WO2005031283A1

WO2005031283A1 - Device and method for measuring the level of glass in a vitrification furnace for radioactive wastes

Info

Publication number: WO2005031283A1
Application number: PCT/EP2004/010508
Authority: WO
Inventors: Winfried Tobie; Wolfgang GRÜNEWALD; Günther Roth
Original assignee: Forschungszentrum Karlsruhe Gmbh
Priority date: 2003-09-23
Filing date: 2004-09-18
Publication date: 2005-04-07
Also published as: DE10343852A1

Abstract

The invention relates to a method for measuring the level of glass in a vitrification furnace for radioactive wastes. The aim of the invention is to provide a corresponding method that is uncomplicated while being more reliable. To this end, the inventive method consists of leading a first flow of gas via an opening of a line into the vitrification furnace just beneath a maximally permitted filling level, and a second flow of gas via a reference line into the gas chamber of the vitrification furnace above the predetermined maximally permitted filling level, and consists of measuring a pressure difference between the first and the second flow of gas, whereby a rising glass level completely closes the opening starting from a predetermined filling level, and after the opening has been closed, a predetermined pressure difference level indicates that the maximally permitted filling level has been reached, whereby the end of the line inside the vitrification furnace is downwardly open and has a horizontally oriented edge area.

Description

Device and method for glass level measurement in a glazing furnace for radioactive waste

The present invention relates to a method for glass level control in a glazing furnace for radioactive waste according to the first and a device for carrying out the aforementioned level control according to the fifth claim.

The commercial application of nuclear technology for energy generation poses the problem of the disposal of highly radioactive waste. As a result of the nuclear conversion through fission and neutron capture, a large number of highly radioactive fission products such as artificial isotopes or actimides are generated from the nuclear fuel in a reactor. The fission products gradually accumulating in the course of fuel consumption are of no use for the process of energy generation and represent a radioactive ballast in the remaining fuel matrix, which must be disposed of after the fuel elements have been removed from the reactor. Several technologies have been developed for the disposal of the highly radioactive fission products enriched in the spent nuclear fuel.

With regard to the disposal of spent nuclear fuel, general reference is made to [1].

One of these technologies is based on the glazing of radioactive waste [2]. It involves cnemic separation of the fission products from the reusable part of the fuel elements. In a first step, the fuel elements with boiling nitric acid are dissolved, followed by multi-stage extraction processes in which .vie uranium and plutonium are extracted from the solution. Let the remaining nitric acid, ie aqueous solution, be concentrated with the highly radioactive fission products and the non-volatile radioactive components incorporated in glass, ie by glazing in a solid form that is suitable for transport and disposal - bergefuhrt. Glass not only has an extremely good solubility for the radioactive fission products, but also a solidification area without a sudden phase change during the transition between the liquid and solid phases, which practically prevents re-excretion of fission products already dissolved in the glass.

Due to the high level of radioactive radiation, the glazing takes place shielded in a remote-controlled glazing furnace. In the glazing furnace, a glass melt is heated to a temperature above the solidification range in a closed melting vessel by the flow of electrical current. The solution with the highly radioactive fission products is continuously fed into the melting vessel via a feed line, specifically into the gas space above the glass melt.While the non-radioactive aqueous solution components evaporate on the surface of the melting bath and can escape from the melting vessel via an exhaust line, the non-evaporable radioactive components after previous chemical calcination in a process zone in the area of the melt pool surface integrated into the glass melt. At the lower end of the melting vessel there is also an outlet channel for continuous removal of the now radioactive glass melt, the so-called tapping of the glass melt. Furthermore, a feed line for filling glass into the melting vessel is required. When a predetermined glass level is displayed, the removal from the melting furnace is initiated into a mold. Knowledge of the full glass level is therefore an important process parameter when glazing radioactive substances.

Due to the extreme thermal and corrosive conditions 1 melting vessel, reliable glass full level measurement in a glazing furnace for radioactive waste has so far been an unsolved problem despite various approaches.

An electrical system for glass fullness control is described as an example in [3]. It includes a separately heated bed tenkammer of the melting vessel, in which the glass melt surface vc of the process zone is decoupled and in which three electrodes are inserted. Depending on the height of the glass full level, the side chamber and therefore the electrodes are more or less immersed in the glass melt, the level being determined by a simple resistance measurement between c

Have electrodes determined and monitored.

However, this system is very complex, since a special shape of the melting vessel and its own heating for the side chamber e are required. Furthermore, the solution proved to be comparatively prone to failure due to an increased tendency of the melt to form slags and due to the changing composition of the glass melt, especially in the secondary chamber. Reproducibility is also significantly affected.

An optical system is described as an example in [4]. It comprises a neutron source and a neutron receiver, which are positioned at the height of the glass melt. The connecting neutron beam is immersed in the molten glass as the fill level increases, where the neutrons are increasingly absorbed by the molten glass. Here too, installation and operation are prone to failure and comparatively expensive, given the risk of contamination of the transmitter and receiver alone.

Proceeding from this, it is an object of the invention to propose a method for glass full level measurement in a glazing furnace for radioactive waste, which has a reduced reliability with little effort. Another object of the invention relates to a device for level measurement, which also has a simple structure.

The object is achieved by a method with the features of the first patent claim and a device with the features of the fifth patent claim. Subclaims indicate preferred executions of the method and the device. The invention relates to a method and an apparatus for

Glass level control in a glazing furnace for radioactive waste, based on a pneumatic differential pressure measurement ar two places in the glazing furnace. Here, a first gas stream is introduced via a line into the melting vessel of the glazing furnace just below a maximum permissible fill level, and a second gas stream is fed via a reference line into the gas chamber of the melting vessel in the glazing furnace, i.e. always above the specified maximum permissible fill level, ideally at the upper end of the sealed melting vessel. Both gas streams are continuous and are preferably set to a constant, equally large flow rate. The pressure in the line and the reference line are measured in cold areas outside the glazing furnace on the first and second gas streams. In the simplest embodiment, the differential pressure can be determined by simply subtracting the two measurement signals.

If the glass full level in the glazing furnace rises close to the maximum permissible level, the opening is completely immersed, i.e. the exit opening of the first gas stream into the glass melt, thus interrupting and backing up the first gas stream. As a result, there is a measurable increase in the static pressure in the first gas stream and thus a significant change in the measured differential pressure, which until then had ideally been close to zero. The increasing differential pressure signal can be easily recognized by setting a limit for the pressure differential measurement signal and a corresponding reaction, e.g. an opening of the drain channel for the removal of the gas melt from the melting vessel (tapping the glass melt) can be initiated.

An opening that is open towards the bottom is advantageous, ie the end of the line for the first gas stream, which also has a horizontally oriented edge region. In this way, the entire opening is completely immersed riding with a very low level difference. In addition, this design makes it more difficult for glass melt or reaction products to penetrate the opening, thereby considerably reducing the risk of the opening gradually becoming clogged. It also proves to be particularly advantageous if the cross-section of the opening has an enlarged cross-section compared to that of the line or if the end of the line has a bell shape in the melting vessel, for example.

The aforementioned enlarged cross-section of the opening has the following advantageous effects: on the one hand, this significantly reduces the risk of clogging of the opening by viscous intermediate reaction products of the glazing process, which form in a glazing furnace as a layer on the entire liquid glass surface and for the glazing process, ie. H. in the case of incorporation of fission products in the glass with simultaneous controlled evaporation of the non-radioactive volatile components are absolutely necessary. On the other hand, the larger cross-section makes the glass full level check more sensitive. Since a differential printer is only increased by completely closing the opening, i.e. by completely immersing or wetting the edge of the opening, an influence of the topography differences of the above-mentioned layer can be significantly reduced simply by extending the circular immersion line.

It goes without saying that the portions of the line and reference line located in the melting vessel consist of a material which is corrosion-resistant and high-temperature-resistant and which prevails there.

Static pressures in the line and reference line, ie apart from the gas streams in flow-free areas, are preferably used for the differential pressure measurement. The differential pressure between the first and second gas stream is as a static differential pressure, which influences the flow on the

Differential pressure eliminated. Within the scope of the invention, a technical implementation of this feature is proposed by positioning the pressure sensors at the end of a branch from the line and reference line. A gas cushion is also formed in the branch, which acts as a filter between the gas and the pressure transducer, for example, buffering short-term pressure peaks. This filter effect can also be optimized depending on the application, whereby an increasingly large volume of the branch not only buffers increasingly larger pressure jumps, but also extends the time between the occurrence and the detection of a pressure change.

The invention is explained below using an exemplary embodiment 6. Show it

1 a and b are schematic representations of a glazing furnace for radioactive waste with a device for checking the glass fullness at different glass fullness heights,

Fig. 2 is a detailed view of the line with a downward opening in the melting vessel, and

Fig. 3 shows the time course of the differential pressure signal during the increase of the glass full level with subsequent tapping of the glass melt.

A glazing furnace of the type mentioned at the outset consists, as shown in FIGS. A and b, of a melting vessel 1 with Joule heating 2 for a glass melt 3 and a gas space lying thereover. The glass full level 5 of the glass melt 2 is heated during glazing. solution process between a minimum and a maximum level β or 7. This is done by appropriately controlling the inflow of the solution with the highly radioactive fission products sc such as glass and the tapping of the glass melt. For this purpose, there is a corresponding feed pipe 8 fu at the uppermost point of the melting vessel the solution and glass (in practice two separate supply lines in the de

Gas space) and at the lowest point of the melting vessel, a heated outlet channel 9 is provided for tapping the glass melt. In order to derive the vei vaporizing non-radioactive constituents of the solution as process exhaust gas, an exhaust gas line 10 from the gas space 4 is additionally inserted into the melting vessel 1.

As shown in FIGS. 1 a and b, the device for checking the fill level of the glass protrudes into the gas space 4 from above. It comprises a line 11 and a reference line 12, which flows into the melting vessel 1 of the glazing furnace. While the reference line merges into the gas space near the highest point, the lower open end of the line ends just below or in the area of maximum level 7. The opening has a horizontally aligned edge area, so: the process of completely immersing the opening in the melt as the glass full level rises in the shortest possible time and, on the other hand, lowering the glass full level to below the opening also releases the full cross section of the opening in the shortest possible time.

The exact positioning of the opening relative to the maximum le vei 7 can be optimized. It is to be determined in such a way that the upper level 7 is reliably exceeded by the melt even at the maximum possible rate of increase of the glass full level and taking into account the abovementioned generally described buffer effect in the branches 17 in front of the pressure transducers 16 and the reaction times for tapping the glass melt is avoidable. In a tried-and-tested embodiment, the opening is arranged approximately 10 mm below the maximum level permitted.

In an advantageous embodiment, the lower end of line 11 consists of a bell-shaped extension 13. In principle, all metals and ceramics resistant to high-temperature corrosion are suitable as materials for line 11, reference line 12 and bell-shaped extension 13. in the As part of the exemplary embodiment, a nickel-based

Alloy INCONEL 690 used with good result.

Line 11 and reference line 12 serve to introduce a gas flow into the gas space 4, both gas flows in the sense of high accuracy and reproducibility of the method with comparative pressure measurement ideally being identical in composition, volume flow and pressure with the aid of a suitable center. These means include a gas supply unit for the provision of a constant and iso ble gas volume flow with connection to the line and the reference line. An inert gas is particularly suitable, but also a Ga which has a neutral effect on the glazing process, e.g. Nitrogen, argon, but also compressed air, with the volume flows being adjustable via flow limiting valves 14 and the pressure via corresponding pressure reducers 15 within the gas supply unit outside the glazing furnace.

In the exemplary embodiment, a compressed air stream with an identical composition was selected as the first and second gas stream, which was in each case introduced into the line and into the reference line at a pressure of 0.5 bar and 20 Nl / 1 volume flow.

Outside of the glazing unit, the line and reference line are each equipped with a pressure sensor 16, which at the end is connected to the line and reference line in each case by a branch 17 which is decoupled from the gas flow. Pressure transducers and branches are representative of general means for determining the internal pressures in line 11 and reference line 12 as well as the pressure difference and for compensating for the differential pressure.

2 shows, as an example of a line lead-through, a lead-through of line 11 with bell-shaped extension 1. ^" into the new fullness measurement as a detailed construction drawing in a sectional view. The device is placed in a gas-tight and remote-controlled manner on the furnace top 21 by a ceramic bushing 19 in an electrically insulated manner on a furnace neck 20. Like the bell-shaped extension 13, the gas supply line consists of a high-temperature resistant INCONEL 690 nickel-based alloy

1 a shows the glazing furnace in a state in which the glass full height is below the opening of the bell-shaped extension 13. The opening is not closed by the glass melt. Since both the first and the second gas stream can escape unhindered from the exhaust pipe from the melting vessel, the differential pressure Δp is approximately zero.

1b, on the other hand, shows the glazing furnace in a state in which the glass level is above the opening of the bell-shaped extension 13. The opening is thus closed by the glass melt, so that the first gas flow is interrupted and accumulated in line 11; the differential pressure .DELTA.p thus increases and is of great magnitude zero.

The time course of the differential pressure Δp when the glass level rises with subsequent tapping of the glass is shown in the context of the advantageous embodiment as a Δp / t diagram in FIG. 3. If the full glass level is below the opening, this will not be closed by the glass melt; the differential pressure Δp is approximately zero (range I, noise). After the glass full level has risen, the opening closes briefly but gradually, leading to a moderate increase in the differential pressure Δp (area II). If the opening is completely closed by the glass melt, the differential pressure Δp increases continuously (area III) in order to initiate a tapping of the glass melt at a limit (here around 7.5 mbar). When tapping, the glass level drops rapidly to the initial state (area IV). reference numeral

1 melting pot

2 joule heating

3 glass melt

4 gas space

5 glass fullstand

6 minimum levels

7 maximum level

8 inlet pipe

9 drain channel

10 exhaust pipe

11 line

12 reference line

13 bell-shaped extension

14 flow limiting valve

15 pressure reducers

16 pressure transducers

17 junction

18 gas flow

19 ceramic bushing

20 furnace nozzles

21 furnace ceiling

Claims

Claims:

1.Procedure for checking the glass level in a glazing furnace for radioactive waste, comprising the following process steps: a) introducing a first gas stream via an opening of a line into the glazing furnace just below a maximum permissible fill level, b) introducing a second gas stream via a reference line into the gas space of the glazing furnace above: half of the specified maximum permissible fill level, and c) measurement of a differential pressure between the first and the second gas stream, an increasing glass fill level completely closing the opening from a specified full level and a predetermined differential pressure level after closing the opening When the maximum permissible fill level is reached, d) the end of the line in the glazing furnace is open at the bottom and has a horizontally aligned edge area.

2. The method according to claim 1, characterized in that each of the first and second gas streams is a compressed air stream having an i dentic composition and is set via a valve by a pressure reducer to 0.5 bar pressure and 20 Nl / h volume flow.

3. The method according to claim 1 or 2, characterized in that the measurement of the differential pressure between the first and the second gas stream is carried out apart from the gas streams in current-free areas.

4. The method according to any one of the preceding claims, dadurci characterized in that the opening about 10 mm below the m < maximum permissible fill level is arranged.

5. Device for checking the level of glass in a glazing furnace for radioactive waste, comprising a) a line which opens into the glazing furnace and ends just below a maximum permissible fill level, for introducing a gas stream, b) a reference line which merges into the glazing furnace and ends above the specified maximum permissible fill level for introducing a gas flow, c) a gas supply unit for providing a constant and isobaric gas volume flow with connection to the line and the reference line, and d) means for determining the internal pressures in the line around the reference line and the pressure difference and for comparing the differential pressure, e) the end of the line in the glazing furnace being open at the bottom and having a horizontally oriented edge region.

6. The device according to claim 5, characterized in that the end of the line in the glazing furnace has an opening with an enlarged line cross-section.

7. The device according to claim 5 or 6, characterized in that the end of the line is bell-shaped and the area of the opening is made of a high-temperature resistant nickel base alloy.

8. Device according to one of claims 5 to 7, characterized in that the opening is approx. 10 mm below the maximum permitted level. Device according to one of claims 5 to 8, characterized in that the means for determining the internal pressure include pressure transducers for detecting static pressures which, apart from the gas streams, are arranged at pipe sockets which branch off from the pipe or the reference pipe outside the glazing furnace.