A. Kaliatka, S. Rimkevicius, E. Uspuras Lithuanian Energy Institute (LEI) Safety Assessment of Shutdown Reactors at the Ignalina NPP

Transcription

1 A. Kaliatka, S. Rimkevicius, E. Uspuras Lithuanian Energy Institute (LEI) Safety Assessment of Shutdown Reactors at the Ignalina NPP

2 Outline Introduction Specific of heat removal from shutdown RBMK-type reactor RELAP5-3D model of shutdown and cooled RBMK-1500 reactor of Ignalina NPP Analysis of loss of heat removal in finally shutdown reactor of unit 2 of Ignalina NPP Analysis of station blackout case at shutdown reactor Analysis of water flow rate blockage through FC at shutdown reactor Analysis of heat removal from shutdown reactor of unit 1 of Ignalina NPP when circulation circuit is filled by air Conclusions

3 INTRODUCTION (1) Ignalina NPP two units of RBMK-1500, commissioned in 1983 and 1987, shutdown for decommissioning at the end of 2004 and In time period Ignalina NPP produced up to 82% of electric energy for Lithuania.

4 INTRODUCTION (2) A decree endorsed by Lithuanian Parliament in June, 2007 about construction of a new NPP in Lithuania in cooperation with Latvia, Estonia and Poland.

5 INTRODUCTION (3) The Ignalina Nuclear Power Plant is a twin-unit with two RBMK-1500, graphite moderated, boiling water, multichannel reactors. P nom = 7 MPa. Design thermal power 4800 MW. Electrical power 1500 MW.

6 INTRODUCTION (4) Acceptance criteria for accident analysis in shutdown reactor Parameter Acceptance criterion Subcriticality Not less 0.02 Fuel channel power, MW Below 4.25 Fuel linear power, W/cm Below 485 Calculated temperature of the graphite stack, о C not more than 760 o C (when the reactor space is pressurized by nitrogen and helium) no more than 350 o C (when the reactor space is pressurized by air) Maximal fuel cladding temperature, о C Below 700 Maximal fuel channel pressure tube temperature, о C Below 650 Maximal Excess pressure in the RCS, MPa Below 10.4 Heat-up rate of reactor and RCS, о C/h Below 30 Cooldown rate of the reactor and the RCS, о C/h Below 30 The annual effective dose to the population, msv Less than 1

7 SPECIFIC OF HEAT REMOVAL FROM SHUTDOWN RBMK-TYPE REACTOR Treedimensional diagram of one loop of reactor cooling system, with schematic of compartment s, covering main equipment Ventilation stack Ventilators Air filter Air cooler DS compartments Ventilation (exhaustion) DS 1 DS 2 Ventilation (air supply) Biological shielding Reactor hall Fuel channel DS compartments DS 3 DS 4 Schematic of air ventilation in reactor hall and DS compartments Downcomers Steam - water piping SWP corridor Initial stage: temperature of water in FCs is о С and temperature of graphite stack is 120 о С (~ 1 day after reactor shutdown). the constant decay heat level (13.99 kw in one fuel channel) was assumed.

8 ANALYSIS OF LOSS OF HEAT REMOVAL IN FINALLY SHUTDOWN REACTOR OF UNIT 2 OF IGNALINA NPP (1) Open steam discharge valve 1 Feedwater Heat removal by ventilation system 8 Main safety valves Feedwater Reactor 3 5 RELAP5-3D Nodalization scheme of shutdown and cooled RBMK-1500 reactor of Ignalina NPP

9 ANALYSIS OF LOSS OF HEAT REMOVAL IN FINALLY SHUTDOWN REACTOR OF UNIT 2 OF IGNALINA NPP (2) m Control and Protection System channel FC with fuel assemblies Channels with absorbers Radial reflector Reflector cooling channel Steam Water Piping 75 o C Reactor 20 W/m 2 K RELAP5-3D Nodalization scheme for modelling of heat removal from SWP Two most likely initiating events were analysed: station blackout case; blockage of water flow rate in the group of fuel channels.

10 ANALYSIS OF LOSS OF HEAT REMOVAL IN FINALLY SHUTDOWN REACTOR OF UNIT 2 OF IGNALINA NPP (3) Temperature, о С Station blackout during reactor cooling by coolant natural circulation mode saturation temperature cladding graphite collumn center of fuel pellet FC wall Coolant flow rate, kg/s disconection of heat removal natural circulation mode through single RCS loop steam through steam discharge valve water boiling in FCs Fig. 1. Behaviour of temperature of the core components Water volume in DSs, m left RCS loop right RCS loop Fig. 2. Behaviour of coolant flow rate through FCs of one RCS loop Pressure, kpa core inlet core outlet DS Fig. 3. Behaviour of water volume in both DS of one RCS loop Fig. 4. Behaviour of pressure in RCS

11 ANALYSIS OF LOSS OF HEAT REMOVAL IN FINALLY SHUTDOWN REACTOR OF UNIT 2 OF IGNALINA NPP (4) Coolant flow blockage in group of fuel channels of one RCS loop core middle bottom water piping steam water piping 10 steam water steam Coolant flow rate, kg/s Coolant velocity, m/s Fig. 1. Coolant flow rate through fuel channel water Fig. 2. Change of rate of separate coolant phases in FC 600 Water volume in DSs, m affected RCS loop intact RCS loop Temperature, о С affected RCS loop intact RCS loop Fig. 3. Change of water volume in DSs Fig. 4. Change of peak fuel cladding temperature in FCs

12 ANALYSIS OF LOSS OF HEAT REMOVAL IN FINALLY SHUTDOWN REACTOR OF UNIT 2 OF IGNALINA NPP (5) The process of reactor core heat-up at shutdown reactor is very slow. The increase of temperature of fuel claddings starts not earlier as hours after the beginning of accident. In both beyond design basis cases (station blackout case and blockage of water flow rate in the group of fuel channels) the operators have enough time to find the possibilities to provide make-up of RCS by water, using non-regular, non-designed water sources. Before mentioned time moment (12 18 hours after the beginning of accident) none of acceptance criteria are exceeded.

13 ANALYSIS OF HEAT REMOVAL FROM SHUTDOWN REACTOR OF UNIT 1 WHEN RCS IS FILLED BY AIR (1) Opened steam discharge valve Heat removal by ventilation system Opened steam discharge valve Opened pressure header Reac -tor 3 5 Schematic representation of RCS of RBMK-1500 with scheme of reactor core cooling by air natural circulation (the air path is marked in grey)

14 ANALYSIS OF HEAT REMOVAL FROM SHUTDOWN REACTOR OF UNIT 1 WHEN RCS IS FILLED BY AIR (2) Environment Heat removal by ventilation system Reactor RELAP5-3D Nodalization scheme 10

15 ANALYSIS OF HEAT REMOVAL FROM SHUTDOWN REACTOR OF UNIT 1 WHEN RCS IS FILLED BY AIR (3) Analysis of heat removal from the shutdown reactor when circulation circuit is filled by air Temperature, o C Group distribution header Core middle part Core exit Inlet into SWP Outlet from SWP Start of ventilation system Air pressure, kpa Start of ventilation system Group distribution header Core middle part Core exit Outlet from SWP Fig. 1.Behaviour of air temperature in RCS Air flow rate, kg/s Start of ventilation system Total flow rate FC without fuel assemblies FC with fuel assemblies Fig. 3. Air flow rate through FC with and without fuel assemblies Fig. 2. Behaviour of air pressure in RCS Temperature, o C Start of ventilation system Upper fuel assembly Lower fuel assembly Fig. 4. Behaviour of fuel temperature in lower and upper fuel bundles

16 ANALYSIS OF HEAT REMOVAL FROM SHUTDOWN REACTOR OF UNIT 1 WHEN RCS IS FILLED BY AIR (4) The results of performed analysis demonstrate, that if the conditions for natural air circulation will be fulfilled and ventilation system in drum separator compartments will be available, the reactor core will be cooled by natural circulation of air. The maximum reached fuel temperature is 282 o C such temperature of fuel is during normal reactor operation, thus the fuel claddings remain intact during this accident. The contamination of surrounding area is minimal, none of acceptance criteria are exceeded.

17 CONCLUSIONS The performed analysis using RELAP5-3D code shows that the RBMK-1500 reactor after shutdown can be cooled by natural circulation of water or air. Two bounding events were analysed for cooling of core by water after reactor shutdown: station blackout and coolant flow blockage in the fuel channels of one RCS loop. The increase of temperature of fuel claddings starts not earlier as hours after the beginning of accident. The performed analysis demonstrated, that partially reloaded reactor, in long term after shutdown can by cooled by natural circulation of air.

18 Thank you for your attention Questions?