CNE Cernavoda Response to Fukushima Event/EU Stress Test Requirements Sorin Holostencu IAEA Technical Meeting on Operational Experience with Implementation of Post-Fukushima Actions in Nuclear Power Plants, Vienna, 27-29 March 2017
Requirements in response to Fukushima nuclear accident In response to Fukushima nuclear accident, the governmental and nongovernmental organizations involved in nuclear industry requested immediate actions to be taken INPO (Institute of Nuclear Power Operations) and WANO (World Association of Nuclear Operators) issued two similar documents (IERL 11-1/ SOER 2011-2) on 14 and 17 march, 2011, respectively, by means of which all the affiliated plants were required to: Verify the response capability to severe accidents as a consequence of external events considered beyond design basis (e.g.: Earthquake); Verify the response capability to station blackout (SBO); Verify the response capability to internal and/or external flooding; Verify the response mode to fires or flooding caused by earthquake. CNE Cernavoda performed the WANO requested assessments by May 2011 (including provision on-site of 2 Mobile Diesel Generators); 2
On 25 March, 2011, EUROPEAN COUNCIL decided that Nuclear safety of all EU Nuclear Power Plants has to be revised according to transparent and extended risk assessments, the so-called STRESS TESTS ; ENSREG (EUROPEAN NUCLEAR SAFETY REGULATORS) issued in late May 2011 a set of technical specifications based on which regulatory authorities of each member state to require the license holders to reassess the design basis margins of the nuclear safety (STRESS TEST); In this context, CERNAVODA NPP forwarded to CNCAN (ROMANIAN REGULATORY AUTHORITY) a preliminary Stress Test Report on 12 August 2011, and final report on 28 Oct. 2011 Both preliminary and final Stress Test reports have been prepared by a reunite team of Cernavoda NPP experts and plant designers, AECL Canada and ANSALDO Italy. 3
Reactor Unit Type Capacity MW(e) Construction Start First Criticality Operating Status Cernavoda-1 CANDU-6 (PHWR) 706.5 1980 16th of April 1996 In operation Cemavoda-2 CANDU-6 (PHWR) 706.5 1980 6th of May 2007 In operation Cernavoda - Primary Heat Transport System and Balance of Plant 4
Cernavoda is a CANDU 6 reactor: -380 fuel channels -12 fuel bundles/channel -4 steam generators -2 loops Heat Transport System (with PRZR) -Reinforced Concrete Full Containment Cernavoda (C6) - Reactor Core Structure and Calandria Vault 5
EU Stress Test specifications EU stress tests specification required the EU nuclear power plants to focus on the following issues: 1. External Hazards EARTHQUAKE FLOODING 2. Consequence of Loss of Safety Functions from any Initiating Event LOSS OF ELECTRICAL POWER INCLUDING STATION BLACK-OUT (SBO) LOSS OF THE ULTIMATE HEAT SINK (UHS) COMBINATION OF BOTH 3. Severe Accident Management Issues: MEANS TO PROTECT FROM AND TO MANAGE LOSS OF CORE COOLING FUNCTION MEANS TO PROTECT FROM AND TO MANAGE LOSS OF COOLING FUNCTION IN THE FUEL STORAGE POOL MEANS TO PROTECT FROM AND TO MANAGE LOSS OF CONTAINMENT INTEGRITY Items 1 & 2 above are not limited to earthquake and tsunami, extreme weather conditions were also considered. 6
Topic 1 - External hazards (earthquakes, floods and extreme weather conditions) As part of the response to the WANO SOER 2011-02 Recommendation following the Fukushima accident, walkdowns and inspections of important equipment needed to mitigate fire and flood events were performed at Cernavoda NPP, by the plant staff, to identify any equipment whose function could be lost during seismic events. Several actions to restore minor barrier degradations and administrative procedures improvements have been implemented. As part of Stress Test, new plant walk-downs were performed in May-July 2011 by a team built-up from plant designer (AECL/CEI) engineers and operating organization engineers to confirm the preservation of the original design in terms of seismic interaction and SSCs condition. 7
Topic 1 - External hazards - earthquakes Update of the seismological catalogue, was issued by Romanian National Institute for Earth Physics. The validity of data and the conclusion on the adequacy of the design basis has been confirmed by the recent studies done as part of Probabilistic Hazard Analyses, updated in August 2011 and independently confirmed by the Romanian National Institute for Earth Physics by considering the updated seismological catalogue. The seismic capacity of the plant was reassessed in 2011 by a team built-up from plant designer (AECL/CEI) engineers. 8
Topic 1 - External hazards - earthquakes For Cernavoda site the maximum estimated event has a magnitude of 7.8 (RICHTER Scale) and PGA of 0.18 G. The Design Basis Earthquake (DBE) of Cernavoda units 1 & 2 was established at 0.2 g (PGA) for a return period of 1000 years. A Review Level Earthquake (RLE) was established for PGA of 0.33 G for a return period of one in 10 000 years. To ensure an additional margin, all SSCS in the success path (safe shutdown cooling, containment and monitoring functions) were finally evaluated against the target level RLE of 0.4 G PGA. Detailed analysis (walk-downs, fragility analysis, etc.) confirmed the capability of SSCS on success path to withstand a seismic event of at least 0.4 G (which has a return frequency of 5E-5 events/year) 9
Topic 1 - External hazards - floods and extreme weather conditions Based on the distances between Cernavoda plants and various hydro dams as well as its elevation & distance from the Black Sea, there is no possibility of external flooding due to earthquake and tsunamis. The original design basis for flooding has been re-evaluated by using modern techniques (Aerial Digital Topographic Scanning and Bathymetry of Danube) complemented by classic topographic measurements and systematic walk-downs. Result: extreme water level with a return period of 10E-4 is 13.31m, while site platform has an additional margin of 2.7m. Measures taken to improve protection against flooding by providing sealing doors and on-site sand bags to be used as temporary flood barriers. From 2012 to 2014, 50 flood resistant doors were installed in U1 & U2. Screening and bounding analysis for Cernavoda NPP operation under severe weather conditions. The results show that the worst case scenarios are bounded by SBO or LOHS. The specific procedure in place for extreme weather conditions has been reviewed to include the appropriate proactive actions for plant shutdown. 10
Cernavoda Area digital model.
Topic 2 - Plant Response on SBO and LOHS STATION BLACK OUT (SBO): The plant shutdown is ensured automatically either by SDS 1 or SDS 2, which will perform their design function without the need of being supplied with electrical power; Cooling of the reactor is ensured by thermosyphoning, and the heat transferred to steam generators is discharged to atmosphere via steam discharge valves; the necessary water to the steam generators will be gravitationally fed from the dousing tank inventory using electrical power from batteries to open the valves; The above systems ensure cooling of the reactor for at least 27 hrs, amount of time sufficient to connect electrical power from the mobile diesel generators to the Emergency Water pumps (EWS); Containment function considering SBO is not affected; containment isolation valves will fail close either on loss of their electrical power supply or loss of instrument air; The monitoring of the critical safety parameters will be ensured using electrical power from the batteries, which can continuously supply power for approximately 8 hrs; 12
Topic 2 - Plant Response on SBO and LOHS LOSS OF PRIMARY ULTIMATE HEAT SINK: It means loss of Raw Service Water system (RSW); The shutdown systems can be either manually activated from the control room or automatically ensured by process parameters as designed; The heat sink is ensured in the same way as mentioned in the case of SBO except that the second DB set of diesel generators (EPS) are available and can continue supplying power to Emergency Water pumps (EWS); Containment and monitoring of critical safety parameters will not be affected LOSS OF PRIMARY ULTIMATE HEAT SINK AND LOSS OF ALTERNATE ULTIMATE HEAT SINK: It means loss of Service Water (RSW) and Emergency Water Supply (EWS) systems; Critical safety parameters (3C + M) are ensured similarly to the case of loss of primary ultimate heat sink, except that the necessary water to the steam generators is ensured, after 27 hrs, by the fire water tanks or mobile pumps that can be manually connected to EWS system piping, and can be supplied with water from sources other than Danube (town distribution network or two existing deep ground wells). 13
Topic 2 - Prolonged loss of spent fuel bay cooling: In case of a prolonged loss of Spent Fuel Bay (SFB) cooling, make-up water is required to prevent uncovery of the spent fuel bundles; In this way cooling (boiling and evaporation) and proper disposal of the residual heat is ensured; Based on calculations it was shown that water inventory existing in Spent Fuel Bay is sufficient for 9 days until radiological fields start to increase above normal, and 15 days until first fuel bundle become uncovered; Procedure developed on how to use external make-up water and practiced, using fire trucks and mobile pumps; The ST assessment recommended to install a new make-up water line from outside of SFB building, seismically qualified. 14
SPENT FUEL BAY
Topic 2 - Plant Response on SBO and LOHS CNE Cernavoda implemented several measures to improve the response to SBO and LOHS scenarios: 2 mobile diesel generators (1.4 MW), one for each unit, capable of energizing all EPS loads (EWS pumps, ECI pumps, monitoring panels (2011); Each Mobile diesel generator has a separate 0.4/6 kv transformer, so it can supply either 0.4kV or 6 kv loads. Install connections to the existing EPS buses, in order to connect mobile diesel generators (2011); Developed two new EOP s to cope with SBO and Loss of Spent Fuel Pool Cooling events and revised EOP on Loss of Service Water (2011); Two electrical mobile submersible pumps powered from mobile DG to supply firewater truck under emergency conditions (2011); 16
Topic 2 - Plant Response on SBO and LOHS CNE Cernavoda implemented several measures to improve the response to SBO and LOHS scenarios: Provide mobile diesel engine driven pump and flexible conduits to supply the fire water trucks under emergency conditions (2012); Two additional mobile diesel generators (110kW) for electrical power supply to pumps of two water wells to fill-up firewater trucks under emergency conditions (2012); Install a seismically qualified line to add water to SFB from outside of the building (2011); Provided 2 separate means to manually open the MSSVs after a SBO (hydraulic jack and nitrogen bottles) (2012); 17
Topic 2 - Plant Response on SBO and LOHS CNE Cernavoda implemented several measures to improve the response to SBO and LOHS scenarios: Installed connection facilities required to add water, using fire fighters trucks and flexible conduits, to supply the SGs (2011) Performed a feasibility study (2012) and obtained the Government approval (2013) to change destination of Unit 5 (under preservation), in order to use its existing structures as a new location for the storage of the EME and a new on-site Emergency Control Center; 18
Topic 2 - Plant Response on SBO and LOHS CNE Cernavoda implemented several measures to improve the response to SBO and LOHS scenarios: Improve the seismic robustness of class I & II batteries by consolidating the room walls (2012); Install connections to the SCA instrumentation panels, for the 110kW mobile diesel generators, increasing plant monitoring capabilities after a SBO event (this is additionally to the electrical supply provided from the 1.4MW DG) (2016) Establish agreements with the local and national authorities involved in the emergency response in order to ensure that in case of a SBO coincident with LOHS the plant has absolute priority to grid re-connection and supply of light and heavy equipment and the necessary diesel fuel (from 2011). 19
Flood resistant doors 20
Facilities for opening and keeping MSSVs open in case of SBO hydraulic jack N2 supplies for MSSVs pneumatic actuation 21
Mobile Diesel Generators 22
Mobile Diesel Pump 23
Water Make-up line to Calandria Vault 24
Electrical connection points for Mobile DG s 25
Topic 3 - Severe Accident Management CERNAVODA NPP reactor design is based on the defense-in-depth principle with safety systems, and multiple and diverse heat sinks to cater for severe core damage accidents. The unique design aspects of the CANDU reactor with low pressure, heavy water moderator in Calandria Vessel, surrounding the pressure tubes and large volume of light water in Calandria Vault, surrounding the Calandria Vessel provide heat sink capability which delays the accident progression sequence. Even with moderator inventory boiled off, as long as Calandria Vessel is cooled by Calandria Vault water, the corium will remain in-vessel. The reinforced concrete containment building provides the fundamental barrier protecting the public from the unlikely event of radioactive releases. The LAC fans in the R/B promote forced air circulation for hydrogen mixing during accidents thus avoiding pockets of locally high hydrogen concentrations 26
Topic 3 - Severe Accident Management In the unlikely case of ex-vessel core damage, there is sufficient floor space for debris spread and to keep the debris on the floor submerged with water. Issues related to the loss of cooling to the SFB are well managed with a back-up make-up line, with connections outside the SFB building. A comprehensive emergency response program exists to respond to all emergencies including severe accidents. The on-site emergency control center is well equipped with all the required provisions to respond to all levels of emergencies (alert to general emergency). Where necessary, provisions have been made to bring on site mobile equipment with portable pumps, fire trucks etc., to allow accident mitigation. 27
Topic 3 - Severe Accident Management CNE Cernavoda implemented several measures to improve the severe accident management response: Station specific Severe Accident Management Guidelines based on the CANDU-6 Generic COG SAMG, implemented in 2011; Subsequent revision of SAMG to include Shutdown Conditions in 2015 Validation of the station SAMG through emergency exercises (2011); Established Continuing Training Program for severe accident scenarios (2011); Improved seismic robustness of the on-site Emergency Control Centre (2012); Installed connections to inject water to the Calandria Vessel (via the Moderator Purification lines) (2011 U2; 2012 U1). 28
Topic 3 - Severe Accident Management CNE Cernavoda implemented several measures to improve the severe accident management response : Design modification to install a larger Calandria Vault RD, to enhance the over-pressure protection (2012 U1; 2013-U2); Installed new line to inject water into the Calandria Vault from outside of the R/B (2012 U1; 2013-U2); Install an emergency filtered venting system (EFCVS) for the containment, at each unit (2012 U1; 2013-U2); Installed a hydrogen monitoring system (Hermetis), to monitor R/B condition (2012 U1; 2013-U2); 29
Topic 3 - Severe Accident Management CNE Cernavoda implemented several measures to improve the severe accident management response: Installed passive autocatalytic recombiners (PAR s) (2012U1; 2013U2; Improved the environmental qualification for estimated severe accident conditions / extend the measurement domain, for selected Critical Safety Parameter monitoring loops (from 2012 to 2016); Installed satellite phones in each Main Control Room, Secondary Control Area and the Emergency Control Center (2011) TETRA radio communication system (2011) Install hydrogen vent capability from the spent fuel bays (2012) 30
Topic 3 - Severe Accident Management CNE Cernavoda has implementation several measures to improve the severe accident management response: Reviewed the existing protocols with Public Authorities in order to ensure the necessary support for plant s personnel in case of severe accident, when the roads are blocked due to extreme meteorological conditions, natural disasters (earthquakes, flooding, etc.) or other traffic restrictions; Severe Accident Work Admittance area located 3 km away from the plant - dedicated to accommodate shift emergency personnel for cases that Main Control Room and Secondary Control Area become inhabitable (2013); Set-up an alternative Off-site Emergency Control Center, located in an existing facility in Constanta city (60 km away from Cernavoda NPP) (2015) Improvement of the national arrangements for emergency response in case of radiological or nuclear accident to take account of lessons learned from the Fukushima accident; 31
Containment Filtered Venting System
Containment Filtered Venting System U1 Shielding Wall set
U1 Venturi Vessel Installation Containment Filtered Venting System
Containment Filtered Venting System U1 Stack installation
Passive Autocatalytic Re-combiners
Hydrogen Concentration Monitoring System Panels
Challenges faced in implementation of post-fukushima actions Implement ASAP a modification that require RBLRT and test is not scheduled in next outage Cernavoda has developed an innovative solution to leak check individual cable penetrations, without the need for conducting a full scale Reactor Building Leak Rate Test (RBLRT). This allowed earlier installation of new cabling to improve postaccident monitoring. 38
Challenges faced in implementation of post-fukushima actions To install instrument monitoring loops that will survive SA environmental conditions Design engineers benchmarked outside of the nuclear industry for a solution to preserve the Calandria Vault level and Calandria Vessel (moderator) level indications during severe accident conditions. The new level indication systems measure differential pressure signals using two silicone oil pressure transmitters. This solution eliminates the possibility of instrument line voiding during high-heat, low-pressure conditions that would lead to a loss of level indication. 39
Cernavoda Post-Fukushima Action Plan - remaining actions At this moment, CNE Cernavoda has under implementation only one action, from the post- Fukushima Action Plan: Establish a new location for the on-site Emergency Control Centre and storage of EME (mobile DGs, mobile diesel engine pumps, fire-fighter trucks, radiological emergency vehicles, etc.). ECD: June 2018 40
Expert Feedback on TM 41
Conclusion As per the EU stress tests results, it has been demonstrated that CERNAVODA UNITS 1 & 2 meet the safety requirements as stipulated in the original design, while having sufficient margins against earthquakes, flooding, loss of electrical power and loss of ultimate heat sink. To further increase robustness of the units, design modifications and procedural improvements have been implemented, which represented a significant human and financial effort. SNN-CNE Cernavoda demonstrated its commitment in implementation of the best practices and technologies, in order to be prepared for beyond design basis extreme events, with a paramount focus on the safety of public, workers and the nuclear installations. 42
Thank You! 43