Patent Number: 052992411
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a technology for transmuting transuranium elements and more particularly to a transuranium transmuting reactor core for transmuting the transuranium elements at a fast reactor and also to a transuranium elements transmuting fuel pin and fuel assembly charged into a reactor core of a fast reactor. A spent fuel discharged from a thermal reactor such as boiling water reactor or the like includes transuranium elements (hereinafter called TRU elements) such as neptinium-237 (.sup.237 Np), americium-241 (.sup.241 Am), americium-243 (.sup.243 Am), curium-242 (.sup.242 Cm), curium-244 (.sup.244 Cm) and others which are high-level radioactive wastes, and in minor actinides (hereinafter called MA elements) present after eliminating plutonium (Pu) from the TRU elements, there exists elements such as .sup.237 Np, .sup.241 Am, .sup.243 Am or the like having an extremely long half life such as 2.14 million Years, 432 years, 7,380 years, which cannot be quenched within a short period of time. Thus, it is desired that the MA elements are transformed into elements with a short half life through a nuclear transmutation in a short period of time. A prior art includes technique for transmuting the TRU element which comprises using a fast reactor extremely high in a neutron energy as compared with a thermal reactor and subjecting the TRU elements charged into a fuel charged in a core of the fast reactor to a nuclear transmutation ((1) "Conceptional Design Study on Actinide burning Fast Reactor", T. Osugi et al., JAER1-M 83-217, issued by Japan Atomic Energy Research Institute in December 1983; (2) "Transmutation of Transuranics in FBR", A. Sasahara, T. Matsumura, F7, Fall Meeting Reports, Atomic Energy Society of Japan, 1988). The prior art TRU elements transmuting comprises transmuting the aforementioned MA elements by causing a transmutation shown in FIGS. 9A to 9C to the typical MA elements of .sup.237 Np, .sup.241 Am and .sup.243 Am which are main objects of transmuting at a fast reactor core. In FIGS. 9A to 9C, F.P. denotes fission products, and elements given in a square border around indicates that of being easy to cause a fission against a neutron energy in the fast reactor, namely, that its energy averaged fission cross-sections are about 1 burn or over. The prior art TRU elements transmuting process utilizes a feature of the fast reactor core effectively, and the feature comes in: (1) Since a neutron energy of the fast reactor core is high, a neutron capture is hard to occur in .sup.237 Np, .sup.241 Am and .sup.243 Am and the like, and thus an evil influence of the fast reactor on a neutron economy according to the charging of the TRU elements into the reactor core is relatively small (a neutron capture cross-section getting small according as the neutron energy becomes high as shown in FIG. 18). (2) The fast reactor is generally high by about 1 digit in a neutron flux level as compared with the thermal reactor, therefore the TRU elements can be subjected to a nuclear transformation even if a fission and neutron capture cross section on an energy average is small, and thus a high transmuting efficiency of the TRU elements is ensured. In the prior art transmuting of the TRU elements, nothing has been taken particularly into consideration for charging amount of the TRU elements charged into a fast reactor core and its distribution in core when carrying out a transmuting of the TRU elements. Still, however, only a self-evident technical care on charging the core with the TRU elements as much as possible has been considered for enhancing a transmuting efficiency of the TRU elements. However, if the fast reactor core is charged with the TRU elements as much as possible, then the following problems are capable of resulting therefrom. (1) If the MA elements to be transmuted is added to uranium-plutonium mixed fuel, a melting point of the mixed fuel lowers. Then the melting point drop is capable of causing a fuel melting, thus a measure such as lowering a reactor power or the like will be necessary for avoiding the fuel melting, which may deteriorate the transmuting efficiency of the MA elements. (2) As will be apparent from FIGS. 9A to 9D, the typical MA elements to be transmuted is generally hard to bring about a fission, and hence is transformed into fissionable elements by a neutron capture. Accordingly, if the fast reactor core is charged with the TRU element excessively much, then, as shown in FIG. 19, an amount of fissionable elements produced newly by the neutron capture of the MA element according to a neutron irradiation comes to exceed fissionable elements transmuted by fission, thus an excess reactivity of the fast reactor increasing. Consequently, if the charging amount of the TRU elements and its distribution are not specified properly, an excessive change or distortion may arise on a reactor power distribution and a neutron flux distribution, thus leading to problems on safety and characteristics of the reactor. (3) The TRU elements to be transmuted are easy to cause an alpha-decay in most cases, and an alpha ray energy emitted at the time of the alpha-decay is relatively high at 4 to 6 MeV generally. Accordingly, if the MA elements are added much to a fuel, a calorific value and a source intensity of gamma ray, neutron and others become excessive from the state of a fresh fuel before loading into the fast reactor core. Further, at the time of assembling, storage and transportation of new fuel assemblies in which the MA elements are enclosed, a heat removing of the alpha ray energy becomes difficult and the fuel overheats to lead to a failure in a worst case. (4) When charging a fast reactor uniformly with the TRU elements to be transmuted at the core with a core for which a plutonium enrichment is one kind as a base, a radial distribution of the power density, namely a radial power distribution during operation of the reactor becomes small according as it comes outside, as shown in FIG. 20, therefore a transmuting efficiency of the TRU element and a plant power generation efficiency being unsatisfying. (5) When charging the reactor uniformly with the TRU elements at the core with the fast reactor core for which a plutonium enrichment is two or more than two kinds as a base, a radial power distribution of the core is improved as compared with FIG. 20 by an adjustment of the plutonium enrichment, a flatting requirement can thus be satisfied, however, as shown in FIG. 21, for example, there arises a portion where the power distribution largely fluctuates according to burn-up. On the other hand, a flow rate of a coolant flowed for cooling down the fast reactor core is constant through the lifetime of a reactor plant. The flow rate of the coolant to fuel assemblies is set adaptively to the time when the power is maximized. Thus, when the output distribution fluctuates largely according to the burn-up of the fuel, a heat removing efficiency deteriorates, a heating efficiency gets lowered furthermore, which is not preferable from the viewpoint of an economical operation of the reactor plant. SUMMARY OF THE INVENTION An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art described above and to provide a transuranium elements transmuting reactor core capable of transmuting the TRU elements efficiently without causing a failure of the fuel assemblies, increase of excess reactivity, deterioration of thermal efficiency and others. Another object of the present invention is to provide a transuranium element transmuting fuel and fuel assembly capable of preventing the lowering of the power density of a fast reactor and the distortion of the power distribution of the fast reactor and effectively transmuting the TRU elements. These and other objects can be achieved according to the present invention by providing, in one aspect, a transuranium element transmuting reactor core in which a reactor is charged with a plurality of fuel assemblies at a core and an amount of a transuranium element to be added is controlled so as to prevent a fuel element contained in the fuel assemblies from melting, and in the improvement, the amount of the transuranium elements to be added to the fuel assemblies is controlled so as to keep an excess reactivity of the reactor substantially zero through an operation of the reactor. In another aspect, there is provided a transuranium element transmuting reactor core in which a reactor is charged with a plurality of fuel assemblies at a core and an amount of a transuranium element to be added is controlled so as to prevent a fuel element contained in the fuel assemblies from melting, and in the improvement, charging amounts of .sup.242 Cm, .sup.244 Cm and .sup.241 Am are set so as to satisfy an equation EQU 1.2.times.10.sup.2 .times.M.sub.242 +2.8.times.M.sub.244 +1.1.times.10.sup.-1 .times.M.sub.241 &lt;Q.sub.1 where an upper bound of heating rates of the single fuel assembly outside the reactor is.sub.1 Q from the view point of the fuel assembly integrity, charging amounts of .sup.242 Cm, .sup.244 Cm and .sup.241 Am and also satisfy an equation EQU 1.2.times.10.sup.2 .times.M.sub.242.sup.L +2.8.times.M.sub.244.sup.L +1.1.times.10.sup.-1 .times.M.sub.241.sup.L 21 Q.sub.2 where an upper bound of the heating rates, per unit length of the fuel pellet contained in the fuel pins is Q.sub.2 from the view point of the fuel element integrity, charging amounts of .sup.242 Cm, .sup.244 Cm and .sup.241 Am per the unit length are M.sub.242.sup.L, M.sub.244.sup.L and M.sub.241.sup.L. In a further aspect, there is provided a transuranium element transmuting reactor core in which a reactor is charged with a plurality of fuel assemblies at a core and an amount of transuranium elements to be added is controlled so as to prevent a fuel element contained in the fuel assemblies from melting and in the improvement, a charging density of minor actinides is set to lessen outwards of a core central portion in a core area where a plutonium content is made even. In a still further aspect, there is provided a transuranium element transmuting reactor core in which a reactor is charged with a plurality of fuel assemblies at a core and an amount of transuranium elements to be added is controlled so as to prevent a fuel element contained in the fuel assemblies from melting and in the improvement, a charging density of minor actinides is set high accordingly in an area where a plutonium is enriched high at the core of a plutonium enriched area where a plutonium content varies. In a still further aspect, there is provided a transuranium element transmuting fuel pin wherein a transuranium fuel pin is formed by charging a transuranium fuel material in a fuel clad and the transuranium fuel material includes at least one of fuel materials consisting of an enriched uranium and a uranium-plutonium mixed fuel and a fertile material consisting of a degraded uranium, a natural uranium and a depleted uranium contain transuranium elements such as Np, Am and Cm. In a still further aspect, there is provided a transuranium element transmuting fuel assembly including a wrapper tube and a plurality of fuel pins enclosed in the wrapper tube, each of the fuel pins including a fuel clad, wherein at least one part of the fuel pins are formed by charging a transuranium fuel material in the fuel clad with a transuranium fuel material inside. In a preferred embodiment, the fuel pins enclosed in the wrapper tube comprises transuranium fuel pins charged with the transuranium fuel material and fuel material pins charged with a fuel material consisting of an enriched uranium and a uranium-plutonium mixture fuel, and a radioactive fission product such as Sr or alkaline metals is contained in the transuranium fuel material. In the transuranium element transmuting reactor core according to the present invention, since an amount of transuranium elements to be added to a fuel pin of the fuel assemblies is controlled so as to keep an excess reactivity of the reactor substantially zero through an operation of the reactor, a decrease of effective multiplication factor according to the lapse of time for operation will be prevented, an excessive deterioration or turbulence of the reactor power distribution can be prevented, and as looking for improvement of a power plant capacity factor from enhancing a reliability of the plant, transuranium elements (TRU elements) can be transmuted efficiently. Further, from setting loading amounts of .sup.242 Cm, .sup.244 Cm and .sup.241 Am so as to realize: EQU 1.2.times.10.sup.2 .times.M.sub.242 +2.8.times.M.sub.244 +1.1.times.10.sup.-1 .times.M.sub.241 &lt;Q.sub.1 where an upper bound of the single fuel assembly power assembly outside the reactor is.sub.1 Q from the view point of the fuel assembly integrity, loading amounts of .sup.242 Cm, .sup.244 Cm and .sup.241 Am which can be loaded into the single fuel assembly are M.sub.242, M.sub.244 and M.sub.241, and also to realize: EQU 1.2.times.10.sup.2 .times.M.sub.242.sup.L +2.8.times.M.sub.244.sup.L +1.1.times.10.sup.-1 .times.M.sub.241.sup.L &lt;Q.sub.2 where an upper bound of the heating per unit length of the fuel element contained in the fuel assemblies is Q.sub.2 from the view point of the fuel element integrity, charging amounts of .sup.242 Cm, .sup.244 Cm and .sup.241 Am per the unit length are M.sub.242.sup.L, M.sub.244.sup.L and M.sub.241.sup.L, a melting of the fuel element during operation of the reactor and an overheating or failure of the fuel assemblies outside the reactor can effectively be prevented, and an accident of a control rod and a neutron absorbing material of the control rod can be reduced by a neutron absorption effect of .sup.242 Cm, .sup.244 Cm and .sup.241 Am, an enhancement of heat removing efficiency of the core can thus be realized, an economical operativity is also improved, and a safety and reliability of the core and the fuel assemblies are ensured as well, thus transmuting the TRU elements efficiently. Further, by setting a charging density of minor actinides to lessen outwards of a core center in a core area where a plutonium content is even, and also by setting a charging density of minor actinides high accordingly in an area where Pu is enriched high at the core of a Pu-enriched area where a plutonium content varies, a flatting requirement of a radial distribution of the reactor power can be satisfied, an enhancement of safety and reliability of the core and the fuel assemblies will be realized without causing the excessive deterioration and turbulence of the reactor power distribution, thus transmuting the TRU elements efficiently. In a further aspect, according to the transuranium element transmuting fuel assembly of the characters described above, even if the transuranium fuel material is charged in the transuranium fuel pin, the degradation of the core power density and the distortion of the core axial power distribution can be effectively prevented, thus improving the core cooling efficiency and effectively transmuting the transuranium element.