Patent Number: 054992761
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for minor actinide nuclides incineration by burning them with minimized effects from rare earth elements. 2. Description of the Related Art Spent fuel in a light-water reactor contains minor actinide nuclides with long half-lives. Use of a fast reactor for the minor actinide nuclides incineration is currently being studied. To do this, the minor actinide nuclides need to be separated and extracted from the spent fuel. In the nuclide separation technique, neptunium (Np) of the minor actinide nuclides can be easily separated from the spent fuel by a Purex process. Americium (Am) and curium (Cm) can be separated by a Truex process. However, since americium and curium exhibit almost the same behavior as rare earth elements, when americium and curium are separated from the spent fuel, rare earth elements are also extracted at the same time. FIG. 1 is an illustration showing the percentage of nuclides taken out in each process of spent fuel reprocessing. The Purex process and the Truex process may be combined to provide a method of separating minor actinide nuclides from light-water reactor spent fuel. In this method, plutonium (Pu), neptunium (Np), and uranium (U) are extracted with the Purex process and americium (Am), curium (Cm), and a large amount of rare earth elements are extracted with the Truex process. Thus, when minor actinide nuclides are added to reactor core fuel to make them incinerate, rare earth elements are inevitably mixed at the same time. However, the rare earth elements exist in large quantities (ten times or more) compared with americium and curium, and are large in neutron absorption; if a reasonable amount of americium and curium extracted from spent fuel is added to the reactor core for americium and curium incineration, because of the neutron absorption of the rare earth elements, the reactivity required for burning becomes large, leading to difficult operation control of the fast reactor. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a method whereby minor actinide nuclides with long half-lives can be burnt with a fast reactor core with minimized effects from rare earth elements. To this end, according to the invention, there is provided a method of minor actinide nuclides incineration by adding neptunium of minor actinide nuclides separated from spent fuel to reactor core fuel of a fast reactor and adding americium of the separated minor actinide nuclides and rare earth elements to either or both of radial and axial blankets of the fast reactor. To use a plutonium burner reactor rather than a plutonium breeding reactor, americium and rare earth elements may be added to either or both of radial and axial shields in place of the blankets for nuclear reaction. Curium may be added together with americium and rare earth elements. Preferably, neptunium is added in an amount of 2% to 5% by weight based on the weight of the fuel, and the rare earth elements are added in an amount of 50% by weight or less based on the weight of the fuel. A Purex process is used to separate neptunium and a Truex process is used to separate americium and curium. In the Purex process, neptunium of the minor actinide nuclides can be separated and extracted without containing rare earth elements. In contrast, in the Truex process, americium and curium can be separated, but rare earth elements which are large in neutron absorption are also taken out in large quantities at the same time. If neptunium is then added to the reactor core fuel, it is burnt and made to incinerate with little effect on the reactor core characteristics because rare earth elements are not contained. On the other hand, if americium and curium are added to the blankets or shields, they are burnt with extra neutrons outside the reactor core. Rare earth elements inevitably added at the same time exist only in the blankets or shields and therefore have little effect on the reactor core characteristics. Thus, the minor actinide nuclides can be made to incinerate by limiting addition areas in the fast reactor in response to the type and nature of separated minor actinide nuclides. According to the invention, there is provided a method for an efficient nuclear reaction of americium and curium in a mixture of americium, curium, and rare earth elements, the method comprising the steps of adding neptunium to reactor core fuel of a fast reactor and placing the mixture on the periphery of the reactor core fuel to which the neptunium is added for causing a nuclear reaction to occur. According to the method of the invention, an efficient nuclear reaction can be caused to occur for a mixture of americium and curium which will efficiently cause the elements in the mixture to incinerate. In the method, preferably, neptunium is added to the reactor core fuel in an amount of 2% to 5% by weight based on the weight of the reactor core fuel, and a percentage of the rare earth elements in the mixture of americium, curium, and rare earth elements is 50% or less. In the method, preferably, the mixture of americium, curium, and rare earth elements is stored in either or both of radial and axial blankets, thereby placing the mixture on the periphery of the reactor core fuel. According to the invention, there is provided a method for inciration of americium and curium, comprising the steps of: separating neptunium from spent nuclear fuel, separating americium and curium from spent nuclear fuel, placing the separated americium and curium on the periphery of the separated neptunium in a fast reactor, and causing neptunium to initiate a nuclear reaction for causing an efficient nuclear reaction to occur for the americium and curium. In the method of the invention, preferably, a Purex process is used to separate neptunium from the spent nuclear fuel and a Truex process is used to separate americium and curium from the spent nuclear fuel. In the method, preferably, a mixture of or both of radial and axial blankets, thereby placing the mixture on the periphery of the reactor core fuel.