FR2555911A1 - Process and apparatus for eliminating silicon from a coolant consisting of sodium at high temperature - Google Patents

Abstract

Process and system for eliminating silicon from a coolant system containing liquid sodium at high temperature for a nuclear reactor. The sodium is cooled 15 to a temperature below the temperature of saturation with silicon and is maintained at this low value 16 while strong turbulence is induced by the member 28 in the sodium flow to promote the precipitation of silicon compounds and to facilitate the final separation of the particles of silicon compounds from the liquid sodium.

Description

ST APPARATUS FOR THE REMOVAL OF SILICON D1UN
HIGH TEMPERATURE SODIUM REFRIGERANT.

The present invention relates, in general, to the removal of impurities from a nuclear reactor coolant and, more specifically,
The elimination of silicon from sodium used as a coolant in fast breeder nuclear reactors.

 A typical fast breeder nuclear reactor uses liquid sodium as a coolant to remove the very large amount of heat produced by nuclear fission of fissile materials. The liquid sodium circulates in a closed heat transport system which includes the reactor vessel, a heat exchanger, a piping system suitable for connecting these components in series with each other, and a pump for circulating the refrigerant in this system.

During the operation of the nuclear reactor, various contaminants or impurities formed by the reactions occurring in and with the liquid metallic refrigerant, are entrained or dissolved in the liquid sodium. These impurities can be transported around the loop system and can lead to system corrosion or the formation of solid precipitates which can cause blockage and lead to other adverse effects in the system. To remove some of these impurities, cryogenic traps have been designed and incorporated into these liquid metal cooling systems. Cryogenic trapping is a technique used to produce precipitation or nucleation of impurities under low temperature conditions, in a controlled manner, thereby purifying the metal Refrigerant liquid.

 It was known that silicon, as a chemical element, was one of the many traces of impurities in the closed-loop sodium system.

However, due to its minor presence in certain cryogenic trap deposits, and due to the instability of certain silicon compounds in a medium consisting of metallic sodium, it had not been considered to be a notable disadvantage in operation. sodium-based heat transport systems. However, recent studies have shown, contrary to popular belief, that the transport of large quantities of silicon is responsible for deposits in sodium heat transport systems, particularly in
New systems, which can have significant repercussions on the functioning of these systems.

 Silicon, as a chemical element, is normally found in iron and steel. In the 300 series stainless steels used in the cladding of fuels, pipelines and other internal components of fast liquid metal fast breeder reactors, the silicon content is approximately 0.5 percent by weight. This silicon is extracted from the sodium-wetted surface of new stainless steel and dissolves in sodium at temperatures above 4800 C. In cooler areas of the Sodium Refrigerant System, this dissolved silicon is deposited in the form of 'a crystalline compound. These head losses create increased hydraulic friction and resistance to flow of the refrigerant, which leads to an increase in the pressure drops of the refrigerant, and they can also play a very important role in weakening the heat transfer rate. In addition, the particles resulting from this crystal growth disintegrate and clog the flow control valves, filters and other narrow circulation passages.

 All of the above factors contribute to increased pressure drops and decreased cooling efficiency, which results in overloading the pumps and other components. The conventional cryogenic trapping techniques mentioned above, and which are normally provided for in liquid metal cooling systems, have not proved effective in collecting or eliminating silicon, since they are carried out at too low a temperature. . In addition, the particles entering it tend to be recycled and to dissolve in the circulating refrigerant when they are entrained in higher temperature zones of the system.

 The main object of the invention is therefore to remedy the above drawbacks by providing a new and useful method and a system for efficiently removing silicon from a liquid metal cooling system.

 Another object of the invention is to provide a method for promoting the precipitation of silicon compounds in a controlled manner and environment for their subsequent removal from a liquid metal cooling system.

The invention further aims to provide a new and useful system to facilitate precipitation in the above-mentioned process, by increasing the deposition and growth of crystals of compounds of
silicon, on collection surfaces of the system where
they grow to a size that allows them to be effectively eliminated.

The invention further aims to provide, in the previous system, a composite device,
incorporated, new and useful, to lower sodium
at high temperature, at a temperature below the silicon saturation temperature, as well as to promote the precipitation of crystals of silicon compounds.

 These aims, advantages and characteristics of the present invention, as well as others, will be clearly understood on reading the description below, made with reference to the appended drawings in which the identical parts of the various views are designated by identical reference.

 The subject of the invention is a method and a system for materially reducing the amount of silicon present in a liquid sodium cooling system of a nuclear reactor or in other liquid sodium systems.

 According to the invention, at least part of the sodium is cooled to a high temperature containing dissolved silicon, below the silicon saturation temperature and is kept at this reduced temperature for a given time, while inducing high turbulence. in the sodium flow at reduced temperature to promote the precipitation of silicon. The solid silicon particles resulting from this precipitation are then separated and removed from the liquid sodium.

Figure 1 is a flow diagram of part of a liquid metal cooling system having a preferred form of the silicon removal system of the invention;
Figure 2 is a longitudinal sectional view of a preferred embodiment of the composite economizer / crystallizer apparatus forming part of the present invention;
Figure 3 is a sectional view taken along Line 3-3 of Figure 2;
Figure 4 is a longitudinal sectional view of a separator used in association with the present invention;
Figure 5 is a plan view from above of the separator shown in Figure 4;
Figure 6 is a longitudinal sectional view of a preferred embodiment of the cooling device produced in accordance with the present invention;
Figure 7 is a vertical side view of a diffuser used in the cooler used in association with the present invention; and
FIG. 8 is an anterior vertical view of the diffuser in FIG. 7.

 Referring now in detail to the embodiment described by way of illustration in the accompanying drawings, Figure 1 schematically shows a system, generally designated 10, of the present invention for removing entrained silicon dissolved in a metallic refrigerant liquid such as sodium, used in a heat transport system of a nuclear reactor. As is known, the primary coolant of the reactor, liquid sodium, is heated to extreme temperatures, passing through the core of a vessel containing the nuclear reactor.

This hot liquid sodium flows in the primary system to a heat exchanger used to transfer the heat from the primary system to another refrigerating system or secondary system tightly connected to the primary system for the final conversion into steam. water, to produce electrical energy.

The system 10 comprises an inlet pipe 11 and an outlet pipe 12 connected to the primary cooling system in a high temperature zone since the solubility of silicon in sodium is higher the higher the temperature. Liquid sodium temperature must be above
lower than the saturation temperature in silicon, this being the temperature, for a particular but arbitrary given concentration, in silicon dissolved in sodium, at which there would occur a thermodynamic equilibrium between dissolved sodium and a solid phase containing silicon. The saturation temperature of sodium in silicon depends on the design of the system, its operation and the history of its operation, and may vary by a low temperature corresponding to the melting point of sodium, at a maximum temperature, determined by the mass of silicon dissolved in sodium. To illustrate the description of the silicon elimination system of the invention, it will be assumed that the temperature of the sodium entering via line 11 is 540 "C and that the saturation temperature of silicon is 480" C.

 Means are provided in the system 10 for cooling the hot liquid sodium entering below the silicon saturation temperature and for promoting its precipitation, in a controlled manner and mainly in a desired area, in the form of crystal growth. of silicon compounds on the walls of the drainage pipes. To this end, these means comprise a unit or composite economiser / crystallizer 13 having an upper part acting as an economizer or heat exchanger 15 serving to reduce the temperature of hot sodium and a lower part acting as a crystallizer 16 serving to favor precipitation. The terms "upper", "lower", "above", "below", "vertically", "horizontally", etc., used here, are only intended to facilitate the description of the invention with reference to the drawings, and should not be considered as limiting the scope of the invention.

 As shown in FIG. 2, the economizer part 15 comprises an elongated outer cylindrical jacket 17 and an elongated hollow inner tube 18 concentric with the jacket 17 and defined by a cylindrical wall 20 radially spaced from the wall of the jacket 17. The inner tube 18 defines an axial passage 21 and the space between the jacket 17 and the tube 18, an annular passage 22. The upper end of the tube 18 is provided with a hot sodium inlet orifice 23 connected to the pipe 11 and its lower end forms an integral part of the upper end of the crystallizer part 16 so as to minimize the turbulence in the sodium passing from one to the other.

The lower end of the jacket 17 is shaped so as to have an inlet and return orifice 25 and its upper end is shaped so as to have an outlet orifice 26. Thus, the hot liquid sodium penetrating through the orifice inlet 23 flows into passage 21 in relation to heat exchange with the colder sodium flowing upwards in annular passage 22, to cool the hot sodium to a desired temperature range, below of the silicon saturation temperature.

 The crystallizer part 16 of the apparatus 13 comprises an elongated hollow tube 27 constituting an axial probing and a continuation of the internal tube 18.

A solid central element 28 is mounted concentrically inside the lower part of the tube 27, so as to define a narrowed annular passage 30 between the tube 27 and the central element 28. Although the central element 28 is shown worn and fixed to the inside of the tube 27 by means of radial fins arranged along its circumference and separated by suitable spaces or openings, it should be noted that the specialist in the technique will be able to find numerous mechanical means making it possible to fix the central element 28 inside the tube 27. The inner end of the tube 27 is equipped with an outlet orifice 32 connected to a pipe 33 (FIG. 1).

 To produce optimal precipitation through the formation and growth of crystals, it has been found desirable to maintain the temperature of the cooled sodium exiting from passage 21, in an appropriate range below your saturation temperature in sodium for a given time and to increase the sodium circulation speed to induce a strong turbulence increasing the speed of deposition of the silicon compounds on the surfaces of the interior walls of the tube 27. For this purpose, the empty area, at the interior of the tube 27, located above the central element 28, defines a chamber serving to provide a relatively low speed flow zone 35, making it possible to obtain the desired retention time, and The passage 30 defines a high speed flow zone used to induce a strong turbulence in the sodium flow to increase the mass transfer rate of silicon on the fixed surfaces of the flow zone.

This high-speed flow zone also facilitates the separation or rupture of the particles from the deposit of silicon compounds on the walls of the tube, these being then entrained at the same time as other particles by the sodium flow, leaving the orifice 32 and entering the pipe 33.

 The system 70 includes a flow meter 36 (FIG. 1) in the circuit 33 to monitor the flow in the system, and preferably two pumps 37, which can be controlled at the outlet of the flow meter 36, to circulate the liquid sodium in the system 10 independently of the main or primary cooling system. In addition, a set of pressure gauges 38 and immersion thermocouples 39 are strategically placed at several points in the system 10, to establish and monitor the operating conditions.

 Means are provided in the system 10 for separating and eliminating entrained particles of liquid sodium, including those which have grown in the economizer / crystallizer 13, as well as those which are produced outside the system 10. These means comprise a hydrocyclone, generally designated at 40, located downstream of the economizer / crystallizer 13. As shown in FIG. 4, the hydrocyclone 40 comprises an elongated cylindrical body 41 surmounted at its upper end by an end cap 42, and connected at its lower end, to a receptacle or collector 43. The body 41 is in the form of a conical interior wall 45 defining a conical passage 46 whose diameter decreases progressively downwards. An opening or orifice 47 is formed in the wall 45 near the upper end of the body 41, to receive an inlet reducing element 48 appropriately fixed to the downstream end of the pipe 33. As shown in Figures 4 e t 5, the reducing element 48 is oriented tangentially relative to the upper end of the passage 46 and in a position slightly inclined downward relative to the horizontal plane normal to the longitudinal axis of the body 41. The sodium entraining the particles passes through the reducer 48 and flows through the upper end of the passage 46 in a tangential direction pointing downwards, so as to give the sodium a spiral movement of increasing angular speed, as it s flows downward in the passage 46 This movement propels the heaviest particles towards the surface of the conical wall 45 and, under the effect of gravity, downwards, so that they pass through the orifice narrow outlet 49 and that they enter the manifold 43.

 The manifold 43 comprises a generally cylindrical body 50 having a narrow neck 51, in free communication with the passage of the hydrocyclone 46 and provided with an annular flange 52 provided to adapt to a complementary flange 53 formed at the base of L hydrocyclone 40. A suitable seal C not shown) can be interposed between the flanges 52 and 53. Aligned orifices 54 are formed in the two flanges 52 and 53 to allow the reception of suitable assembly elements (not shown) for removably attach the collector 43 to the hydrocyclone 40. Consequently, the collector 43 makes it possible to permanently keep the separated particles, so that they are not again entrained in the system. periodically disassemble the collector 43 to get rid of the particles which have accumulated there.

 The end cap 42 is welded or rigidly fixed in another way to the upper end of the body 41 and is provided with an axial passage 55 connected to a pipe 56 for passing the purified sodium through it. The purified sodium is then transferred, via line 56 (Figure 1) to an air cooler, designated gLoba Lement at 57, to further cool the purified sodium and thus adjust the thermal profile of the Economizer 15 to which is then directed the cooled sodium.

 As best shown in Figure 6, the air cooler 57 comprises an elongated cylindrical jacket 58 covered with a layer of thermal insulation 60. A tubular cooling coil 61 is suitably mounted inside the jacket 58 and is provided with an inlet port 62 connected to the line 56 and an outlet port 63 connected to a line 65 leading to the inlet port 25 of the economizer 15.

The jacket 58 is provided with a conical inlet end 66 ending in a flange 67 allowing the fixing, by means of suitable assembly elements (not shown) to the flange 68 of a blower 70 (FIG. 1). Air, under a predetermined pressure, is thus blown on the numerous turns of the coil 61 so as to undergo a heat exchange with the liquid sodium which flows there.

 To increase the cooling of the sodium in the coil 61, a plurality of diffusers71 each consisting of a set of stationary radial fins 72, are mounted at axially selected intervals and spaced inside the jacket 58, to give the air flow a spiral path as it travels the coil 61 The spiral movement, imparted to the cooling air, ensures better air distribution and better contact with all of the external surfaces of the coil 61 in which the sodium flows.

 As FIGS. 7 and 8 show better, each diffuser 71 has a structure in one piece, for example produced by a retraction operation, with several fins 72 integral with one another. The fins 72 have a common axis 77 and are spaced on the circumference at equal distances from one another. Each wing 72 is in an inclined position, its lower part 75 lying in a plane practically normal to the axis 73, then gradually tilting, as indicated at 76, towards its most distant end, ending in an end part 77 practically normal to the plane of the internal part 75.

This configuration gives the incident air flow the desired spiral movement. The outer end portion 77 of the fins 72 are welded or otherwise rigidly fixed to the elongated structural elements 78, integral with the inner wall of the jacket 58.

 The cooler 57 cools and conditions the sodium to adjust the thermal profile of the economizer 15. The cooled sodium is transferred, via the channel 65, to the inlet opening of the jacket 25, then in the annular passage 22 in heat exchange relation with the hot sodium flows in the axial passage 21. The cooled sodium passing through the passage 22 is reheated according to the requirements of the main system and again enters the main or primary cooling through the outlet port 26 of the jacket and the outlet port 12.

 In operation, the hot sodium containing dissolved silicon enters the device 13 and is rapidly cooled when it flows through the passage 21 in the economizer part 15, to a temperature below the silicon saturation temperature.

This temperature of the cooled sodium is maintained for a given time, thanks to the low-speed flow zone 35, in the crystallizer part 16.

Instead of providing a low-speed flow zone 35 designed for this purpose in the crystal part
Reader 16 as is the case in the embodiment cited by way of example, the desired retention time could be obtained in an economizer or heat exchanger designed to produce a relatively low cooling rate. In both cases, it has been found that keeping the temperature below the silicon saturation temperature and for a determined time is desirable in this process in order to optimize the crystallization of silicon compounds.

The sodium flow then increases notably when it crosses the high flow flow zone defined by the annular passage 30. This also has the important effect of subjecting the sodium flow to strong turbulence, promoting precipitation. Consequently, the cooling of the sodium, in a desired temperature range, lower than the silicon saturation temperature, the maintenance of this temperature for a given time, then the induction d: strong turbulence, together lead to the precipitation of the silicon in the form of crystalline silicon compounds, mainly on the walls of the tube 27 and on the central element 28 near the passage 30. The size of the crystal lins continues to increase during this process until the force produced by the sodium flow and the turbulence generated in it tears them away or separates them from the wall of the tube in the form of solid particles.

 The sodium laden with particles leaving the crystallizer part 16 enters the hydrocyclone 40 where the particles, under the effect of centrifugal and gravitational forces, separate in large quantities from the liquid sodium and are deposited in the collector 43. The purified sodium is then transferred to the cooler 47 for a further decrease in the sodium temperature, in accordance with the desired thermal profile, required in the economizer part 15. The sodium leaving the cooler 57 is then directed to the outer jacket of the economizer 15 where it is reheated according to the requirements of the main or primary system and it re-enters the primary system.

 The example below is given by way of illustration of the parameters used in the process of the invention, so that the precipitation of silicon, the crystalline growth and the formation of solid particles are optimal. Using the temperatures indicated above, hot sodium containing dissolved silicon enters Economizer 15 at about 5400 C and its temperature there is lowered from about 0.55 to 55 "C below the saturation temperature in silicon from 480 "C up to a temperature greater than or equal to 425" C. This desired temperature is maintained for a given time of approximately 1 to 30 seconds.

To establish the desired thermal profile of the economizer 15, the temperature of the purified sodium is again lowered in the cooler 57, to a temperature in the range, from about 4079 C to 463 C.

The temperature of this cooled sodium rises again
When passing through the jacket side of the economizer, up to a temperature of approximately 520 ° C. or other corresponding to the requirements set by the primary cooling system.

 From what has been mentioned above, it appears that the aims of the invention have been fully achieved. The invention therefore results in a- a new and improved method and system for removing silicon from a refrigerant consisting of liquid sodium. Lowering the sodium temperature to a temperature below the silicon saturation temperature and keeping the sodium at this reduced temperature produces optimal precipitation in the form of crystals of silicon compounds on the surfaces of the drainage pipes. In addition, the strong turbulence conferred on the sodium flow at low temperature promotes crystal growth and particle formation up to large sizes which significantly increase the elimination of silicon from the primary sodium cooling system. .

Claims (18)

REVENDICATIOllâ  1. Process for removing the silicon from a refrigerant consisting of sodium Liquid at high temperature, containing dissolved silicon, characterized in that it comprises: The cooling of at least part of this sodium at high temperature, to a reduced temperature, below the silicon saturation temperature, maintaining this sodium at this reduced temperature for a given time, Inducing a strong turbulence in the flow of this sodium at this reduced temperature, to promote precipitation of particles of silicon compounds, and the separation of these particles from the bulk of this sodium Liquid.  2. Method according to claim 1, characterized in that the temperature of this refrigerant consisting of liquid sodium at high temperature is lowered by a value of at most 550 C, compared to this saturation temperature of silicon.  3. Method according to claim 1? characterized in that this reduced temperature of this refrigerant consisting of liquid sodium is maintained for a time of 1 to 30 seconds.  4. Method according to claim 1, characterized in that this purified liquid sodium is again heated and brought back into a primary cooling system with liquid sodium.  5. System for removing silicon from a flow-through refrigerator consisting of high-temperature liquid sodium containing dissolved silicon, characterized in that it comprises: means (15) for cooling at least part of this sodium to high temperature, at a reduced temperature below the silicon saturation temperature, means for maintaining this sodium at said reduced temperature for a given time, means for producing a strong turbulence in the flow of sodium at this reduced temperature, for favor the precipitation of particles of silicon compounds, and means for separating these particles from the major part of this Liquid sodium.  6. System according to claim 5, characterized in that iL comprises means (40) for eliminating these particles from this system.  7. System according to claim 5, characterized in that these cooling means, these means for maintaining a temperature and these means for producing turbulence, are incorporated in a single composite unit (13).  8. System according to claim 7, characterized in that these cooling means comprise a heat exchanger forming part of this element and comprising an axiaL passage for cooling this coolant at high temperature which passes through you.  9. System according to claim 8 characterized in that these means for maintaining a temperature comprise a tube (27) forming part of this element and having a passage extending this axial passage to define a flow zone at low speed downstream of said axial passage.  10. System according to claim 9, characterized in that these means for producing a turbulence, comprise a narrowed passage (3Q) in this tube, defining a high-speed flow zone downstream of this flow zone at low speed.  11. The system as claimed in claim 8, characterized in that it comprises means (57) for further cooling this sodium refrigerant downstream of these separation means, before restoring the heat transfer between this sodium refrigerant and this high temperature refrigerant, through this heat exchanger.  12. Apparatus for precipitating impurities in a high temperature liquid containing dissolved impurities, characterized in that it comprises a jacket (17), a tibe (18) mounted in this shirtS spaced radially from thatGci and defining between them a passage annular 22 having an inlet and an outlet, this tube defining an axial passage (21) having an inlet and an outlet, this tube having an axial extension (27) defining a chamber in communication alec this outlet opening of the axial passage and continuing by forming a low-speed flow zone, and a central element (28 mounted in this extension near its end remote from this outlet opening of the axial passage, defining a passage narrowed 30 providing a high speed flow area.  13. Apparatus according to claim 12 character - rised in that this annular passage 22 has an axial length less than that of this axial passage.  14. Apparatus according to claim 12, characterized in that this central element is aligned coaxially with respect to this extension, this narrowed passage 30 having an annular cross section.  15. Apparatus for cooling a liquid, characterized in that it comprises: an elongated jacket (5) having an inlet orifice and an outlet orifice near these opposite ends, a tubular coil (61) mounted in this jacket and having an inlet and an outlet for circulating a liquid therein, means for supplying air under pressure in this jacket in heat exchange relation with this liquid, and mounted means in this jacket to direct the air flow in the direction of The length of this jacket, in a spiral path, along this tubular coil.  16. Apparatus according to claim 15, characterized in that these means, for directing the air flow, comprise several diffusers (71), spaced axially along this jacket.  17. Apparatus according to claim 16, characterized in that each diffuser (71) comprises a plurality of fins (72) arranged radially relative to the axis of this diffuser and spaced from each other along  the circumference.  18. Apparatus according to claim 17, characterized in that each fin (72) has an inner part located in a plane practically normal to this axis of the diffuser, in that it is gradually inclined towards the distant end and in that 'it ends with an end portion located in a plane parallel to a plane passing through this axis.