Patent Number: 
Section: description

A detailed description will be presented below for a nuclear fuel assembly designed to be employable either for a UO2/light water reactor or for a MOX nuclear fuel/light water reactor in accordance with three independent embodiments of this invention. Referring to FIG. 4, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel or MOX nuclear fuel in accordance with the first embodiment of this invention, will be described below. A thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific reactor is operating. Thus, possibilities exist that the entire requirements of this invention may not be allowed. In the first embodiment, however, a serious attention is paid to realize the feature and advantages of this invention as much as possible independently from the current legal restriction. In other words, the MOX enrichment grade of the only one kind of MOX nuclear fuel rods is selected as high as 14 weight % and the ratio of the quantity of the MOX rods with respect to the total quantity of the nuclear fuel rods, is selected to be as low as 33%. Referring to FIG. 4 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the first embodiment of this invention, the assembly having MOX nuclear fuel rods and U O2 rods and having a burn-up capacity of 70 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in approximately 14 weight %. The quantity employed is 24. This is the only one kind of MOX nuclear fuel rods employed for this assembly. Symbols 2 and 3 show UO2 rods each of which kinds contains U235 in 4.9 weight % and 4.5 weight % respectively. The quantity employed is 24 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 3.5 weight % and gadolinium in 3.5 weight % respectively. The quantity employed is 20. Thus, the MOX nuclear fuel rods accounts for 33% of the total quantity of the nuclear fuel rods contained in the nuclear fuel assembly. Each rod is approximately 4 m in length and approximately 11 mm in the external diameter. The nuclear fuel assembly is a square of which the length of each side is approximately 15 cm. The production process of the foregoing nuclear fuel assembly in accordance with this invention is nearly identical to that which is presently available. Firstly, Purex process or some other dry or wet spent nuclear fuel reprocessing processes are employed to separate Pu-s out of a spent fuel. The quantity of fissionable Pu-s or Pu239 and Pu241 contained in the separated Pu-s which usually is determined by a mass analysis process is 60 through 70%. An oxidation process is conducted to produce Pu-oxides. Secondly, the powder of PuO2 and UO2 are mixed to make the resultant grade of enrichment to a desired value. Thirdly, MINAS process or SBR process is conducted to well commingle the powder of PuO2 and UO2. Molding and sintering process are conducted to convert the powder of PuO2 and UO2 to sintered pellets of PuO2 and UO2. The product pellets are charged in a zircaloy sheath and the both ends thereof are sealed to produce fuel rods. MOX fuel rods, UO2 rods and other parts are fabricated to finish nuclear fuel assemblies. As is illustrated in FIG. 4, only one kind of MOX fuel rods 1 of which the enrichment grade is as high as 14 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2 and 3 are UO2 rods and since the rods represented by symbol 1 alone are MOX fuel rods, the ratio of the quantity of MOX fuel rods with respect to the total quantity of the nuclear fuel rods is as high as 33%. As a result, the grade of enrichment of 14 weight % is remarkably higher than that of the prior art of 5 weight %. On the other hand, the ratio (33%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less in comparison with that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. Namely, it is to restrict fission to occur at the beginning of a reactor operation period. In other words, the gadolinium fuel rods are effective to reduce the possibility of fission to occur at the beginning of the reactor operation period but thereafter they lose the function and transit themselves to a fissionable fuel. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which has remarkably large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is remarkably small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. Referring to FIG. 5, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel alone or MOX nuclear fuel in accordance with the second embodiment of this invention, will be described below. As was described earlier, a thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific thermal reactor is operating. In the second embodiment as well, however, a serious attention is paid to realize the feature and advantages of this invention as much as possible within the limitation to observe the legal restriction presently effective generally in the world. In other words, the MOX enrichment grade is selected to be 6 weight % and the ratio of the quantity of the MOX rods with respect to the total quantity of the nuclear fuel rods, is selected to be 25%. Referring to FIG. 5 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the second embodiment of this invention, the assembly having MOX nuclear fuel rods and UO2 nuclear fuel rods and a burn-up capacity of 45 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which kinds contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in 6 weight %. The quantity employed is 16. This is the only one kind of MOX nuclear fuel rods employed for this assembly. Symbols 2, 3 and 4 show UO2 fuel rods each of which kinds contains U235 in 4.0 weight %, 3.5 weight % and 3 weight % respectively. The quantity employed is 28, 8 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 2 weight % and gadolinium in 2 weight % respectively. The quantity employed is 16. Thus, the MOX nuclear fuel rods accounts for 25% of the total quantity of the nuclear fuel rods employed in the nuclear fuel assembly. The production process of the foregoing nuclear fuel assembly in accordance with this embodiment is entirely identical to that for the first embodiment. As is illustrated in FIG. 5, only one kind of MOX fuel rods 1 of which kind the enrichment grade is 6 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2, 3 and 4 are UO2 fuel rods and since the rods represented by symbol 1 alone are MOX fuel rods, the ratio of the quantity of MOX fuel rods with respect to the total quantity of the nuclear fuel rods is 25%. As a result, the grade of enrichment of 6 weight % is higher than that of the prior art or 5%. On the other hand, the ratio (25%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less than that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. Namely, it is to restrict fission to occur at the beginning of the reactor operation period. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which only one kind has large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. Referring to FIG. 6, an exemplary arrangement of nuclear fuel rods in a nuclear fuel assembly designed to be employable for a thermal reactor which is allowed to employ either UO2 fuel alone or MOX nuclear fuel in accordance with the third embodiment of this invention, will be described below. As was described earlier, a thermal reactor currently in operation in any country in the world is required to observe the laws and regulations regarding the length of operation cycle thereof and the burn-up thereof, effective in the specific country in which the specific thermal reactor is presently operating. In the third embodiment, the best efforts are used to realize the feature and advantage of this invention as much as possible within the limitation of the design of the presently operating reactor. Referring to FIG. 6 illustrating a horizontal cross-section of a nuclear fuel assembly in accordance with the third embodiment of this invention, the assembly having MOX fuel rods and UO2 fuel rods and having a burn-up capacity of 45 GWd/ton (heavy metal mass ton), symbol 1 shows highly enriched MOX nuclear fuel rods each of which contains U235 in 0.225 weight % and the fissionable Pu-s or Pu239 and Pu241 in 6 weight %. The quantity employed is 24. This is the only one kind of the MOX nuclear fuel rods employed for this assembly. Symbols 2, 3 and 4 show UO2 fuel rods each of which kinds contains U235 in 4 weight %, 3.5 weight % and 3 weight % respectively. The quantity employed is 20, 8 and 4 respectively. Symbol G shows gadolinium rods each of which contains U235 in 2 weight % and gadolinium in 2.2 weight % respectively. The quantity employed is 16. Thus, the quantity of the MOX nuclear fuel rods accounts for 33% of the total quantity of the nuclear fuel rods employed in the assembly. The production process of the foregoing nuclear fuel assembly in accordance with this embodiment is entirely identical to that for the first and second embodiments. As is illustrated in FIG. 6, only one kind of MOX fuel rods 1 of which only one kind the enrichment grade is 6 weight % are arranged at the area at which the effects of the moderator are less. Since the rods represented by symbols 2, 3 and 4 are UO2 fuel rods and since the rods represented by symbol 1 alone are the MOX fuel rods, the ratio of the quantity of the MOX fuel rods with respect to the total quantity of nuclear fuel rods is 33%. As a result, the grade of enrichment of 6 weight % is higher than that of the prior art or 5%. On the other hand, the ratio (33%) of the quantity of the MOX fuel rods with respect to the total quantity of the nuclear fuel rods is remarkably less than that of the prior art or 80%. The function of the gadolinium fuel rods is identical to that of the prior art. As is identical to the first and second embodiments, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. In conclusion, the nuclear fuel assembly in accordance with this embodiment is provided with only one kind of MOX nuclear fuel rods each of which only one kind has relatively large magnitude of the enrichment grade of the fissionable Pu-s or Pu239 and Pu241, and the quantity of the MOX nuclear fuel rods is small. As was described earlier, the production cost of this nuclear fuel assembly is much less and the value of the spent fuel of this nuclear fuel assembly is considerably large. The above description has clarified that this invention has successfully provided an improvement applicable to a nuclear fuel assembly employable either for a thermal neutron reactor employing UO2 as the nuclear fuel and light water as the moderator/coolant or for a thermal neutron reactor employing the MOX nuclear fuel as the nuclear fuel and light water as the moderator/coolant, wherein the production cost is much less and the value of the spent fuel thereof is much larger than that of the nuclear fuel assembly available in the prior art.