Abstract
The paper gives a description of conservative analysis of initiating event associated with uncontrolled dilution of boric acid concentration in the reactor VVER-1000/320 of Temelín NPP. This event is included in the group of beyond design basis accidents. The aim of analysis is to determine also the time interval which is necessary for interventions leading to the deterrence of fuel damage. The failure of operator intervention to isolate dilution routes at intervals shorter than 30 min is assumed. Since the response of the whole NPP system influences the course of safety important parameters of the reactor core, the calculations were made by an externally coupled version of the 3D reactor dynamic code DYN3D and the thermohydraulic system code ATHLET. It is shown that, in addition to exceeding the DNBR limit of more than 99 min from the start of the transient, the remaining safety acceptance criteria will not be violated until the end of the calculation.
Kurzfassung
Das Papier gibt eine Beschreibung der konservativen Analyse des auslösenden Ereignisses im Zusammenhang mit der unkontrollierten Verdünnung der Borsäurekonzentration im Reaktor VVER-1000/320 des Kernkraftwerks Temelín. Dieses Ereignis ist Teil der Gruppe der Unfälle, die über die Auslegungskriterien hinausgehen. Ziel der Analyse ist es, auch das Zeitintervall zu bestimmen, das für Eingriffe zur Verhinderung von Brennstabschäden notwendig ist. Es wird davon ausgegangen, dass es dem Operator nicht gelingt, die Verdünnungswege innerhalb von 30 Minuten zu isolieren. Da die Reaktion des gesamten KKW-Systems den Verlauf der sicherheitstechnisch wichtigen Parameter des Reaktorkerns beeinflusst, wurden die Berechnungen mit einer extern gekoppelten Version des 3D-Reaktor-Dynamikcodes DYN3D und des thermohydraulischen Systemcodes ATHLET durchgeführt. Es wird gezeigt, dass außer der Überschreitung des DNBR-Grenzwertes von mehr als 99 Minuten ab Beginn der Transiente die verbleibenden Akzeptanzkriterien bis zum Ende der Berechnung nicht verletzt werden.
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