Russian regulatory approach to evaluation of passive systems used for specific BDBA S (SBO, loss of UHS) during safety review of NPP

Transcription

1 Federal Environmental, Industrial and Nuclear Supervision Service Scientific and Engineering Centre for Nuclear and Radiation Safety Member of Russian regulatory approach to evaluation of passive systems used for specific BDBA S (SBO, loss of UHS) during safety review of NPP Denis Rogatov SEC NRS

2 CONTENT 1. Introduction 2. VVER-1200/AES-2006-M and VVER-1200/AES-2006-P design solutions 2.1. Evolution of VVER in years 2.2. AES-2006 safety systems 2.3. Configuration of passive safety systems. Novovoronezh NPP Configuration of passive safety systems. Leningrad NPP-2 3. Key regulatory requirements to evaluation of passive systems 4. Key findings of safety review in regard to passive systems (by the example of Novovoronezh NPP-2) 4.1. Novovoronezh NPP-2. Key findings of safety review (Air-cooled PHRS & HA-2) 4.2. Complex of additional calculated and experimental substantiations. Example 4.3. Novovoronezh NPP-2. Results of safety review 5. Conclusion 2

3 VVER-1200/AES-2006-M and VVER-1200/AES-2006-P design solutions 3

4 Evolution of VVER in years NPP-91 NPP-92 Serial VVER Tianwan NPP, China (CPR) Kudankulam NPP, India Leningrad NPP-2 Novovoronezh NPP-2 4

5 AES-2006 safety systems in new VVER design Leningrad NPP-2 4х100% (Typical structure of safety systems) Double-walled containment Passive part of the ECCS (hydro accumulator tanks of the 1 st stages) Water-cooled SG Passive heat removal system (PHRS) Containment PHRS Emergency hydrogen removal system Active systems Passive systems Novovoronezh NPP-2 2-trays (some - with redundancy of separate elements inside train) Double-walled containment Passive part of the ECCS (hydro accumulator tanks of the 1 st stages) Hydroaccumulator tanks of the 2 nd stages Air-cooled PHRS Emergency hydrogen removal system Molten core catcher (in case of severe accidents) Mobile equipment 5

6 Configuration of passive safety systems Novovoronezh NPP-2 Air-cooled PHRS 1st stage HA 2nd stage HA PCHR PHRS heat exchanger Steamgenerator RCP Molten core catcher Reactor Spent fuel pool 6

7 Configuration of passive safety systems. Leningrad NPP-2 Water-cooled PHRS 1 Heat is removed with evaporating water from the tanks to the atmosphere tank; 2 steamlines; 3 condensate lines; 4 SG PHRS valve; 5 Containment PHRS heat exchangers; 6 Steam generator; 7 Cut-off valve (large and small for SG PHRS) 7

8 Key regulatory requirements to evaluation of passive systems 8

9 Key regulatory requirements to evaluation of passive systems When designing the systems (elements) of NPP and Reactor Installation, it is necessary to give preference to the systems (elements) with design based on the passive principle of action and inherent safety features (self-control, heat retention, natural circulation and other natural processes) as well as on the fail-safe principle (Para NP ) New VVER NPP designs: Passive heat removal systems - design solutions to cover specific BDBA (SBO, loss of UHS) Reliability of safety functions implementation reduce the number of active components and (or) support systems reduce maintenance and testing efforts sources for failures may be reduced 9

10 Key regulatory requirements to evaluation of passive systems The established limits for design basis accidents shall not be exceeded at any initiating event considered by the NPP design with a coincidental independent failure of one of the following safety system elements according to the single failure principle: an active element or a passive element with mechanical movable parts, or a passive element without movable parts whose probability of safety function performance failure is equal to 10-3 or higher or one human error independent of the initiating event. Failures of elements (systems which they make part of) may not be considered if high level of their reliability is demonstrated or when the element (system) is in outage for a determined period of time for maintenance and repair. The reliability level is considered to be high if indicators of reliability of elements (systems) are not lower than appropriate indicators of the most reliable passive elements of safety systems without movable parts. (Para NP ) 10

11 Key regulatory requirements to evaluation of passive systems Passive safety systems (as the active systems) and elements shall be capable of performing their functions within the NPP designs cope and with due consideration of the natural effects, external human induced events typical for the NPP site and (or) under possible hydraulic, mechanical, thermal, chemical and other impacts occurred as a result of accidents at which work of the considered systems and elements is required (Para NP ) Passive systems mainly designed to fulfill their function in case of SBO or/and loss of UHS: special attention should be directed to the hazards which can lead to SBO and loss of UHS 11

12 Key regulatory requirements to evaluation of passive systems All safety systems shall be designed and protected to tolerate common cause failures by applying the principles of diversity, redundancy and independence (Para NP ) Principles applied to combination of active and passive systems Active system Safety function 33% 33% 100% Passive system 33% 33% 100% 12

13 Key regulatory requirements to evaluation of passive systems Passive safety systems should be tested, including their active components, during commissioning and in operating plants to the extent feasible to assure their compliance with the design parameters Direct and full checks are preferable but if performance of direct and/or full checks not be possible indirect and/or partial checks shall be carried out Adequacy of indirect and/or partial checks shall be validated in the NPP design (para NP ) 13

14 Key regulatory requirements to evaluation of passive systems Reliability analyses of fulfillment of functions by the passive safety systems shall be presented in SAR (para NP ) General approaches of fulfillment qualitative and quantitative reliability analysis of safety important systems, including features of the analysis of the reliability of systems with passive elements: RB Regulatory guide. Recommendations in order of execution reliability analysis of systems and components of nuclear power plants important to safety and their functions 14

15 Key regulatory requirements to evaluation of passive systems Confirmation of the actual characteristics of passive systems is performed during the commissioning of the NPP unit, while the characteristics of equipment and systems, design limits and conditions will be specified (para NP ) Based on the results of commissioning, a final version of the FSAR should be developed, that contains information on the results of the carried out tests, including the refined information of the systems. The Licensee should justify in the FSAR that the passive systems, according to the results of commissioning, correspond to the initially presented data, and the characteristics of the systems obtained from the results of calculations and experiments are confirmed (para NP ) 15

16 Key regulatory requirements to evaluation of passive systems Passive safety systems should be described in Safety Analysis Reports (SAR) at the same level of detail as the other safety systems; When safety function can be fulfilled by either active or passive safety system the SAR (Chapter 15) should contain substantiation for both cases: a) when only active systems are in operation and b) when only passive systems are in operation; False actuation (starting) of passive system should be evaluated as one of initiating event (as a rule the false actuation of PHRS SG is considered) NP Requirements for the content of safety analysis report for nuclear power plants with VVER type reactors (new version) 16

17 Key regulatory requirements to evaluation of passive systems Requirements for safety valves, hydraulic test, materials used to manufacture of passive system components without any difference from active systems, are applied for (elements under pressure of the working medium) of passive systems NP Rules for design and safe operation of equipment and pipelines of nuclear power installations (new version) In case of passive systems (their pipelines) penetrating the containment and communicating with the reactor coolant or containment atmosphere (isolation valves and etc.) NP Rules for arrangement and operation of nuclear power plant confining safety system (new version) 17

18 Key regulatory requirements to evaluation of passive systems Technical and administrative decisions made for NPP safety ensuring shall be well proven by the previous experience or tests, investigations, operating experience of prototypes. (Para NP ) Main regulatory requirement for passive systems evaluation 18

19 Key regulatory requirements to evaluation of passive systems SAR (or some topical reports referenced in the SAR) shall present: Sufficient substantiation of systems operability by calculations, experimental studies and (or) operational experience) Applicability of thermo-hydraulic computer programs (codes) used for safety justification: codes shall be verified and certified Analysis of the compliance of experimental conditions with the actual conditions of the system 19

20 Key regulatory requirements to evaluation of passive systems When the calculations and experimental studies of passive safety systems are implemented, the following issues should be taken into account: sufficiency of heat removal from reactor: with no time limitation or the providing explanation for time limitation 24, 72 or more hours (for water-cooled PHRS: the measures on inventory make-up are to be envisaged) in any weather condition (for air-cooled PHRS: temperatures or wind speed) potential interactions of active and passive systems or/and different passive systems foreseen to operate simultaneously: potential negative interactions i.e. interactions worsening their reliability 20

21 Key regulatory requirements to evaluation of passive systems possible delays in systems activation caused by various reasons: such time interval shall not exceed time period when the accidents evolving into the severe condition is being prevented because of natural processes related to coolant heating and boiling-off (in SGs, in primary circuit) possible negative effects of non-condensable gases: measures to prevent collection of them into the heat exchangers 21

22 Key findings of safety review in regard to passive systems (by the example of Novovoronezh NPP-2) 22

23 Novovoronezh NPP-2. Key findings of safety review (Air-cooled PHRS & HA-2) Substantiation of the PHRS & HA-2 characteristics: start-up characteristic, start-up time under the minimum and maximum external temperatures; reliability of the PHRS lock valve and air controller some others PHRS efficiency in case of LOCA, active ECCS failure, and non-condensable gases entering heat exchange area of the steam generator Efficiency of PHRS heat exchangers in case of the impact of non-condensable gases inside PHRS s steam condensate tract 23

24 Novovoronezh NPP-2. Key findings of safety review (Air-cooled PHRS & HA-2) PHRS efficiency in case of multifactor impact of on-site fire (high temperature and air humidity decrease on the entrance of the PHRS air intake): possible downflow air movement streaming around the containment, reverse flow in the PHRS air ducts on the outer side of the compartment opposite the fire can deteriorate performance of a PHRS heat exchangers Efficiency of the HA-2 system, taking into account joint operation of PHRS and HA-2 (calculated and experimental substantiations of the efficiency of passive safety systems operation in case of LOCA, taking into account the interaction of primary circuit, passive systems (HA-2 and PHRS), containment and the impact of non-condensable gases); PHRS efficiency in case of extreme wind impact 24

25 Complex of additional calculated and experimental substantiations. Example General view of one of the experimental units Aerodynamic stand that provide substantiation of the PHRS protection from the wind impact. The base of the stand - air tunnel (tube) CIAM* (outlet nozzle 25 m 2 ). The stand ventilation system creates a high-speed air flow up to 300 km/h. In the wind tunnel placed model of the main building (scale 1:90), including reactor compartment, turbine building, vent pipe, etc. * CIAM - Central Institute of Aviation Motor Development Vienna, June 6-9,

26 Novovoronezh NPP-2. Results of safety review Applicant: fulfilment of full-scale complex of the calculated and experimental substantiations of the HA-2 and PHRS performance efficiency Additional information: SAR, including Chapter 15 Accident analysis Topical reports referenced in SAR Regulatory Body (with technical support organisation): Assessment of results of background calculations, tests and experimental justification based on specially constructed experimental stands Positive conclusions of the final review 26

27 Conclusion Passive safety systems in new VVER design are one of the important elements (along with active systems) of DiD and provide significant contribution to the safety, especially in case of the BDBA (DEC) involving SBO and loss of UHS Experience of safety review has shown that: available regulatory requirements valid are sufficient to assess passive systems safety and the effectiveness of their performance of safety functions main attention was paid to the verification of sufficiency and completeness of the justifications of passive systems (experimental and calculated) and their potential interactions with other systems In Future important issue that should remain in the sphere of the regulator's attention is the evaluation of the experience of operating passive systems 27

28 Federal Environmental, Industrial and Nuclear Supervision Service Scientific and Engineering Centre for Nuclear and Radiation Safety Thank you for your attention!