Burning and Freezing of coolants

The intensity of processes in the reaction zone is determined by two mechanisms (a) air oxygen diffusion into reaction zone, and (b) opposite direction sodium vapour diffusion from sodium surface into the reaction zone. 

The former mechanism basically controls processes at temperatures lower than 650°C. With temperature increase the latter mechanism becomes more and more significant. The higher sodium temperature, the more vapour generation rate. The reaction zone extends from the sodium surface resulting in the decrease of heat transfer to sodium. This, in its turn, decreases evaporation rate. Studies have shown that steady state is achieved at the pool sodium temperature of 720°C to 745°C. During burning process 15 to 25% of the combustion product mass leaves the reaction zone in the form of fume. Nevertheless sodium mass remains almost constant because of simultaneous mass increase due to sodium oxidation. 

Heavy coolant freezing. One of the most dangerous events resulting in coolant freezing is the secondary steam header depressurization. Rarefaction wave propagates at sound velocity in the secondary circuit on the initial stage of such accident. 

Secondary circuit water boiling up and abrupt increase of heat removal from the primary coolant occurs to result probably in its freezing. Steam turbine drive of feed water pump and by-pass pipelines connecting steam generator inlet and steam header through orifices are provided in the secondary circuit to decrease consequences of such accidents. 

Engineering features are provided in the reactor design to prevent lead freezing due to malfunctions or personnel errors in start-up and transient modes. 

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