Sodium interacts with oxygen according to the reactions:

2Na + iOa-» NaaO 8H = -104 kcal/mole (4.39)

2Na + 02 -» Na2Oa 8H = -20 kcal/mole (4.40)

The interaction is characterized by low flames and dense white oxide smoke, which itself can create a visibility hazard for workers in the area. Once started, enough heat is liberated to maintain the reaction and gradually raise the temperature of the molten sodium.

4.5.1 Pool Fires*

If sodium in a pool is exposed to oxygen, either because sodium is spilled in an air atmosphere or because what should have been an inert atmosphere was somehow contaminated or exchanged for air, then the possibility of a sodium fire is present.

However, the right conditions for that fire must be present. The sodium has an ignition temperature below which it will not ignite but will slowly oxidize. If the sodium surface is undisturbed, the ignition temperature is about 550°F, whereas this might fall to 400°F if the surface were disturbed. On the other hand some cases in which a sodium fire did not occur up to 800°F have been reported. There must be not less that 4% of oxygen pre­sent; otherwise there will only be considerable incandescence and a lot of smoke. This would occur all the way down to 0.1 vol% of oxygen. Humidity in the atmosphere is an important catalyst for a sodium fire.

If, however, a fire does start, the combustion rate is about 0.10-0.3 lb/min — ft2 of surface. This rate would increase with the depth of the pool and decrease with the surface area of the pool. A fairly standard rate used in calculations is 5 lb/hr-ft2.

Heat is generated at the surface and is dissipated from the sides and bottom of the sodium pool, so that there is a temperature gradient within the pool which might reach about 30°F/in. if it were quiescent. Above the surface of the sodium, there is a more severe gradient in “still” air of up to 500°F/in. Sodium fire modeling should take these possibilities into account.

When the sodium has burned completely, it liberates between 4100 and 4850 Btu/lb, although in most cases it does not seem to burn completely, and among the debris there is always unburned sodium.