Transuranium elements transmuting reactor core, transuranium elements transmuting fuel pin and transuranium elements transmuting fuel assembly

Abstract: In a transuranium elements 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, the amount of the transuranium elements to be added to the fuel element is controlled so as to keep an excess reactivity of the reactor substantially zero through an operation of the reactor. 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. The 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. A transuranium elements transmuting 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 an uranium-plutonium mixed fuel and a fertile material consisting of a natural uranium and a depleted uranium contain transuranium elements. In a transuranium elements transmuting assembly including a wrapper tube and a plurality of fuel pins enclosed in the wrapper tube, each of said fuel pins includes a fuel clad. 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.

Description: The present invention relates to a technology for transmuting transuranium elements in a fast neutrons reactor and more particularly to a fast neutrons reactor comprising a transuranium transmuting reactor core for transmuting the transuranium elements. The invention also relates to a transuranium elements transmuting fuel pin and fuel assembly charged into a reactor core of such a fast neutrons reactor. A spent fuel material discharged from a thermal reactor such as boiling water reactor or the like includes transuranium elements (hereinafter called TRU elements) such as neptunium-237 (²³⁷Np), americium-241 (²⁴¹Am), americium-243 (²⁴³Am), curium-242 (²⁴²Cm), curium-244 (²⁴⁴Cm) and others which are high-level radioactive wastes. Furthermore in minor actinides (hereinafter called MA elements) present after eliminating plutonium (Pu) from the TRU elements, there exist elements such as ²³⁷Np, ²⁴¹Am, ²⁴³Am or the like having an extremely long half life such as 2.14 million years, 432 years, 7,380 years, respectively, 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 reaction in a short period of time. The prior art including techniques for transmuting TRU elements comprises a process of using a fast neutrons reactor extremely high in a neutron energy as compared with a thermal reactor. The process comprises the step of 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 above prior art TRU elements transmuting process comprises transmuting the aforementioned MA elements by causing a transmutation shown in Figs. 9A to 9C to the typical MA elements of ²³⁷Np, ²⁴¹Am and ²⁴³Am which are main objects of transmuting at a fast neutrons reactor core. In Figs. 9A to 9C, F.P. denotes fission products, and elements given in a square box indicate that of being easy to cause a fission against a neutron energy in the fast neutrons 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 neutrons reactor core is high, a neutron capture will hardly occur in ²³⁷Np, ²⁴¹Am and ²⁴³ Am and the like, and thus detrimental influence of the fast neutrons 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 when the neutron energy becomes high as shown in Fig. 18). (2) With respect to the neutron flux level, the fast neutrons reactor generally exceeds, by about 1 digit, the thermal reactor. Therefore the TRU elements can be subjected to a nuclear transformation even if the 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 with respect to the charging amount of the TRU elements charged into a reactor core of a fast neutrons reactor and its distribution in the core when carrying out a process of transmuting of the TRU elements. However, only a self-evident maximum-possible technical care when charging the core with the TRU elements has been considered for enhancing the efficiency of transmuting the TRU elements. However, if the fast neutrons reactor core is charged with the TRU elements as much as possible, then the following problems may result therefrom: (1) If the MA elements to be transmuted are added to a uranium-plutonium mixed fuel, the melting point of the mixed fuel is lowered. Then the melting point drop may cause a fuel melting. Thus a measure such as lowering a reactor power or the like will be necessary in order to avoid melting of the fuel which may deteriorate the transmuting efficiency of the MA elements. (2) As will be apparent from Figs. 9A to 9D, it is generally difficult to bring about a fission of the typical MA elements to be transmuted. Hence, these elements are transformed into fissionable elements by neutron capture. Accordingly, if the fast neutrons reactor core is charged with the TRU element excessively much, then, as shown in Fig. 19, the amount of fissionable elements produced newly by the neutron capture of the MA elements upon neutron irradiation exceeds the amount of fissionable elements transmuted by fission. Thus, the excess reactivity of the fast reactor increases. Consequently, if the charging amount of the TRU elements and its distribution are not specified properly, an excessive change or distortion of the reactor power distribution and the neutron flux distribution may arise, thus leading to problems on safety and characteristics of the reactor. (3) The TRU elements to be transmuted may easily cause an alpha-decay in most cases, and the alpha ray energy emitted at the time of the alpha-decay is relatively high, generally 4 to 6 MeV. Accordingly, if greater amounts of the MA elements are added to a fuel, the calorific value and the intensity of gamma-rays, neutrons etc. emanating from the source 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, the removing of heat resulting from the alpha-ray energy becomes difficult and the fuel overheats which fact leads to a failure in the worst case. (4) When uniformly charging a fast neutrons reactor with a core for which a plutonium enrichment is basically intended with the TRU elements to be transmuted at the core, the radial distribution of the power density, namely the radial power distribution during operation of the reactor, becomes small, as shown in Fig. 20, therefore the transmuting efficiency of the TRU elements and a power generation efficiency of the plant being unsatisfying. (5) When uniformly charging the reactor with the fast neutrons reactor core for which a plutonium enrichment is intended as one of two or more than two kinds of operation, with the TRU elements at the core, the radial power distribution of the core is improved as compared with Fig. 20 by an adjustment of the plutonium enrichment, and 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, the flow rate of a coolant flowed for cooling down the fast neutrons 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 due to the burn-up of the fuel, the heat removing efficiency deteriorates, and the heating efficiency of the plant gets lowered furthermore. This, however, is not preferable from the viewpoint of an economical operation of the reactor plant. An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art described above and to provide a fast neutrons reactor comprising 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 etc.. Another object of the present invention is to provide a transuranium element transmuting fuel pin and a fuel assembly which are used in/or charged to respectively, said reactor and are capable of preventing the lowering of the power density of a fast neutrons reactor and the distortion of the power distribution of the fast neutrons reactor and are effectively transmuting the TRU elements. These and other objects can be achieved according to the present invention by providing, in one aspect, a a nuclear fast neutrons reactor comprising a transuranium elements-transmuting core which is charged with a plurality of fuel assemblies at least some of which contain an amount of one or more transuranium element(s) to be added to an ordinary fuel material, wherein the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am charged is set to satisfy the equation ┴1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 ┴ < ┴Q 1 ┴ wherein Q₁ is the upper bound of heating rate (in W/assembly) of a single fuel assembly outside the reactor and M₂₄₂, M₂₄₄ and M₂₄₁ are the charging amounts (in g/assembly) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, and the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am charged is set to also satisfy the equation ┴1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 ┴ < ┴Q 2 ┴ wherein Q₂ is the upper bound of heating rate per unit length of a fuel pellet contained in a fuel assembly (in W/cm) , and M₂₄₂, M₂₄₄ and M₂₄₁ are the charging amounts (in g/cm) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, per unit length. The invention, in a preferred embodiment, concerns a nuclear fast neutrons reactor, wherein the charging density of minor actinides decreases from a core central portion outwards of the core in a core area where a plutonium content is fixed. The invention concerns, in another preferred embodiment, a nuclear fast neutrons reactor, wherein, at a core having two or more plutonium-enriched areas, the charging density of minor actinides is increased in each of the plutonium-enriched areas. The invention is furthermore directed to a transuranium elements-transmuting fuel pin for a nuclear fuel assembly of a fast neutrons reactor mentioned above comprising a fuel clad being charged with a transuranium fuel material, said transuranium fuel material including at least one of fuel materials consisting of an enriched uranium and an uranium-plutonium mixed fuel and a fertile material consisting of a natural uranium and a depleted uranium containing transuranium elements such as Np, Am and Cm, wherein the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am contained in said fuel assembly charged in said fast neutrons reactor is set to satisfy the equation |1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 | < |Q 1 | wherein Q₁ is the upper bound of heating rate (in W/assembly) of a single fuel assembly outside the reactor and M₂₄₂, M₂₄₄ and M₂₄₁ are the charging amounts (in g/assembly) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, and the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am contained in said fuel assembly charged in said fast neutrons reactor is set to also satisfy the equation |1.2 x 10 2 x M L 242 + 2.8 x M L 244 + 1.1 x 10 -1 x M L 241 | < Q 2 wherein Q₂ is the upper bound of heating rate per unit length of a fuel pellet contained in a fuel assembly (in W/cm), and M L / 242, M L / 244 and M L / 241 are the charging amounts (in g/cm) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, per unit length. Furthermore, the invention is directed to a transuranium elements-transmuting fuel assembly for a nuclear fast neutrons reactor including a wrapper tube and a plurality of fuel pins enclosed in the wrapper tube, wherein the fuel pins enclosed in the wrapper tube comprise transuranium fuel pins charged with the transuranium fuel material and an uranium-plutonium mixed fuel material and fuel pins charged with an enriched uranium material and/or an uranium-plutonium mixed fuel material, the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am contained in said fuel assembly charged in said fast neutrons reactor being set to satisfy the equation |1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 | < |Q 1 | wherein Q₁ is the upper bound of heating rate (in W/assembly) of a single fuel assembly outside the reactor and M₂₄₂, M₂₄₄ and M₂₄₁ are the charging amounts (in g/assembly) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, and the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am contained in said fuel assembly charged in said fast neutrons reactor being set to also satisfy the equation |1.2 x 10 2 x M L 242 + 2.8 x M L 244 + 1.1 x 10 -1 x M L 241 | < |Q 2 | wherein Q₂ is the upper bound of heating rate per unit length of a fuel pellet contained in a fuel assembly (in W/cm), and M L / 242, M L / 244 and M L / 241 are the charging amounts (in g/cm) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, per unit length. Finally, another preferred embodiment of the invention concerns a transuranium-elements transmuting fuel assembly, wherein Sr and/or alkali metals as radioactive fission product(s) is/are contained in the fuel pins. In a preferred embodiment, the fuel pins enclosed in the wrapper tube comprise transuranium fuel pins charged with the transuranium fuel material and fuel pins charged with a fuel material consisting of an enriched uranium and a uranium-plutonium mixture fuel, and also comprising a radioactive fission product such as Sr or alkaline metals in the transuranium fuel material. In the nuclear fast neutrons reactor comprising a transuranium element transmuting reactor core according to the present invention, since an amount of transuranium elements to be added to the fuel pins of the fuel assemblies is controlled so as to keep an excess reactivity of the reactor substantially zero during the operation of the reactor, a decrease of effective multiplication factor with the lapse of time for operation will be prevented, and an excessive deterioration or turbulence of the reactor power distribution can be prevented. By looking for improvement of a power plant capacity factor from enhancing the reliability of the plant, transuranium elements (TRU elements) can be transmuted efficiently. Further, by setting the loading amounts of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am to a value so as to satisfy the relationship: |1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 | < | Q 1 | where in Q₁ is the upper bound of the heating rate of a single fuel assembly (in W per assembly) outside the reactor from the view point of the fuel assembly integrity, and M₂₄₂, M₂₄₄ and M₂₄₁ are the loading amounts of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am (in g per assembly) which can be loaded into the single fuel assembly, and also to satisfy the relationship |1.2 x 10 2 x M L 242 + 2.8 x M L 244 + 1.1 x 10 -1 x M L 241 | < |Q 2 | wherein in Q₂ is the upper bound of the heating rate per unit length of the fuel pellet contained in the fuel assemblies (in W/cm) from the view point of the fuel element integrity, and M L / 242, M L / 244 and M L / 241 are the charging amounts of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am and per the unit length (in g/cm), 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. In addition, an accident of a control rod and the neutron absorbing material of the control rod can be reduced by a neutron absorption effect of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am. Furthermore, an enhancement of heat removing efficiency of the core can thus be realized, the 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 the charging density of minor actinides to lessen outwards of the core center in a core area where the plutonium content is remainning constant, and also by increasing the charging density of minor actinides in each area where Pu is highly enriched at the core of a Pu-enriched area where the plutonium content varies, a flatting requirement of the radial distribution of the reactor power can be satisfied. Furthermore 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 of the transuranium element transmuting fuel assembly having the features 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 elements. For a better understanding of the present invention and to show how the same is carried out, reference will be made, by way of preferred embodiments, to the accompanying drawings, in which: Fig. 1 is a longitudinal sectional view exemplifying a nuclar fast neutrons reactor comprising a transuranium element transmuting reactor core relating to the present invention; Fig. 2 is a plan view showing a nuclear fast neutrons reactor of the present invention to which the transmuting reactor core is applied; Fig. 3 is a view showing a fuel assembly charged into the fast reactor core; Fig. 4 is an elevational section showing a fuel pin (fuel element) enclosed within the fuel assembly; Fig. 5 and Fig. 6 are views exemplifying an arrangement of the fuel pins enclosed in the fuel assembly; Fig. 7 is a graph showing the relation between the MA content in the fuel pellet and the fuel pellet melting point; Fig. 8 is a graph showing the relation between the TRU transmuting rate and the MA content when the reactor power is adjusted to the MA content for prevention of a melting of the fuel; Figs. 9A to 9D are views showing a nuclear transformation pass of typical TRU elements; Fig. 10 is a graph showing in which manner the effective multiplication factor is varying during the reactor operation at different mean MA contents of the core; Fig. 11A is showing the decay constant of main MA elements and the emitted energy per decay; Fig. 11B is showing heating rates resulting from a decay of these MA elements when each MA element is added to the fuel pellet; Fig. 12 is showing the neutron capture cross section of the main elements in a fast neutrons reactor core; Fig. 13 is showing the main specification characteristics of a first embodiment of the TRU elements transmuting reactor core relating to the present invention; Fig. 14 is a graph showing the radial distribution of the MA charging density to a core in the first embodiment; Fig. 15 is a graph showing the radial distribution of the power density in the first embodiment with reference to the beginning and the end of a reactor operation cycle; Fig. 16 is a graph showing the radial distribution of the MA charging density to a core in a second embodiment; Fig. 17 is a graph showing the radial distribution of the power density in the second embodiment as compared with a prior art; Fig. 18 is a graph showing the relation between the neutron energy and the neutron capture cross section; Fig. 19 is a graph showing the relation between the excess reactivity of a reactor and the number of days having passed for the reactor operation in the prior art; Fig. 20 is a view showing the radial distribution of the power density when the Pu enrichment is one kind of operation in the prior art; Fig. 21 is a graph comparing the beginning of the operation cycle with the end of the operation cycle for radial distribution of the power density where the Pu enrichment is one of two kinds of operation in the prior art. Fig. 22 is an elevational sectional view representing schematically one embodiment of a transuranium elements transmuting fuel assembly relating to the present invention; Fig. 23 is a sectional plan view taken along the line XXIII - XXIII of Fig. 22; Fig. 24 is an elevational sectional view showing a TRU fuel pin which is a transuranium element transmuting fuel pin with which the fuel assembly is charged; Fig. 25A and 25B are comparative figures showing a TRU fuel disposition characteristic and an axial power distribution of the TRU fuel pin corresponding with each other; Fig. 26 is a graph showing the power change with the number of days of operation of the TRU fuel assembly consisting only of the TRU fuel pins; Fig. 27 is an elevational sectional view showing a first alternative of the TRU fuel pin; Fig. 28 is an elevational sectional view showing a second alternative of the TRU fuel pin; Fig. 29 is a sectional plan view corresponding to Fig. 23 which shows a first alternative of the TRU fuel assembly or a transuranium element transmuting fuel assembly; and Fig. 30 is a sectional plan view corresponding to Fig. 24 which shows a second alternative of the TRU fuel assembly. One embodiment of a nuclear fast neutrons reactor comprising a transuranium element transmuting reactor core relating to the present invention will now be described with reference to the accompanying drawings. Fig. 1 represents a basic concept of a nuclear reactor such as pool type nuclear fast neutrons reactor enclosing the transuranium elements transmuting reactor core of the present invention. The present invention is applicable to a core of the nuclear reactor of a type to remove heating from cooling the core by flowing a coolant thereto. The coolant then includes liquid Na, liquid NaK, He gas and the like. Cooling is intended for the core for which a fission is caused chiefly by a fast neutron. Fig. 1 represents an example of a pool type nuclear fast neutrons reactor. However, it is needless to say that also a loop type fast reactor may be employed in this case. The nuclear reactor includes a reactor vessel 10 which is charged with liquid sodium (Na) 11 as a coolant. A core 12 is provided at a central portion in the reactor vessel 10, and an upper core structure 13 is provided above the core 12. The upper core structure 13 is supported by a roof slab 14 working as a shielding plug to cover the top of the reactor vessel 10. A primary coolant pump 15 and an intermediate heat exchanger 16 and others are suspended on the roof slab 14. A cover gas such as inert gas or the like is enclosed between the roof slab 14 and a free liquid surface of the liquid Na 11. The core 12 enclosed in the reactor comprises, as shown in Fig. 2 for example, two region cores 12a, 12b. A fuel assembly 17 relatively high in plutonium content is disposed in the outer peripheral area 12a and a fuel assembly 18 relatively low in plutonium content is disposed in the central area 12b. For charging amount of the MA elements of the TRU elements to be transmuted which is charged into the fuel assemblies 17, 18, the charging density of the MA elements is controlled at each area of the fuel assemblies 17, 18. The reference numeral 19 denotes a control rod. As exemplified in Fig. 3, the fuel assemblies 17, 18 have a plurality of fuel pins 21 enclosed in a bundle within a hexagonal wrapper tube 20 as a fuel element in a wide sense. A coolant inlet 22 is formed on the lower portion of the wrapper tube 20, and a coolant outlet 23 is formed on its upper portion. With opposite ends sealed up with an upper end plug 27 and a lower end plug 28, the fuel pin 21 enclosed within the wrapper tube 20 is that for which a plurality of fuel pellets 26 are inserted in a row, as shown in Fig. 4, within a fuel clad 25 as a fuel element. The fuel pellets 26 are retained elastically by a spring 29 within the fuel clad 25, a fuel stack portion 30 being constructed as an effective fuel length. The fuel pellet 26 used as fuel element is a sintered ceramics, material and, hence, is an oxide-type fuel material. A so-called metallic fuel is also available for the fuel element instead of the oxide fuel matter, and the fuel element is not necessarily a thoroughly sealed type covered by the fuel clad 25. For example, a so-called vented fuel element (fuel pin) which discharges gaseous fission products (hereinafter called FP gas) such as He gas or the like from the fuel element by fission is also acceptable. On the other hand, the fuel assemblies 17, 18 are charged with the TRU elements (transuranium elements) to be transmuted, and a concrete case for controlling the charging amount of a main MA elements of the TRU elements to be charged is exemplified as follows: (1) The amount of the elements is controlled so that the MA elements to be transmuted which are added to the fuel pellets 26 of the fuel assembly will be distributed. (2) There are prepared fuel pellets with the MA elements to be transmuted relatively high in density and fuel pellets with the elements relatively low in density or almost not added, and the charging number of each fuel pellet to a fuel pin is controlled, thereby controlling the charging amount. An array order of the fuel pellets is arbitrary. (3) There are prepared fuel pins 21a with the MA elements to be transmuted relatively high in charging amount and fuel pins 21b with the MA elements relatively low in charging amount or almost not charged, and the number of the fuel pins 21a, 21b enclosed within the wrapper tube 20 is controlled as shown in Fig. 5 and Fig. 6. Thereby control of the charging amount of the MA elements per the fuel assemblies 17 and 18 is possible. If the percentage by weight of minor actinides (element with the atomic number 92 or over excluding uranium and plutonium; hereinafter called MA element) to total actinides with the atomic number 92 or over is called minor actinides content (hereinafter called MA content), the relation in the melting point between the MA content and the fuel pellet 26 is as indicated in Fig. 7. Accordingly, the melting point of the fuel pellets 26 drops as the MA content increases. Thus, the melting point of the fuel pellets to which the MA elements are added for transmuting the MA elements drops as compared with the fuel pellets to which the MA elements are not added, thereby facilitating the melting, of the fuel material. It is then necessary that the reactor power be lowered in a nuclear reactor so as to prevent the fuel elements from being melted during operation of the nuclear reactor regardless of the melting point drop, and the neutron flux level of the nuclear reactor lowers due to a decrease of the reactor power. The transmuting rates of the TRU elements are proportional to the MA content and the neutron flux level. When taking an adjustment of the reactor power further into consideration for prevention of a melting of the fuel element, the transmuting rates of the TRU elements are not always to rise monotonously along with an increase in the MA content. As shown in Fig. 8, there exists a peak portion at which the TRU elements transmuting rates are maximized correlatively to the MA content. Then, an MA content P MA at which the TRU elements transmuting rates are maximized is effective and hence may result in realizing a transmuting of the TRU element most efficiently without causing a melting on the fuel element. On the other hand, in the case of ordinary nuclear fast neutrons reactor core, fissionable elements contained in the fuel are lost in accordance with burnup and thus fission products are accumulated, and therefore, an effective multiplication factor indicating the degree of a criticality of the nuclear reactor (fast neutrons reactor) decreases during the lapse of time for the reactor operation. However, in the case of fast neutrons reactor core operation for transmuting the TRU elements, main MA elements ²³⁷Np, ²⁴¹Am, ²⁴³Am, ²⁴²Cm of the TRU elements to be transmuted are transformed, as shown in Figs. 9A to 9D, into a fissionable elements easy to cause a fission by fast neutrons. Consequently, the effective multiplication factor comes to decrease moderately during the lapse of time of reactor operation. As shown in Fig. 10, when the MA content becomes excessive, the TRU elements are transformed into the fissionable elements too much, and the effective multiplication factor may increase during the lapse of time of reactor operation. Accordingly, the change of the effective multiplication factor during the lapse of time of reactor operation may be the lapse of time of reactor operation may be controlled by the MA content. Therefore, such reactor core will suppress a change in the effective multiplication factor during reactor operation and keep the excess reactivity of the reactor substantially zero during the reactor operation which result may be obtained by controlling the MA content. Further, the heating rates resulting from the decay of the MA elements of the TRU element can be calculated from a decay constant of each MA element and the energy emitted per decay. The decay constant of the MA elements to be transmuted and the energy emitted per decay are shown in Fig. 11A. The heating rates per gram of each MA element may be calculated by means of data given in Fig. 11A as shown in Fig. 11B. The MA elements of the TRU elements which contribute influentially to heating are ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, and hence, it is understood that the heating rate of the TRU elements will be calculated by taking these MA elements into consideration. On the other hand, if the upper bound of the heating rates per single fuel assembly capable of removing heat is Q₁ (W), then amounts of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am which may be charged into the single fuel assembly must satisfy the following equation: |1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 | < |Q 1 | wherein M₂₄₂ is the amount (g) of ²⁴²Cm charged into a single fuel assembly; M₂₄₄ is the amount (g) of ²⁴⁴Cm charged into a single fuel assembly; and M₂₄₁ is the amount (g) of ²⁴¹Am charged into a single fuel assembly. Then, assumed that the heating rate per one single fuel assembly is within the range capable of removing heat, if the heating is one-sided at a position, the fuel element is capable of being damaged , and therefore, there is a limit to the charging of the MA elements to be transmuted from the viewpoint of preventing a fuel rupture. That is, if the upper bound of the heating rate per unit length (cm) in an axial direction of a local one fuel pellet is Q₂ (W), the charging amount of the MA elements to be transmuted at the position must satisfy the following equation: |1.2 x 10 2 x M L 242 + 2.8 x M L 244 + 1.1 x 10 -1 x M L 241 | < |Q 2 | wherein M L / 242 is the amount (g) of ²⁴²Cm charged per unit length (cm) in axial direction of local one fuel pellet; M L / 244 is the amount (g) of ²⁴⁴cm charged per unit length (cm) in axial direction of local one fuel pellet M L / 241 is the amount (g) of ²⁴¹Am charged per unit length (cm) in axial direction of local one fuel pellet Accordingly, to prevent a failure of the fuel assembly due to overheating, it is necessary that the charging amount of the MA elements of the TRU elements to be transmuted is controlled so as to satisfy the above Eqs. (1) and (2) at the same time. For example, an upper bound of the heating rate per a single fuel assembly, where the length of the fuel stack general as the fuel assembly is 100 cm, and the number of the fuel pins charged into the single fuel assembly is 271 pieces is about 5 kw/assembly. In this case, an upper bound of addable amount of main MA elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am which are contributive to heating to the fuel pellets is: ²⁴²Cm: 0.05 percent by weight ²⁴⁴Cm: 2 percent by weight ²⁴¹Am: 50 percent by weight Accordingly, in order that the fuel assembly with the TRU elements added thereto may not cause overheating and failure of the fuel element for transmuting of the TRU element at the time of assembling, storage and transportation, if percentages by weight of the MA elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am to be transmuted to a total weight of the heavy metal elements of a fresh fuel pellet are f₂₄₂, f₂₄₄ and f₂₄₁, then it is necessary that the equation f 242 0.05 + f 244 2 + f 241 50 < 1 be realized. So far as an amount of the main MA elements of the TRU elements added to the fresh fuel pellet satisfies Eq. (3), the fresh fuel assembly will never be overheated or damaged. Further, as will be understood from Fig. 9, the MA elements of the TRU elements to be transmuted function generally as a neutron absorber, and its degree is stronger than uranium 238 (²³⁸U), as shown in Fig. 12, in the fast reactor core. On the other hand, if the ratio of weight of plutonium element, or the plutonium content, to the weight of total heavy metal elements in the fuel pellet is called the plutonium (Pu) enrichment, in the fast neutrons reactor core, since the neutron flux level normally decreases according as it goes outside from the core center, the power density lowers according as it goes outside in the same Pu enrichment area. Accordingly, in the same Pu enrichment area at the fast neutrons reactor core operating for transmuting the TRU elements, the charging density of the MA elements to be transmuted which functions as a neutron absorber will be lessened gradually outside from the reactor core, thereby satisfying a flatting requirement. Thus, fluctuation and change of the reactor power are suppressed, and reliability and safety of the nuclear reactor can be enhanced. Further, in the fast neutrons reactor core having the Pu enrichment in two kinds or more, the area higher in the Pu enrichment has a high content of fissionable elements, and the fissionable elements are transmuted quickly by burnup in accordance with the reactor operation. Thus in the area high in the Pu enrichment, the power density according to the operation lowers more. On the other hand, the MA elements to be transmuted are transformed, as shown also in Figs. 9A to 9D, into fissionable elements by neutron capture. Accordingly, from increasing the charging density of the MA elements to be transmuted in an area higher in Pu enrichment, a decrease of the power density during the operation of the nuclear reactor will be compensated for with the transformation of the MA elements to be transmuted into fissionable elements, thus moderating the decrease of the power density during the reactor operation. Fig. 13 shows the main specification characteristics of a fast neutrons reactor core to which the TRU elements transmuting reactor core according to the present invention is applied, and Fig. 14 shows a charging density radial distribution of the MA elements charged into the transmuting reactor core. The mean MA content of the fast reactor core in the example shown in Fig. 13 is, for example, 5 percent by weight. As shown in Fig. 8, the MA content is set so as to minimize an excess reactivity of the reactor to substantially zero, for example, in the range where a fuel fusion does not occur during operation of the reactor. When the MA content is 5 percent by weight, the amount of the fissionable elements produced by a neutron capture of the charged MA elements is moderate to be balanced just sufficiently with the amount transmuted by fission. Therefore, an excess reactivity of the reactor during operation of the reactor being checked to be about 0.5% ▵K/K at most. In the case of an ordinary fast neutrons reactor core equivalent to the present embodiment in a reactor thermal power and the operation cycle length, the maximum excess reactivity during the operation is about 3% Δ K/K. In the example, since the MA elements have a neutron absorbing effect as a control rod, a required reactivity worth of the control rod may be minimized, thus the number of control rods and the required amount of neutron absorber such as boron, hafnium or the like which is charged into the control rod being reduced, and the economic efficiency will be enhanced. If the excess reactivity of the reactor is low, a reactivity insertion into the reactor at the time when the control rod is drawn erroneously can be minimized, thereby ensuring more safety. Further, the excess reactivity of the reactor can be kept low during the reactor operation, and thus the reactivity loss of the reactor due to the reactor operation, namely the decrease of the effective multiplication factor according to burnup can be minimized. Thus, the period of continuous operation of the reactor or the operation cycle length can be prolonged, and the plant capacity factor or besides the transmuting efficiency of the TRU elements may be enhanced. Further, in the present embodiment, the charging density of the MA elements varies, as shown in Fig. 14, according to the position where the fuel assembly is charged into the reactor core. However, when a fresh fuel assembly outside the reactor is taken particularly into consideration, even in the case of the fresh fuel assembly whereby the heating rates are maximized, the amounts of MA elements are set within the range satisfied by the Eqs. (1) and (2) or (3) so as not to increase the heating rates excessively by a decay of the MA elements. Accordingly, the maximum heating rates of the fresh fuel assembly outside the reactor are about 5 kW/assembly, as shown in Fig. 13, and hence, a trouble such as failure due to the overheating of the fuel assembly or the like will not result at the time when assembling, storing and transporting the fresh fuel assembly. Still further, in the present embodiment, a radial distribution of the charging density of the MA elements at the reactor core is made less, as shown in Fig. 14, according as it goes outside of the reactor core in the same Pu-enriched area. Besides, since the MA elements function as a neutron absorber, the neutron flux level getting large toward the core central portion is suppressed normally by the MA elements, and as a result, the radial distribution of the neutron flux becomes flatter. Accordingly, as shown in Fig. 15, the radial distribution of the reactor power density is flattened during the period of the reactor operation cycle as compared with a prior art shown in Fig. 21. Thus, a local power peak will be kept from arising, and hence a thermal tolerance of the fuel assembly can be ensured thoroughly, thus enhancing the economical efficiency such as compacting and lightening the reactor core in structure. Further, in the present embodiment, in the area higher in Pu enrichment, the charging density of the MA elements is increased as compared with the area lower in Pu enrichment. As shown in Fig. 14, the area higher in Pu enrichment has fissionable elements resulting from the neutron capture of the MA elements more than that. On the other hand, the consumption due to the burnup of the fissionable plutonium is more with the area high in Pu enrichment where a high amount of fissionable plutonium is present. Accordingly, the distribution of the MA elements charging density shown in Fig. 14 is set at every area of Pu enrichment so that the area with much consumption of the fissionable plutonium has much fissionable elements produced by the neutron capture of the MA elements. Therefore, a net decrease of the fissionable elements according to the reactor operation is suppressed, and, as shown in Fig. 15, the fluctuation of the power density according to operation is suppressed. Thus, if the fluctuation of the power density is minimized during the operation of the fast neutrons reactor (nuclear reactor) or burnup, the heat removing efficiency is improved, the enhancement of economical efficiency is realized, and thus fuel temperature is decreased, and safety and reliability of the reactor core can be enhanced accordingly. Fig. 16 is a graph showing characteristics of a second embodiment of the TRU elements transmuting reactor core according to the present invention. Even in the case of a reactor core where the Pu enrichment is one kind of operation, the fast neutrons reactor core represented in this embodiment has a radial distribution of the MA element charging density flattened preferably as compared with the prior art as shown in Fig. 6 and also has a radial power distribution of the core flattened as shown in Fig. 17. Accordingly, by employing the present invention, the reactor power distribution is flattened despite Pu enrichment being one kind of operation and therefore, the plant efficiency will be enhanced, and the economical efficiency such as decrease in fuel fabrication cost or the like may also be enhanced. Needless to say, a characteristic similar to the first embodiment applies to those features other than the reactor power distribution. Further, as another embodiment, the axial distribution is applied to the MA elements charging density as in the case where the radial reactor power distribution is flattened according to the present invention as described above, thereby flattening the axial reactor power distribution. If the power distribution need not to be flattened, the MA content of the whole core will be set according to the present invention even in case a distribution is not applied particularly to the MA elements charging density, thereby realizing effects such as decrease in excess reactivity of the reactor through the operation, prevention of overheating and failure of the fresh fuel assembly outside the reactor and the like. As described above, in the transuranium element transmuting reactor core relating to the present invention, since an amount of transuranic elements added to the fuel element of fuel assemblies is controlled so as to maintain an excess reactivity of the reactor substantially zero during the operation of the reactor, the decrease of the effective multiplication factor according to the lapse of time for the operation is prevented, the excessive deterioration and turbulence of the reactor power distribution can be prevented, the reliability of the power plant is thus enhanced, and as seeking an improvement of the plant operating efficiency, transuranium elements (TRU elements) can be transmuted efficiently. Then, by setting the charging amounts of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am into the fuel assembly so as to satisfy the relationship |1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 | < |Q 1 | where Q₁ is the upper bound of heating rate (in W/assembly) of the single fuel assembly outside the reactor, and M₂₄₂, M₂₄₄ and M₂₄₁ are the charging amounts (in g/assembly) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am which can be charged into the single fuel assembly, and the amount of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am charged is set to also satisfy the relationship : |1.2 x 10 2 x M L 242 + 2.8 x M L 244 + 1.1 x 10 -1 x M L 241 | < |Q 2 | where Q₂ is the upper bound of heating rate of per unit length of the fuel pellet contained in the fuel pins (in W/cm), and M L / 242, M L / 244 and M L / 241 are the charging amounts (in g/cm) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am per the unit length, the melting of the fuel pellets during the operation of the reactor and an overheating or failure of the fuel assemblies outside the reactor can be prevented. Moreover, the neutron absorption effect of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am is available for eliminating accidents of the control rod and the neutron absorbing material of the control rod and also for realizing the improvement of the heat removing efficiency of the core, whereby the economical efficiency will be enhanced, the safety and the reliability of the core and the fuel assemblies will be enhanced as well, and the TRU elements can be transmuted efficiently. Further, by setting a charging density of minor actinides to lessen outwards of the core center in a core area where a plutonium content is remaining constant and also by increasing the charging density of minor actinides in an area where Pu is highly enriched at the core of the Pu-enriched area, the radial distribution of the reactor power can be flattened, the excessive deterioration or turbulence of the reactor power distribution will never result, the enhancement of safety and reliability of the core and the fuel assemblies may be realized, and thus the TRU elements can be quenched efficiently. Further embodiments of the present invention will be described hereunder in conjunction with Figs. 22 to 30, with reference to transuranium elements transmuting fuel pins and fuel assemblies. Fig. 22 shows an example of a transuranium elements transmuting fuel assembly according to another embodiment of the present invention which is charged into the fast neutrons reactor core. The fuel assembly 110 has a coolant inlet 111 at the lower portion thereof for letting in the coolant such as liquid sodium (Na), liquid NaK, helium (He) gas or the like and a coolant outlet 112 at the upper portion thereof. A plurality of fuel pins 114 are enclosed in the bundle within a wrapper tube 113 square) tubular or, for example, hexagonal in section which works as a fuel channel. The fuel pin 114 enclosed within the wrapper tube 113 may be constructed only of a TRU (transuranium elements) fuel pin 115, or of the TRU fuel pin 115 and an ordinary fuel material pin 116 otherwise as shown in Fig. 23. In the case of TRU fuel assembly having constructed the fuel assembly 110 only of the TRU fuel pins 115, nothing will be taken into consideration for arrangement of the TRU fuel pins 115. The administration on manufacture and assembling of the TRU fuel pins 115 is facilitated, the number of fuel assemblies for which a special measure such as heat removing, shielding or the like is required may be minimized, thus enhancing the economical efficiency on a core administration and the fuel handling. Then, in case the TRU fuel assembly 110 is constructed of the TRU fuel pin 115 and the ordinary fuel material pin 116, the change in reactor power according to the reactor operation is minimized by the presence of the TRU elements, and the TRU elements can be transmuted efficiently without lowering a cooling efficiency of the core as, in addition, keeping the integrity of the reactor internal structure. Fig. 23 exemplifies the case where the TRU fuel pins 115 are dispersed and disposed almost uniformly within the wrapper tube 113 of the TRU fuel assembly 110. By dispersing and disposing the TRU fuel pins 115 uniformly, a relatively low-temperature coolant around the TRU fuel pins 115 and a relatively high-temperature coolant around the usual fuel material pins 116 are mixed acceleratedly in the presence of the TRU elements, the fuel clad temperature of the ordinary fuel material pin 116 is decreased, the cooling efficiency of the core can be enhanced, the ordinary fuel material pin 116 can be made to last so long, and the economical efficiency and safety of the nuclear fast neutrons reactor can thus be enhanced. As shown in Fig. 24, the TRU fuel pin 115 has a fuel clad 118 charged with a TRU fuel material 119, and its upper and lower portions closed by an upper end plug 120 and a lower end plug 121. The TRU fuel material 119 has at least one of an ordinary fuel material and a fertile material consisting of degraded uranium, natural uranium and depleted uranium contain TRU elements (trans- uranium elements) of minor actinides elements such as Np (neptinium), Am (americium), Cm (curium) and the like. Here, the ordinary fuel material refers to a fuel material consisting of an enriched uranium and an uranium-plutonium mixed fuel (uranium fuel having enriched plutonium). An oxide fuel such as uranium oxide or the like is used for the fuel material, however, a metallic fuel may be employed instead of the oxide fuel. Further, the ordinary fuel material pin 116 has the fuel clad charged with a plurality of so-called fuel pellets (or metallic fuel otherwise) having sintered the ordinary oxide fuel material, and the upper and lower ends closed by the upper end plug and the lower end plug, a gas plenum part being formed on at least one portion, or upper and lower portions, for example, in the fuel clad. The TRU fuel pin 115 and the ordinary fuel material pin 116 have been described with reference particularly to a closed type one closed by the upper and lower end plugs. However, does not necessarily need to always be a full-closed type, and hence a so-called vented type fuel pin which is capable of discharging gaseous fission products (hereinafter called FP gas) generated by the fission outside the fuel pins 115 and 116 may be employed. Fig. 24 exemplifies the TRU fuel pin 115 working as a transuranium elements transmuting fuel pin. The fuel pin 115 is of the type having a multiplicity of TRU fuel areas provided axially. A TRU fuel 119a high in content of Np of the minor actinides elements is disposed in a core upper portion area of the fuel clad 18. A TRU fuel 119c high in content of Am and Cm is disposed in a core lower portion area, and a TRU fuel 119b is also disposed in a core height center area high in a neutron flux level. An ordinary fuel 122 is disposed among the TRU fuels 119a, 119b, 119c to constitute the TRU fuel material 119. A gas plenum part 123 is formed on an upper portion of the fuel clad 118, the upper and lower ends being closed by the upper end plug 120 and the lower end plug 121. In the gas plenum part 123, a spring is installed, as occasion demands, so as to stably hold the TRU fuel material 119. By disposing the TRU fuels 119a, 119b, 119c with which the fuel clad 118 of the TRU fuel pin 115 is charged inside as indicated in Fig. 25A, a core axial power distribution curve A of the nuclear fast neutrons reactor is as indicated by a full line and a peak power is decreased more than a core axial output distribution curve B indicated by a broken line in the case of the ordinary fuel material pin 116, thus the reactor characteristic and the transmuting efficiency of the TRU elements being improved. Typical minor actinides elements ²³⁷Np, ²⁴¹Am, ²⁴³Am, ²⁴²Cm, ²⁴⁴Cm of the TRU fuels 119a to 119c with which the TRU fuel pin 115 is charged inside to capture neutrons to produce fissionable elements as shown in Figs.9A to 9D when the fuel pin is charged into the fast reactor core for operating the reactor thus transmuting the TRU elements. Further, due to the TRU fuel pin 116 having the fuel clad 118 charged with the TRU fuel pellets 119 inside, there arises a problem that the melting of the fuel drops due to the presence of the minor actinides elements. The fuel melting will not result even at the transient conditions of the nuclear fast neutrons reactor according to the normal core design. In the TRU fuel pin 115 shown in Fig. 24, the TRU fuel 119a high in content of low melting point MA elements is disposed in the core area with high fuel temperature, namely in the core upper portion area in the case of the metallic fuel, and in the core height center area in the case of the oxide fuel. The TRU fuel 19c high in content of high melting point MA elements is disposed in the core lower portion area where the fuel temperature is low. By disposing the TRU fuel 119c containing Am and Cm with a comparatively large drop of the melting point for the oxide fuel in the area where the fuel temperature is low, the drop of the fuel melting point due to the content of the TRU element such as Am and Cm or the like does not exert an influence directly on determination of the core power density, and the TRU elements can be transmuted efficiently without lowering the core power density. On the contrary, the melting point of Np is lower than that of Am and Cm for the metallic fuel. Typical minor actinides elements ²³⁷Np, ²⁴¹Am, ²⁴³Am, ²⁴²Cm, ²⁴⁴Cm of the TRU fuel pellets 119 with which the fuel clad 18 of the TRU fuel pin 115 are charged inside are transformed into fissionable elements, as shown in Figs. 9A to 9D, by a neutron absorption. Therefore, the power of the TRU fuel assembly constructed only of the TRU fuel pin 115 sharply increases as indicated by a symbol I in Fig. 27 according to the number of days for operation. Accordingly, the power of the TRU fuel assembly constructed only of the TRU fuel pin 115 is low at the time of start for operation but increases largely during the operation. Then, as shown in Fig. 24, in the TRU fuel assembly 110 for which the TRU fuel pin 115 and the ordinary fuel material pin 116 are disposed mixedly, since the ordinary fuel material pin 116 generates power, the power of the TRU fuel assembly 110 is also relatively high at the time of start for the reactor operation as indicated by a symbol II in Fig. 26. Since the decrease in the fissionable material of the ordinary fuel material pin 116 and the transformation of the TRU fuel element 119 into a fissionable material are offset, the degree of power fluctuation of the TRU fuel assembly 110 due to the operation is smaller and smoother than the fuel assembly constructed only of the TRU fuel pin 115. In a core design, a coolant flow rate of the fuel assembly charged into the core is specified so as to remove heat at the time of maximum power of the fuel assembly. The TRU fuel assembly constructed only of the TRU fuel pin 115 does not generate the power corresponding to the coolant flow rate at the time of start of the operation or for a several time after the start, an outlet temperature is consequently low, and therefore, the coolant outlet temperature does not rise, thus leaving a problem on the cooling efficiency of the core and the integrity of the reactor internal structure. However, in the case of the TRU fuel assembly for which the TRU fuel pin 115 and the ordinary fuel material pin 116 are disposed mixedly as shown in Fig. 23, the power coming up substantially to the maximum power is generated at the time of the start of the operation or for a certain time after the start, and therefore, the outlet temperature of the TRU fuel assembly 110 will not drop extremely. Thus the problem of the cooling efficiency of the core and the integrity of the reactor internal structure can be avoided. As shown in Fig. 24, by mixing a radioactive fission product (F. P) such as strontium (Sr), alkaline metals (Cs or the like), technetium (Tc) or the like into the TRU fuel pellets 119 with which the fuel clad 118 of the TRU fuel pin 115 is charged inside, the transmuting of the TRU elements and also the transmuting of a long-lived radioactive fission products in the reactor and the internal administration of the reactor are realizable. The disposal and administration of the radioactive waste products will be facilitated as compared with the case where these are carried out outside the reactor. For example ⁹⁹Tc is transformed in the fast neutrons reactor by neutron capture (neutron absorption) into a stable element which is not radioactive, and ⁹⁰Sr and ¹³⁷Cs become: ⁹⁰Sr → ⁹⁰Y → ⁹⁰Zr (stable) ¹³⁷ Cs → 137m Ba → ¹³⁷Ba (stable) by natural decay while residing in the reactor, and ⁹⁰Zr and ¹³⁷Ba are stable materials, which are removed from a fuel by a process of spent fuel reprocessing. In the TRU fuel pin 115 shown in Fig. 24, the case where TRU fuels 119a, 119b, 119c are dispersed and arranged axially is exemplified. However, a TRU fuel pin 115A may be constructed as shown in Fig. 27 so as to minimize the influence to be exerted on the reactor power distribution by the TRU elements. Like reference characters are applied to the like portions in Fig. 24, and a further description will be omitted here. The TRU fuel pin 115A exemplifies the case where the TRU fuels 119b containing the TRU element which constitute the TRU fuel pellets 119 are disposed even axially at a relatively low content (several percent by weight or below). The TRU fuel pin 115 for which the TRU fuels are disposed uniformly has the advantage that the administration on fabrication and transportation is facilitated, the fabrication costs can be reduced, and the influence to be exerted on the axial power distribution is uniform and minimized. Further, the TRU fuel pin 115A has a high TRU elements loading amount per pin, and thus is available for enhancing the transmuting efficiency of the TRU elements. A TRU fuel pin 115B shown in Fig. 28 exemplifies the case where the TRU fuel pellets 119 with which the fuel clad 118 is charged inside has the TRU fuel 119b disposed only in the core height center area, and such disposition is effective in decreasing a peak power and flattening the core axial power distribution. By disposing the TRU fuel 119b in the core height center area where a neutron flux density is high, the power peak is reduced preferably by the distortion of the core axial power distribution, the power distribution is flattened, and the reactor characteristics are improved to enhance the transmuting efficiency of the TRU elements. Then, in this case, the content (percent by weight) of the TRU elements contained in the TRU fuel 119b is relatively high to stand, for example, at 10% or above as in the case of the TRU fuel pin 115 of Fig. 24. Further, in the first embodiment of the transuranium elements transmuting fuel assembly 110, the TRU fuel pins 115 are dispersed and disposed uniformly in the TRU fuel assembly 110 as shown in Fig. 23. However, the fuel pins 115 (115A, 115B) will be disposed intensively in the center area of the TRU fuel assembly 110, as shown in Fig. 29, and the ordinary fuel material pins 116 will be disposed around the TRU fuel pins 115 otherwise. In case the TRU fuel pins 115 are disposed intensively in the center area, since the TRU fuel assembly 110A has the neutron flux density maximized at the center area, the transmuting efficiency of the TRU elements can be enhanced. In the TRU fuel assembly 110A, the fuel pins disposed around the TRU fuel pins 15 (15A, 15B) may comprise a fertile material pin consisting of natural uranium and depleted uranium. By disposing such fertile material pins, alpha-rays and neutrons emitted from the TRU fuel pins 115 are shielded, and thus measures on transportation and shielding of the TRU fuel assembly 110A may be relieved. A TRU fuel assembly 110B shown in Fig. 30 exemplifies the case where the TRU fuel pins 115 (115A, 115B) are disposed at an outer peripheral position abutting on a inside wall of the wrapper tube 113. The ordinary fuel material pins 116 are disposed inside of the TRU fuel pins 116 arrayed as above. Generally, the coolant flow rate per one fuel pin is larger on the wall side of the wrapper tube 113 than in the center area, and temperature of the fuel clad 118 on the wall side becomes lower than the center area. When the metallic fuel is employed as the fuel of the TRU fuel assembly 110B, it is necessary that the fuel clad temperature be adjusted as low as possible for the prevention of an eutectic reaction of the fuel clad 118, with the metallic fuel slug. The TRU fuel assembly 10B shown in Fig. 30 is that for which the TRU fuel pins 115 are disposed along the inside wall of the wrapper tube 113 with a coolant flowing much therethrough, the fuel clad temperature is reduced thereby, the economical efficiency of the TRU fuel assembly 110B is thus secured to prolong the lifetime. The TRU fuel pins 115 (115A, 115B) have typical minor actinides elements easy to cause alpha-decay contained much in the TRU fuel material 119. Thus a relatively high alpha-ray energy is emitted at the time of alpha-decay, and therefore, special measures on fabrication, heat removing and shielding which are different from the ordinary fuel will be necessary for the TRU fuel pins 115 (115A, 115B) and the TRU fuel assemblies 110 (110A, 110B). Thus, for the TRU fuel pin 115, the TRU fuel pellets 119 are constructed to contain the TRU elements or specific minor actinides elements at a predetermined content. For fuel temperature and others to keep in order, it is preferable that the TRU fuel pin 115 be provided with a TRU fuel material area in the area lower than the core height center more than the area upper than the core height center. Further, from inserting a control rod into the core, the transmuting efficiency of the TRU elements may be enhanced by providing the TRU fuel area in the core height center area where a neutron flux level gets high, thereby flattening the core axial power distribution. Further, by disposing the TRU fuel in the area lower than the core height center level whereat the neutron flux level becomes high, the melting of the TRU fuel can be avoided effectively and securely. As a described above, in the transuranium element transmuting fuel pin and fuel assembly relating to the present invention, an improvement of core characteristics and an enhancement of the core cooling efficiency may be achieved without causing deterioration of the core characteristics according to various restrictions such as fuel melting during the operation of the reactor, excessive change of the core axial power distribution, distortion of the distribution and the like. Thus the safety and reliability of the fuel assemblies and fuel pins to be charged into the core may thus be achieved, and the transmuting efficiency of the TRU elements can be improved and enhanced.

Claims: A nuclear fast neutrons reactor comprising a transuranium elements-transmuting core which is charged with a plurality of fuel assemblies at least some of which contain an amount of one or more transuranium element(s) to be added to an ordinary fuel material, wherein the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am charged is set to satisfy the equation |1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 | < |Q 1 | wherein Q₁ is the upper bound of heating rate (in W/assembly) of a single fuel assembly outside the reactor and M₂₄₂, M₂₄₄ and M₂₄₁ are the charging amounts (in g/assembly) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, and the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am charged is set to also satisfy the equation |1.2 x 10 2 x M L 242 + 2.8 x M L 244 + 1.1 x 10 -1 x M L 241 | < |Q 2 | wherein Q₂ is the upper bound of heating rate per unit length of a fuel pellet contained in a fuel assembly (in W/cm), and M L / 242, M L / 244 and M L / 241 are the charging amounts (in g/cm) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, per unit length. The nuclear fast neutrons reactor according to claim 1, wherein the charging density of minor actinides decreases from a core central portion outwards of the core in a core area where a plutonium content is fixed. The nuclear fast neutrons reactor according to claim 1 or claim 2, wherein, at a core having two or more plutonium-enriched areas, the charging density of minor actinides is increased in each of the plutonium-enriched areas. A transuranium elements-transmuting fuel pin for a nuclear fuel assembly of a fast neutrons reactor, comprising a fuel clad being charged with a transuranium fuel material, said transuranium fuel material including at least one of fuel materials consisting of an enriched uranium and an uranium-plutonium mixed fuel and a fertile material consisting of a natural uranium and a depleted uranium containing transuranium elements such as Np, Am and Cm, wherein the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am contained in said pin charged in said fast neutrons reactor is set to satisfy the equation |1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 | < |Q 1 | wherein Q₁ is the upper bound of heating rate (in W/assembly) of a single fuel assembly outside the reactor and M₂₄₂, M₂₄₄ and M₂₄₁ are the charging amounts (in g/assembly) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, and the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am contained in said fuel pin charged in said fast neutrons reactor is set to also satisfy the equation |1.2 x 10 2 x M L 242 + 2.8 x M L 244 + 1.1 x 10 -1 x M L 241 | < |Q 2 | wherein Q₂ is the upper bound of heating rate per unit length of a fuel pellet contained in a fuel assembly (in W/cm), and M L / 242, M L / 244 and M L / 241 are the charging amounts (in g/cm) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, per unit length. A transuranium elements-transmuting fuel assembly for a nuclear fast neutrons reactor including a wrapper tube and a plurality of fuel pins enclosed in the wrapper tube, wherein the fuel pins enclosed in the wrapper tube comprise transuranium fuel pins charged with the transuranium fuel material and an uranium-plutonium mixed fuel material and fuel pins charged with an enriched uranium material and/or an uranium-plutonium mixed fuel material, the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am contained in said fuel assembly charged in said fast neutrons reactor being set to satisfy the equation |1.2 x 10 2 x M 242 + 2.8 x M 244 + 1.1 x 10 -1 x M 241 | < |Q 1 | wherein Q₁ is the upper bound of heating rate (in W/assembly) of a single fuel assembly outside the reactor and M₂₄₂, M₂₄₄ and M₂₄₁ are the charging amounts (in g/assembly) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, and the amount of the transuranium elements ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am contained in said fuel assembly charged in said fast neutrons reactor being set to also satisfy the equation |1.2 x 10 2 x M L 242 + 2.8 x M L 244 + 1.1 x 10 -1 x M L 241 | < |Q 2 | wherein Q₂ is the upper bound of heating rate per unit length of a fuel pellet contained in a fuel assembly (in W/cm), and M L / 242, M L / 244 and M L / 241 are the charging amounts (in g/cm) of ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am, respectively, per unit length. The assembly according to claim 5, wherein Sr and/or alkali metals as radioactive fission product(s) is/are contained in the fuel pins.

Inventors: ARIE KAZUO, KAWASHIMA MASATOSHI, SATO YOSHIAKI, SUZUKI MASAO, ARIE, KAZUO, KAWASHIMA, MASATOSHI, SATO, YOSHIAKI, SUZUKI, MASAO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: G21C1
IPC codes: G21C1, G21C3, G21C5