KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Institute for Neutron Physics and Reactor Technology LEADER, WP5: Task 5.5 – Analyses of representative DEC events of the ETDR (ALFRED). Pre-final results & conclusions by KIT (SIM-LFR) E. Bubelis, M. Schikorr (KIT)

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten DEC transients analyzed for ALFRED using SIM-LFR code TR-4 : UTOP, 250 pcm in 10 sec, no reactor trip TO-3 : PLOF+FW temp drop (335 o C → 300 o C), reactor trip TO-6 : PLOF+FW flow increase by 20%, reactor trip, DHR-1 T-DEC1 : ULOF, SCS in operation, no reactor trip T-DEC3 : ULOHS, PPs operating, no reactor trip, DHR-1 T-DEC4 : ULOHS+ULOF, no reactor trip, DHR-1 T-DEC5 : SA blockage, determine max acceptable SA flow reduction factor, no reactor trip T-DEC6 : SCS failure, PPs operating, DHR-1 & DHR-2 fails, reactor trip

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten TR-4 : UTOP, 250 pcm in 10 sec, no reactor trip For reactivity insertion of 250 pcm in 10 sec at BOC, as well as EOC core conditions, ALFRED reactor peak fuel pin cladding is not expected to fail (rupture), but local fuel melting is observed in the center of the peak fuel pins (pellets).

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten TO-3 : PLOF+FW temp drop (335 °C → 300 °C), reactor trip Protected LOF with FW temperature decrease to 300 o C will lead to a safety issue as Pb-freezing will have to be expected ~3.8 hours into the transient. During this time period operator has sufficient time to deactivate sub- systems of DHR-1 system, in order to prevent lead freezing at the outlet of the MHX.

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten TO-6 : PLOF+FW flow increase by 20%, reactor trip, DHR-1 Protected LOF with FW flowrate increase by 20 % presents no issue as related to Pb-freezing in the short term. However, in the long term, lead temperature at the outlet from the MHX will continue to decrease further, eventually approaching lead freezing temperature (~327 o C) several hours into the transient.

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten T-DEC1 : ULOF, SCS in operation, no reactor trip Under BOC and EOC conditions, the ULOF transient in the ALFRED reactor can be accommodated, implying minimum clad failure time of ~1.6E+5 sec under the minimum coolant flow conditions (flow undershoot conditions). This transient is not a critical transient for the ALFRED plant.

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten T-DEC3 : ULOHS, PPs operating, no reactor trip, DHR-1 The main concern is the average vessel wall temperature which in ~ 1 hour transient time approaches ~670 °C, thus questioning the long term structural integrity of the vessel under these elevated temperature conditions. Sufficient grace time (>> 30 min) is available for the manual operator intervention to terminate this transient by manually shutting down the reactor and cooling down the primary cooling circuit.

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten T-DEC4 : ULOHS+ULOF, no reactor trip, DHR-1 The ULOOP (ULOF+ULOHS) transient revealed no additional relevant safety issues for ALFRED reactor as observed during the ULOHS. A large influence in coping with this DEC transient is associated with the negative fuel, diagrid, pads, lead and CRs drivelines expansion reactivity feedback effects, effectively reducing reactor power to a safe power level (~ 3-4 % of the nominal power) in a timely manner. Sufficient grace time (>> 30 min) is available for the manual operator intervention to terminate this transient by manually shutting down the reactor and cooling down the primary cooling circuit.

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten T-DEC5 : SA blockage, determine max acceptable SA flow reduction factor, no reactor trip As a result of the SA blockage, the flow rate will initially decrease to ~ 12 % nominal, gradually recovering to about 20% flow rate at ~50 sec into the transient due to changing SA pressure conditions. The power remains at 100% nominal throughout the transient. Peak pin will fail ~56 sec into the transient as the cladding temperature will reach 1000 °C, with a peak pin fission gas pressure of ~23 bar. Extreme case: Flow area of hottest SA 97.5% blocked; w/o radial heat transfer; EOC

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten T-DEC5 : SA blockage, determine max acceptable SA flow reduction factor, no reactor trip For flow blockages of < 75%, no pin failures nor fuel melting is expected, even under unprotected conditions. For flow blockages of > 75%, peak power pins clad failure has to be expected, but fuel melting is not expected even for a flow blockage of 97.5%. Case: Flow area of hottest SA blocked 20 – 97.5 %; w/o radial heat transfer

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten T-DEC6 : SCS failure, PPs operating, DHR-1 & DHR-2 fails, reactor trip Protected SCS failure transient, even assuming the complete failure of the DHRS, presents no imminent safety issue to ALFRED in the first several hours, following the event. However, all primary cooling circuit temperatures, will start increasing slowly and will reach ~500 o C in ~3 hours. Operator has sufficient time for manual intervention in order to prevent overheating of the whole primary system.

Institute for Neutron Physics and Reactor Technology (INR) E. Bubelis and M. Schikorr – LEADER WP5 meeting, Petten Conclusions General: The analysis of DEC transients performed for the Pb-cooled ALFRED design demonstrated the forgiving nature of this plant design, ascribable to the combination of: 1. the inherently, large thermal inertia of the Pb-cooled primary system, 2. the detailed focus on the optimization of all safety relevant systems, in particular emphasizing appropriate designs of all relevant control and safety systems (as well as components), and 3. optimizing the neutronic core characteristics of the ALFRED “core system” thereby assuring various reactivity feedback effects (fuel, diagrid, pads, Pb-coolant and CRs drivelines expansion reactivity) that effectively depress the reactor power under all adverse DEC transient conditions, 4. as the ALFRED core is specifically designed to accommodate the ULOF transient.