AREVA’s Technology for Safety Improvement

Marina Welker Company: AREVA GmbH Address: Henri-Dunant-Str. 50, 91058 Erlangen, Germany Phone: +49 (0) 9131 900-95606 Email: [email protected]

Summary – Mitigation of severe accidents in nuclear power plants (NPPs) focuses on protection of the public, the operators and plant structures/ systems. Apart from pressure reduction in the containment, also monitoring systems are required to provide the status of the plant and its equipment / systems. In addition, during severe accidents monitoring systems support the operator and authorities in making the right decisions and timely initiate the appropriate measures to mitigate the impact of a severe accident. Based on more than 30 years of experience in this field AREVA offers nuclear operators high-performance products and services to guarantee the safety of their plants.

1. INTRODUCTION Severe accidents in NPPs are linked with serious impacts on the plant itself, plant operators, the environment and the public. As Fukushima showed, they could lead to political decisions impacting the macroeconomic situation. This is another reason to pay special attention to measures which improve the safety of the plant and mitigate possible implications. In the unlikely case of a severe accident, the ability to monitor critical plant parameters and manage events becomes essential for plant operators and authorities. AREVA has developed and installed a number of dedicated technical solutions to address this need.

2. USEFUL TECHNICAL SOLUTIONS IN CASE OF SEVERE ACCIDENT 2.1. Hydrogen in the containment: generation and control In 1979 the nuclear accident of Three Miles Island directed the attention to the adverse effect on containment integrity by high H2-concentration after DBA. After the Chernobyl accident in 1986 the discussion was extended additionally taking into account severe accidents. High production of hydrogen in the reactor containment leads to a high concentration of combustible gas which might ultimately affect the integrity of the containment. The hydrogen concentration has to be kept within certain limits even under severe accident conditions. Knowing the concentration of hydrogen and reducing it is the key success factor to maintain the integrity of the containment. When considering long-term accident mitigation, passive safety systems are advantageous as they require neither operator action nor power supply. AREVA has developed passive components to reduce hydrogen, based on the principles of catalytic oxidization. There are two principles available: recombination and ignition. Historically, the igniter was the first product on the market, but the following years showed that there was more interest in the recombiners, being the safer technique. However, passive igniters as well as a combination of passive igniters and recombiners can also be provided.

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The AREVA Passive Autocatalytic Recombiner (PAR) features a gas-treating capacity of up to 1500 m3/h per PAR unit and a simple, cost-effective design which is easy to integrate into existing NPPs. The PAR design is based on thin stainless steel plates coated with catalyst at a large reaction surface. It allows for low start temperatures and has proved to be very efficient in severe accident conditions. To promote natural convection the PAR has a metal housing with a gas inlet at the bottom and a lateral gas outlet at the top. Upon contact with the catalyst in the lower part of the housing, the gas mixtures containing hydrogen are recombined. The heat from this reaction causes a reduction in gas density that leads to buoyancy-driven flow, thus supplying the catalyst with a large amount of hydrogen that ensures a highly efficient recombination. The AREVA PAR is available in different sizes, which allows for the best possible arrangement in the various compartment areas.

To date, AREVA has delivered and installed PAR-based H2 control systems in more than 140 NPPs worldwide, including equipment for testing and maintenance. Excellent test results were achieved even under adverse conditions such as oil fire or oil aerosols from the main coolant pump, boric acid, or a significant high concentration of organic carbon-hydrogen compounds. The AREVA PAR has been subject to extensive and long-term testing in order to evaluate it for various accident scenarios in both PWR and BWR. The most important qualification test was performed during the PHEBUS FPT3 (Fission Product Test 3) in Cadarache (France) by a third party group with participation of various authorities, research institutes and different PAR suppliers. Catalysts of various manufacturers were tested under real severe accident conditions with a degraded core for the first time. Within the scope of the EC/IRSN Core Melt Program PHEBUS (with real molten core), AREVA’s PAR showed the best performance concerning

 very fast start-up,

 highest efficiency and

 absolute constant hydrogen depletion rates. 2.2. Detailed information about the situation inside the containment Monitoring of the containment atmosphere, e.g., the level of hydrogen and other combustible gases, is required in case of severe accident in order to timely provide the baseline decisions for mitigating actions, e.g., filtered venting. All containment atmosphere monitoring systems from AREVA are fully qualified for BDBA conditions but can be used for normal operation as well. For hydrogen monitoring inside the containment AREVA offers the Containment Atmosphere Hydrogen Monitoring System WS85. WS85 safely measures the H2 concentrations at different locations in the containment during normal operation as well as under severe accident conditions. Thus it is possible to continuously and simultaneously monitor the area and time distribution of H2 concentrations during accident progression. The WS85 system can be installed in all types of reactors. The areas to be monitored and the position of the sensors are individually assessed for each different NPP layout. If in addition to the hydrogen concentration the oxygen concentration must be determined as well, the WS85 system can be extended to a WS85 Plus solution: in addition severe accident proven O2 sensors will be installed together with H2 sensors in this case.

If more parameters (e.g., H2, O2, CO, CO2, steam concentration) must be detected, the AREVA HERMETIS Monitoring System can be installed as it is proven to operate under core melt

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accident conditions. HERMETIS provides the following critical information for the severe accident management / emergency operations:

 released hydrogen quantities

 distribution of hydrogen

 distribution of steam

 containment atmosphere combustion regime (locally)

 potential of hydrogen combustion

 efficiency of hydrogen countermeasures such as recombiners / igniters

 occurrence of global convection

 indication of molten core concrete interaction / failure of reactor pressure vessel The system works with in-situ micro sampling based on capillary pipe technology. Thus, only fully metallic components are placed in harsh environment, and the entire measuring equipment is installed outside of the containment and easily accessible for operators. The superheated micro samples are transported to the analysis module placed outside the containment. With the analysis module being located outside the containment measurement errors are significantly reduced. No containment isolation valves are required due to the use of capillary technology. The system needs no operator action and allows easy maintenance and in-service inspection. The system measures the concentration of, e.g., hydrogen, oxygen, CO (and CO2, if needed) and steam. For the management of further progressing severe accidents a precise understanding of nuclide-specific radioactivity inside the containment is essential. In this situation, the atmosphere of the containment may be mixed with radiolysis products (hydrogen, oxygen), steam, noble gases, radioactive aerosols and iodine. The activity of the containment atmosphere is dominated by the noble gas activity. To estimate the potential risk to the environment in case of containment pressure release, the aerosols and iodine concentration must be determined in addition to the noble gas activity. The AREVA Post-Accident Sampling System PASS is designed to provide representative gaseous and liquid samples from the containment atmosphere and in-containment refuelling water storage tank (IRWST) or sump / wetwell following a DBA or BDBA event. The potentially highly radioactive samples are diluted to permit further analysis with normal laboratory equipment. The analysis of the samples will provide details on the containment situation, the core damage state and potential risk for the environment. An in-situ sampler collects the aerosol and elemental iodine in a scrubbing liquid. The iodine and aerosol sample is flushed to a dilution module (located outside containment) and a sample box where it can be taken for laboratory analysis. The in-situ sampling technology makes the samples transportable over long distances without significant sample measurement errors. Noble gases and organic iodine are also flushed to the outside sample module. The different samples taken from the containment atmosphere contain:

 aerosol bound radionuclides

 non-aerosol bound (gaseous) iodine isotopes

 radioactive noble gases

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Liquid samples from the IRWST (sumps / wetwell) contain suspended and dissolved fission products. Furthermore, the information about activity of the IRWST spray water during containment heat removal system operation is provided by these samples. The system is operated using the control panel installed in the control cabinet. The desired dilution factor (1:10 / 1:100 / 1:1000) for the gaseous samples can be selected using this panel. With this technique samples can be taken based on an activity of up to 1016 Bq/m³ inside the containment atmosphere. In addition to samples from the containment atmosphere, samples of the containment sump (wetwell / IRWST) are taken. For them a dilution factor (1:100 / 1:1000) can be selected at the control panel. This allows a handling of activities up to 3.7 x 1014 Bq/m³ inside the sump (wetwell / IRWST). 2.3. Containment venting and its control Despite all applied measures to mitigate the progression of severe accidents the containment pressure can increase until the integrity of the containment is jeopardized. In such a case a controlled pressure discharge should be the method of choice. Therefore, for almost 30 years AREVA has installed Filtered Containment Venting Systems (FCVS) that allow for filtering and retention of aerosols and radioactive components of the discharge. However, even during operation of a FCVS, radioactive substances, e.g., noble gases, are released to the environment. In order to monitor the released amount of radioactive material the AREVA Passive Gaseous Effluent Monitoring System (PEGASUS) can be applied to the FCVS. PEGASUS can be used for any kind of vent line discharging active gases, not only to FCVS outlet line. PEGASUS provides online measurement of the radioactivity within the discharged gas allowing for a determination of the released amount of radioactive nuclides. Optionally an aerosol / iodine sampler can be installed in addition to the radioactivity measurement. A distinct analysis of the samples serves to differentiate the various nuclides discharged to the environment. Being able to take these samples is of great importance in particular regarding a detailed estimation of the long-term soil contamination. Moreover, the information provided by PEGASUS allows conservation of evidence for the authorities. PEGASUS is a dedicated tool which uses cost-efficient and low- energy consuming detectors and analysis modules to precisely monitor the activity released to the atmosphere. The robust design includes a backup power supply for operation after severe accidents in case of station blackout. 2.4. Special protection for operators in control rooms In case of a severe accident in NPPs radioactive releases may leak into control rooms – such as main and emergency control rooms. These radioactivity releases could be caused by, e.g.,

 discharge of activity, e.g., of noble gases, from the containment to the atmosphere of the plant,

 containment leakages or

 containment bypass to adjacent buildings. To ensure that the workforce’s exposure to radiation doses does not exceed specified limits (according to OECD recommendation, doses within the control room should be kept below 100 mSv over a 7-day period), special measures need to be taken both during and after a severe accident to prevent activity from entering the control rooms. AREVA has developed a Control Room Air Filtration System (CRAFT) to protect control rooms and emergency response centers – or other rooms designated for prolonged occupancy throughout the duration of accidents – from radioactive releases. CRAFT filters airborne radioactive

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pollutants (e.g., iodine, aerosols) and poisonous gases and also retains noble gases by means of dynamic adsorption. CRAFT is based on proven technology for treatment of active gases, which has been used by AREVA since the 1970s.

CRAFT has no limitation of operation time. It is designed to operate during the complete accident progression and is suitable even in case of multiple venting scenarios. Moreover, CRAFT covers scenarios on NPP sites with more than one unit. CRAFT can be used for all types of rooms which need protection, independently on the type of NPP. The number of persons inside the room is the main factor for the dimensioning of CRAFT. The system could be installed inside or outside existing buildings. It could be provided as a mobile solution as well. Another important characteristic of CRAFT is its low maintenance effort.

3. EXPERIENCE WITH THE PRESENTED TECHNICAL SOLUTIONS All technical solutions presented in this document have been tested and qualified for operation under severe accident conditions. All these systems are available as integrated tailor- made solutions. They could be part of an integrated safety upgrade. Moreover, the safety management processes in NPPs can be improved by applying these technical solutions.

Table 1. AREVA’s experience with the presented technical solutions

Technical Number of Type of NPP Countries solution NPP units Argentina, Belgium, Brazil, Bulgaria, China, Czech BWR, EPR, Republic, Finnland, France, Germany, Japan, PAR >140 PHWR, PWR, Netherlands, Romania, Russia, Slovakia, South Africa, VVER South Korea, Spain, Sweden, Switzerland, UK, Ukraine ABWR, BWR, Bulgaria, China, Czech Republic, Finnland, Germany, WS85 / WS 85 >50 EPR, PHWR, India, Japan, Slovakia, Spain, Swede, Switzerland, Plus PWR, VVER Ukraine EPR, PHWR, HERMETIS 7 Bulgaria, China, Finnland, Romania PWR, VVER BWR, EPR, PASS 13 China, Finnland, Germany PWR ABWR, BWR, Argentina, Belgium, Brazil, Bulgaria, Canada, China, FCVS >80 EPR, PHWR, Finnland, Germany, Japan, Netherlands, Romania, PWR, VVER South Korea, Spain, Switzerland PEGASUS 3 PWR Spain 4 engineering ABWR, BWR, CRAFT Japan studies PWR

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4. CONCLUSION AREVA has broad experience in severe accident analysis and mitigation. Thus, AREVA can offer the full range of know-how concerning management of severe accidents in NPPs. With this AREVA helps NPP operators to upgrade their plants to meet stricter requirements that supervisory authorities have imposed or may impose. Detailed information regarding AREVA’s entire portfolio of severe accident management solutions can be found in AREVA’s Safety Alliance Catalogue and on our website. This portfolio represents AREVA’s worldwide contribution to safety improvements of NPPs. ACKNOWLEDGEMENTS Firstly, I would like to express my sincere gratitude to Axel Hill and Norbert Losch for their support and immense knowledge. Secondly, I would like to thank the colleagues of the AREVA Severe Accident Team for comments, questions and discussions. Sincere thanks also to Carolina Perez from AREVA Spain for her support during the preparation for this conference. ABBREVIATIONS

ABWR Advanced Boiling Water Reactor BDBA Beyond Design Basis Accident BWR Boiling Water Reactor CRAFT Control Room Air Filtration System DBA Design Basis Accident European Commission / Institut de Radioprotection et de Sûreté Nucléaire – Joint EC/IRSN Research Centre EPR European Pressurized Water Reactor FCVS Filtered Containment Venting System HERMETIS Containment Atmosphere Monitoring System IRWST In-Containment Refuelling Water Storage Tank NPP Nuclear Power Plant OECD The Organization for Economic Co-operation and Development PAR Passive Autocatalytic Recombiner PASS Post-Accident Sampling System PEGASUS Passive Gaseous Effluent Monitoring System PHEBUS FPT3 Joint In-pile Severe Accident Research Program Fission Product Test 3 PHWR Pressurized Heavy Water Reactor PWR Pressurized Water Reactor VVER Water-Water Power Reactor, Pressurized Water Reactor WS85 Containment Atmosphere Hydrogen Monitoring System WS85 Plus Containment Atmosphere Hydrogen and Oxygen Monitoring System

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REFERENCES. Dr.Janis Endres, Axel Hill, Nuclide Specific Activity Monitoring of Containment Atmosphere and Semi Passive Effluent Monitoring during Containment Venting, The 10th International Topical Meeting on Nuclear Thermal-Hydraulics, Operation and Safety (NUTHOS-10), Okinawa, Japan, December 14-18, 2014 AREVA Safety Alliance, Products, Services and Solutions, November 2012, AREVA 2012 European comission, Joint Research Center, T.Haste; F.Payot; C.Manenc; B.Clement; Ph.March; B.Simondi-Teisseire; R.Zeyen, Nuclear Engineering and Design Vol. 261 P. 333-345, Phebus FP programme, http://publications.jrc.ec.europa.eu/repository/handle/JRC75884, Published Elsevier Science SA, 2013 OECD/NEA THAI Project, Hydrogen and Fission Product Issues, Relevant for Containment Safety Assessment under Severe Accident Conditions, NEA/CSNI/R(2010)3, 22-Jun-2010, http://www.oecd.org/officialdocuments/publicdisplaydocumentpdf/?cote=NEA/CSNI/R%282010%29 3&docLanguage=En

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