STEAM GENERATOR

Abstract: A steam generator (1, 1A, 1B) suitable for in a liquid metal fast breeder reactor such as the superheater (1A) and/or the evaporator (1B) comprises a barrel (3), a heating transfer tubes (2), an inner shroud (5, 41, 51) and an outer shroud (8). The heating medium flows into the barrel (3) through an inlet pipe (10) and flows out the barrel (3) through an outlet pipe (9), wherein the inner shroud (5, 41, 51) forms the opening (5a, 41a, 51a) in the upper end thereof, and both the inlet pipe (10) and the outlet pipe (9) for the heating medium are provided at the upper portion of the barrel (3). In order to reduce the difference in temperature or thermal stress in the inner shroud (5, 41, 51), both the inlet pipe (10) and the outlet pipe (9) are provided at the upper portion of the barrel (3). The heating medium flows out from the barrel (3) to the outlet pipe (9) through the upper opening (5a, 41a, 51a) of the inner shroud (5, 41, 51). The region in the vicinity of the inner shroud (5, 41, 51) is filled with a stagnant or expended heating medium such as a sodium.

Description: Steam Generator The present invention relates a steam generator, and more particularly, to a steam generator which is suitable for employment in a liquid metal fast breeder reactor. A conventional large scale herical coiling type steam generator in a liquid metal fast breeder reactor is known in, for example, Japanese Patent Laid-Open Publication No. 28467/1980. High temperature liquid metal sodium serving as a heating medium flows into a longitudinal placement type steam generator through a sodium inlet nozzle provided at the upper end of a barrel of the steam generator. The liquid metal sodium descends through a tube bundle portion around which is vertically and helically wound a multiplicity of heat transfer tubes. This tube bundle portion is disposed in a region which is so formed as to be surrounded by a cylindrical inner shroud and an outer shroud. The thus lowered sodium then flows out through a sodium outlet nozzle. Feçd-water is supplied through a feed-water inlet nozzle into a feed-water inlet chamber and is then led into a mulitiplicity of the heat transfer tubes, whereby the water is raised. At this time, the feed-water which has flowed in at low temperature is subjected to a thermal exchange with respect to the sodium which flows at high temperature on the outside of the heat transfer tubes. With this step, the feed-water is preheated, boiled and superheated, thus vaporizing the feed-water at high temperature and at high pressure. The thus produced vapor is collected in a vapor outlet chamber and then fed out from a vapor outlet nozzle to a turbine. Fig, 3B shows a general example of a secondary sodium system loop of a fast breeder reactor which includes the above-described steam generators la and lb. This system loop is constituted by an intermediate heat exchanger 23, the steam generators la and lb, a circulating pump 24 and pipes 31a, 32a and 33a. The intermediate heat exchanger 23 performs the thermal exchange in regard to the primary sodium and the secondary sodium. The steam generators la and lb produce vapor by effecting a thermal exchange between the feed-water and the secondary sodium with high temperature which is transmitted from the intermediate heat exchanger 23. The circulating pump 24 circulates the secondary sodium. A conventional steam generators shown in Fig. 3B are constituted by a superheater la and an evaporator lb which have the same structure with respect to each other. The evaporator lb evaporates the feed-water by virtue of the heat evolved by the sodium; and the superheater la has a function to convert the vapor produced by the evaporator lb into the superheated vapor by further heating it with the sodium. The secondary sodium which has been heated at high temperature by means of the intermediate heat exchanger 23 is transmitted via a hot leg pipe 31a, a middle leg pipe 32a and a cold leg pipe 33a to the superheater la or the evaporator lb. The middle leg pipe 32a extended form the lower end of the superheater la passes through a floor (from the point f to the point f2) on which the superheater la is installed and is then descended. Furthermore, the middle leg pipe 32a reverses the piping direction thereof at the lower portion of the floor and again passes through the floor so as to reach the upper portion of the evaporator lb. The cold leg pipe 33a extended from the lower end of the evaporator lb passes through the floor (from the point f3 to the point f4) on which the evaporator lb is installed and is then descended. In the large scale steam generator, as described above, the sodium pipe is large in diameter; hence, it is required to avoid an increase in the length of a supporting skirt. For this reason, through-holes are formed in the floor, and further, a bent portion of the sodium pipe is provided beneath the floor such as to be led round. The large through-holes are formed respectively in the floor on which the superheater la and the evaporator Ib are installed. In such a case, elavorations are required so as to obtain the strength of the floor when designing the structure of the reinforced concrete or disposing the reinforcing bar as compared with an arrangement wherein no through-hole is formed therein. Moreover, the sodium pipe is led round on the upper, lower and lateral sides of the steam generator, this involving a large space of pipe arrangement. Namely, such arrangement is the very factor which enlarges the structure itself. In the above-described pipe arrangement, if the cold leg pipe 33a in the vicinity of the sodium outlet nozzle of the evaporator lb is damaged, all of the sodium within the evaporator lb flows out into the atmosphere where it is virtually certain to ignite. Also, in as much as the through-holes are formed in the floor, some of the leaked sodium would flow into other equipment, spreading the damage and increasing the danger. Moreover, as an another example, a large scale steam generator of having a coiling type in a liquid metal fast breeder reactor is known in the publication papers. (ASME 80-C2/NE-31 '.5 team Generator Design and Experience in the SNR-Project", page 10, Fig. 7). The sodium within the longitudinal placement type steam generator of this publication papers flows into through the sodium inlet nozzle provided at the lower end of the barrel of the steam generator and is then raised within the inner shroud with which the sodium inlet nozzle is directly communicated, thereby reaching an upper sodium plenum chamber. Thereafter, the flow direction of the sodium is converted, so that the sodium descends through the tube bundle portion around and then flows onto the outside of the steam generator through the sodium outlet nozzle provided beneath the barrel separately the sodium inlet nozzle. In this example, the sodium flows both inside and outside of the inner shroud, and there is a great difference in temperature between the inside flow and the outside flow thereof, whereby a great thermal stress is presumably imparted to the inner shroud. Since the tube bundle portion is supported by the inner shroud, the inner shroud must be shielded from the thermal stress in so far as is possible. Since the inner shroud is connected to the barrel, it is necessary to measure for the purpose of absorbing the difference in the thermal expansion between the inner shroud and the barrel. Moreover, the sodium pipe is provided on the lower side of the apparatus; hence, it is reasonable to assume that all the sodium within the steam generator would flow out if the pipe were seriously damaged, the flow mostly occurring in the damaged region. As mentioned above, some technical problems still remain unsolved in both prior arts. Further, there is no idea to utilize the inner shroud so as to the sodium or heating medium flow out therethrough. If the heating medium flow out from the upper end portion of the inner shroud, it is difficult to reduce the difference in temperature about a region in the vicinity of the inner shroud. An object of the present invention is to provide a steam generator wherein a difference in temperature about a region in the vicinity of an inner shroud can be reduced. Another object of the present invention is to provide a steam generator wherein a thermal stress on an inner shroud can be reduced. Still object of the present invention is to provide a steam generator wherein a compact piping structure can be obtained. Stillmore object of the present invention is to provide a steam generator wherein a space required for disposing the pipes can be reduced. Further object of the present invention is to provide a steam generator wherein an amount of leaked heating medium can be minimized. Furthermore object of the present invention is to provide a steam generator wherein no through-hole can be formed within the floor on which the steam generator is installed. The present invention is a steam generator comprising: a barrel having an inlet portion and an outlet portion for a heating medium; a multiplicity of heat transfer tubes being formed vertically in a helical configuration and being disposed in the barrel; an inner shroud formed a lower end opening being disposed such as to pass through the central portion of the helical heat transfer tubes and supporting the helical heat transfer tubes, the lower end opening being formed such as the heating medium flows into the inner shroud through the lower end opening; and an outer shroud being disposed on the outside of the helical heat transfer tubes and forming a passage for the heating medium between the inner shroud and the outer shroud; characterized in that both the inlet portion and the outlet portion for the heating medium are provided respectively at the upper portion of the barrel, an opening is formed in the upper end portion of the inner shroud so as to the heating medium flows out through said upper end opening of the inner shroud, and a region in the vicinity of the inner shroud is filled with a stagnant heating medium or an inert gas. The advantages offered by the present invention are a compact and secure steam generator which has effects wherein it is possible to reduce the difference in temperature about a region in the vicinity of the inner shroud, the thermal stress on the inner shroud, the space for installing the steam generator as well as the amount of pipe, and to decrease the amount of leaked heating medium if the pipes are damaged and further to minimize the damage from a heating medium fire, should one occur. Fig. 1 is a longitudinal sectional view of a steam generator of a first embodiment according to the present invention; Fig. la is a section view of along line a-a in Fig. 1; Fig. 2 is a structural view which shows a steam generator constituted by a superheater and an evaporator in combination according to the present invention; Fig. 3A is an isometric diagram of a secondary sodium system loop according to the present invention; Fig. 3B is an isometric diagram of a secondary sodium system loop according to the prior art; Fig. 4 is a longitudinal sectional view of a steam generator of a second embodiment according to the present invention; Fig. 5 is a longtitudinal sectional view of a steam generator of a third embodiment according to the present invention; and Fig. 5a is a sectional view of along line a-a in Fig. 5. A preferred embodiment in accordance with the present invention will hereinafter be described with reference to the accompanying drawings. Fig. 1 shows a steam generator 1 of a first embodiment according to the present invention. In the steam generator 1, a multiplicity of heat transfer tubes 2 pass through the lower portion of a cylindrical barrel 3, these heat transfer tubes 2 being communicated with feed-water inlet chambers 4. The feedwater inlet chambers 4 are provided at the outside lower portion of the barrel 3 of the steam generator 1. The heat transfer tubes 2 are vertically and helically wound around a cylindrical inner shroud 5 having an upper end opening 5a and a lower end opening 5b in the barrel 3, thereby forming a helically coiled tube bundle portion 6. These helical heat transfer tubes 2 further pass through the upper portion of the barrel 3 so as to be communicated with - vapor outlet chambers 7. A cylindrical outer shroud 8 is provided on the outside of the helically coiled tube bundle portion 6, such outer shroud 8 being designed for the purpose of shielding the heat and forming an offtake for sodium or heating medium. A dual-purpose sodium inlet-outlet pipe or nozzle is provided at a single upper portion above the barrel 3. A sodium outlet pipe or a sodium outlet nozzle 9 is coaxially provided within a sodium inlet pipe or a sodium inlet nozzle 10. Such sodium outlet pipe 9 passing through a bent portion 11 of the sodium inlet pipe 10 and further being connected to each other with the aid of a metal bellows 12 as a displacement absorption mechanism. The sodium inlet pipe 10 includes an opening 13 which is so formed in an upper sodium plenum chamber 14 such as to be communicated therewith. The sodium outlet pipe 9 extends downwardly and passes through the upper end opening 5a of the inner shroud 5. Such sodium outlet pipe 9 includes another opening 15 which is formed in a lower sodium plenum chamber 16 such as to be communicated therewith. The sodium outlet pipe 9 has a double pipe structure 18 with respect to the length ranging from the metal bellows 12 to the upper portion of the inner shroud 5. An inert gas for medium as a thermal insulating material is encapsulated in a gap 17 formed between the inner pipe and the outer pipe of the double pipe structure 18. A body supporting skirt 19 is provided at the lower portion of the barrel 3, the body supporting skirt 19 supporting the steam generator 1. No through-hole is provided in the concrete floor 20 on which the steam generator 1 is installed. In this first embodiment, the steam generator 1 is the non-liquid level type, and thus the inside portion of the barrel 3 is filled with the sodium. The initial method of filling the steam generator 1 with the sodium is usually on of two types: one is the pressurizing method wherein the pressurized sodium injected into the barrel 3 of the steam generator 1; and the other is the vacuum pull-in method wherein a high vacuum inside of the barrel 3 of the steam generator 1 pulles the sodium into the barrel 3. When the sodium is put in by employing the pressurizing method, the inner region of the inner shroud 5 and the similar region formed by the metal bellows 12 and the sodium outlet pipe 9 are each closed at one end thereof, thereby enclosing the fluid. It is also possible for the sodium to be filled in by the vacuum pull-in method. The abovedescribed two regions are filled with stagnant or expended fluid regions by either method. The high temperature sodium serving as a heating medium passes through a gap 21 formed between the sodium inlet pipe 10 and the sodium outlet pipe 9, both of which are provided on the upper side of the barrel 3. Such high temperature sodium flows into the barrel 3 of the steam generator 1 and then descends through the helically coiled tube bundle portion 6 around which is helically wound with the helical heat transfer tubes 2. Thereafter, the high temperature sodium reaches the lower sodium plenum chamber 16. The high temperature sodium is subjected to the thermal exchange with respect to the water and the vapor within the helical heat transfer tubes 2. Feed-water is supplied through a feed-water inlet pipe or nozzle into a feed-water inlet chamber 4 and is then led into a mulitiplicity of the helical heat transfer tubes 2, whereby the water is raised. At this time, the feed-water which has flowed in at low temperature (about 240or with about 150 ata) is subjected to a thermal exchange with respect-to the sodium which flow at high temperature (about 500or with about 45 ata) on the outside of the helical heat transfer tubes 2. With this step, the feed-water is preheated, boiled and superheated, thus vaporizing the feed-water at high temperature (about 487or with high pressure about 33 ata). Then the sodium becomes at low temperature (about 320 C with about 4LC5 5 ata). The thus produced vapor is collected in a vapor outlet chamber 7 and then fed out from a vapor outlet pipe or nozzle to a turbine. The low temperature sodium (about 3200C with about 4¯ ata) which has reached the lower sodium plenum chamber 16 changes its flow direction and flows through the opening 13 of the sodium outlet pipe 9 into the sodium outlet pipe 9. Then, this low temperature sodium is raised and flows out from the steam generator 1. There is a great difference in temperature between the sodium descending outside the inner shroud 5 and the sodium ascending inside the sodiumn outlet pipe 9. However, the region which is formed such as to be surrounded by the inner shroud 5 and the sodium outlet pipe 9 is filled with the stagnant or expended fluid such as sodium or an inert gas, whereby a difference in temperature about the region between the inner shroud 5 and the outlet pipe 9 is reduced and also a great thermal stress is not imparted on the inner shroud 5. The inner shroud 5 is joined to the barrel 3 and further to the sodium outlet pipe 9. Accordingly, the metal bellows 12 is provided on the sodium inlet pipe 10 in order to absorb the difference in the thermal expansion between the sodium outlet pipe 9 and the barrel 3. The sodium inlet-outlet pipe or nozzle has a double structure for the purpose of obtaining a preferable sodium flux in the upper sodium plenum chamber 14 and a uniform sodium flux at the upper end portion of the helically coiled tube bundle portion 6. The sodium inlet pipe 10 and the sodium outlet pipe 9 are concentrated at the upper portion of the barrel 3 respectively. A bulkhead or patitioning plate 22 is provided in order to avoid the damage to the sodium inlet pipe 10 and the sodium outlet pipe 9 if the water-vapor system pipes rupture. In the steam generator according to the present invention, Fig. 2 shows a connecting arrangement of the pipes for sodium, this pipe arrangement being based on a superheater 1A and an evaporator 1B. The steam generator 1 is constituted by the superheater 1A and the evaporator 1B which have the same structure with respect to each other. Fig. 3A shows a general example of a secondary sodium system loop of a fast breeder reactor which includes the above-described steam generator 1 of the first embodiment in the present invention. This system loop is constituted by an intermediate heat exhanger 23, the steam generator 1, a circulating pump 24 and a hot leg pipe 31A, a middle leg pipe 32A and a cold leg pipe 33A. The intermediate heat exhanger 23 performs the thermal exchange in regard to the primary sodium and the secondary sodium. The steam generator 1 produces vapor by effecting a thermal exchange between the feed-water and the secondary sodium with high temperature which is transmitted from the intermediate heat exchanger 23. The circulating pump 24 circulates the secondary sodium. The high temperature sodium moved from an intermediate heat exchanger 23 flows in via the sodium inlet pipe 10 of the superheater 1A and then flows out via the sodium outlet pipe 9. The sodium outlet pipe 9 of the superheater 1A is communicated with the sodium inlet pipe 10 of the evaporator 1B through the cold leg pipe 33A. The sodium transmitted from the superheater 1A flows via the sodium inlet pipe 10 of the evaporator 113 into the evaporator 113 and then flows out through the sodium outlet pipe 9. Where the steam generator' 1 of the first embodiment according to the present invention is adopted, as shown in Fig. 2, there is no necessity for any through-hole to be formed in the floor 20 on which the steam generator 1 constituted by the superheater 1A and the evaporator 1B is installed; hence, the strength of the installation floor 20 can securely be maintained by adjusting the thickness thereof. Furthermore, it is practicable to install the superheater 1A and the evaporator 1B in the same chamber by an arrangement wherein these two, that is the superheater 1A and the evaporator 1B, as a combined installation are brought closer each other. In addition, no sodium pipe passes through the floor 20 on which the steam generator 1 is installed. With this arrangement, the space for installing the steam generator 1 is made smaller than that of the prior arts. Moreover, all the sodium pipes, that is the hot leg pipe 3LA, the middle pipe 32A and the cold pipe 33A, are provided on the upper side of the superheater 1A and the evaporator 1B. Therefore, the sodium within the steam generator 1 does not flow out at all even if the sodium pipes 31A, 32A and 33A are damaged. The amount of leaked sodium can be minimized as compared with the prior arts wherein the sodium pipe in the proximity of the sodium outlet nozzle is damaged. No through-hole is formed in the floor 20 on which the steam generator 1 is installed, this preventing any leaked sodium from flowing into other parts other of the equipment, thereby greatly reducing the possibility of fire caused by the leaked sodium. Fig. 3A is a schematic diagram of a secondary sodium system loop of one embodiment which employs the steam generator 1 constituted by the superheater 1A and the evaporator 1B according to the present invention. As shown Fig. 3A, in comparison with Fig. 3B which : shows the schematic diagram of the secondary sodium system loop of the prior art, the piping in this case can be eliminated when the above first embodiment of the present: invention is adopted. If the constitution of the steam generator takes only one unit, that is, in the case of the continuous percolation, it is possible to reduce the number of pipes as well as the space for disposing the pipes in regard to the secondary sodium system loop by dint of adopting the present invention. Moreover, it obviously reduces the extent of damage of a sodium fire occurs. Fig. 4 shows a steam generator of a second embodiment according to the present invention. It is shown in Fig. 4 an arrangement wherein an inner shroud 41 provides a metal bellows 42 thereof and a metal bellows 42 as a displacement absorption mechanism is provided within the barrel 3. The inner shroud 41 has an upper end opening 41a and a lower end opening 41b. The metal bellows 42 absorbs the difference in thermal expansion between the sodium outlet pipe 9 and the barrel 3. Such being the case, the sodium outlet pipe 9 is connected to an upper end opening 41a of the inner shroud 41 by means of the metal bellows 42. Fig. 5 shows a steam generator of a third embodiment according to the present invention. Fig. 5 shows an arrangement wherein the whole inside area of an inner shroud 51 is employed as an offtake for sodium. In this modified embodiment, the inner shroud 5 takes a double pipe structure as shown in Fig. 5a in order that the thermal exchange between the sodium moving on the outside of the inner shroud 51 and the sodium moving on the inside of the inner shroud 51 is not permit to occur. A gap 52 formed between the inside pipe and the outside pipe of the inner shroud 51 is filled with an inert gas. The inner shroud 51 has an upper end opening 51a and a lower end opening 51b. The sodium flows into the lower end opening 51b of the inner shroud 51 and flows out through the upper end opening 51a of the inner shroud 51 and the outlet pipe 9. In this case the inner shroud 51 and the outlet .' pipe are integrally formed therewith.

Claims: Claims: 1. A steam generator (1, 1A, 1B) comprising: a barrel (3) having an inlet portion and an outlet portion for a heating medium; a multiplicity of heat transfer tubes (2) being formed vertically in a helical configuration and being disposed in said barrel (3); an inner shroud (5, 41, 51) formed a lower end opening (5b, 41b, 51b) being disposed such as to pass through the central portion of said helical heat transfer tubes (2) and supporting said helical heat transfer tubes (2), the lower end opening (5b, 41b, 51b) being formed said inner shroud (5, 41, 51) so as to the heating medium flows into said inner shroud (5, 41, 51) through the lower end opening (Sb, 41b, 51b); and an outer shroud (8) being disposed on the outside of said helical heat transfer tubes (2) and forming a passage for the heating medium between said inner shroud (5, 41, 51) and said outer shroud (8); characterized in that both the inlet portion and the outlet portion for the heating medium are provided respectively at the upper portion of said barrel (3), an opening (5a, 41a, 51a) is formed in the upper end portion of said inner shroud (5, 41, S1) so as to the heating medium flows out through said upper end opening (5a, 41a, 51a) of said inner shroud (5, 41, 51), and a region in the vicinity of said inner shroud (5, 41, 51) is filled with a stagnant heating medium or an inert gas. 2. A steam generator (1, 1A, 1B) comprising: a barrel (3) having an inlet portion and an outlet portion for a heating medium; a multiplicity of heat transfer tubes (2) being formed vertically in a helical configuration and being disposed in said barrel (3); an inner shroud (5, 41, 51) formed a lower end opening (5b, 41b, 51b) being disposed such as to pass through the central portion of said helical heat transfer tubes (2) and supporting said helical heat transfer tubes (2), the lower end opening (5b, 41b, 51b) being formed said inner shroud (5, 41, 51) so as to the heating medium flows into said inner shroud (5, 41, 51) through the lower end opening (5b, 41b, 51b); an outer shroud (8) being disposed on the outside of said helical heat transfer tubes (2) and forming a passage for the heating medium between said inner shroud (5, 41, 51) and said outer shroud (8); an inlet pipe (10) being disposed on the inlet portion of said barrel (3); and an outlet pipe (9) being disposed on the outlet portion of said barrel (3) characterized in that both said inlet pipe (10) and said outlet pipe (9) are provided respectively at the upper portion of said barrel (3), an opening (5a, 41a, 51a) is formed in the upper end portion of said inner shroud (5, 41, 51), said outlet pipe (9) passes through said barrel (3) at the outlet portion for the heating medium and also passes through the upper end opening (5a, 41a, 51a) of said inner shroud (5, 41, 51) so as to the heating medium flows out through the inside of said outlet pipe (9), and a region by formed the inside of said inner shroud (4, 41, 51) and the outside of said outlet pipe (9) is filled with a stagnant heating medium or an inert gas. 3. A steam generator (1, 1A, 1B) as claimed in claim 2, wherein said outlet pipe (9) is coaxially provided within said inlet pipe (10). 4. A steam generator (1, 1A, 1B) as claimed in claim 2, wherein said outlet pipe (9) is formed double pipe structure (18) having an inner pipe and an outer pipe, and a gap (17) formed between the inner pipe and the outer pipe of the double pipe structure (18) is encapsulated with a thermal insulating material. 5. A steam generator (1, 1A, 1B) as claimed in claim 2, wherein a displacement absorption mechanism (12) is provided on said inlet pipe (9) so as to absorb the difference in the thermal expansion between said barrel (3) and said inlet pipe (10). 6. A steam generator (1, 1A, 1B) as claimed in claim 2, wherein a displacement absorption mechanism (42) is provided within said barrel (3) so as to absorb the difference in the thermal expansion between said barrel (3) and said inner shroud (41). 7. A steam generator (1, 1A, 1B) as claimed in claim 2, wherein said inner shroud (51) being formd a double pipe structure so as to prevent the thermal exchange between the heating medium of the outside of said inner shroud (51) and the heating medium of the inside of said inner shroud (51), and a gap (52) formed within said inner shroud (51) is filled with an inert gas. 8. A steam generator (1, 1A, 18) as claimed in claim 7, wherein said inner shroud (51) and said outlet pipe (9) are integratedly formed therewith. 9. A steam generator (1, 1A, 1B) as claimed in claim 1 or 2, wherein the steam generator (1) is at least one of the selected from a superheater (1A) and an evaporator (1B) of a liquid metal fast breeder reactor, and the heating medium is a sodium. 10. A steam generator (1, 1A, 1B) as claimed in claim 9, the sodium moved from an intermediate heat exchanger (23) flows in via said inlet pipe (10) of the superheater (1A) through a hot leg pipe (31A) and then flows out said outlet pipe (9) through a middle leg pipe (32A), and said outlet pipe (9) is communicated with said inlet pipe (10) of the evaporator (1B) through a cold leg pipe (33A).

Inventors: ABE, KOUJI, KASHIWAKURA, JUN, TACHI, YASUO, TSUDA, HIROSHI
Applicants: HITACHI, LTD.
Application year: 1985
Main classification: F22B1
IPC codes: F28F9, F28D7, G21C1, F22B1