Patent Number: 
Section: claims

1. A rotary actuator comprising:a driven element comprising an axis of rotation and a plurality of recesses, the recesses being spaced around the axis of rotation by a spacing angle; andfirst, second and third latch elements, each latch element being movable between a retracted position, and an extended position in which it engagingly locates against one or more of the plurality of recesses;wherein the first latch element is angularly offset from the second and third latch elements respectively in rotationally opposite directions around the axis of rotation by an offset angle, the offset angle being greater than the spacing angle;whereby sequential movement of the first, second and third latch elements between respective retracted and extended positions causes rotation of the driven element. 2. The rotary actuator as claimed in claim 1, wherein each of the plurality of recesses is positioned in a co-planar arrangement around the axis of rotation. 3. The rotary actuator as claimed in claim 1, wherein each of the first, second and third latch elements is biased towards the respective retracted position. 4. The rotary actuator as claimed in claim 1, wherein each of the first, second and third latch elements has an actuating portion being configured to move the latch element between retracted and extended positions. 5. The rotary actuator as claimed in claim 4, wherein each actuating portion is one of a mechanical, hydraulic or pneumatic actuator. 6. The rotary actuator as claimed in claim 1, wherein each of the first, second and third latch elements comprises a circular engaging element, the engaging element being arranged to engagingly locate in one of the plurality of recesses. 7. A control rod assembly for a light water nuclear reactor comprising:a control rod, having a plurality of lateral recesses, the recesses being spaced apart from one another along the control rod;a rotary actuator as claimed in claim 1; anda brake element configured to engage with at least one of the plurality of recesses to prevent linear movement of the control rod. 8. A method of using a rotary actuator, the rotary actuator comprising:a driven element comprising an axis of rotation and a plurality of recesses, the recesses being spaced around the axis of rotation by a spacing angle; andfirst, second and third latch elements, each latch element being movable between a retracted position, and an extended position in which it engagingly locates against one or more of the plurality of recesses;wherein the first latch element is angularly offset from the second and third latch elements respectively in rotationally opposite directions around the axis of rotation by an offset angle, the offset angle being greater than the spacing angle;the method comprising the steps of:(a) moving the first latch element from its retracted position to its extended position; and(b) moving the second latch element from its retracted position to a partially extended position in which it contacts the driven element; and(c) returning the first latch element from its extended position to its retracted position whilst simultaneously continuing the movement of the second latch element from its partially extended position to its extended position, thereby causing a first rotation of the driven element around the axis of rotation in a first direction. 9. The method as claimed in claim 8, comprising the further steps of:(d) moving the third latch element from its retracted position to a partially extended position in which it contacts the driven element; and(e) returning the second latch element from its extended position to its retracted position whilst simultaneously continuing the movement of the third latch element from its partially extended position to its extended position, thereby causing a second rotation of the driven element around the axis of rotation in a first direction. 10. The method as claimed in claim 9, comprising the further steps of:(f) moving the second latch element from its retracted position to its partially extended position in which it contacts the driven element; and(g) returning the third latch element from its extended position to its retracted position whilst simultaneously continuing the movement of the second latch element from its partially extended position to its extended position, thereby causing a first rotation of the driven element around the axis of rotation in a second direction. 11. The method as claimed in claim 10, comprising the further steps of:(h) moving the first latch element from its retracted position to its partially extended position in which it contacts the driven element; and(i) returning the second latch element from its extended position to its retracted position whilst simultaneously continuing the movement of the first latch element from its partially extended position to its extended position, thereby causing a second rotation of the driven element around the axis of rotation in a second direction. 12. A method of deploying a control rod for a light water nuclear reactor, the nuclear reactor comprising a control rod assembly having a control rod and a rotary actuator, the method comprising the step of:(1) using the rotary actuator according to the method of claim 8 to drivingly rotate a shaft, the shaft being coupled to the control rod, so as to control the insertion of the control rod into the nuclear reactor. 13. The method as claimed in claim 12, the control rod assembly comprising a brake element, the brake element being movable between a retracted position and an extended position, the method comprising the further step of:(2) moving the brake element from the retracted position to the extended position, thereby preventing rotation of the shaft and maintaining the control rod in a fixed position within the nuclear reactor. 14. The rotary actuator as claimed in claim 1, wherein when the first latch element is in the extended position to engagingly locate against one or more of the plurality of recesses, the second and third latch elements respectively are not in the extended position.