The above comments refer to an ideal single zone core, but present designs of sodium-cooled fast breeders comprise two zone cores in which the zones differ in enrichment; therefore, they have separate power and worth distri­butions. At the boundary between the zones there is a discontinuity in the

f See Nicholson (12a).

reactivity worth and in the power, so that fuel moving outward across the boundary between the zones experiences an increase in worth.

The discontinuity in power density produces an energy generation that is also discontinuous. A pressure discontinuity is produced that relieves itself as a compression wave inward and a rarefaction wave outward. Both waves can cause material to move inward near the interzone boundary.

Even in a homogeneous core there are also local areas in which flux and worth gradients do not coincide; this too can cause fuel material to implode rather than to disperse.

Calculations with the hydrodynamic code VENUS (12b), which includes a calculation of the reactivity effect of actual fuel movement rather than relying on a constant density material model for the expansion, have shown that the implosion effect is relatively unimportant. The inward implosion does not continue with ever increasing violence, but it rapidly explodes with a relatively small total energy production.

A calculation of the energy release, excluding consideration of the surface effects, may only be about 20% less than a calculation that includes all the surface or boundary effects.