Patent Number: 053612795
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to the drive mechanism that positions control rods within the nuclear core of a boiling water reactor. More particularly, a control rod drive which is completely contained within the reactor pressure vessel is described. In boiling water reactors, the control rod drives are traditionally positioned outside and below the reactor pressure vessel. Since the control rods are each positioned within the pressure vessel, each drive must include a coupling that penetrates the pressure vessel. This design has several disadvantages. One of the most noticeable drawbacks is the requirement that the containment structure (as well as the reactor pressure vessel) must be very high in order to provide sufficient storage space for the control rods below the reactor fuel bundles. Another major drawback is the existence of a very large number of items that must penetrate the reactor walls (i.e. the connecting rods). SUMMARY OF THE INVENTION It is a general objective of the present invention to provide an internal control rod drive arrangement for a boiling water reactor that permits more compact pressure vessel designs and substantially reduces the number of vessel penetrations required to facilitate control of the control rod positioning. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a control rod drive system is disclosed for positioning the control rods relative to fuel rods positioned in a nuclear core of a boiling water reactor. In a first aspect of the invention the control rod drives each include a jack rod and a hydraulic jack. The jack rod is coupled to a control rod. It may be either integrally formed with the control rod or coupled via a connector. The hydraulic jack is positioned within the pressure vessel above the nuclear core. The jack is capable of lifting, lowering and holding the jack rod to position the control rod. In a preferred embodiment, the jack includes a hydraulically operated holding mechanism and a hydraulically operated lifting mechanism. The holding mechanism has holders arranged to selectively engage the jack rod to hold the jack rod in place. The lifting mechanism includes lifters arranged to selectively engage the jack rod and a lift cylinder arranged to lift or lower the jack rod when the holders are disengaged and the lifters are engaged. In a second preferred aspect of the invention a hydraulically operated hold control valve is also provided for each control rod drive. The use of hold control valves facilitates the use of an addressing system that dramatically reduces the number of hydraulic lines that are required to operate the system. The hold control valve is positioned within the jack and is moveable between an open position and a closed position. In the open position a communication path is formed between a first hydraulic line and a holding line. The holding mechanism includes a holding piston arranged to move the holders between engaged and disengaged positions. A A first side of the holding piston is influenced by the first hydraulic line and a second side of the piston is influence by the holding line. The lifting cylinder is influenced by a second hydraulic line. In a preferred embodiment, the hold control valve includes a plunger that is moveable between an open and a closed position and a biasing spring for biasing the plunger towards the closed position. Additionally, both the first and the second hydraulic lines are arranged to influence the plunger in a direction towards the open position. In a third aspect of the invention, an open grid is provided within the pressure vessel at a position above the nuclear core of the boiling water reactor. The grid includes a multiplicity of mounting surfaces with each mounting surface supporting one of the hydraulic jacks. The hydraulic lines required to control the jacks may be strung along the sides of the grid beams. In one preferred embodiment, each mounting surface includes an alignment foot that mates with an associated alignment recess in the associated control rod drive and a pair of spaced apart raised hydraulic ports that mate with associated port recesses in the associated control rod drive. The alignment foot and the raised hydraulic ports cooperate to align the control rod drive. In a separate preferred embodiment, the control rods are cruciform in shape and each mounting surface and each control rod drive has a cruciform opening therein through which the control rod may be withdrawn during maintenance operations.