Safety analysis results of the DBC transients for ALFRED Pre-final results and conclusions

1. Safety analysis results of the DBC transients for ALFREDPre-final results and conclusions E. Bubelis (KIT) of behalf of Task 5.4 of LEADER WP5

2. DBC transients analyzed for ALFRED TR-2 : Spurious withdrawal of the most reactive control rod TR-3 : Reactivity insertion (100 pcm) due to fuel loading error TD-1 : Spurious reactor trip TD-2 : Turbine trip TD-3 : Loss of AC power (PLOOP) TD-5 : Loss of one primary pump (AC power available) TD-7 : Loss of all primary pumps (PLOF) TD-8 : Partial flow blockage of the hottest FA, with reactor trip delay TO-1 : FW temp drop (335 oC → 300 oC), reactor trip TO-4 : FW flow increase by 20%, reactor trip TRB-1: Steam system piping break. SGTR analysis E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

3. TR-2 : Spurious withdrawal of the most reactive control rod KIT results 83 pcm in 549 sec There are no safety related issues evolving from this transient. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

4. TR-3 : Reactivity insertion (100 pcm) due to fuel loading error KIT results ~667 sec There are no safety related issues evolving from this transient. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

5. TD-1 : Spurious reactor trip, SCS available CEA/ENEA results Reactor trip at t = 0 sec The maximum temperature gradient experienced by the clad is of 8 °C/s in the first 10 s time interval of the transient. There is no risk of lead freezing in the primary system, since the feedwater temperature remains at its nominal value of 335 °C. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

6. TD-1 : Spurious reactor trip, switchover to DHRS KIT results Reactor trip at t = 0 sec There are no safety related issues evolving from this transient. In the long term, lead temperature at the outlet from the MHX will continue to decrease further, eventually approaching lead freezing temperature (~327oC) several hours into the transient. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

7. TD-2 : Turbine trip Turbine trip at t = 0 sec KIT results Reactor trip at t = 2 sec There are no safety related issues evolving from this transient. In the long term, lead temperature at the outlet from the MHX will continue to decrease further, eventually approaching lead freezing temperature (~327oC) several hours into the transient. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

8. TD-3 : Loss of AC power (PLOOP) PSI results Reactor trip at t ~ 2 sec The maximum fuel and clad temperatures achieved during the PLOOP transient are 1984 oC and 554 oC, respectively. Coolant temperature at MHX outlet, being considered to be the lowest one in the primary system, will approach the freezing point of liquid Pb after ~4.5 hours into the transient. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

9. TD-5 : Loss of one primary pump CIRTEN results Following the postulated initiating event the mass flow rate in the affected flow path experiences a flow reversal, as high as 1000 kg/s. An increase of ~ 20°C is computed by the code at the core outlet and for the max clad temperature due to the reduction of the core mass flow rate by 7%. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

10. TD-7 : Loss of all primary pumps (PLOF) ENEA results Reactor trip at t ~ 4 sec The protection and safety systems are able to bring and maintain the plant in safe conditions in the short and long term. After 3 hours the lead temperature at the MHX outlet is still above the lead solidification point. Manual or automatic control of the IC unit operation is needed in the long term to avoid the risk of lead freezing. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

11. TD-8 : Partial (70%) flow blockage of the hottest FA, with reactor trip delay (1, 2, 3, 5, 10 sec) KIT results Reactor trip signal at t = 3 sec The clad of the peak pins of the ALFRED reactor has a very large margin to clad failure (rupture) during the simulated peak power SA 70 % flow blockage transient. Reactor safety is ensured by the reactor trip, as well as by DHRS, efficiently removing the decay heat. Reactor trip delay even by 10 sec, does not imply additional safety issues for ALFRED reactor. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

12. TO-1 : FW temp drop (335 oC → 300 oC), reactor trip ENEA results Reactor trip at t ~ 2 sec There is no risk of lead freezing in the whole primary circuit in the short and medium term. After 3 hours the lead temperature approaches the lead freezing point (T = 327 °C); therefore, manual or automatic control of the DHR-1 system operation is needed for avoiding the lead freezing in the long term. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

13. TO-4 : FW flow increase by 20%, reactor trip CEA/ENEA results No reactor trip signal generated The maximum MHX power increase is about 10% of nominal value. Core power somewhat increases and comes to equilibrium with the MHX power in about 5 minutes. After this time a new steady-state conditions is reached in both primary and secondary systems, without exceeding any reactor trip threshold set-point. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

14. TRB-1: steam system piping break (depressurization of all 8 SG units) PSI results SG power will significantly increase at the beginning of the SLB transient mainly due to water evaporation in SG pipes. After ~ 4.0 hours, the lowest coolant temperature in the primary system (at MHX outlet, prim. side) will be lower than the freezing point of liquid Pb. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

15. TRB-1: SGTR analysis for ALFRED, 1 tube rupture CIRTEN results The steam generated by one bayonet tube rupture accident flows upward in the 1SG and reaches the cover gas passing through the gas vents. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

16. TRB-1: SGTR analysis for ALFRED, 7 tubes rupture CIRTEN results As a consequence of the large amount of steam produced in the case of seven broken tubes and SIMMER-III code model limitations, traces of steam flow from the 1SG directly and indirectly to the core. The steam enters the core downcomer mostly by the inner path, as it was shown by the void fraction calculated in the cells beneath the core. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

17. TRB-1: SGTR analysis for ALFRED Conclusions: In the SIMMER-III simulation, the water injected into the liquid metal fully vaporizes in contact with it at the injector tube exit region enhancing the local peak pressure. In reality, all the injected water would not vaporize instantly when contacting lead (only part of it will do so), but will be encapsulated in water vapor bubbles and transported to the cover gas region instead. This fact should be kept in mind when analyzing the SGTR simulation results obtained using SIMMER-III code. The very low amount of vapor entering the core, the limited vapor residence time and the negative coolant expansion reactivity effect in the optimized ALFRED reactor core do not result into a hazard of positive reactivity insertion. In reality, in order to reduce the potential of steam transport to the core, a mechanical device located at the steam generator tube outlet promotes the separation between lead and steam. A dedicated scaled facility should be foreseen to analyze in depth the SGTR phenomena further as part of the future R&D activities for LFR. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

18. Conclusions • As related to the DBC transients, all selected transients examined proved that the ALFRED plant can accommodate a rather wide range of accidental events. • The ALFRED plant has proved to be able to enter a safe shutdown phase after every DBC accident analyzed. • The core temperatures (clad and fuel) always remain well below the safety limits and no significant vessel wall temperature increase is predicted. • ALFRED is a very forgiving plant design, and there is an extended time margin (grace time) of several hours for a possible manual operator intervention even under worst accidental conditions (potential of Pb-freezing in the long term, in case of uncontrolled decay heat removal by the DHRS). E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten

19. Conclusions (continued) • The very important issue for the critical ETDR (ALFRED) is a tight and continuous operational control of the secondary coolant conditions (feedwater inlet temperature, feedwater flowrate) in order to assure prevention of freezing of the lead coolant at the coldest location of the primary loop, namely at the outlet of the primary side of the main heat exchanger. • In general, the safety analysis performed for the Pb-cooled ETDR (ALFRED) design demonstrated the extremely forgiving nature of this plant design, ascribable to the inherently, large thermal inertia of the Pb-cooled primary system and optimization of safety relevant control, safety systems and components. E. Bubelis – LEADER/ELECTRA Safety Review Workshop, Petten