Severe Accident Management at the Loviisa NPP - Application of - - PowerPoint PPT Presentation

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Severe Accident Management at the Loviisa NPP - Application of - - PowerPoint PPT Presentation

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Severe Accident Management at the Loviisa NPP - Application of Integrated ROAAM and PSA level 2 Satu Siltanen, Tomi Routamo, Petra Lundstrm and Harri Tuomisto Fortum Nuclear Services Ltd. Workshop on Evaluation of Uncertainties In Relation To

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 1

Severe Accident Management at the Loviisa NPP - Application of Integrated ROAAM and PSA level 2

Satu Siltanen, Tomi Routamo, Petra Lundström and Harri Tuomisto Fortum Nuclear Services Ltd.

Workshop on Evaluation of Uncertainties In Relation To Severe Accidents and Level 2 Probabilistic Safety Analysis, Hotel Aquabella, Aix-En-Provence (France) 7-9 November 2005

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 2

Overview

  • How Integrated ROAAM has been the key element when

constructing well balanced SAM strategy for Loviisa NPP and how the use of Integrated ROAAM analysis has affected level 2 PSA studies

  • PSA and ROAAM - advantages in combining two approaches
  • ROAAM - credible treatment of phenomenological uncertainties
  • Integrated ROAAM - balance between accident prevention and

mitigation of containment threatening phenomena

  • Going further than ROAAM - Level 2 PSA source term calculations
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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 3

Contents

  • SAM strategy of Loviisa NPP

– unique features of the plant – main aspects of Integrated ROAAM approach – phenomenological uncertainties using ROAAM (shortly) – overall SAM approach, SAM safety functions

  • Level 2 PSA

– influence of the Integrated ROAAM study

  • Summary and conclusions
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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 4

Loviisa NPP - unique features

  • VVER-440 with ice condenser

containment – steel shell – design pressure 1.7 bar (ultimate failure pressure well above 3 bar) – low leakage (0.2 %/d) – 57 000 m3 – 835 000 kg ice

  • Low decay power level
  • Narrow cavity, no penetrations in

the RPV lower head

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 5

Integrated ROAAM approach - safety goal

"Containment failure at the plant is a physically unreasonable Containment failure at the plant is a physically unreasonable event for any accident sequence that is not remote and event for any accident sequence that is not remote and speculative" speculative" – at a sufficient level of evidence to the contrary, certain postulated events can be considered "physically unreasonable" (safety goal component focused on physics) – at a certain high level of reliability, system failure can be considered as "remote and speculative" (equipment/system based component of safety goal)

  • Sufficient measures have to be taken at the plant to ensure that

high consequence hazards can't happen

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 6

Integrated ROAAM - implementation

  • complete system analysis (level 1 PSA)
  • "safeguards tree" (an event tree that assesses the availability of

safeguards systems)

  • use of a screening frequency (determines which accident classes

can be considered as "remote and speculative")

  • showing that containment failure is "physically unreasonable" or if

this is not the case, finding the measures to achieve this goal (ROAAM in issue resolution context)

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 7

1E-7 1E-6 1E-5 1E-4 1E-3 1E-3 1E-2 1E-1 1E+0

Conditional containment failure probability Core damage frequency 1/r-yr Mitigation window Prevention regime Remote and speculative sequences

SAM not ok SAM ok SAM ok

Input from PSA level 1

SAM not ok

For Loviisa NPP

  • screening

frequency 10-6 1/r-yr

  • design target for

failure of each safeguards function < 10-2 / demand

The idea of Integrated ROAAM

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 8

Phenomenological uncertainties using ROAAM

  • Understanding the phenomena underlying physics (careful studies

involving modelling and experimental verification) The key elements of ROAAM: – Physically-based decomposition that allows transparency in separating out the essential portions of epistemic uncertainty – Probabilistic framework (causal relations and intangible parameters – Causal relations (key physics) represent well-posed

  • problems. Uncertainty can be reduced to the parameter level

(no major modelling uncertainty) – "Splinter" scenarios in combination with conservative estimates of epistemic uncertainty, so as to obtain convincingly conservative results

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 9

ROAAM - studies in Loviisa NPP

IVR:

– Extensive research program (analytical and experimental studies) for demostrating coolability of corium on the lower head of the reactor pressure vessel – RPV failure can be considered as "physically unreasonable" situation when requirements (depressurization, lowerin of RPV lower head insulation and neutron blocks) are fullfilled

Hydrogen management

– Significant experimental and analytical efforts – Current strategy (forcing open ice condenser doors, passive catalytic recombination and deliberate ignition) entailed significant plant modifications – Containment failure "physically unreasonable" with the new hydrogen management strategy in place

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 10

Overall SAM approach in Loviisa NPP

  • Prevention of core damage and sequences with imminent threat
  • f large radioactive releases and mitigation of containment

threatening phenomena along the lines of Integrated ROAAM

  • SAM safety functions

– successful containment isolation – primary system depressurization – mitigation of hydrogen combustions (absence of energetic events) – cooling of reactor core or core debris – mitigation of slow overpressurization

  • Hardware and I&C modifications, SAM guidelines, SAM

handbook, revision of emergency prepardeness organization, versatile training approaches

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 11

Level 2 PSA

  • Status of work

– at the moment internal hazards fot at-power states for unit 1, – weather and internal flood hazards (under work, finished by the end

  • f next year)

– internal hazards extension for normal refuelling shutdown states. – Fire risks (after extensive I&C renewal has proceeded far enough)

  • Simplification of CET due to ROAAM (number of CET end states

reduced)

  • The usage of ROAAM in development of the SAM strategy

assures that phenomenological uncertainties are covered well during simplification process

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 12

CET

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 13

Level 2 PSA - source term evaluation

  • The key issue in reducing environmental releases is to ensure

containment integrity (main goal of the ROAAM approach as well)

  • further than ROAAM - issues related to source term evaluation
  • Fortum has developed own spreadsheet based tool for source

term analyses – Loviisa specific – issues essential considering SAM strategy – source terms for each accident progression category – reduced amount of sequence-specific input data – wide uncertainty analyses of buil-in parameters carried out as Monte Carlo simulations (specific time dependent confidence levels of release fractions for each fission product group)

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 14

Example of source term uncertainty result

Caesium release into environment

1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 1E-2 1E-1 1E+0 0 h 10 h 20 h 30 h

5 % 25 % 50 % 75 % 95 % 99.5 %

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 15

Level 2 PSA

  • Testing the SAM strategy
  • containment sequences can be handled well (fraction of

sequences with contaiment failure is small)

  • specific problem with containment bypass sequences

– may (or may not) as such to lead large releases – not enough water for cavity flooding – cavity failure and significat environmental releases – studies of aerosol retention in the bypass route – still further actions to either bring down frequency of bypass sequences or modify EOPs to ensure flooded cavity

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Satu Siltanen / Fortum Nuclear Services Ltd. 8.11.2005 16

Summary and conclusion

  • Fortum sees significant advantages in combining Intergrated ROAAM

and PSA when studying severe accidents

– PSA determines the relevat sequences to be mitigated in the SAM approach and is therefore strongly integrated in the ROAAM work – ROAAM increases the credibility an transparency of the level 2 PSA study (significant simplification of CET) – With PSA level 2 also source term behaviour is to be analyzed (extremely important when dealing with containment bypass sequences)

  • ROAAM has significant advantages in dealing with phenomenological

uncertainties, especially in the mitigation regime. PSA is the obvious choice in the preventive regime

  • Integrated ROAAM captures the idea behind Finnish safety goal in an

ideal way - with Integrated ROAAM a sound balance between preventive and mitigative measures can be found