Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Patent
1979-11-13
1984-11-20
Nelson, Peter A.
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
376917, G21C 332
Patent
active
044838182
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
This invention relates to fuel assemblies, and more particularly to a fuel assembly suitable for enhancing the economy of fuel.
2. Background Art
The fuel of a boiling water reactor is received in a large number of long rod-shaped members. These fuel rods are held in the flow of the coolant within a reactor core with their longitudinal direction set vertical and at predetermined intervals from one another, and thus form a fuel assembly. The coolant is in the subcooled state in the lower part of the core which serves as an inlet for the coolant. As the coolant rises within the core, it becomes the states of subcooled boiling and saturated boiling. Therefore, the void coefficient increases from the lower part towards the upper part of the core, and it reaches approximately 70% in the upper part of the core serving as an outlet for the coolant (void distribution). As a result, the heating of neutrons proceeds more and the reactivity becomes higher in the lower part than in the upper part of the core, so that the position of a power peak arises in the lower part of the core. In the case where the power distribution is not uniform over the entire core, only a domain of the highest power operates at the highest efficiency, with the result that the mean power becomes lower than the theoretical value. Further, a larger and more expensive core and more fuel are required for a given power level. In order to minimize the cost of the fuel as well as the whole apparatus, it is desirable that the power distribution is flat in the axial direction.
In order to flatten the axial power distribution, the following method has been commonly practised. That is, by inserting control rods into the reactor core from the lower end of the core, the fuel reactivity (infinite multiplication factor K.sub..infin.) in the lower part of the core is lowered so as to cancel the vertical unbalance of reactivities attributed to void coefficients.
On the other hand, the rate of plutonium production attendant upon the burn-up of the fuel rod is comparatively high in a void region, so that the deterioration of the reactivity attendant upon the burn-up becomes as illustrated in FIG. 1. In FIG. 1, a curve A indicates the reactivity deterioration in the upper part of the core, and a curve B the reactivity deterioration in the lower part of the core. In a region of low degree of burn-up indicated by L (a range in which the mean burn-up degrees of the fuel are from approximately 10,000 MWD/T to 18,000 MWD/T), the deterioration of the reactivity in the upper part of the core is smaller than the deterioration in the lower part of the core. In consequence, the difference between the reactivities in the upper and lower parts of the core tends to gradually decrease with the burn-up, and the power distribution as above stated flattens. In this manner, in the low burn-up degree region, the power distribution tends to flatten without making the reactivities unequal between the upper and lower parts of the core, and it can also be easily controlled with the control rod.
In contrast, in a region of high degree of burn-up indicated by H in FIG. 1 (in which the mean burn-up degrees of the fuel are at least 30,000 MWD/T), the difference between the reactivities in the upper part of the core and the lower part of the core tends to gradually increase. Therefore, the unbalance of the reactivities caused by the unequal void coefficients is canceled excessively, the reactivity in the upper part of the core becomes greater than that in the lower part of the core, and the power distribution becomes one having a power peak in the upper part of the core. Moreover, since the control rod is inserted into the core from the lower end of the latter in the boiling water reactor, the power difference in the upper and lower parts of the core becomes increasingly great, and so does the power peak value. The distortion of the power distribution in which the power peak appears in the upper part of the core spoils the safety of the reactor for the reas
REFERENCES:
patent: 3145149 (1964-08-01), Imhoff
patent: 4229258 (1980-10-01), Takeda et al.
patent: 4244784 (1981-01-01), Takeda et al.
Kawai Toshio
Yamashita Jun-ichi
Yokomi Michiro
Hitachi , Ltd.
Nelson Peter A.
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