Provision of containment integrity at Russian VVER NPPs under BDBA conditions

Transcription

1 Provision of containment integrity at Russian VVER NPPs under BDBA conditions IAEA Technical Meeting Severe Accident Mitigation through Improvements in Filtered Containment Venting for Water Cooled Reactors 31 August -3 September 2015

2 Contents Introduction Existing requirements applied for Russian NPPs Russian Regulatory requirements EUR requirements Results of BDBA analyses for operating VVER-1000 Measures on improvement of operating VVER NPPs safety Possible FCVS designs for VVER NPPs Results of BDBA analyses for new VVER designs Additional technical solutions Conclusions 2 2

3 Introduction The Fukushima Daiichi accident demonstrated that, in the absence of alternatives to reduce the containment pressure build-up (due to steam and incondensable gas accumulation during an accident), the venting of the containment becomes an essential accident management measure for the preservation of its structural integrity. This accident have led to consider the implementation of filtered containment venting on NPPs, where these systems are not currently applied, as an enhancement of the response capability to severe accident situations. At present, Russia has 6 VVER-440 units and 11 VVER-1000 units in operation. Up to now, non of them are equipped with FCVS. 3 3

4 Russian Regulatory Requirements General Safety Rules OPB-88/97 (in force) It The reactor and NPP systems and elements containing radioactive substances shall be accommodated entirely in leak proof compartments for localizing radioactive substances released within their limits during design basis accidents. In this or any other case of arrangement it is necessary that during normal operation and design basis accidents the corresponding reference exposure doses are not exceeded as well as allowable release rate and content of radioactive substances in the environment. The necessity and acceptability of the directed release of radioactive substances under beyond design basis accidents shall be substantiated in the design. It Measures on detecting and preventing formation of explosive concentrations of gases in rooms accommodating localizing safety systems shall be envisaged 4 4

5 Russian Regulatory Requirements General Safety Rules new revision (draft, not yet implemented) It The reactor and NPP systems and elements containing radioactive substances shall be accommodated entirely in leak proof compartments for localizing radioactive substances released within the hermetic envelope during design basis accidents. The directed discharge of radioactive substances outside the hermetic envelope is allowed under severe beyond design basis accidents only for purpose of protection of hermetic envelope integrity if measures on ensuring radiation safety for population are provided. 5 5

6 Russian Regulatory Requirements Localizing safety systems NP If in the nuclear power plant design a provision for the discharge of the working medium from one accident confinement area compartment to its another compartment or discharge of the working medium beyond the accident confinement borders (apart from discharge of the working medium through steam passive condensers) is made, such accident confinement areas shall be equipped with the safety and (or) relief devices (dump valves, rupture diaphragms, release (nonreturn) valves, etc.) with decontamination of the working medium being discharged from the accident confinement area The hydrogen explosion safety systems are intended to fulfil the following main functions: - to prevent formation of the explosion-hazardous mixtures in the accident confinement area by holding the hydrogen concentration in the mixture below the explosion safety limits, - preventing formation of the explosion initiation source in the accident confinement area, - providing protection from explosion in the accident confinement area, monitoring hydrogen concentration in the accident confinement area. The fulfilment (non-fulfilment) of these or other functions shall be established by the nuclear power plant design and shall be validated in the nuclear power plant safety validation Report Proceeding from the functions established by the nuclear power plant design, the following main systems shall be used for assurance of hydrogen explosion safety: - systems for combustion of the hydrogenous mixtures in the accident confinement area; - systems for withdrawal of the hydrogenous medium from the accident confinement area (including decontamination of the working medium and its discharge into the environment); - systems for mixing the medium in the accident confinement area; - systems for phlegmatization during and after the accident. The list of the systems required to ensure hydrogen explosion safety shall be established by the nuclear power plant design and shall be validated in the nuclear power plant safety validation Report The confining safety systems shall be designed with allowance for the pressure which is produced in the process of combustion of the hydrogenous mixtures. The systems (component elements) and compartment can be protected from destruction with the use of the pressure release devices, fire barriers, hydraulic locks. 6 6

7 EUR Requirements rev. D Chapter Filtered venting systems The need of using a system of discharge with purification shall be considered in the design for the worst scenario of Severe Accident If a containment filtered venting system is provided, it shall have the following performance capabilities: - The flow capability shall be determined such that it can reduce containment pressure to an acceptable level - The flow capability shall be such that the containment filtered venting system can relieve the full residual power after a time consistent with the passive capabilities of the containment (at least 12 h). - The retention capability of the filter shall be such that the release criteria can be met 7 7

8 Results of BDBA analyses for operating VVER-1000 In order to reveal the weak points in the NPP s designs in case of BDBA, the stress-tests were carried out to analyze the NPP response In addition to events happened at Fukushima plant: loss of power supply, including major station black-out (SBO) loss of ultimate heat sink combination of the above-said events the loss of coolant accidents were considered as the worst case BDBA BDBA analyses were carried out both for power operation conditions and for shut-down states Large break LOCA combined with loss of all AC power sources Long-term blackout with full core unloaded to SFP Long-term blackout during power operation Long-term blackout during refueling 8 8

9 Results of BDBA analyses for operating VVER-1000 without accounting for additional technical means Äàâëåí èå, Ï à Äàâëåí èå, Ï à Âðåì ÿ, ñ Âðåì ÿ, ñ VVER-1000 (mod. V-320) containment pressure under station blackout at power operation VVER-1000 (mod. V-320) containment pressure under LB LOCAВ 9 9

10 Additional technical post-fukushima means suggested for operating VVER-1000 For BDBA management and mitigation of their consequences, in accordance with the developed by Concern Rosenergoatom Actual measures for mitigation of BDBA consequences the additional mobile equipment is placed at the NPP sites : Mobile diesel-generator unit (MDGU) MDGU-2,0 MWt, capable to provide electric power supply to the equipment required to overcome BDBA. Mobile pump units (MPU): MPU 150/900 for boron solution supply to the reactor ; MPU 500/50 for service water supply to consumers and/or water pumping from «bottom elevations» in case of flooding; MPU 150/120 for water supply to steamgenerators; MPU 40/50 for boron solution supply to spent fuel pool and/or water pumping from «bottom elevations» in case of flooding. Depending on the purpose of MPU, the sources of water are: emergency sump or water storage tanks, refueling water storage tanks, demineralized water storage tanks, service water intake channels. MPU 150/900 are stationary placed near the main reactor buildings, MPU 500/50, MPU 150/120 and MPU 40/50 are stored in hangars and in case of necessity are brought promptly to the connection points. The Mobile diesel-generator unit MDGU-2,0 MWt is placed at special ground near the stand-by diesel power plant. All additional mobile equipment maintains operability after SSE

11 Scope of equipment delivery to Russian NPPs Установка «Большой поток» испытана на Калининской АЭС 29 Mobile 2.0 MWt DG (6kV; 0,4 kv; 220V DC) 37 Mobile 0.2 MWt DG (0,4 kv) 36 Mobile high pressure pump units with different capacity and pump head 1 High capacity separate pump unit 80 Monoblock pumps with different capacity and pump head

12 Results of BDBA analyses for operating VVER-1000 with account for additional technical means Äàâëåí èå, Ï à The results of analyses: In case of station blackout, loss of ultimate heat sink or LOCA the use of additional technical means, delivered to NPP as a post-fukushima measures, prevents reaching the containment limiting pressure Âðåì ÿ, ñ VVER-1000 (mod. 320) containment pressure under LB LOCA with account for water supply to SFP from mobile pumps

13 Summary of the analyses results Accident condition LOCA+Blackout Blackout with full core unloaded to SFP Blackout during power operation Blackout during refueling Required additional equipment MDGU 2,0 MWt MPU 40/50 MDGU 2,0 MWt MPU 40/50 MDGU 2,0 MWt or MPU 150/900 MPU 150/120 MPU 40/50 MDGU 2,0 MWt or MPU 150/900 MPU 500/50 MPU 40/50 Time reserve for actuation (requirements based on TH calculations) Real time needed for actuation (basing on on site emergency exercises) <1 1 1,5 <1 1 1,5 2 < , ,5 1 1,5 4 4,5 < ,5 2 1, ,5 Comment Prevention of containment failure Prevention of fuel damage and containment failure

14 Measures on prevention of hydrogen explosion Implementation of hydrogen monitoring systems at operating VVER units Installation of passive catalytic hydrogen recombiners at VVER NPPs Avoidance of initiators of hydrogen explosion in containment

15 The conclusions of the NPP General designer on the expediency of FCVS implementation at operating NPPs basing on the analyses results Due to specific design of VVER-1000 containment and spent fuel pool, without account for management actions the available time reserve to possible containment failure is in the range from 2 to 4 days The comparison of the time estimates during which the additional technical means for BDBA management shall be deployed and the real time required for bringing them into action (basing on the results of emergency exercises at NPPs) demonstrates the capability of successful implementation in case of accidents The BDBA management using additional technical means allows to exclude the containment failure caused by pressure increase and provides integrity of the core and fuel in the spent fuel pool. The availability of several ways of accident management utilizing additional technical means, its high reliability, gives confidence in high probability of successful performing required safety functions. Taking into account that usage of additional technical means stops the containment pressure build-up, the installation of containment venting system is a supplementary protection measure that doesn t contribute much to NPP safety

16 Development of initial data and organization of activities for FCVS implementation The initial input data for designing of FCVS are defined in assumption that additional technical means are unavailable An Industry Decision on the way of implementation of FCVS is issued The initial technical requirements to FCVS for operating VVER NPPs are developed The mode of operation of FCVS is determined (periodical short-run operation with succeeding recharging). The decision is taken that first FCVS will be installed at Rostov NPP unit 1 and after that FCVSs will be implemented at all other operating VVER NPPs The Technical Assignment for delivery of non-standard process equipment for Rostov NPP unit 1 is developed The FCVS implementation schedules are developed for all NPPs The engineering-constructor documentation for upgrading of standard systems that will be used for connecting FCVS is developed. The activities for buying FCVS equipment are on the way

17 Initial technical requirements to FCVS Parameter Steam-gas mixture flowrate Working pressure Gas mixture composition, %: Requirements for VVER ,5 m 3 /s ~ 4,5 kg/s at 0,45 MPa (abs.) and 250 О С Containment pressure not more 0,45 MPa (abs.) With account for hydrogen recombiners Requirements for VVER-440 (mod. B-213) 35 t/h (10 kg/s) 0,15 0,2 MPa (abs.) Steam ,5 77,5 Air (N 2 +O 2 ) (10-20) Carbon monoxide Hydrogen ,5 5,5 Relative humidity, % Total mass of aerosols, coming to filter, kg

18 Initial technical requirements to FCVS Parameter Radioactivity of fission products, Cu : Requirements for VVER-1000 Requirements for VVER-440 (mod. B-213) I-131 (aerosols) I-131 in volatile form (organic -20%, molecular 80%) Cs Sr Trapping efficiency, % Iodine - 99,9 Cesium - 99,95 Strontium - 99,0 Aerosols - 99,9 Molecular iodine - 99,9 Organic iodine - 99,0 Fission products heat release, kw

19 Selection of the FCVS design solution Two filtration methods are considered: 1. «Wet» filtration

20 Selection of the FCVS design solution 2. «Dry» filtration The process diagram of filtering system using dry filter

21 Selection of the FCVS design solution 2. «Dry» filtration Iodine sorption filter Aerosol filter Ceolitic filter Thermoxide-58 filter Fizkhimin filter

22 Ceolitic filter Thermoxide-58 filter Fizkhimin filter Filter for cleaning gaseous fission products (developed in 90 at the stage of technical design А ) 1 Catalyzer for hydrogen burning 2 Sorbent grains 2 4 mm 3 Sorbent grains 1,6 2 mm 4 Sorbent grains 0,4 1 mm 5 Tubular electric heater 6 Net 1,4 мм 7 Net 0,45 мм 8 Filtering element 9 Grate 10 Sampling pipe 11 Working medium entrance 12 Working medium exit Mass of filter 45 tons Thermoxid-58 sorbent load 17 tons

23 Selection of FCVS type and layout solutions For wet filtration (AREVA) there are difficulties in placement of the equipment due to its weight and dimensioning specifications: At VVER-1000/В-320 NPPs two layout variants are possible: in auxiliary building compartments or near the RB wall from the outside. At small series VVER-1000 NPPs the equipment can be mounted in SWT building

24 Comparative analysis Placing in auxiliary building advantages: constant readiness to operation in case of BDBA (the system is already filled); absence on necessity to construct additional building for placing of equipment Placing outside the RB advantages: No need in relocation of existing pipes and communications; Possibility to perform erection works in periods between planned maintenance. disadvantages: The main scope of work is possible only during planned maintenance; Difficulties of installing large size equipment requiring removal of the roof, provision of additional sanitary zone, relocation of existing pipes and communications, inconvenient conditions of works. disadvantages: Less availability for operation (need to fill the Venturi tank); Need to confirm the possibility to design the system as of 1 st seismicity class accounting for connections with other systems; Additional justification of possibility to design pipelines and equipment containing highly radioactive substances outside the RB is required; Possible problems with start up of the system at negative temperatures

25 Selection of FCVS type and layout solutions If the «dry» filtration is selected, then the equipment can be placed inside the containment and in compartments of auxiliary building as well

26 The issues to be solved when implementing FCVS Ready FCVS designs that fully meet the laid requirements do not exist The decisions about the design and type of FCVS for VVER NPPs are not finally taken. The layout solutions require additional elaboration (difficulties of placing and mounting). Difficulties in buying non-standard import equipment and long manufacturing period. Licensing of equipment and erecting works

27 New VVER NPP designs New VVER NPP designs: AES-2006 ( VVER.1200 ) VVER-TOI ( VVER.1300TOI ) Passive Heat Removal System All these designs are based on AES-92 design Distinctive feature - wide application of passive safety systems Primary circuit Core catcher 27

28 Main technical and economic parameters of new VVER NPP designs Unit Electrical power VVER MWt VVER.1300TOI MWt Unit thermal power MWt MWt Service life of reactor unit 50 years 60 years DBE/SSE 6/7 points 7/8 points Airplane crash (design basis event) 5,7 ton 20 ton NPP autonomy in case of BDBA 24 hours 72 hours 28 28

29 Main passive safety systems of new VVER NPP designs Outer containment Inner containment Annular space Second stage HA Passive Heat Removal Passive System Heat Removal System HA-2 HA-3 HA-1 PHRS heatexchanger Main circulation pump Primary Primary circuit circuit Core catcher Core catcher 29

30 Results of BDBA analyses for new VVER NPP designs The results of analyses for Fukushima conditions: no core damage time reserve for bringing active safety systems or additional technical means (post-fukushima measures) into operation is ~ 10 days Containment pressure in case of station blackout

31 Safety analysis of new VVER NPP designs for more burdened conditions as compared to Fukushima NPP Loss of all AC electric power sources combined with LOCA of primary side main circulation pipeline PHRS Hydroaccumulator-2 (960 m 3 ) Under the stated emergency conditions the residual heat is removed from the reactor core by the jointly operated passive heat removal system (PHRS) and the second-stage and third-stage hydroaccumulators Hydroaccumulator-3 (720 m 3 ) Hydroaccumulator-1 (200 m 3 ) The NPP autonomy (no damage to the core) in this mode of operation is determined by water inventory in the HA-2 and HA-3 hydroaccumulators and equals to 72 hours at least. Ultimate containment pressure can be reached after ~ 5 days

32 Conclusions and further directions of work on safety improvement of new VVER NPP designs In order to improve the new VVER NPP robustness against low probability and hypothetic events and increase the autonomy period in case of BDBA, the designs are provided with additional technical means for heat removal containment, such as: from the core, spent fuel pool and - Mobile diesel-generator unit; - Alternate intermediate circuit pump; - Mobile fan cooling tower and foreseen their connection to the regular equipment provided for by the design for design-basis accident management

33 Measures on improving new VVER NPP designs robustness against extreme external impacts

34 Final Conclusions For operating VVER NPPs, taking into account that in particular accident modes the time needed for connection of additional technical means is comparable with the time period during which it shall be brought into operation, the political decision on equipping NPPs with FCVS was taken. For new NPP designs, taking into account large time reserves (provided by operation of passive safety systems) for enabling additional technical means under accidents resulting in containment pressure increase, the decision on inexpediency of FCVS implementation was taken

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