Patent Application
20050183675
The invention relates to a steam generator and especially to a steam generator of a pressurized-water-cooled nuclear reactor comprising an emergency feed water device.
Steam generators such as steam generators of pressurized-water-cooled nuclear rectors comprise an outer jacket of generally cylindrical shape arranged vertically in the building of the nuclear reactor, that is to say with the axis of the outer jacket vertical.
The steam generators of pressurized-water nuclear reactors enable the feed water to be heated and vaporized by heat exchange with the pressurized cooling water of the nuclear reactor, forming the primary heat exchange fluid which flows inside tubes of an exchange bundle. The tube bundle is arranged inside a bundle wrapper of generally cylindrical shape which is arranged coaxially inside the outer jacket.
The outer jacket of the steam generator generally comprises a lower cylindrical part containing the bundle of heat exchange tubes arranged inside the bundle wrapper, an upper cylindrical part having a diameter greater than the diameter of the lower part in particular containing separators and steam dryers and a part with a frustoconical junction between the lower part and the upper part of the outer jacket.
Steam generator feed water is introduced into the outer jacket and channelled, so as to enter the exchange bundle, at the lower part of the bundle and of the bundle wrapper. The feed water then flows from the bottom upwards inside the bundle wrapper in contact with the outer surface of the tubes, such that it is heated then evaporates and is in the form of steam in the upper part of the outer jacket of the steam generator. The steam recovered in the upper part is separated from the water droplets that it may contain and dried then sent to the reactor turbine.
The feed water is generally introduced in the upper part of an annular space made between the tube bundle wrapper and the outer jacket of the steam generator or a skirt for guiding the feed water and flows in the annular space to the lower part of the bundle. The annular space communicates, at its lower part, with the inner space of the bundle wrapper containing the tube bundle.
To obtain good efficiency of the steam generator and satisfactory operating conditions, it is necessary to distribute the feed water stream over the circumferential direction of the annular feed space of the steam generator. To do this, the feed water device of the steam generator comprises a header having a wall of generally toroidal shape which is placed inside the outer jacket of the steam generator, in a coaxial arrangement, vertically in line with the upper part of the annular feed water space. The header is connected to a feed water pipe penetrating the outer jacket of the steam generator and its wall comprises a plurality of openings in its upper part which are distributed over the periphery of the header and associated with means making it possible to direct the feed water to the upper part of the annular feed space of the steam generator.
During normal operation of the steam generator, feed water is sent continuously and with a substantially constant flow rate to the lower part of the tube bundle, and the flow of feed water, which is heated then vaporized on contact with the tubes of the bundle, cools the pressurized water flowing in the tubes constituting the primary fluid of the nuclear reactor.
In the case of degraded operation of the secondary feed water circuit of the steam generator, the feed water flow rate provided by the feed water device with a toroidal header may become insufficient, irregular, or may even be interrupted.
In this case, it is necessary to provide the steam generator, in a very short time, with emergency water making it possible to compensate for an insufficient or zero flow rate of the normal feed device of the steam generator.
To do this, it has been proposed to use an emergency feed water device for the steam generator which has a structure similar to the normal feed device. Such an emergency feed water device for the steam generator comprises a header having a toroidal wall which is placed above the annular feed space of the steam generator and which comprises a plurality of openings in the upper part of the toroidal wall, in order to distribute the water. The emergency water distribution header may be placed above or below the toroidal-walled header of the normal feed device.
A drawback of this arrangement is that the emergency water of the steam generator coming from a water reserve connected to the toroidal header which is at a fairly low temperature, for example 7° C., is brought into contact with the outer wall of the steam generator which is at a temperature of about 290° C. to 300° C. in the upper part of the steam generator, before the emergency water has been able to be heated inside the steam generator. This may result in thermal shock which can be damaging for the structure of the steam generator.
The aim of the invention is therefore to provide a steam generator comprising an outer jacket of generally cylindrical shape arranged with its vertical axis having a lower cylindrical part containing a bundle of heat exchange tubes connected to a primary fluid circuit and arranged in a bundle wrapper which is substantially coaxial with the outer jacket, an upper cylindrical part having a diameter greater than the diameter of the lower part in particular containing separators and steam dryers and a part with a frustoconical junction between the lower part and the upper part of the outer jacket, at least a first and a second device for supplying feed water to an annular space between the bundle wrapper and one of a guide skirt and the outer jacket, via the upper part of the annular space which communicates with the inner space of the bundle wrapper at its lower part, each one comprising a feed water distribution header in the annular space arranged inside the outer jacket, having a toroidal wall substantially coaxial with the outer jacket and with the bundle wrapper, the toroidal wall being penetrated by a plurality of openings into its upper part and at least one feed water supply pipe in a space inside the toroidal wall of the header penetrating the outer wall of the steam generator, the first feed device being used during normal operation of the steam generator and the second feed device being used in order to feed the steam generator with emergency water, the emergency feed water of the steam generator being supplied without producing thermal shock, in particular on the outer jacket of the steam generator.
With this aim, the second toroidal header of the second emergency feed water device placed inside the upper part of the outer jacket comprises, fastened to each of its openings of its toroidal wall, a tube for injecting water having an open lower end part fastened to the opening of the toroidal wall, a straight part which is inclined with respect to the vertical axial direction in the direction of a central part of the steam generator and an upper end open at the end of the straight part of the tube for injecting emergency water inside the steam generator.
According to particular features of the invention:
In order for the invention to be better understood, a steam generator of a pressurized-water nuclear reactor made according to the invention will be described by way of example with reference to the appended figures.
Inside the lower part 2a of the smaller-diameter outer jacket 2 is placed the bundle 3 of the steam generator, inside a bundle wrapper 4 of overall cylindrical shape arranged coaxially inside the outer jacket 2 of the steam generator.
The tube bundle 3 consists of tubes folded into a U shape fastened at the lower end into a tube sheet 5 separating the inner space of the jacket 2 of the steam generator from a two 6 in parts making it possible to supply the tubes with primary water and to recover the primary water having flowed through the tubes.
Feed water is introduced inside the steam generator so that it can flow in contact with the tubes of the bundle 3 in the vertical direction from the bottom upwards, so as gradually to be heated and vaporized.
During normal operation of the steam generator, feed water is introduced by a feed device 8, in the upper part of an annular space 7 defined between the lower part 2a of the outer jacket of the steam generator or a guide skirt 13 and the bundle wrapper 4.
Above the tube sheet 5, the bundle wrapper 4 provides a passage for the feed water introduced in the upper part of the annular space 7 and reaching, by flowing in the vertical direction from the top downwards, the tube sheet 5. The feed water enters the bundle wrapper, thereby coming into contact with the tubes of the bundle 3 and flowing in contact with the bundle in the vertical direction and from the bottom upwards.
Heat exchange occurs between the primary water flowing in the tubes of the bundle 3 and the feed water flowing in contact with the outer surface of the tubes of the bundle 3.
The heated then vaporized feed water forms steam in the upper part of the bundle wrapper, the steam then being discharged inside the upper cylindrical part 2b of the steam generator containing separators and steam dryers.
According to a known arrangement, the normal feed water device of the steam generator, generally denoted by the reference 8, comprises a toroidal header 10 connected to a feed water supply pipe 9 in the toroidal header penetrating the outer jacket 2 of the steam generator inside a throughpipe 11. The normal feed water device 8 of the steam generator may advantageously be made as described and claimed in patent application No. 9805843 by Framatome.
The upper part of the wall of the toroidal header 10 which is arranged with its axis of revolution in the vertical direction and coincident with the axis of the outer jacket is penetrated by openings distributed over the circumference of the toroidal header 10.
The main feed device 8, which is shown in
Preferably, the annular space 7 for supplying feed water to the inner space of the bundle wrapper of the steam generator may be defined between the bundle wrapper 3 and a guide skirt 13 of cylindro-frustoconical shape placed coaxially with the outer jacket 2 and with the bundle wrapper 3 of the steam generator.
The diameter of the toroidal header 10 is such that the openings for passage of feed water that are located in the upper part of the toroidal header on a parallel circle are arranged vertically in line with the entrance of the annular feed space 7, the feed water guide means 12 emerging at their lower part, between the flared part of the skirt 13 and the bundle wrapper 4, in a substantially centred position.
As can be seen in
The separating devices 15 may advantageously consist of cyclone separators, which are each connected to the upper part of the bundle wrapper 4 by means of a steam pipe 15′.
Below the stage of the separators 15, the large-diameter upper part 2b of the outer jacket of the steam generator defines an inner space of the steam generator whose peripheral part 14 around the steam pipes 15 connected to the upper part of the bundle wrapper 4 is a completely free space.
For the embodiment shown, a second feed water device 16 constituting an emergency feed water device for the steam generator, is placed inside the space 14, above the main feed water device 8.
In other embodiments, the second feed device may be below the toroidal header.
Where the main supply of feed water provided by the main feed device 8 becomes insufficient, or where this supply is no longer guaranteed by the device 8, the emergency feed water device 16 may be controlled in order to provide continuity of supply of water from the steam generator and cooling of the steam generator.
The emergency feed water device 16 comprises a toroidal header 17 placed with its axis of revolution along the axis of the outer jacket of the steam generator, that is to say in a coaxial arrangement with respect to the toroidal header 10 of the main feed water device.
The mean diameter of the toroidal header 17, which is equal to the diameter of the parallel circle located on the upper part of the toroidal header 17, may be substantially greater than the mean diameter of the toroidal header 10, that is to say than the diameter of the parallel circle along which the openings for passage of water from the toroidal header are arranged.
The mean diameter of the toroidal header 17, which is generally between the inner diameter of the smaller-diameter lower part of the outer jacket 2 of the steam generator and the inner diameter of the larger-diameter upper part 2b may, for example, be equal to the inner diameter of the intermediate part 2c of the outer jacket 2 of the steam generator, at the axis of a pipe 11 penetrating the outer jacket 2 of the steam generator, to which the supply pipe 9 of the toroidal header 10 of the main feed water device of the steam generator is connected.
A pipe 18 penetrating the outer jacket 2 of the steam generator in its larger-diameter part 2b, at the toroidal header 17 of the emergency feed water device 16, makes it possible to provide emergency feed water coming from a reserve of water, from an emergency water supply pipe 19 inside the toroidal header 17 of the emergency feed device 16.
As can be seen, in particular in
The toroidal jacket 17 is penetrated by circular-shaped openings 21 for passage of water, the centres of which are arranged on the upper parallel circle of the toroidal header 17 whose diameter constitutes the mean diameter of the toroidal header. For example, in a preferred embodiment, fifty openings 21 are provided, distributed over the circumferential direction of the torus on the upper parallel circle. Each of the emergency water distribution devices 20 comprises, at an opening 21, a water injection tube 22 fastened in a sealed manner by welding to the wall of the toroidal header 17. Each of the water injection tubes 22, made for example by folding a straight tube, comprises a short first portion or lower portion 22a and a second portion or upper portion 22b, the length of which is greater than the length of the portion 22a and the axis of which makes an angle α with the axis of the first portion 22a.
Preferably, the angle α is between 5° and 20°, it being possible for this angle, for example, to be 10°.
The end of the lower portion 22a of each of the tubes 22 is fastened, along an opening 21, to the upper part of the wall of the toroidal header 17 in a direction parallel to the direction of the axis of revolution of the toroidal header 17, that is to say a vertical direction inside the steam generator in its service position.
The lower portion 22a of the tube 22 is fastened in the opening 21, in an orientation such that the upper part 22b is inclined with respect to the direction of the axis of revolution of the torus, in the direction of the central part of the steam generator.
Preferably, as can be seen in
Thus a strong fastening for the tube and high accuracy in orienting the axis of the upper part 22b of the tube 22 are provided.
The supply pipe 19 comprises a sleeve 19a fastened by welding, at one of its ends, to the pipe 18 and assembled end to end, at its second end, to a pipe 19b for connecting the toroidal header 17. In this way, the water supply pipe 19, consisting of the sleeve 19a and of the pipe 19b, put the end part of the pipe 18 located outside the outer jacket of the steam generator in communication with the inner distribution volume of the toroidal collector 17. The emergency feed water supply pipe 19 is fed with emergency water, from the water reserve, by a channel connected to the end part of the pipe 18 located outside the outer jacket of the steam generator.
Thus a completely sealed junction is obtained for the supply pipe 19 and for the pipe 18, which makes it possible to prevent any leakage of cooling water between the pipe 18 and the water supply pipe 19. Such leaks could occur where a sleeve mounted so that it can slide inside the pipe penetrating the wall of the steam generator is used, for the purpose of avoiding the appearance of stresses due to differential expansions between the emergency feed water device and the outer jacket of the steam generator.
When a sleeve 19 secured, at one of its ends, to the pipe 18 is used, it is necessary to provide means of supporting the toroidal header of the emergency feed water device allowing movement under the effect of the differential expansions between the emergency feed device and the outer jacket of the steam generator.
At its end, the bracket 26 comprises a recess 27 whose height in the vertical direction is substantially greater than the diameter of the toroidal header 17. On each side of the recess 27, the bracket 26 comprises a lower supporting arm 26a and an upper arm 26b. A cavity 28 is machined in the inner surface of the lower arm 26a directed towards the recess 27, a cavity 28 intended to accommodate a wedge 29 which is fastened by welding in the cavity 28 and whose height can be calibrated such that, in the mounted position shown where the wedge 29 supports the toroidal header 17 by means of an upper bearing surface, there is a clearance in the vertical direction between the upper part of the toroidal header 17 resting on the wedge 29 and the inner surface of the arm 26b directed towards the recess 27.
As can be seen in
The supporting device consisting of the brackets 26, at which the travel of the toroidal header 17 in the vertical direction and in the horizontal directions with respect to the edge of the recess 27 can be adjusted, allows expansion of the toroidal header 17 and movement with respect to the outer jacket of the steam generator both in the horizontal directions (radial direction of the toroidal header 17) and in the vertical direction.
Thus it is possible to reconcile a mounting with rigid fastening of one end of the water supply channel in the toroidal header 17 with respect to the outer jacket of the steam generator and movement due to the differential expansions between the toroidal header and the outer jacket of the steam generator during operation.
When supplying the inner volume of the toroidal header 17 with emergency feed water, the water introduced into the header is made to penetrate the openings in the upper part of the wall of the header in order to be distributed inside the volume of the part 14 of the steam generator by means of the water injection tubes 22 of the emergency water distribution devices 20.
The emergency water coming from the storage reservoir may accidentally convey foreign bodies whose introduction inside the bundle of the steam generator via the annular space 7 needs to be prevented.
For this purpose, as shown in
Debris with a size greater than 4 mm remains inside the toroidal header 17 and emergency feed water injected by the tubes 22 inside the upper part of the steam generator does not contain debris or migrant bodies which may be large enough to damage the tubes of the bundle.
Furthermore, the filter element 30 has the advantage of creating a slight pressure drop on the flow of emergency water and of making it possible more quickly to bring together the jets on the wall of the tubes 22 and consequently to decrease the length of the tubes 22.
The main advantage sought for the steam generator according to the invention comprising an emergency feed water device is to produce emergency water jets having a path inside the steam generator with enough length before the jet encounters a wall or internal equipment of the steam generator for the jet to be heated to a temperature close to the average temperature of the steam generator. In particular, a sought-after advantage is to avoid bringing the surface of the upper part of the outer wall of steam generator in contact with the low-temperature water capable of causing cracking of thermal origin. Within the scope of the example described, the toroidal header of the emergency feed water device of the generator according to the invention is placed in an area of the upper part of the steam generator not containing separating or drying devices and the jets of emergency feed water are injected into the steam generator by tubes which are slightly inclined towards the central axis of the steam generator. In this way, the jets, given the feed pressure of the emergency water, have a very long path, with no impact on equipment of the steam generator, before reaching the upper part of the bundle wrapper.
The height of the jets formed at the outlet of the tubes of the emergency feed device and the path of the jets may be controlled by adjusting the number and the diameter of the holes penetrating the upper wall of the toroidal header.
For example, for a steam generator of a pressurized-water nuclear reactor of recent design, it is possible to use a toroidal header of the emergency feed water device having an average radius greater than two metres, whose wall is drilled in its upper part with fifty holes having a diameter of 15 mm distributed over the circumference of the torus. In this case, it is possible to use tubes, each one fastened to the openings of the toroidal header, having a length of 150 mm and exhibiting a part inclined by 10° with respect to the vertical towards the inside of the steam generator.
The jets formed by the water penetrating the openings of the toroidal header may be subject to significant contraction, so that these jets may be detached from the inner wall of the tubes. In this case, in order to limit the height of the jets at the outlet of the tubes and in order for the jets to be joined back together inside the tubes, the number of holes can be increased or the diameter of the holes can be increased without changing the number thereof. However, it turned out that it was preferable not to increase the diameter of the holes significantly since the increased diameter of the jets resulted in less reheating of the water inside the steam formed in the upper part of the steam generator. As indicated above, it is also possible to create a slight pressure drop at the inlet of the tubes, for example by using a filter element as shown in
The flow rate of the emergency feed water may be maintained in all cases at a value high enough to prevent any stratification effect inside the toroidal header according to the invention.
Furthermore, it is possible to prevent any risk of water hammer in the emergency feed device by providing a rising path for the emergency feed water from the inlet pipe in the steam generator up to the opposite end of the toroidal header. This inclination together with the presence of openings penetrating the wall of the toroidal header in its upper part makes it possible to discharge the steam which would have been able to form inside the toroidal header in phases where the emergency feed water device is not used.
The invention is not strictly limited to the embodiment which has been described. Thus provision can be made for an angle of inclination of the tubes with respect to the vertical towards the central part of the steam generator which differs by 10°, it being possible for this inclination advantageously to be between 5° and 20°. The tubes may have a different shape from the shape described. For example, the tubes may be completely straight tubes fastened in an inclined position with respect to the vertical in an opening penetrating the toroidal header near its upper part. For bent tubes as described with respect to
The invention is applicable to any steam generator requiring the use of an emergency feed water torus.
The steam generator according to the invention may comprise a first normal feed water device different from the device which has been described in and which comprises means for guiding the feed water consisting of a jacket arranged around part of the header and at least one guide wall extending the jacket downwards. In particular, the first header of the first normal feed water device of the steam generator may comprise water injection tubes fastened to the openings penetrating the toroidal header in its upper part. Such tubes may, for example, have the shape of a J so as to direct the feed water to the annular space of the steam generator. However, such an arrangement is less favourable than the device described above, in so far as the injection tubes extend the toroidal header upwards, such that the inner space of the upper part of the steam generator above the first feed device is not completely free to allow a path for the jets of the second emergency feed water device without interfering with the internal equipment of the steam generator, as for the example described.
In some cases, the main torus is in the cylindrical part of the outer jacket at a level which is generally higher than that of the main torus, as described above. In this case, the secondary torus may be placed either above the main torus, or below.
| Number | Date | Country | Kind |
|---|---|---|---|
| 03 03964 | Mar 2003 | FR | national |
