EP0252826B1 - Process for the radioactive decontamination of an oil - Google Patents

Abstract

1. Process for the radioactive decontamination of an oil containing radioelements, comprising the stage of passing said oil through a pulverulent material in the presence of an acid.

Description

The present invention relates to a process for radioactive decontamination of an oil applicable in particular for the decontamination of oils used in nuclear installations.

In fact, in nuclear installations, the oils used in machines such as the primary pumps for example, can, after a certain time, be contaminated by radioactive elements. The level of activity varies from 3.7.106 to 3.7.104 Bq / m3 approximately, while we admit that the non-contamination threshold is 3.7.10 ³ Bq / m3. In the oils used on the primary pumps, the main contaminant is xenon 133, but this has a fairly short period (5.3 days) and, after this period, the oils are at about the same level of activity which is around 3.7.104 Bq / m3. In addition to xenon 133, the other radioelements likely to be found in contaminated oils are notably: manganese 54, cobalt 58, cobalt 60, niobium 95, iodine 131, cesium 134, cesium 137 and cerium 144.

Document GB-A-1 524 693 describes a process for the radioactive decontamination of hydrocarbons in gaseous or liquid state, containing radioelements. The method involves passing the hydrocarbons through a space where there is a solid or liquid contact material, which consists, for example, of porous granules and which is inert with respect to the hydrocarbons.

Another method currently used to get rid of contaminated oils is to incinerate them. This incineration produces on the one hand ash which can be evacuated in storage drums and, on the other hand, gaseous products which should be treated. For this, they are passed through so-called "absolute" filters, that is to say filters which retain practically all dust and solid particles even the finest.

If the disposal of ash does not pose any particular problems, the treatment of large quantities of gas would require large, therefore expensive, installations. Also, we are satisfied with installations of reduced dimensions, which makes it possible to obtain only low flow rates of treatment in the radioactive decontamination of oils.

The object of the present invention is to remedy these drawbacks by proposing an inexpensive radioactive decontamination process for oils which makes it possible to obtain an oil whose activity rate is less than 3.7 × 10 3 Bq / m3.

According to the main characteristic of the process which is the subject of the invention, intended for the decontamination of an oil containing radioelements, this oil is passed through a pulverulent material in the presence of an acid.

Preferably, the pulverulent material is an earth containing diatoms or bentonites and its particle size is less than 0.5 mm. As for its mass, it is preferably between 0.5 and 5% of the mass of the oil to be treated.

The acid used can be sulfuric or phosphoric hydrochloric acid and its aqueous concentration is preferably greater than or equal to 70%.

In the present description, the word "oil" must be taken in its most general sense and designates both a lubricant and a lubricant base such as a mineral, animal or vegetable oil.

The mechanism of decontamination of an oil by the process of the invention can be explained as follows: the acid reacts with the oil to form products such as tars and the radioelements are fixed on these tars. The latter are retained by the pulverulent material and it is therefore an oil, at least partially freed from the radioelements that is collected. Optionally, as will be seen later, it may be necessary to recycle the oil through the pulverulent material until it is completely decontaminated.

As indicated above, in the present description, the expression "completely decontaminated" or "decontaminated" means that the activity rate of the oil is less than 3.7 × 10 3 Bq / m3.

• (a) - mélanger l'huile avec le matériau pulvérulent,
• (b) - faire passer le mélange à travers un filtre apte à retenir au moins une partie du matériau pulvérulent, et
• (c) - répéter l'étape (b) autant de fois qu'il est nécessaire pour obtenir la décontamination complète de l'huile.

In a first embodiment, the method which is the subject of the invention comprises the following steps consisting in:

• (a) - mix the oil with the pulverulent material,
• (b) - passing the mixture through a filter capable of retaining at least part of the pulverulent material, and
• (c) - repeat step (b) as many times as necessary to obtain complete decontamination of the oil.

• (d) - placer le matériau pulvérulent sur la face amont d'un filtre,
• (e) - faire passer l'huile à travers ce filtre recouvert du matériau pulvérulent, et
• (f) - répéter l'étape (e) autant de fois qu'il est nécessaire pour obtenir la décontamination complète de l'huile.

In a second embodiment of the method which is the subject of the invention, it comprises the following steps consisting in:

• (d) - place the pulverulent material on the upstream face of a filter,
• (e) - pass the oil through this filter covered with pulverulent material, and
• (f) - repeat step (e) as many times as necessary to obtain complete decontamination of the oil.

The expression "upstream face" or "downstream face" of the filter used in the present description must be understood with respect to the direction of flow of the oil through the filter. On the other hand, it is understood that steps (c) and (f) are optional because it may be, in certain cases, that a single passage of the oil or of the mixture through the filter is sufficient to remove all contamination.

Finally, in most cases, it is advantageous to heat the oil before passing it through the pulverulent material.

The invention will appear more clearly on reading the description which follows, given by way of purely illustrative and in no way limitative example, with reference to the appended drawing, which comprises a single figure which is a schematic view in vertical section of a device. used for implementing the process which is the subject of the invention.

If we refer to the drawing, we see that the availability sitive object of the invention consists first of a preparation tank 10 equipped with an agitator 12 which can be set in motion by a motor 14 and a heating means, for example an electrical resistance 16. Du lowest point of the tank 10 leaves a pipe 18, fitted with a tap 20, which connects the tank 10 to a pump 22. From this latter share another pipe 24 fitted with a tap 26. The pump 22 can make circulate the liquid contained in the tank 10 in the direction of the arrows indicated in the figure. A pipe 28 fitted with a tap 30 connects the pipe 18, from a point located between the tap 20 and the pump 22, to the pipe 24 at a point of the latter located downstream of the tap 26 relative to the direction of circulation of the liquid imposed by the pump 22. The pipe 24 opens into a pipe 32 which is broken down into two parts. A first part 32a equipped with a tap 34 returns to the tank 10 at the top of the latter while a second part 32b equipped with a tap 36 opens into a filtration tank 38.

This comprises a set of filters 40 which, in the example shown here, are flat filters placed in a vertical position. These filters are arranged in groups of two, such as filters 40a and 40b and thus define an internal space 42 which communicates at its lower part with a collector 44. The filters are fixed at their lower part to the wall of the collector 44 and at their upper part to a frame 46 which can be set in vibration by a vibrator 48. The role of this vibrator will be explained below in the rest of this description.

A deflector 50 is placed at the lower part of the filtration tank 38, below the collector 44, at the point where the pipe 32 opens. The position of this deflector 50 is such that it forces the oil entering into the tank 38 to pass through the bottom thereof before returning to the area where the filters are located. Finally, the tank 38 is closed at its lower part by a hatch 52 which is movable between a closed position 52a shown in solid lines and an open position 52b shown in broken lines.

The manifold 44 communicates with a pipe 54 placed outside the filtration tank 38 and equipped with a tap 56. The pipe 54 opens, at its end opposite to the tank 38, at the upper part of the preparation tank 10. On the pipe 54, at a point located between the filtration tank 38 and the tap 56, is connected an evacuation pipe 58 equipped with a tap 60, which opens out inside a receiving tank 62 used to recover the decontaminated oil.

We also see in the figure a pipe 64 which starts from the upper part of the filtration tank 38 and which is divided into two branches. A first branch 66 equipped with a tap 68 returns to the preparation tank 10 at the upper part thereof. A second branch 70, equipped with a tap 72, is in communication with an air source which supplies dry and lubricated air, through the tap 76, to the vibrator 48 and to the hatch 52.

• Le robinet 20 étant fermé, on introduit d'abord l'huile à traiter dans la cuve de préparation 10. Si nécessaire, on chauffe l'huile à l'aide de la résistance 16 jusqu'à ce que la température désirée soit atteinte : une température de l'ordre de 110°C convient dans presque tous les cas. Afin d'homogénéiser le produit à traiter, celui-ci est agité grâce à l'agitateur 12 mis en mouvement par le moteur 14. Lorsque la température désirée est atteinte, on introduit dans l'huile la quantité voulue de matériau pulvérulent, par exemple de la terre. Le chauffage permet d'une part d'améliorer la viscosité de l'huile et, d'autre part, d'éliminer l'eau ou d'autres solvants qui ne seraient pas miscibles avec l'huile. En effet, ces solvants pourraient avoir un comportement néfaste vis-à-vis de la terre, ce qui pourrait nuire à la qualité de la décontamination. D'autre part, l'agitation améliore le contact entre le matériau pulvérulent et l'huile à traiter.

The course of a decontamination operation with such a device is done as follows:

• With the tap 20 closed, the oil to be treated is first introduced into the preparation tank 10. If necessary, the oil is heated using the resistor 16 until the desired temperature is reached: a temperature of the order of 110 ° C. is suitable in almost all cases. In order to homogenize the product to be treated, it is agitated by means of the agitator 12 set in motion by the motor 14. When the desired temperature is reached, the desired quantity of pulverulent material is introduced into the oil, for example of the earth. Heating makes it possible on the one hand to improve the viscosity of the oil and, on the other hand, to remove water or other solvents which would not be miscible with the oil. Indeed, these solvents could have a harmful behavior with respect to the ground, which could harm the quality of the decontamination. On the other hand, the agitation improves the contact between the pulverulent material and the oil to be treated.

When the mixture is sufficiently homogeneous, the taps 72 and 76 being closed, the taps 20, 26, 36, 56 and 68 are opened, all the other taps being closed. The pump 22 is then started, which has the effect of circulating the mixture from the preparation tank 10 to the filtration tank 38 through the pipes 18, 24 and 32b. The mass of oil gradually invades almost the entire volume of the filtration tank 38. The level of the oil thus rising in the filtration tank, part of the oil ends up flowing through the lines 64 and 66 and returns to the preparation tank 10. The lines 64 and 66 constitute a vent which makes it possible to ensure that the liquid occupies almost the entire volume of the filtration tank.

On the other hand, most of the oil passes through the filters 40 and enters the spaces 42 located between the filters 40a and 40b of each group of two filters. This has the effect that part of the pulverulent material is deposited on the upstream face of each filter: the upstream face of the filter is that which is on the side opposite the space 42. The oil which has thus been filtered passes into the manifold 44 and, from there, into the pipe 54 and returns to the preparation tank 10.

Since the filters 40 are formed so as to retain at least part of the pulverulent material mixed with the oil to be treated, a first layer of this material, called "pre-layer", is deposited on the upstream face of the filter. It is therefore at least partially purified oil which is found in the collector 44 and returns to the tank 10. The pump 22 being always on, the oil is thus recycled through the filters. At each pass, a new quantity of pulverulent material is retained either by the filter itself, or by the already deposited layer. A “cake” of powdery material is thus formed. As will be seen below, the filter and the layer of soil deposited on the upstream face thereof retain the radioelements contained in the oil.

After a certain number of cycles, the oil which passes through the filters 40 and returns to the cu ve 10 is completely decontaminated, that is to say that its activity is less than 3.7 × 10 ³ Bq / m3. This can be easily determined by analysis using samples taken from the preparation tank. When the oil is decontaminated, the tap 60 is opened and the tap 56 is closed. Thus, the pump 22 sends the decontaminated oil to the receiving tank 62 through the line 58. When the receiving tank 62 is full, decontaminated oil can be recovered and disposed of.

It should be noted that, since the oil is forced through the filters or through the cake which has deposited on them, a certain pressure, of the order of about 5 bars, prevails in the filtration tank. 38. The lines 64 and 66 and the tap 68 play the role of a vent which makes it possible to maintain the pressure inside the tank 38 within reasonable limits and to prevent it from reaching too high values. .

As the oil is expelled into the receiving tank 62, the level drops in the preparation tank 10. When this level has reached a predetermined value, the tap 56 is opened and the tap 60 is closed. open the tap 72 in order to send compressed air to the filtration tank and maintain the pressure inside the latter, the tap 26 is then closed and the pump 22 is immediately stopped. The tap is then opened 34 so that, under the effect of the pressure of the compressed air, the remainder of oil in the filtration tank is returned to the tank 10 via line 32. When there is has more oil in the tank 38, which can be determined by simple visual observation when there is no more oil entering the tank 10 through the lines 54 and 32, the valves 34 and 56 are closed. At this time, compressed air is sent to the tank 38 to dry the cake which has settled on the he filters, either of the valves 34 and 56 can be opened in order to let air escape.

When the cakes are dry (the time required can be easily determined by preliminary tests), the tap 68 is opened to reduce the pressure inside the tank 38. All the taps are then closed except tap 68 which allows '' introduce air under normal pressure inside the tank. The hatch 52 which opens from the closed position 52a to the open position 52b is opened, and the vibrator 48 is started up. Under the effect of these vibrations, the layers of pulverulent material which are deposited on the filters detach themselves from it and fall into a barrel 78 which has been previously placed under the tank 38. When the barrel 78 is full, it can be evacuated to a storage place.

Optionally, instead of mixing the pulverulent material with the oil inside the tank 10, this material can be placed directly on the upstream face of the filters 40 and then circulate the oil as before the process is exactly the same than the one just described.

We will now describe some tests which have been carried out in the laboratory in order to test the effectiveness of the process which is the subject of the present invention.

EXAMPLE 1

• - densité apparente de la terre non tassée : 450t40 g/1
• - densité apparente de la terre tassée : 670±60 g/I
• - poids spécifique : environ 2,4 kg/I

In this test, 200 cm ³ of an oil was treated from a primary pump of a nuclear power station and whose initial activity was 2.7.104 Bq / m3. To this oil was added 5 g of a soil, the characteristics of which were as follows:

• - bulk density of the uncompressed soil: 450t40 g / 1
• - bulk density of packed earth: 670 ± 60 g / I
• - specific weight: about 2.4 kg / I

Physical and chemical properties

• - humidity (2 h, 110 ° C): 7% maximum
• - fire losses (1000 ° C): 7% maximum
• - pH (10% suspension): 2.5 - 3

Particle size analysis (sieving_

• - 150 m (DIN 40): 97%
• - 70 m (DIN 80): 88%
• - 60 m (DIN 100): 80%

Chemical composition - SiOz, Al ₂ O ₃ , FezOa, MgO, CaO, Na ₂ 0, KzO.

The values indicated are average values.

It is a bentonite washed with hydrochloric acid, then calcined, sold by the company Süd-Chemie AG of Munich under the reference TON-SIL @ OPTIMUM FF.

The mixture was stirred for 30 minutes at room temperature, which was of the order of 22 _° C. The mixture was then filtered under vacuum on filter paper. A cake formed, which was retained by the filter, and the activity of the filtrate was measured, which was less than 3.7 × 10 3 Bq / m3.

EXAMPLE 2

200 cm ³ of the same oil as in Example 1 were first heated with vigorous stirring until the temperature stabilized around 110 _° C. Then 3 g of the same earth was added as in Example 1, and the mixture was stirred for 30 minutes at 110 _° C. The mixture was then filtered under the same conditions as above, and a filtrate was obtained whose activity was less than 3.7 × 10 3 Bq _{/ m3} .

EXAMPLE 3

• - couleur : blanc
• - densité : humide..... 320 g/1 apparente.. 180 g/l

200 cm ³ of the same oil as in the previous examples were mixed at room temperature (that is to say about 22 _° C.) with 3 g of an earth activated with sulfuric acid. For this, a few drops of concentrated sulfuric acid were added to the 200 cm3 of oil before mixing. As for the land, it had the following characteristics:

• - White colour
• - density: wet ..... 320 g / 1 apparent .. 180 g / l

Granulometry

• - refusal to the 600lim sieve = 1.0% max.
• - refusal to a 104µm sieve = 5% average
• -pH = 10
• - specific surface = 1.5 - 2 m ² / g
• - porosity = 75 - 85%
• - chemical analysis =
  • Si0 ₂ 91.2%
  • AL ₂ 0 ₃ + Fe ₂ O ₃ 4.6%
  • CaO + MgO 0.8%
  • Na ₂ O + K ₂ 0 2.5%
  • H ₂ 0 0.1%
  • loss on ignition 0.3%
  • permeability in darcies about 1.1%

It is a diatomaceous earth of lacustrine origin. First extracted selectively and then ground, the ore is then sintered, that is to say it undergoes calcination with the addition of prior flux. This treatment produces a larger particle and, therefore, more permeable. The material is then cycloned to obtain different particle sizes.

The mixture was stirred for 30 minutes, then filtered under the same conditions as above. The activity rate of the filtrate was below the contamination threshold, that is to say below 3.7.103 Bq / _m3 .

EXAMPLE 4

200 cm3 of the same oil as above were mixed first with a few drops of concentrated sulfuric acid, then with 3 g of the earth used in Example 3. The oil was heated with stirring to a temperature about 110 _° C before being mixed with the earth. The mixture was stirred for 10 minutes at 110 _° C, then was filtered under the same conditions as above. After cooling, the total activity of the filtrate was measured, which was less than 3.7 × 10 3 Bq / m3.

It should be noted that, for these laboratory tests, a single pass through the filter was sufficient to completely decontaminate the oil. This is due to the fact that the filter papers used were extremely fine pore papers, and therefore capable of retaining all of the soil. In the case of industrial use with the device illustrated in the drawing, the filters are wider mesh filters which retain only part of the soil, and it is therefore necessary to recycle the oil until that all the earth or all the powdery material is deposited on the filter.

• Les radioéléments contenus dans les huiles à traiter peuvent se trouver soit sous forme de particules solides, soit sous forme de composés dissous, soit sous forme de composés se trouvant à l'état colloïdal. Les particules solides peuvent être présentes naturellement dans l'huile ou avoir été formées par réaction de l'acide avec l'huile, comme indiqué plus haut. La décontamination se fait par l'action conjuguée de trois effets :
• Il y a d'abord un effet mécanique de filtration, les filtres arrêtant la terre ou les particules solides contenant les radioéléments, cet effet de filtration étant de plus en plus important au fur et à mesure que l'on recycle l'huile puisque l'action du gâteau qui se dépose progressivement s'ajoute à celle du filtre lui-même.

The decontamination mechanism can be explained as follows:

• The radioelements contained in the oils to be treated can be found either in the form of solid particles, or in the form of dissolved compounds, or in the form of compounds in the colloidal state. The solid particles may be present naturally in the oil or may have been formed by the reaction of the acid with the oil, as indicated above. Decontamination is done by the combined action of three effects:
• First there is a mechanical filtration effect, the filters stopping the earth or the solid particles containing the radioelements, this filtration effect being more and more important as the oil is recycled since the he action of the cake which is gradually deposited is added to that of the filter itself.

On the other hand, when an earth containing diatoms is used as a pulverulent material, the particles containing the radioelements are absorbed or adsorbed on the skeleton of the diatoms since the fluid is forced into the pores of the latter. This is all the more true in the case where the mixture is recycled and ironed through the filter since, as the earth or the pulverulent material is deposited on the filter, there is a pressure increase.

Finally, there is a chemical effect, especially when using an activated earth with an acid: in fact, the radioelements can react with the activating acid or the constituent compounds of the earth, which leads to precipitation within the powder material and further improves absorption or adsorption.

We will now give other examples of tests carried out in the laboratory under the same conditions as Examples 1 to 4, but with other oils.

EXAMPLE 5

200 cm3 of an oil from a lifting reducer were heated for 15 minutes at 110 ° C. and then mixed with 5 g of the earth used in example 1. The mixture was stirred at this temperature for 30 minutes. Passage through a filter paper such as those used in Examples 1 to 4 made it possible to reduce the activity of this oil, which was initially 5.2.10 ⁴ Bq / m ³ , to a value less than 3.7.103 Bq / m3.

EXAMPLE 6

The same oil as in Example 5 was heated at 110 _° C for 15 minutes, then mixed with a pulverulent material consisting of 4g of the earth used in Examples 1 and 2 mixed with 2g of the earth used in Examples 3 and 4.

Again, we were able to reduce the activity rate, which was initially 5.2.104 Bq / m ³ to a value below 3.7.103 Bq / m3.

EXAMPLE 7

200 cm ³ of an oil used on a steam generator crane and having an activity of 3.104 Bq / m ³ were heated at 110 _° C for 15 minutes, then mixed with 3g of the earth used in Example 5. Here too, filtration on paper made it possible to obtain an oil having an activity of less than 3.7 × 10 3 Bq / _m3 .

Thus, the process which is the subject of the invention has particularly advantageous advantages, the first of which is that it is inexpensive to implement since the device used can be produced at using simple and readily available elements. On the other hand, such a device consumes little energy. In addition, the treatment capacity is important since it is possible to treat several cubic meters of contaminated oil per day whereas with the incineration methods of the prior art, in order to avoid having too bulky installations and too costly, we are satisfied with installations of reduced dimensions which can treat only a few liters per day. Finally, after treatment, a good quality oil is recovered in the receiving tank which can be reused, in the nuclear installation itself from which it originates, possibly with the introduction of some suitable additives.

Finally, it should be understood that the invention is not limited to the sole embodiments which have just been described, but that numerous variants can be envisaged without thereby departing from the scope of the invention. This is how a person skilled in the art can choose the shape and the nature of the filters according to the nature of the oil to be treated and adapt the dimensions of the installation and the power of the pump as a function of the flow rate to be treated. or replace any element of the device with an equivalent element.

Claims (9)

1. Process for the radioactive decontamination of an oil containing radioelements, comprising the stage of passing said oil through a pulverulent material in the presence of an acid.

2. Process according to claim 1, characterized in that the pulverulent material is an earth or clay containing diatoms or bentonites.

3. Process according to claim 1, characterized in that the pulverulent material has a grain size below 0.5 mm.

4. Process according to claim 1, characterized in that the mass of the pulverulent material is between 0.05 and 5% of the mass of the oil to be treated.

5. Process according to claim 1, characterized in that the acid used belongs to the group consisting of hydrochloric, phosphoric and sulphuric acids.

6. Process according to claim 1, characterized in that the aqueous concentration of the acid used is equal to or above 70%.

7. Process according to claim 1, characteïized in that it comprises the following states:

(a) mixing the oil with the pulverulent material,

(b) passing the mixture through a filter (40) able to retain at least part of the pulverulent material, and

(c) repeating stage (b) the number of times necessary for obtaining the complete decontamination of the oil.

8. Process according to claim 1, characterized in that it comprises the following stages:

(d) placing the pulverulent material on the upstream face of a filter (40), .

(e) passing the oil through said filter (40) covered with the pulverulent material, and

(f) repeating stage (e) the number of times necessary to obtain the complete decontamination of the oil.

9. Process according to claim 1, characterized in that the oil is heated before passing it through the pulverulent material.

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