Damage to the head plug can be minimized to a certain extent by paying some attention to the design of the vessel and its internals.

The vessel could be allowed to rupture at any earlier stage of the accident. The SL-1 explosion interpretive tests showed that a vessel jump of 11 ft resulted from a liquid hammer if the vessel did not rupture, whereas no significant jump resulted from a case when the vessel did rupture. False rupture could be arranged in the vessel above the cover gas-liquid sodium interface by the use of carefully selected rupture disks if it seemed preferable to seek this design solution to the relief of invessel pressures.

The core barrel and thermal shields could be minimized in order to avoid the gun barrel effect whereby the force of the explosion would be directed upward. Radial vessel deformation reduces damage to the plug.

A sodium slug suppressor plate above the core could allow the plug to

feel the effects of the slug movement before the sodium has time to accelerate if the suppressor plate were rigidly connected to the plug. This would allow the plug to absorb the energy gradually rather than be subjected to an impact. However the plug would also be subjected to shock, against which it would otherwise be insulated by the height of sodium and the cover gas. A European design of LMFBR does include such a sodium slug suppressor. It has not been yet demonstrated that its inclusion is an advantage.

Gas volumes included in the vessel may attenuate pressure waves, but they would have associated hazards during normal operation if they were to become connected to the primary circuit, by being a source of gas which might be introduced into the core.

To minimize the result of the impact on the vessel plug, the plug may be equipped with energy absorbing honeycomb crush shields and it may be bolted down. The head design may be such as to avoid allowing the sodium access above the operating floor by a reentry design to direct sodium to drains and from there it could be directed to vaults. Finally, outside the vessel a missile dome or shields would protect the containment from pene­tration.

Table 5.14 shows some of the safety features used in U. S. fast reactors to hold down the head plug and contain missiles. No system uses rupture disks, and it is debatable whether any plant should go to that extent to design for a hypothetical event.

5.6 Engineered Safety Features

A list of safety features was given in Section 3.4.3 conforming to the definitions made in Section 3.4.1. Here we are interested in consequence limiting systems for various circumstances.

To avoid criticality following a hypothetical core melt-down, it could be necessary to maintain subcriticality by dispersing the fuel, by retaining the molten fuel in catchers where it might be cooled, and by providing terminal cooling to ensure that the fuel would remain where it was safe and could be cooled until the decay heat decreased.

To contain the effects of blast, head hold-down systems, blast shielding, missile barriers, and double containment are obvious design aids. Aerosol settling devices, filtration systems, and hold-up volumes would also aid the reduction of radioactive effluent following an energy release that released fuel from the system.

To avoid sodium fires and other reactions, the absence of air from the vaults and the containment, and the absence of water from these areas are self-evident solutions. The retention of sodium below its ignition tempera­ture at those times when a sodium fire is more likely to occur (during main­tenance) can be just as effective. Procedural controls are, of course, extremely important in separating sodium from oxygen and keeping it separate during start-up of the plant, during operation, and during maintenance.