ADS Safety and Comparison to Fast Breeders

1. ADS Safety Enrique M. González Romero CIEMAT Workshop on Applications of High Intensity Proton Accelerators Fermilab 20-X-2009 1

2. Outline • ADS Safety from its differences to Fast Reactors • Generic ADS Safety considerations • Specific results for XT-ADS 2

3. ADS vs Fast Breeder reactor from a Sefety perspective • Subcritical systems: • Subcritical Core • Different Kinetics (Source follower vs Delay neutrons feed) • Different Dynamics (Source driven vs. Feedback driven) • They host a Spallation Source • Large amount of inert material in the core center • Liquid metal volume in the core center • Vacuum penetration and pipes reaching the center of the core and connecting to the accelerator • Very high energy neutrons and protons (Shielding and activation) • Neutron flux peaked around the source (large variations on power densities) • Driven by a high energy high intensity proton beam • Beam trips (short – self recovered? - interruptions of the power) • Beam can be lost along the transfer line • Variation of the beam characteristics (Intensity, Energy, Position, focusing, shape) affecting source efficiency and power (overpower). • Using special fuels for Transmutation • Inert matrix or Low fertile (U,Th) content and high Pu & MA content • Very active and self heating fuels – Fuel cycle Recycling facilities • More graceful handling of low feedbacks and positive reactivity coefficients Design constraints Engineered sols 3

4. np follows to the source (or reactivity level) with a time constant of μs to ms. s(t) = S (constant) Point kinetics approximation Hypothesis: n(x,t) = f(x,t)/w=f(x)n(t) and Ci(x,t)=gi(x)ci(t) So the power will adapt to the level corresponding to the equilibrium neutron population within μs to ms (Sext/a). Then it will follow the population of delay neutrons with characteristic times of seconds to reach the total equilibrium power. In this second stage, thermal feedbacks can change reactivity an the final equilibrium power. 4

5. ADS vs Fast Breeder reactor The main difference between these two systems is expected on its kinetic and dynamic behavior after source or system perturbations and in particular in accident conditions. Change of beam intensity 5

6. Beam trips 6

7. Application of MUSE exper. results to industrial scale ADS Detector Detector Note that L might be different from MUSE to industrial ADS. E.g. For XADS (Pb-Bi) L = 2.3 ms whereas for MUSE L = 0.6 ms. 7

8. We can use beam trips to estimate reactivity by two methods: a • Prompt-Decay Constant Method: • Point Kinetics Model. • Source-Jerk Method: • Point Kinetics Model. βeff and Λ calculated using MCNPX 2.5.0 and JEFF 3.1 libraries. • Source-Jerk Method: • Prompt-Decay Constant Method: ρ = 7.35±0.30 $ ρ = 7.2±0.8 $ ρ(PNS) = 7.19±0.10 $ • Obtained with standard PNS: EUROTRANS Yalina (JIPNR- Minsk) – First Beam-Trip recording Beam Current Core Power Neutron Source

9. Yalina - Reactivity monitoring using beam trips in stable reactivity conditions ρ=7.26 ± 0.15 $ r(Beam trips)=7.26 ± 0.15 $ r(PNS)=7.19 ± 0.10 $

10. 10

11. 11

12. 12

13. Temperature transients after Beam trips Temperature difference in the Above Core Shielding of the ADTF after a 10s beam trip followed by a ramp back Temperature difference in the IHX tube Sheet Rim of the ADTF after a 1000s beam trip followed by a ramp back ASME Thermal Fatigue results Careful beam restoration + Trip resistant reactor design a must ! From F. Dunn, Consequences of Return to Power after a Beam Interruption in the Blanket of an Accelerator Driven System. ADTTA01. 13

14. Insertion of reactivity, keeping constant beam intensity: for example by the movement of the reactor elements, changes on the fuel temperature, or changes in the total mass or density of ADS elements,… Fast follow-up in the power proportional to the change on the prompt neutron multiplication, followed by a slower adjustment due to the delayed neutrons and the possible dynamic feedbacks. Dks/ksDM/M -/ (Prompt neutron multiplication and reactor power change) Initial  = –10b (ks=0.97-0.95), =1bDP/P=10% In a critical reactor =+1b makes the reactor prompt critical and =-1b stops the reactor. Smaller sensitivity of the power to reactivity changes: Advantage:Reduced sensitivity to unscheduled modification of the reactor operating conditions. (low b fuels) Inconvenience: Reduced effectiveness of the reactor feedbacks to compensate perturbations and to bring the reactor back to its normal conditions of operation. Very fast reaction to source/reactivity changes  Introduce artificial feedback mechanisms?Active control required !! In accident condition, warranty that the external source will be shut down! Then power will decay accordingly to the neutron generation time of the fundamental mode of the final configuration, down to the delayed neutrons levels followed by their consequent decay. 14

15. Reactivity insertion with beam ON Linear scale Logarithmic scale 15

16. 16

17. 17

18. Doppler Feed-back D D 18

19. ADS vs Fast Breeder reactor Stationary operation For the subcritical system, the fluence and the averaged power will be more concentrated surrounding the source (normally in the center) resulting in a higher peak-to-mean factor on the power density space distribution. This effect increases when keff decreases.  Difficulties on the power extraction by the coolant at the hottest point, with the final effect of limiting the maximum power of the system or with the increase on the cost of the cooling system.  Can be partially mitigated using several regions of fuels with different enrichments or by distributing the source to several spots. The neutron spectrum and the spectral indexes are not expected to change substantially (except close to the spallation source). 19

20. ADS control 20

21. LBE-Cooled XADS system 21

22. 22

23. Shutdown to Ambient Temperatures KeffHFP = 0.9728 Keffamb = 0.9857 23

24. Plant Startup from CZP 24

25. Protected Transient Overpower at Hot Full Power (PTOP) +300 pcm +2000 pcm 25

26. Protected Loss of Heat Sink and Flow 26

27. Unprotected Loss of Flow (ULOF) (a serious problem for higher power densities) For these calculations the primary pump was assumed to fail and the secondary heat transport system fully functional. The core inlet temperature was assumed to remain constant at 300 °C throughout the transient. The beam shut-off system is assumed to fail. The time response of the flow-rate subsequent to primary pump failure was presumed to correspond to the flow rate as calculated by RELAP 27

28. Unprotected Transient Overpower at HFP A step reactivity insertion of 2000 pcm. 28

29. Unprotected Beam Overpower to 200% at HFP (UBOP) 29

30. Unprotected Loss of Heat Sink (ULOH) & Unprotected Loss of Heat Sink and Flow (ULOH & LOF) At about 3000 sec into the transient peak pin clad temperatures will reach 1000 °C. Clad melting assumed at 5200 s. Later the vessel temperature can reach ~ 900 °C, where its integrity could become an issue 30

31. LBE-Cooled XADS system • Favorable thermophysical properties of the coolant LBE • very high boiling temperature, low vapor pressure at operating conditions, reasonable value for the melting temperature, high specific heat and thermal expansion coefficient, good heat transfer characteristics, • High thermal inertia, or large heat sink / heat buffer characteristics associated with • the coolant Pb-Bi (LEB) as reflected by: • the large coolant mass times heat capacity in the core region, the large coolant mass times heat capacity in the primary system (pool type design), • Excellent natural convection characteristics of the plant design due to: • the favorable sub-assembly design ( pin diameter and pitch ), the large height difference between heat source (core) and heat sink (IHX), namely ~ 3 m, • Modest liner power ratings: • peak pin averaged linear power rating = 131 W/cm, peak power rating = 154.1 W/cm. A combination of these features assures that the core damage frequency is very low (perhaps <10-5) and that the core and primary system temperatures for almost all scenarios are below the temperature limits established to avoid clad melting and radioactivity release in the vessel. There are 2 scenarios for which the core temperatures can reach high values and melting of clad could occur in the peak channel. These are (a) the unprotected almost- complete blockage of a subassembly in the core and (b) the unprotected loss of heat sink coupled with loss of flow. Both can be solved by activating a beam trip with long grace times 31

32. LBE-Cooled XADS system Reactivity Coefficient Issues The analysis performed showed that reactivity coefficients do not play a significant role in the dynamic response during protected or unprotected transients of the XADS system while the system is at hot full power (HFP) or hot zero power (HZP) conditions. The reason for this is that the reactivity changes due to temperature feedbacks at HFP are small when compared to the margin to criticality, (or the level of sub-criticality), especially for Keff < 0.97. Reactivity coefficients gain in importance when Keff> 0.98 since then the dynamic response of the sub-critical system approaches the behavior of critical systems. For ADS designs without active control rod systems, however, reactivity coefficients play a dominant role during the shut-down and startup procedures. The reactivity coefficients alone determine the increase in the level of sub-criticality when the sub-critical assembly changes from the hot full power (HFP) operating condition to the cold zero power stand-by condition (CZP). All reactivity coefficients must then be known to a high degree of precision in order to assure that the system does not reach criticality during the shut-down procedure under BOC conditions with Keff relatively high at HFP, namely BOC Keff ~ 0.97. This issue is of particular concern in the case of use of minor actinide fuels in the XADS 32

33. 33

34. CONCLUSIONS (generic) • • The ADS’s dynamic response to changes in reactivity or neutron source strength is the area of greatest difference in safety characteristics between fast reactors and ADSs and an area where no precedents exist in the fast reactor experience base. Main differences: • (a) the external neutron source which can provoke rapid and, depending on the sub-criticality level, large neutron-kinetic responses, unmitigated by the delayed neutrons, and • (b) the fertile-free fuel which features very weak reactivity feedbacks, especially from the Doppler effect. • • This puts high demands on the control actuators, the fuel behaviour, and the heat removal processes. In particular, the strong dissimilarity of the neutron-kinetic and thermo-structural time constants requires the fuel to be capable of sufficient adiabatic heat storage. • • The weak Doppler effect exhibited by fertile-free fuel affects the energetics of hypothetical core disruptive accidents. If such accidents have to be taken into account in the safety analysis of an ADS, a prompt quenching mechanism will have to be developed. • • The management of the surplus neutrons in sub-critical cores with fertile-free fuel by means of neutron leakage and/or absorption involves delicate trade-offs which affect core design. This applies particularly to TRU burners which feature a high burn-up reactivity loss. • • Partially fertile fuels with low fractions of U or Th are being considered to address some of these issues and the fuel fabricability and performance. 34

35. CONCLUSIONS (LBE cooled ADS) Safety can only be assessed for specific, LBE studied in PDS-XADS FP6 project. The system showed in general very good safety characteristics and graceful recovery from accidental conditions. For the low power density LBE cooled ADS, the core damage frequency is very low (perhaps <10-5) and that the core and primary system remain within safe temperatures for almost all scenarios . The 2 problematic scenarios are (a) the unprotected almost- complete blockage of a subassembly in the core and (b) the unprotected loss of heat sink coupled with loss of flow. Both can be solved by activating a beam trip with long grace times. However several accidental scenarios were very sensitive to the power density (ULOF and others), and can limit the power density and the maximum ADS power. Myrrha (XT-ADS) studies and hopefully operation for higher power densities. 35

36. Bibliography: • 0) Accelerator-driven Systems (ADS) and Fast Reactors (FR) in Advanced Nuclear Fuel Cycles A Comparative Study. NEA 3109 (2002) • Accelerator and Spallation Target Technologies for ADS Applications A Status Report, NEA 5421 (2005) • Fuels and Materials for Transmutation A Status Report ,NEA 5419 (2005) • Análisis dinámico del reactor experimental de fisión nuclear MUSE-4. Villamarin-Fernandez, D. Ph.D.Thesis in Universidad Complutense de Madrid (2004). • Assessments of the kinetic and dynamic transient behavior of sub-critical systems (ADS) in comparison to critical reactor systems. W.M. Schikorr. Nuclear Engineering and Design 210 (2001) 95–123 • PDS-XADS Deliverable 45: Transient Analyses of the MOX-fueled 80 MWth LBE-cooled PDS-XADS Compiled by M. Schikorr (FzK) (2004) 36