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The lessons learned from the Fukushima Daiichi accident support safety enhancements to cope with events that go beyond the design basis. Nevertheless the fundamental concepts of defense-in-depth still remain valid for nuclear safety. In case of higher uncertainties of external hazards, the effective implementation of the defense-in-depth requires additional means.

Concerning the structures, systems and components, technology and concepts exist which can cope with this type of accidents. With respect to severe accident mitigation, most of the technologies required to cope with Fukushima type accidents is considered to be already available, too.

External hazards

From the technological point of view it has to be stated that every measure that needs to be installed to cope with stricter requirements for both earthquake and tsunami hazards is available. This is explicitly demonstrated by the already started or even finished measures for the enforcement of the plants up to now.

The main issue for the enforcement of the plants is to find out the design requirements which have to be considered concerning the beyond design basis conditions.

Enforcement of structures, systems and components

The main issue of multi-unit sites is to identify weak points of individual units. They are considered to be as follows:

• Common cause failures that lead to the failure of safety related systems and/or components, and

• Connections among units that may affect intact structures, systems or compo-

nents from hazardous conditions of other units which consequently may lead to their failure

The PSA is considered to be the tool that enables to identify the areas which must be considered to strengthen the safety of multi-unit sites.

Since after an external event like an earthquake the offsite electricity supply is very difficult to guarantee, for such case the solution is mainly to use mobile equipment which is to be stored in the vicinity of the plant with the guarantee that it can be connected to the respective plugs at the plant under all circumstances. Only in cases where an offsite electricity source is very closely located to the plant site it can be considered to harden the source and the connection appropriately.

For the enforcement of onsite emergency energy supply many examples exist for bunkered systems, which were back-fitted and therefore are already provided for existing plants. So, the technology for such components is available; for example, diverse diesel generator systems with appropriate reliability for their function exist.

The main issue to strengthen the safety related structures, systems and components (SSC) in case of extreme external events is as follows:

• enforce the design of existing SSCs

• add alternative and/or additional SSCs

• use bunkered solutions

• provide passive components which need no electricity supply

For all these measures the technology is available and there exist a number of executed solutions for existing reactors. It is a matter of individual plant design what measure could be appropriate to strengthen existing SSCs considering also the impact on the economics effects of the plant.

Severe accident mitigation measures

The use of catalytic recombiner can be regarded as the most suitable hydrogen hazard mitigation strategy for nuclear power plants in the future because of its passive behavior, its well-known physical phenomenology, its effi y under both beyonddesign-basis and design-basis accident conditions, its start-up at low hydrogen concentration, and its simple use without supplementary constraints in normal operation. All venting systems have passed a number of qualification tests and most of them were already installed in NPPs, meaning that they have successfully passed a licensing process. Decisive criteria for the selection of one of the systems have to be defined by the respective utilities under consideration of their regulatory requirements.

For existing reactors the back-fitting of RPV outside cooling is a very complex and expensive measure, and may be only possible from the technical point of view for very limited applications. It is expected that in most cases for the cooling an active system must be provided. In such cases it is proposed to use such an additional active system to inject water into the vessel instead injecting it for outside cooling. For existing BWRs, the method proposed by the Nordic countries could be a solution if it is assured that the cavity around the RPV can be filled with water passively and the water tightness of the compartment can be maintained.

In other cases, the proposal considering melt concrete interaction could be a solution, which may lead to an extension of the time the melt can be contained within the containment boundaries or even will be stabilized within the containment. For both solutions further effort of research and development is required.

Core catchers are mainly proposed for Generation 3+ reactor systems. Up to now already some concepts have been successfully developed and licensed, such as those for the VVER and EPRTM reactors, and are implemented in ongoing projects.

Appropriate instrumentation qualified for severe accident conditions is one of the main prerequisites for an efficient severe accident management. In order to improve existing measures, it is required to consider this issue and implement severe accident related instrumentation for hydrogen monitoring or radioactivity monitoring. In addition the instrumentation that reliably indicates the state of the plant such as temperature, pressure and water level measurement have to be qualified for severe accident conditions at elevated temperatures and radioactivity doses. It should be noted that management, command and control of severe accident for reducing the socio-psychological impact is important, although it is not

addressed in this chapter.

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