Analysis of melt-coolant energetic interaction in the course of the in-vessel phase of severe accidents on NPP with VVER
Safety is one of the main factors determining the competitive ability of nuclear-power engineering. At the same time, during last 35 years three severe accidents (SA) took place in different countries all over the world. It evidence that the probability of SA can't be neglected and the comprehensive investigations of this type of accidents are essential.
In the course of SA the energetic melt-coolant interaction (steam explosion) could took place. These phenomena could endanger the reactor vessel integrity and could be reason (trigger) for hydrogen explosions. This issue have been investigated in details since 1990-th. In connection with the events at the Fukushima Daiichi studies in this area have intensified, and problems considered solved, were reanalyzed, taking into account the current level of science and technology. GIDROPRESS" carried out an analysis of the energetic melt-coolant interaction on the in-vessel stage of severe accidents at NPP with WWER.
In the framework of in this study the methodology of an estimation of loads on the elements of WWER reactor pressure vessel during SA with energetic melt-coolant interaction has been developed, basing on international experience and taking into account the current level of knowledge.
On the basis of the methodology analysis of loads on the elements of the WWER-1200 reactor pressure vessel for a series of key accidents scenarios have been carried out, where:
- availability of the trigger needed for the implementation of a powerful steam explosion is postulated (conservatively), despite the fact that the likelihood of such a trigger for WWER technology due to its constructional features is negligible;
- conservative assessment of loads in terms of integrity of the reactor vessel and its fastening elements were performed. As part of the methodology SA is modeled using computer code SOCRAT/B1, and energetic interaction is taken into account conservatively with the use of semi-empirical correlations.
The analysis results showed that:
- the most dangerous in terms of the energy potential of the steam explosion is the case of through-penetration of core melt in the cylindrical part of the core barrel;
- the major limiting factor of the power of steam explosion is the amount of water in the downcomer area and in the reactor pressure chamber during the steam explosion;
- loads occurring during the energetic melt-coolant interaction in key SA scenarios are not endanger the integrity of the WWER-1200 reactor vessel and its fastening elements;
- since there are no integrity loss of reactor vessel in case of steam explosions during key SA, it can be concluded that the containment damage according to the α-mode scenario is impossible for WWER-1200.Full version (russian)