By virtue of their short generation times fast reactors may be considered as particularly safe, provided that their reactivity feed-back is governed by a prompt negative contribution from effects inherent to the system. In large ceramic fueled reactors this is to be ensured by the Doppler coefficient of reactivity. It is generally known that stability may be jeopardized by sufficiently large delayed contributions of either sign to the total reactivity feedback. Yet one has to cope with the fact that Uranium-Plutonim oxide fueled fast breeder reactors with core volumes larger than a few hundred liters exhibit sizable positive void coefficients if they are cooled by steam. The present paper derives stability criteria which allow one to determine the maximum permissible ratio of magnitudes of the positive steam void coefficient and the negative Doppler coefficient of reactivity. Two models are considered, according to the types of instability to be prevented.
展开▼
