Technical potentialities of integration of reactor vessel external cooling in operating WWER-440 plant - Assessment results

Transcription

1 Technical potentialities of integration of reactor vessel external cooling in operating WWER-440 plant - Assessment results Speaker Pantyushin S.I. ERMSAR

2 1. Introduction. Current status A core catcher or a ULR (corium retention device) is used in the projects of 2000-s to localize and retain corium within the containment for NPP with VVER-1000 (V-412, V-428), VVER-1200 (V-392М, V-491) and TOI (V-510) a similar engineering solution is applied for the projects of AREVA company - EPR. For medium-power projects of NPP with RP VVER-640 and RP VVER-600 the in-vessel corium retention concept (due to external cooling of the reactor vessel) is accepted. Application of the in-vessel corium retention system has been studied (option of the design) within the framework of development of a conceptual design for RP VVER TOI. A provision is made for in-vessel corium retention in projects of Westinghouse (USA) АР-600 and АР-1000 and Korean project of APR

3 1. Introduction. Current status For Russian designs of VVER-440 (V-179,V-213,V-230) and VVER-1000 (V-320,V-338,V-187) no provisions were made for measures on retention of corium release beyond the containment boundaries at late stage of severe accidents. Engineering solutions about application of in-vessel corium retention system/ devices have been taken at Loviisa NPP, Paks NPP, Mochovce NPP, Dukovany NPP (NPP with RP VVER-440). By the present time all is justified and realized at Loviisa NPP. 3

4 1. Introduction. Current status After accident at Fukushima NPP the following tasks were set by the Operator to the OKB GIDROPRESS - analysis of current situation; - evaluation of engineering feasibility for application of system for corium retention and reactor vessel cooling (SUROK) at operating Russian Units with - RP VVER-440; - RP VVER By the end of 2012 the work was completed. Based on the results of this work the Operator - the Rosenergoatom Concern - will consider the issue on realization of the system at operating NPP with VVER. 4

5 1. Introduction. Current status To date two systems for corium retention are designed in Russia: - corium retention device (ULR) - system for corium retention and reactor vessel cooling (SUROK) Adaptation of the corium retention device system to operation NPP is impossible due to a design feature (there is a need to increase in the containment building height up to 8 m) Steam out Water storage tank Reactor room Reactor vessel flooding height 5 flow intensifier

6 2. In-vessel corium retention concept General provisions: 1. It is necessary to retain corium within the boundaries of the containment (reactor compartment) and to reduce the fission product yield in order to mitigate consequences of severe accidents. 2. The in-vessel corium retention is a strategy of severe accident management and an engineered feature for retention of molten material within the reactor coolant boundaries. Water Coolant Metal Oxides vessel 6

7 2. In-vessel corium retention concept General provisions [continued]: 3. For corium retention within the reactor vessel boundaries it is necessary to provide: - external cooling of the reactor vessel - steam removal from the reactor concrete cavity - heat removal from the containment to the end absorber - makeup of emergency pool; - monitoring of basic parameters (flooding level of reactor vessel, containment pressure, and etc.) Oxides: UO 2, ZrO 2, Metals: steel, Zr, U convection and heat into liquid stable temperature stratification thermal radiation cooling 7

8 2. In-vessel corium retention concept General provisions [continued]: 4. Design success criteria: - heat flux from corium through the reactor vessel wall to water must not exceed CHF; - reactor vessel wall shall withstand load from corium and coolant being within the reactor boundaries; - stress in the reactor vessel wall thinned due to partial burn-off must not exceed a threshold of strength damage. Meeting the criteria must be justified in detail design and verified by experiments. 8

9 2. In-vessel corium retention concept Tasks for design study: 1. To optimize the boundaries of pressurization circuit 2. To provide steam passage in an area of a supporting truss 3. To realize the solutions on heat transfer intensification on the outside surface 4. To make impossible early fuel melting in the core (specific times are determined for each RP) 9

10 2. In-vessel corium retention concept Tasks for design study (continued): 5. To provide water collection from the maximum possible quantity of sources (except for spent fuel pool). 6. To enable water supply from an external source. 7. Application of emergency instrumentation. 10

11 3. OKB GIDROPRESS tasks For operating NPP with VVER a possibility must be determined for application of SUROK with regard for an actual configuration, for this purpose, to : - develop design and process solutions aimed at assurance of effective functioning of SUROK; - state general requirements for modernization and upgrading of RP equipment, common-station systems and building structures; - determine together with the General design organization of NPP a necessary scope of design and calculation work during application of SUROK. 11

12 4. Evaluation of engineering feasibility for realization at NPP with RP VVER-440 Materials used : Reports on safety justification of NPP with RP VVER-440 as of ; Results of R&D on justification of in-vessel retention for designs of V-407,V-510 and V-498; Public publications on materials of NPP Paks, NPP Loviisa designs, АР-600; Results of R&D within the framework of justification of hydrogen safety of NPP with RP VVER-440 (as to analysis of severe accidents); Results of analyses of beyond basis design accidents within the framework of justification as per Guide on BDBA management 12

13 4. Evaluation of engineering feasibility for realization at NPP with RP VVER-440 (V-213) Pre-accident parameters of initial state - initial thermal power of the reactor 1471 MW (107 % N nom ) - decay heat rate nominal (MS ISO ) Initial data and assumptions accepted in cacluations Design Governing conditions Time of exceeding the maximum design limit of fuel rod damage Beginning of fuel damage Beginning of corium coming onto the reactor vessel bottom head Technical method for evaluation of corium coming onto the reactor vessel bottom head NPP with VVER-440 (V-213) RP, implemented at Kola NPP (Units 3 and 4) LB LOCA (Dnom 500) and with superposition of NPP blackout 30 min 2 h 00 min 5 h 30 min 6 h 00 min Salvo (instantaneous) with selection of components with the most unfavorable combination 13 13

14 4. Evaluation of engineering feasibility for realization at NPP with RP VVER-440 (V-213) Results of calculation analyses the minimum residual thickness of the vessel is 40 mm; the maximum heat flux is below the critical one; the minimum value of DNBR is 1,1; strength conditions for load-carrying layer of metal are met (integrity is ensured). THE CALCULATION ANALYSES SHOWED THAT THERMOHYDRAULICS AND THERMOMECHANICS CRITERIA ARE MET REQUIREMENT: - reactor vessel flooding level not less than 2,6 m from the pole 14 14

15 5. Proposed engineering solutions for SUROK for NPP with RP VVER-440 (V-213) Water sources for passive flooding the reactor concrete cavity sump underneath the vessel are а) Primary leak + boron solution from two lower rows of bubblervacuum system by gravity from SG box floor through additional pipelines and penetrations in the reactor concrete cavity b) From low-pressure ECCS accumulators by gravity through additional pipelines and penetrations in the reactor concrete cavity Stop members on the pipelines are controlled from MCR (ECR), as well as manually by personnel 15

16 5. Proposed engineering solutions for SUROK for NPP with RP VVER-440 (V -213) high-pressure ECCS tank low-pressure ECCS tank а) Nominal level low-pressure ECCS pump 16 б) TL system

17 5. Proposed engineering solutions for SUROK for NPP with RP VVER-440 (V -213) Versions for removal of the steam generated during cooling the reactor. Version 1. Steam removal upwards via an annular slot between the vessel and the supporting truss. Upper level of the core The version needs for the minimum labour hours and modernizations, but the channel flow area is insufficient 17

18 5. Proposed engineering solutions for SUROK for NPP with RP VVER-440 (V -213) Versions for removal of the steam generated during cooling the reactor. Version 2. Steam removal upwards via channels between the reactor supporting truss beams Steam out Dnom= holes The version ensures a required flow area but needs for significant labour hours and dose commitment 18

19 5. Proposed engineering solutions for SUROK for NPP with RP VVER-440 (V -213) Versions for removal of the steam generated during cooling the vessel. Version 3 (bypass). Steam removal downwards via dry channels laid below the level of flooding through the sealed door of the reactor core barrel into SG rooms. Steam out Steam out SG room The version ensures a required flow area and keeps a supporting truss in unchangeable state, but needs for significant labour hours and dose commitment Steam header To room A

20 5. Proposed engineering solutions for SUROK for NPP with RP VVER-440 (V -213) 20 RP REQUIREMENTS FOR NPP Provision of sufficient tightness of the containment ; Application of system of passive heat removal from the containment rooms; Application of additional components of emergency instrumentation; Provision of emergency instrumentation and SUROK with reliable power supply; Avoidance of the accidents caused by core melting at high pressure in the reactor coolant system (feed&bleed procedure); available additional sources of water at NPP and lines for water supply from these sources into SUROK.

21 CONCLUSIONS A set of pre-design work has been implemented on evaluation of engineering feasibility for realization of SUROK at Russian NPPs with VVER: design success criteria have been established; tasks for design study have been stated; VVER states under governing conditions has been asessed; process requirements for implementation of SUROK have been stipulated; possible flow sheets, main design solutions on implementation of the system at NPP with RP VVER-440 have been elaborated. 21

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