Sodium-water reactions in steam generators

Unless protective action is taken a leak in an SGU tube will both grow (i.e. the hole will enlarge so that the leak rate increases) and also escalate (i.e. leaks in other tubes will be caused, again increasing the overall leak rate). Safety against leaks is therefore ensured by appropriate means of detecting leaks quickly so as to prevent or minimise escalation, and a protective system to isolate and make safe the affected SGU. 

Foerster et al. [6.11] review the current understanding of SGU leak behaviour and the design provisions necessary to protect against the consequences of leaks. 

Sodium-water reactions. When sodium and water interact various chemical reactions compete for example 

The first of these reactions dominates under the conditions in an SGU, so that the main reaction products are sodium hydroxide and hydrogen, but depending on the initial conditions and quantities of the reaaants and on the final pressure, some sodium oxide or hydride may be present. If the mixing ratio of sodium and water is not stoichiometric (i.e. equal numbers of moles), there will also be excess water or sodium, either of which may be present as liquid or vapour. 

Classification of leaks. The behaviour of a leak and the opportunities for detection depend on its size. It is convenient to define four classes as follows. 

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