1. Introduction
In the history of nuclear power plants, while there have been numerous catastrophes, three major accidents have been reported, namely, Three Mile Island (1979), Chernobyl (1986) and the recent Fukushima (2011). The crucial reasons for NPP accidents are reactor core melt down, release of radioactive isotopes, unexpected external events that disrupt the normal functioning of the plant & equipment failure. Fukushima Daiichi has by far been the worst NPP accident reported. In terms of understanding its effect, a large scale earthquake in March 2011 generated a 45 ft high tsunami that stroked the Fukushima NPP plants, resulting in the blackout of all the units in the power station [1]. In fact all the reactors were scrammed by the station blackout but the emergency core coolant system failed to cool the core and remove the decay heat due to which the core heated up and melted. The molten core generated steam and hydrogen which caused over pressurization of the containment. Hydrogen combustion resulted in explosion of two units leading to the release of radioactive materials into the environment which had a huge impact on the society. In nuclear power plants, when the core starts to melt, the reactor containment could be drastically affected by the excessive pressure caused by the heat formation inside the containment over a long time. Once the primary containment fails the secondary containment is usually unable to retain radioactive material inside it since the secondary containment is not as protective as the primary containment. Due to this accident, iodine-131, considered to be a major harmful component, was released into the atmosphere. Radioactive iodine is toxic for normal human life as it causes overactive thyroid and may engender thyroid cancer [2]. This disaster led to reinforce the necessity of further improvements regarding safety in nuclear power plant designs. To reduce the risks generated by severe NPP accidents which occur due to over pressurization and emission of fatal elements, filtered containment venting system (FCVS) can be incorporated in NPPs as a Severe Accident Mitigation Method (SAMM). FCVS is a system that retains the fission products and allows the release of excess pressure and can also remove decay heat during the time of pressure build up in case of a NPP accident. It avoids the rupture of the containment caused by the build-up of pressure by passing the vented exhaust through a scrubber filter, by controlling the discharge of fission products it protects human life inside and nearby the plant and eases the task of the emergency teams. By employing cooling provisions it removes the decay heat and by timely venting reduces H2accumulation and explosion risk. FCVS are generally used in severe accidents belonging to the category of overall applied SAM strategy for PWRs (Pressurized Water Reactors) and BWRs (Boiling Water Reactors). But they are also used in design-basis accidents for some pressurized heavy water reactors (CANDUs) [3]. Several countries have already initiated the process of modifying their nuclear power plants to FCVS. Switzerland, Finland, France, Germany and Sweden have previously installed FCVS in their operating nuclear power plants during 1980–1990. Subsequently Netherlands, Bulgaria, China and Canada started to modify their PWRs, VVERs (Water-Water Energetic Reactors) and HPWRs (Pressurized Heavy Water Reactors) type nuclear power plants during 1990–2011. So the incident in Fukushima Daiichi in March 2011 resulted in the inspection and review of NPP safety worldwide and this instilled a clear awareness of installing FCVS in every nuclear power plant [4].
The primary intention of this paper is on discussing existing technologies of filtered containment venting system and carrying out a comparative study on them. It also identifies the FCVS implemented in different countries and highlights the legislations regarding it. The paper presents a critical review on the existing technology, specifies the current challenges and mentions the improvements that can be done in this respect.
2. Types of filtered containment venting systems
Aim of a FCVS is to maintain containment integrity, control the excessive pressure generated inside the containment and retain the airborne activity of vented gases efficiently by means of controlled venting with the help of a filtration system and preserve the containment function despite the severity of the accident. Venting also aids in removing decay heat and minimizes H2 risk. By the release of steam, air and non condensable gases like H2 into the atmosphere from the filtration system installed on the vent lines, the tragic containment failure can be prevented. In comparison with its alternatives like hardened vents and severe accident confinement strategies, FCVS extends higher certainty on regulation and permits regular implementation. Hence FCVS plays a crucial role in controlling severe accidents and is essential to preserve the environment and human life and minimize on and off site contamination. Filtered containment venting system was introduced by AREVA [5]. FCVS has DFof 1000 for aerosol and more than 200 for iodine. It has two staged processes, i.e. high speed venturi scrubber and the filter. All components are placed in the pressure vessel and controlled under sliding pressure conditions.
2.1. High speed sliding pressure venturi (HSSPV) scrubber
This high speed sliding pressure venturi scrubber was developed by SIEMENS-AREVA and installed in most of German, Finland and other European countries’ NPPs. AREVA provides FCVS with a combined two staged process, the benefit of HSSPV (wet stage) with an efficient deep-bed fibre filter (dry stage) is made use of. The components are installed in a pressure vessel and operate in sliding pressure conditions thus leading to a compact system. The venturi scrubber unit is controlled at a pressure approximately equal to the pertaining confinement pressure. It is connected with the containment either by an isolation valve with a rupture disc or two isolation valves and a venting line. Whenever the pressure exceeds an approximate of 50 kN/m2, the rupture disc opens in the discharge line followed by the filtered vent gas entry into the venturi scrubber and gets consumed into a pool of water [6]. When the gas passes through the throat, gas flow produces suction which causes liquid water to be entrained with it and forms droplets. High interaction of water and gas occurs in the throat. Large amount of iodine is removed in the venturi scrubber because of the absorption of iodine in the scrubbing liquid. In addition with caustic soda, other additives in water increase the iodine scrubbing in the pool of water. Small amounts of aerosols and liquid droplets contained in the ejector of venturi scrubber are removed from the gas by the droplet separation unit and a micro-aerosol filter at the downstream. Water droplets are captured and discarded in the first part of the filter unit. The second part of the filter unit captures the aerosol particles which are very small to retain by any other scrubber or droplet separator devices [6]. An orifice plate is kept downstream of the metal fibre filter section. The gas in the downstream is expanded to atmospheric pressure while the filtration process is operated close to containment pressure. This orifice plate regulates the gas at a high speed inside the venturi section and at low speed at the metal fibre filter section. Hence it acts as a passive speed controller. The new FCVS Plus is the extension of the existing FCVS integrated with a passive superheating module and a molecular sieve (I-CATCH). As a result, the retention of organic and elemental iodine is significantly increased and permits a modular design. The AREVA FCVS significantly reduces the risk of clogging in the dry stage as majority of the aerosols are contained in the wet stage. The integration of the scrubber with a downstream droplet separation permits a dry gas condition for the downstream metal fibre fine aerosol filtration stage hence avoiding wet operation of the metal fibre filter sections ergo ensuring a reliable operation. The dry stage significantly reduces possible aerosol and iodine re-volatilization effects. The heat of more than 99% of the isotopes is absorbed by the scrubber liquid in the wet stage where it is trapped. This facilitates passive cooling and temperature control by evaporation. Thus the heat transfer from the FCVS to the atmosphere is drastically minimized which is crucial in SBO (station blackout) scenarios where the heating, venting and air condition (HVAC) would probably be out of order. The residual isotopes (less than 1%) trapped in the second stage is very unlikely to attain self-ignition temperature at hot spots inside this stage as the amount is very low. Flow diagram of the AREVA filtered containment venting system is shown in Fig. 1. Different Filtered containment venting system developed by AREVA is shown in Table 1.
Fig. 1. Principle flow diagram of the AREVA filtered containment venting system [6].Table 1. Different Filtered containment venting system developed by AREVA [5].
| FCVS Plus | FCVS Standard | FCVS Basic | I-Catch | |
|---|---|---|---|---|
| Filtration stages | High-velocity venturi scrubber | High-velocity venturi scrubber | High-velocity venturi scrubber | N/A |
| Metal fibre filter | Metal fibre filter | Demister | N/A | |
| Molecular sieve | – | – | Passive superheating and molecular sieve stage (I-Catch process) | |
| Decontamination factors (DF) | ||||
| Fine aerosols | >10,000 | >10,000 | >100 | – |
| Elemental iodine | >1000 | >200 | >200 | – |
| Organic iodine | >10-1000 | ∼5 | ∼5 | >10-1000 |
2.2. CCI filtered containment venting system
CCI filtered containment venting system was developed by SULZER CCI (IMI CCI nuclear) together with the Swiss “Paul Scherrer Institute” and installed in some Swiss NPPs [4]. This system includes an inlet basket inside the containment, inlet piping between the filter vessel and containment, containment isolation valves, auxiliary systems, instrumentation and control systems, filter vessel and clean gas piping. CCI filter vessel was made of a stainless steel vessel and has a three staged filtration system. Water pool section of a wet scrubber is stage-1, stage-2 is used for mixing elements and acts as a recirculation zone and gas space above the wet scrubber. Stage-3 is a three staged moisture separator. In stage 1, the wet scrubber removes the particulate effluents emitted from the containment with high efficiency. The contaminated gas moves through a large number of nozzles located inside a riser which is enclosed by an annulus region surrounded by the riser and vessel wall. Water is pushed by the gas, creating a water circulation between the annulus and the riser and forms gas bubbles. Total mass transfer of gas and water is increased by the recirculation of gas bubbles improving the decontamination factor (DF). It also arrests any flame propagation from the containment with the help of water present in the filter. In stage 2, steam condensation occurs in the gas space which increases the water level during the initial venting phase. Larger droplets formed by the bubbles burst at the surface of water due to low gas velocity and thus does not go through the separation unit. The mass transfer rate is increased by the co-current scrubber within the core section and by large residence time through trapped bubbles in the recirculation zone. In stage 3, the moisture separation unit removes the water droplets and aerosol in the gas stream. Sodium thiosulfate is added to the filter vessel to reduce elemental iodine (I2) and organic iodine (R-I) which are volatile. Sodium thiosulfate decomposes the dissolved elemental iodine and organic iodine into non-volatile iodide ions, increasing the pH of water leading to high reaction rate with CH3I and I2. Sodium hydroxide is used to maintain high pH. This system belongs to two filter systems. In generation I, only sodium thiosulfate was used as chemical additive. Later filter generation II was conceptualized by connecting tanks filled with chemical additives to increase the iodine decontamination factor [7], where decontamination factor is the ratio of initial specific radioactivity to final specific radioactivity that is a result of a separation process. Its unique feature is faster reduction and efficient retention due to decomposition of all iodine species to I− and prevention of the re-volatilization of the iodide ions respectively. A DF of 10 implies that 90% of the radioactive nuclides have been removed; it is one of the major aspects that form the basis of NPP safety design. A standard is set for each radioactive substance in a country by their respective regulatory body and only if the minimum DF is achieved can a NPP be installed. For example the DF of inorganic iodine should be greater than 200 for NPPs in Spain. This design offers high DF for both aerosols and iodine also preventing re-vaporization of both. The few disadvantages are risk of H2 combustion due to high pressure vessel during outlet throttling of venturi nozzle and its complex structure. CCI containment venting vessel is shown in Fig. 2 and flow diagram of CCI containment venting system is in Fig. 3.
Fig. 2. CCI containment venting filter vessel [7].
Fig. 3. Flow diagram of CCI Filtered containment venting system [7].2.3. Sand bed filter
Sand bed filter was developed by the Electricity de France (EDF) and The Institute for Radiation Protection and Nuclear Safety (IRNS). Later a dry metallic pre filter was added to increase the decontamination factor (DF) of aerosol to 1000 and it was installed in the French, EDF power plant [4]. Sand bed filter designed as deep bed filtration operates under dry condition. It is a cylindrical tank made of 316L stainless steel. Due to its large dimension, the sand bed filter is kept outside the containment. After the operation, the contents inside the filter generate high level of radiation. Metallic pre filter is able to keep 90% of radioactive material inside [8]. In the sand bed filter, the gas is uniformly dispersed throughout the filter by a constant mesh Kevlar lattice covering the sand bed. The gas then enters into the strainers made of stainless steel placed in the expanded clay and then goes to the rectangular collector in the filter area. A radioactive material measuring device is installed on the connecting pipe between the sand bed filter and the plant stack. The filter can withstand a pressure drop of 500 mbar. Re-vaporization and adsorption in the exterior of the sand bed filters and upstream piping balance the removal of the elemental iodine but it’s unable to remove the organic iodides. DF of this filter is almost 100 for aerosol particles. From a safety perspective, whenever containment pressure reaches its design limit, the pre-heating and conditioning fan are firstly stopped and connected valves are closed. Then two isolation valves of the containment are manually opened and at this time the pressure and temperature of the containment and the release activity during the venting are continuously evaluated by the operators. On the other hand, venting system is manually stopped whenever the pressure is within the safe level. But, this filter faces some issues like risk of hydrogen combustion, safety issue of venting system on site accessibility and resistance to hazards of the FCVS at the time of accident i.e., earthquake [7]. Schematic view of sand bed filter is shown in Fig. 4.
Fig. 4. Sketch of the sand bed filter of the FCVS system [7].2.4. Dry filter method
Dry filter method (DFM) was presented by the Karlsruhe Nuclear Research Centre KRANTZ-TKT [8]. DFM consists of modular filter stages. Two different types of filters are used for aerosol filtering and gaseous iodine retention. In the first stage, there is a deep bed metal fibre filter which removes the aerosols present in the venting gas using metal fibre fleece as the filter medium both consisting of stainless steel. It consists of multi staged design containing metal fibres of large diameter present in the initial stage, subsequently the diameter decreases, the diameters decrease from 65 μm to 12 μm and 12 μm–2 μm in the two stages respectively. Maximum amount of the aerosols are retained in first stage, while the next filter stages increases overall filtering efficiency. Iodine is removed in the 2nd stage by an iodine filter fixed downstream of the aerosol filter, which is made of silver doped zeolite, nearly spherical in shape specifically designed for iodine absorption. The iodine is chemisorbed by silver bound to the zeolite. The system iodine removal efficiency is optimized and steam humidity is eliminated by the specific passive expansion of the venting gas. Fig. 5 shows a schematic view of interior configuration of containment of a DFM system. The iodine filter is placed outside of the containment, while the aerosol filter is located inside the containment. Aerosol filter is composed of various modules which are arranged in parallel configuration. Aerosol filter stages are shown in Fig. 6. German PWR installs 3 aerosol filter modules. Initially the gas enters the aerosol module, leaves the containment and enters the iodine filter. Finally, clean gas escapes to the environment after removing iodine. DFM offers, DF > 10000 for aerosols, DF > 1000 for elemental iodine and DF > 40 for organic iodine. Modification of zeolite filter should be done to get the desired retention efficiency of organic iodine [7], [9]. Thus the DFM venting filter system consists of a combination of metal fibre and zeolite iodine filter modules and is designed in modules for easy adaption to meet plant requirements and safety regulations, which includes individual sizing of modules to fit into existing buildings. The system requires low maintenance cost, provides small pressure drop, high DF even for organic iodine (approximately 40), resists high temperature and radiation, exclusive passive mode of operation.
Fig. 5. Schematic view of the Dry Filter Method for containment filtered venting with aerosol filter inside containment [4], [7].
Fig. 6. Aerosol filter stages in DFM aerosol filters [7], [9].2.5. FILTRA MVSS
Westinghouse and Alstom presented a filtered vented containment system to mitigate the consequences of accidents occurring due to the over pressurizationof the containments of nuclear power plants called FILTRA MVSS (Multi Venturi Scrubber System). It consists of various filtration steps: multi venturi scrubber, moisture separator, water pool and sintered metal fibre filter located in a tank. Aerosols are captured in the water pool and removal of iodine is done by adding the chemicals in water tank with the use of the highly efficient venturi scrubber. Sodium thiosulphate is added to remove the essential iodine and organic iodine [7]. The demister in the tank reduces more than 97% of the droplets and thus significantly contributes to the overall DF while the metal fibre filter is used for filtering particles less than 0.8 μm. Filtra MVSS is designed as fully passive mode for 24 h; in that time it requires neither water supply nor electrical power supply, only a rupture disc is needed to actuate the system [10]. Other advantages include withstanding high seismic loads, high decay heat capacity, versatile as it gives same DF at high temperatures independent of the vent flow rate, modular design. It is installed in all the Swedish nuclear power plants and Muhleberg BWR (Boiling Water Reactor) station in Switzerland because of its 99.9% removal efficiency of iodine and aerosols. Later it was further improved by replacing the gravel bed moisture separator with a standard demister. After the moisture separation step, a set of sintered metal fibre filter cartridges were added, leading to a tremendous increase in the radioactivity removal, DF > 10,000 was obtained for elemental iodine and DF ≈ 2 for organic iodine. Zeolite filter which enhances the removal efficiency of organic iodine can be added if required. A typical flow diagram of a FILTRA MVSS is shown in Fig. 7. Generation 2 FILTRA MVSS module is depicted in Fig. 8.
Fig. 7. System configuration of FILTRA MVSS venturi scrubber [7], [10].
Fig. 8. Integrated FILTRA MVSS scrubber module [7], [10].2.6. SVEN scrubber system
SVEN scrubber system, presented by Westinghouse is used to remove micro size radioactive particles using sintered metal fibre filters (MFFs), making them a suitable primary containment ventilation by submerging in liquid. The contaminated gas is passed through a metallic filter cartridge which is submerged in the liquid inside a pressure vessel and cooled [11]. More than 99% of aerosols (DF > 100) are removed by this filter without clogging or exhibiting large pressure drop. Scrubber liquid cools the decay heat from captured aerosols and after that the gas passes through a liquid volume, where scrubbing of liquid occurs. Elemental iodine is eliminated by the sodium thiosulphate present in the scrubber liquid. Sodium hydroxide is used to increase the pH level of liquid. Splash shield and demister are installed above the liquid to remove moisture. Metal fibre HEPA filters is placed in the upper part of the SVEN tank which removes the smaller aerosols of size 0.3 μm. Small amount of organic iodine is present in the exit gas from the SVEN tank. In the upper part of the tank, above the scrubber liquid, a splash shield and a demister are installed to eliminate moisture from the vent flow, downstream to which there are a set of fine MFFs at the tank top to remove smaller aerosols. To achieve the desired DF for organic iodine, gas is routed through the silver impregnated zeolite beads. Integrated SVEN filtered containment venting system is shown in Fig. 9 and System Configuration of the SVEN scrubber is shown in Fig. 10. The advantages of this system are easy availability of filter media, possibility of scaling to fit into existing buildings, localized manufacturing of equipment when necessary and high DF values.
Fig. 9. Integrated SVEN filtered containment vent system [7], [11].
Fig. 10. System Configuration of the SVEN scrubber [11].3. Current global scenario of implementation of filtered containment venting system
Subsequent to the Fukushima incident, the Nuclear Energy Agency (NEA) Committee on the Safety of Nuclear Installations (CSNI) initiated a number of significant actions; particularly stress tests were performed across the globe that led many countries to consider the implementation of FCVS. Despite serving the same purpose, each county has its own safety system, varying in design and structure, owing to the difference in regulatory guidelines; while some countries adopt stringent measures others take up bit more flexible ones with cost issues being one of the major reasons. As of July 2018 nuclear power plants operate in 31 countries worldwide with a total of 453 nuclear reactors in operation, equalling 3975649 MWe of installed capacity, a brief description of the safety systems and legislations accepted in various countries is given below.
3.1. Sweden
All the present eight Swedish Nuclear power reactors in operation have a FCVS using Multi venturi scrubber filters of the FILTRA MVSS type in operation, providing about 35% of the total electricity [12]. They provide a minimum DF of 500 for BWR and 1500 for PWR. Since 2010 the Swedish Radiation Safety Authority (SSM) governs both the nuclear safety regulation and radiation protection. The 1980/1981 Energy Bill contains the fundamental policy guidelines for severe accident management and release mitigation. It dictated that at least 1000 of the core inventory of each radionuclide isotope excluding noble gases must be contained in the containment when venting during a severe accident. The new guidelines released in 1986 imposed a limit on the radioactive release to the environment as a maximum 0.1 percent of the reactor core content of Cesium-134 and Cesium-137. It was required by the licensees to construct strategies to protect the reactor containment function and permit the reactor to reach a stable condition where the core is cooled and covered by water. It was also essential that the containment function remains intact during the first 10–15 h after core damage.
3.2. Netherlands
Netherlands has one PWR in running condition and it has employed a wet scrubbing HSSPV type FCVS. It runs following the rules and regulations set by the 1963 Nuclear Energy Act [13].
3.3. France
About 75% of the electricity in France is obtained from nuclear power which is derived from 58 PWRs which employed FCVS by the early 1990s. This was a result of the plans and actions subsequent to the Three Miles occurrence which led to the France’s Institute of Radiological Protection and Nuclear Safety (IRSN) to establish secondary measures to control accidental situations. The FCVS include a metallic filter inside the containment and a sand filter to completely retain aerosols [14]. Formed in 2006 the Nuclear Safety Authority (Autorite de Surete Nucleaire– ASN) is the regulatory authority in charge of nuclear safety and radiological protection. The basic design criteria of the FCVS included: manual initiation of the system, 5bar (abs) initiation pressure and more than 90% (DF > 10) filter efficiency for aerosols and molecular Iodine. Though there are Safety Guides which are a limited set of technical regulations for nuclear establishments there are no legally binding rules or guidelines for FCVS in France.
3.4. Germany
The national body which is concerned with licensing and governing nuclear facilities in Germany is the Federal Ministry of Environment (BMU). But the license for NPPs is provided by the states which are in charge for the execution of federal laws. 1 to 1.2 times containment design pressure is the accepted critical containment pressure for venting. The reactors use either the wet (Sliding pressure venturi scrubber) or the dry filter (Metal fibre filter) [15]. The wet filters primarily comprise of a venturi scrubber system with a metal fibre droplet filter unit similar to the FILTRA/MVSS and the dry filter system. The system should be initiated manually; the filter has DF > 1000 for aerosol and DF > 10 for molecular iodine and the initiation pressure should be same as CV test pressure, these are the basic design criteria laid down by the Reaktor Sicherheits Kommission (RSK), the German Reactor Safety Commission: “Guidelines 4.6.25 section 2.2.1 and 4.6.32 BI 9.3.3.2.5” in the KTA (Kerntechnischer Ausschuss) Safety Standards and Standard Series 1503, 3401, 3404, 3413 and 3601. Additionally all the isolation valve must be able to be opened closed or adjusted under all severe accident conditions.
3.5. Switzerland
Switzerland owns five reactors on four sites, three PWRs and two BWRs which supplies approximately 35% of the country’s electricity. Three types of FCVS are currently in operation in the Swiss plants, the HSSPV, Filtra-MVSS and the SULZER-CCI-type. The basic requirement was that the aerosol and iodine retention factor should be greater than or equal to 1000 (99.9%) and 100 (99%) respectively [16]. The principal legislation governing nuclear energy is the 1959 Atomic Energy Act. From 2009 the Swiss Federal Nuclear Safety Inspectorate (ENSI) oversees the nuclear safety and security of the nuclear plants in the county. The following basic requirements were laid down by the “Hauptabteilung für die Sicherheit der Kernkraftwerke” (HSK) for containment integrity and mitigating measures: prevent uncontrolled radioactive release due to loss of integrity of the containment during severe accident (Prevention of long-term contamination by Cs); nominal relief capacity of 0.8% of thermal power; maximal relief capacity of 1% of thermal power; aerosol retention factor: ≥ 1000 (99.9%); iodine retention factor: ≥ 100 (99%).
3.6. Bulgaria
Bulgaria having two nuclear reactors on one site has implemented a HSSPV FCVS in them, this was in response to the new operation license issued by the Nuclear Regulatory Agency of Bulgaria in 2003 [17]. The two main bodies the Nuclear Regulatory Agency (NRA) associated with the safe uses of Nuclear Energy Act 2002 and the Kozloduy Nuclear Power Plant PLC are responsible for safety, radiation protection and management of nuclear wastes.