1. Introduction

The nuclear technology gives us the perspective of the effective use of natural resources of energy, but on the other hand, it is a risk for the environment [1–4]. The first accident in nuclear research facilities dates back to 21 May 1946. A nuclear critical accident occurred at the Los Alamos Scientific Laboratory in New Mexico. Severe problems arrived after an accident in Chernobyl Nuclear Power Plant in Ukraine on 26 April 1986. The last massive nuclear power plant (NPP) accident was on 11 March 2011 in Fukushima [5–7]. Following a major earthquake

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(a magnitude 7.1), a 15-m tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, hence causing a nuclear accident.

In view of the analysis results of the Fukushima accident [5, 6], the owner and operator of nuclear power plants (NPPs) in Slovakia re-evaluated the safety and reliability of structures and technology of all objects, according to the recommendations in "Stress tests" on all units in operation or under construction [7]. The main plant technological equipment (except for main circulation pumps) for all units was manufactured either in Czech Republic or in Slovakia [3].

In the process of design, construction and operation of plants, significant improvements in safety were made compared to the original project, including enhanced resistance to external hazards [1–3, 8]. Lately, in addition to previous safety improvements, many proposals have been implemented in the reconstruction process of NPP structures with reactor VVER 440/V213 in Slovakia for mitigation of severe accidents [3]. Fifty-one per cent of the overall production of electricity in SR comes from the nuclear power plants (Table 1).

The NPP buildings with the reactor VVER 440/213 consist of the turbine hall, middle building, reactor building and bubble condenser [1]. The NPP containment is limited by the area of reactor building and bubble tower (Figure 1). In the case of the loss-of-coolant accident (LOCA), the


Table 1. Source ratio on annual electricity consumption in SR.

Figure 1. Section plane of the containment with reactor VVER440/213.

steam expands in the containment space, and the overpressure and temperature loads affect the containment structure [1]. The structural integrity and the tightness of the containment must be guaranteed in the case of the technology accident.
