Protection system with multiple actuation modes for the shutdown of a nuclear reactor

Abstract

A protection system (1) for the shutdown of a nuclear reactor comprises a first volume (V1) and a second volume (V2), communicating with each other at respective lower ends and containing a process fluid (F) and a pressurised gas; and a gas regulation system (13) to regulate the gas pressure in the volumes (V1, V2); the first volume (V1) comprises a neutron absorber (16) immersed in the process fluid (F) in a lower portion (15) of the first volume (V1) and having a lower density than the process fluid (F), and a constraint element (20) placed above the absorber (16) and pushed downwards by the lowering of the level of the process fluid (F) due to the pressure of the gas contained in the first volume (V1) to keep the absorber (16) in the lower portion (15) against the buoyancy thrust exerted by the process fluid (F); the constraint element (20) is vertically sliding in the first volume (V1) as the gas pressure in the first volume (V1) varies so as to vary the vertical position of the absorber (16) to bring the absorber (16) to face in use to an active portion (A) of the core (C) of the reactor (R).

Description

[0001] PROTECTION SYSTEM WITH MULTIPLE ACTUATION MODES FOR THE

[0002] SHUTDOWN OF A NUCLEAR REACTOR

[0003] Cross-Reference to Related Applications

[0004] This Patent Application claims priority from Italian Patent Application No . 102024000013540 fi led on June 13 , 2024 , the entire disclosure of which is incorporated herein by reference .

[0005] Technical Field

[0006] The present invention relates to a protection system for the shutdown of a nuclear reactor, comprising one or more shutof f devices characterised in that their actuation can be commanded by the operator and / or determined by a plurality of accidental modes .

[0007] Background

[0008] A nuclear reactor is normally provided with at least two independent shutof f systems . The most common shutof f system consists of rods adapted to insert absorbent material into the core . In the case of a fast reactor, it is also possible to control the power by moving absorbent material closer to or away from the core periphery .

[0009] For example , the solutions described in International Patent Applications PCT / IB2017 / 052611 and PCT / IB2022 / 061692 are known . Although these solutions are ef fective , it is nevertheless desirable to have alternative solutions , possibly equally or more simple as well as ef fective , that can also be used in conditions other than those for which the known solutions were speci fically designed .

[0010] An aim of the present invention is therefore to provide a protection system for the shutdown of a nuclear reactor which is relatively simple and yet particularly ef fective under various operating conditions , including situations in which the pumps of the nuclear reactor operate at variable speeds , and is capable of acting ef fectively and with high reliability even in the event of accidental sequences of various types , including those leading to an increase in the temperature of the core and of the primary coolant , and to violent overpressures in the primary system generated, for example , by the break of a steam generator tube .

[0011] Summary

[0012] The present invention therefore relates to a protection system with multiple actuation modes for the shutdown of a nuclear reactor as defined in the appended claim 1 and, for its auxiliary features , in the dependent claims .

[0013] The invention al so relates to a nuclear reactor , in particular a nuclear reactor cooled by liquid metal or molten salts , comprising this protection system, as defined in claim 9 .

[0014] The invention provides an extremely simple , effective and reliable solution and allows the shutdown of a nuclear reactor to be commanded either by an operator or as a result of a plurality of accidental modes , including those leading to an increase in the reactor temperature or an overpressure of the primary coolant .

[0015] In particular, when applied to reactors with a steam generator inside the reactor tank and operating with a secondary-side pressure higher than the primary-side pressure , the invention ensures passive reactor protection even upon pressurisation accidents of the reactor .

[0016] Furthermore , for example in case of reactors wherein the level of the hot collector is , in operation, lower than the level of the cold collector, the invention makes it possible to carry out the protective action even after the circulation pumps of the primary fluid of the reactor have stopped .

[0017] The present invention in particular applies advantageously to reactors in which the process liquid is of high density, such as lead or lead-bismuth eutectic .

[0018] In summary, the protection system according to the present invention comprises one or, preferably, several shutof f devices extending substantially vertically and comprising a first volume and a second volume , for example coaxial and concentric to each other .

[0019] The two volumes communicate at respective lower ends . The first volume , e . g . radially inside , communicates at the top with a gas regulation system supplying pressurised gas; the second volume, e.g. radially outside, has a closed bottom and communicates at the top with the gas ceiling of the reactor. Both volumes contain, in their lower portion, the same process fluid which can conveniently be the same primary fluid of the reactor.

[0020] In the lower part of the first volume, immersed in the process fluid, there is a neutron absorber (e.g. boron carbide spheres) which is placed, under normal reactor operating conditions, below the active portion of the core of the reactor; above the absorber, the first volume contains process fluid facing the active portion of the core functioning as a neutron reflector to facilitate reaching the reactor critical mass, and above pressurised gas. The buoyancy of the absorber in the higher-density process fluid is prevented by a constraint element. The higher gas pressure in the first volume if compared to the second volume results in a different level of the process fluid, higher in the second volume, lower in the first volume where the absorber is kept in the lowest portion.

[0021] The gas upper portion of the first volume has a conduit communicating with the primary fluid of the reactor, which is intercepted by a fusible plug made of a low-melting material (e.g. aluminium) ; and a gas supply conduit provided with a supply valve. The second volume has a discharge conduit , which can be put in communication with the supply conduit of the first volume either by opening a connection valve or by a connection device actuated by an overpressure in the primary system of the reactor .

[0022] The neutron absorber can move from the initial lower position in the f irst volume to a position facing the active portion of the core ( so as to perform the shutof f function) following rebalancing of the gas pressures in the two volumes .

[0023] This can occur as a result of various actuation modes : opening of the connection valve commanded by an operator ; increase in temperature of the primary fluid of the reactor, which causes the plug to melt , resulting in depressurisation of the first volume ; increase in pressure in the reactor R ( e . g . in the accidental case of a break of a steam generator tube ) , which actuates the connection device , resulting in rebalancing of the pressure in the two volumes .

[0024] The invention is also applicable to reactors provided with primary fluid circulation pumps that draw from the hot collector and wherein the primary fluid is , under normal operation of the reactor, at a lower level in the hot collector than in the cold collector ; in fact , in the event of a rise in the level of the primary fluid following the shutof f of the circulation pumps , the particular construction arrangement of the protection system of the invention allows the primary fluid to spill into the second volume of the shutof f device , with a consequent rise also of the level of the process fluid in the first volume and consequent rise of the absorber until it faces the active portion of the reactor core . This solution is also applicable to reactors wherein the pumps are operated at a variable speed, because the spill level can be set so that it only operates below a predetermined limit speed, leaving the possibility of operating at a variable speed above said limit speed .

[0025] Brief Description of the Drawings

[0026] The invention is further described in the following non-limiting embodiments , with reference also to the attached figures , wherein :

[0027] - Figure 1 is a schematic overall view in longitudinal section of a protection system for a nuclear reactor in accordance with the invention, shown in the normal reactor operating condition;

[0028] - Figure 2 is a schematic view in enlarged scale of a detail of the protection system of Figure 1 ;

[0029] - Figure 3 is a schematic overall view in longitudinal section of the protection system in Figure 1 , shown in the reactor shutdown condition;

[0030] - Figure 4 is a schematic overall view in longitudinal section of a variant of the protection system of the invention, shown in the normal reactor operating condition;

[0031] - Figure 5 is a schematic overall view in longitudinal section of the variant of the protection system in Figure 4 , shown in the reactor shutdown condition;

[0032] - Figure 6 is a schematic overall view in longitudinal section of a further variant of the protection system of the invention, shown in the normal reactor operating condition;

[0033] - Figure 7 is a schematic overall view in longitudinal section of the variant of the protection system in Figure 6 , shown in the reactor shutdown condition;

[0034] - Figure 8 is a schematic overall view in longitudinal section of the variant of the protection system in Figure 6 , shown while being mounted in the reactor .

[0035] Description of Embodiments

[0036] Figure 1 shows a protection system 1 for the shutdown of a nuclear reactor R, in particular a nuclear reactor cooled with liquid metal or molten salts , under accidental conditions .

[0037] The reactor R may be of a substantially known type and is therefore not shown or described in detail for the sake of simplicity . In so far as it i s relevant to the present invention, the reactor R to which the protection system 1 is applied has a tank V which houses a core C, formed of a plurality of fuel elements placed side by side and having an active portion A, as well as further known components not shown or described as not being relevant to the present invention; the reactor R operates with a primary fluid P for cooling the core C, which fills the tank V to a level H, above which there is a cover gas G .

[0038] The protection system 1 comprises one or more shutof f devices 2 which in use are placed inside the core C of the reactor R, or laterally and radially outside the core C ; preferably, the protection system 1 comprises a plurality of shutof f devices 2 , only one of which is shown in Figure 1 and in the following figures for exemplary purposes .

[0039] The shutof f device 2 comprises a casing 3 , for example (but not necessarily) substantially cylindrical , extending along and about a longitudinal axis X and placed in use within the core C or facing it . The casing 3 extends between a lower end 4 , closed by a bottom wall 5 , and an upper end 6 , closed by a head 7 .

[0040] The casing 3 is provided with an internal separation structure 10 delimiting within the casing 3 an internal volume VI and an external volume V2 ( defined by respective chambers ) coaxial and concentric around the axis X ; volumes VI , V2 are separated from each other by the structure 10 and communicate with each other at respective lower ends placed at the lower end 4 of the casing 3 .

[0041] In the non-limiting example shown, volume VI is arranged along and around the axis X and volume V2 is arranged radially outside around volume VI.

[0042] Volume VI is open at the bottom to communicate with volume V2, which is instead closed by the bottom wall 5.

[0043] At respective upper ends placed at the upper end 6 of the casing 3, the volumes VI, V2 have a supply conduit 11 and a discharge conduit 12 respectively, communicating with a gas regulation system 13 described below.

[0044] Volume V2 also has, at the upper end 6, holes 14 through which the volume V2 communicates with the outside of the shutoff device 2, i.e. with the outside of the casing 3, and in particular, in use, with the gas ceiling of the reactor R, i.e. the upper area of the reactor R occupied by the cover gas G (in other words, in use the shutoff device 2 is arranged so that the holes 14 are placed above the level H of the primary fluid P) .

[0045] The casing 3 contains a process fluid F, which partially occupies both volumes VI, V2 ; each volume VI, V2 also contains, above the process fluid F, a pressurised gas; the process fluid F can conveniently be the same primary fluid P used in the reactor R.

[0046] In particular, the volume VI comprises a lower portion 15, close to the end 4 and containing a neutron absorber 16 immersed in the process fluid F; an intermediate portion 17 placed above the lower portion 15; and an upper portion 18, communicating with the gas regulation system 13 .

[0047] The absorber 16 consists , for example , of a plurality of bodies , e . g . spheres , of boron carbide or another neutronabsorbing material .

[0048] The absorber 16 has a lower dens ity than the process fluid F, so that when immersed in the process fluid F it tends to float in the process fluid F . Above the absorber 16 there is a constraint element 20 which holds the absorber 16 in the lower portion 15 of the volume VI against the buoyancy thrust exerted by the operating fluid F .

[0049] In the non-limiting embodiment shown, the constraint element 20 compri ses a plate 21 that retains the absorber 16 and is connected via a rod 22 to a ballast 23 positioned in the upper portion 18 of the volume VI and sliding vertically in the volume VI ; the upper portion 18 contains pressurised gas from the gas regulation system 13 .

[0050] The process fluid F reaches a level Hl in the volume VI , such that the absorber 16 is immersed in the process fluid F below the plate 21 of the constraint element 20 .

[0051] The gas contained in the volume VI is kept , by means of the gas regulation system 13 , at a pressure higher than the pressure of the gas contained in the volume V2 : the higher gas pressure in the volume VI with respect to the volume V2 determines a di f ferent level of the process fluid F, higher in the volume V2 ( level H2 ) and lower in the volume VI ( level Hl) where the constraint element 20 keeps the absorber 16 in the lower portion 15 of the volume VI.

[0052] The upper portion 18 of the volume VI, containing gas, has a communication conduit 24 communicating with the outside of the shutoff device 2, i.e. with the outside of the casing 3 and, in use, with the primary fluid P of the reactor R; the communication conduit 24 is closed by a fusible plug 25 made of a low-melting material (e.g. aluminium) , which bears the operating temperature of the primary fluid P under normal operating conditions of the reactor R, but melts at a predetermined temperature above said operating temperature.

[0053] The gas regulation system 13 is for regulating the gas pressure in the volumes VI, V2 and consequently the position of the constraint element 20 and, thus, of the absorber 16 retained by the constraint element 20.

[0054] In particular, the gas regulation system 13 comprises a supply branch 30, connected to the volume VI via the supply conduit 11 and provided with a supply valve 31; and a discharge branch 32, connected to the volume V2 via the discharge conduit 12.

[0055] In a variant not shown, the branch 32 communicates directly with the cover gas G of the reactor R, which in turn communicates with the gas inside the volume V2 via holes 14. In general, therefore, the branch 32 is in fluid communication with the volume V2, e.g. via the discharge conduit 12 or via the cover gas G and the holes 14 .

[0056] The two branches 30 , 32 are connected to each other via a connection valve 33 and via a connecting device 34 .

[0057] As shown in Figure 2 , the connecting device 34 , which can be conveniently positioned outside the tank V of the reactor R for ease of access , comprises a set of chambers substantially aligned and f luid-tightly separated from each other : a chamber 35 in communication with the supply conduit

[0058] 11 via the branch 30 , a pair of chambers 36 , 37 in communication with the branch 32 and the discharge conduit

[0059] 12 via respective auxiliary conduits , and a chamber 38 in communication via an opening 39 with the environment outside the reactor R .

[0060] The chambers 35 , 36 , respectively communicating with the supply conduit 11 and the di scharge conduit 12 and thus with the volumes VI , V2 respectively, are adj acent to each other and f luid-tightly separated by a breaking septum 40 ; and the connecting device 34 comprises a breaking mechanism 41 configured to break the septum 40 as a result of an increase in pressure of the cover gas G of the reactor R ( causing a consequent increase in pressure in the volume V2 communicating with the cover gas G) to thus put the supply conduit 11 and the discharge conduit 12 into communication, leading to the rebalancing of gas pressure in the volumes VI , V2 for the actuation of the protection system 1 . The chamber 38 is interposed between chambers 36, 37 and is separated from chamber 36 by a fixed wall 42 and from chamber 37 by a movable wall 43 provided with one or more elastic elements 44 placed inside the chamber 38 . The breaking mechanism 41 comprises a punch 45 that extends from the movable wall 43 into the chamber 38 and through a passageway formed in the fixed wal l 42 to the septum 40 where it has a piercing tip 46 .

[0061] The punch 45 pierces the septum 40 as a result o f the movement of the punch 45 caused by an imbalance of pressures between the cover gas G of the reactor R and the external reference environment ( outside the reactor R) .

[0062] In use , the shutof f device 2 and in particular the casing 3 are placed in the reactor R in such a way that , under normal operating conditions of the reactor R, the absorber 16 , housed in the lower portion 15 of the volume VI immersed in the process fluid F, i s below the active portion A of the core C, constrained by the constraint element 20 ; the intermediate portion 17 of the volume VI is facing the active portion A of the core C and contains process fluid F acting as a neutron reflector to facilitate reaching the critical mass of the reactor R; the upper portion 18 contains pressurised gas supplied through the gas regulation system 13 .

[0063] In the volume V2 the process fluid F reaches the level H2 , which is higher than the level Hl of the process fluid F in volume VI due to the di f ference in gas pressure in volumes VI , V2 (higher in volume VI than in volume V2 ) .

[0064] The holes 14 are placed above the level H of the primary fluid P so that the volume V2 communicates with the gas ceiling of the reactor R, i . e . the upper area of the reactor R occupied by the cover gas G .

[0065] The protection function of the protection system 1 by means of the shutof f device 2 is performed by passing the absorber 16 from the lower portion 15 of the volume VI , where the absorber 16 is located below the active portion A of the core C, to the intermediate portion 17 of the volume VI , where the absorber 16 is facing the active portion A of the core C, as shown in Figure 3 .

[0066] This passage can occur as a result of various actuation modes .

[0067] For example , the opening of the connection valve 33 , e . g . commanded by an operator, puts the gas contained in volume VI into communication with the gas contained in volume V2 , thus leading to the rebalancing of the levels Hl , H2 of the process fluid F in volumes VI , V2 ( Figure 3 ) and the consequent rise of the constraint element 20 which thus leaves the absorber 16 free to rise .

[0068] The same rise of the absorber 16 can also occur as a result of : - an increase in the temperature of the primary fluid P of the reactor R in which the protection system 1 is immersed; the increase in temperature causes the melting of the low-melting material with which the plug 25 is made , resulting in the depressurisation of the volume VI ;

[0069] - a pressure increase in the reactor R ( e . g . in the accidental case o f a break of a steam generator tube housed inside the reactor R) which pressurises the chamber 37 of the connecting device 34 : by means of the holes 14 , the increase in pressure propagates into the volume V2 and, via the discharge conduit 12 , to the chamber 37 which expands by displacing the movable wall 43 against the action of the spring elements 44 and causing the punch 45 to perforate the septum 40 . As a result , the chambers 35 , 36 reach a pressure balance and consequently the gas contained in volumes VI , V2 also balances to the same pressure .

[0070] Figures 4 and 5 show an alternative embodiment that is particularly suitable for use with a reactor having a hot collector that is at a lower altitude than the cold collector in normal reactor operation, i . e . when the circulation pumps draw primary fluid P from the hot collector to return it to the cold collector .

[0071] In this variant , the casing 3 is provided with a further plurality of holes 48 which put the volume V2 in communication with the outside of the shutof f device 2 , i . e . with the outside of the casing 3 , and are located below the holes 14 ; in particular, the holes 48 are located in such a way that , in use , when the circulation pumps of the primary fluid P in the reactor R are in operation, they have respective lower edges 49 at a higher level than the level H that reaches the primary fluid P in the area contiguous to the holes 48 ( i . e . near the holes 48 ) and higher than the level H2 of the process fluid F within the volume V2 .

[0072] In the event of a shutdown of the circulation pumps of the primary fluid P, as shown in Figure 5 , the level of the primary fluid P rises and reaches a level H ' by overflowing through the holes 48 within the volume V2 ; the level H2 of the process fluid F within the volume V2 stabili ses at the same level H ' causing the level Hl of the process f luid F to also rise in the volume VI causing the absorber 16 to rise .

[0073] The supply branch 30 communicating with the supply conduit 11 is provided with a buf fer volume 50 which prevents over-pressurisation of the volume VI to allow the constraint element 20 to rise and the absorber 16 to act .

[0074] To rearm the protection system 1 it is consequently necessary to start up the circulation pumps of the reactor R, pressurise the volume VI until excess process fluid F is released into the protection system 1 , and partially release the pressure in the volume VI to re-establish the correct level Hl for rearming the protection system 1 . The present invention is also applicable when the protection system 1 is external and facing the core C (i.e. the casing 3 of the shutoff device 2 is located laterally and radially external to the core C) and can also be used when the space on the roof of the reactor R does not allow the vertical insertion of a cylindrical or similar geometry component. In these cases, the casing 3 may have a shape other than the substantially linear shape described above, in particular, the casing 3 may have a substantially S shape or similar configuration.

[0075] For example, in the embodiment shown in Figures 6 and 7, the casing 3 comprises two portions 51, 52 axially offset (i.e. vertically) and offset from each other, i.e. extending along respective axes X (vertically in use) parallel to each other .

[0076] The upper portion 51 comprises the head 7 of the casing 3 and is located radially external to the lower portion 52 housing the absorber 16.

[0077] Advantageously, the head 7 of the casing 3 is housed in a penetration 53 formed on the roof T of the reactor R and wider than the head 7, so that it can first be inserted vertically (figure 8) , radially away from the core C, to then take the protection system 1 adjacent to the core C by means of a radial approaching translation movement.

[0078] The advantages of the present invention are clear from the foregoing :

[0079] - the shutdown of the reactor by the system of the invention can occur upon command by the operator, protection logic, or even in a totally passive mode to cope with various accidental sequences , depending on the embodiments thereof ;

[0080] - one or more plant parameters that go beyond pre-set limits can, depending on the embodiments of the invention, activate the system; the system can be rearmed according to its embodiments , but only with the intervention of the operator and not by the possible return within the design limits of the parameter that actuated it ;

[0081] - the system can be positioned inside the core and also on the periphery of the core ;

[0082] - the actuation by the operator, of the protection logic or of the increase in temperature and pressure of the reactor or the shutof f of the primary fluid circulation pumps , does not require the movement of mechanical parts inside the reactor tank;

[0083] - the actuation resulting from the pressure increase in the reactor is simply the result o f a break of a septum that is located on the roof and at a distance from the reactor core and is easily replaceable ;

[0084] - in some embodiments , the protection system does not allow the reactor to start i f the pump speed has not reached a pre-set minimum value .

[0085] Finally, it is understood that further modi fications and variations may be made to the protection system and reactor herein described and shown, without departing from the scope of the attached claims .

Claims

CLAIMS1. A protection system (1) for the shutdown of a nuclear reactor, comprising at least one shutoff device (2) provided with a first volume (VI) and a second volume (V2) , communicating with each other at respective lower ends and containing a process fluid (F) and a pressurized gas above the process fluid (F) ; and a gas regulation system (13) to regulate the gas pressure in said volumes (VI, V2 ) , the gas pressure in the first volume (VI) being higher than the gas pressure in the second volume (V2) ; and wherein the first volume (VI) comprises a neutron absorber (16) immersed in the process fluid (F) in a lower portion (15) of the first volume (VI) and having a lower density than the process fluid (F) , and a constraint element (20) placed above the absorber (16) and pushed downward by a lowering of the level of the process fluid (F) due to the pressure of the gas contained in the first volume (VI) to keep the absorber (16) in the lower portion (15) against the buoyancy thrust exerted by the operating fluid (F) ; the constraint element (20) being vertically sliding in the first volume (VI) as the gas pressure in the first volume (VI) changes so as to vertically vary the position of the absorber (16) to bring the absorber (16) to face in use to an active portion (A) of the core (C) of the reactor (R) .

2. The protection system according to claim 1, whereinthe volumes (VI, V2 ) are at least partially concentric and coaxial around an axis (X) and the second volume (V2) radially surrounds the first volume (VI) on the outside.

3. The protection system according to claim 1 or 2, wherein the shutoff device (2) comprises a casing (3) provided with an internal separation structure (10) delimiting within the casing (3) said volumes (VI, V2 ) , separated from each other by the structure (10) and communicating with each other at a lower end (4) of the casing ( 3 ) .

4. The protection system according to any one of the preceding claims, wherein said volumes (VI, V2 ) have, at respective upper ends, a supply conduit (11) and a discharge conduit (12) , respectively, communicating with the gas regulation system (13) .

5. The protection system according to claim 4, wherein the gas regulation system (13) comprises a supply branch (30) connected to the first volume (VI) via the supply conduit (11) and provided with a supply valve (31) ; and a discharge branch (32) , connected to the second volume (V2) via the discharge conduit (12) ; the two branches (30, 32) being connected to each other via a connection valve (33) , the opening of which leads to rebalancing of gas pressure in said volumes (VI, V2 ) for actuation of the protection system6. The protection system according to claim 4 or 5, wherein the gas regulation system (13) comprises a connecting device (34) having chambers (35, 36) which are in fluid communication with the first volume (VI) and the second volume (V2) , respectively, and are separated by a breaking septum (40) ; and a breaking mechanism (41) configured to break said septum (40) as a result of an increase in pressure in the second volume (V2) consequent to an increase in pressure, in use, in the reactor (R) to thus connect the supply conduit (11) and the discharge conduit (12) leading to the rebalancing of gas pressure in the volumes (VI, V2 ) for actuation of the protection system (1) .

7. The protection system according to any one of the preceding claims, wherein the second volume (V2) has, at an upper end thereof, holes (14) through which the second volume (V2) communicates with the outside of the shutoff device (2) so that, in use, the second volume (V2) is in fluid communication with a cover gas (G) of the reactor (R) .

8. The protection system according to any one of the preceding claims, wherein the first volume (VI) has, in an upper portion (18) containing gas, a communication conduit (24) communicating with the outside of the shutoff device (2) and closed by a fusible plug (25) made of a low-melting material, e.g. aluminum, which melts at a predetermined temperature above an operating temperature of the primaryfluid (P) of the reactor (R) in normal operating condition, so as to put the first volume (VI) in communication with the primary fluid (P) of the reactor (R) leading to the rebalancing of gas pressure in the volumes (VI, V2 ) for actuation of the protection system (1) .

9. A nuclear reactor (R) equipped with a protection system (1) for the shutdown of the reactor under accidental conditions, wherein the protection system (1) is a protection system according to any of the preceding claims.

10. The reactor according to claim 9, wherein the protection system (1) comprises at least one shutdown device (2) located laterally and radially outside with respect to a core (C) of the reactor (R) , and / or inside the core (C) , and facing at least partially an active part (A) of the core(C) .

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