To protect the containment building integrity against missiles that might be generated at the reactor vessel head following a core disruptive accident, the source and strength of possible missiles must be assessed. If critical components do become effective missiles, then either barriers or adequate height may have to be provided within the containment design.

Missiles might arise from control rod drives or their shafts, from unused head penetration plugs, from head restraint bolts which snap under impulse by the sodium slug, from whipping cables, or from small components asso­ciated with head equipment.

The energy imparted to each is calculated by a momentum transfer, and the trajectory of the missile may be calculated by the usual mechanics. If the trajectory meets a containment building, the velocity and energy at impact can be calculated in order to find the penetration the missile produces.

The real characteristic of interest is the areal density in lb/ft2, since the missile is more penetrating if it has a large mass and small effective dia­meter. Assuming a missile with weight M (lb), velocity v (ft/sec), area A (in.2) and diameter D(in.), there are a large number of penetration correlations available to relate the energy of the missile to the barrier needed to contain it (32).

For steel, the usual correlation used is the Stanford equation (33)

E/D = (£/46,500) (16,000г2 + 1500 WT/WS) (5.32)

where E is the missile energy, S is the ultimate tensile strength of the steel (psi), T is the barrier thickness (in.), W is the length of a steel square side between rigid supports (in.), and Ws is the length of a standard width (4 in.).

For concrete, the Petry formula is commonly used (34)

d = (KM/A) log10(l + t^2/215,000) (5.33)

This gives the penetration depth d (in.) when K, an experimental coefficient, is obtained for the particular case under consideration. However, the Petry formula is rather optimistic compared to a large number of other formulas that have been derived largely by military and naval research. A recom­mended correlation is

d = (282M/2(£U)Z)2) (D)°-215(v-lO"3)1-* + (D/2) (5.34)

where Sn is the compressive strength of concrete (psi) (55).

Some reactor systems have included specific missile shields built above the vessel head. The Enrico Fermi reactor missile shield completely en­closed the head and included an aluminum absorber section to reduce the missile impact energy. The component is illustrated in Fig. 5.18.

It should be noted that the most efficient missile barrier is distance and therefore it may be possible to specify a minimum ceiling height for the containment or any attendant hot cells so that penetration does not become a problem. Such a possibility will depend largely on layout and refueling method.

External missiles are treated in much the same way. Sources of missiles from such items as a runaway turbine, components of overflying planes, and tornado-driven debris are assessed for the energies involved and their penetrations are calculated by the same formulas as those given above. For very large and improbable external missiles, barriers cannot be provided but it is still necessary to know the probable consequences in order to ensure that the reactor fuel itself would not be damaged and that no radioactive fission products would be released to the atmosphere. Generally, the concern is not with the reactor itself, which is well protected by the massive head plug, but with the spent fuel storage pit which may only be covered by a thin lid, but which, nevertheless, contains a large inventory of fission products.