Device for the automatic disconnection of a control rod and a nuclear reactor absorber cluster

Automatic uncoupling device between the pommel (18) of an absorber cluster (6) for introduction into the fuel elements (8) of the core (7) of a pressurized water nuclear reactor and a control rod (5) transmitting to the absorber cluster (6) movements induced by a control mechanism (4) located in a tight enclosure placed above the sealing cover (2) of the reactor vessel (1), said uncoupling device incorporating an attachment head (37), a mobile locking member (32), able to occupy a locked state and an unlocked state and whereof an axial displacement leads to a change of state, and a thermal module, characterized in that the thermal module deforms axially under the effect of the temperature and is placed between the attachment head (37) and the locking member (32) in order to form an assembly mobile in the axial direction and comprising a tight, water-filled bellows (35) and the attachment head (37) comprises at least one radial attachment finger (47) projecting by means of a small spring (48) and sliding within a cylinder (49) fitted in the sheath (28) of the control rod (5), said cylinder having an internal portion and from top to bottom a chamfered widening (53), as well as an axial notch (54) having a U-shaped section over part of its height, in order to receive the attachment finger (47), a second spring (44) placed above the attachment head (37) and tending to permanently push downwards so that the absorber cluster (6) is automatically uncoupled in the case of a reduction in the temperature of the water of the reactor vessel (1) below a first given temperature, the attachment finger (47) then abutting in the axial notch (54) and the locking member (32) rising under the effect of the shortening of the bellows (35), due to the contraction of the water to the upper position permitting the radial engagement of the balls (31) in the annular groove (30) of the locking member (32), the recoupling of the control rod (5) with the pommel (18) of the absorber cluster (6) taking place when a pressure is exerted on the top of the control rod (5), the latter firstly being downwardly displaced by a height equal to the axial portion (64a) of the slot (64) of the cylinder (49) and then, under the action of the helical portion (64b) of the slot (64) on the guide pin (51) rotating the cylinder (49) by a fraction of a turn freeing the attachment finger (47) from the U-shaped notch (54), thus permitting a downward displacement of the assembly of the attachment head (37) of the thermal module of the locking member (32).