Patent Number: 054992761
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is an illustration showing an example of a fast breeder reactor for carrying out a method according to the invention. The reactor core structure adopts a homogeneous 2-region core wherein outer reactor cores 12 have higher enrichment of plutonium (Pu) than inner reactor cores 10 for flattening power distribution. Axial blankets 14 are disposed on the top and bottom of the reactor core and a radial blanket 16 is disposed on the outer peripheral surface of the core. Minor actinide nuclides are extracted by using the above-mentioned known technique. To prevent rare earth elements adversely affecting reactor core characteristics from mixing with reactor core fuel, in the invention, neptunium separated and extracted by the Purex process is added to the reactor core fuel and americium, curium, and rare earth elements extracted by the Truex process are added to the axial blankets and/or radial blankets. FIG. 2A shows a structure wherein neptunium is added to all of the reactor core fuel, while americium, curium, and rare earth elements are added to the axial blankets 14. FIG. 2B is a structure wherein neptunium is added to all of the reactor core fuel, while americium, curium, and rare earth elements are added to the radial blanket 16. FIG. 2C is a structure wherein neptunium is added to all of the reactor core fuel, while americium, curium, and rare earth elements are added to the axial blankets 14 and the radial blanket 16. A burner reactor intended only for burning plutonium has no blankets; instead it comprises shields located axially and radially. In this case, neptunium may be added to the reactor core fuel and americium, curium, and rare earth elements may be added to either or both of the axial and radial shields, as in the fast breeder reactor described above. In the invention, if neptunium is added to the reactor core fuel in an amount of about 2%-5% by weight,based on the weight of the fuel and rare earth elements are added to the blankets or the shields in an amount of about 50% by weight,based on the fuel weight, the operation characteristics become substantially equal to those of a fast reactor core with normal mixed oxide (MOX) fuel not containing minor actinide nuclides or rare earth elements. By the way, americium and curium are nuclides having a large neutron generation amount, gamma dose, and heat generation amount; their manufacturing requires a cell surrounded by thick shields. In contrast, neptunium has a very small neutron generation amount, gamma dose, and heat generation amount; manufacturing of neptunium does not require any large-scaled shields, and a glove box is sufficient for the purpose. Therefore, according to the invention, reactor core fuel with severe manufacturing specifications can be manufactured in a glove box and a rise in costs can be suppressed. Curium-244 occupying most of the neutron generation amount has a comparatively short half-life of 18.1 years. Thus, curium-244 may be made to decay by storage in order to reduce the manufacturing load before it is loaded into the reactor core. Table 1 lists reactor core characteristics of a 1000-MWE class fast breeder reactor using oxide fuel. Case 1 is an example in which neptunium, americium, curium, and rare earth elements are all added to the reactor core fuel. In contrast, Cases 2 to 4 in the invention are examples where neptunium is added to the reactor core fuel, while americium, curium, and rare earth elements are added only to the axial blankets, only to the radial blanket, and to both the axial and radial blankets, respectively. TABLE 1 __________________________________________________________________________ PARAM- ETER CASE 1 CASE 2 CASE 3 CASE 4 __________________________________________________________________________ ADDITION Np: 5% Np: 5% Np: 5% Np: 5% TO REAC- Am,Cm: 0.7% TOR CORE RARE EARTH FUEL ELEMENTS: 20% ADDITION NONE Am,Cm: 0.7% NONE Am,Cm: 0.7% TO AXIAL RARE EARTH RARE EARTH BLANKETS ELEMENTS: ELEMENTS: 20% 20% ADDITION NONE NONE Am,Cm: 0.7% Am,Cm: 0.7% TO RADIAL RARE EARTH RARE EARTH BLANKET ELEMENTS: ELEMENTS: 20% 20% BURN UP 4.18 0.82 1.02 0.84 REACTIVETY (%.DELTA.k/kk') MAXIMUM 390 360 380 360 LINEAR HEAT RATE (W/cm) BREEDING 0.97 1.22 1.22 1.23 RATIO MA INCIN- 104 104 104 105 ERATION AMOUNT (kg/cycle) __________________________________________________________________________ In Case 1 in which 5 wt. % of neptunium, 0.7 wt. % of americium and curium, and 20 wt. % of rare earth elements are added to the reactor core fuel, the burn up reactivity becomes 4.2% .DELTA.k/kk', which drastically exceeds the limit value 3% .DELTA.k/kk', which is the guide for control rod design, making it difficult to control the operation, and introducing a safety problem. Also, the breeding ratio drastically lowers to 0.97 and necessary performance as a fast breeding reactor cannot be demonstrated. In contrast, although the minor actinide nuclides are added in the same amount, if they are added to the reactor core fuel and blankets separately in response to the type of minor actinide nuclide as in the invention (Cases 2-4), the burn up reactivity becomes about 1% .DELTA.k/kk' , which sufficiently falls below the limit value 3% .DELTA.k/kk'. Also, the breeding ratio can be kept at 1.2 or more and necessary performance as a fast breeding reactor can be demonstrated. As described above, the invention is characterized by the fact that neptunium is added to the reactor core fuel, and that americium (and curium) and rare earth elements are added to the blankets or shields. Therefore, minor actinide nuclides with long half-lives extracted from the spent fuel can be burnt until they incinerate without the load of separating minor actinide nuclides and rare earth elements.