Patent Number: 046408118
Section: description

DETAILED DESCRIPTION In FIG. 1, is seen a sealed vessel 1 which is fixed at its lower part (not shown) on the cover of the reactor vessel. Inside the sealed vessel and along its axis is disposed a control shaft 6 provided with grooves and equally spaced in the vertical direction. Three magnetic coils 2,3 and 4 are arranged around the sealed vessel. The coil 2, called the holding coil permits the actuation of a set of three holding pawls 8 disposed at 120.degree. around the control shaft 6. The holding pawls 8 are mounted rockable through upper axles 14 on a support 11 fixed in the sealed vessel 1. The holding pawls 8 are actuated through the intermediary of links 9 constituting connecting elements articulated through axles 12 on the pawls at one of their ends and on a movable magnetic piece 10 at their other ends. The energization of the coil 2 permits the movable magnetic piece 10 to be attracted and maintained in its top position against a fixed magnetic piece 15. The double-toothed pawl 8 is shown in FIG. 1 in its open position. The coil 3 permits the closure of three pawls 16 disposed around the control shaft 6, called transfer pawls, mounted rockably on a support 17 through horizontal upper axle 18. The transfer pawls 16 are actuated through the intermediary of links 19 articulated through axle 20 on the pawls at one of their ends and on a movable magnetic piece 22 at their other ends. The magnetic piece 22 is mounted movable in the vertical direction on the support 17. The energization of the coil 3 permits the movable piece 22 to be attracted and maintained in its top position against a part of the support 17 made of magnetic material. THe double-toothed pawl 16 is shown in its closed position in FIG. 1. In this position the set of pawls 16 connects the control shaft 16 and the support 17. The support 17 is mounted movable in the vertical direction on the support 11 fixed in the sealed vessel 1. The coil permits the movable support 17 to be moved between a top position in which it is stuck by magnetic attraction to a fixed piece 23 and a bottom position to which it is returned by a spring 25. Generally speaking, the movable magnetic pieces are returned to the bottom position by springs. The transfer pawls 16, the amplitude of movement of which is equal to the distance between two grooves 7 of the control shaft 6, permits the control shaft 6 and the control rod which is connected to it to be moved upwards and downwards in a vertical movement, the amplitude of which is equal to the pitch of the control shaft grooves. After each pitch or step, the holding pawls 8 resume charge of the control shaft 6, which permits the transfer pawls to be returned into their initial position after having been opened by means of the coil 3. The movements of the control shaft 6 are executed step by step by means of the three coils 2,3 and 4 energized in succession. In FIG. 2, is seen a pawl 8 (or 16) in its closed position where it holds the control shaft 6, the two teeth 27 and 28 of the pawl being engaged in two successive grooves 7a and 7b of the control shaft 6 that bears on the teeth 27 and 28. When the teeth of the pawl are being engaged in the grooves 7, the teeth 27 and 28 are not in contact with the control shaft 6 due to the fact that the groove 7 are wider than the teeth and that the position of the pawl is such that the teeth are centered with respect to the position of the grooves 7. A subsequent upward movement of the pawls permits the control shaft to bear on the pawl teeth. The wear of the pawls and of the control shaft is thus reduced as there is no friction between them as explained in the U.S. Pat. No. 3,158,766. As seen in FIG. 2, the vertical distance between the tips of the two teeth 27 and 28 equals the pitch of the control shaft 6 or the step interval of the mechanism. In FIG. 2 the tooth 3 and the lower part of a pawl of the prior art has been drawn in dotted lines to show the differences between the pawl of the invention and the pawl of the prior art. The distance between the upper axle 14 and the lower axle 12 of the pawl has been kept unchanged so that the mechanism could remain unchanged except the substitution of pawls. An opening 32 of same shape is also provided on both pawls to permit the rocking of the link 9 (or 19) around the lower axle 12. For the pawl of the prior art the horizontal axis of axle 12 is at the same vertical level as the tip of the tooth 30. It has been discovered by applicant that the physical phenomena responsible for the wear and the breakage of the pawls and of the control shaft are very complex and related to shock and vibration. It has been also discovered that by providing the pawl with two teeth like 27 and 28 the resistance to wear and to fatigue breakage of the pawl is greatly increased if the lower axle 12 is centered on an axis situated about at a vertical distance h=1/5p below the tip of the upper tooth 28. With this position of the lower axle, the increase in resistance to wear and strength is obtained even if the contact between the two teeth and the control shaft did not occur at precisely the same time due to inaccurate machining of these parts. Thus, the increase in the wear resistance cannot be explained only by the increase of the surface of contact between the pawls and the control shaft. This result has been illustrated in FIG. 3 where the mean wear of a certain number of pawls belonging to different mechanisms have been represented as a function of the number of steps of displacement of the control shaft actuated by the pawls. The wear is determined as the means depth of the metal worn off the teeth of the pawls. The number of steps is given in million steps (10.sup.6). Curve A relates to pawls belonging to mechanisms of the prior art (one-toothed pawls) and Curve B to pawls belonging to mechanisms of the invention (double-toothed pawls), the two types of pawls being used in the same conditions to move control rods of a 1300 MW Pressurized Water Reactor. The wear rate of the pawls of the invention is rather decreasing and tends to be stabilized after about 5 million steps. Finally, the pawls of the invention are still operating after more than 10 million steps. On the contrary the sets of pawls of the prior art had to be stopped before at 6 million steps due to fatigue rupture or fatigue cracks. Applicant's experiments have shown that comparable results are obtained if there are provided two teeth on each pawl spaced by a distance equalling the pitch of the control shaft and if the lower axle of the pawl is centered at a vertical distance comprised between 1/6 and 1/4 of the pitch P below the tip of the upper tooth. If another arrangement of the lower axle is chosen in a double-toothed pawl, the wear is reduced compared to a one-toothed pawl but the risks of breakage remain great and operation over more than 10 million steps cannot be guaranteed. Thus the control rod mechanism of the invention is suitable for use under high stresses and with frequent displacements of the control rod as in the high-power reactors built in recent years. The invention is applicable to reactors comprising control rods moved step by step for the regulating of the power of the reactor.