EP1453607B1 - Method for treating a nuclear graphite contaminated - Google Patents

Abstract

A process for the milling of a nuclear graphite contaminated with radioelements includes subjecting the graphite, immersed in a liquid medium, to high-voltage pulses. The liquid medium has a resistivity such that, owing to the effect of the energy conveyed by the said pulses, electric arcs are initiated and, upon contact with the graphite, break the carbon-carbon bonds that make up the graphite. The number of high-voltage pulses is set so as to obtain a given particle size.

Description

TECHNICAL AREA

The present invention relates to a method for treating a nuclear graphite contaminated with radioelements by grinding said graphite, placed in immersion in a liquid medium, in particular graphite from the natural uranium-graphite-gas (called UNGG), recovered during dismantling or nuclear graphite from nuclear sites during nuclear remediation operations.

STATE OF THE PRIOR ART

The general field of the invention is therefore that of the treatment of nuclear waste, such as nuclear graphite contaminated with radioelements.

At present, one of the processes for treating nuclear graphite consists in subjecting said graphite to dry fragmentation, in air, by means of conventional grinding means, such as impact mills or roll mills, so that to obtain a powder, which is then subjected to combustion, so as to completely destroy the contaminated graphite. Such a process is described in document FR-A-2 691 524.

• ce procédé est un procédé très coûteux, dans la mesure où le graphite présente une dureté telle, qu'il engendre une usure rapide des pièces mécaniques entrant dans la constitution des broyeurs, ce qui nécessite un renouvellement fréquent desdites pièces ;
• ce procédé engendre la formation de particules très fines de graphite, qui, lorsqu'elles sont en suspension dans l'air, peuvent engendrer une explosion, lors de la survenance d'une étincelle ;
• ce procédé engendre une pollution importante due notamment à la volatilité des particules de graphite, ces particules pouvant être chargées éventuellement en métaux lourds radioactifs, en ⁶⁰Co et en ¹³⁷Cs, ce qui nécessite un confinement de la station de broyage, afin d'éviter toute fuite à l'air libre d'éléments radioactifs contaminants ; toutefois, la mise en place d'un tel confinement ne permet pas pour autant d'éviter la dispersion de radioéléments volatils, tels que le tritium, qui peuvent s'échapper par les systèmes de ventilation de la station.

However, this treatment method has the following drawbacks:

• this process is a very expensive process, since graphite has such a hardness that it causes rapid wear of the mechanical parts involved in the construction of grinders, which requires frequent renewal of said parts;
• this process causes the formation of very fine particles of graphite, which, when suspended in the air, can cause an explosion when a spark occurs;
• this process generates a significant pollution due in particular to the volatility of the graphite particles, these particles being able to be loaded possibly with radioactive heavy metals, in ⁶⁰ Co and in ¹³⁷ Cs, which requires a confinement of the grinding station, in order to avoid any air leakage of contaminating radioactive elements; However, the establishment of such a confinement does not allow to avoid the dispersion of volatile radioelements, such as tritium, which can escape through the ventilation systems of the station.

STATEMENT OF THE INVENTION

Thus, the object of the present invention is to provide a method for treating a nuclear graphite contaminated with radioelements, which does not have the drawbacks of the prior art and which does not require, in particular, the use of mechanical parts. and which does not cause dispersion of radioelements and also controls the risk of powder explosion.

To this end, the subject of the invention is a method for treating a nuclear graphite contaminated with radioelements, said method comprising a step of grinding comprising subjecting said graphite, immersed in a liquid medium, to high voltage pulses, said liquid medium having a resistivity such that, under the effect of the energy carried by said pulses, electric arcs are triggered and ensure, in contact said graphite, a rupture of carbon-carbon bonds constituting this graphite, the number of high voltage pulses being set so as to obtain graphite particles with a given particle size.

It is specified that according to the invention, the term "high voltage pulses" electrical pulses can convey a voltage of the order of one to several kilovolts resulting in the creation of arcs in a liquid medium having adequate resistivity properties to the formation of arches. Thus, for the purpose of carrying out this method, it is advantageous to use liquid media whose resistivity is greater than 1 MΩ.cm.

This method has the advantage of being feasible without the use of mechanical grinding parts, which minimizes the costs of implementing this method compared to the embodiments of the prior art.

In addition, this treatment method has the advantage of being carried out in a liquid medium. As a result, the graphite powders resulting from grinding are trapped in this liquid medium, which makes it possible to avoid the aforementioned powder explosion phenomenon. In addition, the radioelements released during the grinding of the graphite remain confined in the liquid medium, by example, by isotopic exchange, as is the case of tritium.

In addition to the release and trapping of radioelements, the method according to the invention makes it possible to obtain, at its end, graphite particles with a given particle size, which can be either subjected to combustion in order to completely destroy them or to be recovered. with a view to possible reuse, for example, as a basic product for the elaboration of engineered geological barriers for the long-term storage of highly radioactive products. These particles can also be stored under zero leaching conditions by runoff.

According to the invention, in order to obtain a grinding of the nuclear graphite in the form of more or less fine particles, the person skilled in the art can easily choose the conditions of application of the high voltage pulses (energy, frequency, duration and number of pulses sent ) depending on the nature of the starting nuclear graphite, it being understood that the higher the energy of the pulses, the more the number of pulses to be applied will be less, to obtain a given grain size.

According to the invention, the energy conveyed by each pulse may advantageously be between 10 and 100 kJ, preferably equal to 1 kJ.

According to the invention, the high-voltage pulses may advantageously have a duration ranging from about 200 ns to 100 μs, preferably with a duration of 1 μs.

According to the invention, the high voltage pulses may have a frequency ranging from 1 Hz to 1000 Hz, preferably 10 Hz. It is understood that this frequency will be fixed precisely by the skilled person, depending on the generator used.

According to a particularly advantageous embodiment of the invention, a liquid medium that can be used in the context of this process is water. It is understood that the water used in the context of the invention will advantageously have resistivity qualities such that an electric arc can be triggered under the effect of high voltage pulses. For example, the water used may be partially deionized, so as to have a lower conductivity than untreated water.

Advantageously, the process of the invention may also comprise a step of treating the liquid medium, in which the grinding of the graphite takes place, this treatment being a conventional treatment intended, especially when this liquid medium is water, to clean up the liquid medium released radioelements and maintain its resistivity, these treatments being within the reach of the skilled person. The treatment of the liquid medium, intended to sanitize said medium of the radioelements contained therein, may be that which is usually practiced in the "STEL" (Liquid Effluent Treatment Station) nuclear centers, where depending on the case, practice precipitation of dissolved elements, neutralization of liquids, evaporation of water, desiccation of precipitates.

BRIEF DESCRIPTION OF THE DRAWINGS

• Figure 1 shows a particular device for grinding a conductive carbonaceous material.
• FIG. 2 illustrates the granulometry curves of graphite powders obtained in two tests carried out, applying for each a different number of pulses.

DESCRIPTION OF A PARTICULAR EMBODIMENT OF THE INVENTION Example

Figure 1 illustrates a particular device implemented in the context of this example.

This device comprises a sealed reactor 1 of non-conductive material, for example polyethylene. The bottom of the reactor is a conductive plate constituting the earth electrode 2 connected to a high voltage generator 3 of the Marx generator type. This generator supplies a high-voltage electrode 4 whose distance from the earth electrode 2 can be adjusted. A block of nuclear graphite 5 rests on the bottom of the reactor, said block being totally immersed. in a liquid medium 6. High voltage pulses are sent substantially towards the block 5, thus releasing fragments 7 of said initial block 5. The high-voltage pulses are embodied in the form of arcing between the high voltage electrode and the electrode connected to the the earth, the potential difference applied between these two electrodes being a function of the distance between these two electrodes.

An output 8 of the gases possibly produced during grinding is provided in order to avoid any phenomenon of overpressure.

In the reactor described above, a block of nuclear graphite with a mass of about 60 g is placed. A Marx generator used delivers pulses of the order of 1 kJ, a frequency of 10 Hz and a duration of 1 μs.

The nuclear graphite block is covered with water, so as to be totally immersed.

• une première série en fixant le nombre d'impulsions à 720;
• une deuxième série en fixant le nombre d'impulsions à environ 5000.

Two series of tests were carried out:

• a first series by setting the number of pulses to 720;
• a second series by setting the number of pulses to about 5000.

The results of these tests are grouped together in FIG. 2, which represents the distribution of the particle size of the graphite powder obtained. The size ∅ (in μm) of the graphite particles obtained is plotted on the abscissa of the graph, in logarithmic scale, and the percentage% of the number of particles of a given size relative to the total number of particles appears on the ordinate of the graph. The size of the graphite grains obtained is determined by the Coulther method, based on the principle of laser scattering. In this example, the levy performed only at the top of the reactor, without stirring the assembly.

Curve (a) represents the particle size distribution formed for a pulse number of 720, while curve (b) represents the distribution for a pulse number of about 5000.

For the 720-shot test, an average grain size of the order of 800 μm is obtained. For the test with approximately 5000 pulses, an average grain size of the order of 100 μm is obtained. These two tests clearly show that the grinding efficiency increases with the number of pulses.

Depending on the desired particle size, a person skilled in the art will experimentally determine, once the energy, the frequency and the duration of the pulses are set, a suitable number of pulses.

Claims (5)

1. Process for the treatment of a nuclear graphite contaminated with radioelements, the said process comprising a milling step consisting in subjecting the said graphite, immersed in a liquid medium, to high-voltage pulses, the said liquid medium having a resistivity such that, owing to the effect of the energy conveyed by the said pulses, electric arcs are initiated and, upon contact with the said graphite, break the carbon-carbon bonds that make up this graphite, the number of high-voltage pulses being set so as to obtain graphite particles of a given particle size.
2. Treatment process according to Claim 1, in which the energy of the high-voltage pulses is from 10 J to 100 kJ.
3. Treatment process according to either of Claims 1 and 2, in which the high-voltage pulses have a duration ranging from 200 ns to 100 µs.
4. Treatment process according to any one of the preceding claims, in which the high-voltage pulses have a frequency ranging from 1 Hz to 1000 Hz.
5. Treatment process according to any one of Claims 1 to 4, in which the liquid medium is water.

Images