Steam generator, particularly for pressurized water nuclear reactor

Steam generator, particularly for a pressurized water nuclear reactor, which has in per se known manner a water box in which circulates the hot water from the reactor core, and then enters a series of heat exchange tubes installed in a tube plate and extending within a secondary ferrule traversed by the secondary water, wherein the water box, secondary ferrule and tube plate are three independent, disassemblable assemblies.

BACKGROUND OF THE INVENTION 
The invention relates to a steam generator, particularly for a pressurized 
water nuclear reactor. 
In the nuclear industry, steam generators are known, which are used in 
conjunction with pressurized water reactors and whereof one per se known 
type is described with reference to FIG. 1. FIG. 1 shows the enclosure 1 
of the reactor containing core 2 and in which circulates, on the one hand, 
at 3 the rising hot water leaving the core in the direction of arrow F 
and, on the other hand, the descending cold water circulating between 
envelope 1 and the reactor basket 4. 
In per se known manner, steam generator 5 has a water box 6, in which 
circulates the water coming from reactor 1 whilst, above said water box 6, 
there is a ferrule 7 traversed by secondary water entering at 8 and 
leaving at 9 with respect to said secondary ferrule 7. A pipe 10 connects 
reactor 1 to the water box 6 and has two concentric annular passages, 
namely a central passage 11 by which the hot water enters water box 6 and 
a peripheral passage 12 by which the cold water from water box 6 leaves 
and returns to the reactor 1 between the basket and the enclosure to be 
reinjected at the bottom of the core. 
In known manner, water box 6 is separated from the secondary ferrule 7 by a 
tube plate 13, in which are vertically installed a certain number of 
inverted U-shaped heat exchange tubes 14 and traversed by primary fluid. 
The water box 6 is subdivided into two parts, namely a hot compartment 15 
and a cold compartment 16. The secondary water which travels in rising 
manner within the secondary ferrule 7, heats and evaporates progressively 
whilst rising along the U-tubes 14. 
In all existing steam generators of loop reactors, such as described 
hereinbefore, or integrated reactors of the CAP type, for which the 
primary vessel acts as a water box, the secondary ferrule 7 is directly 
welded to tube plate 13, which is jammed on to the water box 6. As a 
result of the large pressure difference (approximately 100 bars) to which 
the tube plate 13 is exposed, stresses are accumulated in the angular 
connecting zone of said plate 13 to secondary ferrule 7. In order not to 
weaken this zone, it is consequently not possible to perforate the plate 
too near to its junction with the secondary ferrule 7 in order to install 
tubes there, which increases the overall dimensions of the steam generator 
in the manner shown in FIG. 2, where it is possible to see that a minimum 
space e is necessary between the side wall of the secondary ferrule and 
the first tube 14 installed in the tube plate 13. 
Moreover, the secondary part of such a steam generator cannot be 
disassembled and it is therefore very difficult to repair it or even clean 
it when necessary after a certain period of operation and its inspection 
whilst in service is a complicated, dangerous operation. 
SUMMARY OF THE INVENTION 
The present invention relates to a steam generator for a pressurized water 
nuclear reactor which permits, by using simple means, the formation of a 
connection between the secondary ferrule and the tube plate without 
accumulating lateral stresses and which also makes the steam generator 
dismantlable in an integral manner. 
The present invention therefore specifically relates to a steam generator, 
particularly for pressurized water nuclear reactors, comprising in per se 
known manner a water box, in which circulates the hot water leaving the 
reactor core in order to then penetrate a series of heat exchange tubes, 
installed in a tube plate and extending within a secondary ferrule 
traversed by secondary water, the water box, the secondary ferrule and the 
tube plate being three independent, dismantlable assemblies, wherein the 
secondary ferrule is terminated in its lower part by an annular neck 
having on the one hand a double conical bearing surface cooperating by two 
O-rings with a female groove having two conical bearing surfaces hollowed 
from the upper surface of the tube plate and on the other hand, on its 
outer periphery, a conical bearing surface associated with a locking ring; 
the upper part of the water box is terminated by a flange provided with a 
planar connecting bearing surface with two interposed O-rings, on the 
inner surface of the tube plate; and a system of tie bolts, certain of 
which pass through handling brackets on the periphery of the tube plates, 
whilst studs installed in the flange of the water box and nuts ensure, by 
means of the locking ring, the assembly and tight locking of the three 
aforementioned independent assemblies. 
According to an important feature of the present invention, the handling 
brackets installed on the periphery of the tube plate have recesses, 
whereof the conical profile makes it possible to receive the conical 
collars equipping the studs so that, during a dismantling manipulation of 
the water box alone, it is possible to handle the tube plate-secondary 
ferrule assembly without breaking the seal thereof. 
In the steam generator according to the invention, the secondary ferrule, 
the tube plate and the water box are consequently independent and 
completely disassemblable assemblies. Moreover, the sealing of the water 
box and the secondary enclosure of the steam generator take place in an 
overall manner and by independent means. Thus, in the generator according 
to the invention, it is possible to completely eliminate stresses from the 
angular connecting area of the tube plate to the secondary ferrule, which 
makes it possible to increase the number of heat exchange tubes contained 
in said ferrule, whilst the integral SO dismantlability of the steam 
generator makes it much easier to use than the prior art welded 
generators.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 shows the secondary ferrule 7, the tube plate 13 and the water box 
6. According to the invention, the lower part of the secondary ferrule 7 
is terminated by an annular neck 20 having a double conical bearing 
surface 21, 22 in its lower connecting part to tube plate 13. Thus, the 
latter has a corresponding female groove 23, which also has two conical 
bearing surfaces for cooperating with the conical bearing surfaces 21, 22 
of neck 20. Two metal O-rings 24 ensure the seal between tube plate 13 and 
secondary ferrule 7 when, in the manner shown hereinafter, the assembly is 
secured by tie bolt 25 and locking ring 26. For this purpose, the upper 
part of neck 20 also has a conical bearing surface 27 cooperating with a 
corresponding bearing surface of locking ring 26. In the lower part of the 
device, water box 6 is provided with a connecting planar bearing surface 
28 having two metal O-rings 29 interposed between the water box 6 and the 
tube plate 13. 
The vertical locking of the three aforementioned locking of the three 
aforementioned assemblies is brought about with the aid of tie bolts 25, 
nuts 30 and studs 31 which, according to the invention, are located in 
water box 6. 
As can be seen in FIG. 4, on the periphery of tube plates 13 are provided a 
certain number of handling brackets, such as 32, which have recesses 33 
with a conical bearing surface 34 for the studs 3l which, for this 
purpose, have a conical collar 35, which abuts against the corresponding 
conical part 34. Thus, when it is desired for any reason, e.g. for a 
maintainance manipulation, to dismantle the water box 6 without breaking 
the seal of tube plate 13 and secondary ferrule 7, it is possible to 
handle said plate 13 using tie bolts 25 traversing brackets 32, whilst 
maintaining a tight contact with secondary ferrule 7. The sequence of the 
manipulations is then as follows. The first stage is to loosen the nuts 30 
in fours and in several passes with the aid of extending jacks, without 
loosening them beyond a minimum torque imposed by the maintaining of the 
secondary sealing effect. In the final pass, the studs 31 corresponding to 
the brackets 32 are completely loosened and then unscrewed, followed by 
raising in the corresponding recesses 33, so that the conical collar 35 of 
each of them comes into contact with the conical bearing surface 34 of the 
bottom of the recess of the handling brackets 32. The studs 31, 
corresponding to the brackets 32, are then retightened, so as to again 
secure tube plate 13 and secondary ferrule 7, without permitting the 
loosening of the O-rings 24. The other studs 31 are now completely 
loosened. 
According to a preferred embodiment of the invention, there are three 
handling brackets 32 on the periphery and each of them has two recesses 
33a, 33b for the studs 31. These recesses 33a, 33b make it possible to 
protect the threads of studs 31 during handling operations. The number of 
studs used for taking up the locking action is determined in such a way 
that it is possible to obviate any possibility of any of them jamming 
during the manipulation. 
FIG. 4 shows the tie bolt 36, 37, 38, 39, 40 and 41 which, via the 
corresponding studs 31 and nuts 30, ensure the locking of the secondary 
ferrule and the water box 6 on to the tube plate 13 and whereof two tie 
bolts 39, 40 are also used for the locking handling of the tube 
plate-secondary ferrule assembly during the disassembly of the water box 
6. 
The aforementioned steam generator has a certain number of advantages, the 
most important of which are the easy access to the secondary tubes 14 and 
to the internal structures of the generator. However, it also has a 
certain number of further advantages, which are listed below: 
with regards to the arrangement of the bundle of tubes, the elimination of 
the connection area at the bottom of the ferrule, which makes it possible 
to gain space for the installation of the tubes; 
the second ferrule neck works in compression and consequently more 
favourably than a conventional flange; 
the system makes it possible to prestress the tube plate, by giving it a 
downward sag at the time of locking; 
the double conical bearing surface ensures surface contacts making it 
possible to retain low contact pressure levels, as well as sliding 
conditions of a very satisfactory nature (sliding is stopped as soon as 
the play is taken up by the external conical bearing surface); 
the double slope system makes it possible to lower the stress level and 
consequently reduce the thickness of the tube plate, under otherwise 
identical conditions; 
from the manufacturing standpoint, the fact of the complete separation of 
the plate-tube bundle assembly constructional problems from those of the 
secondary ferrule assembly.