Design Safety Improvements at Swiss BWRs after Fukushima

Transcription

1 Design Safety Improvements at Swiss BWRs after Fukushima IAEA Technical Meeting on Evaluation of NPP Safety Design in the Aftermath of the Fukushima - Daiichi Accident IAEA Technical Meeting, Vienna August 2013 Willem van Doesburg BKW / Switzerland

2 Content Existing plant safety features and post-fukushima improvement targets Evaluation of design safety and resulting plant improvements (carried out + planned) for the Mühleberg NPP (KKM) Leibstadt NPP (KKL) 2

3 Plant improvements Swiss BWRs Beznau 1 (KKB1) PWR 365 MWe Dez 69 Beznau 2 (KKB2) PWR 365 MWe Mar 72 Mühleberg (KKM) BWR 373 MWe Nov 72 Gösgen (KKG) PWR 970 MWe Nov 79 Leibstadt (KKL) BWR 1165 MWe Dec 84 3

4 Existing Safety Features All Swiss NPPs have Special Emergency (SE) heat removal systems (KKL: SEHR / KKM: SUSAN) additional to the normal ECCS and heat removal systems These special emergency systems provide alternative cooling and decay-heat removal with autarky > 10h The systems are bunkered, designed to withstand APC, SSE, flooding as well as acts of sabotage The systems constitute an appropriate countermeasure to a severe natural event including long-term T-SBO All CH-NPP have Filtered Containment Depressurization (Venting) Systems with adequate retainment factors for I-131 and Cs-134/137 4

5 Swiss AM development timeline 5

6 Post-Fukushima Safety Targets 6 Assure robustness to withstand and cope with any severe Swiss natural hazard ( yearly extreme weather, flooding, earthquake events or combinations) Assure control of T-LOOP during minimum 72 h without loosing safe shutdown condition Assure additional SAM-hardware and necessary auxiliary material as well as SAM staff preparedness (availability, level of training, procedures) so that a BDB event (with T-SBO) can be controlled during minimum 72 h without challenging safe shutdown condition a BDB event - if safe shutdown is not possible - will be limited to the lowest consequential level: avoid major core damage avoid RPV-damage avoid H 2 -explosion avoid major radiological releases to the environment Assure safety of long-term fuel pool cooling

7 Improvements after Fukushima accident Made on the basis of: Original Licensing Requirements Experiences Fukushima Additional ENSI Requirements European Stress Test Results Own initiatives 7

8 Mühleberg NPP (KKM) GE - BWR4 design, MK1 containment Original plant design similar to the Fukushima- Daiichi NPPs Various design extensions (e.g. outer torus, drywell spray and flooding system, 2 extra PRVs, emergency control room, alternative scram/shutdown system) Hilltop water reservoir ( 300 m 3, not classified) Special emergency cooling/heat removal (bunkered) systems, 2 redundant trains with one DG each, classified Filtered containment venting system via outer torus both passive (rupture disc) and active 8

9 KKM Post-Fukushima conformity evaluations 9 Evaluation of coolant supply to safety and auxiliary systems Evaluation (and implementation) of site specific and of central external storage depot for emergency tools and equipment Spent fuel pool cooling evaluations for possible improvement measures Upgrade auxiliary DG for supplying power to 1 SUSAN train Depot near site for mobile equipment (DGs, pumps, etc.) Deterministic evaluation of response to yearly flooding event Deterministic evaluation of response to yearly seismic event Deterministic evaluation of combination of seismic and flooding event (dam break) Development of explicit feed&bleed procedures (multiple paths) and other AMM Evaluation of protection against H-deflagration Evaluation of possibility for diverse water supply Evaluations for extreme weather conditions (wind, rain, temperature, lightning, tornados..)

10 KKM measures taken / planned 10 90cm high protection walls (protect up to level of yearly precipitation event) Retrofitting three additional water intake pipes ( periscopes ) in the SUSAN SE water inlet structure Provision of additional injection option (inlet shaft) in the SUSAN SE water inlet structure, and portable pumps for injecting water 2 alternative SAM - injection feeds to the fuel storage pool Diverse water supply for the SUSAN SE system Special emergency cooling system for spent fuel pool Additional reactor coolant & decay heat removel system Seismic stabilization of the embankment dam upstream of KKM

11 KKM: mobile flood protection walls Mobile flood protection equipment of the service pump building - outside and inside the building. entire mounting time approx. 3 h. 11

12 KKM: debris safe suction pipes for SUSAN flow direction river Aare river suction pipe (periscope) protection poles max. height for drift of stones, mud and sand SUSAN-intake river bed Normal Operation: outlet of hot main cooling water 12 2 suction pipes installed, each with 3 protection poles

13 KKM: debris safe suction pipes (cont d) Protection Poles 13 before installation after installation outage 2011

14 strainers KKM: additional water inlet shaft for SUSAN Installed during outage 2011 Aare river water level Special bunkered emergency heat removal system (SUSAN) building 14 SUSAN water intake structure

15 KKM: 2 alternative SAM - injection feeds to the fuel storage pool (outage 2011) SFP SFP 15

16 KKM: 2 alternative SAM - injection feeds to the fuel storage pool (cont d) 2 redundant injection conduits Penetrations (2x DN80) made in the wall of the reactor building during outage 2011 System ready for use as of 31 March

17 KKM: (special) emergency power equipment, firmly installed Hydro-Electric Plant (HEP) near KKM: two 16kV lines, continuously on stand-by Emergency DG (1800 kva), air cooled Special Emergency system (SUSAN): two redundant strains, DG (800 kva) + battery unit for each strain Auxiliary DG (on the roof of the SUSAN building, i.e. seismic-proof), for recharging of SUSAN batteries Upgraded (2012), now 1000 kva DG installed Diesel fuel and oil tanks 17

18 KKM: new auxiliary 1000 kva DG 18

19 KKM, mobile and other equipment 3 mobile DGs (10 kva) Pumps (2 x 1800 l/min, 2 x 10000l /min) and various smaller pumps Hoses / flexible tubes (up to 5 km length), connection materials All-terrain vehicle with crane Boric acid, diesel fuel + oil supplies 19 Mostly available at net control center - location < 1 km from site Further supplies available at joint central storage location (Reitnau)

20 KKM: tools / equipment (1): large pump 20

21 KKM: tools / equipment (2) 21

22 KKM: tools / equipment (3): 3 mobile DGs 22

23 KKM: tools / equipment (4): mobile diesel oil tanks 23

24 KKM: new system for level and temperature measurement in SFP 24 Based on regulatory (ENSI) requirement for accident resistant measurement system Improve existing level measurement New level measurement: hydrostatic pressure difference (air bubbles tube) Redundant temperature measurement: Pt-100 (Pt resistance thermometer) ENSI-approval for implementation recently obtained

25 KKM plan: diverse water supply SSE-safe water intake from groundwater river Saane SSE-safe underground channel Connect to the SUSAN- SE system building 25

26 KKM plan: Seismic Stabilization of the Embankment Dam upstream of KKM 26 Will provide additional margin against yearly seismic event Final plans submitted, awaiting approval

27 KKM plan: additional SSE-safe Emergency Fuel Pool Cooling System Reaktorgebäude Sekundärcontainment EPCS PRV SRV SV PRV Primärcontainment 3x EPCS 2x 3x 2x 2x Aare SUSAN Dieselgeneratoren Äusserer Torus TCS Äusserer Torus ICWS CWS 27 ALPS Niederdruckeinspeisesystem SRV Sicherheits- und Abblaseventile EPCS BEB-Kühlsystem PRV Druckentlastungsventile SUSAN Spezielles unabhängiges System Aare CWS Kühlwassersystem zur Abfuhr der Nachzerfallswärme TCS Toruskühlsystem

28 KKM plan: additional coolant injection and heat removal system (1) 28 Issue: all emergency coolant injection systems presently located in the basement of the Reactor Building (-11 m level) Postulated BDB flooding of the RB basement would incapacitate emergency coolant injection Possible solution: installation of ALTERNATE COOLANT INJECTION SYSTEM (ACIS) ACIS capability equivalent to existing ALPS (low pressure injection) ACIS to be located at higher elevation Design for AMM

29 KKM plan: additional coolant injection and heat removal system (2) 29

30 Leibstadt NPP (KKL) GE BWR6 design, MK3 containment Original plant design with design extensions: Special emergency cooling/heat removal (bunkered) system (SEHR), 2 redundant trains with water supply from deep groundwater spring and one DG for each train Filtered containment venting system 30

31 Leibstadt NPP (KKL) EU Stress Test - results: even without measures taken after the Fukushima accident KKL has a high degree of design provisions against DB accidents and a remarkable high level of SAM provisions KKL demonstrates high safety margins against all BDB external and internal accident initiators, especially in a T-SBO case 31

32 Leibstadt NPP (KKL) 32 Only few ENSI requirements on further SAM modifications Accident resistant water level and temperature measurement system in spent fuel pools (outage 2013) Review H 2 management effectiveness and, if necessary, backfitting ( ) Seismic enforcement of FCVS (2014) Improve SAM procedures / documentation (2013) Integration of Reitnau external SAM storage facility hardware in KKL SAM process and logistics (2013) KKL has proactively initiated some further improvements

33 KKL: Emergency SFP - measurement system for water level & temperature Regulatory (ENSI) requirement Redundant Separation Safety Class 3, Seismic Class I Implementation: Level: air bubbling - measurement Temp: Pt-100 probe Foreseen for outage

34 KKL: Hydrogen control Parameter Goal Action Igniters & PARS: H2- concentration during SA Low H2-concentration even under SBO condition (passive protection) No detoriation of FCVS during venting! 1. Improve MELSIM higher nodalisation 2. Simulate spatial H2 distribution during core melt accident (Containment & FCVS) 3. Evaluate need for additional PARs 4. Install 34

35 Leibstadt NPP (KKL) Proactive current projects 35 2 SAM DGs to feed emergency power trains (batteries) >>5h (done) 2 additional diverse SRV for fast & stable pressure relief (2017) Backfitting of additional alternative injection points for SAM in seismic stable environment ( ) Facility for fast alternative Boron injection via CST (2015) Seismic resistant garages for fire trucks & fire fighting device (2017) Seismic resistant location for SAM devices ( DGs, pumps, tools etc.) (2013) Pressure equalization drywell containment for harmonized SAM flooding ( ) Improvement RCIC logics for continuous operation (2014)

36 KKL: alternate Injection with external accesses/adapters to regular systems 1. Injection to Fuel Pools 2. Injection to 1. RCIC, CRD, HPCS suction lines 2. Re- filling CST 3. Injection to ESW B 3. Injection to RPV via Feed Water Line Mainly extension/adaptation of already available nozzles / access points for mobile or fire fighting equipment 36

37 KKL: Alternate Supply Systems e.g. to increase reliability of pressure relief of RPV in a severe accident situation realized in 2011 Purchase of Alternate Power Supply a. Power supply for e.g. SRV with mobile SAMG Diesel Generator (150kVA) Alternate Air supply a. DG powered mobile air compressor Mobile pumps, etc. 37

38 KKL: Alternate Power Supply in case of T-SBO, with SSE-safe connection plugs in both DG buildings - realized in 2011 Regular Emergency Diesel Generator «SAM DG» 150kVA battery charger 38 Safety relevant DC-consumers: motors, solenoid valves (SRV), instrumentation, emergency lights, etc.

39 KKL: plan for further SAM optimizations after Fukushima: long-term depressurization of the RPV DC-MOV-SRV Most important requirements in a SA situation: External connection plugs for supply with compressed air ( mobile compressor unit) to keep safety relief valves (SRV) permanent open External connection plugs for power supply for opening of solenoid valves with mobile SAMG Diesel generator (150kVA) 39 Relevant modification: 2 additional, diverse DC-motor operated safety relief valves (MOV-SRV) for stable «open position» to depressurize the RPV for extended time