Patent Number: 047553464
Section: summary

BACKGROUND OF THE INVENTION The invention relates to devices for displacing and holding a cluster of control elements movable vertically in the core of a nuclear reactor, such a cluster being part of a set of two coaxially movable clusters fulfilling different functions. With such sets, driving, power, control regulation, compensation of fuel exhaustion and shut down of a nuclear reactor can be better assured. Particularly, sets in which one of the clusters is formed of so called "black" elements having very high neutron absorption and the other clusters is formed of "grey" elements, have been proposed. The cluster of "black" elements is inserted in the core of the reactor by lowering it into it lowest position thus for shutting down the reaction and maintining it stopped. The displacement device may be partially shared by both clusters (French Pat. No. 2 106 373). It has also been proposed to have a separate device for each cluster (French Pat. No. 2537 764) one cluster including absorbent element and the other cluster elements containing fertile material. But none of the prior arrangements allows full freedom of displacement of one of the clusters, when the other cluster is in one of its endmost positions, except if complex mechanisms are installed. SUMMARY OF THE INVENTION It is an object of the invention to provide a device for vertically displacing a first cluster of control elements into and out of the core of a nuclear reactor, said device allowing to bring said first cluster and to hold it in one or other of its two endmost positions, while leaving full freedom of adjustment for a second cluster whatever which of the two endmost positions is occupied by the first cluster, and by using only simple means. To this end, the invention provides a displacing and latching device which comprises a vertical shaft movable along its axis by a fluid pressure difference and carrying support means cooperating with at least one head of said first cluster to be moved and resiliently holding and retaining means for the head on the support means. The device further comprises a mechanism for latching in the top position, carried by a fixed cluster guide tube and having at least one resilient finger for securing the head and retaining said head against gravity and a bolt movable in the direction of displacement of said first cluster, resiliently based towards a position in which it prevents release of said resilient finger and cooperating with a mechanism controlling a second cluster when said first cluster is moved upwardly beyond the highest position taken by the second cluster in normal operation. With this arrangement, the cluster to be moved is not subjected to the acceleration and shocks of the control mechanism of the other cluster, usually of electromagnetic type for step by step movement. The invention also provides a device for displacing and latching a control element cluster movable vertically into and out of the core of a nuclear reactor, said device comprising a vertical shaft movable along its axis by the action of a fluid pressure difference and having means for supporting at least one head of said cluster and a first resilient finger for securing and retaining the head on the support means, further comprising a high position holding mechanism carried by a fixed cluster guide tube and having at least one resilient finger for securing the head and retaining said head against the action of gravity and a vertically movable bolt, resiliently biased towards a position in which it prevents disengagement of a second resilient finger and cooperating with a mechanism controlling a second cluster so as to be brought into a position in which it releases the second resilient finger when the second cluster is moved above the highest position taken during normal operation. The high position securing mechanism will be generally carried by a fixed guide tube, belonging to the internals of the reactor and placed above a fuel assembly. The guide tube and an end piece of the assembly will be preferably both provided with sides for guiding the cluster support means, on which the support means are not simultaneously engaged, so as to accomodate possible eccentricity. The device may comprise cluster position detection means comprising a counter weight movable axially between a low position which it occupies when at rest and a high position in which it is pushed back by the head or each head of the external cluster. A sensor detects the presence of the counter weight in its top position.