Since the original Bethe-Tait calculations (5), the calculation of the energy release has improved significantly in the following ways:

(a) Doppler feedback coefficients are now included; they provide con­siderable amelioration of the energy release.

(b) Reactivity changes are calculated from a worth function D(r) rather than from a one-energy-group perturbation theory and in the latest codes the reactivity changes are continuously computed by a diffusion equation calculation.

(c) The power function is now included as a shaped function.

(d) Two-dimensional geometry is used instead of spherical geometry.

(e) Improved nonthreshold equations of state are now used.

(f) It is to some extent possible to investigate nonhomogeneous auto­catalytic effects to see what aggravation of the accident might be caused.

It has been pointed out that the history of energy release calculations has been marked by a succession of advances in calculational techniques that generally tend to diminish the energy releases calculated and a succession of conjectural physical occurrences, such as an implosion of the core, which tend to give greater explosive values. Fortunately over the years, although the energy release has been a saw-toothed curve, the tendency has generally been downward. So that, although the fast reactors considered have grown larger, the energy releases calculated have not grown markedly. Present day explosive values are at a level where they can be contained in containment barriers which the system might possess for other reasons (e. g., the reactor vessel).