Core reactivity behaviour, Dnb margins and fuel temperature after main steam line break, in subsequent cycles

In the analysis of the hypothetical accident 'main steam line break'is necessary to demonstrated that the accident is controlled and that the core is at no time endangered. In this paper a cycle-independent analysis procedure is deduced, which in subsequent cycles allows to prove integrity of the core. The method is based on the 'concept of fictitious super-criticality at zero load' or 'ΔρFUEN concept'. The ΔρFUEN concept incorporates the elaboration of correlations between ΔρFUEN (the excess reactivity at the instant of maximum core subcooling) and the most important variables of the core after going recritical. The correlations are constructed by a fictitious variation of boron concentration at EOC1 and checked by exemplary calculations for selected follow-up cycles. The correlations allow and estimate to be made of the margins to DNB and centerline fuel melting solely on the basis of a reactivity balance. (author)