Patent Number: 
Section: description

As shown in FIGS. 1 and 2, the cell contains the pollution source 1 (melting pot and calciner, represented diagrammatically), and the overhead travelling-crane 2 to be protected from said pollution source 1. Said cell is filled with air. The technical problem facing the inventors was to limit significantly the contamination of hoists in such cells. The air heated and contaminated by the melting pot and the calciner 1 rises in the cell as it would in a chimney, and insofar as the air is charged with radioactive particles, it contaminates the crane 2 at the top of the cell, thereby making any maintenance operations performed on this equipment much more complex. Experience has shown that the availability of hoists present in cells containing hot pollution sources is related directly to the degree to which they are contaminated. In the context of the example, and of the entire research conducted by the Applicant, and whose results are given further on in the present text (table of the example), the cell had the following dimensions: In the prior art (FIG. 1), a ventilation system is installed for protecting said crane 2. Air is fed into the top of the cell (above the melting pot) and is extracted at the bottom of the opposite wall. Said air is injected at a temperature of 28xc2x0 C. at a flow rate of 4,300 Nm3 per hour (Nm3/h). According to the invention (FIG. 2), a confinement barrier 3 is created in a horizontal plane by maintaining a temperature difference that is large enough between the bottom portion (cold zone) 4 and the top portion (hot zone) 5 of the cell. Said temperature difference is maintained by a suitable ventilation system and must be such that the resultant of the gravitational forces that are applied to a volume of cold air penetrating into the hot zone 5 is greater than the inertial forces that are applied thereto, thereby causing the volume of cold air to fall back towards the bottom of the cell until it reaches equilibrium and thus preventing it from contaminating the crane 2 in the top portion 5 of the cell. The confinement barrier 3 is a mixing zone in whose volume the vertical temperature gradient is much steeper than the temperature gradients in either of the two volumes 4 and 5 lying outside of this zone 3 (see FIG. 3). In general, the thickness of said mixing zone must be less than 15% of the height of the cell. Said thickness is defined as a function of the geometrical characteristics of the cell, such as: the height of the cell; the height of the volume generated by the displacement of the crane; and the height occupied by the equipment included in the polluted zone. For any given level of disturbance of the zones (namely the hot zone and the cold zone), the larger the temperature difference between the zones, the smaller said thickness. The bottom volume 4 and the top volume 5 are swept by means of ventilation (the fluids involved are hot air at 60xc2x0 C. and cold air at 28xc2x0 C., see example below). The following can be specified concerning the ventilation: 1) General Principle The ventilation is designed as if it were to guarantee the desired rate of air renewal in two distinct and superposed cells (4 and 5) separated by a physical volume whose thickness is the thickness of the mixing zone 3. The two virtual cells (or zones) are fed with a flow of air that has undergone the usual treatment undergone by air for ventilating reprocessing units. The intake openings A and D are disposed such that, they produce, respectively in zone 5 and in zone 4, a continuous vertical low-speed flow (a few centimeters per second (cm/s)) whose random component needs to be as small as possible. The flow in the bottom (cold) zone 4 is directed upwards, and the flow in the top (hot) zone 5 is directed downwards. The extraction slots B and C are situated on either side of the mixing zone 3: the hot-air extraction slots B are situated just above the top interface of the said mixing zone 3; and the cold-air extraction slots C are situated just below the bottom interface of said zone 3. The extraction slots B and C stabilize the level of said mixing zone 3. This stabilization requires the ratios of the intake flow rates to the extraction flow rates of the hot air circuit and of the cold air circuit to be controlled sufficiently accurately. The simplest solution to implement for this purpose is to recycle the hot air. This also offers the advantage of saving heat energy and of reducing the size of the heater units. 2) The Physical Characteristics of the Air that Determine the Dimensioning of the Ventilation a) Air in the Hot Zone 5 Feed Flow Rate Since zone 5 does not contain any pollution source, it is necessary: on the one hand, for the flow speed (vertically downwards) through a horizontal cross-section in the vicinity of the top plane of the mixing zone 3 to be higher than the speeds of the Brownian motion and of the turbulent diffusion of the polluting particles that have penetrated into said mixing zone 3; and on the other hand, for the flow rate of hot air to be high enough to compensate for losses by convection with the walls. Thermally insulating the side walls of the top zone 5, thereby reducing the heat exchange and the stray convection currents, contributes to the stability of the mixing zone 3. Temperature The temperature in the top zone 5 must be as high as possible, and it is limited only by the constraint that the motors of the hoist must be cooled. b) Air in the Cold Zone 4 Feed Flow Rate The flow rate of the air fed into the bottom volume 4 is a function of the intensity and of the type of the pollution sources, and mainly of the total power given off by the heat sources that they contain; it being necessary for the resulting rise in the mean temperature of the air to be compensated by the flow rate of cold air. In each case, the maximum allowable value for the flow rate is determined by the constraint that it is necessary to limit the thickness of the mixing layer 3 (and thus the speed of penetration and the upward speed of the flow of air through a horizontal cross-section). Temperature Since low temperatures are a factor favorable to reducing the thickness of the mixing layer 3 and to increasing its stability (provided that it remains positive), there is no lower limit to the temperature of the air in the bottom zone. However, for reasons of simplicity and to limit investment, air at ambient temperature is generally used. The temperature taken into account for dimensioning purposes must then be the temperature corresponding to the meteorological maximum recorded on the site during a reference period of sufficient length. Level of Turbulence The turbulence is greater than in the hot zone 5 because of the presence of the heat sources and because of the way the intake openings D are disposed, and it is characterized by the maximum value of the root mean square of the random speed at the interface with the mixing zone 3, which is the parameter determining the height of said zone 3. Concerning the apparatus used to implement the ventilation, the following can be specified. 1) Feed and Design of the Ventilation Openings In order for stratification to be effective, it is essential for the intake flow rates through the ventilation openings A, D and for the extraction flow rates through the extraction slots B, C to be distributed uniformly. The feed ducts to the intake openings A, D and the extraction ducts from the extraction slots B, C must be designed as a function of this constraint (fan blading, ducts of varying section, etc.). Therefore, and because of increase in overall size that could otherwise occur, studying duct dimensions is an essential element in the design of the apparatus, it being necessary for this work to precede the work on the civil engineering of the cell. The cell may be designed with thermally-insulating double inner walls (e.g. made of expanded glass); the gap of about 0.4 m between these walls and the structural walls of the cell then being available for the ducts and apparatus for distributing the intake air. In addition, it is necessary for the speeds at the delivery sections of the intake openings A, D to be uniformly distributed. This may be obtained by lining the delivery orifice with two or three layers of perforated sheet metal having transparency of about 20%, the layers being a few millimeters apart. In general, the intake openings A, D, which are xe2x80x9cinductivexe2x80x9d (i.e. they induce internal circulation movements) must be as far as possible from the mixing zone 3, whereas the extraction slots B, C, whose induction effect on the surrounding environment is very limited in space, may be situated closer to the mixing zone 3 for which they define and stabilize the limits on the vertical walls of the cell. 2) Hot Air Intake Openings (Slots) (see FIGS. 2 and 4) a) General Configuration They must be disposed such that they make it possible to distribute the hot air flow rate across the horizontal sections in the vicinity of the mixing zone 3, so that the flow is as close as possible to laminar flow. To satisfy these conditions (while taking account of the way in which the hoist is fixed), the intake openings may be disposed in the form of narrow slots that are parallel to the longitudinal axis of the cell and that are almost continuous. Their delivery speed, which determines their minimum section as a function of the desired flow rate, must be such that the maximum speeds of impact on the elements making up the crane 2 are not more than 0.4 meters per second (m/s). This avoids any generation of turbulence which would be harmful to the stability of the mixing zone 3. b) Hot Air Flow Rate The hot air flow rate is chosen so as to guarantee a flow speed of about 0.04 m/s through the horizontal cross-sectional area of the top volume 5. In view of the intake speed and of the distribution chosen for the feed openings for feeding the top zone 5, the flow in the vicinity of the top plane of the mixing zone 3 can be considered to be a laminar flow in which turbulent diffusion is negligible, and, even for the finest polluting particles (which diffuse the fastest), the Brownian diffusion speed is much lower than 0.04 m/s. 3) Cold Air Intake Openings (Slots) (see FIGS. 2, 4, 5, and 6) a) General Configuration The cold air delivery openings D are disposed and shaped so as to make the concentration of polluted air and of hot air within the ambient environment in the bottom zone (cold zone) 4 as uniform as possible, while limiting the vertical components of the random speeds of the induced turbulence. The narrow delivery openings D (in the form of vertical slots or xe2x80x9cloopholesxe2x80x9d) are situated in the vicinity of the floor on the long sides of the cell, and are disposed in staggered manner. This layout produces interfitting jets 10 of air having vertical axial planes (see FIG. 6). By a shear effect due to the opposite speeds, these plane-jets produce eddies whose speeds have small vertical components and which cause the currents output by the various sources to mix with the ambient environment (see FIG. 5). It is observed that the eddies having horizontal axes and generating vertical random speeds are produced from an area that is very small (the area of a xe2x80x9cloopholexe2x80x9d) compared with the area of the interface between jets 10. Furthermore, it can be seen that the speed differential between the top portions of the jets 10 and the almost immobile ambient environment in the cell is half as large as the speed differential between opposite jets, thereby generating horizontal currents that predominate considerably relative to the upwardly-directed vertical currents. For these two reasons, the vertical components of the random speeds are significantly attenuated, and the phenomena of penetration into the mixing layer are thus limited; in addition, this layout tends to brake, by dilution, the upward speed of the currents output by the heat sources, this being the most important factor in any possible penetration of the pollution into the protected zone 5. b) Cold Air Flow Rate The value of the mean upward speed of the air is chosen to be about 0.04 m/s in order to limit entrainment of polluting particles by the ventilation air from the bottom zone 4 to those particles whose xe2x80x9caerodynamic diameterxe2x80x9d is smaller than 35 xcexcm. The particles having a larger diameter tend to settle in the cell, they do not adhere to the walls, and they can be removed by vacuum cleaning. This speed defines a flow rate that is a function of the horizontal cross-sectional area of the cell, and that must be high enough, in view of the feed temperature of the cold air and of the power of the heat sources 1, to maintain a sufficiently low temperature in the cold zone 4. In certain applications in which the mean heat power given off by the heat sources per unit volume of the cell is very high, a unit for cooling the feed air may then be necessary to limit its flow rate. In that particular case, the most rational solution may be to use a heat pump for raising the temperature of the hot air while lowering the temperature of the cold air. 4) Extraction Slots B, C (for Cold Air (C) and for Hot Air (B)) The cold air extraction slots C and the hot air extraction slots B are disposed in horizontal lines constituting slots that are almost continuous (gaps between the vertical sides of the suction openings as narrow as possible), the horizontal lines extending facing each other on the long sides. The top level of the cold air extraction slots C defines the bottom plane of the mixing zone 3, while the bottom level of the hot air extraction slots B defines the top plane of said mixing zone 3. The top level of the hot air extraction slots B must be situated about 1 m below the bottom level of the volume in which the hoist moves. When the crane has a vehicle deck, the top level of the hot air extraction slots must be situated below the level of the deck of said crane. With reference to FIG. 3, the following may be specified. A shallow temperature gradient is observed in the bottom volume, because of the presence of the polluting heat source. The desired steep temperature gradient is observed in the mixing zone 3 which constitutes the (virtual) confinement barrier. With reference to FIG. 4, it is specified that, for reasons of simplification, the crane 2 is not shown. The method of the invention has been implemented in the vitrification cell whose dimensions are specified above, with the apparatus described above and shown diagrammatically in accompanying FIGS. 2, and 4 to 6. The following table gives the characteristics of said method and apparatus.