1. “Design and safety analysis of ALFRED”
Accident analysis overview
G. Bandini
ENEA UTFISSM-SICSIS
3rd LEADER International Workshop
Bologna, 6th - 7th September

2. Outline
Introduction
ALFRED design features
Codes and reactor modelling
Steady-state results
Analyzed DBC and DEC transients
Preliminary results from transient analysis
Preliminary Conclusions

3. Introduction
The conceptual design of the Advanced Lead Fast Reactor European Demonstrator (ALFRED) is under development within the LEADER project to meet the safety objectives of GEN-IV nuclear energy systems
For the safety analysis of ALFRED representative accident initiators for Design Basis Conditions (DBC) and Design Extension Conditions (DEC) have been identified by the application of a simplified line-of-defence strategy and on the basis of the design solutions adopted for the ALFRED reactor
The identified event initiators have been categorized according to their frequency and the more representative for each category have been selected for safety analysis to be carried out within the LEADER project
Preliminary results of the analysis of some selected accidental transients within DBC and DEC performed with the RELAP5 and CATHARE codes are presented 3

4. ALFRED: Reactor block Horizontal section Vertical section
Pool-type reactor of 300 MWth power
171 fuel assemblies in the core
8 pump-bayonet tube SG connected to the 8 secondary circuits
Vertical section 4

5. ALFRED: Secondary circuits
From DHR system
To DHR system
Steam lines
Feedwater lines
DHR System (4 x 2 IC loops)
In-water pool isolation condenser (IC)
Valve SG
Feedwater
Steam 5

6. System codes used
The ALFRED accident analysis is performed by various organizations using different codes: KIT-G (SIM-LFR), NRG (SPECTRA), KTH (CFD, RELAP5), JRC (SIMMER, TRACE), PSI (TRACE/FRED), CIRTEN (SIMMER), ENEA (RELAP5, CATHARE), CEA(CATHARE)
RELAP5 and CATHARE (CEA collaboration) codes are used by ENEA for the analysis of selected DBC and DEC transients
RELAP5 (developed in USA) and CATHARE (developed in France) are system codes for thermal-hydraulic transient analysis of light water reactors
The RELAP5 code has been modified by ENEA, Ansaldo and Univ. of Pisa for LFR transient analysis by the implementation of LBE and lead thermal properties - Code validation on CHEOPE, NACIE and CIRCE experiments performed at ENEA/Brasimone
LBE and lead thermal properties have been recently implemented in CATHARE in the frame of an ENEA-CEA collaboration – Code validation on TALL experiments at KTH/Stockholm, NACIE experiments (ENEA/Brasimone) and HELIOS Korean loop experimental data (LACANES Benchmark) 6

7. ALFRED: Reactor modelling
ALFRED Nodalization scheme with RELAP5 (2 secondary loops with weight = 4 with CATHARE)
8 SGs (2 x 4)
8 Secondary loops
(2 x 4)
Primary circuit
8 IC loops (2 x 4)
Steam line
Feedwater line 7

8. ALFRED: Steady-state at EOC
(RELAP5 and CATHARE preliminary results)
Parameter
Unit
CATHARE
RELAP5
Note
Reactor power (thermal) MW
300
Primary flow rate
kg/s
25670
25280
To get average core T = 80 °C
Active core flow rate
24080
23745
Core bypass flow rate
1305
1270
About 5% of primary flow rate
Inner vessel bypass
285
255
About 1% of primary flow rate
Total primary circuit P (P core)
bar
1.43 (0.82)
1.40 (0.80)
Higher CATHARE primary flow rate
Core inlet temperature °C
400
Upper plenum temperature
480
Hot FA outlet temperature
489
Flow rate +18% of average FA
Hot FA peak clad temperature
522
510
Different heat transfer correlation
Hot FA peak fuel temperature
1942
1931
No fuel rod gap dynamic model
Average fuel temperature
1126
1120
Primary lead mass kg
Higher CATHARE lead free level
SG feedwater flow rate
196
192.8
To get Tout steam = 450 °C
SG feedwater temperature
335
SG steam outlet temperature
450
SG outlet pressure
180 8

9. Hot FA: Lead and external clad temperatures
Maximum core temperature (1/2)
(RELAP5 and CATHARE preliminary results)
Hot FA: Lead and external clad temperatures
RELAP5
CATHARE
Maximum clad temperature is below the safety limit of 550 °C for normal operation
ΔT lead-clad is over predicted by CATHARE due to different correlations used for the calculation of heat transfer inside the fuel rod bundle: Seban-Simazaki in CATHARE and Ushakov in RELAP5 9

10. Hot FA: Lead, external clad and internal fuel temperatures
Maximum core temperature (2/2)
(RELAP5 and CATHARE preliminary results)
Hot FA: Lead, external clad and internal fuel temperatures
RELAP5
CATHARE
Maximum fuel temperature is below 2000 °C – Large margin to fuel melting (approximately 730 °C)
Axial distribution of fuel temperature is strongly influence by fuel rod gap dynamic behaviour (fuel swelling and thermal dilatation) not taken into account in this preliminary analysis  constant gap size along the height and during transient 10

11. SG: Axial temperature profile
(RELAP5 and CATHARE preliminary results)
H2O
RELAP5
Steam
Gap
Lead
CATHARE
SG bayonet tube
- HTC on lead side by Seban-Simazaki in CATHARE and Ushakov in RELAP5
- SG heat transfer surface % with CATHARE due to reduced heat transfer capability with respect to RELAP5 11

12. ALFRED: DBC and DEC transients
20 transients (12 DBC and 8 DEC) of a total of 26 transients will be analyzed 12

13. Safety analysis
The main objective of the analysis of the DBC transients is to verify that in all foreseen design basis accident conditions the protection system by reactor scram and startup of the DHR system is able to bring and maintain the reactor in safe conditions assuring:
The core decay heat removal in the short and long term
That fuel rod and vessel temperature limits for each category (DBC1 – DBC4) are not exceeded
The DEC transients are events with very low frequency which include the failure of prevention and mitigation systems like the reactor scram in the so-called Unprotected transients
One of the main objectives of the Unprotected transient analysis is to evaluate the impact of the core and plant design features on the intrinsic safety behaviour of the ALFRED reactor
4 DEC (Protected) transients and 3 DEC (Unprotected) transients have been calculated in this preliminary analysis with RELAP5 and CATHARE codes

14. Main events and reactor scram threshold
ALFRED: Preliminary transient analysis
Main events and reactor scram threshold
RELAP5 and CATHARE (CEA collaboration) codes are used by ENEA for ALFRED DBC and DEC transient analysis
Preliminary RELAP5 and CATHARE results are presented. These results will be updated after comparison with other partner calculations (largest uncertainties for UTOP transient results due to the lack of a fuel rod gap dynamic model) 14

15. DBC (PROTECTED) TRANSIENTS

16. PLOF: Loss of primary flow (1/3)
(RELAP5 and CATHARE preliminary results)
Core mass flow rate
RELAP5
CATHARE
IE: Coast-down of all primary pumps at t = 0 s (pump speed halving time = 2 s)
Reactor scram on low primary pump speed after 3 s  FW and MSIV isolation on secondary circuits – Startup of DHR system (3 out of 4 IC loops)
Core flow rate reduces down to about 15% in 20 s and then progressively to about 4% after s 16

17. PLOF: Loss of primary flow (2/3)
(RELAP5 and CATHARE preliminary results)
Core and IC powers
RELAP5
CATHARE
After reactor scram at 3 s the core power reduces down to decay level (calculated by the code – higher core decay power level with CATHARE)
DHR system is promptly operational - Heat removal by 3 IC loops is about 4 MW with RELAP5 and about 8 MW with CATHARE (much enhanced steam condensation on the inner side of IC tubes) 17

18. Core inlet/outlet and clad peak temperatures
PLOF: Loss of primary flow (3/3)
(RELAP5 and CATHARE preliminary results)
Core inlet/outlet and clad peak temperatures
RELAP5
CATHARE
Initial temperature peak at core outlet due to core flow rate reduction with delayed reactor scram (3 s)
Maximum value of clad peak temperature calculated by RELAP5 (584 °C at 10 s) is well within the safety limits 18

19. PLOF-PLOHS: Loss of AC power (1/3)
(RELAP5 preliminary results)
Core mass flow rate
Core and IC powers
RELAP5
RELAP5
IE: loss all primary and FW pumps with reactor scram at t = 0 s
FW and MSIV isolation on secondary circuits – Startup of DHR system (3 out of 4 IC loops)
Core flow rate reduction like in PLOF and instantaneous transition to core decay level 19

20. Primary lead temperatures Core in/out and clad peak temp.
PLOF-PLOHS: Loss of AC power (2/3)
(RELAP5 preliminary results)
Primary lead temperatures
Core in/out and clad peak temp.
RELAP5
RELAP5
No significant increase in primary lead temperatures
No risk for lead freezing (T > 327 °C) at SG outlet in the initial part of the transient after DHR startup with injection of cold water from IC loop
Clad peak temperature is limited to 534 °C at t = 10 s, within the normal operation limit of 550 °C 20

21. Primary lead temperatures
PLOF-PLOHS: Loss of AC power (3/3)
(RELAP5 preliminary results)
Core, SG and IC powers
Primary lead temperatures
RELAP5
RELAP5
Maximum power removed by 3 IC loops is around 5.4 MW (1.8 MW per IC loop)
Core decay power is efficiently removed by the 3 IC loops after about t = 2500 s
When the IC removed power exceeds the core decay power the primary lead temperatures start to reduce – The minimum lead temperature is calculated at the SG outlet – lead freezing point (327 °C) is reached after about t = s (3.6 h) 21

22. Core in/out and clad peak temp.
POVC: Loss of FW pre-heaters (1/2)
(RELAP5 preliminary results)
Core, SG and IC powers
Core in/out and clad peak temp.
RELAP5
RELAP5
IE: loss of FW pre-heaters with FW temperature (335 °C) down to 300 °C in 1 s
Reactor scram on low FW temperature after 2 s  FW and MSIV isolation on secondary circuits – Startup of DHR with 4 IC loops to evaluate the risk of lead freezing
Core power reduces to decay level after reactor scram - Clad temperatures quickly reduce since the primary pumps remain in operation at nominal speed 22

23. Primary lead temperatures
POVC: Loss of FW pre-heaters (2/2)
(RELAP5 preliminary results)
Core, SG and IC powers
Primary lead temperatures
RELAP5
RELAP5
RELAP5
Maximum power removed by 4 IC loops is around 7.2 MW (1.8 MW per IC loop)
Core decay power is efficiently removed by the 4 IC loops after about t = 700 s
When the IC power exceeds the core decay power the primary lead temperatures start to reduce – Lead freezing point (327 °C) at SG outlet is reached after t = s (4.7 h) 23

24. Primary lead temperatures
PTOC: SG feedwater flow +20% (1/2)
(RELAP5 preliminary results)
Core, SG and IC powers
Primary lead temperatures
RELAP5
RELAP5
IE: SG FW mass flow rate +20% in 1 s (over cooling of primary side)
No reactor scram since the scram threshold set-points are not reached
Increase in SG heat removal capability and core power balances at 313 MW power level after about t = 300 s
Maximum core temperature decrease at core inlet is of 14 °C 24

25. Clad peak and fuel temperatures Total reactivity and feedbacks
PTOC: SG feedwater flow +20% (2/2)
(RELAP5 preliminary results)
Clad peak and fuel temperatures
Total reactivity and feedbacks
RELAP5
RELAP5
No significant fuel peak temperature increase
Clad peak temperature reduces by 10 °C
Core power evolution is determined by total reactivity behaviour – Negative reactivity feedbacks mainly by doppler and fuel expansion - Positive reactivity feedbacks by radial core and coolant expansion 25

26. DEC (UNPROTECTED) TRANSIENTS

27. ULOF: Loss of primary flow (1/6)
(RELAP5 and CATHARE preliminary results)
Core mass flow rate
RELAP5
CATHARE
IE: Coastdown of all primary pumps without reactor scram
The secondary circuits remain in operation in forced circulation
After an initial small core flow rate undershot natural circulation stabilizes in the primary circuit - RELAP5 and CATHARE codes predict similar stable natural circulation flow values  RELAP5 = 23.7% and CATHARE = 23.2% of nominal flow rate 27

28. ULOF: Loss of primary flow (2/6)
(RELAP5 and CATHARE preliminary results)
Core, SG and IC powers
RELAP5
CATHARE
The core power initially reduces due to negative reactivity feedbacks and then stabilizes at 205 (CATHARE) – 210 (RELAP5) MW in equilibrium with SG power
The SG power initially decreases due to reduced primary flow and then increases according with lead temperature increase at SG inlet 28

29. Core inlet and outlet temperatures
ULOF: Loss of primary flow (3/6)
(RELAP5 and CATHARE preliminary results)
Core inlet and outlet temperatures
RELAP5
CATHARE
Initial lead temperature increase at core outlet  max calculated value near 700 °C by RELAP5 at 15 s
Max core outlet temperature stabilizes just above 600 °C
The core inlet temperature progressively decreases and then stabilizes at 344 °C 29

30. Clad peak and max vessel temperatures
ULOF: Loss of primary flow (4/6)
(RELAP5 and CATHARE preliminary results)
Clad peak and max vessel temperatures
RELAP5
CATHARE
The initial clad peak temperature increase is below 750 °C  max calculated value is 738 °C by RELAP5 at 12 s
Clad peak temperature stabilizes below 650 °C – highest value calculated by CATHARE due to different heat transfer correlations used by the codes
No safety concern for clad and vessel wall temperatures

31. Fuel average and peak temperatures
ULOF: Loss of primary flow (5/6)
(RELAP5 and CATHARE preliminary results)
Fuel average and peak temperatures
RELAP5
CATHARE
Peak and average fuel temperatures reduce according to the decrease of core power level

32. Total reactivity and feedbacks
ULOF: Loss of primary flow (6/6)
(RELAP5 and CATHARE preliminary results)
Total reactivity and feedbacks
RELAP5
CATHARE
The negative control rod and core radial expansion (pads at core top) feedbacks induced by temperature increase at core outlet are mainly counterbalanced by positive Doppler and fuel expansion feedbacks (fuel temperature decrease)

33. ULOF+ULOHS: All SGs and PP trip (1/5)
(RELAP5 and CATHARE preliminary results)
Core mass flow rate
RELAP5
CATHARE
IE: Loss of offsite power (all SG FW and PP trip) without reactor scram
Startup of DHR system on the secondary side (4 IC loops)
The core mass flow rate initially reduces down to 20% of nominal value and then progressively reduces down to a residual flow rate of about % 33

34. ULOF+ULOHS: All SGs and PP trip (2/5)
(RELAP5 and CATHARE preliminary results)
Core, SG and IC powers
RELAP5
CATHARE
The core power reduces down due to negative reactivity feedbacks induced by core temperature increase – The power suddenly reduces down to about 170 MW and then progressively towards IC power level
Significant thermal inertia of secondary circuit contributes to remove power from the primary system in the initial phase of the transient 34

35. Core inlet and outlet temperatures
ULOF+ULOHS: All SGs and PP trip (3/5)
(RELAP5 and CATHARE preliminary results)
Core inlet and outlet temperatures
RELAP5
CATHARE
Initial core outlet temperature peak at about 700 °C and then progressive temperature increase up to about 800 °C
Temperature increase at core inlet is limited below 500 °C after 3600 s due to very low natural circulation flow rate in the primary circuit and large primary system thermal inertia 35

36. Clad peak and max vessel temperatures
ULOF+ULOHS: All SGs and PP trip (4/5)
(RELAP5 and CATHARE preliminary results)
Clad peak and max vessel temperatures
RELAP5
CATHARE
Initial clad peak temperature increase below 750 °C and then progressive temperature increase up to about 800 °C – Clad rupture may occur (to be verified)
Max vessel temperature increase is limited below 500 °C after 3600 s due to very low natural circulation flow rate which stabilizes in the primary circuit and the large primary system thermal inertia

37. Total reactivity and feedbacks
ULOF+ULOHS: All SGs and PP trip (5/5)
(RELAP5 and CATHARE preliminary results)
Total reactivity and feedbacks
RELAP5
CATHARE
The negative control rod, core radial expansion (pads) and coolant expansion feedbacks induced by temperature increase at core outlet are mainly counterbalanced by positive Doppler and fuel expansion feedbacks (fuel temperature reduction with decreasing core power)

38. Total reactivity and feedbacks
UTOP: Reactivity insertion (1/6)
(RELAP5 and CATHARE preliminary results)
Total reactivity and feedbacks
RELAP5
CATHARE
IE: Insertion of 250 pcm in 2 s without reactor scram (beta = 335 pcm)
The secondary circuits remain in operation in forced circulation
The inserted reactivity is mainly counterbalanced by negative Doppler and fuel expansion feedbacks induced by fuel temperature increase
Total reactivity reaches a maximum of about 175 pcm at 2 s and then reduces according to negative feedbacks 38

39. UTOP: Reactivity insertion (2/6)
(RELAP5 and CATHARE preliminary results)
Core power
RELAP5
CATHARE
The core power increases up to 870 MW (about 300%) in 2 s and then quickly reduces down to about 450 MW (150%) at t = 10 s 39

40. UTOP: Reactivity insertion (3/6)
(RELAP5 and CATHARE preliminary results)
Core, SG and IC powers
RELAP5
CATHARE
After the initial transient the core power progressively reduces and stabilizes at about 380 MW in equilibrium with SG removed power
SG power increases according to temperature increase at SG inlet on primary side and consequent steam outlet temperature increase on the secondary side (constant FW flow rate) 40

41. Core inlet and outlet temperatures
UTOP: Reactivity insertion (4/6)
(RELAP5 and CATHARE preliminary results)
Core inlet and outlet temperatures
RELAP5
CATHARE
After an initial jump of about 40 °C the core outlet temperature progressively increases according to core temperature increase at core inlet
The max core outlet temperature stabilizes at about 600 °C

42. Clad peak and max vessel temperatures
UTOP: Reactivity insertion (5/6)
(RELAP5 and CATHARE preliminary results)
Clad peak and max vessel temperatures
RELAP5
CATHARE
After an initial jump of about 60 °C the clad peak temperature progressively increases and stabilizes below 650 °C
The max vessel temperature remains below 500 °C
There is no safety concern for maximum clad and vessel temperatures

43. Fuel average and peak temperatures
UTOP: Reactivity insertion (6/6)
(RELAP5 and CATHARE preliminary results)
Fuel average and peak temperatures
RELAP5
CATHARE
The fuel peak temperature reaches a maximum value of 2600 °C in the initial part of the transient and then progressively reduces around 2400 °C
Fuel melting seems excluded (MOX melting point ~ 2673 °C)
Fuel rod gap dynamic behavior (not modeled) may significantly affect the UTOP transient results  further confirmation is needed using a more realistic gap model

44. Preliminary conclusions (1/2)
The preliminary accident analysis for ALFRED has confirmed the good inherent safety futures of the design that mainly rely on:
Low pressure drops in the primary system with enhanced natural circulation after primary pump trip
Large primary system thermal inertia for slowing down the transients
Redundant systems working in natural circulation for core decay heat removal
Significant negative reactivity feedbacks for limiting the core power and temperature increase during transients 44

45. Preliminary conclusions (2/2)
In particular the preliminary transient analysis for ALFRED has confirmed that:
In case of DBC (Protected Accidents) the prompt reactor scram actuation by the protection system and the startup of the decay heat removal system is able to maintain the core and vessel temperatures within the safety limits with adequate margin
In case of DEC (Unprotected Accidents) the core degradation and vessel failure is excluded and a large grace time is left to the operator to take the opportune corrective actions for bringing the plant in safe conditions in the medium and long term
The preliminary results must to be confirmed by further analysis, taking into account the fuel rod gap dynamic behaviour and enlarging the analysis to the whole set of representative accident initiators for DBC and DEC