Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Patent
1993-04-26
1995-04-25
Wasil, Daniel D.
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
376170, G21C 3328
Patent
active
054105803
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND ART
The present invention relates to a fuel assembly adapted for a thermal neutron type reactor and more particularly to a fuel assembly utilizing fuel rods enriched in plutonium.
In view of effective usage of resource and energy security, there is a schedule for the utilization of plutonium recovered through reprocessing of spent fuel as a fuel in a thermal neutron reactor.
The plutonium radiates .alpha.-rays having high radiation intensity and it is hence necessary to prevent a human body from being internally exposed and also radiates neutrons and .gamma.-rays through decay and spontaneous fission. For this reason, production or fabrication of the fuel including the plutonium should be performed in a sealed environment in comparison with uranium fuel. In addition, many considerations must be paid for equipment and manufacturing processes. For example, multiple shielding equipment is required and strict attention should be paid for its decontamination and maintenance. Accordingly, it is extremely disadvantageous from economical and other view points to manufacture many kinds of fuel pellets and fuel rods containing the plutonium in different concentrations.
From another view point, since severe conditions are placed on the fuel rods containing the plutonium with respect to the conveyance, measurement control and criticality control, it is desired to reduce the number of fuel rods containing the plutonium by using a large containing ratio of plutonium in one fuel rod.
From the above view points, in a fuel assembly in which plutonium of a predetermined amount obtained through the reprocessing of spent fuel is utilized for a thermal neutron reactor, it is advantageous to substitute fuel rods each having high enrichment, in such a fuel assembly utilizing enriched uranium as shown in FIG. 7, with fuel rods containing the plutonium such as uranium-plutonium mixed-oxide fuel (MOX).
FIG. 7A shows a fuel arrangement in the radial direction and FIG. 7B shows a fuel arrangement in the axial direction. In these figures, reference numeral 1 denotes a channel box, 2 denotes a fuel rod, a symbol Ui (i=1-4) represents a uranium fuel, G is a fuel rod containing a burnable poison and W is a water rod.
In the substitution of the highly enriched uranium fuel rods in the uranium fuel rod assembly with the MOX fuel rods, the enrichment of the fissionable plutonium is experimentally set such that the reactivity characteristics as the plutonium fuel assembly and the peaking factors in the radial and axial directions becomes approximately the same as those of the uranium fuel assembly, but as a result, the following relationship will be established approximately.
Namely, in the case where the fuel rod having enrichment ei of the uranium fuel assembly is substituted with the MOX fuel rod having the enrichment Pi, supposing that the concentration of U-235 of the uranium of the MOX fuel is eB, the following equation (1) will be established. ##EQU1## I: Numbers of the fuel rods to be substituted.
In this equation Q becomes 1.2 to 1.5.
In another case where the fuel rod having high enrichment of the uranium fuel assembly is substituted with the MOX fuel rod, there is a case in which blanket portions having low concentration of U-235 are arranged to upper and lower ends of the uranium fuel rod. However, in the MOX fuel rods, there is a possibility of increasing the number of MOX fuel rods when a predetermined amount of the recovered plutonium is treated with the MOX fuel rod by providing the blanket portions having a lower U-235 concentration than natural uranium, thus being disadvantageous from an economical view.
In the thermal neutron reactor, when it is required to mix the plutonium with the uranium fuel and then to cause a fission reaction, nuclides other than U-235 for causing the fission reaction with respect to the thermal neutrons are plutonium isotopes of Pu-239 and Pu-241. The plutonium further includes Pu-240 absorbing the thermal neutrons and minute amount of Pu-238 and Pu-242.
The plutonium isotope Pu-241 is
REFERENCES:
patent: 3802995 (1974-04-01), Fritz et al.
patent: 3844886 (1974-10-01), Crowther
patent: 3933582 (1976-01-01), MacNabb
patent: 4652416 (1987-03-01), Millot
Kabushiki Kaisha Toshiba
Wasil Daniel D.
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