MELCOR Analysis of Helium/Water/Air Ingress into ITER Cryostat and Vacuum Vessel

Transcription

1 TM MELCOR Analysis of Helium/Water/Air Ingress into ITER Cryostat and Vacuum Vessel C. H. Sheng and L. L. Spontón Studsvik Nuclear AB, SE Nyköping, Sweden Abstract This accident is postulated starting with two large holes respectively on vacuum vessel and cryostat simultaneously with water and helium cooling pipe breaks. One base case, eight parameter studies and one very conservative case are studied by MELCOR. The analysis results showed that the cryostat and vacuum vessel pressures for this accident remain below the design limits of 2 kpa. The consequential radioactive releases of tritium and W dust stay well below the no-evacuation limits, which are 9 g of tritium and 16 kg of W dust. 1. Introduction The magnetic fields store large energies (5 GJ in normal operation, can increase during offnormal events). During magnet faults, the energies can be released in both mechanical and thermal form. The magnetic energies are located in proximity to the first and second barriers which enclose significant inventories of radioactive materials. These confinement barriers are the vacuum vessel (VV), cryostat vessel (CV), water cooling pipe, and ducts between the VV and CV walls. In this case, it is assumed that a magnet fault could simultaneously entail holes in both the VV and CV walls with cross section areas of the square meter order; and breaks in both VV water cooling loops and the helium cooling loop for the superconducting toroidal field (TF) coil system. This assumption is considered as overly conservative, in fact as hypothetical. On the other hand, damage mechanisms associated with magnets faults leading to such extensive damage should not be completely excluded from analysis on the basis of present magnet expertise. This accident has been previously analyzed by Okada [1], Sheng [2] and Sponton [3] with MELCOR However, due to one identified error (the vacuum vessel heat structure mass 416 t was double specified) in the input deck [1, 2 and 3] was found and due to the fact that a new version of MELCOR has been released, this accident is re-analyzed using the latest.bdba 1 and.dba 2 versions, with the modified input deck. Analyzing this accident with both versions (.bdba and.dba) is for comparison, final documentation and quality assurance purposes. The result from this work is compared with those from earlier analysis [1, 3]. 1 bdba beyond design base accident (best estimate). 2 dba design base accident (conservative).

2 TM Accident Description 2.1 Base Case The base case sequence analysis was postulated to proceed as follows: The analysis starts with a postulated, simultaneous occurrence of two holes, one in the VV wall and one in the CV wall towards the Cryostat Space Room (CSR) which in turn is connected to the environment, both directly and indirectly via the Gallery. At the beginning of the event sequence, the radioactive materials at risk (tritium in HTO and activated tungsten dust) reside inside the VV. They amount to 1 kg of tritium and 35 kg of dust. A plasma disruption coincident with hole occurrence is also assumed and is accompanied by the generation of 5 kg of dust inside the VV which add to the 35 kg. Heat stored in the plasma flows into the plasma-facing components. The hole in the VV wall is conservatively assumed to damage both loops of the vessel s PHTS so that the total cooling water inventory becomes connected to both the VV and the CV entailing water/steam discharge into these vessels. Further, it is postulated that the hole in the VV wall is simultaneously accompanied by a break inside the CV of one (out of the two) helium cooling loops of the superconducting TF coil system. The rate of helium spill from its start up to 2.5 s amounts to 231 kg/s, followed by 6 kg/s until the total helium mass from the broken loop (2.6 t) has been discharged into the CV. The loss of helium from the TF coil cooling system is accompanied by a fast discharge of the coils. Also the Poloidal Field (PF) coils and the Central Solenoid (CS) coils undergo fast discharges. In the course of these discharges, the temperature of the TF coils rises from 4.5 K to 55 K, whereas there is almost no temperature increase of the PF and CS coils. Water discharging from the VV PHTS comes into contact with the structures at cryogenic temperatures inside the CV. Air from the CSR flows into both the CV and the VV. In the early sequence phase, the pressure in the CSR drops due to this outflow. Vacuum breakers and/or doors are opened by under-pressure, causing air to flow into the CSR from the Gallery. Overall, the pressure inside the VV and the CV rises. This is most important for the mobilisation and dispersion of the radioactive materials initially residing inside the VV. To mitigate VV pressurization, a VV Pressure Suppression System (VVPSS) is connected to the VV. The Suppression Tank (ST), half of which filled with water, is connected to the VV by two lines. They are isolated in normal operation by rupture disks and valves. In case, the VV is pressurized by an accident, steam generated in the VV flows into the ST and condenses there. Water spilled into the VV is routed to a Drain Tank (DT), connected to the VV by two lines, closed in normal operation by a rupture disk or valve. Since the cryogenic structures inside the CV act as heat sinks, air-ice and water-ice can form on the structures surfaces, thus moderating the pressure rise inside the CV. Upon receipt of a signal from tritium monitors in the CSR, the HVAC system serving this room and the Gallery stops operation with an assumed delay of 3 s (to account for

3 TM instrument response times and valve closure) followed by actuation of the Standby Vent Detritiation System (S-VDS) with a delay of 3 seconds to account for heat-up of the recombiners. Dust and tritium in the CSR and the Gallery can be released into the environment by leakage, in case the pressure in these compartments is higher than that in the environment. 2.2 Parameter Studies Calculations are done for eight individually most conservative cases. Every case is characterized by one conservative parameter, all other parameters having their base case values. Each parameter study has been given a name, see list below. These are used in the figure legends in the presentation of the results. Finally, one very conservative case scenario is analysed, which is a combination (i.e. simultaneous occurrence) of all eight cases postulated for the individual parameter studies. HeliumMass VVBreakDistr HoleAreaVV HoleAreaCV ElevVVHole The total helium inventory of the TF cooling system (5.2 t) is discharged, not only the 2.6 t contained in one out of the two cooling loops. The cooling water discharged from the VV primary heat transport system (VV-PHTS) is entirely discharged into the VV, not split equally between VV and cryostat. The cross-sectional area of the hole in the VV wall is 3 m 2, not 1 m 2. Note: 3 m 2 is about the free cross-sectional area that can exist inside a VV port. The cross-sectional area of the hole in the CV wall is 3 m 2, not 1 m 2. The hole in the VV wall is located at a lower port (to which ITER allocates elevation level zero ), not at an equatorial port. DelaySVDS The actuation of the S-VDS is delayed by 8 hours (not by 3 s) relative to sequence initiation. DelayHVAC LeakRateModel The radiation monitors send their signals only when the concentrations reach their set points (.8 g/room for dust and g-t/m3 for tritium). This leads to a delay of HVAC ducts closure by 67.1 s (not by 3 s) and of the S-VDS actuation by s (not 3 s). The pressure threshold in the leak rate model is 3 Pa, not 2 kpa

4 TM MELCOR Model This accident is analyzed by a modified version of MELCOR code. The VV, VV pressure suppression system (VVPSS), CV, drain system, CSR, gallery, VV-PHTS, the heating, ventilating, air conditioning system (HVAC), the vent detritiation system (VDS) and the environment are included in the MELCOR model. The nodalization is shown in Figure 1 and 2. Space between VV & VVTS 4m 3 Cryostat 74m 3 Environment Vacuum Vessel 135m 3 Cryostat Space Break Room 51m 3 1.m 2 Leakage HVAC S-VDS Leakage Gallery 6m 3 Break 1.m 2 Vacuum Breakers Area:.4m 2 Start to open: p=. 5kPa Fully open: p= 1.kPa Doors Area:2m 2 Start to open: p=2 kpa Fully open: p=2 kpa Figure 1 Model of Tokamak Building m 3 Outlet Ring Manifold m 3 Water-to-Air Heat Exchanger Pressurizer Upper 6m 3 Junction m 3 3m 3 Break into Cryostat m 2 Break into VV m 2 Inlet Ring Manifold m 3 Lower Junction m 3 Figure 2 Model of VV PHTS ( 2 loops). Circulation Pump 4. Results and Discussions Figure 3 shows the comparison of the pressure histories for the base case in VV, CV and CSR obtained by different performers. The cryostat pressure starts increasing because of the simultaneous ingress of the water, helium and air and reaches a maximum value of about 3 MPa at about 36 s. The pressure increasing is limited by the VVPSS and the drain tank. After about 4 s, the cryostat is stabilized at a pressure slightly less than one atmosphere

5 TM by the HVAC system serving the cryostat space room and the gallery. The pressures in the VV and cryostat space room closely track that of the cryostat because of the large breach area (1 m 2 ) created by this event. Sheng2 Sheng1 Sheng3 Sponton Okada Cryostat Sheng2 Sheng1 Sheng3 Sponton Okada VV 5 Sheng2 Sheng1 Sheng3 Sponton Okada Cryostat Sp.R Time (s) Figure 3 Pressure history of the base case in the cryostat, the vacuum vessel and the cryostat space room (detail meaning of the legend shown in Table 1). Figure 4 shows the pressure history of all the cases in VV and CV. Case HeliumMass shows the highest pressure increase in the VV and CV by the increasing helium ingress. The cryostat and VV pressures for this accident remain below the design limits of 2 kpa for all the parameter studies. 5 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) Figure 4 Pressure histories of all the cases in the cryostat and the vacuum vessel.

6 TM Table 1 and 2 show the tritium and dust releases result for all the ten cases run by different MELCOR versions and input decks by different authors; the water property table used by Okada is different from the others. From Table1 and 2 it is clearly shown that the very conservative case has the largest tritium and dust releases among all the ten cases from different authors calculations. TABLE 1 TOTAL RELEASES OF TRITIUM, ALL CASES. (Without Extra Heat Structure) (With Extra Heat Structure) MELCOR1.8.2 Tritium (g) (With Extra Heat Structure) dba bdba dba bdba bdba (Okada) (Sponton) (Sheng1) (Sheng2) (Sheng3) Property table 1 Property table 2 Property table 2 Property table 2 1-BaseCase ConservativeCase (2.577)* HeliumMass (1.534)* VVBreakDistr HoleAreaVV HoleAreaCV ElevVVHole DelaySVDS DelayHVAC LeakRateModel * Release in parenthesis resulting from when helium is being injected with double mass flow rate compared to base case. TABLE 2 TOTAL RELEASES OF DUST, ALL CASES. (Without Extra Heat Structure) dba bdba (With Extra Heat Structure) MELCOR1.8.2 Dust (g) (With Extra Heat Structure) dba bdba bdba (Okada) (Sponton) (Sheng1) (Sheng2) (Sheng3) Property Property Property table 1 Property table 2 table 2 table 2 Property table 2 1-BaseCase ConservativeCase (549.8)* HeliumMass (257.3)* VVBreakDistr HoleAreaVV HoleAreaCV ElevVVHole DelaySVDS DelayHVAC LeakRateModel * Release in parenthesis resulting from when helium is being injected with double mass flow rate compared to base case.

7 TM The total weight of the systems involved in this accident analysis is t, and is the value used in Sheng1 and Sheng2, i.e. columns 3 and 4 of Table 1 and 2. The columns named Okada, Sponton and Sheng3 have an extra 4 16 t VV heat structure (hs12 in the input file) added to the whole system. The mass of heat structure influences the thermo hydraulic behavior like pressure and temperature of an accident, and also influences the amount of the radiological release to the environment. Comparing the release from Sheng2 and Sheng3 where the same MELCOR version.bdba was used but with different mass of heat structure, it shows that larger mass in the system results in more tritium and dust releases. Comparing the release from Sheng3 and Sponton where the same input deck was used but run with respectively.bdba and MELCOR1.8.2.bdba,.bdba gives less tritium release and larger dust release than MELCOR1.8.2.bdba. Except for the very conservative case, the case HeliumMass reveals the most influence on both dust and tritium releases among all the other cases. This strong influence of discharged helium mass appears to be caused by the fact that more evaporating helium currently increases the pressure in the VV and CV, forcing more dust and tritium out, and also pressurizing the Gallery. The tritium and dust releases have three channels, i.e. via S-VDS, HVAC and leakage. The analyses show that the release to environment is mainly contributed by the leakage which is pressure dependent. 5. Conclusion Vacuum vessel + cryostat large hole failure combined with water and helium cooling pipe breaks accident is analyzed with. One extra VV heat structure is removed from the previous input deck and its influence on the radioactive releases is analyzed. One base case, eight parameter studies and one very conservative case are studied. Among all the parameter studies, except the very conservative case the HeliumMass case results in the largest tritium and dust releases to the environment. The calculation results showed that the cryostat and VV pressures for all the cases remain below the design limits of 2 kpa. The radioactive releases of tritium and W dust stay also well below the no-evacuation limits, which are 9 g of tritium and 16 kg of W dust. 6. References [1] OKADA H., MELCOR Analysis on Vacuum Vessel + Cryostat Consequences of Failure Event (Rev. 5), ITER/SEHG Memo, (IDOMS G 84 RI W ) (23). [2] SHENG, C., MELCOR analysis of helium/water/air ingress into ITER cryostat and vacuum vessel(final report with radiological release analysis). Studsvik Nuclear AB, Sweden (Technical Note N-3/42) (23). [3] SPONTON L & SHENG, C., MELCOR Analysis of helium/water/air ingress into ITER cryostat and vacuum vessel Parameter study. Studsvik Nuclear AB, Sweden (Technical Note N-5/7) (25).