, the tubular protrusion 38 is preferably corrugated in order to extend the creepage distance even further and to minimize the possibility of sprayed material being deposited in the valleys of the corrugated portion of the structure.

A number of metal feedthrough pins 39 ^{extend axially through the insulator body 17 1} to establish an electrical connection to the thermocouples within the glow discharge chamber 34. The lead-through pins 39 are hermetically inserted into the insulator body and are connected to the lines 25, for example by welding. Insulator beads 41 are threaded onto the lines 25 in order to create electrical insulation between the thermocouple lines and the earthed jacket 16. A heat-resistant tube insulator 42 also surrounds the inner conductor 18 and line 22 to isolate the line and center conductor 18 from the grounded tube wall 16.

In another embodiment is the central tungsten electrode

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hollow and partially filled with silver so that when the central electrode 18 is heated to operating temperature, the silver evaporates and jils heat pipe acts to make a uniform Temperature distribution within the central electrode 18 to so that hot spots are prevented.

The benefit of a glow discharge heated nuclear reactor fuel rod thermal simulator According to the invention, in contrast to resistance-heated simulators, is that the Simulator structure according to the invention is easier to manufacture and the tolerances are less critical. Furthermore you can higher heat flux densities are easier in the construction according to the invention be included. This latter feature is particularly important for reactor fuel rod thermal simulators for fast breeders where the heat flow requirement is several times that for water-cooled or gas-cooled fuel rod simulators amounts to.

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Claims (10)

Claims 1.y Heat simulator for nuclear reactor fuel rods, consisting of a tubular structure which is to be heated to simulate the thermal properties of a nuclear reactor fuel rod, and a heating device which is arranged in heat exchange relationship with the tubular structure in order to convert this tubular structure to nuclear fuel To heat rod simulation temperature, characterized in that the heating device has devices with which a glow discharge can be built up within the tubular structure in order to heat the tubular structure to the nuclear fuel rod simulation temperature. 2. Simulator according to claim I _1, characterized in that the glow discharge build-up device has an electrode structure which is arranged in the tubular structure in an electrically insulated manner from this, a device with which a piece of the tubular structure containing the electrode structure is hermetically sealed, and that a gas filling carrying a glow discharge fills this sealed part of the tubular structure and the spaces between the electrode structure and the tubular structure. 3. Simulator according to claim 2, characterized. that a power supply is provided with which an electrical voltage carrying a glow discharge and a corresponding current is applied between the tubular structure and the internal electrode structure in order to support a glow discharge in the gas filling between the internal electrode structure and the tubular structure. 4. Simulator according to claim 2 or 3, characterized in that the gas filling consists of hydrogen and / or helium. ... / A2 409881/041 1 5. Simulator according to one of claims 1 to 4, characterized in that the electrode structure made of tungsten, molybdenum and / or their - Alloys. 6. A simulator according to any one of claims 1 to 5, characterized in that the tubular structure comprises outer and inner concentric tubes arranged in heat exchange relationship with one another, and that the inner tube consists of tungsten, molybdenum and / or their alloys. 7. Simulator according to claim 6, characterized in that the outer tube is made of stainless steel. 8. Simulator according to one of claims 1 to 7, characterized in that the gas filling has a pressure within the sealed part der.Rohrstruktur which is between 10 and 10 Torr at the operating temperature of the fuel rod simulator. 9. Simulator according to one of claims 1 to 8, characterized in that the product pd at the operating temperature of the fuel-rod simulator is between 0.3 and 1.0 Torr centimeters, wherein d the distance in cm between the electrode structure and the latter surrounding tubular structure in the glow discharge area and ρ - the pressure of the glow discharge gas filling in Torr at the operating temperature of the glow discharge. 10. Simulator according to one of claims 2 to 9, characterized in that the electrode structure is hollow and silver is arranged in the hollow electrode, so that a uniform temperature distribution is achieved within the hollow electrode. 4098817041 1 L e r s e i t e