Summary of OECD/NEA Status Report on Filtered Containment Venting

Transcription

1 Summary of OECD/NEA Status Report on Filtered Containment Venting Technical Meeting on Severe Accident Mitigation through Improvements in Filtered Containment Venting for Water Cooled Reactors Vienna, Austria, 31 August 3 September 2015 T. Nitheanandan Canadian Nuclear Laboratories

2 Content Introduction Objectives of FCVS as SAMM FCVS Safety Requirements Venting strategies and procedures Main FCVS design requirements Filtration efficiency (required or target) Existing filtration technologies/implementation Current Implementation in OECD Countries Risks related to FCVS Potential improvements of FCVS Conclusion Acknowledgement 2/22

3 Introduction In May 1988, CSNI organised an experts meeting in Paris to facilitate international exchanges on Filtered Containment Venting System (FCVS) concepts, research and development, and their analysis. The proceedings NEA/CSNI/R(1988)148 was published in the same year The basic premise of FCVS is that a catastrophic failure of the containment structure can be avoided by discharging steam, air and non-condensable gases like hydrogen to the atmosphere provided, filtering systems are implemented on the vent lines to reduce significant radioactive releases to the environment The FCVS concept is already integrated in some Nuclear Power Plants 3/22

4 Introduction Post Fukushima stress tests led many countries to consider the implementation of Filtered Containment Venting Systems to prevent significant radiological releases as part of Severe Accident (SA) management measures Some countries consider improving existing systems to enhance robustness to hazards and H 2 risk, safe use and efficient filtration for prolonged or repetitive use during SAs Recent advances in research and technology can be used to design new FCVS that can deal with more challenging conditions than the current designs Post-Fukushima OECD/NEA action resulted in a CSNI Report NEA/CSNI/R(2014)7 published in July /22

5 Objectives of FCVS as SAMM Preventing containment overpressure failure as an additional line of defence using controlled venting through a filtration system and maintaining the containment function until stable conditions are reached, no matter how severe the accident Limiting radioactive releases to the environment Protect on-site workers and the public while minimizing the extent of land contamination Reduce the burden on on-site and off-site emergency response Venting can also remove decay heat more effectively than other methods of heat removal in smaller BWR containments In some cases, timely venting of the containment can reduce H 2 risk 5/22

6 FCVS Safety Requirements FCVS is not explicitly mandated and not considered as the primary measure to prevent containment pressurization Releases can be prevented either by isolating the containment or filtering the discharge - at least one of the 2 strategies to be used The containment shall be protected from overpressure and off-site releases should be minimized in a SA In some cases, unfiltered and uncontrolled releases from the containment to be precluded or if venting is provisioned as SAMM, the system has to be provided with efficient filtration Alternative approach: SA capable venting systems without prescribed filtration (such as Hardened Containment Venting System), notably wet-well venting for some BWRs 6/22

7 FCVS Safety Requirements (Cont d) In some cases, prescribed delay time for venting (e.g., > 1 day) In some cases, vent capacity (decay heat removal, pressure decrease rate) and opening and closing pressures are prescribed In some cases, maximum extent of land contamination is prescribed: frequency of accidents with releases exceeding Bq of 137 Cs to be < or releases to be < 0.1% of 134 Cs and 137 Cs core inventory or contamination to be limited to a few tens of km 2 In some cases, compatibility with protection measures for the population is prescribed (in conjunction with delayed venting) 7/22

8 FCVS Safety Requirements (Cont d) In some cases, compatibility with plant worker exposure limits are prescribed In some cases, FCVS Decontamination Factors (DFs) are prescribed: usually set to > 1,000 for radioactive aerosols, > 100 for molecular iodine In some cases, resistance to hazards is considered 8/22

9 Venting strategies and procedures Timing is critical, and it is dependent on reactor type and design, accident progression, venting objectives, guided by SAMG decisions taken by competent authorities As little as a few hours for small containments (some BWRs) Usually, a day or more for larger containments (some BWRs, PWRs, PHWRs) Pressure: vent opening and closing pressures based on containment design pressure and known margins to containment failure Opening/cycling: multiple or long venting considered as long as no other cooling system is recovered Passive or active actuation: line to be closable, to be opened and closed certainly and without difficulty 9/22

10 Venting strategies and procedures (Cont d) Operation: guided by controls and instrumentation Fast accident progression Early venting as SAMM Sudden pressure increase (HP RPV failure, energetic events) Early venting 1 day Wait for more favorable conditions (in and out of containment) Coordinate with on and offsite emergency response Delayed venting Pressure approaching containment failure limits 10/22

11 Main FCVS design requirements Should allow reliable containment decay heat transfer and pressure decrease (for SA thermalhydraulic conditions including energetic events) Pipes, supports, valves shall be designed for safe and reliable operation and for dynamic loads Sufficient aerosols (tens to several 100 kg), gaseous iodine (few g to tens of g) and heat load (a few to hundreds of kw) capacities should be set (vent timing and duration critical) and avoid FCVS shared by two units Autonomy time to cope with prolonged or multiple uses: No maintenance for at least 24 h for liquid type filters (provisions to be made for 72 h) Minimize the risk of potential re-vaporization or re-volatilization of filtered contaminants of compounds such as cesium and iodine 11/22

12 Main FCVS design requirements (Cont d) Use of high temperature and radiation resistance materials, such as metallic or ceramic materials Improve combustible gas safety, particularly: Exclude combustion propagation from the containment Reduce the hydrogen ignition probability by ensuring high selfignition threshold temperature inside the system Pre-heat FCVS before the initiation of the vent operation to minimize risk of reaching deflagrable hydrogen concentration due to steam condensation Investigate whether pressure boundary design (piping and housings) can cope with the H 2 combustion loads 12/22

13 Main FCVS design requirements (Cont d) System maintenance (liquid systems) and operation to be done safely during SAs Robustness to hazards such as against site specific earthquake conditions, internal or external flooding, extreme temperatures at the NPP, partial or total loss of DC and AC power, loss of plant pressurized air/nitrogen supply, loss of plant demineralised or normal water supply, projectiles generated by wind, risks due to falling aircraft, etc. 13/22

14 Filtration efficiency - required or target Decontamination factors > 1,000 for radioactive aerosols including small size and hygroscopic aerosols (10,000 may be desirable for early venting with high loads or to further reduce long-term land contamination) > 100 for molecular iodine > 10 for organic iodides Gaseous ruthenium and noble gases filtration issue to be assessed more thoroughly 14/22

15 Filtration efficiency - required or target (Cont d) Filtration technologies Existing technologies allow reaching such DFs for aerosols and molecular iodine under prescribed conditions Extensive qualification already exist for postulated SA conditions (ACE tests) Less knowledge exists for organic iodides and ruthenium retention Filtration efficiencies for more challenging conditions (high loads, long-term operation, energetic events) need to be further assessed (re-volatilization, re-vaporization) 15/22

16 Existing filtration technologies/implementation Solid filtration: metal fibre filter, sand bed filter (coupled with metal fibre filters) Liquid filtration: chemically-doped pool scrubber for gaseous iodine retention and heat sink, usually coupled with a droplet separator and deep-bed fine aerosol filter Pre-filtration in some cases (e.g., wet-well scrubbing, pre-filters inside or outside containment), in-containment filtration limits radioactivity transfer and on-site worker exposure Some flexibility in implementation (depending on systems): inside or outside the containment (or partly), possibility to add filtration stages Extensive qualifications performed (ACE tests), some still required 16/22

17 Technologies/implementation Metal Fibre Filter Scrubbers CCI Scrubbers MVSS Example of Dry Filter Method implementation 17/22

18 Current Implementation in OECD Countries Country NPPs no FCVS Metal + FILTRA Comments HSSPV DFM SULZER CCI EFADS sand bed MVSS Implementation of HSSPV+ planned in 5 units between , under evaluation for Belgium 7 PWR remaining two units which were just granted operation beyond 2015 Brazil 3 PWR Bulgaria 2 VVER 1000 Canada Czech Republic Finland 19 PHWR Single unit (1) Multi units (18) no FCVS planned but design not yet selected installed planned HSSPV for Point Lepreau single unit, DFM planned in 4 Darlington units 4 EFADS shared among 18 reactor units, EFADS are designed for DBA 4 VVER 440 No FCVS planned 2 VVER 1000 Under assessment in conjunction with SAMM for corium cooling (in or ex vessel) 2 VVER 440 FCVS is not feasible in VVER 440 due to the steel shell of containment that is vulnerable 2 BWR to sub atmospheric pressures; EPR plant under construction will be equipped with FCVS France 58 PWR Metal pre filter inside containment, sand bed filter outside containment 7 PWR Germany DFM in 2 PWR units 2 BWR 24 PWR PWRs: FCVS planned as Specialized Safety Facility (no design decided yet) Japan HSSPV and other scrubber system planned for BWRs. Additional FCVS planned as 26 BWR Specialized Safety Facility Mexico 2 BWR Hardened CVS from wet and dry well (Mark II) under implementation Netherlands 1 PWR Romania 2 PHWR HSSPVplanned Slovakia 4 VVER 400 Under assessment with other SAMM 6 VVER 440 Under assessment for some VVERs after Fukushima. Not considered for other reactors Russia 11 VVER 1000 (RBMK, ) Slovenia 1 PWR DFM installed in 2013 in Krško NPP, used in passive mode with rupture disk South Korea Spain Sweden Switzerland Taiwan Ukraine USA 19 PWR Only Wolsong 1 Unit equipped with HSSPV. Other units are planned to be equipped 4 PHWR with FCVS by 2018, development of a Korean system under consideration 6 PWR PWR: FCVS implementation is planned by BWR BWR: Hardened venting available. Filter implementation is planned in PWR 7 BWR 3 PWR HSSPV type at Gösgen 2 BWR F MVSS type at Mühleberg 2 PWR 4 BWR 2 VVER VVER PWR Preparation of guidance documents on Hardened CVS for BWR Mark I & II. 35 BWR Implementation by 2018 or earlier 18/22

19 Risks related to FCVS Improper venting time: too late (following other containment failures) or too early (unintended release of radioactivity) De-inerting by gas back flow into the containment (e.g., vent closure failure), conducive to H 2 combustion H 2 combustion risk in FCVS: steam condensation (heating failure) system not designed to withstand combustion loads reflux from other systems Failure of closure: more risk with automatic termination, manual termination requires accessibility Early and long-term venting challenging for filter loadings Containment-impairment risk and threat to other systems during earthquake 19/22

20 Potential improvements of FCVS Venting strategies and operations: Development of realistic assessment of containment behaviour under SA conditions (including for energetic events) Redundancy and protection of opening and closure of the systems Limitation of H 2 combustion risks (hardened heating or inerting, isolated stack to chimney exhaust, design to withstand combustion loadings) Pre-filtration in the containment Filtration reliability for challenging conditions (high dose and thermal loads, energetic events) and for all contributors to releases Avoiding FCVS shared between two or more units Designing systems to be robust to hazards (earthquake) Developing robust instrumentation to guide optimized FCVS use 20/22

21 Conclusion OECD/NEA reference Report to serve as a guide for FCVS implementation: FCVS recognized as important, valuable alternative SAM for preservation of the containment and reduction of radiological impact (ease emergency response, reduce radiological costs and health effects, increase societal acceptability) FCVS are to be used in conjunction with other SAM strategies Desirable DFs reachable with existing simple and robust technologies but, safe and reliable filtration for challenging conditions to be established: on-going national and international research on filtration technologies (e.g., PASSAM, ) Reference source term and instrumentation could be developed further to guide venting strategies and emergency response 21/22

22 Acknowledgments All contributors to the OECD FCVS working group Belgium (BEL V, GDF-SUEZ, Westinghouse), Canada (CNL, CNSC), Czech Republic (UJV), EC, Finland (STUK), France (EDF, IRSN), Germany (AREVA, GRS, RWTH-Aachen), Japan (JNES, JAEA), Korea (KAERI), Mexico (CNSNS), Russian Federation (FSUE VO Safety), Slovakia (UJD), Slovenia (NSA), Spain (CIEMAT, IBERDROLA), Sweden (SSM), Switzerland (ENSI, IMI Nuclear, PSI), USA (USNRC) OECD/NEA Secretariat 22/22