PSHA developments in low seismicity regions: Case history of the UK

1. PSHA developments in low seismicity regions: Case history of the UK RMW Musson

2. Outline • Part One – Early hazard studies in the UK for strategic facilities • Part Two – The evolution of “national” models • Part Three – Towards the future

3. PART ONE

4. Early NPPs in the UK • First operating civil reactors started in 1956 • Ten sites operating between 1956 and 1979 • None of these were designed against earthquakes; the UK was considered aseismic • Ironically, some sites were close to significant historical epicentres!

5. Early NPP sites

6. Ignorance is bliss

7. Kessock Bridge

8. The Great Glen Fault

9. Browitt, Burton & Lidster (1976) • It did: • Provide the first modern reassessment of historical British earthquakes from original sources • Query conventional wisdom concerning earthquakes and the GGF • It did not: • Assess parameters in a transparent way • Calculate hazard, except …

10. Burton & Browitt (1976) • Estimated 100 year event as ~ 5.0 mb • Selected some “comparable accelerograms” from Californian earthquakes

11. 1979 - Irving • First use of recognisable PSHA in UK for nuclear industry - influential • Assumed spatial uniformity of seismicity • “Average” hazard ~ 0.25 g at 10-4 ann. prob. • 1982 update – split UK into ten source zones on a purely geographical basis • “Without recourse to tectonic regionalization it can be concluded … that the rough regional boundaries … can be used to rank regions in order of their seismic density per unit area …”

12. Busy years: 1981-1984 • Around 1979-1980 it was realised that UK seismicity had been neglected • Last published UK earthquake catalogue was 1924! • Four major initiatives to revaluate historical earthquakes: • Principia Mechanica Ltd • Soil Mechanics Ltd • Imperial College • BGS • UK seismic monitoring capability expanded at the same time

13. NPP studies: 1980s • Recognisable PSHA studies for individual NPP sites began in early 1980s • Earliest studies undertaken by private consultants • Several of these combined into a consortium which they referred to as the Seismic Hazard Working Party (SHWP) • Majority of site-specific NPP PSHA studies done by SHWP in 1980s and 1990s • Others by BGS and Halcrow

14. SHWP practice • SHWP used a characteristic methodology: • Extensive background research • Conservative decision making • Exclusive use of Ms magnitudes • Single attenuation model • Simple geometric source models • Regional b values • Gamma distribution for activity rates • This never varied in line with developments elsewhere

15. Sample SHWP source model Redrawn from SHWP (1987)

16. Regulatory environment • Oversight given to what was then the Nuclear Installations Inspectorate (NII) now the Office for Nuclear Regulation (ONR) • NPP operators expected to submit a formal safety case to demonstrate that hazard at the site had been adequately characterised • This would be assessed as part of the periodic review of safety • Retrospective studies undertaken for pre-1980 build

17. Benchmarks • Hazard usually expressed as PGA with annual probability of 10-4 and associated response spectrum (usually fixed shape) • Conservatism expected • Some cognitive anchoring – since Irving, 0.25 g was regarded as “average”

18. Hydrocarbons • Offshore hydrocarbon exploration industry also became interested in seismic hazard after 1980 • Early site-specific hazard study for a shore facility in Scotland • Burton, McGonigle & Neilson (1981) • Hazard calculated by extreme value methods • Funding for expansion of UK monitoring network • Included two sea-bottom stations at Beryl and Statfjord

19. The buoy at Beryl (1980-1985)

20. Offshore PSHA (1992)

21. PART TWO

22. Evolution of PSHA models • Since the 1970s, there has been a discernible trend towards greater use of geological information • Generally considered difficult because of a lack of clear correlations between seismicity and surface geology • Earliest studies were purely based on seismicity

23. Seismicity vs geology

24. 1976 – Lilwall • Based on extreme values (Milne and Davenport 1969) • Therefore no tectonic input

25. Phases of deformation Argued by Muir Wood (1989) that seismicity relates to phases of Quaternary deformation

26. Arup (1993) model • Regional PSHA for UK • Developed logic tree with three sub models: • Uniform • Seismicity-based • Neotectonic (after Muir Wood)

27. GSHAP model

28. 2007 – Musson & Sargeant • UK national hazard map for Eurocode 8 • First attempt to create model starting with seismotectonic regionalisation • Based on seismotectonic model of Chadwick et al (1996) • Zones represent “the surface projections of subsurface volumes of characteristic upper crustal geological structure” • Seismogenic homogeneity maintained by subdividing or merging zones as needed

29. Chadwick model

30. Derived source zone model

31. Latest evolution - SHARE

32. PART THREE

33. A new start for nuclear • No new nuclear build in the UK after 1995 • Government decision to resume NPP construction taken in 2006 • First stage was site selection – mostly existing sites • Seismicity not an exclusionary criterion • New PSHA studies required - no question of returning to old hazard methodologies

34. New regulation? • Regulatory environment unchanged • ONR committed to maintaining standards • Has appointed a standing panel of experts to assess cases • “The UK system … has a goal‐setting approach in which safety objectives and outcomes are set out through legislation, but [the] operators of the plant are responsible for identifying the specific technical measures and procedures necessary to meet these objectives.” • i.e. NOT a prescriptive approach

35. Lessons from Japan • ONR report at http://www.hse.gov.uk/nuclear/fukushima/final-report.pdf • “Our considerations of the events in Japan, and the possible lessons for the UK, has not revealed any significant weaknesses in the UK nuclear licensing regime.”

36. Stress testing • In response to a request from the Council of the European Union, a specification for stress tests for nuclear power stations was developed, and the ONR directed that these test should be undertaken by the nuclear industry in the UK.

37. STF-2 • Stress Test Finding STF-2 states that “The nuclear industry should establish a research programme to review the Seismic Hazard Working Party (SHWP) methodology against the latest approaches … • … This should include a gap analysis comparing the SHWP methodology with more recent approaches such as those developed by the Senior Seismic Hazard Analysis Committee (SSHAC)” • Work in progress!

38. The End