Patent Number: 
Section: claims

1. A rotation apparatus usable with a control drum in a nuclear environment, the control drum having a shaft that is rotatable about an axis of rotation that is horizontal, a reflector portion situated on the shaft, an absorber portion situated on the shaft, and a motor which, when powered, is operable to move the shaft between an operational position wherein the reflector portion faces toward a core of the nuclear environment and a shutdown position wherein the absorber portion faces toward the core, the rotation apparatus comprising:a rotation mechanism which is structured to apply to the shaft in the operational position a force that is structured to rotate the shaft toward the shutdown position, the force being resisted by the motor to retain the shaft in the operational position when the motor is powered, the force not being resisted when the motor is unpowered; anda rotation management system that is structured to resist rotation of the shaft when the shaft is in the shutdown position. 2. The rotation apparatus of claim 1 wherein the rotation mechanism comprises a weight that is structured to be connected with the shaft and that is structured to be movable between a first position in the operational position of the shaft and a second position in the shutdown position of the shaft, the weight in the first position being vertically higher than in the second position and being movable by gravity from the first position to the second position when the motor is unpowered. 3. The rotation apparatus of claim 2 wherein the weight is a counterweight structured to be situated on the shaft such that a center of gravity of the counterweight is spaced from the axis of rotation, the counterweight situated on the shaft being structured to be movable with the shaft between the first position in the operational position of the shaft and the second position in the shutdown position of the shaft. 4. The rotation apparatus of claim 3 wherein the center of gravity is aligned in the vertical direction with the axis of rotation and disposed below the axis of rotation in the second position. 5. The rotation apparatus of claim 4 wherein the rotation management system comprises an electrically conductive structure situated on the shaft and an eddy current brake that comprises a number of magnets situated at opposite sides of the electrically conductive structure in the second position. 6. The rotation apparatus of claim 4 wherein the rotation mechanism further comprises a rotation initiator that comprises a pair of magnets, one of the magnets of the pair of magnets being structured to be situated on the control drum, the other of the magnets of the pair of magnets being structured to be situated on a support on which the control drum is disposed, the pair of magnets having their poles arranged to mutually oppose one another and being positioned to bias the control drum away from the operational position when the shaft is in the operational position, the bias being resisted by the motor when the motor is powered, the bias not being resisted when the motor is unpowered. 7. The rotation apparatus of claim 4 wherein the rotation mechanism further comprises a rotation initiator that comprises a spring that is structured to extend between the control drum and a support on which the control drum is disposed, the spring being positioned to bias the control drum away from the operational position when the shaft is in the operational position, the bias being resisted by the motor when the motor is powered, the bias not being resisted when the motor is unpowered. 8. The rotation apparatus of claim 3 wherein the rotation management system comprises at least one of a hard stop that is structured to be engaged by the control drum in the shutdown position and an eddy current brake that comprises a number of magnets. 9. The rotation apparatus of claim 1 wherein the rotation mechanism comprises a spring that is structured to apply the force to the shaft in the operational position to bias the shaft toward the shutdown position. 10. The rotation apparatus of claim 9 wherein the rotation management system comprises at least one of a hard stop that is structured to be engaged by the control drum in the shutdown position and an eddy current brake that comprises a number of magnets. 11. The rotation apparatus of claim 1 wherein the rotation management system comprises a lock that comprises a first portion and a second portion, the first portion being structured to be situated on one of the control drum and a support on which the control drum is disposed, the second portion being structured to be situated on the other of the control drum and the support, the lock being movable between a locked position wherein the first portion and the second portion are in a fixed relationship with one another and an unlocked position wherein one of the first portion and the second portion is movable with respect to the other of the first portion and the second portion. 12. The rotation apparatus of claim 11 wherein the shaft is in the shutdown position when the lock is in the locked position, the lock in the locked position being structured to resist movement of the shaft away from the shutdown position. 13. The rotation apparatus of claim 12 wherein the first portion is a bolt, wherein the second portion is a receptacle, and wherein the bolt is received in the receptacle in the locked position, the bolt being removed from the receptacle in the unlocked position. 14. The rotation apparatus of claim 13 wherein the receptacle is a cutout formed on the shaft, and wherein the bolt is structured to be situated on the support and to be movable on the support between a first location received in the receptacle and a second location removed from the receptacle. 15. The rotation apparatus of claim 14 wherein the lock further comprises an actuator that is structured to be disposed on the support, the bolt being disposed on the actuator, the actuator being structured to be operable to move the bolt between the first and second locations. 16. A control drum apparatus comprising the rotation apparatus of claim 1, the control drum apparatus being usable in a nuclear environment having a core and further comprising:a shaft that is rotatable about an axis of rotation that is horizontal;a control drum, the control drum having a reflector portion situated on the shaft and an absorber portion situated on the shaft; anda motor which, when powered, is operable to move the shaft between an operational position wherein the reflector portion faces toward the core and a shutdown position wherein the absorber portion faces toward the core.