Method for maintaining a nuclear reactor

Abstract

The invention relates to a maintenance method for a control rod drive assembly. The method comprises the steps of: - removing the initial weld (67) from the omega seal (65) and removing the jacket (25); - performing at least one maintenance operation on the control assembly (17); - cutting an edge (69) of the first lip (61) of the omega seal (65); - obtaining a replacement jacket (25R) comprising a new second lip (71); - creating a new sealed weld (73) between the first lip (61) and the new second lip (71), the new weld (73) being angularly offset by an angle (α) of between 15° and 80° in relation to the initial weld.

Description

[0001] TITLE: Method of maintaining a nuclear reactor

[0002] The present invention relates to the maintenance of the control mechanisms of the control clusters of a nuclear reactor.

[0003] A nuclear reactor typically consists of a pressure vessel containing nuclear fuel assemblies that make up the core of the nuclear reactor. The reactivity of the core is controlled, among other things, by inserting control rods into the core, each rod containing several rods made of neutron-absorbing material.

[0004] Each control cluster is attached to the end of a control rod, itself moved by a Cluster Control Mechanism (CCM).

[0005] The Cluster Control Mechanisms (CCM) each ensure the extraction, maintenance or insertion of a control cluster into the reactor core. They must also allow the free fall of the cluster control rods when the reactor's automatic shutdown circuit breakers open.

[0006] The nuclear reactor thus comprises a plurality of control assemblies, each dedicated to a control cluster. Each control assembly comprises an adapter secured to the pressure vessel cover, a housing for receiving a mechanism for lifting the control rod of the control cluster and a sheath for receiving the control rod. The housing, the lifting mechanism and the sheath constitute the MCG.

[0007] The adapter passes through the tank cover and is rigidly attached to it. It defines a passage through which the control rod moves.

[0008] The lifting mechanism is generally referred to as the lifting mechanism assembly (EML). This mechanism is an actuator whose function is to control, insert or extract the control rod of the control clusters in the reactor core.

[0009] The housing and sheath together form a pressure vessel, in fluid communication with the interior of the pressure vessel through the adapter.

[0010] The housing has a lower end of the housing typically screwed onto an upper end of the adapter. The seal between the lower end of the housing and the upper end of the adapter is achieved by a welded joint, often called a "Canopy" joint.

[0011] The sheath constitutes the upper end of the pressure vessel. It delimits an internal volume in which the control rod is housed in the upper position of the control cluster. This internal volume is closed at the top and communicates with the internal volume of the casing at the bottom.

[0012] In order to create a watertight connection between the sheath and the casing, it is possible to provide a first lip at an upper end of the casing and a second lip at a lower end of the sheath. The first lip and the second lip are welded to each other in a watertight manner and form a seal known as an "Omega" seal.

[0013] Since the EML is made up of moving parts that grip and move the control rod, it is subject to wear and tear that makes it necessary to replace it after several million cycles.

[0014] In most cases, replacements concern cluster control mechanisms (CCMs) that are reaching the end of their service life. Occasionally, CCMs may be replaced due to malfunction.

[0015] One solution for replacing a cluster control mechanism (MCG) on site is to replace the entire MCG by working at the "Canopy" joint. This operation involves replacing the EML (Lifting Mechanism Assembly) and the pressure vessel (duct + casing).

[0016] The RMCG “Canopy” solution can include the following operations:

[0017] - Cutting the “Canopy” seal (located between the housing and the cover adapter),

[0018] - Removal of the MCG in its entirety,

[0019] - Machining of the adapter to restore a weldable profile,

[0020] - Refitting a new MCG (with possible adjustment for pairing with the adapter),

[0021] - Reconstruction of the “Canopy” welded joint,

[0022] - Associated requalifications.

[0023] Another solution could be to intervene at the level of the “Omega” joint by cutting in the axis of the original weld.

[0024] The RMCG “Omega” solution could include the following operations:

[0025] - Cutting of the “Omega” joint (located between the sheath and the MCG housing) in the axis of the weld in order to preserve the initial design of the welded joint (vertical cutting),

[0026] - Removal of the sheath and the EML,

[0027] - Machining of the lip of the casing in order to restore a weldable profile, identical to the initial profile,

[0028] - Refitting a new EML and a new sheath or the original sheath remanufactured on site,

[0029] - Welding of the “Omega” joint identical to the initial design, - Associated requalifications.

[0030] These solutions are not fully satisfactory.

[0031] The RMCG “Canopy” solution requires:

[0032] - Replacement of the complete MCG including the pressure vessel (sheath + casing),

[0033] - Removal / replacement of the cover insulation, which has an impact on the operation schedule.

[0034] The RMCG “Omega”, with vertical cutting of the welded joint, is also not satisfactory on the following points.

[0035] EML replacement tests were carried out by intervening at the “Omega” joint.

[0036] After cutting along the weld axis of the “Omega” joint (vertical cutting), a penetrant test was carried out to ensure a defect-free surface condition. On a set of more than 50 cut MCGs, 38% of the penetrant tests showed non-compliant surface defects in the thin lips.

[0037] The random and unpredictable presence of these defects makes it difficult to prepare a construction site with sufficient control. These defects require systematic repairs and justifications. In the majority of cases, they may not even be repairable, or they may only require the development of complex processes.

[0038] As a backup, intervention at the “Canopy” seal would then be necessary.

[0039] In this context, the invention aims to propose a method for maintaining a nuclear reactor making it possible to ensure the maintenance of a cluster control assembly in a reliable and economical manner.

[0040] To this end, the invention relates to a method for maintaining a nuclear reactor, the nuclear reactor comprising at least one control assembly for a control cluster, the or each control assembly comprising an adapter secured to a cover of a pressure vessel of the nuclear reactor, a housing for receiving a mechanism for lifting a control rod of the control cluster and a sheath for receiving the control rod, the housing having a lower housing end connected in a sealed manner to the adapter, the housing having a first lip at an upper housing end, the sheath having a second lip at a lower sheath end, the first lip and the second lip being welded to each other by an initial weld and together forming an omega joint, the omega joint having an initial shape obtained by rotation of an initial arcuate line around an axis of rotation,said initial arcuate line having an initial geometric center, a first initial arc belonging to the first lip, and a second initial arc belonging to the second lip and connected to the first initial arc by an initial welding point, the initial welding point defining with the initial geometric center a line extending along a reference direction, the maintenance method comprising the following steps:,

[0041] - removal of the initial weld and evacuation of the sheath;

[0042] - carrying out at least one maintenance operation on the control unit;

[0043] - cutting one edge of the first lip;

[0044] - obtaining a replacement sheath with a second new lip;

[0045] - creating a new watertight weld between the first lip and the new second lip, the first lip and the new second lip together defining a new omega seal, the new omega seal having a shape obtained by rotation of a new arcuate line around said axis of rotation, said new arcuate line having a new geometric center, a new first arc belonging to the first lip, and a new second arc belonging to the new second lip and connected to the new first arc by a new weld point, the new weld point defining with the new geometric center a line extending in a new direction angularly offset by an angle of between 15° and 80° relative to the reference direction.

[0046] Thus, the intervention is carried out at the level of the "Omega" seal. As a result, the casing remains in place and is not replaced unnecessarily. Only the EML and possibly the sheath are replaced.

[0047] It is not necessary to remove the cover insulation. This generates a dosimetric gain.

[0048] This also results in a gain in planning compared to the “Canopy” RMCG solution. Furthermore, the risk of default inherent in the “Omega” RMCG described above is much more limited.

[0049] The sealed welded connection between the casing and the sheath is recreated between a new second lip and a surface of the first lip obtained by removing the edge of the first lip. This surface is free of defects, the defects having been eliminated by cutting the edge of the original first lip.

[0050] This particular surface is not a heat affected zone from the creation of the original welded joint, since the edge of the original first lip has been removed.

[0051] The maintenance method may furthermore have one or more of the following characteristics, considered individually or in all technically possible combinations:

[0052] - the replacement sheath is the initial sheath, the second new lip having been created on said initial sheath;

[0053] - the replacement sheath is a new sheath; - the method comprises, between the step of removing the initial weld and the cutting step, a step of checking the surface condition of the first lip in order to detect a possible defect, a width of the edge of the first lip cut in the cutting step being chosen so as to eliminate the possible defect;

[0054] - the rod moves along a displacement axis, the reference direction being parallel to the displacement axis;

[0055] - the step of removing the initial weld, said initial weld is cut along the reference direction or along a direction forming an angle of less than 15° with the reference direction;

[0056] - the new second arc is longer than the initial second arc;

[0057] - the new omega seal has the same shape as the initial omega seal;

[0058] - the new watertight weld is angularly offset from the initial weld towards the first lip.

[0059] Other characteristics and advantages of the invention will emerge from the detailed description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which:

[0060] Figure 1 is a simplified schematic representation of a nuclear reactor;

[0061] Figure 2 is a sectional view of a cluster control mechanism of the nuclear reactor of Figure 1;

[0062] Figure 3 is an enlarged view of the "Canopy" joint of the welded connection between the housing and the adapter on which the cluster control mechanism of Figure 2 is mounted;

[0063] Figure 4 is an enlarged view of the “Omega” joint of the welded connection between the casing and the sheath of the cluster control mechanism of Figure 2, before implementation of the maintenance method of the invention; and

[0064] Figure 5 is a view similar to that of Figure 4, after implementation of the method of the invention.

[0065] The nuclear reactor 1 shown in Figure 1 comprises a pressure vessel 3, in which nuclear fuel assemblies 5 constituting the core 7 of the nuclear reactor are arranged.

[0066] The pressure vessel 3 comprises a lower part 9 and a cover 11 removably attached to the lower part 9.

[0067] Nuclear reactor 1 still has control clusters 13 which can be inserted or extracted from core 7 in order to control the reactivity of the core.

[0068] Each cluster 13 comprises several rods made of a neutron-absorbing material. Each control cluster 13 is fixed to the end of a control rod 15, itself moved by a Cluster Control Mechanism (CCM).

[0069] The Cluster Control Mechanisms (CCM) each ensure the extraction, maintenance or insertion of a control cluster into the reactor core. They must also allow the free fall of the cluster control rods when the reactor's automatic shutdown circuit breakers open.

[0070] The nuclear reactor 1 thus comprises a plurality of control assemblies 17, each dedicated to a control cluster 13.

[0071] Each control assembly 17 comprises an adapter 19 secured to the cover 11 of the pressure vessel, a casing 21 for receiving a mechanism 23 for lifting the control rod 15 of the control cluster 13 and a sheath 25 for receiving the control rod 15 (figures 1 and 2). In figure 1, the sheaths are not shown to reveal the upper ends of the control rods 15.

[0072] The adapter 19 is engaged in a through hole of the tank cover 11 and is rigidly fixed thereto. It passes through the tank cover 11 and defines a passage through which the control rod 15 moves.

[0073] The lifting mechanism 23 is generally referred to as the lifting mechanism assembly (LMA). This mechanism is an actuator having the function of controlling, inserting or extracting the control rod 15.

[0074] The casing 21 and the sheath 25 together form a pressure enclosure, in fluid communication with the interior of the pressure vessel through the adapter 19.

[0075] The lifting mechanism 23, the casing 21 and the sheath 25 form the subassembly generally designated by the term Cluster Control Mechanism (CCM).

[0076] The casing 21 is a tubular part, having a central axis corresponding substantially to the axis of movement X of the control rod 15. It internally delimits an internal volume in which the control rod 15 is engaged. The lifting mechanism 23 is housed between the rod and the wall of the casing.

[0077] In the present description, the terms lower and upper, bottom and top, are understood according to the axis of movement X of the control rod 15.

[0078] This is normally vertical.

[0079] The casing 21 has a lower casing end 27 sealingly connected to the adapter 19.

[0080] More specifically, the lower end of the casing 27 is typically screwed onto an upper end 29 of the adapter 19. The seal between the lower end of the casing 27 and the upper end 29 of the adapter is achieved by a welded joint 31, frequently called a “Canopy” joint (FIG. 3).

[0081] Sheath 25 constitutes the upper part of the pressure vessel.

[0082] It delimits an internal volume 33 in which the control rod 15 is housed in the high position of the control cluster 13. This internal volume 33 is closed towards the top and communicates with the internal volume of the casing 21 towards the bottom.

[0083] The casing 21 has an upper casing end 35 of tubular shape around the movement axis X.

[0084] As seen in Figures 2 and 4, the upper end of the casing 35 has a substantially annular upper surface of the casing 37 perpendicular to said axis of movement X and facing away from the adapter 19.

[0085] The upper casing end 35 further has a radially inner casing surface 39 and a radially outer casing surface 41;

[0086] The sheath 25 has a lower sheath end 43 engaged in the upper casing end 35 and fixed thereto.

[0087] The lower end of the sheath 43 has for this purpose an external thread 45, cooperating with a tapping 47 carried by the radially internal surface of the casing 39.

[0088] The sheath 25 also has a main part 49 located outside the casing 21.

[0089] An annular rib 51 is provided on the lower end of sheath 43, at the junction with the main part 49. It is located above the external thread 45.

[0090] The lower sheath end 43 has an upper sheath surface 53, substantially annular, perpendicular to the axis of movement X, and facing away from the adapter 19.

[0091] This upper sheath surface 53 is defined by the annular rib 51.

[0092] The upper sheath surface 53 extends substantially in the same plane as the upper casing surface 37.

[0093] The lower end of the sheath 43 further has a radially external surface 55.

[0094] This radially external surface 55 is defined by the annular rib 51.

[0095] It bears against the radially internal surface of the casing 39.

[0096] The annular rib 51 further defines a lower sheath surface 57, substantially annular and perpendicular to the axis of movement X. The lower sheath surface 57 bears on a shoulder 59 formed in the radially internal surface 39 of the upper end of the casing 35. As illustrated in FIG. 4, before implementing the method of the invention, the casing 21 comprises a first lip 61 at the upper end of the casing 35.

[0097] The sheath 25 has a second lip 63 at the lower end of the sheath 43.

[0098] The first lip 61 and the second lip 63 are welded to each other in a sealed manner by an initial weld 67.

[0099] Together they form an omega 65 joint.

[0100] The upper surface of the casing 37 carries the first lip 61.

[0101] The upper surface of sheath 53 carries the second lip 63.

[0102] The omega joint 65 has an initial shape obtained by rotation of an initial arcuate line 65a around an axis of rotation.

[0103] The axis of rotation corresponds approximately to the X axis of movement of the control rod.

[0104] The initial arcuate line 65a is shown in Figure 4. It corresponds substantially to the section of the omega joint 65 in a radial plane containing the axis of rotation X.

[0105] The initial arcuate line 65a has a certain thickness, corresponding to the thickness of the omega joint 65.

[0106] The initial arcuate line 65a has an initial geometric center C.

[0107] This initial geometric center C, when the initial arcuate line 65a is an arc of a circle, corresponds to the center of the arc of a circle.

[0108] When the initial arcuate line 65a is not an arc of a circle, the initial geometric center C is, for example, the isobarycenter of all the points of the initial arcuate line. Alternatively, the geometric center C is the isobarycenter of all the points of the internal surface of the initial arcuate line 65a, or the isobarycenter of all the points of the external surface of the initial arcuate line 65a.

[0109] The initial arc line 65a has a first initial arc 61a belonging to the first lip 61, and a second initial arc 63a belonging to the second lip 63 and connected to the first initial arc 61a by an initial welding point P.

[0110] The initial weld point P belongs to the initial weld 67.

[0111] The initial weld 67 has a shape obtained by rotation of the initial weld point P around the rotation axis X.

[0112] The initial welding point P defines with the initial geometric center C a line L extending along a reference direction.

[0113] As seen in Figure 4, the initial welding point P has a certain surface area.

[0114] To define the line L, we consider, for example, the isobarycenter of all the points forming part of said surface. Typically, the reference direction is parallel to the X axis of displacement of the rod.

[0115] The first and second lips 61, 63 each extend all around the X axis and have closed contours. They are curved towards each other.

[0116] The first and second initial arcs 61a, 63a each typically have a quarter-circle shape. The initial arcuate line 65a is a semicircle, centered on point C.

[0117] Thus, the initial shape of the omega 65 joint corresponds to a half torus, coaxial with the X axis.

[0118] The first and second lips 61, 63 delimit with the upper surface of the casing 37 and with the upper surface of the sheath 53 a cavity extending all around the axis X.

[0119] This 65 joint is called omega because of its shape.

[0120] The maintenance method aims to intervene on the control assembly 17 to carry out a maintenance operation on it.

[0121] The maintenance method includes the following steps:

[0122] - removal of the initial weld 67 and evacuation of the sheath 25;

[0123] - carrying out at least one maintenance operation on the control assembly 17;

[0124] - cutting of an edge 69 of the first lip 61;

[0125] - obtaining a replacement sheath 25R comprising a second new lip 71;

[0126] - creation of a new watertight weld 73 between the first lip 61 and the second new lip 71, the first lip 61 and the second new lip 71 together defining a new omega seal 73.

[0127] In the step of removing the initial weld 67, said initial weld 67 is cut along the reference direction L or along a direction forming an angle of less than 15° with the reference direction L.

[0128] The removal is typically carried out remotely, by any suitable means (grinder, saw, machining machine, etc.).

[0129] After evacuating sheath 25, it is possible to access EML 23.

[0130] The at least one maintenance operation typically includes one or more of the following operations:

[0131] - replacement or repair of sheath 25,

[0132] - replacement or repair of the EML 23,

[0133] - repair of the upper end of the casing 35. The method preferably comprises, between the step of removing the initial weld 67 and the cutting step, a step of checking the surface condition of the first lip 61 in order to detect a possible defect.

[0134] This defect is typically the presence of cracks or cavities, which can lead to non-conformity of the new watertight weld 73.

[0135] This control is carried out by penetrant testing, radiography or any other suitable method.

[0136] The edge 69 is the edge of the first lip initially welded to the second lip by the weld 67. It is cut over the entire periphery of the first lip 61. This makes it possible to eliminate the part of the initial weld remaining attached to the first lip 61 at the end of the removal step. This also makes it possible to create a new surface, having a good surface condition, for the purpose of welding the first lip 61 to the new second lip 71.

[0137] The edge 69 of the first lip is cut to a width chosen so as to eliminate any possible defect detected during the inspection step.

[0138] The cutting width here corresponds to the dimension taken in a radial plane, around the axis of rotation.

[0139] Alternatively, the control step is omitted, and the edge of the first lip is cut to a predetermined width.

[0140] Cutting is typically carried out remotely, by any suitable means (laser, saw, machining machine, etc.).

[0141] The replacement sheath 25R is for example the initial sheath 25, the second new lip 71 having been created on said initial sheath 25.

[0142] The second new lip 71 is created by any suitable means: welding an extension onto the initial second lip 63, removing the initial second lip 63 and fixing a second new lip 71 in place of the initial second lip, etc.

[0143] Alternatively, the replacement sheath 25R is a new sheath, never having been used in a nuclear reactor. It is manufactured directly with the second new lip 71, having the required dimensions.

[0144] According to another variant, the replacement sheath 25R is a reused sheath, but coming from another control assembly 17, from the same nuclear reactor or from another nuclear reactor. This sheath is then modified so as to include a second new lip having the required dimensions.

[0145] The new weld is typically performed remotely or manually. The welding process is the same as for the initial weld. Typically, the welding process is TIG / GTAW. The new omega joint 75 has a shape obtained by rotating a new arc line 75b around the rotation axis X.

[0146] The new arcuate line 75b is shown in Figure 5. It corresponds substantially to the section of the new omega joint 75 in a radial plane containing the axis of rotation X.

[0147] The new 75b arc line has some thickness, corresponding to the thickness of the new 75 omega joint.

[0148] The new arcuate line 75b has a new geometric center C'.

[0149] The new geometric center C' is defined as the initial geometric center C.

[0150] The new arc line 75b has a new first arc 61b belonging to the first lip 61, and a new second arc 71b belonging to the second new lip 71 and connected to the new first arc 61b by a new welding point P'.

[0151] The new weld point P' belongs to the new weld 73.

[0152] The new weld 73 has a shape obtained by rotation of the new weld point P' around the rotation axis X.

[0153] The new welding point P' is defined as the initial welding point P.

[0154] The new welding point P' defines with the new geometric center C' a line extending in a new direction L' angularly offset by an angle a between 15° and 80° relative to the reference direction L.

[0155] This offset comes from the fact that the edge 69 of the first lip 61 has been cut.

[0156] Preferably the angle a is between 25° and 70°, more preferably between 30° and 60°.

[0157] The new second arc 71b is longer than the initial second arc 63a.

[0158] The new first arc 61 b ​​is shorter than the initial first arc 61 a.

[0159] As a result, the new weld 73 is angularly offset relative to the initial weld 67 towards the first lip 61, that is to say is angularly offset towards the first lip.

[0160] On the other hand, the new omega seal 75 preferably has substantially the same shape as the initial omega seal 65.

[0161] In particular, the new geometric center C' is substantially at the same position as the initial geometric center C.

Claims

CLAIMS 1. Method for maintaining a nuclear reactor (1), the nuclear reactor comprising at least one control assembly (17) for a control cluster (13), the or each control assembly (17) comprising an adapter (19) secured to a cover (11) of a pressure vessel (3) of the nuclear reactor (1), a casing (21) for receiving a mechanism (23) for lifting a control rod (15) of the control cluster (13) and a sheath (25) for receiving the control rod (15), the casing (21) having a lower casing end (27) connected in a sealed manner to the adapter (19), the casing (21) having a first lip (61) at an upper casing end (35), the sheath (25) having a second lip (63) at a lower sheath end (43), the first lip (61) and the second lip (63) being welded to each other by an initial weld (67) and together forming an omega joint (65),the omega joint (65) having an initial shape obtained by rotation of an initial arcuate line (65a) around an axis of rotation (X), said initial arcuate line (65a) having an initial geometric center (C), a first initial arc (61a) belonging to the first lip (61), and a second initial arc (63a) belonging to the second lip (63) and connected to the first initial arc (61a) by an initial welding point (P), the initial welding point (P) defining with the initial geometric center (C) a line extending along a reference direction (L), the maintenance method comprising the following steps:, - removal of the initial weld (67) and evacuation of the sheath (25); - carrying out at least one maintenance operation on the control assembly (17); - cutting an edge (69) of the first lip (61); - obtaining a replacement sheath (25R) comprising a second new lip (71); - creating a new sealed weld (73) between the first lip (61) and the new second lip (71), the first lip (61) and the new second lip (71) together defining a new omega seal (75), the new omega seal (75) having a shape obtained by rotation of a new arcuate line (75b) around said axis of rotation (X), said new arcuate line (75b) having a new geometric center (C'), a new first arc (61b) belonging to the first lip (61), and a new second arc (71b) belonging to the new second lip (71) and connected to the new first arc (61b) by a new weld point (P'), the new weld point (P') defining with the new geometric center (C') a line extending in a new direction (L') angularly offset by an angle (a) of between 15° and 80° relative to the direction reference (L).

2. Maintenance method according to claim 1, in which the replacement sheath (25R) is the initial sheath (25), the second new lip (71) having been created on said initial sheath.

3. Maintenance method according to claim 1, in which the replacement sheath (25R) is a new sheath.

4. Maintenance method according to any one of claims 1 to 3, in which the method comprises between the step of removing the initial weld (67) and the cutting step a step of checking the surface condition of the first lip (61) in order to detect a possible defect, a width of the edge (69) of the first lip (61) cut in the cutting step being chosen so as to eliminate the possible defect.

5. Maintenance method according to any one of claims 1 to 4, in which the rod (25) moves along a displacement axis (X), the reference direction (L) being parallel to the displacement axis (X).

6. Maintenance method according to any one of claims 1 to 5, wherein in the step of removing the initial weld (67), said initial weld (67) is cut along the reference direction (L) or along a direction forming an angle of less than 15° with the reference direction (L).

7. Maintenance method according to any one of claims 1 to 6, in which the new second arc (71b) is longer than the initial second arc (63a).

8. Maintenance method according to any one of claims 1 to 7, in which the new omega seal (75) has the same shape as the initial omega seal (65).

9. Maintenance method according to any one of claims 1 to 8, in which the new sealed weld (73) is angularly offset relative to the initial weld (67) towards the first lip (61).

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