Experimental Breeder Reactor, EBR-1 received a considerable attention when an oscillatory instability under abnormal operating condition was observed.
The investigations of kinetic characteristics have revealed that a prompt positive power coefficient of reactivity, which is generally believed due to the bowing of fuel elements, and a more delayed negative power coefficient exist.
The controlled oscillator measurements were performed with the result that EBR-1 exhibited the resonance instability.
Though a number of theoretical models of the coefficients has been presented to study the instability, they cannot fit the mechanism of the delayed negative coefficient, which remains unsolved and sought for.
This paper presents the models of feedback reactivity for Mark II and Mark III core loading, respectively.
For Mark II, the main point to be emphasized is the cause of time delay by coolant transport of heat from the core to the structure. The delayed negative reactivity is treated by use of the transfer function which takes into account of axial temperature distribution in the core.
It is demonstrated that this delayed negative reactivity combined with the prompt positive reactivity is sufficient to permit the resonance instability.
For Mark III, the frequency transfer functions of feedback reactivity were separated from the experiment. The model is compared with the result of measurements.
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