Electromagnetic safety device of a screw/nut type

A safety device to be applied to the vertical displacement of, for example, the control rods of a nuclear reactor, comprises the combination of a stator placed on the outside upper part of an impervious, nonmagnetic cylinder within the inside of which a rotor is mounted about its shaft, free in axial translation, with the shaft rotatable with the rotor but fixed in translation, and with the rotation of the shaft transferred to a hollow threaded rod which will transfer rotation to a nut slidable vertically inside the cylinder, supporting the control rods therebelow; the rotor also controlling the opening and closing of a jaw for holding and releasing the hollow threaded rod.

This particular invention is concerned with an electromagnetic device of a 
screw/nut type for use in the vertical displacement of, for example, the 
control rods of a nuclear reactor inside a non-magnetic, impervious 
cylinder. 
Numerous electromagnetic devices of a screw/nut type for allowing the 
movement of the control bars of a nuclear reactor are known. Such devices 
are described, for example, in French Pat. Nos. 2,041,128 and 2,320,656. 
These devices, however, present certain problems, including, for example, 
certain magnetic materials that must be soldered to nonmagnetic materials; 
and the motor must generally be oversized to accommodate the dispersion of 
magnetic field developed by the different active threads; and the nut 
itself has a tendency to be saturated magnetically, etc. 
The goal of this invention is to obviate these problems with a simple and 
inexpensive device; it accordingly being an object of the invention to 
provide a new and improved electromagnetic safety device of the screw/nut 
type that shall not be subject to the above-described problems, and that 
may be applied to the vertical movement of nuclear control rods and 
similar applications. 
Other and further objects will be explained hereinafter and are more 
particularly delineated in the appended claims. 
In summary, however, from one of the important aspects of the invention, it 
embraces a device comprised of a stator placed around the top part of a 
cylinder on the outside, and the following pieces disposed inside the 
cylinder: 
(a) a rotor in the upper part of the cylinder, free in axial translation 
the length of its shaft which it carries in rotation but which is fixed in 
translation, and with the shaft extending from the top towards the bottom 
of the cylinder carrying a hollow threaded rod having a neck at its upper 
end; 
(b) a nut, coordinated with the threaded rod, which is able to slide 
vertically without turning, in the inside of the cylinder and which 
supports the control rods of the reactor; and 
(c) an articulated jaw, one part of which is linked with the rotor and the 
other with part of the shaft. 
This combination is assembled so that when the stator is energized, the 
rotor is drawn to the top, closing the jaw on the neck of the threaded 
rod. By rotating the magnetic field, a rotation will result in the shaft 
of the rotor which, in turn, will cause a like rotation of the threaded 
rod. The effect will be that the simultaneous rotations ensure the 
vertical sliding (without rotating) of the nut. When the stator is not 
energized, the jaw will be open and the combination of the threaded rod 
and the nut (linked with the control rods) will fall to the bottom of the 
impervious cylinder. The length of the shaft should be designed so that it 
penetrates the threaded rod when the rod has fallen to the bottom of the 
impervious cylinder. From this lowered position, the rod may readily 
easily be raised by activating the stator so as to cause rotation of the 
rotor, which turns the shaft and then the hollow, threaded rod. When this 
occurs, the apparatus is designed to keep the jaw open and out of contact 
with the neck of the threaded rod.

In reference now to the different figures, the electromagnetic device of a 
screw/nut type is designed to enable the longitudinal (vertical) 
displacement of the control rods of the nuclear reactor 2, FIG. 1. A 
vertically elongated support container or cylinder 3 is constructed of 
impermeable, nonmagnetic material and the same conditions of temperature 
and pressure exist in the cylinder 3 and in the reactor 2 therebelow. At 
the upper exterior of the cylinder 3, a stator is disposed comprising a 
set of coils 4 designed to generate a transverse magnetic field inside the 
cylinder 3, this field being capable of rotating, as is well known. 
At the upper interior of the cylinder 3, a rotor 5 is disposed for rotation 
about the axis of the cylinder and is preferably in the form of a flange 
of magnetic material, as represented in FIG. 2. A shaft 6 supported in the 
cylinder for rotation about the axis thereof is coupled to the rotor 5 to 
rotate therewith but is fixed in translation. It extends from near the 
base of the cylinder 3, to the top of the cylinder. Key pins 7 shown in 
FIG. 2, ensure the desired connection between the rotor 5 and the shaft 6 
and transmit any rotation of the rotor 5 to the shaft 6 when the magnetic 
field as generated by the stator 4 controls the same. 
A hollow threaded rod 8 is supported on the shaft 6 for translation along 
the shaft and acts as the screw drive of the apparatus. This rod 8 
coaxially surrounds the shaft 6 and is of sectional shape that allows the 
shaft to rotate the rod. In FIG. 3, this sectional shape is represented as 
a square, though obviously another form could be utilized. 
A nut 9, free to slide vertically but without turning in the interior of 
the cylinder 3, has threads that engage the threads of the threaded screw 
rod 8 so that the rotation of the rod results in the translation of the 
nut 9. Pins 10 protrude into slots in the cylinder 3, as shown in FIG. 3, 
allowing the holding of the nut 9 against rotation. The control rods 1 of 
the reactor 2 are attached as, for example, by a cylindrical tube 11, FIG. 
1, to the nut 9. 
An indented section or neck 12 is formed in the upper segment of the hollow 
threaded rod 8 so that when the hereinafter described locking jaw means 13 
is closed or locked to engage the rod 8, FIGS. 1 and 4, the neck 12 will 
act as a support for the teeth of the jaw 13. 
This jaw 13 is comprised of two pivotally interconnected link pieces 14 and 
15 which are respectively pinned to the rotor 5 and the shaft 6 for 
pivotal support in such a way that the teeth of the jaw 13 are inserted 
into the neck 12 of the threaded rod 8 when the rotor 5 is held in the 
raised position of FIGS. 1 and 4 by the energized stator 4. 
When the stator coils 4 are energized, the rotor 5 is drawn by the stator 
to the elevated position of FIGS. 1 and 4, which tends to reduce the 
resistance to the magnetic flux. As this occurs, the lower extremity of 
the rotor 5 pushes the teeth of the upper link of the jaw 13 into the neck 
12 of the threaded rod 8 by downward pivoting action of the lower link 14, 
moving the jaw 13 to its locked position. The weight of the rod 8, the nut 
9, and the control rods 1 is then entirely supported by the rotor 5, which 
maintains a position of axial equilibrium relative to the amplitude of the 
magnetic field generated by the stator coils 4. 
The magnetic field produced by the stator is caused to rotate like an 
asynchronous electric motor or a stepping motor, so that the rotor 5 will 
follow the rotation, carrying the shaft 6 with it. In turn, the shaft 6 
will carry the rotation to the threaded rod 8 which causes a translational 
movement of the nut 9 sliding longitudinally without turning. According to 
the direction of the rotation, the control rods 1 attached to the nut 9 
will thus be raised and lowered smoothly and continuously. 
When, however, the control rods 1 must fall rapidly longitudinally into the 
core of the reactor 2, or when the electric current from the stator coils 
4 fails, safety is ensured by the fact that the rotor 5 will fall to its 
lowest position, since it is no longer held up by field from the stator. 
The rotor 5 will come to rest on shoulders 16 cut for this function on the 
shaft 6. The intermediary lower link 14 becomes elevated to cause the 
upper jaw link 15 to pivot outward to a more vertical position, FIG. 5, 
and to unclamp the jaw 13 from the neck 12. The jaw is thus moved to its 
unlocked position and disengaged from the rod 8. The threaded rod 8 thus 
being no longer held up, falls downward under gravity and as guided by the 
shaft 6, carrying with it the nut 9 and the control rods 1. 
Stops 17 are provided at the bottom interior of the cylinder 3 to retain 
the nut 9 inside the cylinder and thus enable the eventual re-elevation of 
the assembly. 
This re-elevation is accomplished by re-energizing the stator coils 4 so as 
to recreate a rotating electric field. The rotor 5 is then drawn to a 
raised position and transmits the rotation in the appropriate direction to 
the shaft 6 the length of which is adjusted so that it can still reach the 
threaded rod 8 when the rod 8 has fallen to the bottom of the cylinder 3. 
The shaft 6 transmits the rotation to the threaded rod 8. The nut 9 
resting on the stops 17 can go no lower and because it engages the threads 
of the threaded rod 8, it causes the rod 8 to ascend in translation. 
The rotor 5 being in raised position, causes the jaw 13 to close as in 
FIGS. 1 and 4. The upper edge of the threaded rod 8 is bevelled near the 
neck 12 to re-catch the teeth of the jaw 13 again around the neck 12. At 
this time, the threaded rod 8 is stopped in its elevation and the nut 9 
then begins its elevation if the magnetic field continues to turn, thus 
ensuring the elevation of the control rods 1. 
A sleeve 18 (retaining means) provided with a lower lateral pin 19 slides 
in an annular chamber formed inside the shaft 6 below the shoulder region 
16. The sleeve normally is at a rest position on the upper extremity of 
the upwardly held threaded rod 8, FIG. 4, and in such position, compresses 
a spring 20, within the chamber. In the absence of the threaded rod 8, as 
under release conditions of FIG. 5, the spring 20 pushes the sleeve 18 
down the chamber to an operative position at which the pin 19 holds the 
opened jaw 13 in open (unlocked) position as is represented in FIG. 5. 
When the threaded rod 8 is re-elevated, the rod will push the sleeve 18 
back into the chamber, thus freeing the jaw 13 which can close again on 
the neck 12 as before described. The sleeve is preferably of substantially 
the same inner diameter as the threaded rod 8. 
Although this is a preferred construction by which the invention may be 
effectuated, it remains evident that modification by those skilled in this 
art will not depart from the spirit and scope of this invention. For 
example, a helicoidal spring 21 may be designed to go between the shaft 6 
at a region below the shoulder 16 thereof and the bottom of the rotor 5 
near the link part 14, as shown in FIGS. 4 and 5, in order more quickly to 
neutralize the effect of retained magnetization of the apparatus, as in 
the case of an accident. Also, it does not matter what type of mechanism 
is utilized to provide the shaft 6 with only one degree of freedom in 
rotation and none in translation. The pins 10 of FIG. 3, moreover, 
designed to stop the rotation of the nut 9, can be omitted if sufficient 
torque blocking rotation of the screw can be obtained by some other means. 
Finally, it must be noted that this particular invention has the advantage 
of needing fewer electric coils than devices currently in existence for 
similar purposes.