Heterogenous deboration

A heterogeneous deboration is characterized by the formation of a slug of non-borated water or of water with a low boric acid concentration in the primary cooling system. Formation of such a slug is only possible if no circulation, natural or forced, is present, i.e. heterogeneous deboration is only possible if the MCPs and natural circulation are stopped. 

A detection of the deboration is only possible in an early stage if the circulation has not been started. If the circulation is started, the available time is too short for countermeasures. Therefore, attention should be focused on to preventing a slug formation and identifying situations when a slug can occur. Preventive measures include the automatic interruption of boron dilution if one or more MCPs are stopped and also a careful measurement of the boric acid concentration in systems which will be connected to the primary circuit after maintenance. 

Because the condensation of steam always leads to deborated water, any sequence in which steam is forming should be analysed to determine how much and at which location demineralized water can accumulate. The operator should have procedures available which ensure a sufficient mixing of a slug that may form. 

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To keep the core of a PWR subcritical during cold shutdown states, a certain boric acid concentration in the coolant is necessary. An inadvertent deviation from the prescribed concentration, with the tendency to loose subcriticality, is called a dilution or deboration event. The possibilities for diluting the coolant are generally classified as homogeneous (slow) or heterogeneous (fast) deboration. 

For plants dependent on boration to maintain subcriticality, careful consideration should be given to modelling heterogeneous (and possibly homogenous) deboration events. A detailed search for events and actions that cause or contributed to deboration should be performed. 

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