Dr. Martin Sonnenkalb & Dr. Manfred Mertins GRS Cologne. Severe Accident Mitigation in German NPP - Status and Future Activities -

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1 Dr. Martin Sonnenkalb & Dr. Manfred Mertins GRS Cologne Severe Accident Mitigation in German NPP - Status and Future Activities -

2 Content History and status of implementation of Severe Accident Management Programs (SAMP) Consequences on SAMP based on Fukushima accident New GRS research activities after Fukushima accident

3 Consideration of Severe Accidents in current NPPs Severe Accident Management Programs (SAMP) Consideration of Beyond Design Basis Accidents SAMP typically comprises: SAM Measures, hardware modifications/additions & SAM Guidance for crisis teams Differentiation between preventive and mitigative Measures & Guidance Significant differences in realized concepts worldwide Goals of SAMP (IAEA Safety Standard, NS-G-2.15, 2009) To prevent the escalation of the event into a severe accident To mitigate the consequences of a severe accident To achieve a long term safe (controlled) stable state

4 History of Accident Management Development SAM Program developm. - US NRC, EPRI, OGs since 88 NRC-Policy defined in SECY Individual Plant Examination (IPE) Containment Function Improvement IPE of External Events (IPEEE) Development of Severe Accident Management Guidance Consideration of Beyond Design Basis Accidents, but leave NPP as it is - without significant hardware modifications SAMG development based on EPRI Technical Basis Report from 93 (severe accident phenomena, plant deficits, ) Implementation of SAMG/AMP until 98 by OG specific SAMG concepts: WOG, CEOG, B&WOG, BWROG 1986 Chernobyl 1999 SAM development & 2010 implementation 2011 Fukushima

5 History of Accident Management Development SAM Program development - Germany since 86 No formal requirements in licensing and supervision domain Utilities offered to voluntarily realize recommendations on SAM of the German Reactor Safety Commission (RSK) Focus on Prevention of Core Damage with selected Mitigative Measures against well-known phenomena SAMP includes hardware modifications & EOP/SAM Procedures No systematic development of SAM Guidance Decisions on SAM measures have been made in each case individually after intensive discussions in RSK (no criteria) Responsibility for SAMP implementation -> local government 1986 Chernobyl 1999 SAM development & 2010 implementation 2011 Fukushima

6 NPPs: Implemented SAMP of German NPPs - PWR 3 different generations 4 out of operation & 7 in operation (up to 1450 MWe) Large dry containment Selected SAM measures: Bleed & Feed measures Mobile Pumps for SG injection Secured containment isolation Additional power supply Filtered venting PAR system (~60 pieces)

7 Implemented SAMP of German NPPs BWR type 72 NPPs: 2 units with 1300 MWe each Concrete containm. with steel liner Spray system in drywell (not safety relevant) Selected SAM measures: Diverse RPV Depress. & Inj. Syst. Mobile pumps for RPV injection Secured containment isolation Additional power supply Filtered venting (common system) N 2 inertisation of wetwell & PARs in drywell

8 Implemented SAMP of German NPPs BWR type 69 NPPs: 4 NPPs, 2 different generations: 900 MWe or 1300 MWe; all out of operation since spring 2011 Steel containment with spray in drywell (not safety relevant) Selected SAM measures: Diverse RPV Depress. & Injection System Mobile pumps for RPV injection Secured cont. isolation Additional power supply Filtered venting N 2 inertisation of drywell & wetwell

9 Approach used for German PAR Concept RV SRV Triple Loop Reactor Single Loop Pressurizer + Tank 11 Steam EBS -Spray MCP EBS FW 3x SIS VCS 3x Accu's VCS -Spray Steam Collector RPV Failure Accu SIS SRV MCP RV Steam 11 Spray 75 Node Reactor Core FW EBS -Spray VCS -Spray Heater EBS - Extra Borating System VCS - Volume Control System SIS - HP+LP Safety Injection System Burst Disks PORV, SRV1+2 4 different scenarios selected in 98 Integral code analyses to provide source data for containment/cocosys COCOSYS detailed containment analyses for set-up of PAR concept Implementation in all NPPs completed PAR-model re-evaluation (OECD-THAI) & repetition & 2 new scenarios 10/11 Dom * - PAR Position Air System Environment 3 * * * * Venting Filter Environment 1 * FE-Basin * Environment 2 Periph. A * * * * * SG Comp. SG Comp. source data MCP MCP PC Comp. * PC * Comp. * * 1 * Cavity Cavity * Annulus Auxiliary Building

10 Implementation of Filtered Venting in German NPPs Goals Prevent containment over-pressure failure Controlled release of N 2 /H 2 /steam mixture (BWR) Limited radio nuclide releases Implemented in late 80 in most NPPs (some NPPs use metal fiber filters instead) System configuration Connected to BWR wetwell and PWR containment periphery Separate off-gas pipe in all BWRs and some PWRs into the environment Retention factors: aerosols %, element. iodine 99%, organic iodine >60% Interrupted/repeated system use possible No H 2 -combustion within the system Fig. Sliding pressure venting system with venturi scrubber

11 Organization of Accident Management at German NPPs Shift Crisis Team

12 Ongoing Development related to SAMP Implementation of SAMP measures in German NPPs is/was almost completed (see paper & CNS reports of Germany) New German Legal Basis Proposal of new Safety Criteria for Nuclear Power Plants - project by GRS on behalf of BMU between Upgrading of old German regulations for all safety levels of defence in depth concept especially for safety level 4 (severe accidents) extension of requirements on SAM (extension of preventive measures, development and implementation of SAMG) Test of applicability of Revision D between 2008 and 2009 Final preparation of new German Safety Criteria for Nuclear Power Plants, Revision E until end of 2011

13 Future Development Needs related to SAMP Fukushima Accident identified possible weak points National Stress Test performed (March 17 - May 15, 2011) Recommendations of RSK related to SAMP Further development of the accident management concept under external hazard conditions (re-establishment of the supply of threephase alternating current, injection possibilities for the cooling of fuel assemblies, identification of available safety margins, consideration of wet storage of fuel assemblies, etc.) Supplementation of the requirements on accident management by a systematic development of SAMG Optimization of available measures Basics of current SAM concept are still valid RSK recommendation published in 97 and updated in 10 European Stress Test ongoing

14 New Topics related to SAMP Fukushima Accident identified possible weak points : Systematic inclusion of internal/external hazards into SAMP Needed re-check/extension of SAMP measures (also in connection with external hazards): long-term energy supply (mobile systems?) long-term heat removal from reactor core and spent fuel pool long-term heat removal from wetwell of a BWR safe release of off-gases containing combustible gases by a filtered containment venting system use of SAM Measures under long-term SBO conditions development of SAM Measures to protect the building structure surrounding the spent fuel pool in a BWR, which is outside the containment, against hydrogen combustions or to prevent them

15 Extended Requirements on SAMP Extended Requirements based on IAEA SSR 2/1: Application of requirements for Gen. III+ to SAMP of current NPPs A set of design extension conditions (severe accidents) shall be derived on the basis of engineering judgment, deterministic assessments and probabilistic assessments for the purpose of further improving the safety of the nuclear power plant -> enhancing the plant s capabilities to withstand without unacceptable radiological consequences. The plant shall be designed (incl. SAMP) so that it can be brought into a controlled state and the containment function can be maintained, with the result that significant radioactive releases would be practically eliminated* ) if not only protective measures that are of limited scope in terms of area and time shall be necessary for the protection of the public, and sufficient time shall be available to implement these measures. * ) - practically eliminated = a phenomenon is physically impossible or its occurrence is extremely unlikely with a high level of confidence

16 New GRS research activities* ) after Fukushima Spent fuel pool accidents Test of applicability of codes ASTEC & MELCOR Phenomena of interest: radiolysis, conditions for criticality, oxidation of fuel cladding and canisters of BWR fuel assemblies, conditions and progression of fuel assembly degradation Analyses of severe accident scenarios similar to Fukushima Application of codes ATHLET-CD & COCOSYS coupled Analysis of Fukushima accident based phenomena related to German NPPs including application of existing SAM measures Determination of SAMP improvement needs Determination of future code improvement needs * ) sponsored by BMWi

17 New GRS research activities* ) after Fukushima Improvement of codes used for BWR inventory calculation Core inventory calculation for different points in fuel cycle Consideration of higher burn-up and operational conditions Containment structural mechanics analyses Integrity of the containment under dynamic loads Integrity of the containment under the assumption of multiple earth quakes within a given time-frame * ) sponsored by BMWi

18 Summary & future Steps Existing SAMP in Germany was developed in the 90 SAMP consists of significant features -> preventive and selected mitigative measures (e.g. power supply, PARs or inertisation, filtered containment venting Completion of European Stress Test & Definition of findings and NPP improvement needs Completion of new German legal basis: Safety Criteria for Nuclear Power Plants, Revision E Summarizing of Lessons Learned from Fukushima accident (BMWi & BMU projects) Discussion on needed/possible updates of NPPs related to SAMP and other aspects (BMU project)