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Analyzing the Rod Drop Accident in a Boiling Water Reactor

Hsiang-Shou Cheng, David J. Diamond

Nuclear Technology

Volume 56 / Number 1 / January 1982 / Pages 40-54


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The center rod drop accident was calculated for a boiling water reactor using the two-dimensional (R,Z) core dynamics code BNL-TWIGL. Analysts frequently neglect moderator feedback under the assumption that it leads to conservative results. The present study shows that the peak of the power burst and peak fuel enthalpy can indeed be reduced by a factor of 2 or more by including this effect. The magnitude of the effect depends on reactor conditions. Moderator feedback is particularly important when there are voids in the core initially (i.e., at power conditions) or when the core is near saturation condition. When the reactor is initially at zero power and considerably subcooled, moderator feedback will influence the power peak by <10% but will have a much larger effect on the peak fuel enthalpy, which occurs later in time. The moderator feedback is the result of heat conducted from the fuel rod and direct energy deposition. At power conditions, the time constant for heat conduction is small and this is the primary mechanism for changing the steam void content during the accident. At zero power, the initial thermal constant is very large and, hence, any generation of voids at short times is due to direct energy deposition in the moderator. The effect of a different initial power level, flow rate, and inlet sub cooling, as well as the effect of delayed neutron fraction, rod drop speed, and accident rod worth, was calculated. In all cases, with moderator feedback accounted for, the maximum fuel enthalpy during the accident is well below presently established limits. Accident consequences are insensitive to the delayed neutron fraction and rod drop velocity. The parameters of most significance are inlet subcooling and accident rod worth. Most of the analysis used a fixed inlet flow and core pressure. A plant transient calculation was run to see how these parameters varied. The result was fed back into a bounding core calculation, which then showed that the change in pressure and flow increases the peak fuel enthalpy but not to an appreciable extent.

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