Research and experience report 2013. Developments in the technical and legal areas of nuclear monitoring

The research into regulatory safety carried out by the Swiss Federal Nuclear Safety Inspectorate (ENSI) serves to clarify outstanding issues, establish fundamentals and develop the tools that ENSI requires for the fulfillment of its responsibilities. The programme is divided into seven areas: 1) research on fuels and materials covers the reactor core and the multiple successive barriers used for the containment of radioactive materials. It is concentrated on high burn-ups and safety criteria. Research into structural materials is focused on ageing mechanisms. The NORA Project at the Paul Scherrer Institute (PSI) looked into the addition of platinum to the coolant as a way of reducing crack formation in primary circuits; 2) the OECD Projects on internal events and damage encourage international exchange of experience on incidents. Specific data bases facilitate a systematic analysis of relevant operating experience from numerous countries. In the field of damage to the influence of pressurised conduits, an overview report on flow-accelerated corrosion was submitted; 3) ENSI supports international projects on external events, including complex experiments and simulations of aircraft crashes and earthquakes. Earthquakes were simulated by shaking-table tests on models of reinforced concrete structures. The structural load-bearing reserves of safety-critical buildings in the event of an earthquake were studied. The Extreme Events Platform is looking at issues related to flood risks; 4) concerning human factors, the research is looking at the reliability of operator behaviour under various conditions. A methodology developed by PSI analyses operator errors with a negative impact on the course of an accident. Research continued into the optimum way to design simulator tests and into the effects of earthquakes on the reliability of operator behaviour; 5) System behaviour and accident sequences in nuclear power plants are analysed in various conditions ranging from normal operations through to accidents involving core melt-down. Computer models are developed and validated by experiments. The results are used as a basis for the quantitative identification of the plant risk in probabilistic safety analyses. The sequences of events during the accident at the Fukushima-Daiichi nuclear power plant was analysed in order to determine the final state of the reactors; 6) research in radiological protection includes the technology used to measure radiation as well as the development of new methodologies for radionuclide analyses. The development of international norms is contributing to cross-border harmonisation of radiological protection methods; 7) the decommissioning of nuclear installations includes issues related to waste treatment and investigations for a deep geological repository. Research is looking at the properties of the host rock, the design of the repository and the processes which may occur in the long term. The influence of external factors is studied. Considerable progress has been made with the modelling of gases in a geological repository. The understanding of the geo-mechanical properties of the Opalinus Clay and its deformation during tunnel construction was improved. Incidents in nuclear facilities provide information on weaknesses and on the potential for improvements in design and operation. The relevance to Swiss nuclear facilities of particularly instructive events in facilities outside Switzerland was analysed: a) as a consequence of material defects disclosed in the reactor pressure vessels (RPV) of Belgian nuclear power plants, checks made to the RPV in the Muehleberg nuclear power plan found no manufacturing errors; b) at the Tihange-2 nuclear power plant in Belgium, it was found that the well-water supply was not delivering sufficient water to safety-critical systems. The Swiss nuclear power plants were required to check availability of enough water; c) small cracks were found in the welds of stainless steel storage vessels in US nuclear power plants. The primary cause was identified as stress corrosion cracking combined with manufacturing defects. This issue is important for the monitoring of ageing management. ENSI's cooperation with international organisations and regulatory bodies helps it remain abreast of international standards. The ENSI contribution to current increased harmonisation efforts is based on the stringent nuclear safety standards applicable in Switzerland. Steps to implement the lessons from the accident at the Fukushima-Daiichi nuclear plant were continued. The national action plans derived from the EU Stress Test were reviewed and discussed. The Efficiency and Transparency Working Group has drafted proposals for strengthening the Convention on Nuclear Safety. ENSI was actively involved in the development of these proposals. In particular, it will be calling for binding international regulations. The existing regulatory framework is being revised to comply with recent nuclear energy legislation and to harmonise it with international standards