Apart from Fermi steam generator problems (Section 4.6), the only fast reactor which has exhibited a substantial instability is the original core of EBR-I. This proved to be the result of a secondary fuel rod bowing effect, by
which a primary prompt positive reactivity change, produced by an original bowing of the fuel rods, was counteracted by a delayed and negative eifect (10).

When the system was brought to power, the fuel rod first bowed inward while supported by the first shield plate and the lower grid plate, but some time later the rod began to be affected by the expansion of the third shield plate which then effectively moved the fuel element away from the center of the core giving a slow negative reactivity coefficient for large power-to — flow ratios (Fig. 2.38).

The diagnosis of this eifect was very difficult but the problem was solved eventually by using a restrained core to prohibit all bowing, including the

original prompt positive effect. The absence of any instability confirmed the analysis.