Induced nuclear reactions: processes – systems – and elements – Nuclear transmutation – By neutron bombardment
Reexamination Certificate
2000-09-12
2003-01-28
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Nuclear transmutation
By neutron bombardment
C376S172000, C376S173000, C376S428000
Reexamination Certificate
active
06512805
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light water reactor core and fuel assemblies composing a reactor core and, more particularly, to a light water reactor core wherein in a boiling water reactor (BWR), the BWR has the same levels in cost efficiency and degree of safety as those of an existing BWR under operation now, that is, the BWR is oriented to plutonium (Pu) multi-recycle having a breeding ratio near 1.0 or slightly larger and having a negative void coefficient with minimizing modification of the reactor core structure of the existing BWR under operation now, and to fuel assemblies used for the boiling water reactor.
In the inside of the reactor, while fissionable materials such as uranium-235 and plutonium-239 are being consumed by fission reaction, fertile materials such as uranium-238 and plutonium-240 are being converted to fissionable materials. The ratio of an amount of fissionable materials contained in the fuel unloaded from the reactor core to an amount of fissionable materials contained in the fuel loaded from the reactor core is called as a breeding ratio. The breeding ratio in a conventional light water reactor is approximately 0.5. Methods of improving the breeding are studied in order to effectively use the uranium resources.
Japanese Patent Application Laid-Open No.55-10591 and Nuclear Technology, Vol.59, pages 212-227 (1982) propose a method that the breeding ratio is improved in a pressurized water reactor by densely arranging fuel assemblies in a triangular lattice to decrease the water-to-fuel volume ratio. However, the obtained breeding ratio is 0.9 at maximum, and accordingly the fissionable material needs to be supplied in order to continue operation without reducing the output power. In order to further increase the breeding ratio, it can be considered to further decrease the water-to-fuel ratio by narrowing the gap between the fuel rods. However, it is difficult to narrow the gap between the fuel rods because there are limitations in manufacturing the fuel assembly and in securing thermal margin.
On the other hand, Japanese Patent Application Laid-Open No.1-227993 proposes a method that the water-to-fuel ratio is effectively decreased by making use of steam void generated in the reactor core which characterizes a boiling water reactor. It is shown that this conventional technology can increase the plutonium multiplication ratio (the ratio of amount of fissionable plutonium contained in fuel unloaded from the reactor core to an amount of fissionable plutonium contained in fuel loaded in the reactor core, that is, the breeding ratio to fissionable plutonium) up to nearly 1, but it is not shown that the breeding ratio (which is smaller by 4 to 5% than the plutonium multiplication ratio in the case where natural uranium is enriched by adding plutonium) can be increased approximately 1 or larger. In order to continue operation without reducing the output power in the case where the plutonium multiplication ratio is near 1, it is necessary that the natural uranium is enriched by being added with plutonium, and accordingly it is impossible to spent all the uranium resource. Therein, the words that the breeding ratio is larger than near 1 in the present invention means that the breeding ratio is larger than 0.98.
Further, Japanese Patent Application Laid-Open No.8-21890 discloses that in a reactor core comprising fuel which is enriched by adding plutonium or plutonium and an actinide to a uranium containing at least one of a depleted uranium, natural uranium, a degraded uranium and a low enriched uranium, the reactor core has a breeding ratio of near 1.0 or larger than 1.0 and a negative void coefficient. However, the prior art is not a technology which is intended to further improve the ability of a control rod to be manufactured and the cost performance of the control rod.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a reactor core which has the same levels in power generation cost, thermal margin and degree of safety as those of a light water reactor under operation now and fuel assemblies used for the reactor core.
In order to attain the above-mentioned object, the present invention provide a light water reactor core comprising fuel which is enriched by adding plutonium or plutonium and an actinide (hereinafter, referred to as plutonium and the like) to a uranium containing at least one of a depleted uranium, natural uranium, a degraded uranium and a low enriched uranium (hereinafter, referred to as depleted uranium and the like), which further comprises fuel assemblies having fuel rods arranged in a triangular lattice configuration; and large-diameter control rods to be inserted into the fuel assemblies, the large-diameter control rod comprising at least one absorption rod having a transverse cross-sectional area larger than a cross-sectional area of a unit lattice cell of the fuel rod, wherein a breeding ratio of the reactor core is near 1.0 or larger than 1.0, and a void coefficient of the reactor core is negative. According to the present invention, in order to contribute to long-term stable energy supply, it is possible to realize the breeding ratio of 1.0 using the fuel enriched by adding plutonium to depleted uranium (hereinafter, referred to as the first effect). According to the present invention, since it is possible to attain the breeding ratio near 1.0 or larger than 1.0 using the fuel enriched by adding plutonium to depleted uranium, the depleted uranium and the like can be burned as if the plutonium would be used as a catalyst, which can contribute to the long-term stable energy supply.
A first preferable embodiment is a fuel assembly comprising fuel which is enriched by adding plutonium and the like to depleted uranium and the like, wherein a breeding ratio of the fuel assembly is near 1.0 or larger than 1.0. According to the first embodiment, the first effect can be attained. Since it is possible to attain the breeding ratio near 1.0 or larger than 1.0 using the fuel enriched by adding plutonium to depleted uranium, the depleted uranium and the like can be burned as if the plutonium would be used as a catalyst, which can contribute to the long-term stable energy supply.
Further, a second preferable embodiment is a light water reactor core comprising fuel assemblies having fuel rods arranged in a triangular lattice configuration; and large-diameter control rods to be inserted into the fuel assemblies, the large-diameter control rod comprising at least one absorption rod having a transverse cross-sectional area larger than a cross-sectional area of a unit lattice cell of the fuel rod. According to the second embodiment, the first effect can be attained, and at the same time, in order to contribute to the long-term stable energy supply, it is possible to realize the breeding ratio of 1.0 by reducing the water-to-fuel ratio and by using the fuel enriched by adding plutonium to depleted uranium (hereinafter, referred to as the second effect). Further, in the second embodiment, by using the large-diameter control rod, the mechanical strength of the control rod can be increased and accordingly bending and buckling of the control rod can be suppressed when the control rod is inserted or withdrawn. Furthermore, by using the large-diameter control rod, number of absorption rods per fuel assembly can be reduced, and accordingly the control rod can be easily manufactured to reduce the manufacturing cost.
Further, a third preferable embodiment is a light water reactor core comprising a water-excluding region on a surface of the guide tube, the water-excluding region being formed of a substance having a slowing down power smaller than a slowing down power of light water. According to the third embodiment, the second effect can be attained.
Further, a fourth preferable embodiment is a fuel assembly which comprises fuel rods closely arranged in a triangular lattice configuration, a gap between the rods being within the range of 0.7 to 2.0 mm. The light water reactor core may be constructed using
Aoyama Motoo
Ikegawa Motohiko
Miwa Junichi
Takeda Renzo
Carone Michael J.
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Matz Daniel
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