Induced nuclear reactions: processes – systems – and elements – Handling of fission reactor component structure within... – Fuel component
Patent
1999-03-02
2000-10-31
Carone, Michael J.
Induced nuclear reactions: processes, systems, and elements
Handling of fission reactor component structure within...
Fuel component
376419, 376428, 376435, G21C 3326
Patent
active
061413968
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an initial core of a boiling water reactor (BWR).
It is necessary to increase the average enrichment of an initial core to raise the discharge exposure of the initial core. Therefore, in an initial core, which is loaded with a plurality of fuel assemblies having a different average enrichment, the difference in the nuclear characteristics between a high enrichment fuel assembly having a high average enrichment and a low enrichment fuel assembly having a low average enrichment becomes large.
In case a high enrichment fuel assembly and a low enrichment fuel assembly adjoin each other, a thermal neutron is caused to flow from the low enrichment fuel assembly with a high thermal neutron flux to the high enrichment fuel with a low thermal neutron flux. Therefore, because the output of the fuel rods of the high enrichment fuel assembly increases, and the MLHGR (Maximum Linear Power Heat Generation Ratio) and the MCPR (Minimum Critical Power Ratio) in the beginning of burnup become severe, an improvement in the thermal margin becomes a problem.
To improve the thermal margin, fuel rods containing gadolinia (hereinafter called a "Gd fuel rod") are arranged symmetrically in the cross section of a conventional fuel assembly. In case the average enrichment of an initial core is heightened for high burnup, control rods are inserted in the core for a long time, and then they are extracted to suppress excess reactivity.
Therefore, the output of one side near to the control rod is smaller than that of the other side far from the control rod in the cross section of a fuel assembly loaded into a control cell. Burnup of fuel rods on the one side near to the control rod is delayed. This is called control rod history effect. Due to the influence of this control rod history effect, the thermal margin could not be sufficiently secured in a conventional initial core. Particularly, in a case where the high enrichment fuel was loaded into the control cell in a second operation cycle, the thermal margin was severe.
SUMMARY OF THE INVENTION
It is an object of the present invention to secure the thermal margin in an initial core that increases the average enrichment for a high burnup and uses a high enrichment fuel assembly for the control cell in the second operation cycle.
In accordance with the present invention, to achieve the above object, there is provided an initial core comprising a plurality of fuel assemblies having different average enrichment and a plurality of control rods, each fuel assembly having a square-shaped cross section and each control rod having a cross-shaped cross section, wherein a unit cell of a square-form is composed of one first fuel assembly with the lowest average enrichment and three second fuel assemblies with higher average enrichment than the first fuel assembly. A plurality of unit loading patterns are arranged in a central area of the initial core, each unit loading pattern being composed of one unit cell and four control rods arranged at four corners of said one unit cell, and the second fuel assembly obliquely adjoining the first fuel assembly in each unit loading pattern is divided into one side area near to the control rod and the other side area far from the control rod by a diagonal line, the number of fuel rods containing gadolinia in said one side area being 2 or more than the number in said other side area.
To examine an effect according to the present invention, the local peaking factor of the high enrichment fuel assembly that composes the control cell in the second operation cycle was analytically analyzed.
FIG. 5 shows a unit loading pattern of a comparative example. This comparative example includes one low enrichment fuel assembly 7, two high enrichment fuel assemblies 8 and one high enrichment fuel assembly 9a like that of FIG. 2 to be mentioned later. Low enrichment fuel assembly 7 is equivalent to the first fuel assembly and high enrichment fuel assemblies 8 and 9a are equivalent to the second fuel assemblies. In F
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Haikawa Katsumasa
Kanda Akiko
Koyama Junichi
Narita Kenmi
Yamanaka Akihiro
Carone Michael J.
Hitachi , Ltd.
Mun K. Kevin
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