Reassessment of External Events in View of the Fukushima Accident

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1 Reassessment of External Events in View of the Fukushima Accident Michael Knochenhauer Scandpower AB Co-authors: Yvonne Adolfsson and Frida Olofsson, Scandpower AB Tomas Öhlin, Westinghouse Electric (Sweden)

2 Project Background As a consequense of the Fukushima nuclear power plant accident, the EU concluded that the event requires reassessment of the safety in NPPs in Europe. EU and ENSREG agreed upon an initial independent regulatory technical definition of a "stress test" and how it will be applied to the NPPs in Europe, EU Stress Test Specification. The Swedish Radiation Safety Authority (SSM) has required all Swedish nuclear utilities to submit a safety assessment based on the ENSREG specifications. The NOG (Nordic Owners Group) has initiated an analysis of various aspects related to the stress tests with the aim to make sure that no aspects has been disregarded.

3 Project Objectives Initiate assessments within the field of external events in the light of the EU Stress Test Specifications This first phase of the project aims to survey the current status in the field and decide on the necessity for further work. This presentation is made at the conclusion of phase 1-

4 Methodology The reassessment within the present NOG Säk och Miljö project is based on the existing guidance in Westinghouse report NOG Säk och Miljö Metodik för analys av vissa yttre händelser (NOG Safety and Environment Methodology for analysis of certain external events). The review comprises the type of fast evolving initiating events included in the report: Impact on buildings and structures. Electrical environment External flooding Impact on sea water quality For all design basis events the specification of requirements for external events are compiled. Implementation in Nordic reactors is reviewed. Suggestions of areas for further analysis.

5 Reassessment of External Events In the initiating phase of this project, preparatory work is identified for the prospective evaluation of the safety margins, possible cliff-edge effects and the necessity for modifications. Three sub-projects were defined: Extreme water levels in the Baltic Sea and Skagerrak Methodology for the analysis of multiple external events Main heat sink accessibility

6 Extreme water levels in the Baltic Sea and Skagerrak The objective for this work is to evaluate the relevance of the existing analyses which define the external flooding design basis for Swedish and Finnish BWRs. Aspects are presented that should be considered in a flooding analysis of a nuclear facility. The picture shows high sea water levels i Gothenburg Photographer: Tomas Klein, SMHI

7 Extreme water levels in the Baltic Sea and Skagerrak The existing statistical extreme value analysis for recurrence intervals of 100,000 and 1,000,000 years are based on short data series ( years). Such analyses are naturally associated with great uncertainties. Therefore the ambition has been to obtain a more qualitative description of extreme sea level events to use as a benchmark for existing analyses. The work includes the following parts: Description of development in time of observed extreme sea levels Consideration of historical events not included in data series Mechanisms behind extreme sea levels at the Nordic sites Global sea level rise due to climate change

8 Do we have extreme sea levels in the Baltic sea and Skagerrak? Definitely! The picture shows the French ship Caledonia on its way between Copenhagen and Pärnu (Estland) strandad on Bornholm after the devastating storm the night of November 1872.

9 Phenomena affecting water level Air pressure Wind speed Wind direction Wind waves Average sea level Seiche Sjösprång Local effect with short duration caused by a local change in air pressure travelling at the speed of a wave Tide Salinity

10 Historical extreme levels in the Baltic Sea Event (year) Storm Per (2007) Storm Gudrun (2005) January storm (1984) January storm (1983) Christmas Storm (1902) Backafloden (1872) Water level (cm above average) 144, Forsmark 197, Hamina 160, Ringhals 151, Helsinki 150, Göteborg 132, Hanko 177, Kalix 117, Stockholm 206, Lomma 360, Abbekås 280, Pärnu 146, Helsinki Event (year) Water level (cm above average) (1044, 1304, 1320,1449) > 300 (1625) 315, Travemünde (1694) 290, Schleswig (1835) 254, Flensburg Backafloden (1872) Stormfloder (1976, 1986/87, 88, 89) 349, Schleswig Ca 200

11 Methodology for the analysis of multiple external events The work aims to describe a screening analysis in order to identify and prioritize relevant coincident external events applied for deterministic analyses. The methodology is based on existing methodology developed within PSA. Incidence and locations for external event sources.

12 Plant Interfaces Page 12

13 Methodology for the analysis of multiple external events Screening criteria for multiple external events Impact. In order for the multiple event to be relevant to the analysis a situation where the event is challenging any of the safety functions and at the same time degrade at least one safety function is required. Definition. A multiple external event may be included in the definition of a simple external event. Independence. Normally the events included in a multiple event are not independent from each other. Impact on several safety functions. If the events affect the same safety function a further control must be done. The multiple event is relevant only if the combinated strength of the included events are greater than the strength of the separate events.

14 Main heat sink accessibility The objective is to sketch on proposals for measures to prevent different sources of unavailable main heat sink events. Proposals implying minor effort as complementary analyses, minor modifications and improved procedures. More comprehensive solution for diversification is implementation of a backup main heat sink as a cooling tower. The focus of the assessment is on oil spills from ships.

15 Main heat sink accessibility Alternatives for handling the risks from unannounced oil spills Prove that the risk is acceptable using statistical methods. Implement a measurement system of water quality that could detect oil spills. Implement a diversified heat sink. The basic idea is to construct a cooling tower that connects to the secondary cooling circuit. Unannounced oil spills in the Baltic Sea an the Skagerakk

16 Result/Conclusions The existing methodology for external events (NOG Safety and Environment Methodology for analysis of some external events) can be used as a base. Some questions regarding the methodology may need to be further analysed. The final results of the survey may indicate a need for additional safety provisions; either technical or organizational (such as procedures, human resources, emergency response organization and use of external resources).

17 For more information, please contact: Michael Knochenhauer President Scandpower AB T +46 (0) E mkn@scandpower.com W w