South Ukraine’s filtration first

IN SEPTEMBER 2018, A FIRST-of-a-kind installation of a containment filtered venting system for VVER-1000 reactors in Ukraine was completed at South Ukraine 1 and 2. The containment filtered venting system (CFVS) is a dry filtration method that acts as the last barrier for minimising release of radioactive material to the environment should all other systems fail. It is designed and delivered by Westinghouse to protect the containment and facility simultaneously, by depressurising the containment to prevent it from ultimate failure in slow over-pressurisation scenarios.

This system was originally developed by the leading German laboratory of nuclear filtration technologies – the laboratory of aerosol physics and filtration technology of the Nuclear Research Centre in Karlsruhe, now called the Karlsruhe Institute of Technology. It is further developed and offered by Westinghouse and its partner Krantz, and is customised for each plant’s specific requirements.

The basic premise of the CFVS is that catastrophic failure of the containment structure can be avoided by filtering discharged steam, air and non-condensable gases like hydrogen to the atmosphere, decreasing pressure within containment while minimising the radiation and aerosols released. Filtering the gases prior to release reduces both the surrounding population’s risk of exposure to radiation and the potential for land contamination near the site.

Many plants globally have to implement such systems as a safety measure, to enhance severe accident response, following the Fukushima Daiichi accident.

Dry filtered method CFVS

Westinghouse has supplied and installed CFVSs based on the dry filtered method in many nuclear power plants around the world. At South Ukraine 1&2, Westinghouse not only installed the first such system for VVER-1000 units, but the first of the latest developed design of the system, called DFM 2.0 CFVS.

The dry filtration method, or DFM, consists of two passive filtration components, with high decontamination factors for each. One is an aerosol filter using a deep bed metal fibre as the filter medium to remove aerosols such as caesium iodide and caesium hydroxide. The other is an iodine sorption filter using a molecular sieve with zeolites to remove elemental iodine and organic iodine very efficiently. The decontamination factor of the aerosol filter is greater than 10,000, that is, greater than 99.99% retention of contaminants. The iodine filter can be adjusted by design to meet or exceed requirements with a decontamination factor of greater than 10,000 for elemental gaseous iodine (more than 99.99% retention) and a decontamination factor of greater than 100 for organic gaseous iodine (greater than 99% retention).

Each CFVS that Westinghouse delivers is configured based on plant-specific functional requirements. These include the parameters that define the range of operation, location for the installation and seismic requirements, among others such as the composition of the medium to be filtered, that is, the steam-gas mixture generated during an emergency. This mixture contains air, steam from water, hydrogen, radioactive inert (noble) gases, and radioactive and non-radioactive aerosols that result from core melting, cable burning, and high-temperature concrete decomposition products. There are three configurations:

• Aerosol filter located inside containment and iodine filter located outside containment;
• Aerosol and iodine filters combined and located outside containment; 
• Aerosol and iodine filters combined and located inside containment (DFM 2.0 CFVS).

The design is modular, which allows the components to be adapted to the available space of each specific plant layout for any of the configurations. For certain plants, the ability to locate one or both types of filters outside the containment is advantageous. For most plants, including South Ukraine 1&2, the ability of the latest DFM 2.0 CFVS technology to be combined and located together inside containment allows the most effective use of existing structures. This arrangement (Figure 1) also has lower capital cost, fewer components and less maintenance than the configuration with filters outside the containment.

Placing both aerosol and iodine filters inside the containment was made possible by the development of improved zeolite material. This improved zeolite allows the iodine sorption filter to be located inside the containment. Previously available zeolite could not sustain in- containment environmental conditions, such as high- temperature operations and saturation conditions in the steam atmosphere, radiation exposure during standby and also exposure to containment atmosphere during standby. The ability of the improved zeolite material to retain organic iodine was tested and confirmed by TÜV Süd in Germany. The test samples were artificially aged, exposed to radiation and high temperatures and tested for retention at operational pressures up to 6 bar absolute pressure.

The advantages of the latest developed filter design (DFM 2.0) include:

• All fission products except noble gases will stay within the containment.
• There is no possibility flammable mixtures will form in the filters, since they will be in thermal equilibrium with the containment, which prevents condensation.
• Existing components within the plant’s ventilation system can be used, leading to simpler, less expensive systems.
• Filter sizes are significantly reduced, as compared to filters being installed outside of containment, since the steam-gas mixture barely expands, lowering costs substantially.

Arrangement within South Ukraine 1&2

The aerosol and iodine filter stages are combined in a single housing and located within the hermetic part of the containments at South Ukraine 1&2. Each housing contains two aerosol filters and two iodine filters. Each aerosol filter has three pre-filter stages and one main filter stage. The aerosol filters consist of metal fibres with smaller diameters in each filter stage. While the main filter alone can provide the decontamination factor, the pre-filters preserve the storage capacity of the main filter and spread the heat load.

Each iodine filter has one zeolite bed. The adsorption material is composed of small beads of sliver-doped zeolite. The decontamination factor depends on the residence time of the gas passing through the filter bed, and the distance to the dew point in the gas (i.e., how much the temperature is above the value at which airborne water vapour condenses).

The effective area of the iodine filters and bed depth of the silver-doped zeolite are designed to meet the retention requirements for elemental and organic iodine for the whole venting process.

The filters also have to cope with the decay heat from the radioactive aerosols and iodine that is filtered from the gas stream and stays inside the filter. Rows of cooling tubes are installed in front of each aerosol pre-filter stage, between each aerosol pre-filter and main filter stage and zeolite bed. Each row of the aerosol filter stages consists of 21 cooling tubes; the iodine filter stages have 11 cooling tubes each. Through natural convection, the cooling tubes remove heat generated inside the filter stages by the fission products.

The CFVS filters discharge the steam-gas mixture through a 250mm pipe, which runs along the containment outer wall, through a rupture disc and two isolation valves directly to the atmosphere. The pipe runs through a penetration to containment that was already in the reactor compartment.

Except for a low-pressure nitrogen system that is separated from the CFVS with shutoff valves, the CFVS is not connected to other emergency or technological systems. The rupture disc allows the system to be passively activated as it is designed to react to a pressure difference specific to the plant. Subsequent periods of starting and stopping the discharge from containment are conducted manually by opening and closing the hermetic valves. The low-pressure nitrogen systems located at South Ukraine 1&2 are connected to the CFVSs, for more favourable hydrogen safety due to the pipeline being purged with nitrogen gas following each venting cycle.

To assess the safety of the CFVS during flooding conditions, MELCOR simulations were conducted. MELCOR (Methods for Estimation of Leakages and Consequences of Releases) is a fully integrated, engineering-level computer code developed by Sandia National Laboratories for the US Nuclear Regulatory Commission to model the progression of severe accidents in nuclear power plants.

A potential flooding level at South Ukraine 1&2 is at elevation +12.30m. Westinghouse installed the filter modules on steel support structures within a specific room at elevation +13.80m from ground zero. The MELCOR simulations of the severe accident scenarios show that the difference between the maximum calculated coolant level value and the location of the filter modules provides a 1m margin before the filters would be flooded. As a result, the natural circulation that occurs in the filters’ cooling tubes will not be hindered.

To fulfil the requirements specific to the South Ukraine plant, two dual filter housings were needed in the CFVS. Each VVER-1000 unit is equipped with four combined filters installed inside their containments. During the sizing studies for the combined filters for South Ukraine 1&2, the MELCOR severe accident code was used to calculate the local conditions based on the enveloping scenario of several simulations.

The local temperature, pressure and density parameters are linked to the containment atmosphere in the room where they are placed. This is an important feature of the DFM 2.0 CFVS because the decay heat of the radioactive fission products retained in the different filtration stages will be dissipated through the cooling tubes to the containment atmosphere directly. 

The location inside the containment is also positive for hydrogen safety because it eliminates the condensation of steam in the filter stages between venting cycles and excludes the possibility of hydrogen deflagration inside the filters.

Safety and licensing for use in Ukraine

In accordance with Ukrainian nuclear regulatory requirements, the technical specifications, along with the programmes and methods of testing and the preliminary safety analysis report documents, were reviewed and approved by the State Nuclear Inspectorate of Ukraine prior to the installation of the CFVSs at the South Ukraine units.

During this certification process, independent laboratories in Ukraine and Germany supported the certification tests to determine the CFVS’s compliance with the plant technical specification requirements, including attending the factory acceptance tests performed for the CFVS components.

Severe accident scenarios and simulation

The severe accident analyses performed for the preliminary safety analysis report were conducted using MELCOR 1.8.6 on the existing model provided by the plant, with modifications to reflect the current plant status, updated input data and inclusion of the CFVS equipment. Nine accident scenarios were analysed and documented, including the large-break loss-of-coolant accident scenario in combination with full station blackout. Additionally, hydrogen safety was analysed as part of those scenarios. The hydrogen-oxygen mixture in the containment was found not to exceed the deflagration criteria during the entire accident duration. Assessments related to release of radioactive products to the environment were also performed that compare releases with the CFVS implemented and not implemented. These assessments demonstrated that filtered venting of the containment decreases the impact on the population as well as the cost of emergency response measures such as evacuation.

Implementation

Despite the very limited space within containment, installing the DFM 2.0 CFVS does not affect the overall outage schedule. The CFVS and associated piping can be installed during refuelling outages, as was the case at South Ukraine 1&2. Once installed, the system has no impact on plant operation and requires only minimal maintenance. It is fully passive, has no chemicals or liquids and the metal filters and silver-doped zeolite are resistant to the effects of aging. Additionally, since the system is open to the atmosphere within containment, a nitrogen system is not necessarily required, but it can be installed upon request.

Lessons learned during the installation of the CFVS at South Ukraine 2 were applied to unit 1, which shortened the duration of time needed to deliver and install the equipment at this unit. These lessons allowed Westinghouse to improve quality assurance, supply chain management and the logistics associated with import/ export constraints.

By implementing the CFVS, South Ukraine 1 became the first plant in Ukraine that has fully implemented the post-Fukushima measures within the framework of the Complex Consolidated Safety Upgrade Programme for Ukrainian nuclear power plants.  

Author information: Zoran Vujic, Product manager VVER engineering products and services for Westinghouse