EP0736879A1 - Process and device for the continual monitoring of dust activity - Google Patents

Abstract

The process comprises continuously measuring the quantity of dust and its activity as it passes through the ventilation duct (1) of the oven. Also claimed is an appts. that includes an emitter of light (4), e.g. a laser, producing a beam (F) crossing the duct (1) and a light detector (6), e.g. a photovoltaic cell, placed opposite to the beam (F) to detect the density of the dust passing through the duct (1). gamma -radiation detectors (9) are placed around the duct (1) to detect the activity of the dust. A data acquisition box (11) and computer (20) allow the sequencing and treatment of the measurements in real time during the circulation of the fluid inside the duct (1).

Description

Field of the invention

The invention relates to the control of the activity of dust emitted in particular during the dismantling of nuclear installations, in particular resulting from the melting of various scrap metals.

Prior art and problem posed

During the dismantling of basic nuclear installations, pig iron is often melted using large-capacity (6 Megawatts) arc or induction furnaces. This type of oven is also equipped with ventilation which allows it to operate in a confined environment. Three stages of filters then make it possible to recover all of the dust produced during this type of heat treatment.

There is a need to control, or at least to appreciate the events taking place during the merger. It is known to carry out these operations by upstream and downstream balances with respect to the merger phase. Only temperature measurements on heat transfer fluids are currently permanent and carried out during melting. The analyzes during melting of the particles and dust emitted are carried out by sampling.

However, it seems very useful to be able to have information in real time on the kinetics of emission of this dust and, if possible, on the situation of the activity in relation to this emission. This could indeed make it possible to understand what are the mechanisms decontamination and consequently act on the progress of the merger in question.

The object of the invention is therefore to remedy this drawback by implementing such an analysis.

Summary of the invention

• mesurer en continu la quantité de poussières pendant leur passage dans une gaine de ventilation du four ; et
• mesurer l'activité liée à ces poussières.

To this end, a first main object of the invention is a method for controlling the activity of dust emitted during the melting, in an oven, of irradiated elements, the method consisting in:

• continuously measure the amount of dust as it passes through an oven ventilation duct; and
• measure the activity linked to this dust.

The measurement of the quantity of dust can be done by measuring the density, preferably with a photovoltaic cell, while the measurement of the activity can be done with a gamma radiation detector coupled to a multi-channel analyzer.

• un émetteur de rayonnement lumineux placé en regard d'une première paroi de la gaine ;
• un premier détecteur de rayonnement lumineux placé en regard de la paroi opposée à la première paroi de la gaine et délivrant un signal caractéristique de la densité du milieu traversé à l'intérieur de la gaine ;
• au moins un deuxième détecteur, de rayonnement gamma placé en regard de la gaine et délivrant un signal caractéristique de l'activité du milieu traversé à l'intérieur de la gaine ; et
• des moyens d'acquisition et de traitement connectés au premier et au(x) deuxième(s) détecteurs pour recevoir lesdits signaux caractéristiques.

A second main object of the invention is a device for controlling the activity of dust emitted during the melting, in an oven, of irradiated materials and evolving in a ventilation duct, the device comprising:

• a light emitter placed opposite a first wall of the sheath;
• a first light radiation detector placed opposite the wall opposite the first wall of the sheath and delivering a signal characteristic of the density of the medium traversed inside the sheath;
• at least a second detector, of gamma radiation placed opposite the sheath and delivering a signal characteristic of the activity of the medium crossed inside the sheath; and
• acquisition and processing means connected to the first and to the second detector (s) to receive said characteristic signals.

Preferably, the transmitter is a laser diode.

Likewise, the first detector advantageously consists of a photovoltaic cell.

In correspondence, a multi-channel analyzer can be used between the second detector (s) and the acquisition and processing means.

• un boîtier d'acquisition de commande connecté directement au(x) détecteur(s) ; et
• une unité de commande pour le traitement des signaux reçus et la commande des détecteurs.

The latter preferably consist of:

• a command acquisition unit connected directly to the detector (s); and
• a control unit for processing the received signals and controlling the detectors.

• d'un moyen de mesure de tension électrique ;
• d'une mémoire pour enregistrer des valeurs de tension ;
• d'un moyen de calcul de la moyenne des valeurs de tensions électriques ; et
• d'un chronomètre.

The acquisition and processing unit is preferably made up for each detector:

• an electrical voltage measurement means;
• a memory for storing voltage values;
• means for calculating the average of the voltage values; and
• a stopwatch.

A temperature sensor placed in the sheath can complete this device.

List of Figures

• figure 1, deux courbes relatives au déroulement de l'émission de l'activité à mesurer, lors de la fusion de matériaux ;
• figure 2, un schéma du dispositif selon l'invention ; et
• figure 3, un ordinogramme du procédé selon l'invention.

The invention and its technical characteristics will be better understood on reading the following description, supplemented by three figures representing respectively:

• FIG. 1, two curves relating to the progress of the emission of the activity to be measured, during the fusion of materials;
• Figure 2, a diagram of the device according to the invention; and
• Figure 3, a flowchart of the method according to the invention.

Detailed description of an embodiment of the invention

Referring to Figure 1, the implementation of the method according to the invention is based on the examination of the kinetics of dust emission at the time of fusion. In fact, it can be seen that curve A representing the temperature of the dust as a function of time, during the merger, marks a plateau at around 50 or 60 minutes, then resumes its rise to cap at a maximum temperature of the order of 1,400 ° C. In correspondence, it can be seen that curve B in dashed lines, representing the activity of the dust, presents a predominant sharp peak at the moment when the temperature marks this plateau, that is to say around 60 minutes. We therefore deduce that the dust activity is relatively localized in time and that its measurement is therefore facilitated. Indeed, if the activity emission were uniformly distributed over the entire time of the merger, it would be difficult to detect.

This instantaneous and continuous detection of the passage of activity can therefore make it possible to direct the dusts towards selective filters according to their activity, the uncontaminated dusts being recycled to recover the iron and reduce the volume of waste.

The whole phenomenon can be interpreted as the separation at the time of fusion between the oxidized layer containing the contamination with respect to its solid support, the metal. The method according to the invention therefore proposes to continuously measure the fusion in order to be able to measure, at the time desired temperature step, the maximum activity of dust coming out of the oven.

The measurements were carried out on standard fusions (scrap contaminated with Co60 and Cs137, previously "karchérisée") and oxidative fusions (scrap contaminated with uranium, treated in the same way), without graphite at the start to preserve their oxidizing character . The curve in fine lines shows the passage of the Cs137, while the hatched area shows that of the Co60.

The first conclusions from the recorded experiences are quite surprising. A priori, it could be assumed that the thermodynamic properties of Cs137 gave it higher lability, and therefore that it had to appear first compared to Co60. This is not the case for this system. Indeed, we are dealing with previously karcherized scrap metal and we can assume that the labile contamination is gone. Only deep oxidation would remain for the benefit of occluded pores. On the other hand, the active emission occurs at the moment when the merger takes place. It is then reasonable to think that the release of the contents of the pores occurs at the time of the change of state.

This hypothesis is supported by the position of the phenomenon. In the context of a modeling study of the operation of the furnace, it can be observed that the duration and the moment when the transition from the solid state to the liquid state occurs coincides with the recording of the activity in the dust .

The embodiment mentioned here is that used during the operation of the arc furnace of two MARCOULE units by the depositor. This furnace is intended to melt scrap from basic nuclear installations to be dismantled. This arc furnace has an installed power of 6 GW with a capacity of 14 tonnes. It has ventilation allowing operation in a confined environment. One of these ventilation ducts is that shown in cross section in FIG. 2 and referenced 1.

Its section has been shown in a square shape with two opposite side walls 2 and 3. The dust circulates there perpendicularly to the plane of this figure 2.

The density measurement of the dust circulating inside the sheath 1 can be done with many different types of measuring devices such as a light emitter associated with a corresponding radiation detector. In the embodiment described, use is made of the light beam F emitted by a laser diode 4 placed opposite a first lateral face 2 of the sheath 1, outside the latter. This laser diode is positioned using a tube 5 fixed on the first side wall 2 of the sheath 1. The light beam F therefore passes right through the sheath 1 to emerge by a second opposite side face 3. In Regarding this second lateral face 3, a first light radiation detector is installed which is a photovoltaic cell 6 in a position such that it is capable of receiving and detecting the intensity of the radiation of the light beam F after it has passed through. the sheath 1. A second tube 7 makes it possible to install this type of photovoltaic cell by being fixed on the second side wall 3. Thus, when the dust which circulates in the sheath 1 crosses the beam F, they absorb part of it and modulate beam radiation. The photovoltaic cell 6 therefore detects the flux of the resulting radiation and delivers variations in voltage consecutive to these variations in light.

These indications are sent to an acquisition and control unit 11.

où Io est l'intensité transmise par le faisceau en l'absence de poussières, a et k étant des coefficients.It is therefore possible to have information relating to the rate of dust inside the sheath 1, in the form of an electrical voltage characteristic of this density of dust found in this sheath. By way of example, it can be noted that if It is the intensity of the beam passing through the sheath 1 and if V is the voltage supplied by the photodiode, the measurement is carried out using the following formula : $$\text{V = k.Log (1 + a.It / Io)}$$

where Io is the intensity transmitted by the beam in the absence of dust, a and k being coefficients.

After approximating and ignoring the overlap phenomenon, it can be assumed that the concentration C of dust is defined by the following formula: $$\text{C = A. (1-V / Vmax)}$$

It is thus possible to measure a dust concentration of the order of 30 g / m ³ by means of a laser beam one meter long and having a section of 6 square centimeters.

The detection of the activity of this dust is carried out, in this embodiment, with one or more second gamma radiation detectors 9 placed around the sheath 1. They each deliver a signal characteristic of the detected radiation. This signal is sent to a multichannel analyzer 10. The latter allows a selection of one or more detectors 9 and the indications thus selected are sent to the acquisition unit 11.

The processing and exploitation of these signals is therefore ensured by acquisition and processing means constituted by the acquisition unit 11 and a control and signal processing unit 20. This the latter can be constituted by a microcomputer of the "Powerbook" type in real time.

With the help of such a control and signal processing unit, it is possible for an operator to control the start-up, to adjust the duration of the analysis and to synchronize the conditions of acquisition of the measurement, this by real time.

The acquisition unit comprises two voltage measurement means 12a and 12b respectively receiving the signal from the photovoltaic cell and the signals from the multi-channel analyzer 10 constituted by the detection signals of gamma radiation. Two memories 13a and 13b are also used, connected respectively to the measurement means 12a and 12b. They can thus record series of measured values relating to the two types of signals.

The box is advantageously supplemented with means for calculating the averages 14a and 14b, connected to the output of the memories 13a and 13b to provide a characteristic indication of the phenomenon measured over a determined period.

The acquisition unit is advantageously completed by a chronometer 15 which controls the voltage measurement means 12a and 12b and the averaging means 14a and 14b.

The output of these averaging means 14a and 14b is connected to the input of the processing unit 20, in this case a "Powerbook" microcomputer.

The assembly is completed by an electrical supply device 21 for the diode 4. This is controlled by the control and signal processing unit 20, but can also be controlled by the acquisition and command 11.

It is also possible to use a temperature sensor 22 placed inside the sheath 1 to possibly correct, at the level of the voltage measurement means 12a and 12b, the measured values.

The operation of such a device can be as follows. It is shown diagrammatically by the flowchart in FIG. 3 and is provided by the acquisition and control unit 11, as well as by the control and signal processing unit 20 also appearing in this figure.

The acquisition program is stored in the microcomputer constituting the control unit 20. It is thus possible to define a time interval over which the average of the measured voltages is calculated, as well as a total duration of the measurement.

On the right of the flowchart, we see that once the acquisition of the data is done, the reading of the voltages takes place, regulated by the stopwatch. Depending on the conditions of RS232 computer transmissions, the information is stored.

The stopwatch delays the storage of information. After a determined period value, this information is sent to the microcomputer. On the right side of the flowchart, we see that the microcomputer is in direct dialogue with the acquisition and control unit. This microcomputer receives the values accumulated by the acquisition and control unit, the average being calculated by determined time intervals. The cycle continues until the operation is completed.

In fact, depending on the type of dust to be measured, the type of dust density measurement can be varied. In the previous example, we described the use of a laser in the visible domain. The screen formed by the dust was measured. One could also consider measuring the density of dust by measuring the optical density, that is to say the density of the volume traversed by the laser beam. It then suffices to change the wavelength of the radiation and of the photovoltaic cell. So we could choose an optical measurement system, characteristic of a completely gaseous radioactive effluent, that is to say being in the form of vapor.

An infrared laser makes it possible to see the density and the radioactivity at the same time in the case of ruthenium oxide measurement.

With the system according to the invention, it is understood that it is possible for the operator to act in real time on the ventilation operating conditions as a function of the measurements carried out in real time on the ventilation duct. The elements used in this system are relatively simple and can be easily managed by a microcomputer.

Claims (10)

Method for controlling the activity of dust emitted during the melting, in an oven, of irradiated elements, the method consisting in: - continuously measure the quantity of dust during its passage through a ventilation duct (1) of the oven; and - measure the activity linked to this dust. Method according to claim 1, characterized in that the measurement of the quantity of dust is carried out by measuring the density of the dust with a photovoltaic cell (6), while the measurement of the activity is carried out with at least one detector gamma radiation (9) coupled to a multichannel analyzer (10). Device for controlling the activity of dust emitted during the melting in an oven of irradiated materials evolving in a ventilation duct 1, comprising: - a light emitter (4) placed opposite a first wall (2) of the ventilation duct (1); - a light radiation detector (6) placed opposite a wall opposite (3) to the first side wall (2) of the ventilation duct (1) and delivering a signal characteristic of the density of the medium traversed to the interior of the ventilation duct (1); - at least one gamma radiation detector (9) placed opposite the ventilation duct (1) and delivering a signal characteristic of the activity of the medium passed through inside the ventilation duct (1); and - Acquisition and processing means connected to the first and (x) second (s) detectors (6, 9) and receiving said signals characteristic of the density and activity of the dust. Control device according to claim 3, characterized in that the readers (4) are laser diodes. Control device according to claim 3, characterized in that the first detector (6) is a photovoltaic cell. Control device according to claim 3, characterized in that the processing acquisition means comprise: - an acquisition and control unit (11) connected to the first detector (6) and second (s) detector (s) (9); and - a control unit (20) for processing the signals and controlling the detectors. Device according to claim 6, characterized in that it comprises a multi-channel analyzer (10) placed between the second detector (s) (9) and the processing acquisition unit (11). Device according to claim 6, characterized in that the processing unit (20) is a microcomputer. Device according to claim 6, characterized in that the acquisition and processing unit comprises: - two voltage measurement means (12A and 12B); - two memories (13A, 13B) for storing the measured voltage values; - two means of calculating the average of the values (14A, 14B); and - a stopwatch (15). Device according to claim 3, characterized in that it comprises a temperature sensor placed inside the sheath (1) and connected to (x) means (s) of acquisition and processing.

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