WO2018002305A1 - System for the mechanical locking of the hydraulic mechanism for moving the control rods of an smr-type nuclear reactor - Google Patents

Abstract

The invention relates to a system for locking nuclear reactor control rods in the shutdown position thereof. The invention guarantees safety during the transportation and/or handling of a nuclear reactor together with its core (combustible elements) according to the invention, in particular an SMR reactor in which the nuclear control rods are controlled hydraulically, including in the event of the reactor overturning.

Description

 MECHANICAL LOCKING SYSTEM OF THE MECHANISM

 HYDRAULICS FOR DISPLACING THE CONTROL BARS OF A

 NUCLEAR REACTOR, SMR TYPE

 Technical area

 The present invention relates to a mechanical locking system of the hydraulic mechanism for moving the control rods of a nuclear reactor.

 The invention aims to ensure the mechanical locking in the inserted position of the control rods in the spaces between fuel assemblies, corresponding to the stopping of any nuclear reaction.

 The main application targeted by the invention relates to so-called small or medium power nuclear reactors or SMR in English (acronym for "Small Modular Reactor"), whether modular or not.

 In general, although described with reference to the main application, the invention applies to all nuclear reactors, especially those with a higher power than SMR reactors, and that is to the technology (pressurized water reactors (PWRs). ), boiling water (REB), fast neutron (RNR), and the fuel cycle implemented.

 Prior art

 Control or control rods in a nuclear reactor are generally constituted by cylindrical tubes containing one or more neutron-absorbing materials, that is to say which absorbs neutrons with substantially the same dimensions as the rods or fuel assemblies housed within the reactor. reactor.

 These control rods have the function of absorbing neutrons so that the neutron flux decreases, and thus the number of fissions in the reactor core decreases, which causes a decrease in the thermal power of the heart.

The displacement of these bars, generally vertical, by insertion or withdrawal within the assemblies of fuel rods allows rapid power changes (reactivity of the core) and even a nuclear reactor shutdown in case of emergency. In most of the existing ones, more particularly the PWR reactors, the insertion of the control rods is from the top to the bottom while in others, in particular in the REB reactors, it is carried out from bottom to top for reasons of mechanical interference between components. The integration of the control rods and their mechanism of displacement within the heart of a nuclear reactor requires in fact an accounting with the coolant of the primary circuit of the reactor.

 It is already known to be able to implement a hydraulic mechanism for moving the control rods, which has the advantage of being compatible with the coolant, in particular with the water of a nuclear reactor, and to avoid mechanical and / or electromagnetic displacement. Thus, it is the controlled circulation of a coolant flow, in particular water, in the mechanism that allows the displacement of all the control clusters of the reactor, that is to say nuclear control rods and emergency stop bars. Subsequently and throughout the application, the term "control rods", the set of bars that drive the nuclear reactions including those only intended for emergency stop reactions.

 The US Pat. No. 5,277,619 of the company SIEMENS thus proposes such a hydraulic mechanism which allows a stable and precise control of the displacement stroke of the nuclear control rods. This mechanism essentially comprises two cylinders mounted coaxial, one of which forms a piston adapted to move axially inside the other forming the fixed cylinder. The piston is made with an alternation of projections and depressions on its outer periphery while the cylinder comprises an alternation of projections and depressions on its inner periphery, the projections and hollow of the cylinder being of complementary geometric shapes with those of the piston and define between them an annular space with a constriction inside which the displacement fluid, which is the water of the primary circuit, can circulate. The constriction thus defined makes it possible to move the control rods in a stable and precise manner because the displacement fluid can flow in one direction or the other by displacing the piston in small increments. In case of emergency stop required, the flow of fluid inside the mechanism is interrupted, which causes the downward translation of the piston and therefore these control rods which can fall by gravity in the reactor, that is to say over the entire height of the fuel assemblies. The nuclear chain reaction is then also interrupted.

There are SMR type nuclear reactors already in operation around the world. Many SMR projects are also under development, notably in the United States, Russia and France. According to the official definition given by the International Atomic Energy Agency, these are nuclear reactors whose power is less than 300 Megawatts electric (MWé).

 They are designed to be manufactured in the factory in the form of modules, their modular appearance to make them transportable to be installed on site, especially at sea as envisaged in some projects.

 These SMR reactors can be constructed as an isolated unit or in modules subsequently assembled to form a so-called multi-module high power plant.

 The inventors have identified a problem likely to be encountered for these SMR reactors, some of which can be transported and / or handled with their fuel. We can mention here the project known under the name "Flexblue" on behalf of the company DCNS which provides for such SMR reactors.

 When transporting and / or handling an entire SMR reactor with its fuel, an incident could cause the reactor to overturn, and thereby disengage the control rods from their fully inserted position in the core in the spaces between assemblies fuels, that is, stopping any nuclear reaction. This would result in reactivating the nuclear reaction, which would be uncontrolled.

 A mechanical lock of these control bars therefore appears obligatory.

 There is therefore a need to improve SMR type nuclear reactors, in particular to ensure mechanical locking of the nuclear control rods during transport and / or handling of these reactors in order to avoid any disengagement of said bars.

 The object of the invention is therefore to at least partially meet this need. Presentation of the invention

 To do this, the subject of the invention is a nuclear reactor, in particular a SMR reactor, comprising:

- nuclear control rods joined together in the form of cluster (s); - A holding plate, fixed inside the reactor vessel, in which is formed at least one through opening forming a movement guide of a cluster;

 at least one hydraulic mechanism for displacing at least one cluster comprising two cylinders mounted coaxially, one of which forms a piston secured to the cluster and adapted to move axially inside the other forming the fixed cylinder, the piston comprising an alternation of projections and depressions on its outer periphery while the cylinder comprises an alternation of projections and recesses on its inner periphery, the projections and hollow of the cylinder being of complementary geometric shapes with those of the piston define between them an annular space with a constriction within which a displacement fluid can flow in order to move the piston relative to the fixed cylinder; the piston further comprising on its outer periphery a groove of semicircular section; the cylinder further comprising on its inner periphery a groove of semi-circular section; the groove of the piston being intended to be in a position opposite that of the cylinder when the control rods are in their position to stop the nuclear reactions;

 mechanical locking means of the piston with respect to the cylinder, comprising an elongated flexible element whose length is at least equal to the circumference of the piston groove, and at least one spherical ball fixed on the flexible element;

 means for moving the mechanical locking means to wind the flexible element with its ball, inside the cavity formed between the groove of the piston and that of the cylinder and thereby holding the control rods in their position stopping nuclear reactions.

 Preferably, in their stop position of the nuclear reactions, the control rods are in a low position obtained by fallout by gravity.

According to an advantageous embodiment, a plurality of spherical balls are fixed on the elongated flexible element by forming an open bead of beads, which may also be designated as being a string of beads. Preferably, these balls are regularly spaced apart from each other along the string. More preferably, the distance between the first ball and the second to last of the string, considered in the direction of the winding, is substantially equal to the circumference of the groove of the piston so that the abutment between this first ball and the penultimate ball determines the effective lock.

 An important advantage to realize the locking means in the form of a bead of beads is that it can allow to easily determine mechanically a bad initial positioning of the control rods, that is to say does not correspond to a stopping position of the nuclear reactions. Thus, a bad position of the control rods can result in insertion of too much length of the cord or in the impossibility of introducing the collar between the piston and the cylinder.

 According to another advantageous embodiment, the supply piping of the displacement fluid which opens between the piston and the fixed cylinder is that of supply of the flexible element.

 In other words, the invention consists in defining a flexible mechanical locking element of the displacement piston of the nuclear control rods, by locking said piston relative to its fixed cylinder by means of a spherical ball supported by the flexible element introduced beforehand. .

 The flexible element is advantageously introduced in the manner of an endoscope, by the circulation piping of the displacement fluid, which has the considerable advantage of not having specific piping to achieve. In addition, when introducing the flexible element so that it is modified in its locking configuration, it is possible to maintain inside the piping the displacement fluid without flow. The displacement fluid is advantageously the coolant of the reactor, especially water in the case of a pressurized water reactor.

 Thus, the flexible locking element according to the invention can easily be brought from outside the reactor vessel into the pipe of the hydraulic displacement circuit.

 When dimensioning the elongated flexible element and the locking ball or balls supported by the flexible element, it is of course ensured that a sufficient clearance is defined between the outer diameter of the balls and the inside diameter of the piping. The displacement fluid is thus allowed to be expelled by the flexible element during its progression in the pipework.

It goes without saying that it is also ensured that the flexibility of the element with the ball (s) therein is sufficient for the progression in the pipework to take place. do without difficulty, that is to say without mechanical jamming, and taking into account the dimensioning and shape of the pipe.

 Finally, given the relatively large shear forces that can be subjected to the flexible element in its locking configuration due to the weight of the piston and the cluster or bars supporting control rods, it is ensured that the element locking hose is resistant to these shear forces

 Thus, by virtue of the invention, a mechanical function of locking the control rods in their position of stopping the nuclear reactions is efficiently and easily provided.

 In this way, it is possible to guarantee the safety of transport and / or handling of a nuclear reactor with its core (fuel elements) according to the invention, in particular a SMR reactor, with hydraulic control of the nuclear control rods, even in reversal of the reactor.

 detailed description

 Other advantages and characteristics of the invention will emerge more clearly from a reading of the detailed description of exemplary embodiments of the invention, given for illustrative and nonlimiting purposes, with reference to the following figures among which:

 - Figure 1 is a schematic longitudinal sectional view of a nuclear reactor SMR according to the invention implementing a pressurized water technology (PWR);

 - Figure 2 is a schematic longitudinal sectional view of a portion of the reactor according to Figure 1 wherein the locking system of the invention is intended to be implemented;

 FIG. 3 is a diagrammatic view of FIG. 2 showing the hydraulic displacement mechanism of the control rods of the nuclear reactor according to the invention;

 FIG. 4 is also a schematic view of the detail of the hydraulic displacement mechanism according to FIG. 3 showing the relative arrangement of the piston inside the fixed cylinder;

FIG. 5A is a diagrammatic sectional detail view showing the operation of the hydraulic displacement mechanism according to FIGS. 3 and 4, in an operating control configuration of the nuclear reactor; FIG. 5B is a diagrammatic sectional detail view showing the operation of the hydraulic displacement mechanism according to FIGS. 3 and 4, showing the positioning of the piston of the mechanism with no flow of displacement fluid, this position corresponding to a position of the bars of FIG. nuclear control in a shutdown configuration of the nuclear reactor;

 FIG. 5C corresponds to FIG. 5B and furthermore shows the piston locking means of the hydraulic displacement mechanism, in its insertion position wound around the piston which locks the position of the nuclear control rods in a stop configuration nuclear reactor;

 - Figures 5D and 5E show situations of poor positioning of the piston with respectively too great length of the inserted locking means or on the contrary an impossibility of insertion of the latter.

 Throughout the present application, the terms "vertical", "lower", "upper", "lower", "high", "below" and "above" are to be understood by reference with respect to a tank of a nuclear reactor according to the invention as it is arranged in vertical configuration of operation. Thus, in an operating configuration, the upper portion of the reactor vessel is that located above the core.

 It goes without saying that only a portion of the components of a SMR type nuclear reactor is schematically illustrated in the figures to explain the invention.

 FIG. 1 shows a nuclear reactor with an SMR type architecture, implementing a pressurized water technology as heat transfer medium.

 Such a reactor 1 comprises a reactor vessel 10 filled with water, called primary fluid, and inside which is present the reactor core 11. The fuel assemblies not shown are inserted into the core 11 while being maintained at the reactor. vertical from the bottom of the heart to an upper plate 12 called the output plate of the heart.

 As diagrammatically illustrated in FIGS. 1 and 2, a set of guides 13 is arranged while being held between the output plate of the core 12 and another plate above 14, called the guide holding plate.

Above this plate 14 holding the guides 13 is (are) arranged (s) one or more mechanisms 2 hydraulic displacement of the nuclear control rods. Finally above this housing space of the hydraulic displacement mechanism or mechanisms 2 is arranged a water pressurization device 17, separated from the bottom of the reactor 1 by a separating plate 16 provided for this purpose.

 FIG. 3 shows in detail the relative arrangement of the hydraulic displacement mechanism 2 of the control rods in the nuclear reactor 1. Thus, it can be seen in this figure that the control bar guide assembly comprises guides in the form of tubes 18 fixed between the two respective holding plates 12, 14.

 A plurality of nuclear control rods 19 in the form of a single cluster can slide inside a tube 18 by actuation of the hydraulic displacement mechanism 2. The displacement fluid is advantageously the heat transfer fluid of the reactor and therefore in the illustrated example of SMR reactor, constituted by water.

 More specifically, the mechanism 2 comprises two cylinders 20, 21 mounted coaxial, one of which forms a piston 20 adapted to move axially inside the other forming the fixed cylinder 21 by supplying a flow rate. the inside of the supply pipe 22 provided for this purpose.

 The piston 20 is secured to a cluster by means of a tip 23, preferably removable. The fixed cylinder 21 is fixedly mounted in a through opening 15 of the holding plate 14 of the clusters.

 This through opening 15 through which the piston 20 can slide thus forms a guide for moving a cluster of bars.

 Apertures 15A make it possible to let the heat transfer fluid from the heart to the steam generator.

 As shown in FIGS. 5A to 5E, a seal 3, preferably metal with respect to the operating temperature, mounted inside the fixed cylinder 21 seals around the piston 20.

 In Figure 4, the embodiment of the piston 20 and the fixed cylinder 21 in which the piston 20 is slidable under the effect of a displacement fluid which circulates with a certain flow rate between them.

The piston 20 comprises an alternation of projections 24 and recesses 25 on its outer periphery. The fixed cylinder 21 also comprises an alternation of projections 26 and recesses 27 on its inner periphery.

 The projections 26 and recesses 27 of the cylinder 21 are of complementary geometrical shapes with those 24, 25 of the piston define between them an annular space with a constriction within which the displacement fluid can flow with flow from the bottom to the top in order to move the piston 20 relative to the fixed cylinder 21.

 Preferably, as in the illustrated embodiment, the projections 24 and recesses 25 of the piston 21 as well as the projections 26 and recesses 27 of the fixed cylinder 21 are made in the form of circumferential grooves 28, 29. More preferably, these grooves 28, 29 are of trapezoidal section.

 As symbolized by means of the arrows in FIG. 5A, the reactor water is brought with a certain flow rate into the pipe 22 between the grooves 28 of the piston 20 and the grooves 29 of the fixed cylinder 21.

 Depending on the flow introduced, it is possible to maintain at equilibrium in a certain position, that is to say at a certain height, the piston 20 and thus the control rods 19 integral with the piston 20.

 By varying the flow rate of water introduced, the position of the piston 20 can be varied, and thus the operation of the reactor is controlled since the control rods 19 are more or less introduced over the height of the fuel assemblies within the core. .

 The hydraulic displacement mechanism 2 with the grooves 28, 29 respectively of the piston 20 and the cylinder 21, as illustrated, has the advantage of allowing a very stable and very precise displacement of the piston 20 and therefore at the end of the piloting of the nuclear reactor 20.

 The inventors have thought to introduce a new mechanical locking function of the movement of the piston 20 and therefore of the control rods 19 secured thereto.

This new mechanical locking function makes it possible to respond to any unforeseen situation of reversal of the reactor during transport and / or handling which could lead to the undesired displacement of the nuclear control rods 19 from their position dropped by gravity within the core. reactor which corresponds to a stop of the latter. This mechanical locking of the piston 20 relative to the cylinder 21 is provided by a system 4 described below.

 The system 4 firstly comprises, in addition, a groove of semicircular section 40 formed on the outer periphery of the piston 20 and also a groove of semicircular section 41 formed on the inner periphery of the cylinder 21.

 As illustrated in FIG. 5B, the groove 40 of the piston 20 is in a position facing the groove 21 of the cylinder 21, forming a cavity 42 of substantially circular cross-section, when no flow is delivered to the displacement fluid of the piston 20 , and that the control rods 19 fell by gravity into a low abutment position in the core 11 allowing the stopping of nuclear reactions.

 The system 4 also comprises a locking means 43 consisting of an elongated flexible element 44 whose length is at least equal to the circumference of the groove 40 of the piston 20, and at least one spherical ball 45, 45.1, ... 45.i, 45.i + 1 attached to the flexible element 43.

 According to an advantageous embodiment, as illustrated in FIG. 5C, a plurality of spherical balls 45, 45.1, ... 45.i, 45.i + 1 are fixed on the elongated flexible element 20 by forming an open bead of beads. says beads rosary.

 When the grooves 40 of the piston 20 and the groove 41 of the cylinder 21 are in their coinciding position facing each other as shown in FIG. 5B, the string of balls 44, 45 can be introduced mechanically through the pipe 22 which stays under water but without flow.

 The string of balls 44, 45 is thus introduced until it is wound inside the cavity formed between the groove 40 of the piston 20 and the groove 41 of the cylinder 21, as illustrated in FIG. 5C.

 In this configuration, the maintenance of the control rods in their position of stopping nuclear reactions is guaranteed.

 As illustrated in FIG. 5C, the locking means 43 are dimensioned so that the distance between the first ball 45.1 and the penultimate ball 45i of the string considered in the direction of the winding is substantially equal to the circumference of the throat. 40 of the piston 20.

Thus, the abutment between this first ball and the penultimate ball determines the effective lock to an operator or robot that handles the string of balls. This insertion length corresponding to an effective locking must always be identical and can be easily calibrated beforehand during manufacture in the factory.

 As illustrated in FIG. 5D, if the control rods 19 and therefore the piston 20 are positioned too high up beforehand, that is to say if these elements have not descended sufficiently, then the non-locking is detected because the winding of the ball chain 44, 45 is too long.

 This therefore results in a stroke of the ball chain 44, 45 in the pipe 22 greater than that provided for the effective locking.

 As illustrated in FIG. 5E, in the event of incomplete preliminary positioning of the control rods 19 and thus of the piston 20, then the non-locking is detected because the insertion of the string of balls 44, 45 is simply impossible in the cavity between grooves 40 piston 20 and cylinder groove 41 21.

 This therefore results in a stroke of the ball chain 44, 45 in the pipe 22 less than that provided for the effective locking.

 Typically for a piston diameter 20 of the order of 50 mm, the inventors have calculated that the stroke difference of the bead ring 44, 45 in the pipe 20 could be of the order of 175 mm more (situation of the figure 5D) or less (situation of FIG. 5E) with respect to the actual effective locking stroke (situation of FIG. 5C).

 A difference value of the displacement stroke of the bead ring 44, 45 sufficient compared to the nominal stroke corresponding to an effective lock, makes it possible to avoid any locking error.

 In addition to this purely mechanical locking control means, it is also possible to control the correct positioning of the nuclear control rods 19 by appropriate sensors, such as ultrasonic or laser sensors.

 Other applications than that which has just been described with reference to a PWR reactor with PWR pressurized water technology can be envisaged within the scope of the invention.

The invention is not limited to the examples which have just been described; it is possible in particular to combine with one another characteristics of the illustrated examples within non-illustrated variants. Other variants and improvements may be envisaged without departing from the scope of the invention.

Claims

 A nuclear reactor (1), in particular a SMR reactor, comprising: - nuclear control rods (19) joined together in the form of cluster (s);  - a holding plate (14), fixed inside the vessel (10) of the reactor, wherein is formed at least one through opening (15) forming a movement guide of a cluster;  at least one hydraulic mechanism (2) for moving at least one cluster comprising two cylinders (20, 21) mounted coaxially, one of which forms a piston (20) integral with the cluster and adapted to move axially at a distance of the other forming the fixed cylinder (21), the piston comprising an alternation of projections (24) and recesses (25) on its outer periphery while the cylinder comprises an alternation of projections (26) and recesses ( 27) on its inner periphery, the projections and recesses of the cylinder being of complementary geometrical shapes with those of the piston define between them an annular space with a constriction within which a displacement fluid can circulate in order to move the piston relative to the cylinder. fixed; the piston further comprising on its outer periphery a groove (40) of semicircular section; the cylinder further comprising on its inner periphery a groove (41) of semicircular section; the groove of the piston being intended to be in a position opposite that of the cylinder when the control rods (19) are in their position to stop the nuclear reactions;  means (43) for mechanical locking of the piston (20) with respect to the cylinder (21), comprising an elongate flexible element (44) whose length is at least equal to the circumference of the groove (40) of the piston, and at least one spherical ball (45, 45.1, ... 45.i; 45.i + 1) attached to the flexible member;  means (22) for moving the mechanical locking means to wind the flexible element with its ball, inside the cavity formed between the groove of the piston and that of the cylinder and thereby the maintenance of the control rods in their stopping position of nuclear reactions. The nuclear reactor of claim 1, comprising a plurality of spherical balls (45, 45.1, ... 45.i, 45.i + 1) attached to the elongate flexible member (44) forming an open bead of beads. says beads rosary. 3. nuclear reactor according to claim 2, the balls being evenly spaced from each other along the string.  4. Nuclear reactor according to claim 3, the distance between the first ball and the penultimate of the chain, considered in the direction of winding, being substantially equal to the circumference of the groove of the piston so that the abutment between this first ball and the penultimate ball determines the effective lock.  5. Nuclear reactor according to one of the preceding claims, the flexible element being a metal cable on which is (are) fi xed (s) or the spherical metal ball.  6. nuclear reactor according to one of the preceding claims, the supply pipe of the displacement fluid (22) which opens between the piston (20) and the fixed cylinder (21) constituting that of supply of the flexible element.  7. nuclear reactor according to one of the preceding claims, the projections (24) and recesses (25) of the piston 21 and the projections (26) and recesses (27) of the fixed cylinder 21 being formed in the form of circumferential grooves ( 28, 29).  8. Nuclear reactor according to claim 7, the grooves (28, 29) being trapezoidal section.  9. Nuclear reactor according to one of the preceding claims, comprising one or more sensors, preferably ultrasonic sensors, for detecting the position of the nuclear control rods.  10. Nuclear reactor according to one of the preceding claims, the control rods being, in their position of stopping nuclear reactions, in a low position obtained by fallout by gravity.  11. A method of mechanical locking of nuclear control rods in a reactor according to one of the preceding claims, comprising the following steps:  - Stopping the feed rate of the mechanism (2) of hydraulic displacement; - insertion by the supply pipe (22) of the displacement fluid with winding of the flexible element (44) with its spherical ball or balls (45) inside the cavity formed between the groove (40) of the piston (20) opposite that (41) of the fixed cylinder (21).  12. Application of the method according to claim 11 for the transport and / or handling of a SMR type reactor.