US3327763A

US3327763A - Evaporator for concentrating radioactive solutions

Info

Publication number: US3327763A
Application number: US396026A
Authority: US
Inventors: Auchapt Pierre; Bouzou Georges; Sautray Roger
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1963-01-24
Filing date: 1964-09-14
Publication date: 1967-06-27
Anticipated expiration: 1984-06-27
Also published as: FR85175E; CH421054A; GB1035130A; FR84487E; NL6410474A; BE653107A; GB1039303A; DE1223963B; ES304195A1; LU46903A1; FR1354259A

Description

June 1967 P. AUCHAPT ETAL EVAPORATOR FOR CONCENTRATING RADIOACTIVE SOLUTIONS a Sheets-Sheet 1 Filed Sept. 14, 1964 mhqmhzwuzou a:

ESQ E5, 02:08 E

INVENTORS 7 /5225 Hum/1 PT GEORQES Bouzou BY ROGER Seam/1r.

' ATTORNEYS J n 27, 967 P. AUCHAPT ETAL 3,327,763

EVAPORATOR FOR CONCENTRATING RADIOACTIVE SOLUTIONS Filed Sept. 14, 1964 3 Sheets-Sheet 2 u: uJ '52 z m a a ,5 33 g 3% Tl CONCENTRATE FIG. 2

VAPOR AND uoum DROPLETS EQUALIZING PRESSURE LINE 248 ATTORNEYS 3 Sheets-Sheet 3 June 27, 1967 P. AUCHAPT ETA-L EVAPORATOR FOR CONCENTRATING RADIOACTIVE SOLUTIONS Filed Sept. 14. 1964 United States Patent 3,327,763 EVAPORATOR FOR CONCENTRATING ACTIVE SOLUTIONS Pierre Auchapt, Georges Bouzou, and Roger Sautray, 1 Bagnols-sur-Ceze, France, assignors to Commissariat a IEnergie Atomique, Paris, France Filed Sept. 14, 1964, Ser. No. 396,026 Claims priority, application France, Sept. 19, 1963,

948,082; Jan. 30, 1964, 962,029

7 Claims; (Cl. 159-14) The present invention is concerned withan evaporator for concentrating radioactive solutions, as applicable especially ,to the concentration of solutions of plutonium.

It is frequently necessary to concentrate radioactive solutions in order to obtain a concentrated residual solution and a distillate which has a low level of activity. Thechief problem which arises from the concentration of solutions of the kind referred to is the danger of criticality which, up to the present time, has imposed narrow limitations on the maximum volume of solution which can be processed in a single oepration. v

The present invention is directed to an apparatus of relatively simple design which permits of rapid concentration of radioactive solutions and also makes it possible to process a high flow rate and a high volume of solution, this being achieved without any attendant danger of crit-icality.

With this end in View, the evaporator according to the invention comprises a vessel which is intended to be partially filled with the solution to be concentrated, 'atleast one thermo-siphon comprising a lower branch which is connected to the base of said vessel, "a heating branch, an upper branch providing a return to the vessel and opening tangentially to said vessel, a pipe'for the evacuation'of the vaporized solvent which is connected tothe top portion of the vessel, a chamber for supplying the solution to be concentrated anda chamber for the withdrawal of the concentrated solution.

The vessel which is intended to be filled withthe solution to be concentrated has a fiat cylindrical shape which is separated into two parts in the direction of its height by a perforated stainless steel plate on which is supported a layer of Raschig rings formed, for example, of boron glass; the vessel is coupled to the thermo-siphon by means of an annular pipe, at least the-internal portion of which is lined with a neutron-absorbing material. If necessary, the outer portion can also consist of a neutron-absorbing RADIO- ma'terial.

In accordance with a particular form of embodiment, the heating branches have a lobed right section which is preferably three-lobed, each lobe being occupied by heating'means which are disposed substantiallyalong the axis of'the lobe and the other branches of which have a flattened cross section. The term"thermo-siphon which is employedv in this description must be considered in a particular sense in which the circulation of liquid which is initated by heating this latter is essentially due to a process of boiling in a vertical branch of a heating circuit. In this latter case, when the apparatus is in the inoperative" condition, the level ofliquid 'at rest'may occupy only the bottom of the fiat, vessel or even be located at a level below the upper branch which provides a return to the ves'sel. It is only when, the apparatus is in operation that, the volume of liquid having increased as a result of boiling, the vessel is thus partly filled with the liquid.

The invention also consists of other arrangements which can usefully be employed in conjunction with those refered to above but which can also be employed independently thereof. Allof these arrangements will become more readily apparent by consideration of the following "ice description of two forms of embodiment of the invention which are given solely by way of example without any limitation being implied. The description relates to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view, partly in section, of an evaporator according to the invention.

FIG. 2 is a diagrammatic view in elevation, partly in section, of another evaporator in accordance with the invention.

FIG. 3 is a diagrammatic sectional view, taken along the line III-III of FIG. 2, of one portion of the :device of FIG. 2 as represented on a large scale.

The evaporator which is illustrated in FIG. 1 shows the apparatus consisting of a vessel 2 which is intended to be partly filled with the solution to beevaporated. The vessel 2 is fitted at the bottom with a vertical connectorpipe 4 and at the top with a connector-pipe 6 for the discharge of vapor one of the heating circuits 14 being provided with a lower, branch 16 which is connected to the connectorpipe 4, with a heating riser 20 and an upper branch 22 providing a return to the vessel 2 which opens tangentially to said vessel and substantially at onehalf the height of this latter. I I

, The heating branch 20consists of a conduit which is of larger diameter than that of the

branches

16 and 22 and in which is mounted a heating rod 28 consisting-of a sheath of silica, for example, in which are fitted electric heating resistors having a suitable power rating and sup plied through lead-wires or conductors such as the conductor 30. Provisionis made between the'sheath 28 and and the wall of the branch 20 for a radial clearance within which the solution to be concentrated-is intended to circulate. The heating is designed to establish the circulationjof the liquid by a rising film effect, a mixture of liquid and vapor being circulated through the heating branch 20. A heat exchanger 42 is located next in sequence to the outle t pipe "6 of vessel 2 and performs the function of a condenser. Said heat exchanger is provided with an outlet 44 for draining-01f the condensate.-

The vessel 2 consists o'fa cylindrical portion 50 which is surmounted by a portion 52 having the shape or a spherical cap, the entire assembly being fabricated of stainless steel. Slightly below the top portion of the cylinder 50 and approximately at the height of the general level 54 of the solution,- p rovision is made fora plate 56 of perforated stainless" steel which supports a layer" of glass Raschig rings as represented diagrammatically at58, and the object of which is to prevent the entrainment of drops of liquid in the vapor phase as well as to forestall any dangerof c-riticality in the exceptional'event that the level of liquid were to'rise within the vessel. The top portion '52 of the vesselis fitted, apart from the'connectorpipe 6, with two additional connector-pipes, namely a pipe 60 whose function will be definedhere'inatter and a pipe 62 which is fitted witha' cock 64 and serves for decontamination. v

The outlet pipe 6 is coupled by means of an elbow 66 to a cyclone '68 of known type which, as a result'of the action of centrifugal force, is designed to retain'the drop lets of liquid which might otherwise have been carried away by the vapor phase. Owing toavery good vapor decontamination factor, this feature makes it possible to obtain a distillate which isas free as possible from radio active materials, andesp'ecially from plutonium. The but tom portion of the cyclone 68 is coupled bymeans of an elbowed tube 70 to the conduit 72 which serves to convey separated liquid towards the heating system. The top portion of the cyclone '68 is coupled to the condenser 42 of a known type which is referred to as a coil'condenser in which a double coil of stainless steelis fed at a suitable rate with cooling water whichis admitted at 74 and discharged at 76.

The admission and withdrawal of solution are carried out through the intermediary of two

similar chambers

78 and 80 which each consist of a glass cylinder which is clamped between two metallic end-plates, the top endplate being designed to carry

safety probes

82, 84 and 86, 88 for checking the top and bottom levels in both chambers. The top portions of said two chambers are connected by means of flexible pipes 90 and 92 which form a T-junction so as to pass through a cooling unit 94 and terminate in the connector-pipe 60 of vessel 2, thus establishing an equalizing pressure relationship between the two chambers and the vessel and providing a first means of controlling the level of liquid in the vessel with a view to preventing abrupt variations in level which could otherwise result from the decomposition into carbon dioxide gas of certain organic elements of the solution. The bottom portion of the feed chamber 78 is fitted with two pipes, namely a pipe 96 for pumping the solution which is stored in the tank 98 by means of the pump 100 and a .pipe 102 for supplying the conduit 72 with solution. The withdrawal chamber 80 is also fitted with two pipes which operate as overflows, namely one pipe 104 which joins the bottom portion of the chamber 80 to the conduit 72 and a pipe 106, the top end 108 of which determines the general level 54 of the liquid and the bottom end of which terminates in a cooling unit 109 and in the outlet 110 1 through which the concentrate is delivered. Said

chambers

78 and 80 are adjustable in height and make it possible on the one hand to observe the general level of the liquid and, on the other hand, to provide a second means of regulating said level according to the density of the concentrate.

In a preferred form of embodiment of the invention which is intended for the concentration of plutonium oxalate mother-liquors and which will be described hereinafter by way of example with reference to FIG. 1, the vessel 2 consists of a cylinder 50 of stainless steel 225 millimeters in diameter and 40 millimeters in height which provides for the liquid an evaporation surface of 398 cm?. Provision is made at 35 millimeters from the base of the separator for a stainless plate having a thickness of 3 millimeters and perforated with holes 8 millimeters in diameter. The said plate supports a layer -8 having a minimum thickness of '60 millimeters and consisting of 10x 10 mm. Raschig rings, formed of glass containing 12.90% B 0 namely 4% boron. The condenser 42 comprises two coils 12 0 of stainless steel having a total length of 13 meters and having a heat transfer surface area of 4,150 cm. and capable of removing 10,500 kcal./h. by means of a flow of water of 925 l./h. (liters per hour), the temperature of the water at the inlet, at 74, being C. and the temperature at the outlet, at 76, being 40 C.

The

chambers

78 and 80 each consist of a glass tube 75 millimeters in diameter and 125 millimeters in height clamped between two metallic end-plates which carry the safety probes, the pressure Supply valves and the liquid inlets and outlets The nest of heating tubes is entirely constructed of stainless steel and consists of three tubes such as the tube 20' which are parallel to each other and welded at their extremities to two

flat collectors

112 and 114 which provide a clearance of 20 millimeters for the passage of the liquid. The immersed heater elements such as the rod 28 which are located at a distance of at least 40 centimeters from the vessel are formed of quartz and centered by means of six shoulder brackets which have not been shown in the drawings and which are welded to the lower base of each tube such as the tube 20. Sealing joints of Teflon which are also not shown in the drawings are fitted at the top portion of the tubes 20 between these latter and the immersion heater rods '28, the distance between the wall of the immersed heater rods and that of the tubes being 10 millimeters for the passage of the liquid. The three immersion heater elements have a unitary power rating of 4 kw. at a maximum voltage of 220 v. Each immersion heater element is controlled in dependence on the

safety probes

82, 84, 86 and 88, mounted on the admission and delivery chambers 78 and is provided 80, and with an alarm amrneter which shuts down the installation.

The heating power can be caused to vary by modifying the voltage developed across the terminals of the heating resistors by means of the 3-phase transformer, the rated power of which is 30 kvA. A manual-switch voltmeter serves to determine the voltage developed across the terminals each resistor.

Subject to possible heat losses which are in any case small and of the order of 3% since the apparatus is heat-insulated, the volume of distilled liquid is proportional to the electric power supplied to the resistors. The table which is given below indicates the hourly volume distilled as a function of the electric input power.

Power in kw.: Output of distillate in l./h. l2 15.5 11 14.3 10 13.0 9 11.7 8 10.4 7 9.1 6 7.8 5 6.5 4 5.2

A number of measurements of overall thermal efficiency have been taken and the results vary between 93 and 97%.

In an apparatus such as that which has just been described, the total quantity of liquid contained is 4.4 liters.

The

probes

82 and 84 of'the chamber 78 have for their object to correct any possible variations in delivery of the proportioning pump 100. In the event that the delivery of said pump decreases, the level drops within the chamber 78, the probe 84 is then uncovered, thereby producing the stoppage of the heating and the lighting-up of an alarm signal. If, on the contrary, the delivery of the pump 100 increases, the level of the liquid rises within the chamber '78 and reaches the probe 82, thus producing the stoppage of the installation and the lighting-up of an alarm signal. A similar safety system operates within the chamber by virtue of probes 86 and 88 in the event of crystallization occurring within the concentrate. Furthermore, if crystallization takes place within the evaporator vessel itself, the liquid no longer circulates, the evaporation process stops, and safety devices mounted on the outlet of the cooling water of the condenser 42 and on the tubes 14 of the heating coil, initiate the total stoppage of the installation as well as the actuation of an alarm signal.

FIG. 2 represents a second evaporator in accordance with the invention which is intended to be placed within a hot cell separated by a leak-tight wall A into two compartments B and C. Compartment B contains the evaporator proper and the devices for retaining those liquid droplets which are entrained by the vaporized solvent and for returning them to the evaporator. Compartment C contains the ancillary devices such as' the condenser for recovery of the solvent and devices for admission of the solution to be concentrated and for draining-01f the solution once it has been concentrated.

The evaporator comprises a vessel 202 formed of two assembled parts which has a substantially cylindrical shape and a small height with respect to its diameter. During operation, the effluent in liquid phase which is to be concentrated occupies the bottom of said vessel to a depth which must remain sufliciently small to prevent any criticality hazard. The vessel 202 contains boron glass Raschig rings 236, said rings being located above the efiluent and designed to retain any droplets of solution which'could otherwise be carried away by the vaporized solvent.

The vessel 202 is fitted at the bottom portion thereof with a vertical annular tube 204 having a thickness which is small with respect to its diameter and is fitted at the top portion thereof with a connector-pipe 206 for the withdrawal of the solvent in vapor phase. The extremity of the vertical tube 204 is fitted with a pipe 212 for the purpose of filling the vessel with effluent to be concentrated and for returning to the solution the droplets which have been entrained by the vapor. The said pipe 212 is fitted with a device which has not been illustrated but which is of similar design to the admission chamber 78 of FIG. 1 and serves to regulate the level of the liquid within the vessel 202.

' There are associated with the vessel 202 two heating circuits 2'14 and 214', although this number is not given by way of limitation. These circuits are of similar design and, consequently, only the circuit 214 will be described. The circuit 214 comprises a lower branch 216 (as shown in FIGS. 2 and 3) having a common trunk 217 subdividing into two conduits 218 (FIG. 3), two heating elements 220 and an upper branch 222 which also comprises two lateral conduits 224 which return to the vessel 202 via a common trunk. Each branch 220 is associated with a lateral conduit 218 and a lateral conduit 224.

The recovery of the solution once this latter has been concentrated is effected by means of a conduit 226, said conduit being connected to a delivery chamber which has not been shown in the drawings but which is similar to the chamber 80 of FIG. 1 and which is fitted with suction means.

The heating branches 220 of the evaporator according to the invention consist of conduits having a lobed cross section. There is mounted in each lobe an immersion heater rod 228 which is formed of silica, for example, and which is fitted with electric heating resistors having a suitable power rating and supplied with current by means of leads 230. .A radial clearance through which the solution to be concentrated is intended to circulate is formed between the immersed heater rod 228 and the wall of the branch 220. When the immersed heater rods 228 are supplied with current, a circulation of liquid by a rising film effect is established in the direction indicated by the arrows f. This circulation can be accelerated by making provision for a pump, but this complicates the system and is not usually desirable.

The use of a lobed cross section and not a cylindrical section permits a smaller volume for a given heating perimeter, which is an advantageous arrangement from the point of view of criticality hazards.

The lower and

upper branches

216 and 222 have a flat rectangular cross section, namely a height which is small with respect to the width thereof, the object of this arrangement being to rule out any danger of criticality.

The fluid-circuits 214 and 214' which have been shown in FIG. 2 each open tangentially into the vessel 202 in diametrically opposite zones and in opposite directions.

In order to complete the separation of liquid phase and vapor phase as initiated by the boron glass Raschig rings 236, the discharge pipe 206 opens into a cyclone separator 234 which can be of any conventional type. The vaporized solvent flows in the direction shown by the broken-line arrows in FIG. 2 and the vertical flow motion which is generated within the separator tends to remove liquid droplets. Accordingly, after being freed from the liquid droplets which might still have been contained therein, the vapor phase passes out of the separator into a conduit 237 which traverses the partitionwall A and opens into a condenser 242. The droplets which are retained within the cyclone return to the conduit 212 through a pipe 238 which is fitted with a fluid trap 240 (arrows F in FIG. 2). The condenser 242 consists of a heat exchanger of conventional design through which cooling water is circulated. The condensed solvent is withdrawn through the draining-off pipe 244 containing valve 246.

The pressures which prevail within the admission chamber and delivery chamber (which have not been shown in the drawings) and within the vessel 202 are balanced by means of a conduit 248 (as shown in FIG. 2) which is similar to the conduit -92 of FIG. 1.

In order to prevent neutron interaction between the different component units of the evaporator, certain opposite facing walls of said different component units of the evaporator in accordance with the invention are covered with a material which has a high neutron capture cross-section. It is possible, for example, to make use of sheets of cadmium foil which are applied in close contact with the walls.

The arrangements which have just been described make it possible without difiiculty to achieve subcriticality on the one hand of each component unit and on the other hand of the evaporator as a whole while also permitting of small'overall size. It is of interest to note the follow ing numerical values which are given by way of example and which correspond to an apparatus for the continuous processing of solutions of plutonium with an evaporation rate of the order of 60 l/h.

The characteristics of the apparatus are such that each unit satisfies at least one of the following conditions as regards the solution contained therein:

Volume of vessel lit-ers Smaller than 4 Diameter of vessel mm Less than Depth of liquid mm Less than 40 The vessel 202 consists of a cylindrical chamber 450 millimeters in diameter, the bottom of which is filled with solution up to a maximum depth of 35 millimeters during operation and the top portion of which is occupied by boron-type Raschig rings up to a height of approximately 60 millimeters.

Each heating circuit comprises two heating branches having a three-lobed right section and each fitted with three immersion heater elements having a power rating of 4 kw. The lower and

upper branches

216 and 222 have a height of 20 millimeters and a width of millimeters whereas the connector-pipe 204 has an external diameter of millimeters and a radial width of 20 millimeters.

Each unit therefore satisfies at least one of the conditions of subcriticality. In order to prevent any appreciable neutron interaction between the different units, it is merely necessary to provide a clearance of 400 millimeters between these latter; in the apparatus described, the clearances provided are substantially in excess of this value. The presence of neutron-absorbing cladding material such as cadmium and polythene reduces neutron interaction even further. In fact, the geometry of the entire assembly makes it possible to dispose blocks of polythene or any material of the same type between the units referred to in order to take part in the absorption both of neutrons and of gamma radiation which is induced within the cadmium by the neutrons.

Such blocks

250, 252 and 254 are represented diagrammatically in chain-dotted lines in FIGS. 2 and 3.

As will readily be apparent, the invention is not limited to the forms of embodiment which have been illustrated and described by way of example and it will be understood that the scope of this patent extends to any equivalent arrangement.

What we claim is;

1. An evaporator for concentnating radioactive solutions comprising:

(a) a vertical vessel of substantially circular crosssection which is intended to be partly filled with the solution to be concentrated;

(b) a plurality of thermo-siphons each comprising:

(1) a lower solution conveying branch duct communic'ating with the base of said vessel,

(2) an upright solution heating branch duct commnnicating with said lower branch duct-consisting of a vertical pipe and at least one vertical heater rod concentrically disposed therein forming an annular space for movement of said solution andvapor and (3) an upper solution-conveying branch duct communicating said heating branch duct with said vessel and opening tangentially into said vessel; (c)' a pipe for the evacuation of vaporized solvent which is connected to the top portion of the vessel; (d) a respective feed chamber communicating with each thermo-siphon in feed-supplying relation there- (e); a withdrawal chamber communicating with each themo-siphon in concentrate-removal relation thereto, and

(f) means in said withdrawal chamber for regulating the level of the solution in said vessel to a substantially constant height.

2. An evaporator according to claim 1 in which the liquid containing portions are of subcritical dimensions.

3. An evaporator in accordance with claim 1 wherein the, pipe for the, evacuation of the vaporized solvent is provided with a vapor-liquid cyclone separator whose bottom portion is connected to and communicates with the lower branch duct of the ther-mo-siphon by a return line for the liquid and is provided with a condenser located downstream of said cyclone separator on said evacuation pipe for condensing said vapor.

4. An evaporator in accordance with claim 3, wherein blocks of a material absorbing neutrons and gamma radiation are interposed between the vessel and the heating branches.

5. An evaporator for concentrating radioactive solutions comprising:

(a) a vertical vessel of substantially circular crosssection and which is intended to be partly filled with the solution to be concentrated;

(b) at least one thermo-siphon comprising:

(1) a lower branch duct connected to and communicating with the base of said vessel,

(2) a solution heating branch duct communicatingwith said lower branch duct consisting of a vertical pipe having a lobed cross-section and.

one vertical heater rod disposed substantially concentrically in each lobe forming an annular lobe-shaped, rod-surrounding space for upward movement of said solution and generated vapor and (3) an upper branch duct communicating with said heater branch duct and said vessel at their upper ends providing a return to said vessel and openlng tangentially into said vessel; (c) a pipe for the evacuation of vaporized solvent connected to the top portion of the vessel;

((1) means for regulating the level of solution in said.

vessel and communicating therewith; (e) means for supplying said solution to be concentrated to said vessel and communicating therewith; V

and (f) means communicating with a bottom portion of said vessel for withdrawing concentrated solution.

References Cited UNITED STATES PATENTS 1,191,108 7/1916 Kestner 159l4 1,819,517 8/1931 Lichtenthaeler 122 -133 X 2,605,38i1 7/1952 Head 2l9--284 X 3,181,593 5 /1965 Lindley Q l59-1 3,205,934 9/1965 Vincentet a1. l5947 NORMAN YUDKOFF, Primary Examiner.

I. SOFER, Assistant Examiner.

Claims

1. AN EVAPORATOR FOR CONCENTRATING RADIOACTIVE SOLUTIONS COMPRISING: (A) A VERTICAL VESSEL OF SUBSTANTIALLY CIRCULAR CROSSSECTION WHICH IS INTENDED TO BE PARTLY FILLED WITH THE SOLUTION TO BE CONCENTRATED; (B) A PLURALITY OF THERMO-SIPHONS EACH COMPRISING: (1) A LOWER SOLUTION CONVEYING BRANCH DUCT COMMUNICATING WITH THE BASE OF SAID VESSEL, (2) AN UPRIGHT SOLUTION HEATING BRANCH DUCT COMMUNICATING WITH SAID LOWER BRANCH DUCT CONSISTING OF A VERTICAL PIPE AND AT LEAST ONE VERTICAL HEATER ROD CONCENTRICALLY DISPOSED THEREIN FORMING AN ANNULAR SPACE FOR MOVEMENT OF SAID SOLUTION AND VAPOR AND (3) AN UPPER SOLUTION-CONVEYING BRANCH DUCT COMMUNICATING SAID HEATING BRANCH DUCT WITH SAID VESSEL AND OPENING TANGENTIALLY INTO SAID VESSEL; (C) A PIPE FOR THE EVACUATION OF VAPORIZED SOLVENT WHICH IS CONNECTED TO THE TOP PORTION OF THE VESSEL; (D) A RESPECTIVE FEED CHAMBER COMMUNICATING WITH EACH THERMO-SIPHON IN FEED-SUPPLYING RELATION THERETO; (E) A WITHDRAWAL CHAMBER COMMUNICATING WITH EACH THERMO-SIPHON IN CONCENTRATE-REMOVAL RELATION THERETO, AND (F) MEANS IN SAID WITHDRAWAL CHAMBER FOR REGULATING THE LEVEL OF THE SOLUTION IN SAID VESSEL TO A SUBSTANTIALLY CONSTANT HEIGHT.