Patent Number: 059303182
Section: summary

TECHNICAL FIELD The present invention relates to a method and a device for handling fuel assemblies in a light-water nuclear reactor comprising a reactor vessel with a reactor core. More particularly, the invention relates to the handling of fuel assemblies which occurs when fuel assemblies are replaced or moved to a new position when the reactor vessel or parts connected thereto are serviced and therefore have to be emptied of fuel assemblies. DESCRIPTION OF RELATED ART A light-water nuclear reactor plant comprises a reactor vessel which encloses a reactor core. The reactor core comprises a large number of fuel assemblies. More particularly, the core normally comprises between 400 and 1000 fuel assemblies. A fuel assembly comprises a bundle of fuel rods. The fuel rods in turn comprise pellets of a nuclear fuel. A coolant in the form of water is arranged to flow from below and up through the core to cool the fuel rods while nuclear fission is in progress. The heated coolant is evaporated whereupon it is passed to a turbine for conversion into electric energy. After a certain burnup time of the fuel assemblies, it is normal either to reject them or to rearrange them within the fuel core in order to burn them out further. Such a refuelling or rearrangement of fuel takes place upon shutdown of the nuclear power plant. During the shutdown, work is normally also carried out in the reactor vessel and in other systems which are connected to the reactor vessel. Such a shutdown is very costly and takes approximately three to eight weeks. Therefore, it is desirable to do whatever is possible to shorten this shutdown time to the shortest possible time. The refuelling in a nuclear power plant thus comprises (a) replacing burnt-up fuel assemblies with new ones, and (b) rearranging a large number of fuel assemblies in the core to obtain optimum burnup. During refuelling, the fuel assemblies are normally handled one by one. When the reactor vessel is opened to make the fuel assemblies accessible, a handling tool is moved down into the core and is brought to grip a fuel assembly which is to be temporarily placed in a fuel pool. Normally, control rods arranged between the fuel assemblies are left in the reactor vessel. Additional fuel assemblies are lifted out of the core and placed at an arbitrary locations in the pool. Thereafter, new fuel assemblies are lifted from the pool into the reactor vessel to the new empty positions. The fuel assemblies are thus lifted one by one. The fuel assemblies which are to be rearranged within the core are normally moved directly from their old to their new positions. In the event that work has to be carried out in the reactor vessel or in adjacent systems, such as pumps directly connected to the reactor vessel, a suitable number of fuel assemblies have to be lifted out therefrom and be temporarily placed at an arbitrary location in the fuel pool. In certain cases, the whole reactor vessel may have to be emptied of fuel assemblies. The lifting of the fuel assemblies one by one out of and into the reactor vessel, respectively, is one of the independent work operations during the shutdown which takes a relatively large proportion of the total shutdown time. The purpose of the present invention is to provide a method of reducing the time of the fuel handling and hence the total shutdown time. SUMMARY OF THE INVENTION The present invention relates to a method and a device which considerably reduce the time of a shutdown where fuel assemblies are lifted out of or into a reactor vessel. According to one aspect of the method according to the invention, the whole of, or parts of, the reactor vessel is/are transported simultaneously from the reactor vessel to the fuel pool located adjacent thereto. The transport takes place in a forced manner by moving groups containing a plurality of fuel assemblies and/or control rods simultaneously between the reactor vessel and the fuel pool. The groups contain fuel assemblies and/or control rods with a mutual order corresponding to the order of the fuel assemblies in the reactor vessel. According to the invention, a gripper is arranged for lifting and/or transport equipment is arranged, for example, in a reactor hall surrounding the reactor vessel. The gripper comprises a number of gripping devices corresponding to the number of fuel assemblies to be lifted out of or into the reactor vessel in a group. Further, the gripper is adapted to transport the removed group of fuel assemblies between the reactor core and the fuel pool. The group may also contain control rods which are lifted out together with the fuel rods. The gripper is thus loaded with a plurality of fuel assemblies and possibly fuel rods in or above the core, whereupon it transports the group of the fuel assemblies and possibly control rods to the fuel pool where these are lowered for temporary storage in a conventional fuel stand. After positioning the removed fuel assemblies in the fuel stand in the fuel pool, the gripper is again moved to a position above the core where it is arranged in a position for lifting out a new group of fuel assemblies and control rods, if any. When it is time to lift the fuel assemblies and the possible control rods in the reactor core, the above method steps are reversed. The advantage of the invention is that a considerable gain in time can be made by lifting a plurality of fuel assemblies and possibly control rods simultaneously out of/into the reactor vessel. The total time for the handling of fuel assemblies and/or control rods can thus be considerably reduced. This saving of time results in a considerable saving of costs.