Studsvik Nuclear AB, Sweden

Transcription

1 MELCOR Analyses of Helium/Water/Air ingress into ITER Cryostat and VV ChunHong Sheng Studsvik Nuclear AB, Sweden 8th IAEA TM on Fusion Power Plant Safety 10 to 13 July 2006, Vienna, AUSTRIA

2 Space Cryostat Environment between VV & VVTS 40m m 3 Background Vacuum Vessel 1350m 3 Cryostat Space Break 1.0m 2 Room 5100m 3 Air Leakage HVAC S-VDS Leakage Gallery 60000m 3 The magnetic fields store large energies (50 GJ in normal operation, can increase during off-normal events). Break 1.0m 2 Steam Helium Vacuum Breakers Area:0.04m 2 Start to open: p=0. 5kPa Fully open: p= 1.0kPa Doors Area:2m 2 Start to open: p=20 kpa Fully open: p=20 kpa Magnet failure--both mechanical and thermal forms Accident Description Confinement failure: Holes in both VV & CV Helium ingress from TF cooling system Steam ingress from VV-PHTS Objective Whether overpressure in the cryostat and VV will be safely accommodated? Will the radioactive releases be adequately confined? Air ingress This assumption is considered as overly conservative, in fact as hypothetical. 2

3 Heat structures in Tokamak building Cryostat Space Room Cryostat Ceiling 1 m 2 break, Air Ingress S-VDS Exhaust Break, water ingress Cryostat Thermal Shield Ceiling VV- PHTS Cryostat 1 m 2 break Break, He ingress PF Coils HVAC TF Coils Gallery He cooling Ceiling loop Port Cell Cryostat Thermal Shield Floor Gravity Support Gallery Cryostat Floor Cryostat Space Room Gallery Floor 3

4 Model of VV PHTS m 3 Outlet Ring Manifold m 3 Water-to-Air Heat Exchanger Pressurizer Upper 60m 3 Junction m 3 3m 3 Break into Cryostat 0.05m 2 Break into VV 0.05m 2 Inlet Ring Manifold m 3 Lower Junction m 3 Circulation Pump 4

5 Space between VV & VVTS 40m 3 Cryostat 7400m 3 Environment Vacuum Vessel 1350m 3 Cryostat Space Break 1.0m 2 Room 5100m 3 Air Leakage HVAC S-VDS Leakage Gallery 60000m 3 Break 1.0m 2 Steam Helium Vacuum Breakers Area:0.04m 2 Start to open: p=0. 5kPa Fully open: p= 1.0kPa Doors Area:2m 2 Start to open: p=20 kpa Fully open: p=20 kpa Discharged He & steam pressure in Cryostat &VV Air VV and CV CSR, P first Vacuum breakers and/or doors between CSR and gallery are opened by under-pressure. P due to steam and helium ingress Pressure will be mitigated in three ways: coils serve as a cryopump for steam, suppression & drain system This is most important for the mobilisation of the radioactive materials initially residing inside the VV. 5

6 Pressure (MPa) Sheng2 Sheng1 Sheng3 Sponton Okada Cryostat Max P 1.3 bar after 36 s. Pressure (MPa) Sheng2 Sheng1 Sheng3 Sponton Okada VV Back to 1 bar after about 1 hour. Pressure (MPa) Sheng2 Sheng1 Sheng3 Sponton Okada Cryostat Sp.R Time (s) 6

7 Base Case HeliumMass 2.6 t 5.2 t VVBreakDistr Steam to VV & CV Only VV HoleAreaVV 1 m 2 3m 2 HoleAreaCV 1 m 2 3m 2 ElevVVHole 7 m 0 m DelaySVDS 300 s 8 hours DelayHVAC 30 s 67.1 s LeakRateModel 2000 Pa Threshold 300 Pa Finally, one very conservative case scenario is analysed, which is a combination (i.e. simultaneous occurrence) of all eight cases. 7

8 Pressure history for all the parameter study cases Pressure (MPa) Pressure (MPa) Basecase Conserv.Case HeliumMass VVBreakDistr HoleAreaVV HoleAreaCV ElevVVHole DelaySVDS DelayHVAC LeakRateModel Basecase Conserv.Case HeliumMass VVBreakDistr HoleAreaVV HoleAreaCV ElevVVHole DelaySVDS DelayHVAC LeakRateModel Cryostat VV Time (s) HeliumMass shows the highest pressure increase in the VV and CV by the increasing helium ingress mass. 8

9 time (s) Mass (kg) condensed water mass CS TF PF cryostat thermal shield The added weight from the condensed steam mass on the magnets, is far away from threatening the structural integrity. 9

10 Among all the parameter studies, the very conservative case results in the largest tritium and dust releases to the environment. 10

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12 The consequential radioactive releases of tritium and W dust stay well below the noevacuation limits, which are 90 g of tritium and 16 kg of W dust. Okada lowest P but most release, why? 12

13 2000% 1804% Deviation from the Base Case 1500% 1000% 500% 0% Tritium Release 769% 0% 5% 1% -6% 48% 58% -3% % Except for the very conservative case, the case HeliumMass reveals the most influence on both dust and tritium releases among all the other cases. Evaporating helium increases the pressure in the VV and CV. -500% Cases Radioactive material release channels: HVAC S-VDS Mostly Leakage Deviation from the Base Case 2000% 1500% 1000% 500% 0% 1484% Dust Release 0% 86% 15% 18% -7% 24% 6% % % % Cases 13

14 Conclusions The Cryostat and VV pressures---below the design limit 200 kpa. The added weight from the condensed steam mass on the magnets, is far away from threatening the structural integrity. The consequential radioactive releases of tritium and W dust stay well below the no-evacuation limits, which are 90 g of tritium and 16 kg of W dust. 14