Patent Number: 058964304
Section: summary

TECHNICAL FIELD The present invention relates to a method and a device for handling fuel assemblies in a light-water nuclear power reactor comprising a reactor vessel with a reactor core. More particularly, the invention relates to such handling of fuel assemblies which occurs when fuel assemblies are to be replaced or transferred to a new position when the reactor vessel or parts connected thereto are to be serviced and therefore have to be emptied of fuel assemblies. BACKGROUND OF THE INVENTION A light-water nuclear power plant comprises a reactor vessel which encloses a reactor core. The reactor core comprises a large number of fuel assemblies. More particularly, the core comprises normally 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 transfer them within the fuel core in order to burn them out further. Such refuelling or transfer of fuel takes place upon shutdown of the nuclear power plant. During the shutdown, work is normally carried out also 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 limit 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) transferring a large number of fuel assemblies in the core to obtain optimum burnup. During such 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. Further fuel assemblies are lifted out of the core and placed at an arbitrary location 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 transferred 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 adjacently located 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 shutdown when fuel assemblies are to be lifted out of or into a reactor vessel. According to one aspect of the method according to the invention, the whole, or parts of, the reactor core 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 may contain fuel assemblies with an arbitrary order or with a mutual order corresponding to the order of the fuel assemblies in the reactor vessel. To bring about the forced transport, a fuel cassette is provided which accommodates the number of fuel assemblies which are to be simultaneously transported between the reactor vessel and the fuel pool. Such a fuel cassette preferably comprises four, eight or twelve fuel assembly positions. The fuel cassette may be designed so that each fuel assembly position is surrounded by four vertical walls of a neutron-absorbing material forming a sleeve-formed space. The respective sleeve-formed spaces have a length which substantially corresponds to the length of a fuel assembly so that a fuel assembly arranged in the fuel cassette is substantially surrounded by these walls. A fuel cassette comprising, for example, eight fuel assembly positions may be designed with two rows with sleeve-formed spaces in which four sleeve-formed spaces are arranged in each row. The sleeve-formed spaces in a fuel cassette with four or twelve fuel assembly positions are arranged in the same way with two rows of sleeve-formed spaces in which, respectively, two and six sleeve-formed spaces are arranged in each row. Alternatively, the sleeve-formed spaces may be arranged in a single row. The sleeve-formed spaces are provided with a bottom part against which the fuel assembly arranged therein may rest. In one embodiment of the invention, the sleeve-formed spaces are arranged with an opening facing upwardly for inserting and extracting the fuel assemblies. In another embodiment of the invention, a vertical opening in the sleeve wall is provided for loading and unloading the fuel assemblies, respectively, in the lateral direction. In a further embodiment of the invention, this opening is provided with a closable port. When lifting fuel assemblies from a reactor vessel, a fuel cassette of any of the above-mentioned types is arranged at a location in the reactor vessel above the core grid. A handling member is adapted to lift the fuel assemblies one, by one or in groups, and arranging them in the fuel cassette. One example of lifting in groups is the lifting of a core module comprising four orthogonally arranged fuel assemblies and possibly the control rod arranged therebetween. The fuel assemblies are arranged either one or more, preferably four, each in one sleeve-formed space in the fuel cassette. When the fuel cassette is filled with the desired number of fuel assemblies, this is transported to a position in the fuel pool where it is lowered and left in its entirety for temporary storage. A new fuel cassette is arranged in the reactor vessel and is filled with fuel assemblies until the required number of fuel assemblies are moved out of the core for the action to be taken. The advantage of the invention is that a considerable gain in time can be made by lifting a plurality of fuel assemblies simultaneously out of/into the reactor vessel and by transporting a plurality of fuel assemblies simultaneously to and from the reactor vessel, respectively. The time for shutdown of the nuclear power plant can be further reduced in those cases where laterally loaded fuel cassettes are used since the height that the respective fuel assembly has to be lifted for loading and unloading, respectively, can be reduced by approximately four meters. The shutdown time reduction results in a considerable cost saving.