In a practical system, there are a number of sources which should be considered both during operation and during accident conditions. For illustrative purposes we consider a loop-type plant.

(a) Preoperational filling. It is possible that a considerable number of free spaces may have entrapped gas when the primary circuit was initially filled and these entrapped gases could be swept into the coolant stream if they were allowed to remain. However, before operation, cold and hot hydraulic tests of the system at low power will ensure that such gases are removed from the circuit.

(b) Gas entrainment. Cover gases may be entrained from a free surface if a vortex is formed due to the particular flow path of the design. In a loop-type plant the outlet nozzles should be sufficiently immersed to avoid the formation of vortices.

Gas may also be entrained by differences of pressures inside and outside instrument guide tubes that dip below the sodium free surfaces. If the cover gas systems are not regulated well, then it is possible that streams of gas may be introduced within the sodium (4d). However in all cases gas entrain­ment can be prohibited by good engineering design. The problem arises in making sure that all possible cases have been considered. Accident experience has shown that this is not easy.

(c) Gas absorption at a free surface. Gas may be collected by a fluid in motion below a cover gas, and this may later be concentrated within the primary circuit at some high point of the system. However absorption rates are very small, of the order of hundredths of a pound of gas per hour; this may be reduced by ensuring that the sodium in contact with cover gas volumes is nonturbulent.

(d) Fission gas release. The core does contain gas in each fuel pin fission gas plenum and a sudden release of a lot of this high-pressure gas could give rise to a considerable volume of gas in the circuit. For this reason the fission gas plenum is, in most designs, at the outlet of the core rather than at the inlet, so that any fission gas would be immediately swept out of the coolant channels. However, fuel pins are not likely to rupture in large quantities unless something else is seriously at fault, and gas released from small numbers of pins is rapidly removed from the circuit at the vessel free surface and hot and cold purification traps.

(e) Production from oil releases. If a pump lubricant, say the Fluorolube M-10 used in the Fermi pumps, could possibly penetrate to the primary circuit, then in contact with hot sodium, the lubricant would decompose into gaseous products. To avoid this possibility the system is engineered with multiple seals, a tortuous path for any possible leakage, and an oil sump well removed from the sodium coolant. Indeed, if this source were considered possible, then an oil which is compatible with sodium could be used.

(f) External purification and make-up circuits. External purification lines have cover gas systems that may give rise to gas sources, and although in any case it is simple to design the system to avoid a source for primary circuit gas, it is nevertheless difficult to ensure that all possibilities have been covered.

(g) Entry at pipe rupture. In certain loop designs in some low flow cir­cumstances, it is possible that part of the circuit may be at less than atmo­spheric pressure. A leak in this region would result in an inleakage of external gas to the primary. Alternatively, a large break may result in an input of gas, even though that break took place at a part of the circuit which was originally at high pressure. Thus, circuits should be designed to avoid inleakage of gas in the remote event of a rupture of the primary circuit.

If, despite engineering precautions to prohibit gas from entering the primary system, gas were to enter, it is unlikely that after passage through the pipework, the heat exchanger, and the pumps that a coherent bubble would result at the core inlet. Tests have shown that any bubble is dispersed by inlet plenum flows rather than concentrated at a single point. Therefore it is most unlikely that externally introduced gases could give serious re­activity effects. However, it is likely that very small bubbles will penetrate to the core and their effects on the heat transfer should be considered.